New frontiers in wide-band-gap semiconductors and heterostructures for electronics, optoelectronics and sensing

Resume : Outstanding properties of hexagonal polytype of silicon carbide (4H-SiC) make this material most suitable to fabricate low ON-resistance (RON) devices and to improve the energy efficiency in the next generation of power electronics systems. Minimizing the conduction losses in discrete power devices is a fundamental requirement to reduce the overall energy consumption of modern circuits. Switches and rectifiers are key components in power electronics and the SiC Junction Barrier Schottky (JBS) devices are candidates to replace the Si PiN diodes in the 600 - 3300 V blocking voltage range. The JBS rectifier combines a Schottky and PiN diode structure making use of the advantages of both types. Since the core of a Schottky Barrier Diode (SBD) is the metal/semiconductor interface, in the last two decades several studies have investigated the electrical properties of various metallization schemes for Schottky contacts to 4H-SiC. The reduction of the barrier height (i.e., the device turn-on voltage) is one of the current challenges in the SBD technology. In literature, metallization schemes based on low-work-function metals with high melting point (W, Mo, etc.) or adjustable compositions have been investigated to optimize the performance of 4H-SiC Schottky diodes. Currently, one of the most used metal as Schottky barrier for the 4H-SiC-based rectifiers is Ti, which typically gives barrier height values around 1.22 eV and it represents our reference. In this work, the possibility to control the Schottky barrier properties in Ti/4H-SC diodes, by varying either the metal thickness or the post-annealing conditions, is explored. A study of the electrical behavior of tungsten carbide (WC) Schottky contacts on 4H-SiC is also reported. JBS devices were fabricated using Ti or WC as Schottky metals on a commercial n-type 4H-SiC wafer, and by changing the doping concentration of the SiC surface. The role of the different process parameters on the electrical characteristics of these devices was evaluated. The ideality factor and barrier height were extracted from the current-voltage (I–V) measurements on a large set of equivalent diodes, that also allowed to evaluate the process uniformity over large region of the wafer. In addition, structural analysis was performed to evaluate on a micrometric scale the effect of the different processes on the formation of the Schottky barrier. Finally, a design optimization of the JBS to reduce the corresponding increase of reverse leakage is described.

Resume : 4H-SiC is nowadays playing a key role in microelectronics as wide bandgap semiconductor with high critical breakdown field, high-thermal conductivity, and low intrinsic carrier concentration. But, although having already reached the market, SiC-based transistors suffer from low-quality interface with SiO2, the most common gate insulator in silicon technologies. Moreover, SiO2 low dielectric constant can lead to early device breakdown. In the last years, new high-k dielectrics have been introduced in 4H-SiC MIS devices and Atomic Layer Deposition (ALD) has been studied as the most promising thin layer synthesis technique to obtain optimal interface properties. Among all, ALD of binary oxide Al2O3 resulted in amorphous, thermally stable, high-k (~8-10) dielectric with good band offset compared to 4H-SiC. Still, electrical measurements demonstrate that further improvement is needed. Past works focused on AlN ALD layer too, due to its low lattice mismatch with 4H-SiC substrate which potentially may promote low density of interface states. On the other hand, polycrystalline structure at needed thickness or misfit dislocations causes high leakage current. In this work, a new solution is proposed to exploit advantageous features of the two dielectrics: uniform, conform and thickness-controlled Al2O3/AlN bilayers were fabricated on both n-type and p-type 4H-SiC via Thermal or Plasma-Enhanced ALD. Substrates were treated prior deposition with piranha solution (H2SO4:H2O2 4:1 for 10 min) and Buffered Oxide Etch solution (HF:NH4F 6:1 for 5 min). Deposition of 10nm of AlN was performed at 300°C, using trimethylaluminium (TMA) as precursor and NH3 plasma treatment as reaction step. Then, 20nm of Al2O3 were obtained at 250°C by using TMA as precursor and H2O as reactant in Thermal-ALD or O2 plasma in Plasma Enhanced-ALD. On the same substrate, 30 nm of Al2O3 were deposited via Plasma Enhanced ALD for comparison. Effects of oxygen plasma on AlN underlying layer during Al2O3 growth is discussed. Structural properties of the thin films on substrate were studied via transmission electron microscopy, and MIS devices were realized to characterize electrical behaviour, density of interface states, fixed oxide charges and oxide trapped charges. Promising characteristics with respect to single oxide layer are highlighted. In particular, the investigated samples demonstrated a reduction of the oxide trapped charge (almost one order of magnitude down to 10^11-10^12), a slight reduced amount of interface traps density and an increase in the order of 3 units of dielectric constant (from ~7.5 to ~10.4) compared with the Al2O3 30nm layer. These preliminary results encourage the investigation of Al2O3/AlN bilayers as a candidate for future 4H-SiC power MOSFET applications.

Resume : Coating the surface of 4H-SiC with a carbon layer (C-cap) is a consolidated technique to prevent the step bunching phenomenon observed after post implantation annealing. However, surface changes are not completely prevented. Holes on the surface with root mean square (rms) roughness higher than 1 nm was observed in Al ion implanted 4H-SiC annealed at temperatures higher than 1750 °C [1,2]. It was observed that for equal annealing treatments the rms increases for increasing Al concentration and that a prolonged annealing, even at temperatures as low as 1650 °C, also results in increased rms [3]. Some authors attribute surface roughening under the C-cap to a deterioration of the C-cap itself [1,2]. However, the occurrence of material evaporation even under a stable C-cap is not completely ruled out in the literature [4,5]. Indeed, SIMS analyses revealed surface erosion of Al implanted 4H-SiC, subsequent to annealing at 1900 °C and 2000 °C with durations between 15 min and 1 h, while the surface remained smooth [4]. That study attributed the material loss to the lattice damage induced by the ion implantation [4]. By analysing the changes in the C-cap and at the interface between the C-cap and Al ion implanted 4H SiC at different stages of the processing, we confirmed that the C-cap is compact and observed that, after the C-cap removal, the SiC surface is covered by a network of grains which can be removed in a Buffered Oxide Etch (BOE) solution [5]. In this work we investigate the surface of 4H-SiC implanted with different Al concentrations by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Fourier Transform Infrared Spectroscopy (FTIR) in order to detail the mechanism of material loss from the SiC surface. References [1] K. V. Vassilevski, et al., Semicond. Sci. Technol. 20, 271 (2005). [2] M. Spera, et al., Materials Science in Semiconductor Processing 93, 274 (2019). [3] M.Canino, et al., Journal of Microscopy, 280: 229-240 (2020). [4] M.K. Linnarsson, et al., Mat. Sci. Forum 924, 373 (2018). [5] M.Canino, et al., Mat. Sci. Forum 1062, 235 (2022).

Resume : In last years, the research and technological progress on the most common hexagonal polytype of silicon carbide (4H-SiC) has produced huge advancement in power electronics, with the introduction on the market of more efficient devices if compared with traditional Si-based ones. In spite of such a high level of maturity, considerable efforts are still being dedicated to improving device performance, to fully exploit 4H-SiC material potentialities. Already in the device processing phase it is possible to operate towards an amelioration of the device characteristics. In particular, although the ion-implantation technique is a consolidated method for selective doping in 4H-SiC-devices, this process inevitably produces lattice damage and post-implantation thermal annealing treatments are required to restore crystalline structure and achieve dopant electrical activation. Typically, these annealing treatments are carried out in furnaces at high temperature (>1600°C) and this can in turn generate defects detrimental for the electrical activation. Among all the explored unconventional rapid thermal annealing treatments, pulsed-laser annealing methods can provide extremely fast heating ramps and locally much higher temperatures than conventional furnace annealing processes. In this work, we explore the effects of pulsed XeCl-laser irradiation (308 nm, 160 ns, fluence between 1.0 and 2.4 J/cm2) on Al-implanted 4H-SiC layers. In order to obtain a wide picture of the material modifications induced by the laser exposure, we investigated by combining various techniques the morphological (by AFM), structural (by TEM) and electrical properties (by C-AFM and I-V-T measurements on appropriate structures) of high concentration Al-implanted 4H-SiC layers. An evolution of the morphology and local current conduction was observed with the laser fluence increasing, with the appearance of micron-size features producing an increase of the local current up to a saturation condition for the highest fluence irradiated sample. By cross-sectional TEM analyses, we were able to explain these changes in the microstructural properties, with the appearance of a thick polycrystalline layer and two separated layers on the top, consisting of a carbon-rich region and an underlying crystalline-silicon layer. The electrical properties of the 4H-SiC Al-implanted layer were also evaluated, obtaining a total resistance of hundreds of kOhm, a sheet resistance of 1.62E4 kOhm/sq and a resistivity of 160 Ohm×cm at room temperature. This latter value is significantly higher than the value of 0.36 Ohm×cm measured in a 4H-SiC sample implanted under the same conditions but subjected to conventional thermal annealing. The result can be explained either by a very low electrical activation of the Al-implanted species and/or a poor mobility of the carriers. The outcomes of this study can be useful for a fundamental understanding of the mechanisms involved in laser annealing treatments of 4H-SiC implanted layer.

Resume : Silicon carbide (4H-SiC) possesses a favorable combination of physical properties, making it attractive for various power electronics applications. MOS-based electronic devices can be realized by forming silicon dioxide (SiO2) upon thermal oxidation, but this process is accompanied by the release of carbon and by the formation of carbon-related defects near the SiO2/4H-SiC interface, leading to a density of interface traps much higher than in Si/SiO2 system. Hence, the SiO2/4H-SiC interface quality is continuously investigated in order to assess the key parameters determining the current transport in the channel. In this work, we present a reliable method to precisely evaluate both effective channel mobility and interface state density in SiC MOSFETs, by combining I-V and C-V measurements performed on the same device [1], thus overcoming the use of different structures or complex Hall measurements. The C-V measurements are performed in the gate controlled diode configuration with source, drain and body contacts tied together, providing a quasi-ambipolar behavior of the MOS structure and allowing to explore the interface state density over the entire bandgap. The computation of the electrical characteristics is based on the solution of the Poisson equation in the presence of interface states. The electrostatic potential distribution along the depth of the channel determines the amount of trapped charge at interface, the free carrier and space charge distributions. The fit of the I-V characteristics alone has been found not reliable in determining unambiguously the interface state density and carrier mobility, due to the correlations existing between these quantities: to solve this problem, both the I-V and C-V characteristics have been simultaneously fitted using a unique set of parameters in the model. The frequency dependence of the C-V characteristics has been accounted by introducing an effective modulation length, that is the characteristic length over which the carrier density in the channel is modulated in response to a small AC signal. This approach has led to an evaluation of the interface state density and carrier mobility that should be regarded as more reliable and consistent than independent estimate obtained with unrelated methods applied to different devices. The proposed method proved to be very effective in determining these quantities in two different types of devices. The field effect analysis performed on the device characteristics measured at different temperatures also allowed to clarify the temperature dependence of the different regions of the transfer characteristics indicating a predominance of the Coulomb scattering at interface charge mechanism in on-regime (strong inversion) [1]. [1] A. Valletta, F. Roccaforte, A. La Magna, G. Fortunato, and P. Fiorenza “Reliable evaluation method for interface state density and effective channel mobility in lateral 4H-SiC MOSFETs” (2022) Semicond. Sci. Technol. in press https://doi.org/10.1088/1361-6641/ac773c

Resume : The request of reliable Wide band Gap material for power devices and high voltage products is ever increasing during last decades to meet the huge use in many application fields. Such expansion has to been driven by innovative metrology techniques that allow to characterize the material in terms of crystal quality, electrical performance and affordable price of the materials. Silicon Carbide is going to cover a crucial role in such development especially for Electrical Vehicles and power electronics [1]. The enlargement of the seed size from 150 mm to 200 mm is currently underway, such process represents a challenge in terms of improvement of crystal quality. Many advanced inspection methods give us the opportunity to follow the steps forward made to obtain high quality materials by keeping under control the defectiveness of the substrates that are intrinsic generated along the growth process of the ingot. Micro-Raman spectroscopy is a power technique to detects defects on Silicon carbide [2]. It has been used to analyze new size material highlighting its weakness concerning the defect density increase and providing new insights. Such detailed study has been conducted starting from general overview of the substrate material by an optical microscope characterization that revealed, on Si-face, a defect density slightly greater than previous size material. Such initial inspection revealed a slightly higher micropipe density that are strongly connected to sublimation growth, especially linked to doping incorporation and temperature gradient along the reactor chamber. However, the most important additional defect detected on 200mm material is the polytype inclusion which had disappeared entirely on the 150mm material. Such defects are revealed as extended filaments, up to several millimeters, by optical inspections. Such features have been characterized by profilometry, revealing non-planar structures. Voids with hundreds of nanometers of deep are linked to such polytype inclusions. Corona Kelvin (QUAD) technique has revealed, by measuring the reduced surface voltage, a localized lifetime carrier far from 4H-SiC typical value, confirming the polytypic nature of the defects. Micro-Raman spectroscopy has been used to characterize the details of such inclusions. The laser source used (325nm) allows us to study a thin layer of material being the penetration depth in SiC less than 10 µm. The grating used (1800 gr/mm) results in a spectral resolution close to 1 cm-1 that is enough to discern the phonon modes and recognize the nature of polytypes. The results (that will be showed at the conference) show a strong broadening of the transverse optical phonon mode (TO) leading to determine the presence of 6H polytype and probably rhomboidal crystal structures [3] that will be confirmed by transmission electron microscopy (TEM) analysis. The study of Longitudinal optical phonon mode (LO) reveals a slight displacement and considerable broadening that could be linked not to different polytype but to different doping incorporation. Also, as reported in literature [4], several damages, like scratches, on 4H-SiC could reveals Carbon inclusion that are well detected and characterized by Raman spectroscopy. Our analysis is dedicated to the study of such defects too. Reference [1] T. Kimoto & J. A. Cooper, Fundamentals of Silicon Carbide Technology, ISBN: 978-1-118-31352-7 (2014) [2] N Piluso, M Camarda, F La Via, Journal of applied physics 116 (16), 163506 [3] S. Nakashima and H. Harima, Phys. Stat. Sol. (a) 162, 39 (1997) [4] S. Nakashima, T. Mitani, M. Tomobe, T. Kato, H. Okumura, AIP Advances 6, 015207 (2016)

Resume : Atom probe tomography (APT) is a microscopy technique that allows the study of the three-dimensional atomic distribution and composition of a needle-shaped specimen with a curvature of radius around 50nm [1]. Though widely applied in the study of wide bandgap semiconductors, its results should be critically verified through an accurate consideration of the physical effects limiting its performances [2,3]. Concerning the measurement of the atomic composition, carbon in binary semiconductor materials and alloys poses several difficulties in APT [4,5]. Carbon molecules instead of C atoms are very often observed while analyzing in these materials. Furthermore, it still remains unclear how the evaporation mechanism impacts the composition measurement, which is typically poor in C. In this contribution, we investigated the influences of surface dynamics and molecular evaporation on silicon carbide (SiC) in laser assisted APT. Several molecular ions mainly related to carbon are detected, despite the first nearest neighbors of C in the bulk SiC lattice are only Si. We also observe evidence of dissociations of evaporated C molecules. The accuracy of the measurement of atomic positions, calculated by the Fourier transform method, is strongly degraded [6]. This means that the surface is either rough or very disturbed. This is confirmed by the field ion microscopy (FIM) analysis which reveals that the atoms move on the surface of the sample when it is subjected to high electric field. This result might explain the mechanism of the observed molecular formation and the loss of carbon in the calculated composition. [1] D. Blavette, A. Bostel, J.M. Sarrau, B. Deconihout, A. Menand, Nature. 363 (1993) p.432 [2] Mancini L. et al. (2014). The Journal of Physical Chemistry C, 118(41), 24136-24151. [3] Rigutti, L. et al. Journal of Applied Physics, 119(10), 105704. [4] Thuvander, M et al. 2011. « Quantitative Atom Probe Analysis of Carbides ». Ultramicroscopy 111 (6): 604‑8. [5] Estivill, Robert, et al. 2015. « Quantitative Investigation of SiGeC Layers Using Atom Probe Tomography ». Ultramicroscopy 150 (mars): 23‑29. [6] LEFEBVRE, Williams, VURPILLOT, Francois, et SAUVAGE, Xavier (ed.). Atom probe tomography: put theory into practice. Academic Press, 2016

Resume : 3C-SiC is a particularly attractive SiC polytype since it has higher mobility and a lower density of states at the SiC/SiO2 interface than 4H and 6H-SiC. These characteristics along with 2.5 eV bandgap make 3C-SiC suitable for power electronics applications, due to several benefits in MOS devices such as a notable Ron reduction for medium voltage applications working under 1200 V. Current technology is largely based on Si heteroepitaxial growth which thus involves 20% mismatch in the lattice parameter and 8% different thermal expansion coefficient [1]. As a consequence several types of defects, such as dislocations, stacking faults (SFs) and grain boundaries nucleate close to the 3C-SiC/Si interface. In particular, 3C-SiC films grown on (111) Si substrates exhibit poor crystal quality and experience wafer cracks and bowing. The huge tensile stress, which builds up on first layers, hinders the development of (111) oriented 3C-SiC wafers with large surface area and thickness. To address these issues, we provide a new growth strategy together with a comprehensive characterization of the interaction dynamics of defects inside the bulk (111) grown crystal. 3C-SiC growth was performed in a horizontal hot-wall CVD reactor on 4 inches Si substrate. Precursor gases were Tri-chloro-silane (TCS) and Ethylene (C2H4). A two step growth process was applied with the carbonization plateau at 1160 °C followed by CVD growth step at 1400 °C. The growths were carried out by doping 3C-SiC with N which has previously been revealed to hinder the propagation of 4H and 6H-like SFs in 3C-SiC while also supplying compressive stress to the crystal in order to compensate wafer bowing [2-3]. To further improve crystal quality the process was controlled under growth rate from 3 μm/h to 30 μm/h until a ~75 μm layer was reached. Afterwards, Si substrate underwent melting at 1600 °C. The remaining freestanding SiC wafer was used as a seed layer for subsequent N doped homoepitaxial growth at 1600 °C until 220 µm thickness were achieved with doping design suitable for common devices substrates. In previous studies only layers up to 20 μm were considered, whereas the adoption of the above-mentioned innovative approach resulted in the thickest 3C-SiC growths ever achieved on Si (111) substrates. Using 500 °C molten KOH etching, a detailed statistical analysis was carried out by means of an angle-resolved SEM investigation. This allowed to evaluate the evolution of the density of SFs along the entire grown layer and to investigate the dynamics of generation and annihilation of SiC defects. Both μ-PL and μ-Raman investigations were performed to follow the crystal at various growth temperatures during the heteroepitaxial as well as homoepitaxial phases in order to assess dopant distribution in relation to stress and gradual crystallinity enhancement. HAADF-STEM analyses atomically described the behavior of both dislocations, SFs and twin boundaries. Key mechanisms such as stress stimulated reduction of the number SFs layers and self-closure, as well as lack of annihilation between SFs on opposing {111} planes through Lomer and lambda-dislocations, allowed to mark the differences between growths on Si (100) and depict peculiar dynamics in response to growth parameters. [1] Francesco La Via, et al. New approaches and understandings in the growth of cubic Silicon Carbide, Materials, 2021, 14(18), 5348. [2] Y. Umeno, et al. Ab initio density functional theory calculation of stacking fault energy and stress in 3C-SiC Phys. Status Solidi B, 1–6, (2012). [3] Cristiano Calabretta, et al. Effect of nitrogen and aluminum doping on 3C-SiC heteroepitaxial layers grown on 4°◦off-axis Si (100) Materials, 2021, 14(16), 4400.

Resume : Even though the micromachining techniques for silicon (Si) are well developed, silicon micro- and nano-electromechanical systems (MEMS and NEMS) are not suitable for use in harsh environments (high temperature, high frequency, high wear, and corrosive). In extreme conditions, Silicon Carbide (SiC) appears to be a good candidate to replace Si due to its exceptional physical and chemical properties [1]. Furthermore, the high Young's modulus and the relatively low mass density induce a significantly higher resonant frequencies and Quality (Q)-factors in resonant devices at the same geometrical dimensions in comparison with Si or gallium nitride [2]. Q-factor in flexural 3C-SiC resonators critically depends on the presence of stress, in particular on the anchors of the resonator. For characterization techniques, sample preparation can potentially affect the distribution of the stress field as it affects the thickness, morphology, curvature of the sample. In this context, micro-Raman spectroscopy satisfies these requirements. It is a non-destructive and fast optical technique, suitable for characterizing the stress field in epitaxial layers. The shift of the transverse optical mode (TO) in the Raman spectra is strongly correlated to the stress fields [3]. In this work micro-Raman analysis were performed to study the distribution of the stress fields within the epitaxial layer of a set of free-standing double clamped beams (thickness less than 1 μm). We observed that the average stress of the beams is independent of the length, but it depends on its width. In particular, the microstructures show an average tensile stress between 100-350 MPa, for widths less than 6 µm. Increasing the width, the TO Raman shift in the undercut region (close the two anchor points) evidences a different stress profile respect the beam. This difference is due to the processes carried out to realize the microstructure. During the silicon etching, the process also affects the 3C-SiC/Si interface, removing a thin layer of 3C-SiC. This leads to a small variation in thickness between the center and the edges of the structures which induces a reorganization of the stress fields. The role of the orientation of the silicon substrate was also investigated. An almost stress-free value was obtained for long doubly clamped beams (width 16 μm, length [200-1000 μm]) manufactured on 3C-SiC grown on Si (100). While an average residual tensile stress of about 600 MPa was observed on structures manufactured on 3C-Sic grown on Si (111). [1] M. Mehregany, C. Zorman, N. Rajan, C. Wu, Proc. IEEE (1998), 86(8), 1594 [2] K.L Ekinci and M.L Roukes, Rev. Sci. Instrum. (2005), 76, 061101 [3] D. Olego, M. Cardona, Phys Rev B (1982), 25, 1151

Resume : In the last few years, the expansion of the market for silicon carbide (SiC) power devices, driven by the spread of electric vehicles, was so rapid that many suppliers are struggling to meet the demand for SiC power devices, so far produced from wafers with diameters of 150 mm. As a countermeasure, several companies started the production of SiC wafers with a diameter of 200 mm, to obtain a potential increase of the number of devices per wafer of about the 80%. In this framework, a European project called REACTION has been funded with the aim of developing the world’s first 200 mm SiC pilot line facility. STMicroelectronics (ST) and LPE are two of the main partners that collaborate to this project. ST Silicon Carbide AB is developing its own physical vapor transport (PVT) process to grow 200 mm SiC boules with low defect density, and has already produced 200 mm 4H-SiC substrates of good quality. In parallel, LPE has designed and manufactured its latest generation of horizontal hot-wall Chemical Vapour Deposition (CVD) reactor, called PE1O8, to perform uniform epitaxial process on 200 mm SiC substrates. Achieving uniform and reproducible thickness and doping of epi-grown SiC is more challenging on larger substrates. To meet this challenge, computational fluid dynamics simulations are performed by LPE to optimize gas flow and temperature distributions. Combining a tunable multi-zone gas delivery configuration together with the compact design of the reactor, the PE1O8 system ensures an even gas and temperature distribution throughout the substrate. The latest results of epi-layers grown by LPE on ST manufactured 200 mm 4H-SiC substrates are reported in this work. N-type 4H-SiC Si-face wafers with 4° off-cut are used as substrates. 4H-SiC epitaxy is performed at 1600°C, using trichlorosilane (Cl3HSi) and ethylene (C2H4) as silicon and carbon atoms precursors, and hydrogen (H2) as the carrier gas. The reactor’s small graphite mass and the automatic high-temperature load/unload system permit the rapid rump up and cool down, while bake out step is completely avoided, thus, enabling the completion of one standard run in only 60 minutes. Morphological and electrical characterization of the wafers before and after the epitaxial growth are performed using different techniques. By optimizing the process parameters, uniform epi-layers are grown across the 200 mm substrates, with thickness and doping variation (σ/average) as low as 1.2% and 1.9%, and run-to-run variation of 0.5% and 1.1%, respectively. Furthermore, thanks to the improvements achieved in the substrate crystal quality, in the chemistry of precursors, and in the epitaxial process, epi-grown 200 mm wafers with very low defect density comparable to the state-of-the-art 150 mm wafers are obtained, with routinely maintained Total Usable Area of 96%.

Resume : We investigated degradation mechanisms in AlGaN-based ultraviolet (UV) light emitting diodes (LEDs) through a constant current accelerated lifetime test, by means of C-DLTS measurements. These UV-C LEDs have a single quantum well (SQW), an area of 0.1 mm2, an emission wavelength of about 265 nm and they were grown by metal organic vapor phase epitaxy (MOVPE). The constant current stress was carried out at the nominal current density of 100 A cm-2 for 12000 min (about 200 h); during stress we carried out electrical and optical characterizations with logarithmic time steps (at 0 min, 1 min, 2 min, 5 min, …). At specific levels of degradation (i.e. when the optical power reached the 100%, 92%, 85% and 75% of its initial value), we carried out C-V and C-DLTS measurements. C-DLTS were done in three ranges: VMEAS = -2 V to VFILL = 0 V, VMEAS = -1 V to VFILL = -2.5 V, and VMEAS = 1 V to VFILL = -1 V to explore different sections of the active region. It is worth noting that in two measurements we used a filling voltage lower than the measurement voltage, in order to avoid charge accumulations in the structure. Electrical measurements showed that stress induced an increase in parallel resistance components, an increase in the conduction at low forward voltage (ascribed to trap assisted tunneling, TAT) and an increase in drive voltage. The first two effects could be ascribed to an increase in leakage components during the stress test due to generation of defects; on the other hand, the increase in the drive voltage could be associated with a contact degradation or a decrease in the injection efficiency. Stress was found to induce a decrease in optical power at all current levels; this effect was stronger at low measuring current levels, where the OP log-log slope showed an increase from 1 to 2, indicating an increase in non-radiative recombination. Along with the increase in leakage current, these results confirmed the generation of defects in the active region of the device. Another evidence of the defects generation can be found from C-DLTS measurements. After 1000 min of stress, we detected the presence of a new peak in C-DLTS spectra called E3, with an activation energy of about 700 meV and possibly correlated to nitrogen antisites or gallium vacancies (VGa). From these measurements we also found the presence of two defects, H1 and H2, with activation energy of about 475 meV and 150 meV, respectively. From a literature comparison, H2 was ascribed to magnesium-related defects, whereas H1 could be related to Mg or N vacancies. In conclusion, we presented an exhaustive analysis of the behavior of a 265 nm LED during a constant current stress. We were able to identify the degradation of the electrical and optical properties of the device, both of which could be correlated to the generation of defects in the structure. Moreover, by means of C-DLTS analysis, we found the presence of three distinct deep levels, two possibly correlated to Mg, and one ascribed to N antisites or VGa.

Resume : The increase in forward leakage current of InGaN/GaN light-emitting diodes (LEDs) during aging has often been ascribed to the stress-induced increase in the density of traps and/or non-radiative recombination centers (NRRCs), in proximity of the active region of the device. With this work we demonstrate that i) deep levels contribute to sub turn-on forward leakage current, due to the existence of trap-assisted tunneling components (TAT); ii) that the spatial location and energy position of traps strongly influences this process. To this aim, we fully characterized and stressed on-wafer single quantum-well (SQW) LEDs grown on sapphire substrate by metalorganic vapor phase epitaxy (MOVPE). The devices feature a 12% indium molar fraction within the 2.8 nm QW, which allowed to achieve a nominal emission wavelength of about 420 nm. The active region, which also included two n-doped GaN barriers and a 20 nm undoped spacer placed below the electron-blocking layer (EBL), was grown on top of a superlattice underlayer (SL UL) to limit the propagation of defects originating from the buffer region. Such devices were submitted to a room-temperature (RT) constant current stress at 150 mA, corresponding to a stress current density of 167 A/cm^2, for a duration of about 292 h. As a consequence of the accelerated aging procedure, the devices exhibited minor optical degradation in the low current regime, indicating no relevant generation of NRRCs within the QW, whereas a monotonic and strong increase in the forward leakage was observed. By means of numerical simulations, carried out through the TCAD Sentaurus suite from Synopsys Inc., we were able to prove that for the devices under investigation i) the forward leakage is dominated by trap-assisted tunneling, ii) that this process strongly depends on the density of deep levels within the undoped spacer layer, and iii) that the increase in magnitude of the low forward leakage conduction can be ascribed to the stress induced increase in the concentration of a defect related to a deep level located around Ec-1.72 eV, i.e. at the middle of the GaN gap. The presence of such defects was investigated by photoionization cross-section (PCS) spectra, extrapolated through deep-level optical spectroscopy (DLOS) measurements, and is agreement with several other literature reports. The identified correlation between the forward leakage current and the presence of deep-levels within specific device layers indicates that the analysis of I-V curves can provide useful aging indicators, which can be used to analyze the electrical behavior of visible GaN LEDs during their operating life. Acknowledgement This project has received funding from the ECSEL Joint Undertaking (JU) under grant agreement No 101007319. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Netherlands, Hungary, France, Poland, Austria, Germany, Italy, Switzerland. This publication reflects only the author's view and that the JU is not responsible for any use that may be made of the information it contains.

Resume : This paper investigates on the mechanisms that impact on efficiency and stability of visible InGaN-based light emitting diodes (LEDs). To this aim, a 20000 minutes long constant current stress at J = 80 A/cm2 at room temperature was performed, in order to monitor the stress-induced changes in the behavior of the devices under test (DUTs). These devices featured a single quantum well with 20% In molar fraction, corresponding to nominal emission wavelength of 495 nm at I = 100 mA. The electrical (I-V) and optical (L-I, EL spectra) characteristics were monitored before and during stress in order to evaluate the effects of the degradation processes on device performance. From the analysis of the electrical characteristics, we observed an increment in leakage current during stress, suggesting the hypothesis that the ongoing degradation process was inducing an increase in the concentration of the defects that assist leakage conduction through trap assisted tunneling in the sub-turn on forward voltage regime. Regarding the optical performance of the LEDs, we found that the optical power (OP) was decreasing for the low injection regime (J = 0.4 A/cm2), confirming the hypothesis of an increment in the concentration of defects acting as non-radiative recombination centers (NRRCs). The slope of the L-I curve for these injection currents indicates that SRH recombination is dominating. Moreover, there is a clear change of the slope around J = 12 A/cm2, due to the onset of radiative recombination. Numerical simulations revealed that carrier injection is asymmetric within the structure under investigation, meaning that from J= 0.04 A/cm2 to J = 40 A/cm2 the increment in electron concentration scales linearly with the injection current, whereas hole density stays constant around 10^19 cm-3. This result supported the hypothesis that the observed slope change can be related to the transition from an SRH recombination to a radiative recombination regime. Moreover, in the range of high injection level above J = 4 A/cm2, the OP was found to increase during the ageing process. Based on these results, and by also leveraging a first-order analysis of the rate equation within the QW, we developed a hypothesis for the observed increase in OP ad EQE. This process was correlated with the decrease in emission wavelength, highlighting that band filling phenomena is occurring, leading to the screening of the Quantum Confinement Stark Effect (QCSE). Thus we suggest that the OP recovery can be related to the improvement of injection efficiency and/or to a better carrier confinement. Hence, we performed photoluminescence measurement (PL) in the same range of high current level during long stress test. The PL signal shows an increment during the stress test, supporting the idea of an improvement of the carrier confinement, which could be related to the generation of negatively charged defects in the quantum barriers, that modify the band banding and the internal electric field.

Resume : In the recent year, flexible nitride LEDs have been developed with applications in fields such as lighting, screens but also medicine and biology. Said LEDs are made using InGaN/GaN core shell nanowires grown by MetalOrganic Vapor Phase Epitaxy (MOVPE) and can easily be encapsulated in a polymer matrix such as PDMS. This layer is then peeled off to make a flexible structure containing the LEDs. However, the peel off is a meticulous process with significant chances of failure, which requires a trained user and time, making it partially unsatisfactory for industrial applications. One solution could be to grow the LEDs on a 2D material such as graphene, using Van Der Waals epitaxy. In this case, the deposited Gallium Nitride does not form any covalent bond with the substrate, but rather Van Der Waals weak bonds with the graphene, making it easier to peel off. The graphene may also act as a contact for electric measurements, or as a sacrificial layer to chemically peel off the nanostructures. However, Van Der Waals epitaxy is difficult to realize. Indeed, because of a lack of any dangling bond, the surface energy of graphene is low (50mJ.m-2), and the diffusion length of an adatom is extended. The main consequence is that GaN will preferentially nucleate at the defects of the graphene monolayer, which ultimately defeats the purpose of using graphene in the first place. In preliminary trials, graphene grown by CVD on copper and reported on Sapphire proved too defective to be used for Van Der Waals Epitaxy. Graphene grown on SiC is however notably less defective and appears as a solution to this issue. Based previous work realized in our team, tetrahedral structures of GaN have been grown on monolayer graphene on SiC. These structures follow the crystallographic directions of graphene, suggesting that Van Der Waals epitaxy was achieved. This was later confirmed by growth on multilayer graphene, which suppressed any influence for the underlying SiC. LED structures were realized by depositing an array of 5 InGaN QWs on these GaN tetrahedra. Cathodoluminescence revealed a band edge of 3.2eV typical of ZB GaN, as well as a broad QW signal. These preliminary results comfort the possibility of using GaN tetrahedra grown on graphene to make standalone LEDs, with the possibility of making flexible devices.

Resume : Gallium Nitride has become a material of choice in the field of high-frequency power electronics. GaN HEMTs can operate at power levels high enough to determine temperature increase of the order of hundreds of degrees Kelvin due to Joule heating (self-heating). At such high temperatures, the carrier effective mobility and, hence, the device output current are sensibly reduced, leading to a substantial lowering of the attainable power efficiency. In order to successfully design high-power electronics based on GaN, it is crucial to get a precise estimation of the expected operating temperatures. To overcome the difficulties implied in the 3D electrothermal simulations of a realistic device, we adopted a hybrid approach [1], in which the electrical part of the simulation is solved by using an effective model (in this case, a table-based model), while the thermal part is solved by using a finite element model (FEM). It is found that the output characteristics in pulsed regime are substantially affected by self-heating even for measuring delay times as short as 200 ns, since the device temperature has already significantly risen from the start of the voltage pulse. Thus, a procedure for the estimation of "virtual" electrical characteristics in the absence of self-heating effects is presented [2]. Calibrating the effective model used in the hybrid simulations against the "virtual" characteristics or the pulsed ones measured after a finite delay-time, leads to substantially different predicted maximum temperatures. The proposed method provides an accurate estimate of the operating temperature and has been validated by direct comparison with micro-Raman thermographic measurements [2]. The hybrid modelling approach is numerically efficient and has permitted to simulate, on a mid-range workstation, the thermal behavior of realistic device layouts, fully including all the geometric details. Moreover, the adoption of a FEM software environment (COMSOL Multiphysics) has allowed the parametrization of the relevant geometrical dimensions of the simulated device layouts, making very simple to perform numerical studies of the thermal performances in differently scaled devices. Hence, electrothermal simulations of multifinger layouts (HEMTs and power-bar) with different channel lengths, widths, pitch and number of fingers have been easily analyzed. In addition, the simulations of simple circuits including pairs of active devices have allowed quantifying the thermal cross-talk between the devices. By using the hybrid approach, the impact of the signal frequency in pulsed operation on the device thermal response has been analyzed. The reduction of the maximum temperature reached as the operating frequency is increased up to the giga-hertz range has been estimated. [1] A. Chvála et al., Microelectron. Reliab., vol. 78, pp. 148–155, (2017) [2] A. Valletta et al., IEEE Trans. Electron Devices, vol. 68, pp. 3740–3747, (2021)

Resume : It is well known that nitride semiconductors based on gallium nitride (GaN) and its cousins, indium nitride (InN) and aluminum nitide (AlN), are applied for building light emitting diodes, laser diodes as well as high-power and high-frequency transistors. These devices are used in many fields from general lighting, medicine through ecology up to defense. Although nitride-based devices show tremendous technological premise, their reliability and commercial success can still be improved by higher structural quality of crystallized material as well as more homogenious and precise dopining by donors and acceptors. One of the best methods for introducing dopants into semiconductors is ion implantation. The introduced structural damage can be removed by a proper annealing process. The high-temperature treatment enables also electrical and/or optical activation of the implanted dopants. In the case of GaN, annealing at high temperature (~1300°C - 1400°C) seems difficult. This coumpund loses its thermodynamic stability slightly above 800°C at atmospheric pressure. At higher temperature the crystal will decompose. One of the solutions is to anneal GaN at high nitrogen (N2) pressure. Such technology is called ultra-high-pressure annealing (UHPA). In this paper, application of UHPA for GaN crystals and layers implanted by different ions (acceptors and donors) will be presented. The latest results of the implantation with magnesium (Mg), beryllium (Be), zinc (Zn), and calcium (Ca) ions into GaN in order to obtain p-type conductivity will be discussed. Silicon (Si) implantation into GaN for n-type doping will also be analyzed. Structural, electrical and optical properties of implanted GaN after UHPA will be discussed in terms of application for GaN-based devices.

Resume : Worldwide efforts are on to deeply understand the activation of acceptor states in ZnO with the motivation to achieve long sought persistent p-type conductivity. In present, we used LT-CL-SEM mapping, SIMS, RT-Hall, and XRD techniques to compare 1.5 micron thick ZnO and ZnO:N ALD films deposited on HR-Si (100) at 100oC and post-growth annealed at 6 different temperatures 400-900oC under N2/O2 atm. The study was directed to see at which annealing temperature acceptor-related CL becomes dominant over donor-related CL and correlate this with structural and electrical properties of the films. The study was performed for N concentration from 1017 to 1019 at/cm3. XRD revealed that only two crystallite orientations, (100) and (110), with the grain size increasing after each increment of annealing temperature. We observed quenching of free exciton related CL upon higher N doping. For all measured samples, ZnO and ZnO:N, the LT-CL maps reveal a complicated images, in which acceptor- and donor-related CL are separated and mostly derived from different crystallites [1]. It has been found that both annealing medium and temperature influence acceptor-related CL of ZnO and ZnO:N, as it changes concentration of N dopant and H impurity as well as the structural properties of the films. Acknowledgment: The work is supported by research project No. 2018/31/B/ST3/03576 founded by the National Science Centre (Poland). Reference: [1] E. Guziewicz, et al., ACS Applied Materials & Interfaces 9.31, 26143-26150 (2017).

Resume : Metal halide perovskites, although known for many years as a category of materials, they were spotlighted a decade ago as visible-light sensitizers in solar cells. After first reports, perovskites in less than five years reached photovoltaic efficiencies that silicon needed almost 50 years of science and engineering to be achieved. This fact turned the materials’ scientific community to focus on organic-inorganic and all-inorganic metal halide perovskites, and very fast, new scientific knowledge arose, and a variety of potential applications presented [1]. Except the photovoltaics, perovskites used also in LEDs, photodetectors, photocatalysts, lasers, ionizing radiation detectors, as anode materials in batteries [2] and very recently in gas sensors applications [3, 4]. In this work, ligand-free CsPbBr3 microcubes (with sizes 1-3 μm) have been prepared via a facile solution process under ambient conditions as gas sensing elements. The ability to directly grow the microcrystal onto electrodes, provided a simple and low-cost method for the fabrication of high-performance perovskite-based sensors, operating at room temperature, without external stimuli (heating or UV irradiation). We showed that the ambient synthesis conditions resulted in the growth of rounded cube-shaped crystals (RC) compared to the well-shaped microcubes grown at a protective atmosphere. It is revealed that the imperfection of the shape, favored the gas sensing capability and consequently these sensors exhibited superior performance compared to the regular CsPbBr3 cubes (capable to detect 187 ppb ozone concentration) synthesized in inert atmosphere. [3] The RCs were able to detect 4 ppb of ozone concentration with sensitivity of 13% at room temperature. Their response and recovery times at this low concentration, laid at 74 s and 15 s, respectively. RC-based sensor showed also remarkable stability over consecutive cycles and after 3 months its storage in ambient conditions, without obvious alterations in morphology and crystal structure. At the same time, RCs proved to be great hydrogen sensors, exhibiting capability to detect 1 ppm of hydrogen concentration. RC-based sensors, exhibited the lowest reported gas detection limits at room temperature compared to state-of-the art semiconductor materials, providing new opportunities in gas sensing applications. The enhanced sensitivity as well as the ease of the preparation method makes CsPbBr3 RCs as a good candidate for the detection of other types of gases as well. References [1] Polavarapu et al., ACS Nano 2021, 15, 7, 10775–10981 [2] Kostopoulou et al., Journal of Power Sources Advances 2020, 100015-100021 [3] Brintakis et al., Nanoscale Advances 2019, 1, 2699-2706. [4] Argyrou et al., Journal of Materiomics 2022, 8 (2), 446-453 Acknowledgments: FLAG-ERA Joint Transnational Call 2019 for transnational research projects in synergy with the two FET Flagships Graphene Flagship & Human Brain Project – ERA-NETS 2019b (PeroGaS: MIS 5070514).

Resume : Metal halide perovskites (MHP) have been attracted as a potential candidate for photovoltaic (PV) application, due to their several outstanding performances such as long carrier diffusion length, light absorption coefficient and optimum bandgap range. Herein, we explore the ferroelectric nature and the origin of hysteresis in CsPbI3 thin film using PFM and transistor I-V measurements. The PFM results show the spontaneous polarization domain and ferroelectric hysteresis loop (P-E) in CsPbI3 thin films. Since PFM measurements are prone to artifacts, switchable domain polarization is also examined by an external electric field and confirms the presence of ferroelectricity. Light-induced dipole orientation is also confirmed by piezoelectric force microscopy (PFM). Furthermore, using scan rate-dependent I-V measurements, we have observed the contribution of ferroelectricity in hysteresis under dark conditions and the dominance of ionic migration over ferroelectricity in hysteresis in illumination at room temperature (300K). At low temperatures (10K), hysteresis phenomena are also observed due to dipole orientation. Overall, our results imply that both ferroelectricity and ion migration contribute hysteresis behavior of the I-V curve.

Resume : Oxide thin-film transistors (TFTs) have been attracting attention as one of the representative candidates for unit devices for next-generation electronics such as displays, biosensors, and neuromorphic devices due to their optical transparency, mechanical flexibility, and outstanding electrical characteristics. Indium-Gallium-Zinc-Oxide (IGZO) is one of the most representative oxide semiconducting materials because of its excellent uniformity and electrical stability compared to other oxide semiconductors. Recently, rare-metal-free amorphous oxide semiconductors such as Zinc-Tin-Oxide (ZTO) have been in the spotlight because IGZO demands rare metals such as Indium and Gallium and also shows relative low field-effect mobility. In newly emerged rare-metal-free semiconductors such as ZTO, understanding structural relaxation (SR) is important to secure outstanding electrical characteristics. However, in-depth studies regarding SR are not still conducted enough. Therefore, we investigate the effects of SR on electrical properties in solution-processed ZTO TFTs. To demonstrate the effects of SR, the annealing time of ZTO semiconductor was controlled. As the annealing time increased, the saturation mobility increased from 0.38 to 2.41 cm2 V-1 s-1, and the threshold voltage was negatively shifted from 3.08 to -0.95 V. This is originated by an increase in oxygen vacancy as the SR is enhanced. The oxygen vacancy which act as a donor in oxide semiconductor was increased from 21.5% to 38.2% as the annealing time increases. In addition, the metal-hydroxide bonding was slightly deceased from 16.3% to 14.6%. This result indicates that the condensation reaction and densification is more actively conducted in ZTO semiconductors which are relatively annealed in long time. Thus, we expected that the SR of solution-processed ZTO semiconductor might enhance not only the electrical characteristics but also positive bias stress by reducing the localized states near conduction band in ZTO. This study will contribute to the optimization of semiconductor activation for improving the electrical characteristics of newly emerged oxide semiconductors-based TFTs. In addition, it was revealed that the negative bias stress stability deteriorated due to the increase of oxygen vacancy. In contrast, as the annealing time increased, the localized states near conduction band in ZTO decreased owing to the reduction of the free volume and the stabilization of the local atomic condition by additional thermal energy. As a result, positive bias stress stability was improved by increasing annealing time. This study might contribute to the optimization of semiconductor activation for improving the electrical characteristics and stability of newly emerged oxide semiconductors-based TFTs.

Resume : Silica glass is a typical amorphous material, which does not have secondary phases and grain boundaries. Glasses are the most well-known candidates for their better transparency, mechanical stability, thermal resistivity, and chemical durability. However, in the late ‘60s when glass-ceramics were discovered. Since then, many studies have been conducted on glass-ceramics (GC) materials as a completely new field of materials that could improve the needed properties. Doping glass with alkali, alkaline earth metal, transition metal, and/or lanthanide ions allows the development of phosphors for a variety of applications, including lighting and display devices such as light-emitting diodes (LEDs), plasma display panels, safety signals, biological sensing, biological imaging, photocatalysts, and sensors. The aqueous sol-gel synthesis technique of pure silica glass has received much attention in the past decades. Compared to industrial ways of synthesis, it offers a lot of flexible choices in varying the glass morphology and introduction of dopants and requires much lower processing temperatures. Garnet crystal structure is a very important matrix for optical applications. A wide variety of transition and/or rare-earth ions used as dopants to this matrix are extensively discussed over years. Cubic yttrium aluminum oxide is an attractive laser host material and a promising candidate for high-temperature structural applications, of its high chemical and thermal stability. Due to the huge need for new versatile phosphors, there is a demand for studies on modifying the optical properties of phosphors. The varieties of doping explorations are very important in the development of material science [1-4]. In the presented work, particles of yttrium aluminum garnet doped with europium ions were encapsulated in silica glass obtained by the sol-gel method are characterized. The morphology, glass composition, structure modification, and optical properties were analyzed. The main goal was to verify structural changes of the glass when the different concentrations of dopants are used in optically active glass. 1. Skuja, L., Ollier, N., Kajihara, K., Bite, I., Leimane, M., Smits, K. and Silins, A., 2021. Optical Absorption of Excimer Laser‐Induced Dichlorine Monoxide in Silica Glass and Excitation of Singlet Oxygen Luminescence by Energy Transfer from Chlorine Molecules. physica status solidi (a), 218(15), p.2100009. 2. Grandi, S., Mustarelli, P., Agnello, S., Cannas, M. and Cannizzo, A., 2003. Sol-gel GeO2-doped SiO2 glasses for optical applications. Journal of sol-gel science and technology, 26(1), pp.915-918. 3. Inkrataite, G., Kemere, M., Sarakovskis, A. and Skaudzius, R., 2021. Influence of boron on the essential properties for new generation scintillators. Journal of Alloys and Compounds, 875, p.160002.

Resume : The fabrication of low-resistance and thermally stable Ohmic contacts is essential for the realization of reliable GaN power devices. In the particular case of the p-type GaN, a thin Ni/Au bilayer is commonly used for Ohmic contacts. However, Au metal contacts are quite expensive, incompatible with the complementary metal-oxide-semiconductor (CMOS) foundries and also have poor thermal stability. Thus, seeking an alternative that is affordable and thermally stable is crucial. In the present study, we investigate Au-free Ohmic contact formation on p-type GaN using a bilayer Ni/Al-doped ZnO (AZO) thin film as a function of annealing temperature in order to obtain an excellent Ohmic contact on p-GaN. Structural, morphological and electrical properties of the Ni/AZO contacts deposited on the p-type GaN layer were analyzed in detail using X-ray diffraction (XRD), scanning emission microscopy (SEM), atomic force microscopy (AFM) in combination with scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), and circular transfer length model (c-TLM) techniques. Our results show that the contact resistance can be significantly reduced using an AZO/Ni bilayer with a suitable rapid thermal process. We demonstrate that specific contact resistance for AZO/Ni on p-GaN can reach a lowest value of 1.85×10-4 Ω.cm² for a sample annealed at 500°C in air ambient for 5 min. These results also suggest methods for improving the electrical properties of the contacts to p-GaN. Our work demonstrates that the bilayer Ni/AZO contact could be suitable for efficient GaN power diodes or transistors. Keyword: Ni/AZO; p-type GaN; Ohmic contacts; specific contact resistance; power diodes

Resume : Hydrogen titanium oxide (H2Ti3O7) nanotube (HTNT) linked to tungsten oxide (WO3) nanorods, changing the physical property from brittle and super hydrophilic to durable and super hydrophobic. The usage of WO3 nanorods and Nano networks still has challenges with high-temperature substrate adherence. So it can't be used for industrial purposes like gas detection or field emission properties. To solve this issue, ion irradiation is used to solder 10 nm HTNT onto 100 nm WO3 nanorods. On a low-cost glass or silicon substrate, super hydrophobic, conductive, bendable WO3 NRs are developed. The sophisticated and wide-ranging properties of WO3 NRs wrapped by HTNT film make it useful as a transparent electrode in multifunctional flexible electronic devices. Recent advances in heterojunction and heterojunction-based devices such as photocatalytic, sensing, light-emitting, photovoltaic, anti-reflecting, and electrical have gained interest due to their higher energy, storage, conversion, generation, and self-cleaning properties in open environments. Due to diffuse wrapped HTNT on WO3 NRs, a Nano-heterojunction (Nano-network) forms through the bottom-up approach which is well explained by Monte Carlo based the state of art TRI3DYN simulation. The unique topology of wrapped nanowire junctions not only increases the active surface area and reduces contact resistance between WO3 nanorods and HTNT, but also provides the shortest path for ultrafast electron and ion transport. This irradiated thin film surface repels water droplets and has greater electrical conductivity. Wrapped heterojunctions have great mechanical strength and flexibility, allowing them to be used in flexible electronic devices. This hybrid nanostructure solve the most of the problems with portable and artificial noses that can sense dangerous and flammable gas like H2, and able to work at low temperatures for, and with better response-recovery kinetics and sensitivity. This work aims to find the best way to lower the temperature of oxide nanomaterials based gas sensors so that they can be fast, accurate, and cheap.

Resume : Far from being a requirement specific of discrete power components, guaranteeing an efficient thermal management has become a mandatory requirement in several modern electronic applications. Operation at higher temperatures decreases the components lifetime and efficiency, and proper care needs to be taken in the design phase to guarantee that the heat generated during the operation of the device can effectively be transferred from the active regions to the package, where it can be ultimately dissipated to the heat sink or external environment. The thermal management of a particular device can be addressed at different levels. At package level the chip carrier can be replaced by materials with high thermal conductivity () such as AlN or BeO. At board level metal-core printed circuit boards (MCPCB) can replace common FR4 boards for improving the heat transfer. Finally, at system level different techniques are available to the engineer, from the use of heat sinks to active techniques such as forced convection or thermoelectric cooling. Due to its breakdown electric field and thermal conductivity, diamond can be considered the ultimate thermal management material. Diamond is available in the form of plates and can be deposited on foreign substrates by chemical vapor deposition, which facilitates the integration of the material with a particular package. The use of artificial diamond films for the thermal management of power devices, such as p-n junction diodes, LEDs and transistors, has been an active research topic for several decades. The integration of GaN high electron mobility transistors (HEMTs) and diamond films, in particular, is one of the most successful approaches and has involved academic and industrial institutions for more than 20 years. The integration of both materials can be done in two ways: replacing the GaN substrate with diamond or capping the HEMT with a diamond film, close to the gate hotspot. The GaN-on-diamond wafers can be obtained by depositing the diamond film directly on the back of GaN layers, following the substrate removal, by bonding GaN HEMT wafers and diamond substrates, and by growing the GaN layers directly on diamond substrates (GaN epitaxy). This paper will describe each of these approaches, their challenges and their benefits, and will present the evolution of the GaN/diamond devices reported by different groups. As a concluding remark, power amplifiers based on GaN-on-diamond technology are commercially available, attesting the potential and feasibility of integrating diamond and power components.

Resume : Two-dimensional transition metal dichalcogenides (TMDCs) are intensively studied due to their outstanding properties and promising applications in electronics, spintronics, or optoelectronics [1]. However, achieving theoretically predicted properties is still challenging. Doping of impurity atoms has been a conventionally used method to effectively change the electrical and electronic properties of various materials. TMDCs have the generic formula MX2, where M represents a metal atom and X is a chalcogen atom. Impurity atom could replace either M or X atom and act either as donor or acceptor, depending on the relative valency. We focused on the lithium doping of MoS2 layers. In this case, Li behaves as an acceptor and the resulting MoS2 should have a p-type conductivity [2]. For fabrication of lithium doped MoS2, we used a two-step method. In the first step, we prepared MoS2 layers on c-plane sapphire. Subsequently, MoS2 layers were annealed in a mixture of sulfur powder and lithium sulfide serving as a source of lithium. The sample was placed in the middle of the one-zone furnace together with the powders and annealed at the same temperature. MoS2 films were prepared by two different methods – pulsed laser deposition (PLD) and sulfurization of the pre-deposited Mo layer. The thickness of the MoS2 films prepared by PLD was about 1.5 nm (2-3 monolayers). In the case of sulfurization, the initial thickness of Mo was 1 and 3 nm resulting in 4 and 12 nm thick MoS2 films (approximately 6 and 18 monolayers). We compared the structural and electrical properties of both types of films after doping. The presence of lithium in the doped MoS2 layers was confirmed by synchrotron-radiation X-Ray Photoelectron Spectroscopy. Tunable photon energies in the soft X-Ray range (100 eV up to 600 eV) allow detection of Li 1s core levels with desirable sensitivity. The films were characterized by several methods such as Raman spectroscopy, X-Ray Diffraction, grazing-incidence wide-angle X-ray scattering, atomic force microscopy, and optical and electrical measurements. This work has been funded by the FlagERA-JTC 2019 project ETMOS and Slovak Grant Agency for Science VEGA 2/0059/21. 1. S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, and A. Kis, Nat Rev Mater 2, 17033 (2017) 2. L. Loh, Z. Zhang, M. Bosman, and G. Eda, Nano Res. 14, 1668 (2021)

Resume : Environmental pollution is one of the most severe problems humans and other living farms face on our planet. This pollution is induced by mainly toxic metal ions and harmful chemicals from industrial wastes. Degrading environmental conditions below a certain permissible level due to these pollutants becomes very important to detect and stop further increasing pollution. Hg2+ is very harmful even at below micromolar concentrations. It induces severe damage to the digestive tracts and kidneys in the human body. Various types of optical and chemical sensors are available to detect pollutants. We demonstrate an organic field-effect transistor (OFET) based electrical sensor to detect toxic Hg2+ metal ions. It has been shown that structurally simple -conjugated pyridine-end p-phenylenevinylene oligomer can selectively sense toxic metal ions in solution. This molecule is used as the sensing material. Since mercury ions make coordination compounds with pyridine ligands, this molecule senses toxic mercury ions in the solution. Aluminum is used as the gate. A small part of Al is anodized to make a thin alumina layer to reduce the gate leakage current due to its high dielectric constant. Barium titanate nanocrystals are used as a dielectric material for the device. Pentacene is the channel material for OFET. A thin layer of the sensing material is deposited above the channel material. Chromium-gold is deposited for contact as the source and drain of the device.

Resume : Achieving a high degree of spin-valley polarization is the key task for emerging quantum technology. In most of the cases, generation and manipulation of spin-valley polarization have been recorded by individual transition metal dichalcogenides (TMDCs)1,2 or their monolayer (ML) based 2D van der Waals (vdW) heterostructure (HS)3 with a cryogenic temperature regime. In cryogenic limit, the increase of valley lifetime and the decrease of inter/intralayer scattering may cause of achieving a high spin-polarization. However, the challenge remains for the room temperature (RT) limit. Here we demonstrate the mechanism of having a high spin-polarization at the RT regime by manipulation of charge transfer and the reduction of carrier lifetime. Here we have achieved RT spin-valley polarization of ~ 90-95 % with the hBN/TMDC/PbI2 vdW HS. ML TMDCs and a few layers (< 10 L) of PbI2 were encapsulated with thin hBN and measurements have been performed with four different HS with type I (hBN/MoS2/PbI2 and hBN/WS2/PbI2) and type II (hBN/MoSe2/PbI2, hBN/WSe2/PbI2) band alignments. Irrespective of band alignment high valley polarization has been observed with near-resonant excitation (488 nm) at RT. A bit off-resonance (457 nm) more than 50 % spin-dependent signature was recorded for all TMDC/PbI2 based HS. However, individual PbI2 has shown only ~ 10 % polarization even at RT limit. This observation demonstrates that charge transfer is playing a key role in boosting spin-polarization by almost ~ 80 % in the HS system. Helicity resolved PL measurements (circularly polarized excitation) were performed to quantify the amount of spin-valley polarization. Moreover, to ensure the phenomena spin-valley coherence measurement was also performed, and ~ 70-80 % spin-coherence is observed in resonant excitation at the RT regime with the HS. The good agreements between the spin-valley polarization and spin-valley coherence number prove that the TMDCs/PbI2 HS could be a great complement to achieve the high spin selective device performing even in RT limit. However, the effect of thickness of TMDCs and PbI2, temperature, and excitation wavelength dependency has also been verified expensively. We proposed that the interlayer charge transfer and the reduction of career lifetime could be the essential mechanism to show the high value of spin-polarization. To ensure that, time-resolved PL measurements were performed to estimate the lifetime. Here the observed carrier lifetime is significantly lower in HS, whereas the spin relaxation lifetime is higher and this gives rise to a high value of spin-polarization in the TMDC/PbI2 system. We believe that the high degree of spin-polarization at the RT limit observed in this work could be the new paradigm for the practical application-based spin-valley photonics and spin-valley electronics. (1) Jiang, T.; Liu, H.; Huang, D.; Zhang, S.; Li, Y.; Gong, X.; Shen, Y. R.; Liu, W. T.; Wu, S. Valley and Band Structure Engineering of Folded MoS2 Bilayers. Nat. Nanotechnol. 2014, 9 (10), 825–829. (2) Barman, P. K.; Sarma, P. V.; Shaijumon, M. M.; Kini, R. N. High Degree of Circular Polarization in WS2 Spiral Nanostructures Induced by Broken Symmetry. Sci. Rep. 2019, 9 (1), 2784. (3) Zhang, N.; Surrente, A.; Baranowski, M.; Maude, D. K.; Gant, P.; Castellanos-Gomez, A.; Plochocka, P. Moiré Intralayer Excitons in a MoSe2 /MoS2 Heterostructure. Nano Lett. 2018, 18 (12), 7651–7657.

Resume : Transparent conductive oxides (TCO) are a class of materials exhibiting at the same time high transparency, characteristic for classical oxides and low resistivity, typical of metals. Such a combination made them relevant in a number of applications where transparent electrodes are important, such as photovoltaic cells, touch screens, light emitting diodes and lasers. Indium tin oxide (ITO) holds the majority of the market, however its indium content makes it desireable to be phased out in the longer perspective. This is why a number of alternatives have been developed as indium-free TCO films. One of them is aluminium-doped zinc oxide, ZnO:Al, or AZO. Its drawback however is in the fact that it has a band gap od 3.4 eV, smaller than ITO. On the other side, it was shown that it is possible to increase the band gap of ZnO to 4.0 eV by alloying it with Mg to get Zn1-xMgxO at an incorporation of 40% of Mg in the Zn sublattice (i.e. x =0.4) using complex epitaxial techniques. In this work we propose the application of Al doping to ZnMgO to obtain electrically conducting wide bandgap transparent TCO, Zn1-xMgxO:Al, or AZMO. We do it by applying the sputtering technique using co-sputtering of AZO and Mg targets, AZO targets with attached Mg and designed ZnO/MgO/Al2O3 targets with various Mg compositions. We see that the cosputtering and sputtering from a compound AZO/Mg target lead to obtaining black films with a high Mg concentration, although with a band gap not much shifted from the original value for AZO. ToF-SIMS showed a uniform Mg signal and STEM element maps did not detect any multiatomic Mg clusters in the film, suggesting that there might be isolated Mg atoms not built into the ZnO lattice due to the very dinamic sputtering conditions. On the other hand, after annealing, the films became transparent and the band gaps got wider, to around 3.65 eV for the films with the most Mg. Sputtering from compound oxide targets lead to materials that already have high transparency after deposition suggesting this is the preferred route to follow for such materials. The resistivity however is higher than that for unalloyed AZO films at around 600 Ω/square. Since for AZO an increase in the Al2O3 content in the target leads to slight improvements in conductivity, we ordered targets with 4 wt. % of Al2O3 to compare against the standard 2 wt. %. The surprising result showed a significantly increased resistance, to 6.5 kΩ/square showing it is not an efficient way to increase the conductivity of such a material. This work was supported by the National Centre for Research and Development, Poland, project 'OxyGaN' - M-ERA.NET2/2019/6/2020, by the Hungarian NRDI Fund, grant number 2019-2.1.7-ERA-NET-2020-00002 and by the Israel Ministry of Science, Technology and Space in the frames of the M-era.net Programme.

Resume : Beta gallium oxide (β-Ga2O3) is a promising semiconductor material which possess attractive optical and electrical parameters such as ultra-wide energy gap of ~ 4.8 eV, high transparency in visible range and a high value of the critical electric field ~ 8 MV/cm. Despite tremendous progress both in material development as well as development of unipolar devices, the absence of p-type doping capability limits the performance and applications of β-Ga2O3 power devices. To overcome this problem anisotype p-n heterojunction devices, has been proposed based on such a p-type semiconductors as NiO. These devices offers better electrical performance in terms of higher breakdown voltage and lower leakage current. However detailed studies of NiO layer deposition parameters on the properties of heterojunctions is still lacking. In this work we present effect of oxygen partial pressure on the properties of sputtered vertical NiO/β-Ga2O3 heterojunction diodes. The heterojunction diodes were fabricated by sputtering of NiO layers on to n-type epitaxial layer on (001) β-Ga2O3 single crystal wafer. The oxygen content during RF sputtering of thin films using NiO ceramic target in O2/Ar plasma were 0% (pure Ar), 25% O2, 50% O2, 75% O2 and 100% O2. We observed that increasing of oxygen partial pressure during deposition from 0% to 100% cause a decreasing of optical bandgap from 3.52 eV to 3.42 eV as well as decreasing of resistivity of NiO layers. Measured I-V and C-V characteristics shows substantial effect of oxygen partial pressure on the electrical properties of NiO/β-Ga2O3 heterojunction diodes. The diodes with NiO layers deposited in pure Ar shows Schottky-like I-V characteristics with low ideality factor of 1.05 and barrier heigh of 1.27 eV suggesting n-type behaviour of NiO and thus creating isotype n-n heterojunction. Built-in voltage obtained from C-V characteristics was about 1V. On the other hand diodes with NiO layer deposited with oxygen shows anisotype p-n diode characteristics with higher turn-on voltages and higher build-in voltages of about 2V. The best electrical characteristics i.e. low on-state resistance, lack of defects and low ideality factor down to 1.07 were obtained for diodes with NiO layers deposited at 50% oxygen content. All fabricated devices shows very high on-off current ratio over 1012 A/A. We have shown that by optimizing of NiO deposition parameters it is possible to obtain high quality both n-n and p-n heterojunction diodes with excellent electrical performance. This work was partially supported by the Polish National Science Centre within Miniatura 4 programme under agreement nr 2020/04/X/ST5/01014 for project "Investigation of electrical properties of Schottky contacts to gallium oxide using oxygen-containing conductive layers" and by the statutory funds of the Łukasiewicz Research Network – Institute of Microelectronics and Photonics.

Resume : Beta gallium oxide ( β-Ga2O3) is a promising semiconductor material which possess attractive optical and electrical parameters such as ultra-wide energy gap of ~ 4.8 eV, high transparency in visible range and a high value of the critical electric field ~ 8 MV/cm. High optical bandgap of β-Ga2O3 makes it also attractive for application where transparent electronic devices would be beneficial for example optical triggering of power switches. Fully transparent Schottky diodes on β-Ga2O3 were reported recently with transparent indium tin oxide (ITO) ohmic contacts and ITO and InZnSnO (IZTO) as a transparent Schottky contacts. In this work we demonstrate fabrication and characterization of low-resistance all-oxide transparent vertical β-Ga2O3 diodes using ITO and aluminium-doped ZnO (AZO) Schottky contacts. The Schottky contacts were fabricated by sputtering of ITO and AZO layers on to lightly doped n-type epitaxial layer on (001) β-Ga2O3 single crystal wafer. ITO layers was used ohmic contact to Sn-doped n+-β-Ga2O3 wafer. The fabricated diodes were characterized using measurement and analysis of current-voltage (I-V) and capacitance voltage (C-V) characteristics. Electrical, optical and structural characterization of AZO and ITO layers on β-Ga2O3 were also performed. At first, influence of annealing temperature on ITO the ohmic contact formation was studied. We have shown that ITO ohmic contact to n+-β-Ga2O3 substrate was formed after annealing in N2 at 800°C. Both AZO and ITO-based Schottky diodes shows well behaved I-V characteristics. Average Schottky barrier heights and ideality factors were 0.986 eV and 1.05 and 0.951 eV and 1.03 for AZO and ITO Schottky contacts, respectively. The on-off current ratio was about 2×1010 A/A and 1×1011 A/A for AZO and ITO Schottky contact, respectively. Moreover, the on-state resistance was about 6-7 mΩcm2 and 4-5 mΩcm2 for AZO and ITO Schottky contact, respectively, and was 20-35 times lower than for previously reported transparent ITO/β-Ga2O3/ITO and ITO/β-Ga2O3/IZTO Schottky diodes. The built-in voltage and average Schottky barrier heigh determined from C-V characteristics was about 0.91 V and 0.85 V and 1.075 eV and 1.01 eV for AZO and ITO Schottky contacts. The barrier heights determined from C-V characteristics were slightly higher than values determined form I-V curves suggesting some inhomogeneities at AZO or ITO/β-Ga2O3 interface. In overall, we have shown that by optimizing fabrication condition for both transparent Schottky and ohmic contacts it is possible to obtain low-resistance all-oxide transparent diodes with good electrical performance. This work was partially supported by the statutory funds of the Łukasiewicz Research Network – Institute of Microelectronics and Photonics and by the National Centre for Research and Development, Poland, project “OxyGaN” M ERA.NET2/2019/6/2020.

Resume : The near-infrared (NIR) spectral bands promote the absorption of phytochrome proteins which showed a photo-morphogenesis of plant development. However, the commonly reported NIR emitting phosphors fabricated with LED-chip artificial light are poor in thermal and chemical stability. Improving the thermal stability of NIR artificial light is necessary for further application for plant growth supplement as well as non-damaging detection. Here, a surface-emitting NIR alternating current (AC)-driven powder-based electroluminescence (PEL) device is introduced with superior thermal stability from Ga2O3:Cr3+ phosphor. The EL device consists of BaTiO3 insulating layer and Ga2O3:Cr3+ phosphor layer on the ITO glass. which is prepared by a facile and cost-effective screen-printing method. Under AC sinusoidal waveform, the Ga2O3:Cr3+ PEL emitted NIR spectrum, which is optically attributed to the forbidden transition of Cr3+ ions, which is well overlapped with the phytochrome absorption spectrum. The strategy to obtain the optimum condition of the phosphor for PEL purpose is presented with variables of annealing temperature and atmosphere as well as the Cr3+ concentration. Further analysis shows a promising application in harsh environment with thermal stability so-called anti-thermal quenching effect.

Resume : Large-area metal-graphene-metal UV-Visible photodetectors fabricated on quasifreestanding graphene (QFSG)/vicinal SiC (8° off-axis) wafers are applicable to future low-power consumption systems. They demonstrate effective photoresponse under 365 nm (UV) and 405 nm (Visible) light irradiation upon the application of a built-in electric field (self-powered) and low bias (−10 mV). Photocurrent gain and responsivity under UV-Visible light are more significant on QFSG/vicinal SiC than on epitaxial graphene (EG)/SiC(0001)because of the freestanding nature of the topmost layer, absence of a buffer layer, and primary carrier scattering/trapping centers. Further, they are tuned by localized surface plasmon resonance using gold nanoparticles (AuNPs). In the self-power mode and low-bias mode in QFSG/vicinal SiC, the photocurrent is enhanced by 9-fold and 120-fold, respectively, compared to the photocurrent in EG/SiC(0001). The responsivity of QFSG/vicinal SiC after AuNP treatment is ≈1.65 mA/W (at zero bias) and ≈ 20 mA/W (at −10 mV) under 365 nm light illumination (intensity = 18 mW/cm−2), significantly higher than that of EG/SiC (0001). This device shows a similar trend of photoresponse under 405 nm light illumination. These results confirm that this QFSG/vicinal SiC combined with AuNPs possesses potential for application in UV-Visible detection with minimum power consumption.

Resume : For the last few decades, there has been significant progress in the fabrication of different types of sensors for a wide range of applications, such as environmental monitoring, air quality control, and disease diagnostics. However, the issues like stability, sensitivity, and poor detection limit (mainly in ppb level) are hindering the realization of the sensors into products. These limitations demanded some emerging materials which forced the discovery of crystalline hybrid materials with some structural diversity. Metal-organic frameworks (MOFs) have been growing enormously over the last 20 years and attract much more attention for ideal sensing materials due to their high sensitivity, ultrahigh porosity, and large surface area. To date, the most challenging part is fabricating MOF thin film. In this work, a new approach has been adopted by modifying the upper semiconducting layer using MOF molecules. This straightforward approach for fabricating bi-layer OFET comprised of CPO-27-Ni as the upper sensing layer and bottom active semiconducting layer pentacene showed a strong response towards the stimulant (diethyl sulfide (DES)) of chemical warfare agent (CWA), Bis (2-chloroethyl) sulfide (BCES), a highly toxic blister agent. The diffusion of the MOF molecules through the grain boundaries of Pentacene progressively damaged the thin film phase due to the generation of huge compressive stress, whereas the bulk phase keeps on developing gradually. As the DES vapor analytes choose grain boundaries as diffusion paths a Lewis-acid based interaction have been taken place between DES and Ni (II) metal. This kind of device morphology makes these OFET based sensors a potential candidate for the trace amount of sulfur mustard detection below 10 ppm.

Resume : π-Conjugated polymers are being used in several optoelectronic devices, including organic solar cells. By chemical tuning of the physical and electrical properties of the materials one could further understand the importance of inter- and intramolecular interactions that dictate device performance. Generally, their synthesis yields disordered mixtures that behave in unpredictable ways resulting in low luminescence efficiencies. Therefore, designing a more controlled self-assembly would reduce the non-radiative decay processes, specifically charge recombination due to aggregate formation. Encapsulating alkyl chains would introduce more steric bulk and prevent inter-polymer interactions, thus reducing the formation of aggregates and π-π stacking. Benzodithiophene (BDT) is a widely used donor building block for the synthesis of organic photovoltaics (OPV) due to its high power conversion efficiencies (PCEs). Their energy levels are easily tunable due to their rigid and planar structures, thus allowing different band gaps to be achieved. This work presents the first example of an encapsulated phenyl-based BDT co-polymer (E-BDT), where the backbone is protected by an alkyl macrocyclic sheath, as well as its non-encapsulated counterparts (naked, N-BDT and methoxy, OMe-BDT). The absorption and emission profiles of the polymers show a blueshift for both OMe-BDT and E-BDT. Extraction of the Urbach energy with increasing concentration of the encapsulated polymer in film demonstrated a small decrease in energy that indicates the order is maintained, whereas for the naked polymer, a large increase in order was observed with increasing concentration. It is possible that by threading the polymer (donor) with an encapsulating steric shield (E-BDT), its ability to approach acceptors with close enough proximity for charge transfer to occur could be inhibited. However, an OPV device at AM 1.5G using the E-BDT polymer, Y6 acceptor, and 1,8-diiodooctane as an additive exhibited a short-circuit current (JSC) of 24.1 mA/cm2, an open-circuit voltage (VOC) of 0.79 V, and a fill factor (FF) of 60.2%, thus resulting in a high PCE of 11.4%.

Resume : Layered crystals of gallium selenide/sulfide (GaSe1-xSx) are often considered as materials for nonlinear optical application. However, due to the weak bound (Van der-Waals) between the layers, the crystals are not suitable for mechanical processing, such as cutting, polishing, etc. which limits their actual application. Nevertheless, the ability of easy cleavage and excellent optoelectronic properties make these crystals promising in the field of two-dimensional electronics. In recent years, there has been growing interest in the optoelectronic application of two-dimensional (2D) GaSe1-xSx flakes. There are many research works focused on the investigation of optoelectronic properties of the GaSe1-xSx flakes and the production of high-performance photodetectors based on them. The main goal of this work is to study the GaSe1-xSx flakes, obtained by micromechanical and liquid-phase exfoliation of bulk crystals. All GaSe1-xSx crystals were synthesized by melting of stoichiometric amounts of gallium, selenium, and sulfur.

Resume : With the help of first-principles simulations, one can design new materials that can prompt multifunctional properties. In this context, injection of ferromagnetism into ferroelectric materials have drawn attention due to its various implications for the material science community [1-2]. The doping of transition metals into ferroelectrics is one of the easiest ways to induce ferromagnetism due to the exchange splitting among the spin sub-bands through the crystal field of transition metals. In this work, with the help of first-principles Density Functional Theory based simulations, we propose the induction of half-metallic ferromagnetism with a large band gap (~ 2.57 eV) in minority states by transition-metal substitution in sodium bismuth titanate Na_0.5Bi_0.5TiO_3 (TM-NBT). The half-metallic character of the TM-NBT is observed by employing onsite Coulomb repulsion in our calculation. In this work, we also present the optical and magneto-optical properties of the proposed compound. From our simulations we find that, the spectral permittivity is dominated by an absorption peak above 3 eV, originating from the absorptions by minority electrons. However, small absorption peaks near the visible range also emerge for the TM-NBT. We also present the polar MOKE spectra using linear magneto-optical approximation. We find, the most significant Kerr rotation for the compound is 0.5 degrees at a phonon energy of ~3.5 eV in polar geometry. ___________________________ References: 1. Z. Ren, X. Wei, Y. Liu, X. Hou, P. Du, W. Weng, G. Shen, and G. Han, Appl. Phys. Lett. 91, 063106 (2007). 2. D. Thiet, D. Cuong, L. Bac, L. Cuong, H. Khoa, S. Cho, N. Tuan, D. Dung, Mater. Trans. 56, 1358-1361 (2015).

Resume : Recently, ZnO material has been widely used in optoelectronics and thin-film solar cell devices. This is mainly due to its advantages of non-toxicity, low cost and relatively higher conductivity properties compared to other metal oxide materials such as SnO2. However, ZnO typically exhibits low conductivity compared to In2O3 and ITO-based thin films. Therefore, the main objective of this study is to experimentally investigate the impact of the RF sputtering power on the Al-doped ZnO thin films electrical properties. Al-doped ZnO layers were deposited on glass substrate using RF magnetron sputtering technique at different sputter power values ranging from 60 W to 140 W. It was revealed that the sputtering power could modulate the electrical characteristics of Al-ZnO material, tuning favorable conductivity and electron mobility at an appropriate sputter power. Therefore, the present study can open a new pathway for the design and fabrication of optoelectronic and photovoltaic devices. Key words: RF-power; Al-ZnO; Sputtering; thin-film; conductivity.

Resume : Tin Sulfide (SnS) is emerged as interest and suitable material for thin films solar cells (TFSCs) due to its appropriate band gap energy (about 1.3 eV), high absorption coefficient and its native p-type conductivity. In this paper, SnO2 electron transport layer incorporating doping effects is proposed as a prospective buffer layer to improve the optoelectronic performances of SnS TFSCs. The impact of the doping on the optoelectronic properties of SnO2 buffer layer is analyzed using Density Functional Theory (DFT) calculations, including Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA). It is revealed that the optical and electronic properties of SnO2 were substantially affected by the doping types and values, where the band gap energy decreases with increasing the doping. Moreover, it is found that the proposed buffer layer can provide enhanced electronic energy alignment and decreased optoelectronic losses, making it an alternative to the conventional toxic and CdS-based buffer layers for developing low cost and high-performance non-toxic SnS solar cells.

Resume : Surface acoustic wave (SAW) device is highly sensitive to any optical and electrical perturbations on its surface, which has been recently applied to the ultraviolet (UV) detector based on the comparatively longer- wavelength (UVA) photo-conducting layer such as ZnO or GaN film on piezoelectric substrate. Herein, the shorter-wavelength UV (UVC) SAW sensor based on wide-band gap but photo-conducting Zn2SiO4 film on LiNbO3 piezoelectric substrate is reported for the first time. The 100 nm-thick Zn2SiO4 film between the input and output interdigital transducers (IDTs) was deposited using a solution-based spin coating method followed by a rapid thermal annealing process. The SAW UV sensor was found to exhibit the interesting photo-sensing behaviors to solar-blind UVC illumination in a central frequency of more than 50 MHz; (1) a downshift in frequency of more than 30 kHz, (2) a change in insertion loss of less than 1 dB, and (3) a minimum UVC illumination intensity of 1 mW/cm2. Also, the frequency shift shows the rapid increase with the UVC intensity, and then the saturated behavior due to the limitation in the number of photo-generated carriers. The variation in the frequency shift and the insertion loss is attributed to the strong acoustoelectric interaction between surface acoustic wave and free electrons photo-generated within a Zn2SiO4 sensing material. It shows the promise of Zn2SiO4 for the fabrication of low-cost wireless SAW UVC sensor.

Resume : The influence of pressing pressure on the structural, optical, luminescent and electrical characteristics of undoped and manganese doped ZnO and (Mg,Zn)O ceramics has been studied. Both for ZnO and (Mg,Zn)O ceramics, it was found that increasing the pressing pressure from 20 MPa up to 150 MPa results in an increase of direct current conductivity and its independence on the temperature of measurements. The current-voltage characteristics remain linear. Based on the evaluation of electrophysical parameters of ceramics obtained via fitting of the infrared reflection spectra it is shown that pressing pressure rise does not lead to a significant change in free carrier concentration in ceramic grains. Obtained results revealed that the increase in direct current conductivity in un-doped samples is caused by both the formation of conductive channels that shunt ceramic grains and by a decrease in the porosity of ceramics. In Mn-doped samples, the current-voltage characteristics were found to be superlinear due to the influence of barriers at the grain boundaries. In these samples, not only an increase in DC conductivity was observed, but also a decrease in the super-linearity of the current-voltage characteristics, which indicates a decrease in the height of potential barriers. The effect of pressing pressure on the electrical characteristics of doped ceramics is also explained by the formation of some high-conductive regions, which contribute also to reducing the height of barriers. Based on the analysis of luminescence spectra, it is assumed that the increase in pressing pressure leads to the accumulation of interstitial zinc at the grain boundaries, which act as channels with increased conductivity. This statement is confirmed by the analysis of the chemical maps recorded for undoped and doped ceramics.

Resume : For many years, diamond has been trying to find its place in the world of materials for power electronics. It is a wide band-gap material that competes with many other materials and, despite its remarkable intrinsic properties, competition is strong. Its theoretical voltage withstand, high carrier mobility and high thermal conductivity are to its advantage, while the difficulty of obtaining large-area substrates with low defects control is a limiting factor in its use. This paper will present the different steps leading to the fabrication of Schottky diodes by detailing the current limitations but also the successes obtained in recent years. It will discuss and attempt to be objective about the results in comparison with other materials.

Resume : Development of nitride based optoelectronic devices is supported by extensive computational efforts in several areas: simulations of molecular processes at semiconductor surfaces; incorporation of atoms into the solid phase, electric transport in the device structures, strain and spontaneous polarization and related electric fields in optically active structures, light emission and other recombination types. These subject require different analysis tools which will be discussed. Molecular processes require use of ab initio simulations in density functional theory (DFT) formulation. These basic techniques were recently supplemented by new aspects: electric field at surfaces and charge transfer contribution to adsorption energy. The subject was supplemented by incorporation of enthalpy and entropy contributions that allow to obtain pressure/temperature dependence of the surface state. Incorporation of atoms, including dopants was also investigated by ab initio methods. The calculations incorporating above mentioned field and charge effects provide deep insight into the semiconductor doping in the growth stage. That allows to determine the doping problems, especially acute in AlN-rich wide bandgap devices. The use of polarization doping allows to alleviate the doping problems these AlN-rich device structures.

Resume : This study reports on the use of thin films of polycrystalline diamond grown on doped silicon and titanium substrates for in vitro electrophysiological sensing devices. The electrical properties of the polycrystalline diamond surfaces were evaluated using electrochemical impedance spectroscopy, and electrical noise measurements. The polycrystalline diamond surface when immersed in the cell culture medium, establishes an electrical double-layer which in series with the bulk solution (cell culture medium) behaves as a classical Maxwell-Wagner relaxation with a relaxation frequency located at approximately 2 kHz. The low frequency (100 Hz) interfacial capacitance has a value of 10 µF/cm2. Furthermore, the interfacial resistance is low which minimizes the 1/f noise as well as the thermal noise, which is as low as 0.2 µV rms for a sensing electrode with an active area of 0.16 cm2. The high interfacial capacitance associated with the low thermal and 1/f noise makes these polycrystalline diamond coatings particularly suited to measure weak bioelectrical signals generated by non-electrogenic cells. Unlike neurons, non-electrogenic cells generated weak signals (micro-volts) in the mHz frequency range. The diamond coatings with different surface roughness were evaluated to record cell signals generated by the population of glial cells (C6 immortal cell line). Electrophysiological recordings were complemented with a detailed surface morphology analysis to gain insight into the best diamond morphology to minimize the low-frequency electrical noise and achieve bioelectrical recordings with a signal-to-noise ratio above 20 at frequencies as low as 1 Hz.

Resume : The epitaxial growth of high electron mobility transistors (HEMT) on bulk AlN substrates is very attractive to fabricate high-frequency and high-power switching devices. This is due to the high resistivity and good thermal conductivity of AlN as well as the possibility to manage at least in a given thickness or composition range the lattice parameter mismatch between AlGaN, GaN and the substrate. However, few studies have been dedicated to the effect of GaN channel thickness downscaling in Al(Ga)N/GaN HEMTs, especially on bulk AlN substrate. In a previous work, we investigated the effect of GaN channel thickness and AlGaN barrier composition on lateral breakdown voltage of HEMT structures grown by ammonia source molecular beam epitaxy on AlN-on-Sapphire. A similar trend on the three terminal breakdown voltage of transistors was noticed while reducing from 500 nm to 50 nm the thickness of the GaN channel grown on bulk AlN substrate. To go further in the understanding of this trend, HEMTs were grown on bulk AlN substrate with GaN channel thickness varying from 500 nm to 8 nm. Most of these structures contain a barrier layer consisting of a 1 nm AlN spacer layer plus a 19 nm AlGaN layer with a nominal Al content of 30% capped with a thin GaN layer. X-ray diffraction (XRD) shows that reducing the GaN channel down to 20 nm induces a broadening of the diffraction peaks. This indicates a degradation of the crystal quality which is consistent with a drop of the electron mobility. The change of the in-plane lattice parameter with channel thickness shows that the 20-50 nm region is the range corresponding to the most rapid change in strain relaxation rate. The cross-section transmission electron microscopy (TEM) analysis performed on 50 nm and 500 nm GaN channel HEMTs reveals the presence of a 20-30 nm thick contrasted region close to the GaN on AlN interface, with a large number of defects. The present observations indicate that downscaling the channel thickness increases the residual compressive strain in GaN at the expense of the rapprochement to a highly defective region. Nevertheless, the nucleation of defects at this interface is not likely occurring for thicknesses below a critical value estimated around 10 nm. Thus, HEMT structures with 8-9 nm GaN channels using 86% Al content AlGaN barriers are demonstrated with sheet resistances between 980 and 1300 Ohm/sq, which is promising for power switching applications. This work was supported by French technology facility network RENATECH and the French National Research Agency (ANR) through the projects BREAkuP (ANR-17-CE05-0013) and the “Investissements d’Avenir” program GaNeX (ANR-11-LABX-0014).

Resume : Gallium oxide (Ga2O3) is an ultra-wide bandgap semiconductor exhibiting a number of unique properties and promising applications for power and optoelectronics. Importantly, Ga2O3 can be crystallized in different polymorphs having different structure and, consequently, different physical properties that can be potentially exploited by gaining control over the phase transitions. In the present contribution we review our recent activity on radiation phenomena in Ga2O3. Specifically, we have used a combination of experimental methods with DFT calculations to investigate the response of monoclinic beta-Ga2O3 single crystals to ion bombardment. Exploring a wide range of experimental parameters, such as ion species, accumulated dose, ion energy, irradiation temperature and beam flux we demonstrate that ion-beam-induced disorder and associated strain affect the stability of beta-phase leading to the polymorph transformations. As an example, we demonstrated a controlable formation of highly-oriented single-phase orthorhombic κappa-Ga2O3 layer with sharp b/κ interface on the top of the beta-Ga2O3 wafer [1]. Our findings pave the way for a new synthesis technology of the metastable polymorph heterostructures and regularly shaped interfaces in the device components not atchivable by the conventional deposition methods. References: [1] A. Azarov, C. Bazioti, V. Venkatachalapathy, P. Vajeeston, E. Monakhov, and A. Kuznetsov, Phys. Rev. Lett. 128 (2022) 015704

Resume : Beta gallium oxide is an emerging wide bandgap semiconductor with bandgap (4.8 eV), good saturation velocity and a large predicted breakdown field up to 8 MV.cm-1. As shown in the Baliga’s Figure of Merit, the on-resistance of ß-Ga2O3 devices is expected to be lower than that of SiC or GaN at the same breakdown voltage. These properties make Ga2O3 a very attractive semiconductor for high voltage and radio-frequency applications. However, the technological advances of Ga2O3 are limited by the lack of control over defects, impurities and doping level which determine the electronic properties of devices. In the literature, electrical investigations of ß-Ga2O3 crystals grown by Cz and EFG (edge defined film fed growth) method were carried out using deep level transient spectroscopy (DLTS). Several deep levels have been observed, both intrinsic and extrinsic origins have been suggested. But precise identification is still expected. This work aims to describe and report on the electrical properties of ß-Ga2O3 crystals, doped by Si, grown by the Floating Zone method which is crucible free, in theory generates fewer impurities. Electrical characterizations were performed on Schottky diodes doped with Si. DLTS measurements were carried out by varying the parameters to observe the surface and bulk of the substrate distinctly. We observed at least six deep traps in the 77-550K range. The thermal activation energy and the apparent capture cross-section of the traps were extracted from Arrhenius plots: ES (EC – 0.31e eV), E1 (EC – 0.54 eV), E2 (EC – 0.77 eV), E3 (EC – 0.97e eV), E4 (EC – 1.1eV) and E5 (EC – 1.31 eV). Of these levels, ES occurs only near the surface with a high concentration. There have been several previous reports of E1, E2 and E3 and they were observed in substrates grown by EFG and Cz techniques. It is possible that the origin of these deep electron traps is the same since we find similar signatures in materials grown by different crystal growth methods. We also compared two diodes of the same sample. Static analysis and DLTS spectra showed that the higher the concentration of trap ES, the higher the saturation current and the lower the barrier height. Topological analysis of the substrate was carried out and AFM images revealed the presence of scratches on the surface which are expected to be produced by the mechanical polishing of the substrate. They are not evenly distributed on the surface and have different depths. We also carried out DLTS measurement on a diode which have undergone a different polishing cycle (chemical) and trap ES disappeared. We assumed the presence of these scratches could contribute to the defect Es.

Resume : Ga2O3 is a semiconductor with a large bandgap (~4.9 eV at room temperature), a high breakdown field (> 8 MV/cm), and a tunable conductivity from almost semi-insulating to highly conductive. Combining its electrical and optical properties, Ga2O3 has aroused interest in different electronic and optoelectronic applications, such as high-power electronics, UV photodetectors, solar cells, and sensors. Furthermore, and thanks to its monoclinic structure, Ga2O3 presents two easy cleavage planes, (100) and (001), that have been exploited to produce thin, flexible, and transferable nanomembranes with high crystalline quality by mechanical exfoliation. More recent works have been reported about the production of thin membranes based on ion implantation, that contrary to mechanical exfoliation using conventional scotch tape, the thickness of the obtained membranes can be tuned via the implantation energy. In this work, (100)-oriented membranes with different thicknesses in the range from 200 nm to 500 nm were produced using a process based on ion implantation. In more detail, these membranes result from the unrolling of Ga2O3 microtubes formed during implantation upon annealing at temperatures higher than 500 °C. In this work, the microtubes were produced by Cr implantation with different energies from 200 keV to 300 keV, with a fluence of 5x1014 ions/cm2. Using nanoindentation it was found that these thin membranes, produced by this innovative process, present a hardness and a Young Modulus comparable with the values measured by other authors for commercial single crystals produced by Novel Crystal Technology, Inc.. This study of the local physical properties by nanoindentation was complemented with a structural characterization by x-ray diffraction and Raman spectroscopy. Furthermore, a systematic study of the effects induced by argon irradiation on the surface potential, mechanical and morphological properties of these thin membranes will be presented. A correlation of the mechanical properties as a function of the thickness and defect profile will be discussed.

Resume : The rising demand for high-volume data transmission in all sectors, from satellites to mobile Wi-Fi connections, and the need for more efficient and faster power conversion technologies for e-mobility can find a valuable resource in nitride semiconductors. AlScN allows higher current densities and voltages than AlGaN, reducing switching losses in nitrides HEMT. MBE of AlScN has already been reported. We demonstrated in 2019 at Fraunhofer IAF that it is possible to manufacture AlScN epitaxial layers by MOCVD. The epitaxial growth of AlScN was performed on 4-inch sapphire or silicon carbide substrates in a commercial MOCVD reactor. The absence of a precursor with sufficiently high vapor pressure is the main challenge in the MOCVD growth of Sc-containing nitrides. Cyclopentadienyl-based scandium precursors are the best choice among the very few commercially available. Due to their very low vapor pressure, the source material and the gas lines up to the gas injection system must be heated to relatively high temperatures. We succeeded in depositing AlScN epitaxial layers at a typical growth rate of 0.05 µm/h with an Sc-concentration up to 30 % through a modified hardware setup. However, new cyclopentadienyl-based precursors can deliver a higher molar flow while kept at high temperatures and allow the growth of AlSc0.18N layers at a rate of 0.25 µm/h. HRXRD measurements evidenced that 10-50 nm AlScN layers deposited on GaN or AlN templates on Sapphire are strained. The Sc-concentration in the layer and impurities such as carbon and oxygen were determined by SIMS. These measurements showed that an Sc-content above 30% could be achieved and that C and O are incorporated to a high extent, similar to what was observed in MBE-grown samples. We have observed impurities reduction by adopting higher growth temperatures (from 1000 to 1200 °C) and higher V/III-ratios (up to 10000), as commonly observed for other nitride semiconductors. AFM measurements evidenced that AlScN layers have a roughness controllable from 0.5 to 2 nm. Using SiNx as a cap layer on AlScN is the best choice to get an effective but smooth passivation layer (RMS below 0.5 nm). TEM analysis confirmed the absence of sharp interfaces and a sub-optimal lateral inhomogeneity in incorporating scandium across the layer. Zinc blend stacking faults may appear too. HEMT structures with thinner (5-10 nm) AlScN barrier layers and an Sc-content in the 10-15% range had the highest 2DEG mobility up to ~1200 cm2/Vs and a sheet carrier density of 3x1013 cm-2 or higher. The sheet resistance of these samples was typically between 200 and 500 Ohm/sq. HEMT structures were fabricated, as described in our previous work. Compared with AlGaN/GaN heterostructures with the same device architecture, the AlScN-based HEMTs have a 20% higher drain current (1800 mA mm-1) and transconductance (500 mS mm-1).

Resume : Gallium nitride (GaN) is a wide bandgap semiconductor of choice for high-power and high-frequency applications thanks to its outstanding material properties (high breakdown voltage, high electron velocity and good thermal conductivity. The standard AlGaN/GaN High Electron Mobility Transistor (HEMT) device is a depletion-mode (D-mode) transistor. However, enhancement-mode (E-mode) transistors are required for several applications: first for safe power switch applications, second for the co-integration with E-mode devices for high-frequency analog and digital applications where they help to simplify the design of circuits. Yet, an efficient way to shift the threshold voltage is to introduce an additional layer such as p-doped GaN cap on top of the barrier layer. However, such layers need to be removed from the transistor access regions to allow the electron current between source and drain. Plasma-based etching induces some degradation at the surface of the barrier layer. To overcome this problem, we have developed an original process based on the selective evaporation under vacuum (sublimation) of GaN. We show that a dielectric mask can be patterned to define the region where GaN sublimates. The evaporation performed into a molecular beam epitaxy chamber monitored with RHEED stops when the AlN or AlGaN barrier layer is reached. Local evaporation of the p-GaN cap layer can be obtained in the access regions of the enhancement mode transistor with micrometric gate patterns as well as in adjacent larger areas for exposing the depletion mode structure, so that D-mode and E-mode devices are co-integrated on the same substrate. Atomic Force Microscopy and Transmission Electron Microscopy confirm the innocuity of the process achieved below 900°C. By this way the co-integration of E/D mode transistors has been demonstrated with a structure grown on Silicon substrate and composed of a 15 nm Al0.26Ga0.74N barrier layer and a 50 nm p-GaN cap. Furthermore, the local area regrowth of AlGaN can complement the evaporation process to increase the maximum drain current in both E and D mode devices as demonstrated after the selective evaporation of p-GaN on a HEMT structure with a 2 nm AlN barrier layer that was totally depleted from carriers before regrowth. This work was supported by French technology facility network RENATECH and the French National Research Agency (ANR) through the projects ED-GaN (ANR-16-CE24-0026) and the “Investissements d’Avenir” program GaNeX (ANR-11-LABX-0014).

Resume : ScxAl1-xN/GaN high electron mobility transistor heterostructures grown by ammonia source molecular beam epitaxy Caroline Elias, Maxime Hugues, Yvon Cordier Université Côte d’Azur, CNRS, CRHEA, rue B. Gregory, 06560 Valbonne, France. The high electron mobility transistors (HEMTs) have been developed with a lot of success for mobile telecommunications, radars and power switching. To further improve the device performances, the requirement for extremely thin barrier layer with a low lattice mismatch is difficult to fulfil considering the need to keep a high carrier density in the channel two-dimensional electron gas (2DEG). For that purpose, scandium aluminium nitride (ScAlN) alloy is a promising candidate thanks to its large piezoelectric and spontaneous polarization coefficients, but also the possibility to be lattice matched to GaN for scandium content around 18%. In this work, epitaxial growth of ScAlN on GaN with ammonia source MBE is motivated by the possibility to grow nitrides at relatively low temperature under a nitrogen-rich regime. Several ScAlN thin layers with thickness around 25 nm have been grown using different growth temperatures varying with a step of about 50°C on GaN on Sapphire templates. Scandium content in the range of 14%-15% was determined by X-ray photoelectron spectroscopy. The surface roughness of the films assessed by tapping mode atomic force microscopy is with root mean square values below 0.3 nm for 500 nm x 500 nm scans. The structural properties have been studied by X-ray diffraction (XRD) measurements. The 2theta/omega (2Ө/) scans show a single peak around 35.6° accounting for the presence of a single crystalline wurtzite phase while Pendellösung fringes are clearly visible for the layers grown between within a 100°C temperature window below 800°C. This indicates smoother surface and interface with GaN, but also a better crystalline quality as highlighted by the smaller full-width at half maximum values of the rocking curves. XRD reciprocal space mapping around (105) reflection was performed to determine the lattice parameters in- and out-of-plane and confirmed that the ScAlN layers were lattice-matched with GaN. The 2DEG sheet concentration Ns deduced from capacitance-voltage measurements is around 3.0-3.5x1013/cm2 for the lower growth temperatures and suffers a rapid drop for the higher temperatures responsible for crystal quality degradation. HEMT structures with 10 nm ScAlN barriers were grown in the optimal temperature range determined previously. In spite of the barrier thickness reduction, C-V exhibits 2DEG sheet concentration Ns reaching values up to 3.5x1013/cm2 which confirms the quality of the heterostructures. This is definitely a very promising step for the fabrication of the next generation of high-power and high-frequency transistors. This work was supported by French technology facility network RENATECH, the “Investissements d’Avenir” program GaNeX (ANR-11-LABX-0014), and the project GaN for advanced power applications (GaN4AP) supported by H2020 ECSEL JU and the French Direction Générale aux Entreprises.

Resume : High Electron Mobility Transistors (HEMTs) based on gallium nitride (GaN) have gained more and more attention in the last years, owing to their intrinsic properties (namely, high saturation velocity, high critical electric field, high density and high mobility of the charge carriers). These characteristics make GaN-based devices well-suited to high power and high frequency applications. However, the presence of the two-dimensional electron gas (2DEG) at the heterointerface between AlGaN and GaN makes these transistors intrinsically normally-on devices while, for power electronics applications, normally-off operation is usually preferred for safety reasons and compatibility with CMOS circuitry. Several solutions have been proposed to obtain normally-off GaN HEMTs and one of the most promising approach relies on the addition of a p-GaN gate, in order to interrupt the 2DEG below the p-GaN at zero voltage. Given the importance of this extra p-GaN layer, the correct and detailed understanding of its impact on the device behaviour is critical. In this work, different p-GaN process solutions have been studied: experimental measurements and Technology Computer Aided Design (TCAD) simulations have been analyzed, compared and interpreted. In particular, capacitance-voltage (C/V) profilings and gate current (Igss) measurements have been performed in order to reveal the net acceptor concentration, essentially due to activated Magnesium, and the level of the Schottky barrier formed at the interface between metal gate and p-GaN while TCAD simulations of the band diagrams, transcharacteristics (Id/Vg) and gate leakage curves provided many information about the physical phenomena involved in the structure thanks to many possible Design Of Experiments (DoE). TCAD simulations has been performed through 2-D device simulator ATLAS (by Silvaco Inc.). We implemented three main process solution for our GaN HEMT, which are reported below : 1) no intentional Magnesium doping in the GaN cap (grown on AlGaN barrier): the transistors behave as depletion-mode devices, with negative pinch-off voltage (Vpo between -3V and -1V), as expected; in this case C/V curves are flat after the threshold voltage and the Igss contribution is very low (~1E-3 uA/mm at 25°C); 2) intentional Magnesium doping during epitaxial growth of GaN cap: the devices work asnormally-off HEMTs, showing Vpo around 0V and low drain leakage current at Vgs=0V (Idss). C/V curves still exhibit a flat trend with Vgs after the threshold. Gate leakage measurements have shown very similar values to the ones whiteout any Magnesium doping. A set of TCAD simulations has been carried out to explain this behaviour, showing the effect of the net acceptor concentration (in p-GaN and close to AlGaN) on the Vpo. 3) intentional Magnesium doping and a dedicated Rapid Thermal Processing (RTP): the measured Vpo is higher than 0 V and C/V curves exhibit a decreasing trend with Vgs, after the threshold. Igss increases, as predicted from models and simulations.

Resume : Electronic transport properties are fundamental features of any materials targeting optoelectronic applications, and accurate determination of their key parameters is critical for further development of semiconductor compounds. A deep investigation of semiconductors over decades has led to unprecedented technological innovations, though sophisticated characterization tools can be of immense importance for novel photonic architectures. Moreover, a class of oxide semiconductors still needs to be better studied in terms of their charge carrier characteristics. Therefore, a thorough investigation of carrier mobility distribution can be viewed as an attractive electrical technique for separation of distinct carriers. The magnetic-field-dependent Hall effect and resistivity measurements assisted with a mobility spectrum analysis (MSA; see more details elsewhere [1-2]) were proved to determine multichannel conductivity characteristics. This approach was successfully implemented by our group for ternary InAsSb, where a temperature transition associated with a carrier type change was in fact verified to be mainly due to the presence of thermally activated band electrons at near-room temperature regime [3]. However, here the aforementioned approach is applied for wide-band-gap (WBG) semiconductors, namely III-nitrides (GaN) and metal oxides (In2O3- or ZnO-based compounds). In the GaN epistructures, either unintentionally doped or acceptor doped ones, a number of distinct separated electronic carrier channels, electrons and holes, can be revealed with their precise concentration and mobility. Their origins are still discussed. Interestingly, in transparent conductors such as ITO and AZO, an unimodal electron population can be solely detected (associated with a bulk-like conduction) and there is an absence of parallel conductions like in III-nitrides. Additionally, the confirmed experimental datasets of ITO and AZO can be viewed as an important validation of exclusively donor-type defects and/or impurities contributing to total conductivity [4]. Concluding, though not easy for data acquisition and spectrum calculation, this advanced MSA approach can be effectively used to examine fundamental electronic transport properties of relevant WBG semiconductor thin films for their improvement and further development. Acknowledgments: This research was completed with the financial support under the program of the Minister of Science and Higher Education (Poland): “Regional Excellence Initiative” in 2019–2022; project number 014/RID/2018/19, funding amount of PLN 4 589 200.00 (rid.wtc.wat.edu.pl). The study was financially supported from the project: "High-performance AlGaN/GaN-HEMT transistors made with the hybrid MBE-MOVPE technology", number of agreement: 2/Ł-PORT/CŁ/2021, funded by The Łukasiewicz Centre. References: 1. Wróbel, J., et al., Phys. Status Solidi RRL 14 (2020) 1900604. 2. Umana-Membreno, G.A., et al., J. Electron. Mater. 42 (2013) 3108. 3. Złotnik, S., et al., Sensors 21 (2021) 5272. 4. Złotnik, S., et al., Phys. Status Solidi RRL (submitted).

Resume : High quality Atomic Layer Deposited (ALD) Al2O3 films are essential for the gate insulator fabrication deployed in GaN based finFETs. The Al2O3 ALD layers have a crucial influence on the finFETs gate modulation ability. They may determine the transistor threshold voltage and its stability, the voltage span, commonly between 0 V to +5 V, its subthreshold slope and its OFF state leakage. Low ON state conductivity is achieved by high accumulation oxide capacitance and low interface traps density. In our study, we evaluated the properties of deposited films in fabricated in a planar MIS capacitor (MISCAPs) with different pre-treatments prior to the deposition and to different ALD process types, Plasma Enhanced ALD (PEALD) and thermal ALD (ThALD). An established method for gate insulator qualification is MISCAPs capacitance voltage (CV), step stress capacitance voltage scans and capacitance-frequency (CF) characterization. In this work we investigate atomic layer deposited (ALD) Al2O3 films in order to find the deposition conditions, that yield the most suitable electrical properties. We compare between H2, NH3 and Ar plasmas pre-treatment and no pre-treatment as a control while using alternating ThALD / PEALD layers. In addition, we compare between ThALD and PEALD while using H2 and NH3 pre treatments. The MIS capacitors were characterized by forward bias step stress CV sweeps to evaluate the insulator charging effects, to identify possible shifts in the flat-band voltage (VFB) and to gain insight in interface charging phenomena. CF measurements were used for Dit evaluation via the GP / ω method. When comparing between pre-treatments, CV results show that for the forward sweep, Ar contains the most induced interface traps and the highest VFB shift with onset voltage of +4 V. Later comes the control no pre-treatment and the NH3 and H2 are comparable, with a slight advantage (low induced traps and low VFB) to the NH3 pre-treatment with onset voltage of +8 V. For the reverse sweep, the wafer that didn’t undergo any pre treatment contains the most induced traps 6 × 10^11 cm^-2 and has the highest absolute VFB shift of +5 V. The NH3 and H2 are comparable with induced traps 4 × 10^11 cm^-2 and has the highest absolute VFB shift of +2 V after +10 V stress, here again, with an advantage to the NH3 pre treatment. When comparing between ALD process types, PEALD CV curves show a significantly larger hysteresis, which indicate a larger amount of trapping compared to ThALD. On the other hand, ThALD decreases significantly the threshold voltage. ThALD / H2 pre-treatment yields the lowest threshold voltage (4 V) but the lowest VFB, which indicate low interface trapping. The threshold voltage of ThALD with NH3 pre-treatment is 9V. The PEALD MISCAPs were measured up to 10V without difficulty. Dit evaluation shows lowest density of trap states for the ThALD / PEALD mix with NH3 pre-treatment, that was calculated to be 7 × 10^11 cm^-2/eV at E = 0.1 eV below flat-band and remains relatively constant. In conclusion, ThALD / PEALD mix with NH3 yields best results in terms of Dits, threshold voltage stability and in low induced traps shift. These are the most suitable conditions for gate insulator in finFETs.

Resume : Wide bandgap semiconductors as gallium nitride (GaN) have gained research interest for high-voltage, high-power and high frequency application due to their large band gap, high critical field and high electron mobility [1]. Several device concepts have been presented, especially high electron mobility transistors (HEMTs), which consisting of a lateral device structure and are used commercially for some applications. However, the lateral structure suffers from heat dissipation and large areas are needed for power electronic current levels. The vertical structures are introduced where the BV is dependent on the drift layer thickness and not the lateral dimensions [2]. Further improvement could be achieved by confining the channel into a fin, to improve electrostatic control and obtain unipolar normally-off devices [3]. Further, the usage of GaN grown on foreign substrates gives a cost advantage in comparison to fully vertical structures. Here, we present our work on the fabrication of quasi-vertical GaN FinFET on silicon carbide (SiC) substrates. The GaN layer stack was grown on SiC with hot-wall metalorganic chemical vapour deposition (MOCVD) technique, consisting of a n+ (500nm)/n (2000 nm)/n+ (100 nm) stack on an aluminum nitride (AlN) buffer layer. The fin was dry-etched with ICP-RIE with Cl2/Ar gas masked by electron beam defined HSQ (FOX15) resist, followed by a TMAH wet etch to smoothen the sidewalls. The fin width and number per device was varied from 100 nm to 400 nm and 1 to 3 number of fins. A 100 nm bottom spacer of Al2O3 was introduced to move the electrical field away from the fin edges and a 100 nm Al2O3 top spacer to separate gate and source/drain contacts from each other, both grown with atomic layer deposition technique. The gate length was fabricated to around 300 nm in series with around 300 nm ungated fin regions above and below the gate and 1 µm bulk drift layer thickness. The gate stack consists of 80 nm tungsten (W) and 15 nm Al2O3, which was fabricated with ALD and sputter deposition. For structuring of the different layers on the fin the resist planarization technique is used. Finally, a lift-off process for the sputtered Ti/Al/Ni/Au contact formation was performed. Simultaneously fabricated transfer length method (TLM) structures show ohmic contact behaviour with low contact resistance for the drain and source contacts without contact annealing. Evaluations on the transfer and output characteristics show a field effect transistor (FET) behaviour. The extracted threshold voltage (Vt) ranges from around -0.5 V to -4 V depending on the fin width, which indicates a normally-on behaviour. These variations can be explained by the size of depletion zones on each side of the fins, which need to overlap to turn off the device. Specific on-resistances (Ron,sp) below 0.1 mΩ·cm2 (normalized to the fin area) have been obtained. The best subthreshold slope (SS) was extracted to slightly below 100 mV/dec at drain voltage of Vds = 5V. The investigations on breakdown voltage and mobility are currently under progress. The evaluation of our first fabricated devices shows promising results. To bring our device in the normally-off range we are aiming to introduce post gate metallization annealing (PMA) and tuning of gate oxide thickness and material. References [1] H. Amano et al., J. Phys. D Appl. Phys., 51,163001, 2018. [2] S. Chowdhury and U. K. Mishra, IEEE Trans. Electron Devices, 60(10), 3060–3066, 2013. [3] M. Sun, Y. Zhang, X. Gao, and T. Palacios, IEEE Electron Device Lett., 38(4), 509–512, 2017.

Resume : Aluminum oxide (Al2O3) is among the most studied high-κ dielectric because of its interesting physical properties, such as high permittivity (κ ~ 9), high critical electric field (10 MV/cm), and large bandgap (~8.9 eV). With these features, Al2O3 is the gate insulator of choice for GaN-based high mobility electron transistors (HEMTs) operating at high-frequencies and high-power voltages. The reliability of HEMT devices is strongly influenced by the properties of the Al2O3 layer and of its interface with AlGaN, which are generally afflicted by the presence of defects, namely dangling bonds and/or contaminations [1,2]. The deposition technique plays a key role in the quality of Al2O3 and of Al2O3/AlGaN interface, in the trap density associated with defects, and consequently in their electrical behavior. Atomic layer deposition (ALD) with its peculiar deposition mechanism, based on sequential and self-limited chemical reactions, is undoubtedly the best technique to deposit uniform and conformal films of dielectrics with high interface quality. However, the structural and electrical properties and their evolution during the early growth stages of the insulating layers need to be opportunely studied to control the final gate insulator properties. In this work, a comparative study of the early growth stages of Al2O3 layers deposited by thermal (T-ALD) and plasma-enhanced atomic layer deposition (PE-ALD) on AlGaN/GaN heterostructures is presented. In particular, a systematic investigation of ultra-thin (under 6 nm) Al2O3 layers has been carried out. The evolution of morphological and electrical properties of the first Al2O3 atomic layers has been investigated at the nanoscale by conductive- AFM and correlated with the electrical measurements on MIS capacitors. Moreover, the quantification of the trapping phenomena and the evaluation of their effects on the final insulating properties upon increasing the film thickness have been provided. A different insulating behavior has been found for Al2O3 deposited by T-ALD and PE-ALD, which is an indication of a different ALD growth mechanism. Current maps and chemical characterization by XPS provided evidence that the PE-ALD process occurs under an ideal layer-by-layer growth because of the efficiency of the O2-plasma. The T-ALD approach, by contrast, shows a nucleation process similar to the island growth mode [3]. [1] Hossain, T.; Wei, D.; Nepal, N. ; Garces, N. Y.; Hite, J. K. ; Meyer, H. M.; Eddy, D. R. jr; Baker, T, Mayo, A.; Schimitt, J.; Edgar, J. H. Phys. Status, Solidi C, 2015, 11, 565-568. [2] Schilirò, E.; Fiorenza, P.; Bongiorno, C.; Spinella, C.; Di Franco, S.; Greco, G.; Lo Nigro, R.; Roccaforte, F. AIP Adv. 2020, 10, 125017. [3] Schilirò, E.; Fiorenza, P , Greco, G., Monforte F., Condorelli G.G., Roccaforte F., Giannazzo F., Lo Nigro R. ACS Appl. Electron. Mater. 2022, 4, 1, 406–415

Resume : Gallium nitride (GaN) is the material upon which the great success of white and blue solid-state light sources has been built in recent years, including highly efficient light emitting diodes (LEDs) as well as laser diodes (LDs), ubiquitous in modern life. There are constantly ongoing works on the development of the epitaxy and bulk substrates for the GaN-based material system, however device processing has also seen development. One of the most important elements of a device are its electrical contacts, controlling the current flow crucial to its effective operation. For GaN-based LDs, different contact materials are used to contact the n-type, N-face bottom and the p-type, Ga-face top layer and due to the nonsymmetric polarity, the surfaces require different pre-treatment. The standrad ohmic contacts to n-GaN are based on Ti/Al while the ones to p-GaN are based on Ni/Au metallic bilayers. We wish to study improved light extraction by replacing the commonly used metallic contacts with transparent ones based on an transparent conducting oxide (TCO). It has been shown in the literature indium doped tin oxide (ITO) makes good contacts to p-GaN, however it contains In, which is not sustainable. Therefore we study AZO - an In-free alternative TCO and show how it behaves in junctions to both n(N)-GaN and p(Ga)-GaN. Since the application of one contact material for both n and p-type sides of a semiconductor is usually impossible due to inherent physical differences in work function-related barrier heigths, we put strong focus on the interface engineering of the GaN/AZO interface. We follow three strategies: (1) applying ultrathin metallic layers at the interfaces between the GaN and AZO, utilizing e.g. Ti, Ta, Mg, Cr for the n-GaN bulk substrates and Ni, Au, Mg, NiO and Pd for p-GaN epitaxial layers; (2) we study different chemical preparation of the GaN surface before the AZO deposition to remove the native oxides and fill the dangling bonds, including treatments with solutions of HF, HCl, NH4Cl or DMSO; and finally (3) we apply plasma treatment in Ar and CF4 of the surface before the deposition of AZO. We observed that AZO/p-GaN formed ohmic contacts after annealing at 800°C and that an introduction of thin Ni and Au interfacial layers lowered the formation temperatures significantly while increasing the output currents by over an order of magnitude. For the n-GaN side we see how the application of different surface treatments changes the initially Schottky-like character of the AZO/n-GaN junction into a symmetrical, high current characteristic. We discuss the complex interfacial phenomena aided by a wide array of complementary advanced characterization measures, including XRD, HR-TEM/EELS, SIMS, XPS and cross-section SPM and demonstrate how the chosen contacts influence LD emission. This work was supported by the National Centre for Research and Development, Poland, project 'OxyGaN' - M-ERA.NET2/2019/6/2020, by the Hungarian NRDI Fund, grant number 2019-2.1.7-ERA-NET-2020-00002 and by the Israel Ministry of Science and Technology in the frames of the M-era.net Programme.

Resume : Crystallographic defects are one of the main obstacles limiting semiconducting materials development and gallium nitride (GaN) is not an exception. The study of nitride-based devices started seriously in the early 1990`s when GaN was seen to be an excellent material for next generation optoelectronics and for high power and radio frequency (RF) applications. Early expectations in applications other than general lighting based on blue InGaN + phosphor have not fully materialized. One of the key issues in GaN power and RF transistors is the presence of defects which act as carrier trapping centers. In many cases the origins of these traps remain unknown. In this work we describe an ongoing study of trapping states in MOVPE material grown on highly conductive n-type ammono-GaN substrates [1]. The detected states are compared with those in hetero-epitaxialy grown materials. Typically, 100-nm thick silicon doped n-GaN (5x1017 cm-3) was grown, followed by a 1.5-µm thick n-GaN drift layer with a net donor concentration of 2x1016 cm-3. Finally, Ni/Au Schottky contacts were deposited through a shadow mask, and junction spectroscopy measurements were carried out on the diodes. We show that in some homo-epi material relatively fine structure in the DLTS (Deep Level Transient Spectroscopy) spectra can be revealed using Laplace (high resolution) DLTS [2]. This is important in work aimed at identifying the defects as the accurate analysis of electric field dependence of emission rates and depth profiles of the traps is facilitated. During nitride epitaxial growth altering the III/V ratio and the temperature changes the native defect population but also many other parameters. We have considered the effect of MOVPE growth conditions on deep level defects. Further, we have added to our study by introducing native defects, post growth, by electron irradiation. In GaN, electrons with energies below 400 keV displace mostly nitrogen atoms while higher energies displace both lattice species. This is of real importance for identifying VN, VGa, Ni and Gai related defects and their reactions with other species. Although there are several previous publications on electron irradiation induced damage of GaN, very few studies have used homo-epi with low grown-in dislocation density [3], and none has used such material in combination with high resolution Laplace DLTS. In general GaN grown by hetero-epitaxy exhibits inhomogeneous strain which broadens the observed defect parameters. To illustrate the advantages of homo-epitaxy and Lapace DLTS we have analyzed a DLTS signal reported as EE1 at EC-0.13eV associated VN with reported previously by several groups. We have measured this trap in n-GaN Schottky diodes irradiated with 0.4 and 1.5 MeV electrons. We show that the EE1 trap is a donor and its unusual properties can be explained by Laplace DLTS being able to demonstrate that it has two components, possibly due to different configurations of VN. These are not able to be separated by conventional DLTS techniques. Similar analyses are presented for the EE2 trap (Ec-0.98 eV) thought to be related to the nitrogen interstitial and the E3 trap (Ec-0.58 eV) attributed to an iron atom at the gallium site. Acknowledgement This work was financially supported by the National Science Centre, Poland through Project Number 2020/37/B/ST5/02593. [1] www.ammono.com [2] L. Dobaczewski, A. R. Peaker, K. Bonde Nielsen, J. Appl. Phys. 96, 4689 (2004) [3] M. Horita, T. Narita, T. Kachi, J. Suda, Appl. Phys. Lett. 118, 012106 (2021).

Resume : Gated Al2O3/AlGaN metal-oxide-semiconductor junctions show a hysteretic behavior in capacitance vs voltage curves, attributed to near-interface traps deriving from defects within the oxide layer. The origin as well the structural/electronic properties of such defects are still strongly debated in the literature. Here we use Car-Parrinello molecular dynamics and the climbing-image nudged elastic band method to show that Aluminum Frenkel defects can form bistable traps in disordered and stoichiometric Al2O3. Based on these results, we propose a calibrated polaron model representing a distribution of individually interacting levels with an internal reconfiguration mode and coupled to continuous bands of carriers to explain the hysteresis mechanism in Al2O3/AlGaN junctions.

Resume : Vertically operating nitride-based devices require native wafers of specified electrical and structural properties. Highly conductive (with low resistivity) gallium nitride (GaN) crystals of high structural quality should be used for preparing wafers. However, crystallization of bulk GaN is still challenging. Two methods are mainly applied for preparing commercially available GaN substrates: halide vapor phase epitaxy (HVPE) and ammonothermal. This work will cover both of these methods. Properties of ammonothermal GaN (Am-GaN) and HVPE-GaN crystals will be presented. It will be shown that both approaches allow to crystallize material of very high structural quality. In the case of HVPE-GaN growth, the advantages of using native seeds will be demonstrated. The main dopants applied to control the conductivity will be shown for both methods. Wafering procedures, which are necessary to prepare high-quality substrates from crystals, will be briefly presented. Novel ways for improving the ammonothermal process and main factors which are still limiting crystallization of bulk Am-GaN will be presented. It will be shown: 1) how to eliminate unwanted lateral growth during crystallization in vertical directions; 2) how the surface of a native seed should be prepared in order to minimize residual stress in the growing crystal; 3) how to obtain uniform and constant supersaturation in the growth zone with many crystals. The barriers existing for growing truly bulk HVPE-GaN will also be analyzed and compared to those in Am-GaN crystallization.

Resume : Bulk gallium nitride (GaN) is a promising wide band gap semiconductor for the fabrication of vertical power devices. In this work, the electrical behavior of Ni Schottky barrier on GaN epilayers grown on bulk substrates has been investigated. In particular, the forward current-voltage (I-V) characteristics of Ni/GaN vertical Schottky diodes gave average values of the Schottky barrier height of 0.79eV and ideality factor of 1.14. Statistical analyses over a set of diodes, combined with temperature dependence measurements indicated the formation of an inhomogeneous Schottky barrier. A nanoscale electrical analysis, carried out by means of conductive atomic force microscopy (C-AFM), allowed visualizing local inhomogeneities of the current conduction at the surface, which explain the electrical behavior of the barrier. This approach enabled also to determine a density of conductive dislocations in the order of 107cm-2 affecting the electrical performance of the devices. Under reverse bias, a significant dispersion of the leakage current of the diodes was observed. However, the behavior of the best devices could be described by the thermionic field emission (TFE) model. Finally, the impact of the surface roughness on the Schottky barrier inhomogeneity was also discussed. This work was carried out within the ECSEL-JU project GaN4AP (GaN for Advanced Power Applications), under grant agreement no.101007310.

Resume : Vertical GaN based power switching devices, diodes and transistors, are particularly desirable due to their reduced die size, in comparison to lateral heterostructures based devices. This results in a reduction of specific ON state resistance, (Rds_on × A), by one order of magnitude down to 1.0 mΩ∙cm^2. Also, vertical device concept allows for aggressive device scaling in respect to gate periphery length per area, and enables high current densities per unit area. The targeted blocking capability larger than 1 kV demands the growth of drift layers thicker than 10 µm with low residual background doping. However, the drift region conductivity, in particular for thick n- GaN drift layers, may be limited by low mobility, low carrier density, background compensating doping, high defect density and built in potential barriers, having a direct impact on the device electrical performances. Moreover, the trade off between a small active area dictated by the high gate periphery length per area and the resulting small drift region area increased resistance contribution must be addressed. In this work we present the development in n- GaN drift epitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE) on 100 mm diameter c-plane sapphire substrates. For demonstration, ten different wafers with 5.0 µm n- GaN doping concentration ranged between 1 × 10^14 cm^-3 to 9 × 10^16 cm^-3, confirmed by electrochemical capacitance voltage measurements, are grown in three different epitaxial growth iterations. The low carrier density n GaN drift region’s ON state resistance, (Rds_on × A)drift, is directly evaluated by the Areal Vertical Transmission Line Model (AV TLM) like structure. This is a direct method to measure and assess the drift region contribution to the overall device areal specific resistance. This method allows for a fast feedback loop to the epitaxy development of vertical GaN based devices in order to reduce the innovation cycle time and speed up the optimization process and it provides the epitaxial grower an essential and valuable information about one of the most sensitive layers to be grown, the drift region layer. Properties including the relation between the doping and the specific conductivity, growth uniformity across the wafer, and wafer to wafer variations can are extracted. In addition, the AV TLM measurement gives valuable information on the presence of undesired compensation doping and build in potential barriers that hinder the expected device ON state conductivity. As a result of the AV TLM analysis in initial epitaxy growth runs, the origin of the discrepancy between the measured and theoretical values of the drift region (Rds_on × A)drift could be pin-pointed to the drift region only. Thus, improved epitaxial layers could be developed. Further complimentary investigations regarding defect structure and impurity incorporation during epitaxy may give additional insight. The improvement in the (Rds_on × A)drift for drift region with 5 × 10^16 cm^-3 from 1.4 ± 0.6 × 10 ^-3 Ω∙cm^2 to 4.5 ± 0.1 × 10^-4 Ω∙cm^2 and reduced structural defects in the third epitaxy run demonstrate the capability to increase the drift region thickness and reduce the drift region doping concentration while still meeting the desired 1.0 mΩ∙cm^2 specified target for drift region designed for over 1 kV blocking strength.

Resume : Usually GaN layers grown in standard growth conditions possess residual carbon content as high as 1e17 cm-3. The presence of carbon originates from organic precursor chemicals TMGa in Metallorganic Vapor Phase Epitaxy. However, in vertical devices for voltage class of >1 kV the doping level of epitaxial drift layers require net donor concentration in low 1e16 cm-3 or below. Also such intentional donor concentration should be still much higher than carbon acceptor level in order to avoid uncontrolled compensation. It is also well known that if carbon content approaches silicon donor value there is observed abrupt drop of electron mobility. That would lead to increase of device serial resistance in the on state. Therefore we investigated intrinsic residual doping with carbon in MOCVD grown GaN. We tested non-standard growth conditions to promote fast methyl radical elimination with active hydrogen originating from decomposed ammonia and resulted in reduction of carbon doping. These are: high growth pressure, high temperature as well as control of supersaturation by means of high V/III group ratio. It allows to reduce residual carbon doping below 1e16 cm-3. From direct Hall-effect measurement we obtained electron mobility above 1000 cm2/V*s at room temperature for 10 µm thick GaN layer doped with silicon to 1.2e16 cm-3 and grown on semiinsulating ammonothermal bulk GaN substrate. In our works we also used vertical GaN p-n diodes to evaluate quality and capabilities of grown material. For 12.5 µm thick p-n structures on conductive ammonothermal bulk GaN substrates we noticed breakdown voltage of 1950 V, specific on-resistance RON,SP ≤ 0.1 mΩcm2 under high current injection and Baliga Figure of Merit BFOM(VBR2/RON,SP) > 50 GW/cm2. This materials are good candidates for construction of Vertical Trench MOSFET power devices of 1.7 kV class.

Resume : GaN-based power devices have been gaining popularity in recent years thanks to GaN properties such as wide bandgap, high electron mobility and high breakdown field strength, allowing low Ron and high-frequency operation. Lateral GaN devices, which are grown on a foreign substrate such as silicon [1], have already been commercialized and have been able to achieve much better performance compared to their silicon counterparts. However, with these devices it is very challenging to achieve high breakdown voltage (>1kV). The recent availability of native GaN substrates is a promising alternative to boost both breakdown voltage and low Ron, promoting vertical devices. These devices still suffer from premature breakdown and high reverse leakage due to electric field crowding at the junction edge. This issue can be resolved by creating an effective edge termination either by using magnesium implantation [2] to create a p-doped region or by alternate species implantation such as nitrogen [3] or argon [4] to create a resistive region at the junction edge. However, due to the difficulty in creating p-GaN by ion implantation due to implant related defects, which even persist after very high temperature anneals, alternate species of implantation are generally preferred. Fluorine ion implantation [5] is an attractive alternative as it can form a negative fixed charge, owning to its high electronegativity, and thus spread the electric field away from the contact and also modulate the free charge carrier in GaN. In this study, P-N and Schottky structures have been fabricated including edge terminations using multi energy fluorine implantation. The influence of the implant on the electrical characteristics is studied by varying the implant overlap beneath the contact. µ-Raman scanning of the device suggests a reduction in free charge concentration in the implanted region, and C-V measurements show an increase in the built-in potential compared to the device without implantation. The influence of implantation on the electrical characteristics (B-V, I-V, and C-V) is analyzed and TCAD simulations using Synopsys® SentaurusTM are performed to support the interpretation of the results. [1] C. Le Royer, et al., ISPSD 2022 [2] Anderson, T. J., et al., ECS Journal of Solid State Science and Technology 5.6 (2016): Q176. [3] Chen, Chih-Wei, et al., Journal of Electronic Materials 50.9 (2021): 5453-5461. [4] Ozbek, A. Merve, and B. Jayant Baliga, IEEE electron device letters 32.3 (2011): 300-302. [5] Liu, Zirui, et al., AIP Advances 9.5 (2019): 055016.

Resume : High power GaN vertical devices have demonstrated superior switching speed and reduced switching losses in power converting applications compared to conventional Si based and high performance SiC devices. While being promising for high-power applications, truly vertical GaN devices need expensive GaN substrates. Recently, high quality thick n--GaN drift layers has been grown on low cost GaN on Si and GaN on sapphire substrates. Foreign substrates can reduce the overall devices cost by producing semi vertical GaN devices, which can be later converted to truly vertical via back-etching for GaN on Si or laser lifting for GaN on sapphire substrates. However, in order to achieve the high technology standards in blocking voltage and current leakage requested for such devices, many process optimizations need to be fully implemented. One of such process steps is the edge termination needed for same wafer devices separation, this is a key process steps to provide functional single devices to the market. Common edge terminations implement a deep mesa etching combined with passivation of an insulating layer for reliability and device lifetime improvement. This work will report our recent advancement of the edge termination technology. We will present a systematic study of the edge termination to reduce the OFF state leakage current in the GaN on Sapphire quasi vertical Schottky barrier diodes (SBD) in order to increase the device blocking voltage and to reduce the power dissipation. The main current leakage path was identified to be introduced by the sidewall deep mesa etched passivation, allowing the current to flow through the interfacial states between the etched GaN and the SiNx passivation. To suppress the leakage along the sidewalls, an advanced edge termination technology has been developed by combining tetramethylammonium hydroxide (TMAH) wet etching for surface cleaning and refinement, plasma post etching surface treatments, angled ion implantation in order to create an insulating interface layer and multiple dry etching steps. Different passivation layers are tested and characterized. With this advanced edge termination technology, 3.5 orders of magnitude reduced OFF state leakage current has been achieved. A clever edge termination design can also improve the electric field management during high blocking voltage conditions, such termination layouts usually implements some number of field plates configurations, which introduce more process complexity. This work will demonstrate a new narrow mesa edge termination capable of improving the breakdown field of GaN vertical SBDs without introducing any field plates. Simulation results using Silvaco “Atlas” provides further effectiveness demonstration of such edge termination layout. These advancement of our vertical GaN edge termination technology will help to demonstrate the great potential of the GaN on Sapphire device as a new low-cost candidate for high performance power electronics.

Resume : Dislocation analysis on GaN epilayer by high temperature KOH etching for RF technology Ruggero Anzalone1, Jacopo Frascaroli2, Andrea Serafini2, Isabella Mica2, Andrea Severino1 1 STMicroelectronics (STM), Stradale Primosole, 50, 95121 Catania, Italy 2 STMicroelectronics (STM), Via C. Olivetti 2, 20864 Agrate Brianza (MB), Italy, One of the keys for the success in the fabrication of reliable gallium nitride (GaN) -based devices depends on the ability to grow epitaxial films on substrates such as sapphire, silicon or silicon carbide, with a low density of defects. For reducing the cost of the fabrication of GaN based devices, Si substrate is mainly adopted. In fact, the weak match in lattice parameter and thermal expansion coefficient, results in a high density (10E8–10E10 cm2) of threading dislocations within the nitride film. It is assumed that these defects affect the electrical performance of the devices and the optical properties of the material. Thus, devices formed on these materials will have reliability issue. There are three major types of defects identified in GaN material namely screw, threading and edge dislocations. When an etch chemistry preferential for the defects is used, etch pits are formed at the surface in the places where defects are present, giving an estimation of the defect density. Some groups report that mainly threading screw dislocations (TSDs) are the main cause of leakage currents. Others find that also threading mixed type dislocations (TMDs) play a critical role or that neither of both types seem to show an influence on device characteristics, for this reason, investigating the nature of dislocations has become mandatory for GaN-based device development and a deep understanding of dislocation characterization is crucial. Molten potassium hydroxide (KOH) etching, performed at temperature higher than 500°C, is a well-established method for the dislocation counting and evaluation in silicon carbide. Such method was also implemented for Gallium Nitride dislocation study by different groups at low temperature. In this study, the high temperature etching effect of KOH on different GaN/AlGaN samples for threading dislocation (TD) evaluation is reported and deep study of dislocation nature was performed by high resolution TEM. In details, GaN/AlGaN layers epitaxially grown on Silicon floating zone (111)-oriented was studied. To understand the effect of the etching temperature on the dislocations, different temperatures and different etching times have been studied. The temperature was investigated from 420°C to 500°C and the times changed from 60 sec to 180 sec. The short time of etching is due to the rapid etching of silicon substrate on high temperature KOH. To perform the high temperature etching the Silicon back side was protected by Titanium/Nickel thin layers to avoid the very rapid etching of the Si substrate in KOH. SEM post etching was adopted to evaluate the pit density as a function of the temperature and etching time. TEM cross-section images of the samples etched at the highest (500°C) and lowest (420°C) temperature was performed. In both temperatures tested, the TEM images show the presence of dislocations at the bottom of each small or large etch pit. The high-resolution STEM allows the visualization of the crystalline lattice in proximity of the dislocation along the [11-20] crystalline direction and to study the crystalline orientation of the plane involved during the etching.

Resume : Power electronics devices convert and control electrical power, usually higher than 1kW. 1 Si-based power devices have dominated this field, but wide-bandgap semiconductors, such as GaN, are excellent candidates to replace Si in this field. The high critical electric field of GaN enables an improvement in the trade-off between specific Ron and breakdown voltage BV, leading to smaller and cheaper components. Vertical devices are superior to lateral ones at high voltage, as it is possible to increase the BV by increasing the thickness of the drift layer, and hence the footprint of the device remains the same.2 For 1200V BV devices, very thick GaN layers are required (>12µm) meaning that GaN wafers are commonly used. However, silicon wafers are an attractive alternative for GaN vertical structures due to the high cost and limited size of native GaN substrates. However, the strain and doping must be carefully managed. This study develops localised epitaxial growth of GaN-on-Si, by examining growing localised structures on GaN on silicon templates which have three different mask materials (SiO2, SiN, Al2O3). The growth was performed using Metal Organic Vapor Phase Epitaxy (MOVPE) on 200mm diameter Si wafers, growing GaN structures up to 12µm thick without cracks. The focus of this study is to examine the structural and electrical characteristics of the GaN layers by testing different growth patterns, in order to qualify the mask material for the selective growth of low doped and uniform GaN required for high voltage devices. In particular, diffusion of molecules across the mask before incorporation into the films can lead to non-uniform thickness and increased unintentional doping. The different structures have been characterised by Optical and Physical Profilometry and Scanning Electron Microscopy (SEM) to assess their thickness and shape. The localised unintentional doping, in the GaN layer is examined by Scanning Spreading Resistance Microscopy (SSRM) on cross-sections, µRaman Spectroscopy and CV measurements on diodes. The Threading Dislocations Density (TDD), is measured by Cathodoluminescence. In summary, we have validated SSRM as a method for estimating doping when measuring a cross section of structures, with resolution below 5x1016cm-3. We show that compared with SiO2 and SiNx mask, GaN samples with Al2O3 masks have the lowest unintentional doping and thinner structures, and we achieve non-intentional doping values below 5x1016cm-3, which is state of the art for localised growth.3 These structures with Al2O3 mask therefore have great potential for high power devices. 1. Iacopi, F. et al, . MRS Bull. 40, 390–395 (2015). 2. Liu, C. et al,. IEEE Electron Device Lett. 39, 71–74 (2018). 3. Debald, A. et al, Phys. Status Solidi A 217, 1900676 (2020).