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2015 Fall

Materials for electronics and optoelectronic applications away from silicon.


Nitride semiconductors for high power and high frequency electronic devices

The symposium “Nitride semiconductors for high power and high frequency electronic devices “ focuses on nitride-based high power devices and challenges related to electronic transportation, energy efficiency, terahertz emission, and advanced substrates for such application. Physical properties of GaN make this wide-bandgap material very attractive for microelectronics, optoelectronics, and solar applications.

Today, it is well acknowledged that, thanks to its very high breakdown field, high saturation velocity, high electron mobility, and respectable thermal conductivity, GaN material is a revolution in the semiconductor sector. The deployment of nitride technologies is a key issue for Europe to strengthen its competitiveness while addressing societal challenges on transportation, energy efficiency, and renewable energy. Manufacturing capabilities have been limited by the compatibility of the wafer with silicon production environment: diameter, cost, handling, and material quality.

The scope of this Symposium covers the whole value chain of GaN-based devices i.e. from material & equipment to device makers, with a specific focus on advanced substrates and power devices as well as the creation of their production. The concept of this Symposium is to design the expected technological challenges for GaN-based electronics, such as:

  1. compatibility with silicon standard manufacturing device line: wafer damage or breaking issues due to wafer brittleness, stress, and non-standard carrier
  2. requirements for competitiveness: transition to 6” and 8” diameter, how to bring GaN material and technology to a level compatible with high volume manufacturing in terms of yield, robustness, and cost
  3. adequate device manufacturing: thermal management, high voltage, device manufacturing, and challenge of reliability (GaN defects, instability, growth temperature)
  4. GaN-based or GaN epi compatible substrates with high quality and performance in terms of defectivity, reliability, thickness, conductivity, manufacturability, diameter, and yield.

GaN has already been selected for LED and High Performance Solar Cell markets. This material is considered for CMOS nodes below 10 nm and is also particularly attractive for power applications in electronic devices operating at high temperatures, high power, very high frequencies and in a harsh environment. However, there are some barriers connected to GaN-based devices. The first one is the availability, as few GaN transistors are available in mass production. Competing manufacturers’ products are non-standard and there are no second-sources. Secondly the technology so far lacks maturity. An overall device performance and GaN material defect rates need improvement.


Hot topics to be covered by the symposium


  • Substrates for GaN based electronic devices
  • Epitaxy of GaN based structures for electronic applications
  • Progress in Schottky diodes based on GaN
  • Progress in HEMTs based on nitride semiconductors
  • Progress in terahertz devices based on nitride semiconductors


Tentative list of invited speakers


  • P. Guenard (Soitec) “GaN based Advanced Substrates for electronic applications”
  • M. Zając (Ammono) “Ammonothermally grown GaN substrates for electronic applications”
  • P. Coppens (ONSEMI) “GaN-on-Si MISHEMTs in the new 6" pilot line”
  • A. Torres (CEA LETI) “AlGaN/GaN HEMT on GaN-on-Si for power applications”
  • E. Galván (GPTech) “Electric and thermal specifications of GaN power devices used in photovoltaic applications”
  • R. Rodriguez (IUMA) “Numerical simulation and compact physical modeling of AlGaN/GaN power HEMTs”
  • T. Mrotzek (Plansee) “Composite materials for inverse heat sinks”
  • T. Sochacki (TopGaN) “Current status of the HVPE-GaN growth”
  • M. Iwinska (IWC PAN) “HVPE-GaN growth on Smart CutTM substrates”
  • M. Germain (EpiGaN) “GaN growth on Si for RF applications"

Tentative list of invited speakers


  • A. Piotrowska (IET, Poland)
  • T. Skotnicki (WUT, Poland)
  • T. Dietl (IP PAS, Poland)
  • R. Dwilinski (UW, Poland)
  • Z. Sitar (NCSU, USA)
  •  J. Freitas (NRL, USA)
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09:00 Opening remarks    
Authors : Eric Butaud
Affiliations : Soitec SA; STMicroelectronics Tours SAS; ON Semiconductor Belgium BVBA; Green Power Technologies; Plansee SE; EV Group; CEA-Leti; TopGan Lasers; Institute of High pressure Physisc PAS; Information and Communication Systems Institute for Applied Microelectronics University of Las Palmas GC

Resume : The Gallium Nitrides materials development opens opportunities in many application fields, at the cross roads of major technology and societal challenges on transportation, energy efficiency, and renewable energy, where its wide bandgap properties bring significant breakthrough opportunities for Power and photonics applications. Nevertheless, many technology barriers to enable industrial scale exploitation of these materials are still to be broken and limit their implementation. The AGATE program (Development of Advanced GAllium Nitrides substrates and TEchnologies) is a European based initiative which aims to help the introduction and market acceptance of GaN based technologies, answering these challenges, and demonstrate that innovative GaN-based advanced substrates and devices can be manufactured in a standard process line cost-competitive level 150 mm, opening the path to 200 mm. The technology developed in AGATE covers the full value chain, from material supplier to end user, and aims to demonstrate not only that advanced substrates material can achieve quality target through industrial capable processing, but also demonstrates its application through GaN based Power technology development from material to final system through design and device, as well as Laser Diodes demonstration.

Growth I : Michal Bockowski
Authors : Raphaël CAULMILONE, Pascal GUENARD
Affiliations : SOITEC, Parc technologique des Fontaines, 38190 Bernin, France

Resume : GaN based Advanced Substrates by Smart-Cut™ Raphaël CAULMILONE, Pascal GUENARD SOITEC, Parc technologique des Fontaines, 38190 Bernin, France The excellent optical an electrical properties of gallium nitride make it a very attractive material for optoelectronic applications such as LED and lasers as well as for high power and high frequency switching devices. However, lack of closely lattice matched substrates suitable for high temperature epitaxy makes crystal growth difficult. Development of complex buffer layers on SiC, sapphire and eventually on silicon wafers is necessary to achieve high crystal quality, hindering devices development. Additionally those buffer layers are usually of no functional interest for the device itself and are even removed completely in the case of vertical devices. Device processing must also face issues associated with bowing of wafers for the CTE of epitaxy substrate is significantly different from bulk GaN. A peculiar case of epitaxy substrate is the GaN freestanding wafer developed by different companies which is obviously lattice and CTE matched to GaN, can reach very low threading dislocations density and meets requirements for epitaxy processes but low availability and high cost prevented its large scale adoption. Soitec’s Smart-Cut™ technology can be used to alleviate some of those difficulties when applied to GaN. It allows transfer of a thin GaN layer of arbitrary good quality on a carrier wafer whose properties might be chosen independently of lattice parameter, giving more degrees of freedom to build an expitaxy substrate. Since the first demonstration[1] of GaN film transfer by Smart-Cut™, processes have been developed and allow today to produce thin (50nm to 500nm thick) GaN films on a wide variety of carrier wafers. GaN crystal quality has been shown to be maintained and in particular no additional threading dislocation is introduces through Smart-Cut with ion implantation as shown by HR-TEM and confirmed by cathodo-luminescence after regrowth of an LED type of structure by MOCVD. Hence the crystal quality of the engineered substrate depends only of the starting material crystal quality and will show the same extended crystal defects. Smart-Cut™ process has been applied to different GaN starting materials, from conventional GaN templates on sapphire with TDD ranging from 106 to 109/cm² to free standing material from different companies, mainly with c-plane wafers but was also successfully applied to r-plane samples. In the case of GaN templates, due to the wafer transfer process, crystal polarity is flipped and hence temporary bonding of the film in order to transfer on the final carrier wafer. This process is so today applied on 100mm wafers and is easily scalable to 150mm. Alternatively, when using free-standing wafers, the N-face might be used in order to generate Ga-face up engineered substrates which has been demonstrated up to 150mm as well. Regarding carrier wafers, different material have been tested and today baseline processes are setup for transfer on sapphire, molybdenum wafers and composite ceramic wafers. Those two last materials have been chosen for their good CTE matching to GaN and the possibility of easy substrate removal. CTE matching allows to keep wafer bow low enough to ease device processing and the good thermal conductivity of molybdenum might be a benefit for across wafer uniformity of e.g. InGaN composition. Easy substrate removal is also thought to help vertical device processing to reach high yield with robust processes. By carefully choosing the combination of GaN starting material and carrier wafers, engineered GaN substrates might help improving device performance and/or achieving higher processing yield. [1] A. Tauzin et al., Electronics Letters 41, 11 (2005).

Authors : v. Dragoi*, N. Razek*, C. Fl?tgen*, R. Caulmilone**, P. Guenard**
Affiliations : *EV Group, Austria; **Soitec, France

Resume : Various wafer bonding processes were developed over time in order to accommodate specific needs in terms of applications. The types of interactions between two solid surfaces placed into contact involve molecular bonds (direct or fusion bonding), building of a bonding layer during process (anodic bonding), atom bonds (metal thermo-compression bonding), alloying (eutectic or TLP/intermetallic bonding), or gluing using glass paste material (glass frit bonding) or various polymer materials (adhesive bonding with UV or thermally cured polymers). Direct wafer bonding (named also fusion or molecular bonding) is often the process of choice for substrates manufacturing. In this process first the two substrates are placed into contact (usually at ambient conditions) in order to obtain the adhesion due to molecular bonds formed between the two substrates. As the adhesion occurred at room temperature is weak, a thermal annealing is needed as a source of energy to transform the weak room temperature bonds into strong covalent bonds. Thermal steps have a major influence on wafer bonding applicability not only due to the obvious reason that some materials exhibit a limited temperature to which can be exposed without degrading or oxidizing, but also due to the thermal expansion. The stress between the different materials would increase due to different thermal expansion with increasing the temperature. Being a material property, the thermal expansion cannot be avoided but by minimizing the

10:45 Coffee break    
Growth II : Matthias Bickermann
Authors : M. Zając 1, R. Kucharski 1, A. Puchalski 1, J. Krupka 2
Affiliations : 1 Ammono S.A., Prusa 2, 00-493 Warsaw, Poland;2 Institute of Electronics and Microelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland

Resume : Highly resistive bulk GaN substrates are demanded for microwave electronics and electronic devices operating at high voltage (above 1000V). In this communication we show ammonothermal method of highly resistive GaN substrates manufacturing. It uses supercritical ammonia for dissolution of feedstock material and crystallization of GaN on native seeds due to convection-driven transport and supersaturation of the solution. We present two types of such material: 1) substrates of max. 2-inch diameter, in which oxygen donors are compensated by shallow Mg acceptors (type-I), 2) substrates doped with deep acceptors in form of transition metal ions (type-II). In these substrates the oxygen concentration was about 1x1018 cm-3. Both types of material show dielectric properties, with resistivity of at least 1011 Ωcm and outstanding crystallographic quality. This will enable an efficient production of high-power, high-frequency devices based on GaN homoepitaxy. Type-I 1-inch and 1.5-inch substrates were successfully used in fabrication of High Electron Mobility Transistors operating at frequency of 6 GHz [1]. [1] A. Taube et al, Physica Status Solidi (a), DOI: 10.1002/pssa.201431724. Acknowledgements: This work was partially supported by the PolHEMT Project under the Applied Research Programme of the National Centre for Research and Development, Contract Number PBS1/A3/9/2012 and European Space Agency PECS project “Low dislocation Gallium Nitride for space applications”, contract number 4000108320/13/NL/KML.

Authors : T. Sochacki [1,2], M. Amilusik [1,2], M. Fijalkowski [1], B. Lucznik [1,2], M. Iwinska [1], G. Kamler [1], R. Kucharski [3], I. Grzegory [1,2], M. Bockowski [1,2]
Affiliations : [1] Institute of High Pressure Physics PAS, Sokolowska 29/37, 01-142, Warsaw, Poland; [2] TopGaN Sp. z o.o., Sokolowska 29/37, 01-142, Warsaw, Poland; [3] Ammono S.A. Prusa 2, 00-493 Warsaw, Poland

Resume : It was shown that the ammonothermally grown GaN crystals (Am-GaN) can be successfully used as seeds for the Hydride Vapor Phase Epitaxy (HVPE). Crack-free and up to 2-mm-thick HVPE-GaN layers were obtained. Free-standing (F-S) HVPE-GaN crystals sliced from Am-GaN seeds show high structural as well as optical, electrical, and thermal qualities. Since the structural properties of the F-S HVPE-GaN crystals are not different from the excellent structural properties of the Am-GaN seeds, F-S HVPE-GaN can be successfully used as a seed for further HVPE-GaN growth. It is feasible to multiply the ammonothermally grown GaN by the HVPE technology. The F-S HVPE-GaN crystals can also be used as seeds for the ammonothermal growth. In this paper the state of the art of HVPE-GaN growth on Am-GaN seeds will be demonstrated. Particular attention will be paid to the growth rate and its influence on the structural quality and purity of the HVPE-GaN layers. New directions in the development of the HVPE-GaN growth will be presented. The main goal for today is to develop a method of doping by donors and acceptors. It seems that due to the high purity of the HVPE-GaN, the free carriers can be compensated at a very low level of doping. Thus, high-quality HVPE-GaN with a high resistivity should easily be obtained. On the other hand, high-quality HVPE-GaN with the free carrier concentration of the order of 5x1018 cm-3 should also be crystallized.

Authors : Patrick Hofmann1, Franziska C. Beyer2, Christian Röder3, Günter Gärtner4, Frank Habel5, Gunnar Leibiger5, Berndt Weinert5, Stefan Eichler5, Johannes Heitmann2, Thomas Mikolajick1,6
Affiliations : 1 NaMLab gGmbH, Nöthnitzer Str. 64, D-01187 Dresden, Germany; 2 Institute of Applied Physics, TU Bergakademie Freiberg, Leipziger Straße 23, D-09599 Freiberg/Sa., Germany; 3 Institute of Theoretical Physics, TU Bergakademie Freiberg, Leipziger Straße 23, D-09599 Freiberg/Sa., Germany; 4 Institute of Experimental Physics, TU Bergakademie Freiberg, Leipziger Straße 23, D-09599 Freiberg/Sa., Germany; 5 Freiberger Compound Materials GmbH, Am Junger-Löwe-Schacht 5, D-09599 Freiberg/Sa., Germany; 6 TU Dresden, Institute for Semiconductors and Microsystems, D-01062 Dresden, Germany

Resume : High-end electronic products based on GaN intensify the need for electrically conducting material with high crystalline quality. Hetero-epitaxial growth is widely used to supply start templates for the HVPE process. The templates exhibit a high threading dislocation density (TDD) in the range of 10^8–10^9 cm-2, which may have detrimental effects on the devices. TDs act as scattering centres when breaking through functional layers and they lead to cracking of the GaN films grown on the foreign substrate. The latter is an important issue for silicon doping to achieve n-type GaN. Incorporated Si interacts with TDs, causing them to tilt and grow in an inclined manner. This inclined growth is known as effective climb and creates tensile strain in the material. Thus the TDD reduction is necessary to avoid tensile strain generation. In this study, in order to improve crystal quality whilst reducing the TDD, facet-controlled lateral overgrowth seeds have been used as starting templates additionally to an un-doped GaN layer growth prior to the doping experiments. The resulting doped crystals exhibit excellent crystalline quality, confirmed by HR-XRD. Strain performance and charge carrier distribution were studied by confocal Raman spectroscopy, FT-IR Spectroscopy and RT-Hall-measurements. For investigation of deep traps related to the Silicon incorporation DLTS investigations will be presented as well as the study of minority carrier behaviour using UV-light excitation.

12:30 Lunch break    
Epitaxy and Devices I : David Meyer
Authors : Marianne Germain
Affiliations : EpiGaN nv, Kempisceh Steenweg, 293, B-3500 Hasselt, Belgium

Resume : The key advantage of GaN-on-Si material resides in the possibility to develop III-N heterostructures on large diameter substrates, making this technology the most cost-efficient for electronic applications. Highly uniform, low bow, crack-free GaN-on-Si HEMTs structures on wafer diameters up to 200mm have been grown with specifications suited for high performance devices. The key material challenge, with larger and larger wafer diameter, remains the strain engineering in the hetero-epitaxial stack, especially when addressing high voltage operation (650V). Different buffer stacks are required depending also on the final applications (capacitive coupling, leakage current, breakdown voltage…). Higher performance devices are further enabled by the deposition of SiN passivation layer, in-situ grown by MOCVD. This in particular allows for the formation of 2DEG with a very thin (6nm) AlN binary barrier. These SiN/AlN/GaN structures grown on Si substrates exhibit very low sheet resistivity (< 300 Ohm/sq.). We will show latest results on 150 mm and 200mm wafer development for RF/mm wave applications, as well as for High Voltage (Vbkd> 1000V) applications.

Authors : E.Frayssinet1, J.Mohdad1, S.Latrach1,2, S.Chenot1, Y.Cordier1,*
Affiliations : 1. CRHEA-CNRS, rue B.Grégory, 06560, Valbonne, France. 2. LMON, Univ. Monastir, Av. de l’Environnement, 5000 Monastir, Tunisia.

Resume : In the present work AlGaN/GaN HEMTs have been grown by MOCVD with thin buffer layers on Si(111) substrates. Structures were grown with the same layer stack to check the influence of growth parameters. The growth conditions were kept unchanged for the SiN cap, the AlGaN/AlN barrier, the 0.2 µm AlN nucleation layer and the following 0.4 µm GaN buffer grown at low temperature to obtain a carbon rich resistive layer. The remaining part of the GaN buffer including the channel was grown using various conditions. The epilayers structural quality was assessed by XRD and by AFM which revealed some differences especially in case of excessive growth pressure or N/III ratio. CV measurements with a Mercury probe as well as on Schottky diodes revealed differences in the pinch-off regime of the 2DEG located at the AlN/GaN interface. Except in one case, the leakage current between isolated devices confirmed this trend. For the majority of theses structures, Hall effect produced sheet carrier densities of 1xE13/cm² and electron mobility between 1100 and 1400 cm²/V.s depending on the GaN channel growth conditions. The output and transfer characteristics (drain current, transconductance and leakage currents) of the transistors are in agreement with the previous electrical characterizations. Thanks to the combination of structural and electrical characterizations we are then able to determine the optimized growth conditions for such HEMT structures.

Authors : Tomasz Szymański, Mateusz Wośko, Bogdan Paszkiewicz, Regina Paszkiewicz
Affiliations : The Faculty of Microsystem Electronics and Photonics, Wrocaw University of Technology, Janiszewskiego 11/17, 50-372 Wroclaw, Poland

Resume : Due to the lack of large diameter and inexpensive GaN substrates, the growth of GaN-based multilayer structures is commonly carried out on sapphire or SiC. Another heterosubstrate widely used which comprises such properties as low price, availability of large diameters, good thermal and electrical conduction, is silicon. Application of Si substrates can also enable opportunity toward an integration with Si electronics. Along with benefits of using silicon as a substrate there comes many issues that were already attempted to be either eliminated or minimized. Those issues exist mainly due to large lattice mismatch between GaN and Si, and significant difference in thermal expansion coefficients of materials under discussion. Till now, most successful growth of GaN-based structures were performed on (111) oriented Si substrates. The cause of that is attributed to hexagonal like in-plane surface atom arrangement which is called three fold symmetry. Use of Si(111) substrates leads to a GaN c-axis oriented growth. Motivation of GaN growth on Si(11x) other than (111) is control of large spontaneous polarization fields oriented along the hexagonal c-axis that come from non-centrosymmetric nature of GaN compound and presence of polar faces in wurtzite structure. Additionally, one has to take into account that III nitrides are strong piezoelectrics materials, nature of which will intensely influence overall polarization field strength. GaN C-axis inclination from perpendicular direction in regard to substrate surface should reflect the angle between planes of (111) and (11x). In this work several approaches will be presented that lead to GaN growth on Si substrates of chosen orientation. MOVPE epitaxial growth was performed on 2'' Si(111), Si(112), Si(115) substrates simultaneously using a 3x2'' Close Coupled Showerhead MOCVD system. The results for various growth procedures applied will show uncoalesced and coalesced layers and discuss orientation of grown GaN-based multilayer structures. Also theoretical calculation of AlGaN/GaN 2DEG charge in dependence of C-axis inclination will be presented.

15:15 Coffee break    
Devices I : Marianne Germain
Authors : David J. Meyer [1], Brian P. Downey [1], D. Scott Katzer [1], David F. Storm [1], Jason A. Roussos[1], Mario G. Ancona [1], Ming Pan [2], and Xiang Gao [2]
Affiliations : [1] - Naval Research Laboratory, Washington, DC, USA; [2] - IQE RF LLC, Somerset, NJ, USA

Resume : After two decades of international research effort directed towards the advancement of GaN high-electron-mobility transistor (HEMT) technology, microwave solid-state power amplifiers based on GaN are beginning to be widely adopted in commercial and defense markets. With reliable RF performance demonstrated at lower frequency bands (< 12 GHz), there has recently been a push in the research community to seek higher frequency performance in GaN devices by geometrically scaling HEMT dimensions and minimizing parasitics. In order to achieve RF power gain in the millimeter wavelength (MMW) range of 30 - 300 GHz, HEMT device design cannot rely on conventional methods for electric field management, such as source-connected field plates, as they introduce excessive parasitic capacitance and delay into the device. Instead, T-shaped gates with critical dimensions less than 100 nm are used to achieve short intrinsic device electron transit times while still maintaining low gate resistance. To help mitigate electric-field-induced failure susceptibility while retaining the requisite electrostatic charge control of the channel in MMW GaN HEMTs, we have performed several studies involving different AlN and InAlN barrier designs along with the use of various gate insulators. Initial evaluation of the RF operational lifetime reliability of these MMW devices has shown a strong dependence on peak electric field magnitude. In this talk, we will discuss the elements of MMW GaN HEMT design and how they impact device electrical performance and reliability.

Authors : Pawel Prystawko
Affiliations : Institute of High Pressure Physics, PAS Sokolowska 29/37, 01-142 Warsaw, Poland and TopGaN Ltd, Sokolowska 29/37, 01-142 Warsaw, Poland

Resume : Wide bandgap semiconductor system of AlGaN/GaN has a number of features important in manufacturing of electronic devices: strong interatomic bonds, high electron mobility, high electron saturation drift velocity, high critical electric field, as well as thermal stability. Excellent performance in high power, high frequency devices, as well as in power switching has been already demonstrated in planar devices with 2DEG. However, these devices are still far from the GaN/AlGaN material system physical limitations [1]. In my lecture, I will present epitaxial growth and physical properties of HEMTs grown on semi-insulating Ammonothermal GaN substrates [2,3]. Important growth technology steps including MOCVD regrowth interface, insulating buffer, high mobility GaN channel as well as ohmics regrowth will be addressed. Due to very low defect density, these structures can be used to further explore possible advances of high temperature operation, high power density and lowered degradation rate in comparison to standard heteroepitaxal devices. Defect-free advanced substrates allow also for improvement of reliable high power-loaded devices at high frequency, what is attractive for harsh environment/space niche application regardless expensive and limited availability so far. [1] T. Palacios, PSS A, vol. 203, no. 7, pp. 1845 – 1850, May 2006. [2] R. Dwilinski, JCG 310, 3911 (2008). [3] Polish National Centre for Research and Development, PolHEMT Project, Contract number PBS1/A3/9/2012

Authors : A. Georgakilas1,2, A. Adikimenakis1, Ch. Zervos1,2, A. Bairamis1,2, K. E. Aretouli1, K. Tsagaraki1, A. Kostopoulos1, A. Stavrinidis1, E. Tsikritsaki1,2, E. Iliopoulos1,2, and G. Konstantinidis1
Affiliations : 1. Microelectronics Research Group, IESL/FORTH, P.O. Box 1385, 71110 Heraklion, Greece 2. Physics Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece

Resume : Our research targets the physical understanding and technological development of novel HEMT structures based on III-nitride compounds and nano-scale growth control that plasma-assisted molecular beam epitaxy (PAMBE) can provide advantageously. This is the case of HEMT structures with InN channel or AlN barrier, as well as non-conventional HEMT heteroepitaxial materials with thin buffer layer or on novel substrate such as single crystal and polycrystalline diamond. These HEMTs address the standard application fields of GaN HEMTs in RF and power switching electronics, but may also serve new application areas such as sensors and logic. The AlN/GaN heterojunction offers the highest polarization discontinuity, which can be exploited for the realization of high conductivity and ultra-shallow GaN HEMT channels. We will report on the optimization of the growth and design of double barrier AlN/GaN/AlN HEMT structures, with thin 200 nm GaN/ 300 nm AlN buffer layer on sapphire substrate. Initially, the heteropitaxy of AlN-on-sapphire was optimized. Then we compared the electronic properties and device characteristics of HEMT structures with AlN top barrier thickness, varied in the range 1.5-4.5 nm. The maximum drain current was lower in comparison to HEMTs with thick GaN buffer layer, as a result of the negative polarization charge at the bottom of the AlN/GaN interface. However, 1 m gate length devices exhibited maximum drain-source current up to 1.4 A/mm in pulsed I-V measurements. The HEMT devices were not passivated but exhibited rather limited gate and drain lag, of 6-12% and 10-18%, respectively and a small increase of threshold voltage. The high thermal conductivity of diamond substrates makes them ideal substrates for GaN power devices. However, several problems must be overcome in the heteroepitaxy of GaN on diamond. A critical step was to achieve Ga-polarity (0001) heterostructures independently of the crystallographic orientation of the diamond surface, and this was accomplished by optimizing the growth and thickness of the AlN nucleation layer. HEMT devices were fabricated from AlN/GaN heterostructures, on both single crystal and polycrystalline diamond substrates. Transistors with 1 µm gate length, grown on polycrystalline diamond, exhibited drain source current of 340 mA/mm at VGS= +3V and maximum transconductance of 85 mS/mm. The results were inferior compared to other substrates but proved the feasibility of using epitaxial GaN-on-diamond material. Acknowledgement: This work is carried out within the ARISTEIA “NITROHEMT” and KRHPIS "PROENYL" projects, co-funded by European and National resources. The GaN-on-diamond work was initiated in the European NMP project "MORGAN".

Authors : Ankush Bag, Partha Mukhopadhyay, Dhrubes Biswas
Affiliations : Advance Technology Development Centre, Indian Institute of Technology Kharagpur

Resume : The AlGaN and GaN traps have contributed individually on 2DEG confined at the heterointerface. Suppression of third order Intermodulation Modulation Distortion (IMD3) is necessary as it signifies the nonlinearity of the devices at microwave frequency. The normalized ratio of gm2 and gm indicates the generated IMD3 for the device signifying its inherent non-linearity. The positive shift of threshold voltage of Si(111) diodes describes more effect of electron trapping on GaN side of the well. Biasing range of the IMD3 spectrum has been determined considering the CV profile for both heterostructures. The normalized spectrum of IMD3 delineates more distorted terms at depletion region with negative gate bias in case of HEMT on Si as compared to the HEMT on Sapphire. Due to both high lattice and thermal expansion coefficient mismatch growth of GaN on Si, there is more effect of threading dislocations as compared to GaN on Sapphire. These dislocations cause for more traps and related distortion for HEMT on Si. Additionally, 2DEG at the heterojunction is function of electron wave function in quantum well as per Schrodinger’s equations. The wave functions generally penetrate more into comparatively low energy GaN epilayers than AlGaN. Therefore, effects of trapped electrons on confined 2DEG are more for GaN side of the triangular quantum well than AlGaN barrier. The results indicate effect of traps in GaN epilayer profoundly for HEMT on Si as compared HEMT on Sapphire substrates.

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Growth III : Ramon Collazo
Authors : M. Iwinska [1], M. Amilusik [1,2], M. Fijalkowski [1], T. Sochacki [1,2], B. Lucznik [1,2], A. Nowakowska-Siwinska [2], I.Grzegory [1], P.Guenard [3], R. Caulmilone [3], M. Seiss [4], T. Mrotzek [4], M. Bockowski [1,2]
Affiliations : [1] Institute of High Pressure Physics PAS, Sokolowska 29/37, 01-142 Warsaw, Poland; [2] TopGaN Sp z o.o., Sokolowska 29/37, 01-142 Warsaw, Poland; [3] Soitec, Parc Technologique des Fontaines, 38190 Bernin, France; [4] PLANSEE SE, 6600 Reutte, Austria

Resume : Smart Cut™ technology is based on transferring a very thin single crystalline layer from a substrate onto a different handler. This method is successfully used in case of silicon and one of the results is a Silicon-On-Insulator (SOI) engineered wafer. This technology enables fabrication of low-cost, large-area, and high-quality wafers. Currently, attention is drawn to obtaining substrates with a thin layer of gallium nitride for the purpose of GaN-based electronic and optoelectronic devices. Results of HVPE-GaN growth of GaN layers on Advanced Substrates prepared with Smart Cut™ technology will be presented. The substrates consist of handler material (Sapphire or Molybdenum), a bonding layer, and a transferred, about 200-nm-thick Ga-face GaN layer. An approach analogous to growth on MOCVD-GaN/Sapphire templates engaging a technique similar to Void Assisted Separation (VAS) was applied. It is possible to obtain high-quality and crack-free HVPE-GaN crystals on such templates. The process of self-lift-off together with an influence of nitridation time in such growth was analyzed in detail. HVPE crystallization runs performed on Advanced Substrates in different conditions will be discussed. Structural properties of layers obtained on photolitographically patterned Ti masks will be presented. Results of characterization by X-ray diffraction, scanning electron microscopy, defect selective etching, photo-etching, and secondary ion mass spectrometry of the grown HVPE-GaN crystals will be included. Finally, up to 1-mm-thick and crack-free layers together with their properties will be demonstrated.

Authors : Toru Kinoshita1*, Toru Nagashima1, Toshiyuki Obata1, Shinya Takashima2, Reo Yamamoto1, Rie Togashi3, Yoshinao Kumagai3*, Raoul Schlesser4, Ramόn Collazo5, Akinori Koukitu3, and Zlatko Sitar5
Affiliations : 1Tsukuba Research Laboratories, Tokuyama; Corporation, Tsukuba, Ibaraki 300-4247, Japan; 2Fuji Electric Co., Ltd., Hino, Tokyo 191-8502, Japan; 3Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan; 4HexaTech, Inc., Morrisville, NC 27560, U.S.A.; 5Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A.

Resume : Aluminum nitride (AlN) has a wide bandgap of 6.1 eV, high thermal conductivity of 3.2 Wcm−1K−1, and high temperature stability. These unique physical characteristics make AlN an attractive material for high power electronic devices, as well as for deep-UV light emitting devices. Although several groups have reported on deep-UV light-emitting diodes on the AlN substrates, there are very few reports concerning electronic devices on AlN substrates due to the lack of n-type conductive AlN substrates. In this work, we report on the hydride vapor phase epitaxy (HVPE) growth of AlN substrates and their n-type conductivity. In addition, fabrication of vertical Schottky barrier diodes (SBDs) on AlN substrates is demonstrated. Thick AlN:Si layers were homoepitaxially grown by HVPE on c-plane AlN seed substrate prepared by physical vapor transport (PVT). Decomposed Si from quartz was used as n-type dopant. Full width at half maximum values of X-ray ω-rocking curves for (002) and (101) reflections were less than 30 arcsec, respectively, which were comparable to those of the PVT-AlN substrates. Electron concentration at room temperature for AlN:Si with Si concentration of 3 × 1017 cm-3 was 2.4 × 1014 cm-3, which was close to that of the AlN thin film grown by MOCVD. Vertical Schottky barrier diodes with Ni/Au Schottky contacts were fabricated on freestanding HVPE-AlN:Si substrates. A high rectification ratio was observed with a turn on voltage of approximately 2.2 V.

Authors : M. Bickermann, C. Hartmann, A. Dittmar, F. Langhans, S. Kollowa, T. Schulz, M. Naumann, A. Kwasniewski, K. Irmscher, J. Wollweber
Affiliations : Leibniz Institute for Crystal Growth (IKZ) Berlin, Germany

Resume : Single-crystalline aluminum nitride (AlN) is a promising substrate material not only for AlGaN epilayers with high Al content, e.g. for solid-state deep-UV optoelectronics, but also for high temperature and high power applications. This presentation gives an overview of the status of AlN substrate preparation and discusses perspectives and challenges for GaN/AlGaN-based high power electronics. AlN bulk single crystals are grown by the physical vapor transport (PVT) method at temperatures well above 2000°C. Crystals of high structural perfection (dislocation densities < 10^4 cm^-2) can be grown using AlN single crystal wafers as seeds. However, proper control of the PVT growth process is made difficult due to gradual changes of the reactor materials caused by attack of gaseous Al and unintentional incorporation of impurities (O, C, Si) into the growing crystals during growth. The latter determine the electrical, thermal, and optical properties of bulk AlN substrates. In turn, these properties can be adjusted at least partially by providing proper growth conditions or by employing doping. We will present and discuss preparation of bulk AlN substrates for power electronics which are electrically semi-insulating even at temperatures beyond 1000°C, ones that provide weak n-type conductivity (n = 1.2 10^15 cm^-3, µ = 36.5 cm^2/Vs) at room temperature by Si doping, and ones that exhibit transmission in the deep-UV wavelength range (for optical applications).

10:45 Coffee break    
Epitaxy and Devices II : Benjamin Damilano
Authors : R. Collazo[1], P. Reddy[1], B. Haidet[1], I. Bryan[1], Z. Bryan[1], F. Kaess[1], L. Hernandez-Balderrama[1], M. Bobea[1], J. Tweedie[2], R. Kirste[1,2], S. Mita[3], T. Sochacki[4], M. Bockowski[4], E. Kohn[1], and Z. Sitar[1]
Affiliations : [1] Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, USA; [2] Adroit Materials, Apex, NC 27539, USA; [3] HexaTech, Inc., 991 Aviation Pkwy, Suite 800, Morrisville, NC 27560, USA; [4] Institute of High Pressure Physics PAS, Sokolowska 29/37, 01-142 Warsaw, Poland

Resume : Based on Baliga’s FOM, GaN, AlN, and AlGaN-based Schottky diodes are expected to be superior to SiC by a factor of 6 to 200. Advances in native substrates have led to the possibility of vertical devices with low dislocation densities, thus approaching the materials’ ultimate performance. After effectively removing dislocations, new material challenges become apparent: epitaxy morphology on native substrates, point defect control, and surface manipulation. These influence the development of these switches as they determine: thickness and composition, carrier concentrations and mobility at the contact and drift layers, and Schottky barrier and its passivation. Onset of three main surface morphologies was found to depend on the substrate miscut with characteristic differences between AlN and GaN, as determined by surface kinetics. These differences become significant for AlGaN, determining composition and lateral uniformity. Point defect control leads to the achievement of the necessary high carrier concentrations in the back contacts while allowing for controllable low carrier concentrations in the drift layers; a carrier concentration of 2x1016 cm-3 with a mobility of 1100 cm2/Vs was obtained for GaN on sapphire. Homoepitaxial GaN on HVPE/ammonothermal GaN substrates shows a narrow PL DBX peak of 100 μeV, suggesting the higher quality of this material. Results on the performance and further limitations of Schottky diodes based on these materials achievements will be discussed.

Authors : P Kruszewski 1,2, J Jasinski 3, T Sochacki 1,2, M Bockowski 1,2, R Jachymek 2, P Prystawko 1,2, M Zajac 4, R Kucharski 4 and M Leszczynski 1,2
Affiliations : 1 Institute of High Pressure Physics UNIPRESS, Warsaw, Poland 2 Top-GaN Sp. z o.o., Warsaw, Poland 3 The Institute of Microelectronics and Optoelectronics, Warsaw Technical University, Poland 4 Ammono SA, Warsaw, Poland

Resume : In this study, we demonstrate high voltage Schottky barrier diodes grown on highly doped n-type GaN substrate (~1x1E19 cm-3) obtained by ammonothermal method. Such Ammono-GaN crystals are characterized by dislocations density as low as 1E4 cm-2. In our SBDs, thick GaN layers (~150 μm) grown by Hydride Vapor Phase Epitaxy (HVPE) growth technique on Ammono-GaN substrate reproduce this low dislocation density ensuring simultaneously high uniformity, purity and smoothness of GaN film. Moreover, our diodes have been designed in a vertical current flow config-uration, what should also significantly improve the current spreading in the structure leading to more efficient performance and lower power dissipation. First, prior to SBD device fabrication, the HVPE grown GaN layers have been examined elec-trically by means of Hall effect studies. Typically measured values of electron concentration and mo-bility for our structures were in range of 2-8x1E16 cm-3 and 1100-800 cm2/Vs, respectively. The aver-age HVPE-GaN resistivity was around 3x1E-1 Ω*cm. The electrical studies have shown the huge potential in such vertical GaN-based Schottky diodes with the breakdown voltage as high as 700 V and typically around 400 V. Additionally, our results are quite impressive regarding breakdown voltage when one can take into account the fact that there was no any edge termination in our structures such as field plates, guard rings or implantation.

Authors : Boris N. Feigelson, Jordan D. Greenlee, Travis J. Anderson, Jennifer K. Hite, Karl D. Hobart, Fritz J. Kub
Affiliations : Boris N. Feigelson US Naval Research Laboratory; Jordan D. Greenlee National Research Council; Travis J. Anderson US Naval Research Laboratory; Jennifer K. Hite US Naval Research Laboratory; Karl D. Hobart US Naval Research Laboratory; Fritz J. Kub US Naval Research Laboratory

Resume : P-type dopant implantation and activation adds additional complexity to the synthesis of p-type GaN. Advanced annealing processes, such as the Multicycle Rapid Thermal Annealing (MRTA) are required for Mg activation. The MRTA process consists of two separate, successive steps. Both steps are conducted at elevated nitrogen pressure of 25 atm. First, the implanted GaN is annealed conventionally at temperatures at which the capped GaN is stable on the order of 10’s of minutes. This step allows partial restoration of the GaN lattice damaged by implantation, preparing a more stable crystal structure for the next step. In the second step, the annealing temperature is repeatedly pulsed to temperatures above 1000˚ C, accumulating the time GaN is exposed to these high temperatures. In this paper, we introduce a modified MRTA process allowing the realization of a PIN diode using Mg implantation in GaN. The modified MRTA process includes additional step of conventional annealing after multiple rapid heating cycles. It is shown that this step is necessary to remove stresses and defects introduced by the rapid heating and cooling of the GaN. The new modified MRTA process consists of two conventional annealing steps 1 and 3, with the step 2 of rapid heating and cooling pulses in between, thus it is named as symmetrical multicycle rapid thermal annealing (SMRTA). The improvement in crystal quality provided by the SMRTA will be a key enabling step for future Mg-implanted devices.

12:30 Lunch break    
Material and Device Characterization I : Henryk Teisseyre
Authors : M. Seiss, T. Mrotzek, M. Iwinska, M. Bockowski, W. Knabl
Affiliations : Plansee SE; Plansee SE; Institute of High Pressure Physics; Institute of High Pressure Physics, TopGaN; Plansee SE

Resume : Gallium nitride (GaN) is a promising semiconductor for high frequency-, power- and opto-electronics. For its further success not only new types of substrates but also new solutions in the field of thermal management are required. This work is introducing pure molybdenum (Mo) and molybdenum-copper-composites as promising candidates for the named applications, with a focus on the thermophysical properties. The growth of large GaN crystals with high quality is still a challenging task for crystal growers. Therefore another approach is pursued by growing GaN layers on advanced substrates [1]. The deposition of GaN layers is taking place at temperatures higher than 1000°C. The subsequent cooling requires a substrate with an adapted coefficient of thermal expansion (CTE). If the CTE mismatch is too large, wafer breakage or cracking of the deposited layer will occur. The results of the CTE measurements of GaN and Mo presented in this work show that the CTE mismatch between both materials is < 1.5 ppm/K between room temperature and 800°C. Furthermore, the mismatch declines with increasing temperature. Therefore, Mo is a highly interesting candidate as advanced substrate for GaN growth. On the device level of GaN electronics, new approaches for the thermal management are required. The thermal conductivity of GaN (226 W/m/K at RT [2]) is significantly higher compared to current standard semiconductors, such as silicon (156 W/m/K at RT [3]) and gallium arsenide (48 W/m/K at RT [4]). For efficient heat dissipation, the thermal conductivity of the heat sink should be higher compared to the attached GaN die. In addition, the CTE must be adapted to both the GaN and the material of the package housing, e.g. Al2O3. Existing solutions for silicon and gallium arsenide cannot be employed because of the either low thermal conductivity or inappropriate CTE. Therefore, new materials are needed to meet the requirements of GaN devices. Molybdenum-copper multilayer composites can bridge the gap since their thermal conductivity and CTE can be tailored to the application demands. This is achieved by adapting the layer structure of the composite, i.e. layer thicknesses and ratios. Results of the thermal conductivity and CTE of GaN in direct comparison to several multilayer composites with varying layer structures are discussed. Our results show that the molybdenum-copper multilayer composites are suitable materials for GaN packaging. [1] Tomasz Sochacki, Zachary Bryan, Mikolaj Amilusik, Milena Bobea, Michal Fijalkowski, Isaac Bryan, Boleslaw Lucznik, Ramon Collazo, Jan L. Weyher, Robert Kucharski, Izabella Grzegory, Michal Bockowski, Zlatko Sitar, “HVPE-GaN grown on MOCVD-GaN/sapphire template and ammonothermal GaN seeds: Comparison of structural, optical, and electrical properties”, J. Cryst. Growth 394, pp. 55-60, 2014. [2] A. Jezowski, B.A. Danilchenko, M. Bockowski, I. Grzegory, S. Krukowski, T. Suski, T. Paszkiewicz, “Thermal Conductivity of GaN Crystals in 4.2 – 300 K range”, Solid State Commu. 128, pp. 69-73, 2003. [3] C. J. Glassbrenner, G. A. Slack, “Thermal Conductivity of Silicon and Germanium from 3 K to the Melting Point“, Phys. Rev. 134(4A), pp. 1058-1069, 1964. [4] J. S. Blakemore, “Semiconducting and other major properties of gallium arsenide”, J. Appl. Phys. 53(10), pp. 123-181, 1982.

Authors : V. Prozheeva 1, F. Tuomisto 1, H. Li 2, S. Keller 2, and U. K. Mishra 2
Affiliations : 1 Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 AALTO, Finland; 2 Electrical and Computer Engineering Department, University of California, Santa Barbara, CA, USA

Resume : High electron mobility transistors (HEMTs) based on III-nitrides grant higher 2D electron gas (2DEG) densities due to polarization fields intrinsic to the wurtzite structure [1]. Recently investigations were directed from Ga-polar towards N-polar (Al,Ga,In)N heterostructures. The N-polar layout is advantageous for enhancement mode and highly scaled transistors [2, 3]. However, unoptimized N-polar HEMTs suffer from large-signal dispersion and are sensitive to light due to donorlike traps within the device structure [3]. The negative impact of the traps can be mitigated by implementing an elaborate design combined with Si doping. We present results obtained by positron annihilation spectroscopy in N-polar S.I. GaN/AlGaN:Si/AlGaN/GaN HEMTs with graded backbarrier design and different Si doping where the valence band is below the Fermi level. These data are compared to measurements of a simplified HEMT structure where traps formed at the S.I. GaN/AlGaN interface. As shown in [4], polar structures affect the spatial confinement of the positron state. A dramatic change in the positron trapping at the GaN/AlGaN interface takes place when the Si doping of the AlGaN layer is above 5e18 cm-3. Interestingly, no interface traps are detected with positrons in Ga-polar structures. [1] L. Bjaalie et al., New J. Phys. 16 (2014). [2] M. H. Wong et al., Semicond. Sci. Technol. 28 (2013). [3] S. Rajan et al., J. Appl. Phys. 102 (2007). [4] I. Makkonen et al., Phys. Rev. B 82 (2010).

Authors : T. Heikkinen1, F. Tuomisto1, D. Ehrentraut2, M. D’Evelyn2
Affiliations : 1 Department of Applied Physics, Aalto University, P.O.B. 14100, FI-00076 Aalto, Finland; 2 Soraa, Inc., 75B Robin Hill Rd., Goleta, California 93117, U.S.A.

Resume : Vacancy defects in bulk GaN crystals, grown by ammonothermal method in ammono-basic conditions, have been studied with positron annihilation spectroscopy as reported in Ref. 1. Positron lifetimes found in these samples indicate at least two kinds of vacancy defects with smaller open volume than is typical to simple Ga-vacancies. Considering the impurities present in the samples, theoretical calculations of positron annihilation signals [2] give reason to think that the defects in question are Ga vacancy – multihydrogen complexes. We have now applied positron lifetime measurements to ammonothermal bulk GaN crystals grown in ammono-acidic conditions. The studied samples were cut from different growth zones: Ga-face only growth, and N-face sector growth, respectively. Positron lifetime results, again, suggest Ga vacancy – multihydrogen complexes. Both the concentration of these complexes and the amount of hydrogen per complex appear to depend on the growth direction. [1] F. Tuomisto T. Kuittinen, M. Zając, R. Doradziński and D. Wasik, J. Crystal Growth 403, 114 (2014). [2] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).

15:15 Coffee break    
Epitaxy and Devices III : Takashi Matsuoka
Authors : Jason Jones, Matthew Rosenberger, William King, Samuel Graham
Affiliations : Georgia Institute of Technology, Atlanta, USA, University of Illinois Urbana Champaign, Illinois, USA

Resume : AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have recently been under intense research and are becoming attractive devices for high voltage and high-power applications at RF operating conditions. GaN is a wide band gap (~3.4 eV at room temperature) semiconductor with a promising combination of material properties including a high electric breakdown field, good electron mobility, high saturation velocity, relatively high thermal conductivity, and is stable at high operating temperatures; all of which contribute to making these devices very suitable for RF devices where high power and high frequency operation are needed. Development and fabrication of reliable AlGaN/GaN HEMTs has significantly advanced in recent years to enable the production of high quality, commercially available devices in a wide variety of high power and high frequency applications. To further study these devices, however, it is important to investigate the reliability physics associated with AlGaN/GaN HEMTs – especially under the transient operating regime where these devices are predicted to excel over existing technologies. In this work, we present finite element simulation results of the transient temperature, stress, and deformation response of an AlGaN/GaN HEMT built on SiC substrates. The modeling technique involves a small-scale electro-thermal model coupled to a large-scale mechanics model to determine the resulting stress distribution within a device. This technique allows for detailed analysis of the electrical and thermal contributions to stress during both DC and AC operation. The electrical characteristics of the modeled device are compared to experimental measurements of comparative devices, and the bias dependent heating is compared to existing simulation data from literature for validation. The results show critical regions around the gate Schottky contact undergo drastically different transient stresses during pulsed operation. Specifically, stress profiles within the AlGaN layer around the gate footprint undergo highly tensile electro-thermal stresses while stresses within the channel between the gate and drain contact undergo highly tensile electrical stress and compressive thermoelastic stress. Factors such as the operational frequency as well as the thermal boundary resistance at contacts in the device also play a major role in the thermal and deformation response of these devices. Implications on devices reliability will be discussed.

Authors : Quanzhong Jiang1, Duncan W.E. Allsopp1 and Chris R. Bowen2
Affiliations : 1 Department of Electronic and Electrical Engineering, University of Bath, Bath BA2 7AY, UK 2 Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK

Resume : There is a technical need for the epitaxial growth of electronic devices directly on to low-cost polycrystalline diamond (PD) substrates with high thermal conductivity (~2000 W/mK) in order to extract heat efficiently from an operating device to maintain performance stability and increase lifetime. Since a PD surface does not provide sufficient structural periodicity at the wafer scale, there has been limited progress to date. This paper reports epitaxial growth of GaN films on polycrystalline diamond substrates with metal-organic vapour phase epitaxy by using a SixC layer that is formed during deposition of polycrystalline diamond on a silicon substrate. The SixC layer acts to provide sufficient structure ordering information for the formation of a single crystal GaN film at the wafer scale. It is shown that a three-dimensional island (3D) growth process is needed to remove hexagonal defects that are often observed on GaN surfaces. Intensive 3D growth and curvature engineering act as a means for essential reduction of tensile stress in as-grown GaN layers. With such engineering the GaN epitaxial layer thickness was found to be crack-free up to a thickness of 1.1 microns. The twist and tilt can be as low as 0.65 deg and 0.39 deg and are broadly comparable with GaN grown on Si substrates of a similar structure, thereby creating a basis for fabricating GaN based thin-film HFETs with improved heat sinking.

Authors : Mihir Kumar Mahata, Saptarsi Ghosh,Rahul Kumar, Apurba Chakraborty and Dhrubes Biswas
Affiliations : Indian Institute of Technology Kharagpur

Resume : Three AlGaN/GaN heterostructures on c-plane (0 0 0 1) sapphire (Al2O3) were grown in nitrogen Plasma Assisted Molecular Beam Epitaxy (PA-MBE). All structures consist of a nucleation layer (AlN, 50 nm), a buffer layer (GaN, 250 nm) and top AlGaN layer(s). Initial nucleation and buffer layer in all the structures were grown under same growth conditions. Growth variations were performed only in top AlGaN layer(s). Sample S1 has thin AlGaN layer, S2 has thick AlGaN layer and S3 has step graded AlGaN layer. A comprehensive comparative structural investigation was performed by various characterization techniques, including tapping mode atomic force microscopy (AFM), high resolution x-ray diffraction (HRXRD), and photoluminescence (PL). AFM (topographical and phase contrast) imaging was carried out to measure the surface roughness, defects and phase variations to perform comparative analysis of the samples. Crystalline quality and composition of the grown structures were investigated by room temperature photoluminescence (RTPL). Structural parameters of the grown samples were further calculated from HRXRD omega-2theta measurement directly and also by using RADS simulation. XRR results and their simulation using RAFS were employed to calculate the interface roughness in the grown structures. Finally, crystalline quality investigation using dislocation (edge/screw) calculations from rocking curves in different geometries were performed.

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Material and Device Characterization II : Samuel Graham
Authors : Motoaki Iwaya1, Junya Osumi1, Koji Ishihara1, Taiji Yamamoto1, Tetsuya Takeuchi1, Satoshi Kamiyama1, and Isamu Akasaki1,2
Affiliations : 1Faculty of Science and Technology, Meijo University, Nagoya 468-8502, Japan 2Akasaki Research Center, Nagoya University, Nagoya 464-8603, Japan

Resume : High performance group III nitride semiconductor based LEDs and laser diodes most fabricated by MOVPE. In situ monitoring in MOVPE is the key process in device manufacturing. These information can be feedback to growth condition and find its mechanism. So, in situ monitoring is expected key technology to improve the device performance. An in situ monitoring by x-ray can be measured to the lattice constant, nano-structure thin film, crystalline quality and so on. In this study, we examined the novel in situ XRD system for MOVPE growth of GaN. We analyzed of GaInN on GaN by in situ XRD monitoring. We also discuss the growth of GaInN/GaN superlattice on GaN and application of solar cell optimization using long period superlattice by in situ XRD monitoring. As the results, in situ XRD measures the critical thickness of introduction of a+c misfit dislocation in GaInN on GaN system by measuring the FWHM of GaInN XRD peak at only one time experiment. Accordingly, if we employ in situ XRD under various growth conditions, the optimization of the growth conditions will become easier because it would be possible to determine at which misfit dislocation increases by only one growth procedure.

Authors : Y. Tokuda, K. Miyamoto, S. Ueda
Affiliations : Aichi Institute of Technology, 470-0392 Toyota, Japan

Resume : We report a method to distinguish between Ga vacancy (VGa)-related and carbon (C)-related hole traps with similar thermal emission activation energies (0.86-0.88 eV) in MOCVD n-GaN [1]. This method combines minority carrier transient spectroscopy (MCTS) using above-band-gap light with optical deep level transient spectroscopy (ODLTS) using below-band-gap light for Schottky diodes. We used two MOCVD n-GaN on free standing n+-GaN which have the carbon (C) concentration of below ~1x1016 cm-3 (LC) and ~6x1016 cm-3 (HC), respectively. Isothermal MCTS measurements using ~355 nm UV and isothermal ODLTS measurements using ~470 nm blue LED were performed at 300 K for fabricated Schottky diodes. A peak is observed in MCTS and ODLTS at the same thermal emission time constant for the LC sample, while a peak is observed in the MCTS spectrum at the slightly different time constant from that in the ODLTS spectrum for the HC sample. The same time constant is found for LC and HC in ODLTS. This is ascribed to the difference between VGa-related and C-related hole traps in photoionization energy [2,3]. In the LC sample, VGa-related hole traps are dominant with C-related hole traps in concentration below detection sensitivity. There are C-related hole traps in addition to VGa-related hole traps in the HC sample. We will further report the energy levels of these hole traps and their trap concentrations. [1] Y. Tokuda, CS MANTECH, 19 (2014). [2] A. R. Arehart, A. Corrion, C. Poblenz, J. S. Speck, U. K. Mishra, and S. A. Ringel, Appl. Phys. Lett. 93, 112101 (2008). [3] A. Armstrong, A. R. Arehart, D. Green, U. K. Mishra, J. S. Speck, and S. A. Ringel, J. Appl. Phys. 98, 053704 (2005).

Authors : Michitaka Yoshino, Fumimasa Horikiri, Hiroshi Ohta, Tomonari Furuya, Tomoyoshi Mishima, Yasuhiro Yamamoto, Tohru Nakamura
Affiliations : Research Center of Ion Beam Technology, Hosei University; Research Center for Micro/Nano Technology, Hosei University; Dept. of EEE, Hosei University; SCIOCS Co. Ltd.

Resume : GaN high power devices typically need edge termination to reduce the electric field crowding at the edges. A field plate structure is the most widely used technique to increase blocking voltage capability. We demonstrate new GaN high voltage diodes with high-k dielectrics passivation underneath the filed plate. Two-dimensional device simulation was carried out to investigate electric field profiles of pn junction at the mesa etched regions and n-GaN layer near the field plate edges. Simulation results at reverse voltage of 1000V showed that the maximum electric field at the mesa-etched pn junction edges covered with films of dielectric constant k value of 20 was reduced to 1.4 MV/cm from 1.9 MV/cm and depletion layer regions at the field plate edge spread 20 % larger than that of SiO2(k=3.9). Mesa structures of pn junction diodes were fabricated by ICP dry etching, and CeO2 dielectric film with k value of about 20 was deposited by CVD. I-V characteristics of the diode with a field plate showed the breakdown voltage of 2400 V with an increased avalanche current. While breakdown characteristics of the diode covered with SiO2 showed the same breakdown voltage, the diode was suddenly broken at the voltage. This means that the electric field reduces at the periphery of the mesa etched pn junction and was uniformly formed across the whole pn junction. It is clear that high-k dielectric film passivation and filed plate termination are essential techniques for GaN power devices.

Authors : A. Romaniuk1, A. Naumov1,V. Strelchuk1, O. Kolomys1, A. Nikolenko1, H.Hardtdegen2, S. Vitusevich2, and A. Belyaev1
Affiliations : 1 V. Lashkaryov’s Institute of Semiconductor Physics, National Academy of Science of Ukraine, Kiev, 0328 Ukraine 2 Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich, 52425 Jülich, Germany

Resume : A study of electron transport, and phonon radiative properties of one-dimensional and two-dimensional AlGaN/GaN HEMT-heterostructure was done. Current-voltage characteristics (CVC) of the investigated heterostructures under different electric and magnetic fields and optical excitation in the temperature range of 77 - 300 K were obtained. The CVCs showed the effect of current saturation due to the self-heating effect, moreover the effect of negative differential conductivity was observed for the small lengths of the conductive channel. The temperature rise of the samples under electrical load was estimated from CVCs using calculations of the thermal resistance of AlGaN/GaN HEMT structures. The heating effect was also investigated by optical methods (confocal micro-Raman spectroscopy and photoluminescence) using the temperature dependence of the Stokes Raman peak frequency and halfwidth of the E2(high) phonon mode and PL spectra. The temperature dependence of the phonon lines was analyzed taking into account the empirical relationships for the thermally-induced stress due to the difference in the lattice parameters and thermal expansion coefficients of the epilayer and substrate, and decay of the optical phonons in the symmetric three-phonon process. Temperatures derived from the optical studies are compared with the data obtained from the current-voltage characteristics and modeling of 2DEG channel heating in the HEMT structure.

10:45 Coffee break    
Devices II : Piotr Kruszewski
Authors : Tetsuo Narita, Tetsu Kachi
Affiliations : Toyota Central R&D Labs., Inc.

Resume : Recently, power conversion systems in vehicles are of increasing importance with the development of hybrid or electric vehicles (HV/EVs). Higher efficiency in these systems will contribute to energy-saving society in future. Wide-bandgap semiconductors such as GaN are expected as material of new-generation power devices for HV/EVs. There are mainly two major classes of power conversion system in vehicles, such as high-power modules and medium-power modules. A boost converter which is connected to a high voltage battery and a 3-phase inverter for motor driving are classified as high-power modules. GaN vertical power devices are strong candidates for these modules. There are following requirements of performance of the devices in this module: the breakdown voltage of 1.2 kV, the current capability of more than 200 A per device and the specific on resistance of less than 2 m?cm2 beyond the performance of Si-IGBTs. To satisfy these demands, large diameter of high-quality GaN wafer and low carrier concentration control by epitaxial growth are desired. On the other hand, down converters for a low voltage source and a charging system are classified as medium-power modules. High-frequency operation over several hundred kHz and high current density are required as performance of devices. GaN lateral power devices might be suitable for this category. At the meeting, the material characteristics of GaN for high-power modules will be presented mainly.

Authors : Alphonse Torres, Erwan Morvan, Damien Barranger, René Escoffier
Affiliations : Univ. Grenoble Alpes, F-38000 Grenoble France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France.

Resume : Introduction - AlGaN/GaN heterojunction is suitable for high power electronics applications due to superior material properties such as high breakdown field, large carrier density and high saturation velocity. Extra, epitaxy and CMOS-compatible process on 200mm silicon substrate ensure a competitive price in the medium term. Normally-Off or Enhancement-mode (E-mode) AlGaN/GaN HEMT is highly required for safety and design of power electronics circuits. E-mode behavior can be performed through hybrid cascode architecture [2] or monolithic architectures such as GIT [1], monolithic cascode [3] or MOS-channel HEMT structure [4]. In this work, 600V, 50Amps GaN MOS-channel HEMT have been fabricated on 200mm silicon substrate with a CMOS-compatible process. Positive threshold voltage have been demonstrated and impact of GaN doping on threshold voltage was investigated. Devices structures and fabrications - Non-intentionally doped (NiD) 22nm Al0.25Ga0.75N/GaN was grown by MOCVD on 200mm silicon substrate. On one wafer, a [Mg]-doped buried layer was introduced during epitaxy to create a p-type GaN buried layer. Sheet resistance was around 500ohm/sqr for undoped GaN substrate and 600ohm/sqr for [Mg]-doped GaN substrate. MOS-gate structure was realized by a full AlGaN recess under gate. AlGaN layer and partially GaN layer were etched by a chlorine-based ICP-RIE process. For p-type GaN MOS-channel HEMT, etching went down to the [Mg]-doped GaN layer. For both wafers, LPCVD Si3N4 layer wa

Authors : A. Schmid, R. Otto, V. Klemm, D. Rafaja, A. Winzer, A. Wachowiak, J. Heitmann
Affiliations : Institute of Applied Physics, TU Bergakademie Freiberg, 09599 Freiberg, Germany; Institute of Applied Physics, TU Bergakademie Freiberg, 09599 Freiberg, Germany; Institute of Materials Science, TU Bergakademie Freiberg, 09599 Freiberg, Germany; Institute of Materials Science, TU Bergakademie Freiberg, 09599 Freiberg, Germany; Namlab gGmbH, 01187 Dresden, Germany; Namlab gGmbH, 01187 Dresden, Germany; Institute of Applied Physics, TU Bergakademie Freiberg, 09599 Freiberg, Germany;

Resume : High electron mobility transistors with an insulated gate (MISHEMT) offer superior properties compared to non-insulated HEMTs. By the use of a dielectric the gate leakage current can be reduced drastically. However, the integration of a gate insulation into the HEMT fabrication in a “high-k first” approach requires a change in the thermal treatment of the subsequently deposited ohmic metallization. The conventional scheme with a Ti/Al/Ni/Au stack depends on a post deposition annealing at 850°C in order to form low-resistive contacts. Common high-k materials like Al2O3 crystallize at temperatures above 700°C and therefore will exhibit an increased leakage current and a poor breakdown behavior. As a solution the use of a V/Al/Ni/Au electrode is proposed and shown for MISHEMT structures with ALD Al2O3 gate dielectric. With the V-based metallization an ohmic contact formation was achieved after annealing for 30 s at 650°C, which resulted in a low specific contact resistance of 8.9∙10^-6 Ωcm2. Transmission electron microcopy and energy dispersive X-ray analysis were performed to investigate the microstructure and the interdiffusion of elements at the AlGaN/metal interface. A correlation between the microstructure and the electrical properties of the ohmic contacts was found. Beside the penetration of the barrier, the formation of intermetallic phases was observed. Both phenomena strongly depend on the V:Al thickness ratio and can significantly contribute to the contact resistance.

12:30 Lunch break    
Devices III : Tetsuo Narita
Authors : Raúl Rodríguez, Benito González, Javier García, Fetene M. Yigletu, Benjamín Iñiguez, Antonio Lázaro, Antonio Nunez
Affiliations : Raúl Rodríguez, Benito González, Javier García, Antonio Nunez: IUMA-ULPGC Fetene M. Yigletu, Benjamín Iñiguez, Antonio Lázaro: DEEEA-URV

Resume : In this paper, the operating temperature impact on the DC performance of an AlGaN/GaN HEMT grown on sapphire is modelled and numerically reproduced. For that purpose, transfer and output characteristics are measured up to 100 ºC. The transistor thermal resistance is extracted as the ratio between the measured device temperature and the electrical power for a set of similar HEMTs. In order to reproduce the behavior numerically with Atlas, the transistor internal physical parameters are firstly adjusted with the simulator obtaining the measured characteristics at uniform lattice temperatures (in the temperature range studied). Afterwards, once the heat equation is incorporated, different thermal boundary conditions are tested. We find that placing thermal contacts at the gate and at the bottom of the structure becomes convenient to numerically predict the measured device thermal resistance. In relation with compact modelling, the threshold voltage can be extracted from measured transfer characteristics in the linear region through different classic methods (constant-current, extrapolation, transconductance extrapolation, second derivative, etc.) varying the operating temperature. With these linear temperature dependencies of the threshold voltage, the device thermal resistance and the extrinsic ohmic resistances numerically derived, our compact model successfully predicts the measured transfer and output characteristics for the entire temperature range under consideration.

Authors : Herrera, D. B. 1, Galvan, E.1 and Rodríguez, S. 1
Affiliations : 1 GPTech Company, Camino de los Descubrimientos, 17, Seville, SPAIN

Resume : Gallium Nitride (GaN) is an advanced semiconductor material, which represents a huge potential to enable innovations to power converters, including Solar Inverters. The deployment of this technology is the key for the challenges on energy efficiency and renewable energy. A GaN-based three-phase power inverter topology is designed for photovoltaic (PV) applications. Improvements in total efficiency in innovative PV solar power DC/AC inverter have been demonstrated. Performance comparison of high voltage normally-off GaN FETs versus Silicon and Silicon Carbide based devices has been presented. The proposed 100 kW inverter with 20 kHz switching frequency consists of use the new GaN technology to improve the efficiency, to reduce the costs, the system volume and weight and with increased output power. The main objective of this GaN-based PV power inverter is to be able to achieve higher voltage, lower leakage, reduce the power dissipation, passive components and demonstration of high reliability and lifetime technology. Some challenges with the manufacturing capabilities have to be proved to obtain material quality, low cost and high handling. The PV inverter is verified by the GaN thermal design characteristics by PLECS Plexim simulation software and the electrical characteristics, the passive components definitions and the high frequency switching are simulated by PSCAD software. A test-bench prototype is going to be built to verify the high efficiency of the GaN-based.

Authors : D. Gregušová1, E.-Bahat-Treidel2, Š. Haščík1, M. Blaho1, M. Ťapajna1, J. Derluyn3, M. Germain3, O. Hilt2, J. Würfl2, J. Kuzmík1
Affiliations : 1Institute of Electrical Engineering SAS, Dúbravská cesta 9, 841 04 Bratislava, Slovakia: 2Ferdinand-Braun-Institute, Leibniz Institute für Höchstfrequenztechnik, Gustav-Kirchhoff-Strasse 4, 12489 Berlin, Germany: 3EpiGaN NV, Kempische steenweg 293, 3500 Hasselt, Belgium

Resume : Development of AlGaN/GaN-based normally-off high-electron mobility transistors (HEMTs) faces several issues. Among them is a lack of a true pinch-off, low threshold voltage for low on-state resistance devices, high gate leakage of Schottky gates contact, or a threshold voltage drift for HEMTs with a metal-oxide-semiconductor (MOS) gate structures. Inversion-type enhancement-mode GaN-based field-effect transistors is not considered here as an alternative as it features low channel mobility and freedom to manipulate the threshold voltage remains limited. In our approach we propose normally-off AlGaN/GaN MOS HEMTs, where the gate oxide/semiconductor interface is obtained by plasma oxidation of a thin AlN cap layer, and subsequently overgrown by Al2O3 using atomic-layer-deposition at low-temperature. Consequently, inherent MOS-interface is formed with potentially low density of defects and surface donors. In this case there is a possibility to increase the threshold voltage technologically by oxide thickness increasing. Moreover, overgrown Al2O3 kept forward gate leakage current low with a capability of full channel opening. On the other hand, AlN was left intact at the access regions providing 2nd quantum well and low access resistances despite of only 3-nm thick AlGaN barrier. Devices are further optimized by tailored pre- and post-metallisation annealing steps in oxygen or nitrogen atmosphere. Finalized MOS HEMTs grown on Si with in-situ passivation show true pinch -off with < 10-8 A/mm drain leakage current, threshold voltage > 1 V, maximal drain current > 0.5 A/mm, a low hysteresis, and mitigated trapping effects. Support of HipoSwitch EU project no. 287602 and VEGA no. 2/0105/13 are acknowledged

15:30 Coffee break    
Epitaxy and Devices IV : Alphonse Torres
Authors : Takashi Matsuoka, Kanako Shojiki, Takeshi Kimura, Tomoyuki Tanikawa, Ryuji Katayama
Affiliations : Institute for Materials Research, Tohoku University, Sendai, Japan

Resume : A nitride semiconductor with a wurtzite structure is a very promising material for high performance devices in comparison with the conventional semiconductors. The exotic characteristic is a crystallographic polarity. This polarity leads to the polarization in the crystal as pointed out in 1988. This polarization influences the characteristics of the epitaxial growth and all the device performances. Up to now, the growth and devices with group-III polarity have been mainly reported because in the N-polar growth, "{1" "1" ̅"0" "1" ̅"}" and "{1" "1" ̅"0" "2" ̅"}" facets appearing at the edges of the growth islands form a rough surface. In this paper, the N-polar epitaxial growth and its merit are reviewed. For epitaxial growth, the step-flow growth mode has to be promoted. This mode can be realized by using the following optimum conditions; the relatively low V/III ratio for enhancing the migration of Ga adatoms on the surface, relatively high reactor-pressure for forming {11(_)00}, and introduction of off-cut substrates for controlling the step distance. N-polar GaN with the same characteristics of quality, the dislocation density and the p-type carrier concentration as Ga-polar GaN has been successfully grown. The N-polar growth has an advantage in the growth of In-rich materials because one N atom is captured with three Ga atoms at the growth front, while that is captured with only one Ga atom in Ga-polarity. In device applications, the polarization field can improve

Authors : B. Damilano (1), J. Faugier-Tovar (2), E. Frayssinet (1), Y. Cordier (1), F. Semond (1), D. Turover (2)
Affiliations : (1) CRHEA-CNRS, Rue B. Gregory, 06560 Valbonne, France (2) SILSEF SA, 382, rue Louis Rustin, Archamps Technopole, F-74160 ARCHAMPS, France

Resume : Si substrates are attractive for the growth of GaN because of their large size, quality and availability. However, due to the large thermal expansion coefficient mismatch between GaN and Si, the grown layers can crack during the cooling down after growth. This cracking can be avoided by using strain-balancing layers such as AlN or AlGaN interlayers, AlN/AlGaN superlattices,… These approaches are quite complex and require a perfect control of the stress during growth. A simpler approach to avoid cracks is to use mesa-patterned Si substrates. Thanks to the stress relaxation at the mesa free- edges, the cracking of the nitride layers can be limited. The cracking and the final stress of the layers critically depend on the mesa design. Also, depending on the application which is targeted, the pattern has to be adapted. The mesa design optimization can benefit from a mask-less approach using laser lithography. We show that by using such method we can design square mesas with very well controlled sizes from 20x20 µm2 to 500x500 µm2. 2-3 µm-thick GaN layers on 200 nm-thick AlN buffer layers were grown on these mesa-patterned Si substrates by metal-organic chemical vapor deposition. Structural and morphological characterizations show that high quality layers, without cracks, are obtained.

Authors : H. Ben Ammar1, M.P. Chauvat1, P. Gamarra2, C. Lacam2, M.Tordjman2,M.A. di Forte-Poisson2, and P. Ruterana1
Affiliations : 1. Centre de Recherche sur les Ions, les Matériaux et la Photonique UMR 6252, CNRS ENSICAEN UCBN CEA, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex, France 2. III-V Lab, Alcatel-Thales-LETI, Route de Nozay, 91460 Marcoussis, France

Resume : InAlN layers are of a great interest for high electron mobility transistors (HEMT), distributed Bragg reflectors (DBR), and ultraviolet light-emitting diodes (UV LED). Their physical properties are expected to be more interesting in comparison to AlGaN for HEMT applications; this is mainly due to the possibility of obtaining layers which are lattice matched to GaN (LM-GaN) at around 18% Indium content. On the other hand, changing the Indium content may allow to tune the band gap from UV to deep IR. The main issue for InAlN is that degradation takes place even during the growth of nearly LM-InAlN/GaN heterostructures. Previous works have analyzed the influence of various growth parameters, e.g. metal precursor fluxes, ammonia flux, and temperature. However, the mechanisms that govern such degradation are not yet well understood. Recently, two possible explanations for this degradation have been proposed: 1) the influence of the dislocations inside the GaN buffer layer, and 2) an intrinsic formation of pinholes at the coalescence of the growth hillocks inside AlInN layers. In the following, we report characterization studies related to a series of InAlN layers which thickness was around 65nm. The main parameter investigated during the growth was the growth pressure. From, conventional transmission electron microscopy (TEM), and atomic force microscopy (AFM), it was shown that the degradation mechanisms are complex and may critically depend on the growth kinetics.

16:45 Closing remarks    
18:00 Best Student Presentation Awards Ceremony and Reception (Main Hall)    

No abstract for this day

Symposium organizers
Michał BOCKOWSKIInstitute of High Pressure Physics

ul. Sokolowska 29/37 Warsaw Poland

+48 664 446 092
Yusuke MORIOsaka University, Graduate School of Engineering

2-1 Yamada-Oka, Suita Osaka 565-0871 Japan

+81 6 6879 7707
Henryk TEISSEYREInstitute of Physics | Polish Academy of Sciences

Al. Lotników 32/46 02-668 Warsaw Poland

+48 606 666 751
Benjamin DAMILANOCentre de Recherche sur l'Hétéro-Épitaxie et ses Applications (CRHEA) | Centre National de la Recherche Scientifique

Rue Bernard Grégory 06560 Valbonne France

+33 4 93 95 78 29