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Materials for electronics and optoelectronic applications


AlN and AlGaN materials and devices

Introduction and scope:

Due to their outstanding material properties, AlN and AlGaN based materials are designated to enable deep-UV optoelectronics, and expected to provide the next generation of power and high-frequency electronics “beyond GaN”. In this symposium, preparation and design challenges of AlN and AlGaN materials, structures, and devices are addressed.

AlN has a very wide band gap of 6.1 eV, remarkable thermal conductivity (about 320 W/mK), high piezoelectric coefficients, good thermal stability, and forms a complete solid solution with GaN. AlN and AlGaN. Thus, they are the preferred materials for near-(400-300 nm) and mid-(300-200 nm) UV optoelectronics, high-temperature and high-voltage power electronics as well as for novel high temperature and piezoelectric devices. In particular, mid-UV light emitting diodes for water and air disinfection are a hot topic in current nitride semiconductor research. First devices are commercially available, however still costly and with low output power and wall-plug efficiency. AlN/GaN HEMT structures show record charge carrier concentrations and promising properties for power switching. On the other hand, AlGaN deep-UV lasers, sensors, terahertz devices, and power transistors are all still in the research stage.

In many ways, preparation and design of AlN or high Al content AlGaN based materials and devices is remarkably different and often more challenging than the more matured (In)GaN based technology which boosted the solid state lighting, energy-efficient power switching and MMIC domains. The difference in surface mobility of Al adatoms to that of Ga adatoms requires adapted bulk and epitaxial growth technology, often at elevated growth temperatures to grow AlGaN alloys. Doping efficiency and resulting electrical behavior heavily depends on Al content and self-compensation effects. At the same time, structural defects in the material are most detrimental for device operation, and the choice and preparation of substrate materials is critical in this respect. Devices suffer from low current injection efficiencies, issues with contacts and blocking layers, and insufficient light out- or in-coupling for UV LEDs, lasers, and sensors.

The scope of this symposium covers the whole development sequence of AlN and AlGaN based devices, i.e., from substrate preparation and epitaxy to structuring, contacts, device design, and device performance, explicitly including research on the properties of bulk, layers, and nanostructured materials. The symposium seeks to sum up and foster the research activities of this particular material system, and to move the community towards advancing AlN and AlGaN devices to a commercial stage.

Hot topics to be covered by the symposium:

  • Substrates for AlN and AlGaN based electronic devices
  • Epitaxy and preparation of AlN and AlGaN containing structures
  • Properties of AlN and AlGaN layers and nanostructures (quantum dot, nanowires) for deep-UV and electronic applications
  • Contacts and doped layers for AlN and high Al content AlGaN
  • Progress in deep-UV LEDs, and sensors
  • Deep UV laser, edge or vertical emitters, UV distributed Bragg reflectors and applications
  • Electronic devices (HEMTs, power and high frequency) using AlN or high Al content AlGaN layers
  • Materials for passivation and isolation
  • AlN or AlGaN structures for novel applications (piezoelectric devices, terahertz devices, high temperature electronics, and beyond)

List of invited speakers:

  • Andrew Allerman (Sandia National Labs, USA)
  • Shigefusa Chichibu (Tohoku Univ.)
  • Bruno Daudin (CEA Grenoble, France)
  • Sven Einfeldt (FBH Berlin, Germany)
  • Martin Feneberg (Univ. Magdeburg, Germany)
  • Enrique Iborra (Univ. Madrid)
  • Shin-Ichiro Inoue (NICT, Japan)
  • Doug Irving (NC State Univ., USA)
  • Albena Ivanisevic (NC State Univ., USA)
  • Sergey Ivanov (Ioffe Institute, Russia)
  • Koichi Kakimoto (Kyushu Univ., Japan)
  • Robert Kaplan (Sandia National Labs, USA)
  • Lutz Kirste (Fraunhofer IAF, Freiburg, Germany)
  • Erhard Kohn (Univ. Ulm, Germany)
  • Theodore Moustakas (Boston Univ., USA)
  • Sergei Novikov (Univ. Nottingham, UK)
  • Elizabeth Paisley (Sandia National Labs., USA)
  • Nikos Pelekanos (IESL-FORTH, Univ. of Crete)
  • Oliver Rettig (Univ. Ulm, Germany)
  • Eberhard Richter (FBH Berlin, Germany)
  • Jean-Paul Salvestrini (Univ. Metz)
  • Leo J. Schowalter (Crystal IS, USA)
  • Fabrice Semond  (CRHEA-CNRS, France)
  • Max Shatalov (Sensor ET, USA)
  • Zlatko Sitar (NC State Univ., USA)
  • Pawel Strak (Unipress, Warsaw, Poland)
  • Klaus Thonke (Univ. Ulm, Germany)
  • Chris G. Van de Walle (UCSB, USA)
  • Tim Wernicke (TU Berlin)
  • Thomas Wunderer (PARC, USA
  • Marko Zgonik (Univ. Ljubljana, Slovenia)
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Authors : Koichi Kakimoto, Satoshi Nakano, Yoshihiro Kangawa
Affiliations : Research Institute for Applied Mechanics, Kyushu University

Resume : This paper focuses on the crystal growth of AlN grown by physical vapor transport. Analysis of thermos-physical properties for example thermal conductivity of AlN and AlGaN is also focused. To effectively design a furnace for producing large-size AlN crystals, a fully coupled compressible flow solver was developed to study the sublimation and mass transport processes in AlN crystal growth. Compressible effect, buoyancy effects, flow coupling between aluminum gas and nitrogen gas, and Stefan effect are included. Two sets of experimental data were used to validate the present solver. Simulation result showed that the distributions of Al and N2 partial pressures are opposite along the axial direction due to constant total pressure and Stefan effect, with the Al and nitrogen partial pressures being highest at the source and seed crystals positions, respectively. The distributions of species inside the growth chamber are obviously two-dimensional, which can curve a flat crystal surface. Simulation results also showed that AlN crystal growth rate can be increased by reducing total pressure or by increasing seed temperature or by increasing source-seed temperature difference. High nitrogen pressure causes decrease in growth rate, but it is beneficial for obtaining uniform growth rate in the radial direction. Results of simulation also showed that there is an optimized temperature difference in the present furnace for obtaining good homogeneity of growth rate.

Authors : C. Hartmann, J. Wollweber, A. Dittmar, S. Kollowa, K. Irmscher, M. Bickermann
Affiliations : Leibniz Institute for Crystal Growth (IKZ) Max-Born-Str. 2 12489 Berlin, Germany

Resume : Deep UV transparent bulk AlN crystals with high crystalline perfection are considered as the most promising substrate material for UVC LEDs based on AlGaN layers with high Al content. In order to exploit the potential of AlN substrates the absorption coefficients at the emission wavelength of α < 15 cm^-1 are highly desirable since the light is extracted through the AlN substrate. Bulk AlN crystals are grown by PVT at temperatures > 2000°C. Impurities such as oxygen and carbon as well as compensating intrinsic defects lead to optical transitions within the wide band-gap. The carbon related absorption around 265 nm can be quenched for [O] >> [C], due to the shift of the Fermi level. Furthermore, significant absorption tailing off from the band edge into the UV range of interest is presumably caused by oxygen. We will show that the following conditions must fulfilled in order to achieve α(265nm) < 15 cm^-1: [O] < 3 [C] [C] + [O] < 10^19 cm^-3 These conditions can be reached using getter materials for carbon and oxygen. TaC has proven to be highly efficient by converting Al2O(g) to CO(g) by the following reaction: 2TaC + Al2O(g) + N2(g) → Ta2C + CO(g)↑ + 2AlN Remaining volatile carbon species can be efficiently gettered by adding tungsten which reacts partially to W2C during the growth: 2W + C → W2C Best values of α(265nm) = 14 cm^-1 are achieved at [O] = 6.4x10^18cm^-3 and [C] = 1.8x10^18cm^-3. Entire AlN wafers (Ø ≥ 10 mm) with α(265nm) = 25-28 cm^-1 can be grown in a reproducible manner. The combination of the high deep UV transparency with the high structural quality of the grown AlN crystals grown by our PVT growth technology (rocking curve FWHM = 11-18 arcsec) will provide all requirements necessary for the preparation of highly efficient AlGaN UVC LEDs on AlN substrate wafers.

Authors : Sakari Sintonen, Carsten Hartmann, Jürgen Wollweber, Martin Naumann, Matthias Bickermann, Martin Albrecht
Affiliations : Leibniz Institute for Crystal Growth, Max-Born-Strasse 2, 12489 Berlin, Germany

Resume : Physical vapour transport (PVT) is a promising technique for production of high-quality bulk AlN sub-strates for efficient high Al-content AlGaN UV-C optoelectronic and high-power electronic devices. One of the remaining challenges in PVT-growth of bulk AlN is increasing the crystal diameter to a commercially viable size without loss of structural quality. The diameter expansion during a homoepitaxial PVT growth run is typically only 1-2 mm, and it is therefore imperative that during subsequent growths, the structural quality is at least maintained if not improved. In this study, we analyze the dislocation propagation and generation during homoepitaxial PVT-growth of AlN, using synchrotron radiation x-ray topography (SR-XRT) and laser scattering tomography (LST). We use cross-sectional a-plane slices to examine the dislocation generation and propagation, while c-plane wafers provide a large-scale overview of the spatial distribution of the threading dislocations. We show that significant amounts of threading dislocations (TD) are generated at the initial growth interface, particularly near the edges of the crystal. While the maximum threading dislocation density (TDD) is approximately 1x10^3 cm^-2 in the substrate, it is around 5x10^3 cm^-2 near the edges of the homoepitaxially grown crystal. The increased TDD is observed up to 2 mm inwards from the wafer edge, effectively diminishing the diameter of the perfect core area. The maximum TDD is despite a fivefold increase still sufficiently low for most devices, but it is nevertheless important to understand the phenomenon to avoid shrinkage of the usable crystal diameter. We will discuss possible mechanisms behind the generation of TDs and how growth conditions affect the TDD.

Authors : Robert Bondokov, Jianfeng (Jeff) Chen, Murugesu Yoganathan, Takashi Suzuki, Shailaja P. Rao, Toru Kimura, Keisuke Yamaoka, and Leo J. Schowalter
Affiliations : Crystal IS, Inc., 70 Cohoes Ave., Green Island, USA; and Asahi Kasei Corp., 2-1 Samejima, Fuji-shi, Shizuoka, 416-8501, Japan

Resume : Aluminum nitride (AlN) crystals, grown using a Physical Vapor Transport (PVT) method, are used by Crystal IS, Inc., to make single crystal AlN wafers which are then used in the commercial manufacturing of UVC LEDs. In order to obtain two-inch AlN wafers, the AlN crystals were grown with diameter slightly larger than 2”. The wafers show excellent quality as measured by XRD rocking curves with FWHM < 30 arcsec and low UV absorption that is needed to produce high power UVC LEDs. The AlN substrates are used to fabricate high performance, long lifetime UVC LEDs for air purification, water disinfection, and environmental sensing. High quality pseudomorphic epitaxial layers are grown using an MOCVD process to form the device structure. UVC LEDs for instrumentation have been released with wavelengths between 250 and 280 nm with lifetimes greater than 3,000 hours. Recently released SMD product (Klaran™) achieve germicidal power outputs of 15 mW to 30 mW at a nominal wavelength of 265nm with drive currents up to 400 mA. The substrate purities measured by GDMS and SIMS and growth regimes are linked to the high UV transmission. One correlation found is between carbon concentration and UV absorption in 2-inch AlN substrates. Carbon-to-oxygen elemental ratio is the other factor which is linked to UV absorption. Such correlations were obtained through careful preparation of conditions in crystal growth. Similar correlations were previously reported by Hartmann et al. [1]. [1] C. Hartmann, J. Wollweber S. Sintonen A. Dittmar, L. Kirste, S. Kollowa, K. Irmscher and M. Bickermann, CrystEngComm, 2016, 18, 3488-3497

Authors : Sven Einfeldt1, Jens Rass1, Johannes Glaab1, Jan Ruschel1, Mickael Lapeyrade1, Neysha Lobo Ploch1, Tim Kolbe1, Arne Knauer1, S. Hagedorn1, V. Kueller1, Christoph Stölmacker1, Tim Wernicke2, Frank Mehnke2, Martin Guttmann2, Christian Kuhn2, Johannes Enslin2, Markus Weyers1, Michael Kneissl1,2
Affiliations : 1: Ferdinand-Braun-Institut, Leibniz-Institut fuer Hoechstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany; 2: Technische Universitaet Berlin, Institut für Festkoerperphysik, Hardenbergstr. 36, EW 6-1, 10623 Berlin, Germany

Resume : The performance of current ultraviolet (UV) light emitting diodes (LEDs) in the UVB and UVC spectral range is still low. Therefore, their use for many applications is limited particularly when high optical powers and long life times are required such as for water disinfection and UV curing. In this paper we present our recent progress in (i) enhancing the power level of UV LEDs with emission wavelengths in the range from 260 to 310 nm and (ii) pushing the emission wavelength deeply into the deep UVC range below 240 nm. All LED heterostructures were grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates. We discuss the necessity of achieving a low threading dislocation density in the active region by epitaxial lateral overgrowth of the AlN base layer to obtain a high internal quantum efficiency. Moreover, the design of the heterostructure has been improved particularly with respect to a high injection efficiency and an emission spectrum with low parasitic contributions at longer wavelengths by optimizing the p-side of the diode. LED chips of different designs were fabricated by making use of process optimizations, e.g. with respect to the metal contact resistances, and by flip-chip soldering onto ceramic submounts. LEDs with a very wide range of emission wavelengths were realized, from 340 nm down to 231 nm. Optimized UVB LEDs emitting at 310 nm and UVC LEDs at 265 nm show output powers beyond 1 mW at 20 mA. Lifetime studies reveal pronounced differences between LEDs of different wavelength in the burn-in behavior, the long-term drop in optical power, and the change in the leakage current which will be discussed. For an optimized UVB LED the power drops only to 70 % of the initial value after 3,500 hours of operation at 100 mA. Pushing the wavelength even shorter deep UVC LEDs with dominant quantum well emission below 235 nm and peak powers beyond 10 µW have been fabricated and successfully used for gas sensing.

Authors : Max Shatalov
Affiliations : Sensor Electronic Technology, Inc., 1195 Atlas Rd., Columbia, SC 29209, USA

Resume : Ultrawide-bandgap (UWBG) semiconductors (materials with bandgaps greater than 4eV), including high-Al-content aluminum gallium nitride (AlGaN), are becoming a material of choice for development of new generation deep ultraviolet (UV) light emitters, lasers, high power switches, and optical and electronic devices operating in extreme environment. In particular, the efficiency and manufacturability of AlGaN-based deep ultraviolet light emitting diodes (LEDs) progressed substantially to enable new range of applications not addressable by incumbent technologies. In this presentation we will review development of light emitting diodes operating in UV-C and UV-B spectral ranges and discuss challenges limiting the output power and efficiency of UV LEDs related to substrates, doping, and point and extended defects in AlGaN materials. Importance of alloy composition fluctuations for higher efficiency of LEDs and lasers will be discussed based on results of time resolved temperature dependent photoluminescence and light induced grating measurements of Al0.6Ga0.4N layers with different density of dislocations. We will also present study of efficiency, threshold and polarization of the stimulated emission of the low threshold optically pumped lasers emitting above and below 240 nm. Details of the spectral characteristics of surface and edge emission will be compared to evaluate impact of composition uniformity on laser performance. Importance of the LED structure and chip design will be discussed in terms of further improvements of light extraction and LED output power. Current performance of UV LEDs will be related to limitations in device packaging technology. Present and potential commercial applications of UV light emitters will be reviewed.

Authors : Theodore D. Moustakas
Affiliations : Department of Electrical and Computer Engineering, Photonics Center, Boston University

Resume : In this talk I will discuss progress in deep UV LEDs and lasers based on AlGaN films and their quantum wells, grown by plasma-assisted molecular beam epitaxy. A growth mode leading to deep band structure potential fluctuations [1] and resulting in AlGaN multiple quantum wells with internal quantum efficiency as high as 70% [2, 3], and maximum net modal gain in excess of 100 cm-1 [4- 6] under femtosecond optical pumping is discussed. These type of QWs were employed as the active region of deep UV LEDs and laser structures, grown on sapphire or 6H-SiC substrates by MBE. By employing high IQE AlGaN QWs in the active region, unpackaged deep UV-LEDs, grown on sapphire, were obtained with optical output power of about 2 mW at 100 mA drive current [7, 8]. Similarly, laser structures emitting below 250 nm, grown on 6H-SiC substrates, demonstrated stimulated emission at a transparency threshold of about 1017 cm-3 excited carriers [4-6], which is approximately two orders of magnitude smaller than that required to produce population inversion in QWs made of homogeneous AlGaN alloys [6]. To address the issue of electrical injection of such deep UV emitting laser structures, we fabricated devices in the form of grated-index separate confinement heterostructure (GRINSCH), which due to polarization doping of the compositionally graded regions form automatically a p-n junction [9, 10]. The characterization of such devices will be discussed. [1] T. D. Moustakas and A. Bhattacharyya, Phys. Status Solidi C 9, No. 3–4, 580 (2012) [2] A. Bhattacharyya et al., Appl. Phys. Lett. 94, 181907 (2009) [3] W. Zhang et al., J. of Vac. Sci. and Technol. B 30 (2), 02B119-1 (2012) [4] E. F. Pecora et al., Appl. Phys. Lett. 100, 061111 (2012) [5] E. F. Pecora et al., Appl. Phys. Express 5, 032103 (2012) [6] E. F. Pecora et al., J. Appl. Phys. 113, 013106 (2013) [7] Y. Liao et al., Appl. Phys. Lett. 98, 081110 (2011) [8] T. D. Moustakas et al., Proc. of SPIE Vol. 8278, 82780L-1 (2012) [9] H. Sun et al., J. Vac. Sci. and Technol. B 31(3), 03C117, (2013) [10] E. F. Pecora et al., OPTICAL MATERIALS EXPRESS, Vol. 5, No. 4, 809 (2015)

Authors : T. Wernicke1, C. Kuhn1, M. Martens1, C. Reich1, K. Bellmann1, F. Mehnke1, J. Enslin1, M. Guttmann1, A. Knauer2, C. Hartmann3, S. Hagedorn2, T. Markurt3, T. Schulz3, A. Mogilatenko2, U. Zeimer2, G. Kusch4, M. Feneberg5, X.T. Trinh6, P.R. Edwards4, J. Wollweber3, M. Lapeyrade2, J. Rass2, S. Einfeldt2, L. Sulmoni1, R.W. Martin4, M. Albrecht3, N.T. Son6, R. Goldhahn5, M. Bickermann3, M. Weyers2, and M. Kneissl12
Affiliations : 1 Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstr. 36, 10623 Berlin, Germany; 2 Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Straße 4, 12489 Berlin, Germany; 3 Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany; 4 University of Strathclyde, Department of Physics, SUPA, Glasgow G4 0NG, United Kingdom; 5 Otto-von-Guericke-Universität, Institut für Experimentelle Physik, Universitätsplatz 2, 39106 Magdeburg, Germany; 6 Linköping University, Department of Physics, Chemistry and Biology, SE-58183, Linköping, Sweden

Resume : AlxGa1-xN is the material of choice for optoelectronic devices emitting in the ultra violet (UV) spectral range. However, compared to the growth of GaN and InxGa1-xN, new challenges arise in the AlGaN material system. In this contribution, we will present a detailed study on the growth of AlN and AlxGa1-xN by metal organic vapor phase epitaxy on sapphire and on bulk AlN substrates. Depending on the substrate choice, creating or maintaining a low threading dislocation density is crucial, as it influences not only the radiative recombination efficiency but also the strain relaxation of AlxGa1-xN layers and surface kinetics, e.g., during GaN quantum dot growth. Regardless of the substrate, we found distinct growth modes and linked them to the growth temperature, precursor partial pressures, and substrate miscut which directly affect device properties such as the threshold power density of optically pumped UV lasers. The surface growth mode can even be influenced by use of heterostructures, such as suppression of hillock formation by introduction of a superlattice. As effective doping is essential for all UV-emitters, we will show results on n- and p-type doping in a wide AlxGa1-xN composition range and discuss limits induced by self-compensation or DX center formation. Mastering these challenges is inherently necessary for the realization of efficient UV-light emitting diodes, UV-lasers and single photon emitters as we will address in the presentation.

Authors : Jan Ruschel, Johannes Glaab, Moritz Brendel, Neysha Lobo Ploch, Jens Rass, Tim Kolbe, Mickael Lapeyrade, Christoph Stölmacker, Tim Wernicke, Frank Mehnke, Christian Kuhn, Johannes Enslin, Sven Einfeldt, Markus Weyers, Michael Kneissl
Affiliations : Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany; Technische Unversität Berlin, Instiut für Festkörperphysik, Hardenbergstr. 36, EW 6-1, 10623 Berlin

Resume : The lifetime of state-of-the-art UV-B LEDs emitting in the spectral range between 280 - 320 nm is low ranging from less than hundred to a few thousand hours. To improve the lifetime the most important degradation mechanisms need to be clarified. In this paper, we have studied the degradation of (InAlGa)N-based UV-B LEDs with focus on the separation of effects from the active region and the metal-semiconductor contacts. LEDs with peak-wavelengths of 290 nm and 310 nm have been stressed under various operation conditions, such as current and temperature, and analyzed by means of capacitance-voltage (CV) profiling and optical power-current-voltage (PIV) measurements. Within the first up to 250 hours of operation a fast reduction of the optical power can be observed in all LEDs, which is accompanied by distinct changes in the CV profiles and the PIV characteristics. For instance, the reduction of the optical power in the 310 nm LEDs ranges from about -20 % to -30 % at heat sink temperatures of 15 °C and 75 °C, respectively. In contrast, the capacitance change at 0 V shows an inverse temperatures trend, with values of +14 pF at 15 °C and -5.8 pF at 75 °C. To figure out the origin of these changes, test structures have been stressed allowing studying the metal-to-semiconductor p-contacts characteristics separately from the active region. These experiments indicate that the degradation of the p-contact dominates the initial phase of fast degradation of the UV-B LEDs.

Authors : C. Himwas (1), M. Choueib (2), M. den Hertog (1), S. Purcell (3), G. Tocu (2), Le Si Dang (1), E. Monroy (4)
Affiliations : (1) CNRS-Institut Néel, 38000 Grenoble, France (2) Bluescop S.A.S., 75007 Paris, France (3) Université Lyon 1 Claude Bernard, 69622 Villeurbanne, France (4) CEA-Grenoble, INAC-PHELIQS, 38000 Grenoble, France

Resume : The replacement of mercury lamps by solid-state ultraviolet (UV) emitters in the mid-UV spectral range (300-200 nm) is a technological priority that has impelled research on AlGaN/AlN heterostructures. An alternative approach to circumvent the difficulties encountered in carrier injection and transport by AlGaN based-LEDs is an electron-beam pumped UV (EPUV) source where electrons are injected into the semiconductor dice using a miniaturized electron gun. The use of AlGaN/AlN quantum dot superlattices (QDSL) as active media grants high luminescence intensity at room-temperature in the 235-300 nm range thanks to the three-dimensional (3D) carrier confinement. We report the optimization of the MBE-grown AlGaN/AlN QDSL by finding a compromise between the designs providing maximum internal quantum efficiency (IQE = 60%) and maximum QD density (9E11 cm-2). The effect of the variation of the QD height/base-diameter ratio on the optical properties was explored by fitting the experimental data with 3D simulations of the band diagram and quantum confined states. EPUV prototype devices emitting at 260 nm were implemented using a carbon nanotube cathode. The device was operated at 5 kV to minimize x-ray emission. The emission intensity scaling linearly with the injected current confirms efficient heat and charge evacuation. A wall-plug efficiency of 0.3% was deduced.

Authors : Shin-ichiro Inoue
Affiliations : Advanced ICT Research Institute, National Institute of Information and Communications Technology (NICT)

Resume : Deep-ultraviolet (DUV) aluminum gallium nitride (AlGaN)-based light-emitting diodes (LEDs) have attracted considerable interest because of their potential for application to diverse fields, including air/water purification, surface disinfection, bio-agent detection, lithographic microfabrication, and medicine. In this work, we report on the design and experimental demonstration of AlGaN-based DUV-LEDs on transparent AlN substrates with high-light-extraction nanophotonic structures. We show major enhancement of the light extraction efficiency using an AlN hybrid structure of photonic crystals and subwavelength nanostructures and demonstrate the high output power performance with over 90 mW of the DUV-LEDs in CW operation at the peak emission wavelength of 265 nm.

Authors : Albena Ivanisevic
Affiliations : Materials Science and Engineering North Carolina State University, Raleigh, NC

Resume : One of the challenges associated with making implantable devices with long term functionality can be solved if one can find an interface material that not only matched the mechanical and electrical properties requirements but can also be tailored to reduce or eliminate foreign body response. There is a need for an interface that will simultaneously probe chemical and electrical mechanisms associated with neuronal function. We have started to use gallium nitride (GaN) to fabricate such a biointerface. Limited evidence suggests that GaN is an ideal material for this purpose. Advantages of GaN include a wide bandgap of 3.4 eV, availability of Ga bonds for covalent surface modifications, chemical stability, and low electrical drift of GaN biosensors in ionic solutions. Recently we conducted studies to examine the biocompatibility and toxicity of this material after chemical functionalization which can significantly increase its utility. Subsequently we used InGaN with different topographical and chemical gradients to fabricate biomolecular gradients with different adhesion characteristics. We have tested the utility of the interface in physiologically relevant solution conditions. The morphological, mechanical and chemical characteristics of the interface will be discussed based on recent analysis with microscopy and spectroscopy techniques. Cell studies data confirms that the tailored surfaces are suitable as biointerfaces to probe neuronal function and behavior. The last portion of the talk will focus on recent work centered on GaN surfaces with different polarities and the ability to enhance the semiconductor properties using in situ chemical functionalization during etching.

Authors : M. Zgonik, T.Troha, M. Rigler, A. Majkic, D. Alden, I. Bryan, W. Guo, R. Kirste, S. Mita, M. D. Gerhold, R. Collazo, Z. Sitar
Affiliations : Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia; J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia; Department of Materials Science and Engineering, North Carolina State University, 1001 Capability Drive Raleigh, North Carolina 27606, USA; Adroit Materials, 2054 Kildaire Farm Rd., Suite 205, Cary, North Carolina 27518, USA; Engineering Science Directorate, Army Research Office, P.O.BOX 12211, 27703 Research Triangle Park, NC, USA

Resume : The wide bandgap semiconductors GaN, AlN, and their ternary alloys are promising materials for efficient light generation in the deep UV spectral range. As an alternative to direct photoemission, UV light generation can be obtained by nonlinear frequency conversion. AlGaN thin films grown on sapphire substrate form optical waveguides due to the lower refractive index of the sapphire substrate compared to AlGaN. The advantage of using waveguides in nonlinear optics is the pump light confinement over longer distances. Therefore high and constant pump intensity is maintained over the whole interaction length. This is important because the second harmonic (SH) power increases quadratically with the pump power. For achieving high SH efficiencies phase matching between pump and SH wave must be established. Due to material and optical mode dispersion it is necessary to use modal dispersion phase matching (MDPM) technique or quasi phase matching (QPM). Modal phase matching is less efficient but it is relativelly simpler to achieve. It uses the fact that different waveguide modes propagate with different propagation constants. However, it is achievable that different pump and SH mode have the same propagation constant and are therefore phase matched. The efficiency of the process is determined by the overlapping between the interacting modes. The second - most promising techique - is quasi phase matching (QPM) that uses periodic modulation of optical nonlinear coefficient. It is the most efficient method because the fundamental modes at pump and SH frequency interact and have the highest overlapping. QPM is achieved in periodically modulated waveguides - the so-called laterally polar structures - with the period eqal to SH coherence length.

Authors : R. Jayaprakash 1, 2, F. G. Kalaitzakis 2, E. Amargianitakis 3, G. Christmann 2, K. Tsagaraki 2, M. Hocevar 4, 5, B. Gayral 4, 5, E. Monroy 4, 5, N. T. Pelekanos1, 2
Affiliations : 1Department of Materials Science and Technology, University of Crete, 71003 Heraklion, Greece. 2Microelectronics Research Group, IESL-FORTH 71110 Heraklion, Greece. 3Department of Physics, University of Crete, 71003 Heraklion, Greece 4Université Grenoble-Alps, 38000 Grenoble, France 5CEA, INAC-SP2M, 17 av. des Martyrs, 38000 Grenoble, France

Resume : The prospect of low-threshold, inversion-less polariton lasers, making use of the macroscopic occupation at the bottom of the lower polariton branch, has intensified polariton research in the last two decades. Here, we report on ultra-low threshold polariton lasing at room temperature, using an all-dielectric GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated by a photo-electrochemical etching step of an InGaN sacrificial layer, which allows to isolate ultra-smooth membranes containing optimal quality GaN/AlGaN QW’s, to be subsequently embedded in-between high quality dielectric DBR’s. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density, which are the lowest ever reported for a two-dimensional GaN based system. Above threshold, angle-resolved emission spectra reveal a spectacular condensation pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high optical quality of the all-dielectric microcavity accounts for the enhanced characteristics of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonics.

Authors : M. Tollabi Mazraehno, P. Stauss, A. Gomez-Iglesias, L. Höppel, A. Pfeuffer, S. Englisch, M. Hirmer, G. Brüderl, M. Binder, T. Pertsch
Affiliations : Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Max-Wien-Platz 1, 07743 Jena, Germany; OSRAM Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg, Germany

Resume : A hybrid numerical simulation is presented to analyze photoresponsivity, carrier recombination, and parasitic absorption in AlGaN-based UV photodetectors. The proposed computational method combines a transport solver and a full-wave electromagnetic solver to rigorously model UV detection in the AlGaN material system by considering realistic material parameters. Using the proposed hybrid simulation method the effects of photodetectors’ parameters, such as the epitaxial layer design including AlGaN composition, layer thicknesses, doping levels, and geometry on responsivity and efficiency are rigorously investigated for different photodetector designs. Moreover, results of a parametric study on various factors such as carrier lifetime of the material, and applied bias voltage are presented. The detectors are then fabricated based on AlGaN layers grown by MOVPE on c-plane sapphire substrates and analyzed by PL, XRD, AFM, and TEM. The wafers are processed into single chips using standard chip processing. The responsivity of the photodetectors is measured at photovoltaic mode. It is shown that, the experimental results are in excellent agreement with the simulations. Therefore, the method can be used for accurate design refinement at pre-fabrication stage. Furthermore, temperature dependent photo- and electroluminescence measurements are carried out and the dark current dependency of the photodetectors on temperature is investigated.

Authors : Dmytro Kysylychyn1, Giulia Capuzzo1, Tian Li2, Thibaut Devillers1, Alberta Bonanni1
Affiliations : 1Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria; 2Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02 668 Warszawa, Poland.

Resume : Heterostructures based on AlGaN/GaN represent the building blocks of state-of-the-art high-power and of optoelectronic devices working in the visible and ultra-violet range. Extending the operational range of these latter systems, is expected to open wide perspectives for the next generation of devices. We have recently demonstrated room temperature near infrared (NIR) emission originating from Mn-Mgk complexes [1,2] in (Al,Ga,Mn)N:Mg. In the perspective of realizing a viable NIR laser structure based on III-nitrides without the need of employing indium, we report here on the design, fabrication ? by metalorganic vapor phase epitaxy ? and characterization of Al(Mn)GaN/GaN distributed Bragg reflectors (DBRs) optimized for the NIR range. We abridge our work on the modeling, on the epitaxial growth of the heterostructures exploiting the surfactant effect on Mn in (Al)GaN [3] and on the demonstration of enhanced NIR emission from the DBR structures combined with an (Al,Ga,Mn)N:Mg active layer [4]. The work was supported by the European Research Council (ERC, Project 227690) and by the Austrian Science Foundation (FWF, projects #22477, #24471, #26830). [1] T. Devillers et al., Scientific Reports 2, 722 (2012). [2] T. Devillers et al., Appl. Phys. Lett. 103, 211909 (2013). [3] T. Devillers et al., Crystal Growth & Design 15, 587592 (2015). [4] D. Kysylychyn, G. Capuzzo, Tian Li, T. Devillers, and A. Bonanni, unpublished data.

Authors : G. Lukin, T. Schneider, M. Barchuk, F. Zimmermann, E. Niederschlag, O. Pätzold, M. Stelter
Affiliations : G. Lukin Institut für NE-Metallurgie und Reinststoffe, TU Bergakademie Freiberg, Leipziger Straße 32, 09599 Freiberg; T. Schneider Institut für NE-Metallurgie und Reinststoffe, TU Bergakademie Freiberg, Leipziger Straße 32, 09599 Freiberg; E. Niederschlag Institut für NE-Metallurgie und Reinststoffe, TU Bergakademie Freiberg, Leipziger Straße 32, 09599 Freiberg; O. Pätzold Institut für NE-Metallurgie und Reinststoffe, TU Bergakademie Freiberg, Leipziger Straße 32, 09599 Freiberg; M. Stelter Institut für NE-Metallurgie und Reinststoffe, TU Bergakademie Freiberg, Leipziger Straße 32, 09599 Freiberg; M. Barchuk Institut für Werkstoffwissenschaft, TU Bergakademie Freiberg, Gustav-Zeuner-Straße 5, 09599 Freiberg; F. Zimmermann Institut für Angewandte Physik, TU Bergakademie Freiberg, Leipziger Straße 23, 09599 Freiberg

Resume : The high temperature vapor phase epitaxy (HTVPE), which bases on the use of ammonia and thermally evaporated, elemental gallium as precursors, is a Cl-free technique for the growth of GaN films with high growth rates. Conventional HTVPE, however, suffers from significant drawbacks, such as limited possibilities of process control and a strong thermal coupling between the Ga source and the substrate. This presentation deals with a modified HTVPE setup, which was developed to overcome the drawbacks mentioned above. The key component is a novel, inductively heated evaporation cell for gallium, which was designed for an optimal local temperature field and gas flow. In modified HTVPE, the evaporation cell is placed in a relatively large distance from the substrate and the Ga melt is separated from the ammonia flow, so that a flexible control of the deposition process becomes possible. The GaN film growth by modified HTVPE is investigated and the influence of the crucible and susceptor material on the growth process is analyzed. Further experiments were performed at a reduced reactor pressure to study the potential of the method for a deposition with a high growth rate. A low-temperature (LT) nucleation procedure known from MOVPE was applied in this paper for the heteroepitaxial growth on sapphire. Nucleation layers were overgrown under systematic variations of the carrier gas composition, the substrate temperature, and the ammonia flow.

Authors : Rafael J. Jiménez Riobóo1, Ramon Cuscó2, Carlos Prieto1, Sergei V. Novikov3, and Luis Artús2
Affiliations : 1 Instituto de Ciencia de Materiales de Madrid (CSIC), Sor Juana Inés de la Cruz 3, E-28049 Madrid, Spain; 2 Institut Jaume Almera (ICTJA), Consell Superior d’Investigations Científiques (CSIC), Lluís Solé i Sabarís s.n., E-08028 Barcelona, Spain; 3 School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, U.K.

Resume : The determination of the elastic constants and Surface Acoustic Wave (SAW) propagation velocities in nitride thin films has been not only the subject of fundamental research but also a matter of interest for the technological application of these materials. From a practical point of view, it is obvious that having available a non-destructive experimental technique to gather this kind of information is of great interest. In this context High Resolution Brillouin Spectroscopy (HRBS) emerges as a key tool to investigate the elastic properties of these materials. In this work, the principles of HRBS and the application of the technique to the study of bulk and surface elastic properties will be reviewed, and a survey of the bulk and surface elastic information obtained in GaN, AlN, AlGaN, InN, InGaN thin films will be given. In particular, the recent experimental determination by HRBS of the elastic constants of cubic GaN film samples will be presented.

Authors : M. Fialho1, S. Magalhães1, M. P. Chauvat2, P. Ruterana2, K. Lorenz1, E. Alves1
Affiliations : 1 IPFN, Campus Tecnológico e Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066 Bobadela LRS, Portugal 2 CIMAP, UMR 6252, CNRS-ENSICAEN-CEA-UCBN, 6, Bd Marechal Juin, 14050 Caen, France

Resume : Doping of III-Nitride semiconductors with rare earth ions has been explored to face the demand of the semiconductor industry for more efficient optoelectronic devices. The use of ion implantation technique for doping allows the control of dopant profile but generates defects that may alter the structural and physical properties of the implanted material conditioning the role of the dopants. In this context, it is important to study the conditions to minimize the implantation damage during and/or after implantation. In the present work, Al0.17Ga0.83N films grown on (0001) sapphire substrates by Metal Organic Chemical Vapor Phase Deposition were implanted with Tb ions at 150 keV and a fluence of 5x1014 The implantation was carried out with the beam aligned with the c-axis and the implantation temperature varying from RT to 550ºC. Transmission Electron Microcopy results of selected samples showed a large density of basal stacking faults after implantation at 100 ºC while for implantation at 550 ºC the formation of these extended defects is completely suppressed. However, the profile of defect clusters with displacements along the c-axis extend to deeper regions for samples implanted at 550 ºC due to the increase of defect diffusion at high temperature. The defect profiles extracted from Rutherford backscattering spectrometry/channeling measurements and the strain evolution with depth derived from symmetric (0002) 2θ-ω X-ray diffraction scans will be compared.

Authors : Alaa eldin GIBA1,2,3, Philippe PIGEAT1, Stéphanie BRUYERE1, Hervé RINNERT1, Flavio SOLDERA2, Frank MÜCKLICH2, Raul GAGO-FERNADEZ4, David HORWAT1
Affiliations : 1Institut Jean Lamour – UMR CNRS 7198– Université de Lorraine, Nancy, France 2Department Materials Science and Engineering, Saarland University, D-66123 Saarbrücken, Germany 3National Institute of Laser Enhanced Science, Cairo University, 12613 Giza, Egypt 4Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InvestigacionesCientíficas, E-28049 Madrid, Spain

Resume : In this work, Cerium-doped aluminum nitride (Ce-AlN) thin films were prepared at room temperature using radio frequency (RF) reactive sputtering. X-ray diffraction and high resolution transmission electron microscopy (HRTEM) revealed a well crystalline textured microstructure with single (002)out-of-plane orientation. Strong blue emission from the prepared samples was detected when excited by 325 nm laser. Electron energy loss spectroscopy (EELS) has been used to reveal the dominant oxidation state of Ce atoms, which undergoes a change from of Ce4+ to Ce3+ ions after annealing.The chemical composition was analyzed by simulation of Rutherford backscattering spectrometry (RBS) and compared to HRTEM images.A clear correlation between microstructure, compositionand sample photoluminescence (PL) was established.It was found that surface oxidation during the post-deposition annealing plays an important role in the PL response of the samples. We believe that the strong blue emission in this new (oxy)-nitride materials has great potentials for solid state lighting applications due to its thermal and chemical stability as well as the luminescence efficiency. Moreover, the comprehensive approach conducted within this study could serve as a guideline for better understanding and design of the luminescence behavior in rare earth-doped (oxy)-nitride thin films.

Authors : Yuta Kainuma, Akiyoshi Ootake, Kimihiro Yamanaka, Hirohisa Taguchi
Affiliations : Department of Electrical and Electronic Engineering, Faculty of Engineering, Chukyo University;Department of Electrical and Electronic Engineering, Faculty of Engineering, Chukyo University;Department of Electrical and Electronic Engineering, Faculty of Engineering, Chukyo University;Department of Electrical and Electronic Engineering, Faculty of Engineering, Chukyo University

Resume : The semiconductor GaN is a well-known material for use in power devices. Devices based on a GaN material system are referred to as GaN high-electron-mobility transistors (HEMTs). Reports on the high-frequency characteristics and high-temperature resistance of GaN-HEMTs have indicated cut-off frequencies exceeding 190 GHz and stable operation to greater than 300 °C; however, there have been few reports on the high-frequency characteristics at cryogenic temperatures. We have studied the characteristics of InAlAs/InGaAs HEMTs at liquid nitrogen temperatures and reported the effects of high and low temperature on different circuit components. In this paper, we report on AlGaN/GaN HEMTs placed in liquid nitrogen, measuring their DC and RF characteristics. The AlGaN/GaN HEMTs used in the experiment are discrete devices suitable for high-voltage applications at low current. These are also suitable for broadband applications, typically in the 3.5-GHz band. The DC characteristics showed an increase of the drain current at low temperature, reflecting a 210.6% increase in transconductance (Gm). We also measured the S-parameters using a network analyzer to calculate the high-frequency gain. The current gain cut-off frequency (Ft) at room temperature was calculated as 6.5 GHz, increasing by 35.4% to 8.8 GHz when the device was immersed in liquid nitrogen. To clarify the mechanism of the Gm increase, we compared the high-frequency characteristics at room and liquid nitrogen temperature. We confirmed a considerable reduction of the source–gate capacitance in the HEMTs at low temperature, and the possibility that the small signal circuit model of the FET could be partially changed at low temperature.

Authors : D.N.Lobanov, A.V.Novikov, P.А.Yunin, E.V.Skorohodov, M.V.Shaleev, M.N. Drozdov, O.I. Khrykin, O.A.Buzanov*, V.V.Alenkov*
Affiliations : Institute for Physics of Microstructures RAS *JSC «Fomos-Materials»

Resume : Epitaxial A3N structures on piezoelectric substrate are very promising candidate for application in surface acoustic wave devices. Monocrystalline langasite substrates La3Ga5SiO14 (0001) is one of attractive substrates for realization of such devices due to their hexagonal symmetry and low thermal expansion coefficient difference with GaN (~7.5%). One of the main problem of GaN epitaxy on LGS substrate is the instability of LGS surface under typical MOCVD growth condition. In this report the results of technology development of epitaxial growth of GaN on the LGS by MBE PA method are submitted. The study of growth temperature impact at the initial stage of deposition on the crystal quality and morphology of obtained GaN layer has been performed. X-ray measurement has revealed that GaN layer is epitaxial and rotated by 19° around c axis with respect to LGS. Recent results [Cryst Eng Comm, DOI: 10.1039/c5ce00075k (2015)] has shown that such mutual orientation leads to low in-plane lattice mismatch value of 3.2% between GaN and LGS when compared to lattice mismatch between GaN and sapphire (14%) and even GaN and 6H-SiC (3.5%). It has been demonstrated that optimal temperature for initial deposition of GaN layer on LGS substrate to obtain the best crystal quality is about ~ 520 С. As a result of the research the epitaxial GaN/LGS layer with dislocation density ~ 10^11 cm^(-2) and low surface roughness (< 2 nm) has been obtained.

Authors : Sylvia Hagedorn1, Hassan Gargouri2, Viola Küller1, Arne Knauer1, Markus Weyers1
Affiliations : 1 Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, D 12489 Berlin, Germany; 2 SENTECH Instruments GmbH, Schwarzschildstraße 2, D-12489 Berlin, Germany

Resume : Due to the lack of bulk AlN substrates with suitable diameter of at least 2” and in sufficient quantity for production of AlGaN based optoelectronic devices, considerable efforts have been made to grow high quality AlN on foreign substrates. A widely used approach is to grow AlN by metal-organic vapor phase epitaxy (MOVPE) on sapphire. Especially the heteroepitaxial growth start highly affects the crystal quality of the deposited AlN layer. Hence, reaching proper nucleation conditions is necessary. Since the nucleation process is known to be sensitive to the pre-treatment of sapphire in the MOVPE machine (e.g. unintentional pre-treatment by interaction of temperature and parasitic AlN deposits) a homogeneous stable starting layer on the sapphire surface would be a great advantage. Therefore, we investigate the applicability of ALD deposited AlN and Al2O3, using SENTECH ALD System SI ALD LL, to aim for more stable MOVPE nucleation conditions. X-ray diffraction measurements prove the transition of amorphous ALD material into crystalline AlN due to heating the samples in the MOVPE reactor. For pure ALD-AlN layers different AlN crystal orientations can be identified after this treatment, while a mixture of ALD-AlN/-Al2O3 avoids the formation of misoriented crystallites. After the temper treatment 720 nm thick MOVPE AlN is grown to investigate the influence of the ALD nucleation layer on the crystal quality. In-situ metrology is used to monitor the growth process. Structural and optical properties of AlN are determined by atomic force microscopy, x-ray and electron diffraction.

Authors : R. Mohamad1, A. Béré2, H. Ben Ammar1, P. Gamarra3, S. Lacam3, A. Minj1, J. Chen1, and P. Ruterana1
Affiliations : 1 CIMAP, UMR 6252, CNRS-ENSICAEN-CEA-UCBN, 6, Bd Maréchal Juin, 14050 Caen, France 2 Laboratoire de Physique et de Chimie de l’Environnement, Université de Ouagadougou, 03 BP: 7021 Ouagadougou 03, Burkina Faso 3 III-V Lab, 1 Avenue Augustin Fresnel, Campus Polytechnique, 91767 Palaiseau, France

Resume : III-nitride alloys (AlGaN, InAlN, InGaN) are the building blocks for devices such as high power high mobility transistors and light emitters, they are used in the form of quantum wells or thick layers. In particular, recent reports show that the growth of thick layers is difficult and may lead to defective structures. The proposed explanations have relied mainly on the growth modes and the strain relaxation, but not especially on the metallurgical constraints of alloy formation. In this work, we have carried out atomistic calculations of phase diagrams for group-III nitride ternary compounds within the framework of Stillinger-Weber potentials. It appears that, in the bulk form, the critical temperatures (Tc) for these alloys are 2717K for InxAl1-xN, 1718K for InxGa1-xN and 177K for AlxGa1-xN, respectively, which means that as thick layers, the stability of InAlN and InGaN alloys is probably limited. We point out that the Tc may be decreased when the layers are grown under strain, indeed compression and extension have the same effect on the spinodal curve above 1.5% strain but between 1.5% and 2% there is a change in behavior and if compression is 2% Tc increases but if one applies a stress of 2% (extension) Tc decreases. However it appears that Tc increases in case of compressive strain (>=2%), as may be the case of the AlInN alloy. The theoretical analysis will be discussed along with our experimental results on MOVPE InAlN layers by transmission electron microscopy.

Authors : Young Min Jhon, Yong Tae Kim, Young Hwan Kim, and Jung Hee Lee
Affiliations : Sensor System Research Center, Korea Institute of Science and Technology, Seoul, Korea; Semiconductor Materials & Devices Lab., Korea Institute of Science and Technology Seoul, Korea; School of Electronic Engineering, Kyungpook National University Daegu, Korea

Resume : AlGaN/GaN fin-shaped field-effect transistors (FinFETs) with nanowire structures have demonstrated as a promising candidate for a high electron mobility transistor (HEMT), high frequency, and high power applications. To resolve the short channel effect of nanowire fin structure while varying the temperature from 100 to 350K, the I–V curves and the transconductances of AlGaN/GaN-FinFETs have been investigated with different W/H (nanowire fin width/fin height) ratios. When the W/H ratio is large, the current through the 2-DEG channel is dominant. Decreasing the ratio, the contribution of the 2-DEG channel on device current decreases, but contribution of the sidewall MOS channels relatively increases. This characteristics can offer advantages for high temperature applications in electric vehicles. The current of the wide FinFET is controlled by the 2-DEG channel and the mobility has negative, temperature coefficient and the electron density in the GaN MOS channel layer increase with temperature. In contrast, the narrow FinFET has the significant current contribution of the sidewall MOS channels. As a result, the total electron mobility has slightly positive temperature coefficient. Therefore, it is possible to achieve AlGaN/GaN Fin-FETs featuring temperature-insensitive current by controlling the W/H ratio and the gate biasing.

Authors : A. Dittmar, C. Hartmann, D. Klimm, J. Wollweber, M. Bickermann
Affiliations : Leibniz Institute for Crystal Growth (IKZ), Max-Born-Str. 2, 12489 Berlin, Germany

Resume : The PVT growth of bulk Al1-xScxN as lattice matched substrate for AlGaN-UV LEDs A. Dittmar, C. Hartmann, D. Klimm, J. Wollweber, M. Bickermann AlGaN based UV LEDs in the C and B range still suffer from relatively low external quantum efficiency (EQE) and emission power. The main factor contributing to this value is the lack of lattice-matched substrates that results in dislocation densities > 108cm-2 in the epitaxially grown AlGaN layer. Lattice-matched Al1-xScxN substrates with Sc content higher than 2 at.% could close this gap providing low-defect pseudomorphic strained AlGaN layers even with Al contents less than 60%. Physical vapour deposition (PVT) method is the most favorite one for AlN growth. The authors describe the further development of the PVT method for the preparation of Al1-xScxN mixed crystals in this paper. Own basic studies revealed a nitriding of Sc in presence of nitrogen above 1000°C to form pure ScN. Calculations of thermodynamic equilibrium of partial pressures of Sc and Al close to growth conditions provide high differences of PP. Thus, first attempts were carried out to grow bulk Al1-xScxN by PVT using different temperature levels of ScN and AlN source powders inside the setup. As a result, Al1-xScxN single crystals containing 0,4 to 0,7 at % Sc in the axial direction, an uniform Sc distribution within c-plane, rocking curves FWHM of about 21 arcsec and shifted lattice parameters corresponding to Sc content were found. The results demonstrate the basic feasibility to grow Al1-xScxN single crystals by PVT method. However, changes of the setup are necessary to increase the degree of alloying, for example by introducing different temperature zones for AlN and ScN evaporation within the crucible.

Authors : M. Rzin1*, J-M. Routoure1, B. Guillet1, L. Méchin1 R. Kabouche2, E. Dogmus2, M. Zegaoui2, F. Medjdoub2 C. Lacam3, P. Gamarra3 H. Ben-Ammar4, M-P Chauvat4, M. Morales4, P. Ruterana4
Affiliations : 1 GREYC, UMR 6072, CNRS-ENSICAEN-Université de Caen Normandie, 6 bd Maréchal Juin, 14050 Caen Cedex, France 2 IEMN, UMR8520, Av. Poincaré, 59650 Villeneuve d’Ascq, France 3 III-V lab, Thales Research and Technology, 1 Avenue Augustin Fresnel, 91767 Palaiseau, France 4 CIMAP, 6 bd Maréchal Juin, 14050 Caen Cedex, France

Resume : Dependence of 2-DEG channel resistance (RDS) on gate-drain spacing (LGD) for InAlGaN/GaN metal-insulator-semiconductor-high-electron-mobility transistors using in-situ SiN cap layer as gate insulator is studied by dc and low frequency noise measurements. Different LGD of InAlGaN/GaN MIS devices with sub-10-nm barrier layer are studied. RDS increases by increasing LGD which causes a degradation of the maximal drain current and the transconductance. This is assumed to be caused by the electrons that leak from the gate and fill surface traps present in the ex-situ SiN/in-situ SiN interface in the gate-drain access region. The drain current spectral noise density SID has been measured at VGS = 0V for ID from 1mA to 10mA to identify the dominant noise sources in the channel with different LGD. SID varies as 1/fγ type spectra with γ close to 1. The normalized drain current noise (SID/ID2) decreases by increasing LGD by a decade between LGD of 2µm and 30µm. The γ exponent is equal to 1.16 for LGD = 2µm and decreases with LGD to achieve 1.07 for LGD = 30µm. A model of the dependence of SID on LGD is proposed. Two channel noise contributions have been identified depending on LGD: the noise of the gated region of the channel and the noise of the gate-drain access region which is non-negligible for large LGD. In order to use LGD > 10µm to enhance the breakdown voltage of InAlGaN/GaN MIS devices with sub-10-nm barrier layer, an engineering of the SiN passivation layer is needed.

Authors : Kazutada Ikenaga1, Akira Mishima1, Yoshiki Yano1, Toshiya Tabuchi1, Koh Matsumoto1, and Hideto Miyake2
Affiliations : 1 Taiyo Nippon Sanso Corporation Tsukuba-city, Ibaraki 300-2611, Japan; 2 Graduate School of Regional Innovation Studies, Mie University, Tsu-city, Mie 514-8507, Japan;

Resume : To improve the performance of deep ultraviolet LEDs (DUVLEDs), high-quality AlN and AlGaN are required for the underlying layers. Recently, an improvement of the crystal quality of the AlN buffer layer through high-temperature annealing under N2-CO gas ambient was reported [1]. Here, we investigated the characteristics of AlN and Al0.6Ga0.4N layers regrown on buffer layers of AlN/sapphire that were annealed in pure N2 gas. A 300-nm-thick AlN layer was grown by low-pressure metalorganic chemical vapor deposition (SR4000HT, Taiyo Nippon Sanso) and annealed at 1700 °C for 1 h in pure N2 gas using an annealing furnace (STA1700, Taiyo Nippon Sanso). We grew a 3.0-μm-thick AlN layer at 1340 °C on this annealed template, as well as a 2.3-μm-thick Al0.6Ga0.4N layer at 1150 °C. The growth rates for the AlN and Al0.6Ga0.4N were 4.0 μm/h and 3.6 μm/h, respectively. By annealing at 1700 °C, the X-ray rocking curve-full width at half-maximum (XRC-FWHM) of the (0002) plane was decreased from 70 to 38 arcsec, and that of the (10-12) plane was drastically decreased from 1278 to 442 arcsec. Over the regrown AlN on the AlN template, the XRC-FWHMs of the (0002) and (10-12) planes were 133 and 366 arcsec, respectively. The obtained surface morphology was smooth, with a root mean square roughness value of 0.31 nm. No cracks were generated by the regrowth of the 3.0-μm-thick AlN layer, while many cracks were observed in the AlN directly grown on sapphire. Cracks were also not observed in the regrown Al0.6Ga0.4N. These results indicate that the AlN template fabricated by N2 annealing at high temperature can improve the quality and productivity of DUVLEDs. [1] H. Miyake, et al., Appl. Phys. Express 9, 025501 (2016)

Authors : O.F. Kolomys1, V.V. Strelchuk1, B.I. Tsykaniuk1, A.V. Naumov1, V.P. Kladko1, A.E. Belyaev1, Yu.I. Mazur2, E.A. DeCuir Jr.2, M.E. Ware2, G.J. Salamo2
Affiliations : 1.V.E. Lashkaryov Institute of Semiconductor Physics, National Academy of Science of Ukraine, Kyiv, 03028, Ukraine 2. Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA

Resume : Superlattices (SLs) made of GaN and Al(Ga)N have great potential as active elements in many optoelectronic devices, which cover the spectral regions from ultraviolet to infrared. The optical and device properties of such heterostructures heavily dependent on the layer thickness, strain, chemical composition, doping, substrate type and others. Changing these settings allows to control performance parameters device structure in a wide range. In the present work SL structures grown by MBE on GaN/Al2O3 template and containing 5, 10 and 20 periods of GaN/AlN(3 nm) and different well/barrier thickness ratios, were studied by means of scanning confocal photoluminescence (PL), IR reflectivity and Raman Spectroscopy. Strong PL peak in the range 3.0-3.8 eV with a full width at half maximum of about 100 meV was observed. Computational simulation provided a quantitative analysis of electronic transition in the AlN/GaN SL and suggested the observed red-shifted PL emission as a result of quantum confinement Stark and strain effects in these AlN/GaN MQW structures. Most of the observed features in the Raman and IR reflectivity spectra have been identified and assigned to optical phonons of the buffer and superlattice layers. From the frequency shift of phonons of GaN and AlN, the strains in the GaN/AlN superlattice layers were determined. The results of micro-Raman and FTIR measurements show that the GaN well in SL is under compressive stress, while the AlN barrier is under tensile stress.

Authors : R. Zwierz 1, D. Siche 1, K. Kachel 1,+, N. Jankowski 2, Ch. Nenstiel 2, G. Callsen 2, M. Bickermann 1, A. Hoffmann 2
Affiliations : 1 Leibniz Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany; 2 Institute of Solid State Physics, TU Berlin, Hardenbergstraße 36, 10623 Berlin, Germany; + Currently employed by ASM Belgium N.V., Kapeldreef 75, B-3001 Leuven, Belgium

Resume : Thick GaN:C layers were grown by Pseudo-Halide Vapour Phase Epitaxy (PHVPE) [1] to study their suitability as semi-insulating substrates. The carbon doping from the HCN precursor gas was indirectly controlled in a wide range up to 5x10e19 cm–3 with in-situ waste gas analysis by Fourier transform infrared (FTIR) spectroscopy [2]. All runs were conducted at 800 mbar reactor pressure, with 100 sccm N2 transport gas flow from the source to the seed region and with the same rate of N2 gas flow in the HCN synthesis oven. Typical growth temperatures were set to 1010-1050 °C and 1100 °C for seed and Ga source, respectively. The layer thickness d (5 - 150 µm) was controlled by adjusting growth time tg and / or methane flow through the HCN oven; whereas the average growth rate Rav was up to 15 µm/h. The grown layers were characterized by Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectroscopy (SIMS), Photoluminescence (PL), Micro-Raman-Spectroscopy, Hall effect measurements, and High Resolution X-Ray Diffractometry (HRXRD). Like the impurity incorporation (O, Si, Mg and Zn), carbon distribution proved to be inhomogeneous with its increased incorporation on facets of so-called V-pits. Higher impurity incorporation on the facets resulted in Burstein-Moss-shift of near band gap emission, recorded close to and within the V-pits. PL mapping revealed higher intensity of all V-pits in comparison to the undisturbed layer surface. The increase of the average free electron concentration with layer thickness is justified, as the surface of {11-22} V-pits facets to (0001) plane ratio increases with tg. The E2high intensity of Raman spectra decreases with growing carbon concentration and in comparison to unstrained material a shift of ~ 2 cm-1 to higher wave numbers is observed, corresponding to a biaxial compressive pressure of ~ 0.64 GPa. The large FWHM (~ 800 arcsec) of 0002 reflexes of XRD rocking curves is due to the mosaicity from columnar growth and high density of screw and mixed dislocations from the seed-layer lattice misfit. [1] K. Jacobs, D. Siche, D. Klimm, H.-J. Rost, and D. Gogova, Pseudohalide vapour growth of thick GaN layers, J. Cryst. Growth, 312 (2010) 750–755 [2] K. Kachel, D. Siche, S. Golka, P. Sennikov, M. Bickermann FTIR exhaust gas analysis of GaN pseudo-halide vapor phase growth, Materials Chemistry and Physics (2016),

Authors : M. A. Signore1, A. Taurino1, C. De Pascali1, I. Farella1, F. Quaranta1, L. Francioso1, M. Catalano1, A. Campa1, M. Masieri2, M. C. Martucci1, P. Siciliano1
Affiliations : 1CNR, Institute for Microelectronics and Microsystems, Via Monteroni, 73100 Lecce – Italy 2CNR, Institute of archeological heritage-monuments and sites, Via Monteroni, 73100 Lecce – Italy

Resume : Piezoelectric materials have held a key position in the field of materials science due to the increasing demand for their application in various piezo-MEMS devices, such as energy harvesting, resonators, sensors and biosensors. A promising piezoelectric material is aluminum nitride (AlN) thanks to its interesting electronic, electroacoustic, piezoelectric and biocompatibility properties, as well as to its compatibility with the IC-fabrication technology. The aim of this work is the investigation of the growth conditions to obtain AlN thin films with a good piezoelectric response. AlN thin films were deposited on titanium (Ti) underlayer by RF magnetron sputtering in (N2+Ar) atmosphere at room temperature by tuning different deposition parameters. The films structure, microstructure, morphology and piezoelectric response were characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy and capacitance measurements, respectively. A structural switch from (101) to (002) orientation was observed, according to the tuned growth process parameters; consequently, the surface features change from triangular to rounded shape and the piezoelectric response is enhanced. The possibility to use the optimized material for a piezoelectric microcantilever has been evaluated. Preliminary Finite Element simulations were performed to study the frequency response of a bimorph micro-cantilever consisting in a Ti/AlN/Al capacitor supported by a stress-compensated silicon nitride/silicon oxide multilayer cantilever. The effect of different geometrical parameters of the microcantilever (beam length, AlN thickness, capacitor to beam length ratio) on the resonant response was considered.

Authors : A. Wierzbicka1, A. Kaminska1,2, K. Sobczak1, J. Borysiuk1, D. Jankowski1, K. Koronski1, M. Sobanska1, K. Klosek1, Z. R. Zytkiewicz1
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 01-142 Warsaw, Poland; 2 Cardinal Stefan Wyszynski University, College of Science, Department of Mathematics and Natural Sciences, Dewajtis 5, 01-815 Warsaw, Poland

Resume : Aim of this work was to study influence of the thickness of superlattice period on strain distribution in multi quantum wells (MQWs) GaN/AlN structures by X-ray diffraction (XRD), photoluminescence (PL) and transmission electron microscopy (TEM) techniques. Structural properties of GaN/AlN MQWs were studied by XRD. The 2θ/ω scans of 0002 symmetrical reflection and reciprocal space maps of the 1 ̅1 ̅24 asymmetrical reflections were measured. It allowed calculation of the accurate values of lattice parameters of MQWs and of the substrate. High-resolution XRD maps showed that for the narrowest GaN/AlN MQWs a-lattice parameter was similar to that of Si-doped AlN layer under the MQW, so it was almost fully strained. Next the 2θ/ω scans were simulated using Panalytical X’Pert Epitaxy software utilizing dynamical diffraction theory. The fit of these curves allowed to determine thicknesses of the GaN wells and of AlN barriers as well as the quality of the interfaces. Since the blurring of the interfaces causes deviations between experimental and calculated data TEM measurements were used to check the quality of the interfaces in MQWs. Finally, PL spectra showed that due to Quantum-Confined Stark Effect the PL peak energies of the MQWs decreased with increasing of the width of the GaN quantum wells and AlN quantum barriers. Acknowledgements: The authors acknowledge support of the Polish National Science Centre by grant No. DEC-2012/05/B/ST3/03113.

Authors : T. Hanada 1 , T. Iwabuchi 1 , S. Kuboya 1 , H. Tajiri 2, K. Inaba 3, T. Tanikawa 1 , T. Fukuda 4 , and T. Matsuoka 1
Affiliations : 1 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 2 Japan Synchrotron Radiation Research Institute (JASRI), Hyogo 679-5198, Japan 3 X-ray Research Laboratory, Rigaku Corporation, Akishima 196-8666, Japan 4 Fukuda Crystal Laboratory, Sendai 989-3204, Japan

Resume : ScAlMgO4 (SCAM) (0001) is a promising material for the GaN epitaxial growth, which can improve the lattice-mismatch to GaN by one order magnitude compared with conventional sapphire. SCAM is also stable in H2 atmosphere of MOVPE as predicted from the Gibbs-energy change before and after the reaction with H2. SCAM can be easily cleaved along (0001) plane and this is very convenient for fabricating wafers. In this paper, the structure of the topmost layer of a cleaved SCAM grown by the Czochralski method is investigated because the surface condition is most important for growing high quality GaN films. The unit cell of SCAM consists of a ScO2 layer and two Al0.5Mg0.5O (AMO) layers triply stacking along [0001] axis. It was analyzed by the X-ray crystal-truncation-rod scattering method that SCAM is cleaved between two AMO layers. A 2-µm-thick GaN film with a mirror-like surface was successfully grown on the cleaved substrate by MOVPE. The crystal polarity of the GaN layer was identified as Ga-polarity by anomalous-scattering X-ray diffraction. This is consistent with the KOH-etching result as shown in our previous report. This suggests that the atomic configuration of the first GaN layer follows that of a removed upper-half AMO layer. The threading dislocation density (TDD) of a GaN film on a cleaved SCAM substrate has been currently comparable with that grown on sapphire. The further reduction of TDD can be expected under the optimum growth condition of the GaN epitaxial growth.

Authors : Gert Irmer, Christian Röder, Cameliu Himcinschi, Jens Kortus
Affiliations : TU Bergakademie Freiberg, Institute of Theoretical Physics, Leipziger Str. 23, D-09599 Freiberg, Germany

Resume : GaN has attracted considerable interest as material for electronic and optoelectronic devices with high-power and short-wavelength requirements. The properties of wz-GaN as nonlinear optical material are also promising. However, applications based on nonlinear waveguides, frequency doubling, frequency mixing or phase conjugation require for design and optimization of devices an accurate knowledge of the linear and nonlinear optical response. Johnston and Kaminow [1] demonstrated for GaAs that Raman scattering can be used to determine the nonlinear optical coefficients below the bandgap. In case of wz-GaN there have been several experimental and theoretical studies with significant differences concerning the coefficients of the second-harmonic generation (SHG) and the linear electro-optic effect (LEO). In this study Raman scattering experiments were performed in order to determine Raman scattering efficiencies of the polar TO- and LO-phonon modes [2] as they are correlated to the nonlinear optical coefficients in piezoelectric crystals. Due to the wurtzite structure of GaN, symmetry requires three SHG and LEO coefficients to be independently considered. Subsequently, all six coefficients of wz-GaN were deduced for the first time. This work is financially supported by the European Union (European Social Fund) and by the Saxonian Government (grant no. 100231954). [1] W.D. Johnston, Jr. and I.P. Kaminow: Phys. Rev. 188 (1969) 1209 [2] G. Irmer et al.: J. Appl. Phys. 116 (2014) 245702

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Authors : Pawel Strak, Pawel Kempisty, Konrad Sakowski, and Stanislaw Krukowski
Affiliations : Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland

Resume : Density-functional calculations concerning the structure and stability of wurtzite polar Al-terminated AlN(0001) surface are presented. Aluminum and nitrogen adatoms entail states in the bandgap, as well as Al dangling bonds. The Al-Al bonds related to Al adatoms are located in the lower part of the bandgap and are completely filled. The energy of Al broken bond-related band extends for about 2 eV. In the case of N adatom, four states emerge in the bandgap, two are Al-N bonding states, the other two are Al dangling bonds states located high in the bandgap. A quantum overlap repulsion between the conduction band states and aluminum dangling bond states increases the energies of the band states at the surface. The band bending at the surface indicates on an acceptor behavior for p-type and semi-insulating bulk and donor behavior for p-type of the Al broken bond state. Adsorption energy was determained for p- and n-type doping. Adsorption of N adatoms in its lowest energy sites i.e. H3 positions, transfers electrons from the Al broken bond states to the topmost N adatom states. For low nitrogen coverage, i.e. below 0.25 monolayer, the Fermi level is pinned by Al broken bond states located below the conduction band minimum. Adsorption of nitrogen depends on the Fermi level due to the involved charge transfer. For low N coverage the adsorption energy is very high and molecular nitrogen dissociates at the surface. Above 0.25 monolayer coverage the nitrogen adsorption energy gain is reduced as the Al broken bond is empty and energy gain from electron transfer is not possible. For such a coverage the adsorption of N2 is not dissociative and molecules are attached vertically on top of Al atoms. An equilibrium pressure of molecular nitrogen above an Al terminated AlN(0001) surface was determined. It was found that the pressure depends sharply on the Fermi level position, thus on the nitrogen coverage. Nitrogen adsorption mechanism leads to 2x2 reconstruction and the diffusion of N-adatoms is limited by relatively high barriers between lowest energy sites of order of 2 eV. The lowest Al adatom energy site is located in T4 position, and the diffusion barrier is no higher than 0.6 eV so more than three times lower than barrier for nitrogen diffusion. Additional chanel for N diffusion occurs at high Al coverage below the Al adatoms surface.

Authors : S. V. Novikov, A. J. Kent, C. T. Foxon
Affiliations : School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK

Resume : The majority of UV LEDs require AlGaN layers with compositions in the mid-range between AlN and GaN. There is a significant difference in the lattice parameters of GaN and AlN. Therefore AlGaN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlGaN bulk crystals by any standard bulk growth techniques has not been developed so far. There are very strong electric polarization fields inside the wurtzite group III-nitride structures. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende (Al)GaN bulk crystals by any standard bulk growth techniques were not successful. Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used the plasma-assisted molecular beam epitaxy (PA-MBE) technique for bulk crystal growth and have produced, for the first time, free-standing layers of zinc-blende and wurtzite AlGaN up to 100?m in thickness and up to 3-inch in diameter. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

Authors : Sergey V. Ivanov
Affiliations : Ioffe Institute, Polytekhnicheskaya 26, St. Petersburg 194021, Russia

Resume : Despite the great progress made in AlGaN mid-UV emitters (λ<300nm) during several recent years, their output power and emission efficiency are still much poorer in comparison with those of InGaN-based photonics. The AlGaN structures suffer from low carrier localization efficiency, low injection efficiency due to the difficulty in p-type doping of high-Al-content AlGaN layers, and suppressed light extraction efficiency owing to the TM-polarized emission involvement and the UV light absorption in p-GaN injection layers. The structures grown on sapphire also possess rather high defect density which reduces the internal quantum efficiency. The paper considers possibilities provided by plasma-assisted molecular beam epitaxy (PA MBE) for fabrication of high quality AlGaN-based quantum well heterostructures on c-sapphire, demonstrating enhanced internal quantum efficiency (>50%), relatively low lasing threshold under optical pumping (<150kW/cm2) [1], and high light output at electron-beam pumping (160 mW) [2]. In particular, different advanced approaches to the AlN nucleation stage on c-sapphire, the AlN and AlGaN bulk layer PA MBE growth under metal-rich conditions, and the buffer structure design will be discussed, which enable one to lower density of the screw and edge threading dislocations down to ~10^8 and ~10^9 cm-2, respectively, and obtain atomically flat droplet-free surface (rms ~0.4nm). Main impact will be made on the effect of fractional monolayer design of the active region comprising quasi-2D Ga-rich AlGaN quantum wells coherently grown in the Al-rich AlGaN matrix by a sub-monolayer digital allying technique, which provide strong carrier localization both in vertical and lateral directions and are also beneficial for strain-induced dominant TE-polarized emission beyond 280 nm. [1] S.V. Ivanov et al., Semicond. Sci. Technol. 29, 084008 (2014). [2] X. Rong, S. Ivanov et al., Adv. Mater. (2016), to be published.

Authors : F. Semond
Affiliations : CRHEA-CNRS, rue Bernard Grégory, Sophia Antipolis, 06560 Valbonne, France

Resume : AlN is largely used as a buffer layer for III-nitride growth on silicon substrates. Actually, AlN layers on silicon are much more than simple buffer layers. AlN layers on silicon, which are typically 50-200 nanometer thick, have a strong impact on the material quality (defect density) of heterostructures. Also, the strain in epilayers grown on silicon is largely influenced by the quality of AlN buffer layers. For these reasons, epitaxy of AlN on silicon has been largely studied and issues related to growth and quality of AlN have been identified. In this paper, epitaxial growth of AlN on silicon, including the complex nucleation process, will be discussed. Then the influence of the quality of AlN layers on properties of 2DEG AlGaN/GaN heterostructures will be presented. Also, the demonstration of microdisk lasing in the deep UV using very thin GaN/AlN quantum well heterostructures will be discussed. Also, it will be shown that the patterning of silicon substrates provide a solution to grow crack-free and high-reflectivity AlN/AlGaN DBRs for various UV applications. Finally, thanks to the very good control of the epitaxy of AlN on silicon, it is demonstrated that AlN templates on Si offer an interesting platform for various applications.

Authors : Caroline B. Lim, Akhil Ajay, Mark Beeler, Catherine Bougerol, Edith Bellet-Amalric, Jörg Schörmann, Eva Monroy
Affiliations : University Grenoble-Alpes, France and CEA-Grenoble, INAC-PHELIQS, France; University Grenoble-Alpes, France, CEA-Grenoble, INAC-PHELIQS, France and CNRS, Institut Néel, France; University Grenoble-Alpes, France and CEA-Grenoble, INAC-PHELIQS, France; University Grenoble-Alpes, France and CNRS, Institut Néel, France; University Grenoble-Alpes, France and CEA-Grenoble, INAC-PHELIQS, France; I. Physikalisches Institut, Justus-Liebig-Universität Gießen, Germany; University Grenoble-Alpes, France and CEA-Grenoble, INAC-PHELIQS, France

Resume : GaN/AlGaN nanostructures are building blocks for UV GaN-based optoelectronics. The use of nonpolar crystallographic orientations allows enhancing the oscillator strength of the optical transitions, but at the price of increasing the lattice mismatch. In this work, we assess the effect of Al incorporation on nonpolar m-plane GaN/AlGaN multi-quantum-wells (MQWs) grown by plasma-assisted MBE on free-standing m-plane GaN. Three series of samples (A, B and C) are considered, with Al mole fractions in the ranges of 6-8%, 26-44% and 100%, respectively. Series A shows atomically flat surface morphology with no extended defects (TEM) or any indication of strain relaxation (XRD). In spite of the absence of structural defects, HAADF-STEM shows alloy fluctuations up to 30% of the average concentration. Al compositions in the range 26-44% lead to an anisotropic degradation of the surface morphology, with appearance of elongated features that increase the surface roughness. SEM images show cracks that developed during the cooling process as a result of the temperature-dependent GaN/AlN lattice mismatch, with increasing density when increasing the alloy composition. Furthermore, TEM characterizations show the onset of the formation of stacking faults and dislocations, and the appearance of nm-sized Al-rich clusters. The effect of all these structural features on the MQWs optical performance is discussed.

Authors : Simon Fleischmann, Eberhadt Richter, Anna Mogilatenko, Deepak Prasai, Markus Weyers, Günther Tränkle
Affiliations : Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany

Resume : Hydride vapor phase epitaxy (HVPE) is widely used to fabricate GaN substrates because of its high growth rate and high crystal quality. To extend this approach to substrates for UVB LEDs a horizontal HVPE reactor was fitted with an additional aluminum source. Sapphire is used as substrate material because of its UVB transparency, but has a large lattice mismatch to AlGaN in the whole composition range. To cope with interfacial stress, epitaxial lateral overgrowth on isotropically patterned sapphire substrates (PSS) is employed. AlGaN growth on honeycomb-shaped PSS led to non-c-planar oriented domains originating from the sidewalls of the pattern. C-plane AlGaN growth on n-plane sapphire micro facets leads to crystallites protruding from the honeycomb openings and disturbing the coalescence process. To largely avoid these sapphire n-plane micro facets, triangular shaped mesa- and hole-like patterned sapphire was fabricated. Coalesced Al0.8Ga0.2N layers with a total thickness of 10 μm were achieved on such triangular PSS with FWHM of XRD rocking curves lower than 1000” in the symmetric 002-reflection and 1500” in the skew-symmetric 302-reflection. This opens the path to growth of thick AlGaN layers with good crystalline properties.

Authors : E. Richter, S. Fleischmann, S. Hagedorn, D. Prasai, W. John, M. Weyers, and G. Tränkle
Affiliations : Ferdinand-Braun-Institut, Leibniz-Institut fuer Hoechstfrequenztechnik, 12489 Berlin, Gustav-Kirchhoff-Str. 4, Germany

Resume : Hydride vapor-phase epitaxy (HVPE) plays an important role in group III-nitrides because it enables the production of thick films with either high purity or intentional doping. Thus it represents an independent or supplementary method for obtaining buffer layers or substrates. Compared to HVPE of GaN the high reactivity of the aluminum-containing reactants in the Al(Ga)N system results in special demands on reactor design (avoiding quartz) and process parameters. The state of development in the HVPE growth of AlN-containing layers will be discussed highlighting the special challenges and potential solutions. Own studies are presented that include simulation, design and optimization of reactor, patterned substrates and growth processes. A special situation is obtained for AlN, where AlN substrates made by sublimation (PVT) arise as a carrier for subsequent pseudomorphic growth of layers of high Al-content. By contrast, AlxGa1-xN layers in the medium region of composition cannot be grown without lattice relaxation. An overview of the state-of-art Al(Ga)N HVPE, achievable layer thicknesses and material quality will be given and discussed.

Authors : P. Reddy, F. Kaess, R. Kirste, J. Tweedie, R.Collazo, Z. Sitar*
Affiliations : North Carolina State University, Material Science and Engineering Department, Raleigh 27695, NC, USA

Resume : Achieving control of electrical conductivity in n- and p-AlGaN over all doping levels of interest has proven to be challenging. AlGaN exhibits a decrease in free carrier concentration as the dopant concentration increases above a critical doping concentration. This behavior is attributed to the formation of compensating defects due to lowering of their formation energy during doping. These compensators also contribute to impurity scattering, limiting the carrier mobility. A novel scheme to control point defects in wide bandgap semiconductors is presented. The scheme uses above bandgap UV-illumination to change the position of the quasi Fermi levels in a semiconductor, and, thus, increase the formation energy of compensating defects leading to a decrease in incorporation. Using AlGaN as a model system, we demonstrated that over an order of magnitude improvement in the electrical properties could be achieved in these materials by using this scheme for controlling the incorporation of point defects. In n- and p-doped AlGaN films grown by MOCVD, point defects such as hydrogen, carbon, nitrogen or metal vacancies and their corresponding complexes lead to dopant compensation, resulting in a high resistivity and a low mobility in these films. Generally, the energy of formation of a point defect is a function of the process conditions, as described by the chemical potentials of the species involved. In addition, the energy of formation of charged point defects is also a function of the Fermi energy, or the electrochemical potential. We have developed a non-equilibrium process scheme in which the Fermi level is controlled by external excitation in a steady-state condition. We introduce above-bandgap illumination as the excitation source for enhancing the doping capabilities, by controlling the energetics of point defects via Fermi level control during growth. This presentation will focus on the details of the theoretical background and related calculations. In addition, the discussion will also focus on the used experimental setup and influence of UV-light on the growth condition and possible gas-phase interaction. The proposed point defect control scheme is of great interest for all semiconductors in which compensation is the main limiting factor in obtaining the desired free carrier concentrations and conductivity. Samples were characterized using SIMS, photoluminescence, Hall measurements, XRD and TEM. The passivation and self-compensation of Mg acceptors by H and VN was significantly reduced by above-bandgap illumination. The H concentration was reduced by an order of magnitude and no post growth annealing was needed to activate the samples when grown with UV-light. In Si doped (n-type) AlGaN, UV-illumination during the growth, reduced the Si donor compensation by acceptor defects (e.g. C or VGa/Al). Here, UV-growth led to an increase in mobility and free carrier concentrations with a maximum increase of an order of magnitude in AlGaN films grown on sapphire for doping within the self-compensation regime. A significant reduction in PL intensity related to compensating defects was also observed in these n-type films.

Authors : Chris G. Van de Walle
Affiliations : Materials Department, University of California, Santa Barbara, California, USA

Resume : Nitride semiconductors are the key materials for solid-state lighting and power electronics. I will discuss two issues of crucial importance for these applications: point defects and polarization fields. Point defects may act as compensating centers, charge traps, or recombination centers; in bulk crystals, defects may also reduce transparency [1]. Point defects can also be used beneficially, for instance in quantum information science as analogs of the NV center in diamond [2]. I will discuss the theoretical advances that are enabling us to calculate the energetics as well as electronic and optical properties of point defects with unprecedented accuracy [3]. Accurate knowledge of polarization constants is also critical for analysis and device design. We have identified shortcomings in the current implementation of polarization within simulation tools, and present a consistent approach for modeling spontaneous and piezoelectric polarization [4]. Work performed in collaboration with C. E. Dreyer, A. Janotti, J. Lyons, J. Varley, D. Vanderbilt, and Q. Yan, and supported by DOE, LEAST, and NSF. [1] Q. Yan, A. Janotti, M. Scheffler, and C. G. Van de Walle, Appl. Phys. Lett. 105, 111104 (2014). [2] J. B. Varley, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 93, 161201 (2016). [3] C. Freysoldt, B. Grabowski, T. Hickel, J. Neugebauer, G. Kresse, A. Janotti, and C. G. Van de Walle, Rev. Mod. Phys. 86, 253 (2014). [4] C. E. Dreyer, A. Janotti, and C. G. Van de Walle, Phys. Rev. X (in press).

Authors : Douglas L. Irving, Joshua S. Harris, Dorian Alden, Jonathon N. Baker, Benjamin  E.  Gaddy, Zachary  Bryan, Isaac  Bryan, Kelsey J. Mirrielees, Brian D. Behrhorst, Ronny  Kirste, Pramod Reddy, James Tweedie, Toru  Kinoshita, Yoshinao  Kumagai, Akinori Koukitu, Axel Hoffmann, Ramón Collazo, Zlatko  Sitar
Affiliations : Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA; Adroit Materials, Inc., USA; Department of Materials Science and Engineering, North Carolina State University, USA; Adroit Materials, Inc., USA; Tokuyama Corporation, Japan; Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Japan; Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Japan; Institute of Solid State Physics, Technical University Berlin, Germany; Department of Materials Science and Engineering, North Carolina State University, USA; Department of Materials Science and Engineering, North Carolina State University, USA;

Resume : Semiconductors and insulators obtain new functionality through the incorporation of dilute concentrations of impurities. Determining the properties that arise from each defect is a challenge, especially for wide band gap materials. As a result, there has been slow progress in identifying new material combinations (bulk material dilute dopants) that enable novel functionality. To overcome this obstacle, we have integrated results of first principles simulation together with synthesis and characterization to identify and eliminate unwanted defects while at the same time promoting the properties from desirable dopants. In this talk, results of density functional theory (DFT) calculations that utilize hybrid exchange correlation functionals will be presented. These methods are used to determine properties of native and impurity defects as well as defect complexes in both AlN and high Al content AlGaN. The results from DFT are compared to photoluminescence and optical absorption measurements of AlN samples grown by physical vapor transport (PVT), metal-organic chemical vapor deposition (MOCVD), and hydride vapor-phase epitaxy (HVPE) and high Al content AlGaN grown by MOCVD. In addition, routes to mitigate the impact of undesirable compensating defects during growth will be discussed.

Authors : Klaus Thonke a); Matthias Lamprecht a); Ramon Collazo b); Zlatko Sitar b)
Affiliations : a) Institute of Quantum Matter / Semiconductor Physics Group, University of Ulm, Albert-Einstein-Allee 45, 89069 Ulm, Germany; b) Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27606, USA

Resume : Bulk AlN crystals typically contain high concentrations of oxygen, silicon, and carbon – as also still state-of-the art epitaxial layers do, depending on the specific growth conditions. In optical spectroscopy, such crystals show broad bands in the region from 2 – 5 eV in absorption and emission. We investigated in detail the emission bands centered at 2.0 eV and 2.4 eV, which under below-bandgap excitation with a HeCd laser (325 nm) dominate the photoluminescence (PL) spectra both at low and at room temperature. Using time-resolved PL on different time scales ranging from µs to msec at variable temperature and PL excitation spectroscopy, we find clear indications that these transitions occur between different states of the shallow donors Si or O, and a deep acceptor. The donors undergo large lattice relaxation and form DX centers, and similarly the acceptors are subject to Franck-Condon relaxation. The acceptor involved in these PL bands is likely linked to carbon. Depending on temperature, the initial state is either a long-lived (S=1) DX--center state of the donors, a shallow EMT-like conventional donor state, or a free electron – what changes drastically the relaxation time for the optical transitions under below-bandgap excitation. We determined the actual characteristic electron capture times for the DX- states. Based on our data, we develop configuration coordinate diagrams, which also give hints to the source of adjacent optical transitions in the range from 1.8 eV to 2.4 eV.

Authors : Shigefusa F. Chichibu1, Kazunobu Kojima1, Akira Uedono2, and Yoshitaka Sato3
Affiliations : IMRAM-Tohoku Univ.1, Univ. of Tsukuba2, Futaba Corp.3

Resume : In the active layer of blue LEDs, InGaN QWs are exclusively used, since InGaN alloys of low InN mole fractions exhibit bright emissions with internal quantum efficiency higher than 80% in spite of high threading dislocation density (10^8 to 10^9 cm-2). This property is markedly different from the conventional (Al,In,Ga)(As,P) semiconductors or AlGaN alloys. For realizing compact, low power-consumption light sources for high CRI LED-lighting and for sterilization and disinfection purposes, near- to deep-ultraviolet LEDs are indispensable, and are under development using AlGaN. Whereas, as the growth of high-quality crystal is difficult, AlInN alloys have rarely been considered as light-emitting media, although their bandgap energies cover from UV-C to near-infrared wavelengths. In this presentation, we demonstrate planar vacuum-fluorescent-display devices emitting polarized UV-C, blue, and green lights using m-plane AlInN bulk epilayers. The materials are revealed by positron annihilation and time-resolved photoluminescence measurements to contain a large number of nonradiative recombination centers consisting of Al vacancies higher than 1019 cm-3. Such defective materials may not emit any light owing to one-sided nonradiative recombination of excited carriers. However, the films emit the light with the photoluminescence lifetime of 22-32 ps at 300 K. Such defect-resistant radiative performance suggests supernormal localized characteristics of electron-hole pairs in AlInN.

Authors : Martin Feneberg
Affiliations : Otto-von-Guericke-Universität Magdeburg, Institut für Experimentelle Physik, Universitätsplatz 2, 39106 Magdeburg, Germany

Resume : The anisotropy of the optical response of III-nitrides is a direct consequence of their wurtzite crystal structure leading to lifted degeneracy of the valence band. Selection rules and oscillator strengths of inter band transitions involving different valence bands can be described by the material parameters crystal field splitting and spin-orbit interaction energy. Interestingly, the crystal field splitting undergoes a change of sign from GaN (positive) to AlN (negative) when alloying AlGaN. This change of sign can be observed as polarization switching of the low energy absorption onset. Such observations are possible on non- or semipolar AlGaN films allowing to assess ordinary and extraordinary dielectric functions simultaneously. However, such films are usually influenced by epitaxial strain due to the lack of lattice matched substrates for ternary AlGaN. This strain introduces further symmetry breaking to the valence band structure altering the optical answer of the system considerably. This talk will present the current status of ongoing experimental studies on non- and semipolar AlGaN. Experimentally, spectroscopic ellipsometry extending from the far infrared to the vacuum ultraviolet allows access to the elements of the dielectric tensor while perturbation theoretical calculations are used to understand the influence of strain and composition.

Authors : A. Kaminska (1,2), D. Jankowski (1), P. Strak (3), K. P. Korona (4), J. Borysiuk (1,4), E. Grzanka (3), M. Beeler (5,6), K. Sakowski (3), E. Monroy (5,6), and S. Krukowski (3)
Affiliations : 1 Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 01-142 Warsaw, Poland 2 Cardinal Stefan Wyszynski University, College of Science, Department of Mathematics and Natural Sciences, Dewajtis 5, 01-815 Warsaw, Poland 3 Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland 4 University of Warsaw, Faculty of Physics, Pasteura 5, 02-093 Warsaw, Poland 5 Université Grenoble-Alpes, 38000 Grenoble, France 6 CEA Grenoble, INAC-PHELIQS, 17 av. des Martyrs, 38054 Grenoble, France

Resume : High-pressure and time-resolved studies of the optical emission from polar GaN/AlN multi-quantum-wells (MQWs) with various well widths are analysed in comparison with ab initio calculations of the electronic and optical properties. The observed emission properties of MQWs were strongly affected by quantum confinement and polarization induced electric fields. The ambient pressure photoluminescence (PL) peak energies decreased by over 1 eV with QWs widths increasing from 1 up to 6 nm, and the respective PL decay times increased from 1 ns up to 10 µs, due to the strong internal electric field. The pressure coefficients of PL energy were significantly reduced as compared to bulk GaN, and they depended on the QWs widths. The transition energies, their oscillator strength and pressure dependence were modelled for tetragonally strained structures of the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs were also simulated using density functional theory calculations. A good agreement between these two approaches verified by experimental results indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy. Acknowledgements: The authors acknowledge support of the Polish National Science Centre by grant No. DEC-2012/05/B/ST3/03113.

Authors : Felix Kaess, Pramod Reddy, Andrew Klump, Luis H. Hernandez-Balderrama, Dorian Alden, Sebastian Metzner, Alexander Franke, Ronny Kirste, Frank Bertram, Jürgen Christen, Axel Hoffmann, Ramón Collazo, Zlatko Sitar
Affiliations : Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA; Technical University Berlin, Solid State Physics Institute, Hardenbergstr. 36, 10623 Berlin, Germany; Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, 39106 Magdeburg, Germany

Resume : Wide-bandgap semiconductors require precise defect control when used in modern electronic and optoelectronic devices. Compensation mechanisms make it especially challenging to reach either very high or very low free carrier concentrations. This is due to the formation energy dependence of charged point defects on the Fermi level. Control of the Fermi level position during growth offers an elegant way of designing defect profiles in epitaxial films independently of the chemical potential, which is commonly done by changing reactor conditions such as temperature, pressure, or flow of precursors. In this study, the method of defect quasi-Fermi level control was experimentally investigated through illumination of MOCVD-grown epitaxial films with above-bandgap light during growth. The results include the first intensity-dependent study of the effect, matching the data from photoluminescence and cathodoluminescence measurements with a comprehensive, quantitative model developed in our group. Furthermore, the applicability of three different illumination systems have been investigated and the feasibility of lateral defect control could be demonstrated by laser illumination of GaN and AlGaN epitaxial films. The Varshni bandgap shift due to the high temperatures during MOCVD allowed us to use commercially available visible laser diodes, in very good agreement with optical transmission measurements at high temperatures of the epitaxial samples.

Authors : Paweł Strąk, Paweł Kempisty, Konrad Sakowski, Stanisław Krukowski
Affiliations : Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland

Resume : Ab initio simulations were used to determine electron affinity of group III nitride AlN, GaN and InN polar surfaces both metallic and nitrogen sides. It was found that electron affinity at metal and nitrogen faces are drastically different. These values for AlN(0001) and AlN(000-1) are 1.66 eV and 3.14 eV, respectively. Similarly for GaN(0001) and GaN(000-1) 2.34 eV and 5.54 eV while for InN(0001) and InN(000-1) are 2.92 eV and 6.52 eV, respectively. These values are in reasonable agreement with the existing experimental data for AlN and GaN. For InN these values are different. Influence of adsorption of Cs at both metal and nitrogen surfaces was determined. It was shown that Cs drastically decreases affinity. The change of affinity was determined. It was shown that gradual change of the affinity, due to modification of surface dipole layer is followed by finite jump for the coverage corresponding to electronic passivation of the surface, i.e. depinning of the Fermi level.

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Authors : Andrew. A. Allerman, Andrew. M. Armstrong, Albert. G. Baca, Mary. H. Crawford, Jeramy. R. Dickerson, Michael. P. King, Arthur. J. Fischer, Robert. J. Kaplar, and Erica. A. Douglas
Affiliations : Sandia National Laboratories, Albuquerque, NM 87185

Resume : The AlGaN alloy system has been the material system of choice for development of solid-state UV emitters and is gaining interest for high-voltage power electronics. Since the critical electric field scales as the bandgap to the 5/2 power, Al-rich AlGaN alloys with their much larger bandgap compared to SiC and GaN are appealing for high voltage PN diodes and transistors. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer device design options not possible for SiC-based devices. Here we demonstrate the potential of AlGaN alloys of high voltage power devices with both vertical and lateral architectures. Al0.3Ga0.7N PN diodes were grown on sapphire substrates by metal organic vapor phase epitaxy. The PN diode structure consists of an AlN template layer followed by an N+ Al0.3Ga0.7N contact layer similar to that reported for AlGaN-based laser diode structures. Rather than quantum wells, a 4.3 um-thick, lightly n-type doped Al0.3Ga0.7N drift layer is grown followed by a p-type Al0.3Ga0.7N layer graded to a p-GaN contact layer to complete the structure. Employing 1.3 mm-thick sapphire substrates allowed crack-free growth of the 9.1 um epi structure. Quasi-vertical diodes exhibited breakdown voltages in excess of 1600 V with less than 3 nA of reverse leakage current out to 1200 V. To our knowledge, this is the first vertical AlGaN-based PN diode exhibiting a breakdown voltage in excess of 1 kilovolt. An effective critical electric field of 5.9 MV/cm was determined from the epilayer properties and the reverse current-voltage characteristics. We note that a Baliga figure of merit (Vbr2/ Rspec,on) of 150 MW/cm2 found in this study is the highest reported for an AlGaN PN diode. We also report electrical characteristics of 2-dimensional electron gases (2DEGs) formed by Al0.85Ga0.15N/Al0.7Ga0.3N and AlN/Al0.85Ga0.15N heterostructures grown on sapphire substrates. These are the highest Al compositions where a 2DEG has been observed. Unlike previous reports, we show that the sheet resistance of Al-rich, AlGaN heterostructures is independent of the density of threading dislocations when employing growth conditions that minimize the density of electron compensating defects. Sheet resistances as low as 1650 Ohms/sqr. were measured using a Lehighton contactless sheet resistance system in Al0.85Ga0.15N/Al0.7Ga0.3N heterostructures. From capacitance-voltage measurements, using a Hg probe we find that changing the thickness of the Al0.85Ga0.15N barrier layer from 10 nm to 50 nm will shift the pinch-off voltage from ~0 V to -15 V. The addition of Si doping to the Al0.85Ga0.15N barrier results in a negative shift in pinch-off voltage indicating an increase in the sheet carrier concentration in the 2DEG. Based on the pinch-off voltage and sheet resistance we calculate the electron mobility to range from 200 – 250 cm2/Vs and the sheet charge density to range from ~2 x 10^12 to 2 x 10^13 cm-2. The higher electron mobility in these Al-rich heterostructures is consistent with a reduction in alloy scattering predicted by Bajaj[1] as the channel composition approaches AlN. Simple HEMT devices employing Schottky contacts and an etch / N+ GaN regrowth process were fabricated from AlN/Al0.85Ga0.15N heterostructures. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. This exceeds the 370 V breakdown reported for similar simple HEMT devices based on B-Ga2O3[2] but is less than the 1700 V breakdown reported for AlGaN HEMTs with field plates and a lower Al composition[3]. Source-drain current was low and limited by the resistance of the grown contact. Improvements in the contact regrowth process and implementation of field plate architecture are expected to increase transistor performance. 1. S. Bajaj, T.-H. Hung, F. Akyol, D. Nath, and S. Rajan, Applied Physics Letters 105, 263503 (2014). 2. M. Higashiwaki, K. Sasaki, T. Kamimura, M. Hoi Wong, D. Krishnamurthy, A. Kuramata, T. Masui and S. Yamakoshi, Appl. Phys. Lett. 103, 123511 (2013). 3. T. Nanjo, A. Imai, Y. Suzuki, Y. Abe, T. Oishi, M. Suita, E. Yagyu and Y. Tokuda, IEEE T. Electron. Dev. 60, 1046 (2013). This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Authors : R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. R. Dickerson, A. J. Fischer, F. Leonard, A. A. Talin, M. P. King, A. G. Baca, and E. A. Douglas
Affiliations : All authors are from Sandia National Laboratories, Albuquerque, NM 87185 USA

Resume : As the wide-bandgap materials SiC and GaN mature and become commercially available, “Ultra” Wide-Bandgap (UWBG) semiconductors with bandgaps exceeding 3.4 eV are gaining interest as the next-generation materials that will enable dramatic leaps in power electronics performance. This is because the Figure-of-Merit (FOM) comparing breakdown voltage (VB) and area-normalized “specific” on-resistance (Ron,sp), defined as VB^2/Ron,sp, scales as the cube of the critical electric field for vertical devices and the square of the critical electric field for lateral devices. The critical electric field for avalanche breakdown in turn scales approximately as the 5/2 power of the bandgap, implying a very strong dependence of FOM on bandgap. Representative UWBG materials include diamond, Ga2O3, and AlxGa1-xN. The AlGaN system is particularly attractive due to the availability of heterostructures as well as the ability to utilize polarization doping; both of these properties mitigate the issues of energetically deep impurity dopants and consequent incomplete ionization which universally affect UWBG semiconductors. Thus, our group’s focus has been on the development of AlGaN for the fabrication of next-generation power devices with both vertical and lateral architectures. Quasi-vertical AlGaN PiN diodes were grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on sapphire substrates. The key epitaxial layer is the thick, low-doped n- drift layer, since it supports the high voltage in the reverse-bias blocking mode, and it also sets a lower limit for the on-state resistance in the forward-bias conducting mode. Growth of the p-layers follows, and ion implantation into these layers is utilized to form a Junction Termination Extension (JTE) structure to mitigate high lateral electric fields and prevent premature breakdown. The JTE is based on a design developed for vertical GaN PiN diodes, which we have studied extensively using Scanning Photocurrent Microscopy (SPCM). SPCM provides experimental measurements of the internal electric field as a function of position within the device as well as applied voltage. The experimental data are compared to numerical simulations, which not only reveals very rich behavior of the internal electric fields, but also permits informed optimization of the device design. The device fabrication is completed by a mesa-etch down to an n-contact layer and deposition of Ohmic contacts (TiAlMoAu) onto this layer; this is necessary because the substrate is insulating and, due to the lateral current transport in this layer, the device configuration is termed “quasi-vertical” (the conduction through the pn junction and drift layer is in the vertical direction). The top Ohmic contact is PdAu, the surface is passivated with silicon nitride, and a shallow trench isolates the JTE from the mesa sidewalls. For a device utilizing Al0.3Ga0.7N for the active layers and with a 4.3 um thick drift region doped in the mid-10^16 cm^-3 range, VB exceeded 1.6 kV; combined with the measured Ron,sp of 16 mOhm·cm^2, a FOM of approximately 150 MW/cm^2 is obtained. To our knowledge this is the first time a vertical kV-class AlGaN PiN diode has been demonstrated, and the FOM is the highest reported for an AlGaN diode. Further, the current density under forward bias exceeds 3.5 kA/cm^2, and the critical electric field for the Al0.3Ga0.7N is estimated to be 5.9 MV/cm, consistent with the expected EG^5/2 dependence of critical field on bandgap. Lateral High Electron Mobility Transistors (HEMTs) were also grown and fabricated in Al-rich AlGaN alloys. The epitaxial layers are likewise grown on sapphire substrates by MOVPE. Following the growth of an AlN nucleation layer, a 400 nm thick Al0.85Ga0.15N buffer/channel layer is grown, which is capped by a 48 nm thick AlN barrier layer. All layers are unintentionally doped. A combination of mercury-probe capacitance-voltage and contactless resistance measurements indicates a sheet resistance of approximately 4200 Ohm/square and a channel mobility of 250 cm^2/V·s, and the pinch-off voltage is approximately -4.0 V. Circular HEMTs were fabricated using a six-mask photolithographic process. The key step in the process is the fabrication of the source and drain Ohmic contacts, for which n+ GaN:Si is re-grown in trenches etched through the AlN barrier, and a TiAlNiAu metal stack is afterwards deposited. The Schottky gate is NiAu and the surface is passivated with silicon nitride; no field plates are utilized. Drain current vs. drain voltage curves exhibit clear gate control. However, the current density is much lower than expected based on the measured sheet resistance and known device geometry, indicating that further optimization of the Ohmic contacts is necessary (the contact resistivity is estimated to be 0.1 Ohm·cm^2), possibly through the use of compositional grading. Despite this, the devices show excellent performance with Ion/Ioff > 10^7 and VB > 800 V, and to our knowledge are the highest-bandgap transistors ever reported. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Authors : Saptarshi Mandal, Dong Ji, Wenwen Li, Maitreya Dutta, Franz Koeck, Matthew Laurent, Robert Nemanich and Srabanti Chowdhury
Affiliations : School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ-85287; Department of Physics, Arizona State University, Tempe, AZ-85287; Department of Electrical and Computer Engineering, University of California, Davis, Davis, CA 95616

Resume : With our growing dependence on technologies driven by electricity, the role of power conversion is evolving to address higher conversion efficiency, higher temperatures of operation, higher power density and smarter applications. Switches, an integral part of every power converter, directly impact the efficacy of the converter, and indirectly its cost. Widebandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) have shown promising performance owing to their superior material quality (compared to Silicon (Si)) that make them the leading materials for next generation power electronics. While SiC and GaN are making impressive progress wider bandgap semiconductors like Aluminum Nitride and Diamond are attracting research interest. This presentation will focus on a developing technology, the vertical GaN technology and its current challenges, and elucidate some recent developments on an emergent one - diamond. Lateral GaN based HEMTs [1] have been designed into power converters successfully that allowed us to study the outcomes at all levels, all the way up to the system. The two important factors that benefited power converters due to the inclusion of a GaN-switch are 1) higher frequency of operation 2) higher temperature of operation, compared to what Si can offer as it reaches its material’s limit. High frequency operation leads to the reduction in electrical form-factor due to reduced size of passive components (capacitors and inductors). Higher temperature of operation allows reduction of cooling accessories hence reducing the mechanical form factor. Higher power (>15KW, although the limits are yet to be determined) applications benefits from vertical device design to maximize the power density. Vertical topology allows higher On-state currents and off-state voltages over a smaller chip area, where the lateral design become unattractive in both cost and performance due to large chip area. Our recent understanding of vertical GaN devices along with the cost of producing bulk GaN substrates portrays a sustainable solution exists for the high power market. CAVETs have demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design). Our recent research has identified vertical device designs that warranties single chip normally off devices, which will further improve switching performance by eliminating bond wire and PCB trace related inductances. GaN substrates with defect densities lower that 104cm-2 and allowing electron mobility >1100cm2V-1s-1 [2] and recent success on implantation activation [3] techniques create a very encouraging picture for bulk GaN based power devices. Energy bandgap above 3.4eV is very attractive to continue the roadmap of power electronics for higher power application (3kV and up). With a bandgap of 5.5eV, a critical electric field over 10 MV/cm, and thermal conductivity superior to Si, SiC and GaN, diamond offers the most appropriate combination to serve very high voltage power electronics. One of our recent research achievement demonstrated excellent contacts to n-type diamond, which has been challenging due to very low work function of diamond. Phosphorous doped diamond achieved by Koeck et al. [4] on homoepitaxial diamond substrates led to excellent ohmic contacts to n-type diamond. Using the same contact technology on a homoepitaxialy grown p (bottom)-i-n(top) structure, diode operation was achieved blocking over 400V. These early results obtained with diamond clearly show a path beyond GaN and SiC. Reference: [1] S.Chowdhury and U.K Mishra, IEEE Transaction on Electron Devices, 60 3060(2013) [2] P Kruszewski et al. The International Workshop on Nitride Semiconductor (2014) [3] T. J. Anderson et al, Electronics Letts. , 50, 197(2014) [4] F.A.M. Koeck et al, Diamond Relat. Mater. 18, 789(2009)

Authors : Erhard Kohn
Affiliations : North Carolina State University,Dept. of Mat. Sci. and Eng., Raleigh NC 27695-7919, U.S.A. and Ulm University, Dept. of Electron Devices and Circuits, 89081 Ulm, Germany

Resume : Conventionally III-Nitride devices had been based on GaN, grown on foreign substrates. However, recently GaN and AlN substrates of high quality have become available and thus of increasing interest, and heterostructure devices start to cover the entire spectrum of AlGaN and InAlN compositions with electronic properties now reaching from those of classical semiconductors with shallow doping to ultra-wide bandgap compounds with deep doping levels typical for insulators. Their high chemical/thermal stability should allow the realization of devices with extreme properties and applications in harsh environments. Thus, device technologies need also to cover the related spectra like extreme modes of operation. This means that many of the traditional technological building blocks and their materials and processing tools used should be re-evaluated and re-adjusted. This concerns first of all fundamental building blocks like ohmic and blocking contacts and surface passivation with dielectrics. This will be the focus of this talk. However, the field of device processing technologies is rather heterogeneous with electronic, optical, physical and chemical aspects producing often conflicting requirements for processing routines. Thus a selection of cases needs to be made, which may highlight the difficulties and challenges encountered in respect to the conventional procedures used up to now. Thus, the focus will be on the following three topics: (1) Ohmic contacts to P-GaN (with some reference to N-AlGaN), where often non-linear behavior is seen, (2) Ni-based Schottky barriers to n-GaN, which is the standard barrier contact material in HEMTs, but where still often differences in barrier height and low thermal stability are observed and (3) Surface passivation by SiN, which is amorphous (and thus highly defective) with a variable stoichiometry, often resulting in distinctly different interface properties, but which is nevertheless the common passivation material for III-Nitrides presently. This is then contrasted with an InAlN/GaN heterostructure technology of extreme thermal stability, tested up to 1000 °C in HEMT devices. In fact, the intrinsic carrier concentration at 1000 °C of the wide bandgap branch of the III-Nitride materials matrix is below 1014 cm-3 and will thus not impose a temperature limit on most electronic (and perhaps even optoelectronic) device applications, (posing of course also a challenge to the related materials development).

Authors : Elizabeth A. Paisley 1, Christopher T. Shelton 2, Michael Brumbach 1, James M. Lebeau 2, Michael D. Biegalski 3, Hans Christen 3, Stanley Atcitty 1, Ramon Collazo 2, Zlatko Sitar 2, Robert Kaplar 1, Albert Baca 1, Jon-Paul Maria 2, and Jon F. Ihlefeld 1
Affiliations : 1 Sandia National Laboratories, Albuquerque, NM 87185 2 Department of Materials Science and Engineering, North Carolina State University, 1001 Capability Dr. Raleigh, NC 27606 USA 3 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, TN

Resume : Epitaxial MgO-CaO (Mg0.52Ca0.48O) alloys are promising gate oxide candidates for next generation GaN power electronics, owing to its wide bandgap, high dielectric constant and ability to lattice-match to GaN. However, the cubic oxide| hexagonal nitride interface is challenging due to the highly dissimilar structure and symmetry across the interface. Adding to the complexity, the 111-growth direction of epitaxial rocksalt oxides compatible with (0001) GaN is polar, necessitating the formation of rough 100-oriented facets. We have reported the utility of surfactant- MBE and PLD growth, utilizing water vapor, of Mg0.52Ca0.48O alloys on GaN. Using this technique, the 111-surface is hydroxylated, changing the equilibrium habit from cubic to octahedral, enabling 2D growth of (111) MCO on GaN. In this presentation, we will utilize this technique to produce and characterize smooth, commensurate MCO | (Al)GaN interfaces. Band offsets for the highest Al-content samples- Al0.67Ga0.23N- are 1eV ± 0.2 for the valence band and 1.6 eV ± 0.2 for the conduction band. We will further characterize these interfaces through electrical analysis- IV, CV, and Dit curves- that highlight interface properties for each growth ambient. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Authors : G. Greco, F. Roccaforte
Affiliations : CNR-IMM, Strada VIII, n. 5 ? Zona Industriale, 95121 Catania, Italy

Resume : Owing to the presence of the 2DEG and to the high critical electric field, AlGaN/GaN heterostructures are attractive materials for high power and high frequency devices. For this technologies, a good understanding of the behaviour of Schottky contacts to AlGaN/GaN heterostructures is the key to obtain the desired performances, in terms of leakage current, temperature dependent characteristics, etc. However, the behaviour of Schottky contacts to AlGaN/GaN heterostructures can be affected by the high defects density (such as dislocations) of the material, especially if grown on Si substrates. While many works have already discussed the Schottky contacts on GaN layers, Schottky barriers on AlGaN/GaN are still widely debated. In this context, monitoring the temperature dependence of the electrical characteristics can provide useful insights for a better understanding of the current transport mechanisms in these systems. In this work, the forward and reversed I-V characteristics of Ni/Au Schottky contacts on AlGaN/GaN heterostructures grown on Si substrates have been investigated at different temperatures (25-175°C). In particular, the Schottky barrier height has been determined from the forward I-V curves, assuming a ?two diode model? to take into account of the potential well at the AlGaN/GaN heterojunction. The behaviour of the electrical parameters (ideality factor and barrier height) indicated the presence of an inhomogeneous barrier. In addition, a post-metal-deposition at 400°C revealed a modification of the forward I-V curves, which has been discussed considering the possible contribution of dislocations in the diode current.

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Authors : Lutz Kirste; Lars Hahn; Mario Prescher; Martin Walther; Oliver Ambacher
Affiliations : Fraunhofer Institute for Applied Solid State Physics (IAF), Freiburg, Germany

Resume : UV-photodetectors based on the semiconductor material aluminum gallium nitride (AlGaN) are particular suited for monitoring of ultraviolet radiation sources. Typically, AlGaN-detector structures are fabricated on sapphire substrates. Due to the large lattice mismatch, a high density of threading dislocations (TDDs) of ~109 cm−2 is present in AlGaN-layers grown on sapphire substrates. Therefore, freestanding AlN-substrates with low defect density and high UV-transparency are the substrate material of choice for the realization of AlGaN-based detectors with superior properties. Recently, AlN-substrates have become available by various manufacturers. The AlN-substrates differ considerably in the structural perfection, homogeneity and deep UV-transparency. In this work we present a detailed analysis of the structural properties of AlGaN- heterostructures for UV-detectors deposited on AlN/sapphire (0001) templates and freestanding AlN (0001) substrates. The structural perfection of the AlN-substrates is investigated by X-ray topography and HRXRD. The UV-transparency of the AlN-substrates is analyzed by optical transmission measurements. All AlGaN-layers are grown by MOVPE. After growth HRXRD is used for measurement of the Al-concentration and mosaicity of the device structures. We will show that the AlN-substrates contain a heterogeneous distribution of TDDs, basal plane dislocations and grain boundaries. The comparison of AlGaN-layers grown on AlN/sapphire-templates with layers grown on AlN-substrates clearly shows that a significantly improved material quality can be achieved on AlN-substrates.

Authors : Hideto Miyake, Chia-Hung Lin, Yikang Liu and Kazumasa Hiramatsu
Affiliations : Mie University

Resume : We recently reported the formation of high-quality AlN films grown on sapphire substrates by MOVPE by thermal annealing at a high temperature in a carbon-saturated N2-CO gas mixture [1,2]. The TD density in the AlN films was reduced to 4.7×10^8 cm^-2, and the films can be used to fabricate conventional LEDs. However, there have been few reports on the high-temperature thermal annealing of sputtered AlN films grown on sapphire substrates, which has great potential for preparing highly uniform and large-scale AlN films for commercial use. In this work, we studied the effects of thermally annealing sputtered AlN films grown on sapphire substrates on the crystallinity and surface morphology, where the thermal annealing was performed in nitrogen ambient. The effects of annealing were investigated as a function of the sputtered AlN film thickness and the thermal annealing temperature. AlN films were grown on c-plane sapphire substrates by sputtering. The misorientation angle of the 2-inch substrates was -0.2^o toward the [1-100] direction. AlN powder was used as a target, and the ambient was an argon (Ar) and nitrogen (N2) gas mixture with the ratio [Ar]/[N2] = 0.33. The radio frequency (RF) power and growth temperature were 700 W and 650 oC, respectively. The thicknesses of the sputtered AlN films were 170 and 340 nm. Subsequently, the sputtered AlN films were thermally annealed in N2 at 1600 - 1700 oC for 1 h. The (0002) and (10-12) XRCs for the 170-nm-thick sputtered AlN films without and with thermal annealing at 1700 oC were examined. Both the (0002) and (10-12) XRCs of the AlN films had a single sharp peak after thermal annealing owing to the elimination of the tilt and twist components from the sputtered AlN films by high-temperature thermal annealing. The FWHMs of the (0002) and (10-12) XRCs were markedly reduced from 532 to 49 and 6031 to 287 arcsec, respectively. The improved crystallinity is related to the solid-phase reactions that occur at high annealing temperatures, which annihilated domain boundaries and concurrently increased the compressive stress in the films. REFERENCES: [1] H. Miyake, G. Nishio, S. Suzuki, K. Hiramatsu, H. Fukuyama, J. Kaur, N. Kuwano, Appl. Phys. Exp. 9 (2016) 025501. [2] H. Fukuyama, H. Miyake, G. Nishio, S. Suzuki, K. Hiramatsu, Jpn. J. Appl. Phys, 55 (2016) 05FL02.

Authors : Florian Bartoli (1,2,3), Philippe Pigeat (3), Thierry Aubert (1,2), Omar Elmazria (3), Frederic Genty (1,2), Pascal Boulet (3), Jaafar Ghanbaja (3).
Affiliations : 1-Laboratoire LMOPS, CentraleSupélec - Université de Lorraine, Metz, France; 2-Laboratoire LMOPS, CentraleSupélec - Université Paris-Saclay, Saint-Aubin, France; 3-IJL UMR 7198 Université de Lorraine - CNRS, France;

Resume : Surface acoustic waves (SAW) devices are key components in communication systems and are more and more used as wireless and passive sensors including in harsh conditions. AlN/Sapphire structure has been widely investigated for both applications as it allows high-frequency operations. However, this structure shows a relatively low electromechanical coefficient K², close to 0.3%. ScxAl1-xN, which shows stronger piezoelectric properties, is a serious candidate to advantageously replace AlN in SAW applications. The aim of this work is to achieve highly-textured c-oriented ScxAl1-xN thin films with a FWHM rocking-curve below 1°, to maximize the SAW properties of the film. The optimal film composition, with the best compromise between K2 and SAW velocity, will be obtained by testing Sc rates between 0 and 50%. The films are deposited by magnetron sputtering, with a composite target. The microstructure is determined by X-ray diffraction (XRD) measurements (θ-2θ, rocking-curve and pole figure) and transmission electron microscopy. Energy dispersive X-ray as well as Raman spectroscopy methods are used to get the film composition. K² and SAW velocity are determined by RF characterization. Firstly, c-oriented AlN thin films were deposited to get a reference sample without Sc. XRD θ 2θ diagram shows strong (002) orientation with a FWHM rocking-curve below 0.5° for the optimized films. Pole figures show that the films are also textured in the plane. RF measurements confirm the piezoelectricity of the film. Ongoing experiments are dedicated to the optimization of the deposition parameters throughout the whole investigated Sc rate range, to obtain ScxAl1-xN films that are highly textured in and out of the plane such as AlN reference ones.

Authors : Liz Montañez, Andrés Guerra, Rolf Grieseler, Roland Weingärtner
Affiliations : Departamento de Ciencias, Sección Física, Pontificia Universidad Católica del Perú, Av. Universitaria 1801, Lima 32, Peru Technical University of Ilmenau, Institute of Materials Technology, POB 100565, 98684 Ilmenau, Germany

Resume : Crystalline aluminum nitride (AlN) is a semiconductor with excellent properties such as wide bandgap of 6.2 eV, high chemical stability, high thermal conductivity and high dielectric breakdown voltage, which allow this material to be applied in opto-electronics [1]. Due to its optimal low reflection as well as to its fixed negative charges, AlN has also attracted attention as anti-reflective coating and passivation layer for silicon surfaces [2,3]. In this work, the dependence of the structural and optical properties of AlN on different hydrogen content is demonstrated. The layers are prepared by RF-magnetron sputtering in an Ar/H2 atmosphere. Elemental composition was determined by Energy Dispersive Spectroscopy (EDS). The structural properties were investigated by Fourier Transform Infrared spectroscopy, X-ray diffraction at grazing incidence (XRD) and Scanning Electron Microcopy (SEM). The optical characterization was performed through transmittance measurements using a modified envelope method. Hydrogen induces the increase of the grain size and the optical bandgap, showing a good correlation between the optical and structural properties. [1] B.N. Pantha et al, Appl. Phys. Lett. 90 (2007) 3-5. [2] P.M. Kaminski et al, Phys. Status Solidi C 8, N0 4 (2011) 1311-1314 [3] G. Krugel et al, Energy Procedia. 55 (2014) 797-804.

Authors : A. Núñez-Cascajero [1], M. Jiménez-Rodríguez [1], E. Monroy [2] [3], M. González-Herráez [1], F.B. Naranjo [1]
Affiliations : [1] Grupo de Ingeniería Fotónica, Universidad de Alcalá, Departamento de Electrónica, Alcalá de Henares, Madrid, Spain [2] Université Grenoble-Alpes, 38000 Grenoble, France. [3] CEA-Grenoble, INAC-PHELIQS, 17 av. des Martyrs, 38000 Grenoble, France.

Resume : Photoconductors are the most simple semiconductor detectors; their operation is related to change in device conductivity by photogenerated carriers, with an absorption cutoff wavelength related to the semiconductor band gap. AlInN presents a direct band gap that can be tuned with the In composition from the mid-UV to the near IR, while keeping the intrinsic properties of III-nitrides. In addition, the growth of AlInN by RF sputtering presents some advantages due to the low cost of the system and the low temperature of the deposition process, which enables the integration with other materials. In this work, we have developed photoconductor devices based on AlInN layers grown on sapphire by RF sputtering. We have studied the effect of the thickness and composition of the AlInN layer on the photoresponse characteristics. Developed devices show a sublinear dependence of the photocurrent as a function of the incident optical power (I_pc∝P_opt^(-γ) ). In particular, for 20 nm-thick Al0.38In0.62N-based devices a value of γ ~ 0.7 has been estimated for incident light of 405 nm. For these devices, above-the-band-gap (405 nm wavelength) responsivity higher than 0.1 A/W can be obtained for an irradiance of 1 W/m2, and the response drops by more than two orders of magnitude for excitation below the band gap (633 nm). The devices present persistent photoconductivity effects associated to the grain boundaries.

Authors : Enrique Iborra*, Mario DeMiguel-Ramos**, Barbara Díaz-Durán*, Jimena Olivares*, Jesús Sangrador*, Marta Clement*, and Teona Mirea*
Affiliations : *GMME-CEMDATIC, ETSI de Telecomunicación, Universidad Politécnica de Madrid, Madrid, Spain **EDM, Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge, United Kingdom

Resume : Sputtered AlN is a piezoelectric material with applications in electroacoustic devices such as high frequency filters or biological sensors. For most of them, films are deposited with the c-axis of their wurtzite structure perpendicular to the surface. The most commonly-used resonator structure consists in an electrode/AlN/electrode piezoelectric capacitor acoustically isolated from the substrate by an air gap (suspended structures) or by an acoustic reflector (solidly mounted resonators). In these devices the propagating acoustic wave mode is longitudinal, that is, the movement of the material is parallel to the propagating direction, which is parallel to the c-axis. For biosensing applications, which require in-liquid operation, this mode is not suitable due to its high energy radiation to the medium, which gives rise to a reduction of the quality factor. For proper operation, shear mode resonators, where the movement of the material is perpendicular to the propagation direction and parallel to the surface, are need. To generate these modes, it is essential to create a component of the electric field perpendicular to the c-axis. The most straightforward way to do so is to deposit AlN films with grains whose c-axis is tilted with respect to the normal to the surface. To deposit such films, well-controlled directional deposition and rough substrates are needed. In this communication, we present the deposition process and material characterization of tilted grain AlN films. Bulk acoustic wave resonators, made with this material, are also presented together with their application to biosensors operating in-liquid medium. Some details about device design and operation in the detection of proteins are also included.

Authors : Matthias Belloeil, Alexandra-Madalina Siladie, Bruno Gayral, Sonia Mascaros, Nuria Garro, Ana Cros, Bruno Daudin
Affiliations : M. Belloeil; A.-M. Siladie; B. Gayral; B. Daudin: Univ. Grenoble Alpes, F-38000 Grenoble, France and CEA, INAC-SP2M,”Nanophysics and semiconductors” group, F-38000 Grenoble S. Mascaros; N. Garro; A. Cros: Materials Science Institute (ICMUV), University of Valencia, P. O. Box 22085, E46071, Valencia, Spain.

Resume : One motivation for studying the growth and properties of AlGaN nanowires (NWs) is the growing interest in UV-C emission for sanitization applications. Indeed, the versatility of NW growth on a large variety of substrates, the easier doping of NWs with respect to layers, the absence of extended defects, make them particularly attractive and a potential candidate to overcome the limitations of bulk-like layers. Accordingly, we report here on the molecular beam epitaxy growth of AlGaN NW sections grown on top of GaN NWs on Si (111). We show that structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states [1,2]. These carrier localization features are present at very small scale, as they are also observed for AlGaN NW insertions in AlN, less than 2-3 nm thick. Moreover, related micro-photoluminescence (PL) and time-resolved PL data do not show any dependence of decay times with growth temperature. Photon correlation experiments have been performed on the micro-PL sharp lines associated to localized electronic states. Antibunching has been put in evidence, typical of a quantum dot-like behaviour. Interestingly, such a behaviour has been observed for a wide range of AlGaN NW chemical composition and growth temperature. Preliminary results on the p and n-type doping of AlGaN p-n junction (35 % AlN molar fraction) studied by a combination of Raman spectroscopy and Kelvin probe microscopy will be additionally reported, paving the way to the realization of UV NW-based UV emitting devices. References [1] A.Pierret, et al., Nanotechnology 24 115704 (2013). [2] A.Pierret, et al., Nanotechnology 24 305703 (2013).

Authors : Thomas Wunderer
Affiliations : Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304

Resume : High-power optical sources that emit in the UV spectral range have applications that include bio-chem identification, decontamination, medical diagnostic and treatment, communications, and materials curing. Attempts to realize semiconductor UV sources by the conventional approach with a current-injection p-/n-junction diode have encountered numerous difficulties. This presentation will discuss the challenges, illustrate the current status of materials/device development, and describe the necessary building blocks, which include highly conductive p-type cladding layers, sophisticated electron-blocking layers, and high performance active zones. High current densities of more than 20kA/cm2 of a full LD heterostructure have been achieved. Optically pumped lasers have been demonstrated with low pump threshold down to a wavelength of 237nm. However, issues related to effective carrier injection and low absorption losses remain challenging when using high band gap p-type materials. Thus, a laser configuration that omits the use of p-doped materials is an attractive alternative. We will present recent results towards realizing deep-UV lasers in the AlGaN materials system using electron beam excitation as the laser pumping strategy. A custom-built e-beam-pumping system has been developed that enables excitation of AlGaN gain chips with high-energy electrons (10-30keV) and high-power densities exceeding 1MW/cm2. AlGaN gain chip heterostructures for vertical laser emission will be described. Toward development of a compact deep-UV laser source, we report record high pulsed peak optical output power of more than 200mW of spontaneous emission at λ=246nm.

Authors : Jeomoh Kim1, Mi-Hee Ji1, Yuh-Shiuan Liu1, Theeradetch Detchprohm1, Russell D. Dupuis1, Tsung-Ting Kao1, Saniul Haq1, Shyh-Chiang Shen1, Karan Mehta1, P. Douglas Yoder1, Hongen Xie2, Fernando Ponce2, Ashok Sood3, and Nabir Dhar4
Affiliations : 1-Center for Compound Semiconductors and School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Dr. NW, Atlanta, Georgia 30332-0250, USA; 2-Arizona State University, Tempe AZ; 3-Magnolia Optical Technologies, Woburn MA; 4-Night Vision Sensors and Electronic Division, Ft. Belvoir VA

Resume : The field of ultraviolet (UV) photonics at wavelengths <400nm is an area of increasing practical interest and coherent light sources and fast, sensitive photodetectors are needed for many important applications. In this study, AlGaN p-i-n APDs are demonstrated on “free-standing” (FS) n-type (0001) GaN substrates having low dislocation densities. The AlGaN PIN UV-APDs and UV lasers were epitaxially grown on a c-axis n-type free-standing GaN and sapphire substrates using metalorganic chemical vapor deposition (MOCVD). For top-illuminated APD structures, step-graded layers from n-GaN:Si to Al0.02Ga0.98N:Si were introduced instead of a single n-Al0.05Ga0.95N:Si layer as a strain management scheme for crack-free growth. The APD epitaxial layer structure consists of a 0.45-μm thick GaN:Si layer followed by a 0.15-μm thick n-Al0.02Ga0.98N:Si layer for the step grading, a 0.3-μm thick unintentionally-doped Al0.05Ga0.95N layer, a 0.1-μm thick Al0.05Ga0.95N:Mg layer, and a 0.02-μm thick Al0.05Ga0.95N:Mg++ grown on FS-GaN. The growth and doping conditions for epitaxial layers were optimized to achieve improved crystalline and structural quality by modifying the MOCVD growth parameters. The APD wafers were fabricated into circular mesas with various diameters using standard photolithography and inductively coupled plasma etching, followed by SiO2 passivation deposited using plasma-enhanced chemical vapor deposition. The APDs exhibit a maximum avalanche gain >105 at a reverse bias of -102V. We have also fabricated UV optically pumped InGaN/AlGaN MQW lasers at 375nm employing an AlGaAs-GaAs 40 pair n-type bottom DBR operating pulsed at 300K, and will describe the growth, processing, and output characteristics of these lasers.

Authors : O. Rettig, J.-P. Scholz, S. Bauer, F. Scholz, K. Thonke, Haoyuan Qi, Ute Kaiser
Affiliations : Institute of Optoelectronics, Ulm University, Albert-Einstein-Allee 45, 89081 Ulm, Germany; Institute of Quantum Matter / Semiconductor Physics Group, Ulm University, Albert-Einstein-Allee 45, 89081 Ulm, Germany; Central Facility of Electron Microscopy, Ulm University, 89081 Ulm, Germany

Resume : In order to manage and decrease the lattice mismatch induced strain in AlGaN heterostructures for UV-C light emitters, we have started investigations about the growth and basic characterisation of AlBGaN layers. A major problem of this material system is the very poor solubility of boron (B) in Al(Ga)N, which may be increased by MOVPE growth at elevated temperatures of up to 1400 °C. At first, we have carefully optimised the quality of MOVPE-grown AlN templates on sapphire wafers. A multi-step process was applied by alternating slow and fast growth leading to a reduction of the threading dislocation density as determined by TEM. This improvement is also confirmed by XRD showing narrow peaks with FWHM below 80 and 600 arcsec for the (0002) and (10-12) ω-scan, respectively, and also supported by strongly improved photoluminescence (PL) signals. On such templates, first 500 nm thick AlBN layers were grown with various B and Al flow ratios, still using conventional growth temperatures around 1100°C. For layers grown with a TEB/(TEB+TMAl) ratio of 1.7% in the gas phase, XRD scans show a weak peak besides the main AlN peak corresponding to approximately 0.4% boron in AlN assuming a fully relaxed layer, while larger B ratios resulted in polycrystalline layers. First PL investigations show pronounced defects in both AlN and AlBN layers. However, the main defect-related contribution shifts in energy from ~4.4 eV towards 3 eV with increasing boron content, indicating a change in defect generation in presence of TEB in the gas phase.

Authors : Abdallah Ougazzaden1,2, Xin Li2, Suresh Sundaram2, Youssef El Gmili2, Taha Ayari1,2, Gilles Patriarche3, Paul L. Voss1,2, and Jean Paul Salvestrini2,4
Affiliations : 1 Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France; 2UMI 2958, Georgia Tech - CNRS, 57070 Metz, France; 3Laboratoire de Photonique et de Nanostructures (LPN), CNRS, Université Paris-Saclay, route de Nozay, F-91460 Marcoussis, France; 4Université de Lorraine, LMOPS, EA 4423, 57070 Metz, France

Resume : With a “honeycomb” lattice structure similar to graphene, hexagonal boron nitride (h-BN) has attracted great interest for its promising properties such as atomic flat surface free of dangling bonds, high thermal conductivity and electrical resistivity, high neutron capture cross section and wide bandgap around 6 eV, which make it suitable material for graphene electronics, van der Waals heterostructures, neutron detectors and deep UV devices. Since BN atomic layers are hold together by weak van der Waals forces, h-BN can also serve as a buffer layer for the growth of III-nitrides on top and as a release layer at the same time allowing mechanical transfer of device structures to foreign substrates. The quality and the scalability of h-BN are the main issues for its applications. In this work, we have successfully grown 2D layered hexagonal BN in 2 inch wafer scale by MOVPE on sapphire substrates[1]. The h-BN layer shows (0002) and (0004) X-ray diffraction peak with small mosaic spread. The scanning transmission electron microscopy images present highly oriented lattice with hexagonal stacking sequence. The whole wafer exhibits continuous and uniform surface, with growth steps for 3 nm BN and organized wrinkles for 30 nm BN. The growth of layered h-BN on 2 inch sapphire substrates has enabled us to investigate the large-scale mechanical lift-off and transfer of GaN-based device structures[2]. In the present work, high quality InGaN/GaN multi-quantum well (MQW) structures were grown on 3 nm h-BN on sapphire substrate. The nucleation and epitaxial growth process have been studied. Wafer-scale mechanical exfoliation of the device structures using commercially available adhesive tapes was demonstrated. Structural and optical characterization performed before and after the lift-off confirms conservation of structural properties and optical functionalities by this technology. The ability to grow wafer-scale 2D layered h-BN epi-layer is promising for future electronics and optoelectronics. The high quality III-nitride device structures grown on top of BN and subsequent mechanical lift-off from the entire substrate present an exceptional opportunity to realize high-efficiency and low-cost next generation device structures on any platform. [1] X. Li, S. Sundaram, Y. El Gmili, T. Ayari, R. Puybaret, G. Patriarche, P.L. Voss, J.P. Salvestrini, and A. Ougazzaden, Cryst. Growth Des. 16, 3409 (2016). [2] T. Ayari, S. Sundaram, X. Li, Y. El Gmili, P.L. Voss, and J.P. Salvestrini, Appl. Phys. Lett. 108, 171106 (2016).


Symposium organizers

17 av. des Martyrs 38054 Grenoble, France
Matthias BICKERMANNLeibniz Institute for Crystal Growth (IKZ)

Max-Born-Str. 2, 12489 Berlin, Germany
Piotr PERLINInstitute of High Pressure Physics "Unipress", Polish Academy of Sciences

Sokolowska 29/37 Str. 01-142 Warsaw, Poland
Ramon COLLAZONorth Carolina State University

1001 Capability Dr. Raleigh, NC 27695-7919, USA