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International audience; Hexagonal boron nitride (h-BN) can be used as a p-doped material in wide-bandgap optoelectronic heterostructures or as a release layer to allow lift-off of grown three-dimensional (3D) GaN-based devices. To date, there have been no studies of factors that lead to or prevent lift-off and/or spontaneous delamination of layers. Here, we report a unique approach of controlling the adhesion of this layered material, which can result in both desired lift-off layered h-BN and mechanically inseparable robust h-BN layers. This is accomplished by controlling the diffusion of Al atoms into h-BN from AlN buffers grown on h-BN/sapphire. We present evidence of Al diffusion into h-BN for AlN buffers grown at high temperatures compared to conventional-temperature AlN buffers. Further evidence that the Al content in BN controls lift-off is provided by comparison of two alloys, Al 0.03 B 0.97 N/sapphire and Al 0.17 B 0.83 N/sapphire. Moreover, we tested that management of Al diffusion controls the mechanical adhesion of high-electron-mobility transistor (HEMT) devices grown on AlN/h-BN/sapphire. The results extend the control of two-dimensional (2D)/3D hetero-epitaxy and bring h-BN closer to industrial application in optoelectronics.

International audience; Combined photonic and electronic systems require diverse devices to be co-integrated on a common platform. This heterogeneous integration is made possible through several separation and transfer methods where the functioning epilayers are essentially released from their growth substrate. The use of 2D layered h-BN as a mechanical release layer has been demonstrated to be a promising technique for the hybrid integration of III-nitride devices. In this talk we will give an overview of our results on wafer-scale van der Waals epitaxy by MOVPE of different III-N heterostructure devices such as LEDs, HEMTs, solar cells, sensors and photodetectors. Furthermore, mechanical release and transfer techniques of crack-free III-N devices on foreign substrates will be presented along with a comparison between the device performances before and after transfer.

Reliable p-doped hexagonal boron nitride (h-BN) could enable wide bandgap optoelectronic devices such as deep ultra-violet light emitting diodes (UV LEDs), solar blind photodiodes and neutron detectors. We report the study of Mg in h-BN layers as well as Mg h-BN/AlGaN heterostructures. Mg incorporation in h-BN was studied under different biscyclopentadienyl-magnesium (Cp2Mg) molar flow rates. 2&theta, &omega, x-ray diffraction scans clearly evidence a single peak, corresponding to the (002) reflection plane of h-BN with a full-width half maximum increasing with Mg incorporation in h-BN. For a large range of Cp2Mg molar flow rates, the surface of Mg doped h-BN layers exhibited characteristic pleats, confirming that Mg doped h-BN remains layered. Secondary ion mass spectrometry analysis showed Mg incorporation, up to 4 &times, 1018 /cm3 in h-BN. Electrical conductivity of Mg h-BN increased with increased Mg-doping. Heterostructures of Mg h-BN grown on n-type Al rich AlGaN (58% Al content) were made with the intent of forming a p-n heterojunction. The I-V characteristics revealed rectifying behavior for temperatures from 123 to 423 K. Under ultraviolet illumination, photocurrent was generated, as is typical for p-n diodes. C-V measurements evidence a built-in potential of 3.89 V. These encouraging results can indicate p-type behavior, opening a pathway for a new class of wide bandgap p-type layers.

Selective Area van der Waals Epitaxy (SAVWE) of III-Nitride device has been proposed recently by our group as an enabling solution for h-BN-based device transfer. By using a patterned dielectric mask with openings slightly larger than device sizes, pick-and-place of discrete LEDs onto flexible substrates was achieved. A more detailed study is needed to understand the effect of this selective area growth on material quality, device performance and device transfer. Here we present a study performed on two types of LEDs (those grown on h-BN on patterned and unpatterned sapphire) from the epitaxial growth to device performance and thermal dissipation measurements before and after transfer. Millimeter-size LEDs were transferred to aluminum tape and to silicon substrates by van der Waals liquid capillary bonding. It is shown that patterned samples lead to a better material quality as well as improved electrical and optical device performances. In addition, patterned structures allowed for a much better transfer yield to silicon substrates than unpatterned structures. We demonstrate that SAVWE, combined with either transfer processes to soft or rigid substrates, offers an efficient, robust and low-cost heterogenous integration capability of large-size devices to silicon for photonic and electronic applications.

AlGaN/GaN HEMTs have been shown to be efficient sensors for a broad range of physical parameters, in either liquid or dry condition, such as pressure sensor [1,2], gas detection [3,4], pH sensor [5], and more recently used as biosensors for the rapid detection of viruses [6]. These achievements could pave the way for the use of these HEMT transistors in electronic nose development particularly useful for volatile organic compound (VOC) detection. Among VOCs, acetone is one of the most important elements because it can be used as a biomarker for early disease, such as lung cancer, detection. For this purpose, sensors with responsivity in the range below 1 ppm are desired [7]. Up to now, only few works have been reported on acetone detection with rather contradictory observations between its detection at room temperature (RT) [8] or high temperature [9]. In this work, we report on AlGaN/GaN HEMT sensors for acetone concentration below 100 ppm and in a broad range of the sensor temperature varying from (RT) to 300°C. At RT, in presence of acetone, a smooth and monotonic decrease of the current is observed with a rather large responsivity of 15µA/ppm and with large response time (several minutes) and memory effect. This decrease of the current can be explained by the electrostatic interaction between the 2D gas in the HEMT with the dipolar moment of the acetone molecules as described by Neuberger et al. [8]. This decrease is also in agreement with the work of Neuberger. et al [8]. At high temperature (300°C), in contrary to what has been reported in [9], a current decrease is first observed just after the acetone injection and then followed by an increase which saturates and stabilizes at a constant value. In order to clarify this unexpected behavior, a detailed study of the sensors response versus the temperature and acetone injection mode has been carried out. The output of this investigation is that a competition between the current variations induced by both the sensor and gas flow temperature difference from one side and acetone dipolar moment from the other side can explain this transient. At high temperature, the gas flow (especially for high acetone concentration) tends to cool down the sensor inducing an increase of the current, whereas the true acetone effect leads to a decrease of the current. AlGaN/GaN HEMTs – based sensors are shown to allow for very sensitive acetone detection at both room and high temperature. Nevertheless, care must be taken during the characterization and operation of such sensors especially at high operating temperature. Increasing the latter, can help to improve the sensor response suppress the memory effect, but requires the control or the cancellation of the current transient due to the temperature difference between the gas flow and the transistor gate. References [1] Kang, B.S.; Kim, S.; Kim, J.; Mehandru, R.; Ren, F.; Baik, K.; Pearton, S.J.; Gila, B.P.; Abernathy, C.R.; Pan, C.C.; Chen, G.T.; Chyi, J.I.; Chandrasekaran, V.; Sheplak, M.; Nishida, T.; Chu, S.N.G., AlGaN/GaN high electron mobility transistor structures for pressure and pH sensing. physica status solidi (c) 2005, 2, 2684–2687. doi:10.1002/pssc.200461269. [2]. Gajula, D.; Jahangir, I.; Koley, G., High Temperature AlGaN/GaN Membrane Based Pressure Sensors, Micromachines 2018, 9, 207. doi:10.3390/mi9050207. [3] Bishop, C.; Halfaya, Y.; Soltani, A.; Sundaram, S.; Li, X.; Streque, J.; Gmili, Y.E.; Voss, P.L.; Salvestrini, J.P.; Ougazzaden, A., Experimental Study and Device Design of NO, NO2, and NH3 Gas Detection for a Wide Dynamic and Large Temperature Range Using Pt/AlGaN/GaN HEMT, IEEE Sensors Journal 2016, 16, 6828–6838. doi:10.1109/jsen.2016.2593050. [4] Halfaya, Y.; Bishop, C.; Soltani, A.; Sundaram, S.; Aubry, V.; Voss, P.; Salvestrini, J.P.; Ougazzaden, A. Investigation of the Performance of HEMT-Based NO, NO2 and NH3 Exhaust Gas Sensors for Automotive Antipollution Systems. Sensors 2016, 16, 273. doi:10.3390/s16030273. [5] Sama, N.Y.; Bouhnane, H.; Gautier, S.; Ahaitouf, A.; Matray, J.M.; Salvestrini, J.P.; Ougazzaden, A.; Hathcock, A.; He, D.; Vuong, T.Q.P.; Karrakchou, S.; Ayari, T.; Mballo, A.; Bishop, C.; Halfaya, Y., Investigation of Sc2O3 Based All-Solid-State EIS Structure for AlGaN/GaN HEMT pH Sensor, 2019 IEEE SENSORS. IEEE, 2019. doi:10.1109/sensors43011.2019.8956762. [6] Yang, J.; Carey, P.; Ren, F.; Mastro, M.A.; Beers, K.; Pearton, S.J.; Kravchenko, I.I., Zika virus detection using antibody-immobilized disposable cover glass and AlGaN/GaN high electron mobility transistors, Applied Physics Letters 2018, 113, 032101. doi:10.1063/1.5029902. [7] Wilson, A., Advances in Electronic-Nose Technologies for the Detection of Volatile Biomarker Metabolites in the Human Breath, Metabolites 2015, 5, 140–163. doi:10.3390/metabo5010140. [8] Neuberger, R.; Muller, G.; Ambacher, O.; Stutzmann, M., High-Electron-Mobility AlGaN/GaN Transistors (HEMTs) for Fluid Monitoring Applications, physica status solidi (a) 2001, 185, 85–89. doi:10.1002/1521-396x(200105)185:1 3.0.co;2-u. [9] Sun, J.; Sokolovskij, R.; Iervolino, E.; Santagata, F.; Liu, Z.; Sarro, P.M.; Zhang, G., Characterization of an Acetone Detector Based on a Suspended WO3-Gate AlGaN/GaN HEMT Integrated With Microheater, IEEE Transactions on Electron Devices 2019, 66, 4373–4379. doi:10.1109/ted.2019.2936912. [10] Rabbaa, S.; Stiens, J. Validation of a triangular quantum well model for GaN-based HEMTs used in pH and dipole moment sensing. Journal of Physics D: Applied Physics 2012, 45, 475101. doi:10.1088/0022-3727/45/47/475101.

International audience; Boron rich BAlN alloys have been grown on 2-inch sapphire substrates by Metal-Organic Vapor Phase Epitaxy. The surface morphology of BAlN alloys exhibits a transition stage from a completely two-dimensional to a three-dimensional granular surface with an increased trimethylaluminum/group III (TMAl/III) ratio. Only a shift in the position of the 002 plane reflection peak to higher diffraction angles in the 2θ−ω scan along with a decrease in intensity was observed, specifying formation of layered BAlN alloys up to a TMAl/III ratio of 14. AlN phase separation was observed while increasing the TMAl/III ratio to 25, supporting SEM observations. Secondary-ion mass spectrometry measurements confirmed the presence of up to 17% Al in layered BAlN alloy systems. A cross sectional transmission electron microscopy (TEM) study confirmed the layered nature of single phase BAlN alloys. It also revealed the presence of wurtzite Al rich BAlN phases in a matrix of layered hexagonal B rich BAlN. Band to band transition around 5.86 eV has been observed, which shifted slightly to lower energy with increasing Al incorporation. The bowing parameter (C) in boron rich BAlN alloy systems was evaluated to be around 0.65 ± 0.05 eV. Encouraging results were obtained on boron rich BAlN alloy formation, motivating further exploration of growth conditions and study of BAlN fundamental properties for applications in deep UV optoelectronics.Hexagonal boron nitride (h-BN) is a unique III-nitride, which has interesting properties such as a layered structure, high thermal conductivity, and a wide bandgap (∼6 eV).1–5 Even though h-BN is an indirect bandgap semiconductor, it has an impressive deep ultraviolet (UV) emission and, hence, it is very promising for applications in deep UV optoelectronics when compared to direct bandgap AlN and other materials.3,6–13 The AlN alloy system, on the other hand, is the most studied material for applications in the deep UV regime, but growth of high quality materials and p-type doping are challenging.6,14 Both Al rich w-BAlN and B rich h-BAlN alloys may enable a desirable bandgap and lattice/strain engineering for applications. For example, h-BN can be made into a direct bandgap material through strain engineering or alloying with Al, which would enhance emission efficiency in the deep UV for UV LEDs. Alloying boron into AlN could also lead to a type II BAlN/AlGaN heterojunction,15 allowing the achievement of an electron blocking layer. BAlN has also been studied as a promising material candidate for high-reflectivity distributed Bragg reflectors (DBRs) due to strong refractive index modification.16,17 Because of their layered nature and reported intrinsic p-type behavior, the use of h-BN based alloys may give more flexibility to design highly efficient device structures.18–20 Theoretically, it was reported that BAlN alloys can have structural crossover from hexagonal to wurtzite at 50% of boron, and the band-gap transition from indirect to direct would occur at 75% of boron.21,22Apart from these theoretical investigations, boron rich BAlN alloys have not yet been explored experimentally. The growth of boron rich BAlN alloys and the understanding of its basic structural, as well as the optical, properties are of high importance. In this work, we report growth of single-phase boron rich BAlN alloys up to 17% of Al in a Metal-Organic Vapor Phase Epitaxy (MOVPE) reactor. We first studied in detail the relationship between morphology, Al composition, and the trimethylaluminum/group III (TMAl/III) ratio. Second, the bandgap variation in boron rich BAlN and the bowing effect were presented.The BAlN alloys were grown directly on c-plane sapphire substrates without any buffer in an Aixtron MOVPE close coupled showerhead (CCS) reactor. Triethylboron (TEB), trimethylaluminum (TMAl), and ammonia (NH3) were precursors for boron, aluminum, and nitrogen, respectively. 20 nm thick of BAlN layers were grown at 1280 °C and 90 mbar pressure. The TMAl/III ratio was varied from 0 to 25; in order to increase the Al content in gas phase, all other parameters were kept constant.A scanning electron microscope (SEM) was used to study the surface morphology of the samples. The crystalline structure and phase purity of BAlN alloys were examined by high-resolution X-ray diffraction (HRXRD) scans, performed in a Panalytical X'pert Pro Materials Research Diffractometers system with Cu Kα radiation in triple axis mode. The aluminum content was estimated by Secondary-ion mass spectrometry (SIMS) using Cs+ molecular ions. The interfaces of heterostructures were characterized by high-angle Annular Dark Field Scanning Transmission electron Microscopy (HAADF-STEM) performed on an aberration-corrected JEOL 2200FS electron transmission microscope. Prior to this study, cross sectional lamellae were prepared by the FIB process after coating a 100 nm thick carbon for layer protection. Transmission spectra were measured at room temperature with a Perkin Elmer LAMBDA 950 spectrophotometer.

International audience; In article number 1900164 by Abdallah Ougazzaden and co‐workers, III‐Nitride based LEDs have been locally grown on h‐BN and fabricated at a wafer‐scale. This enables a simple and a dicing‐free pick‐and‐place of the devices on a flexible substrate without performance degradation.

International audience; In this work, an all-solid-state electrolyte-insulator-semiconductor (EIS) device is developed for pH sensing performance evaluation of insulating materials. The EIS capacitor incorporates scandium oxide (Sc2O3) sensing film deposited on undoped gallium nitride (u-GaN) by thermal evaporation. The structural and morphological features of the thin films annealed at different temperatures (650-850°C), were investigated through X-ray diffraction and AFM analysis. A gold wire with diameter of 25 µm was bonded on the device and served as the quasi reference electrode for the C-V measurements and pH sensitivity characterizations. After correction of the measured capacitance regarding the pH dependent variation of the standard potential E0(pH) of gold, the Sc2O3 EIS capacitor prepared with an annealing at 650°C exhibited a linear response with a sensitivity of 40 mV/pH while the device with the as-deposited Sc2O3 film showed nonlinear behavior and, those annealed at temperature higher than 650°C were shown to be insensitive to pH variation.

International audience; InGaN nano-structures, grown using nano selective area growth, have been shown to exhibit high crystalline quality, even for high In content InGaN alloy, and reduced polarization charge effect. They are thus very attractive for the realization of high efficiency solar cells. Compared to planar InGaN absorbers, nanopyramid-based absorbers are shown to relax the usual challenging constraint on the doping of the p-GaN layer, which would be needed to overcome the polarization-induced electric field. NP-based solar cells maintain the same performance with ten times lower p-GaN doping. Furthermore, the SiO2 mask used for selective area growth of the nanopyramids is shown to help trap light into the nanopyramids, leading to increased optical absorption and thus efficiency. Last, InGaN nanopyramid absorber-based solar cells can allow for a higher InGaN residual donor concentration than that of the planar InGaN solar cells. Overall, an optimized In0.3Ga0.7N nanopyramid-based solar cell can lead to an efficiency twice than that of a planar InGaN-based solar cells with standard p- and n-GaN doping level. As a proof of concept, an In0.09Ga0.91N nanopyramid-based solar cell has been fabricated and is shown to have larger short circuit photocurrent and open circuit voltage than a state of the art In0.08Ga0.92N-based planar solar cell.

International audience; In article number 1900207, Suresh Sundaram, Abdallah Ougazzaden, and co‐workers demonstrate self‐organized GaN nanorods formation on layered h‐BN templates by van der Waals epitaxial growth. This approach is used to grow III‐N nanowire light‐emitting diodes. In addition, this approach also mitigates transfer processes and scaling issues seen with other 2D materials, since both the 1D (GaN nanorods) and 2D (h‐BN) are grown at the wafer‐scale and in one epitaxial growth run.

International audience; Hexagonal boron nitride (h‐BN) is a promising 2D template that decouples substrate effects from the layer above it by van der Waals epitaxy, because there are only weak forces out of the h‐BN. It also permits convenient mechanical transfer of devices grown on their surface to an appropriate substrate. Here, van der Waals epitaxial growth resulting in the formation of self‐organized GaN nanorods on h‐BN templates is demonstrated. This approach to the growth of III‐N nanostructures avoids transfer processes and scaling issues seen with other 2D materials, since both the 1D (GaN nanorods) and 2D (h‐BN) are grown at the wafer‐scale and in one growth run. Further, this process is used to grow vertical core–shell p‐GaN/InGaN/n‐GaN nano‐PIN device structures on wafer‐scale 2D h‐BN on sapphire and silicon substrates. The high quality of the core–shell nanostructures is confirmed by detailed electron microscopy study, which gives more insight into the nanorod formation mechanism on 2D material. Mechanical transfer of nanostructures on sapphire substrates to copper tape is then demonstrated, with no resulting damage of nanorods. Use of MOVPE grown large‐area h‐BN to realize nanostructures is a significant advancement and can lead to new nanodevice architectures needed for next‐generation optoelectronic devices.

Self-forming, vertically-aligned, ZnO moth-eye-like nanoarrays were grown by catalyst-free pulsed laser deposition on a-SiO2/Si (1 1 1) substrates. X-Ray Diffraction (XRD) and Cathodoluminescence (CL) studies indicated that nanostructures were highly c-axis oriented wurtzite ZnO with strong near band edge emission. The nanostructures were used as templates for the growth of non-polar GaN by metal organic vapor phase epitaxy. XRD, scanning electron microscopy, energy dispersive X-ray microanalysis and CL revealed ZnO encapsulated with GaN, without evidence of ZnO back-etching. XRD showed compressive epitaxial strain in the GaN, which is conducive to stabilization of the higher indium contents required for more efficient green light emitting diode (LED) and photovoltaic (PV) operation. Angular-dependent specular reflection measurements showed a relative reflectance of less than 1% over the wavelength range of 400–720 nm at all angles up to 60°. The superior black-body performance of this moth-eye-like structure would boost LED light extraction and PV anti-reflection performance compared with existing planar or nanowire LED and PV morphologies. The enhancement in core conductivity, provided by the ZnO, would also improve current distribution and increase the effective junction area compared with nanowire devices based solely on GaN.

International audience; The mechanical release of III‐nitride devices using h‐BN is a promising approach for heterogeneous integration. Upscaling this technology for industrial level requires solutions that allow a simple pick‐and‐place technique of selected devices for integration while preserving device performance. An advance that satisfies both of these requirements is demonstrated in this work. It is based on a lateral control of the h‐BN quality, using patterned sapphire with a SiO2 mask, to achieve localized van der Waals epitaxy of high‐quality GaN based device structures. After process fabrication, the devices can be individually picked and placed on a foreign substrate without the need for a dicing step. In addition, this approach could reduce delamination of h‐BN on large diameter substrates because each h‐BN region is smaller, with independent device structures. Discrete InGaN LEDs on h‐BN are grown and fabricated on 2 in. patterned sapphire using a SiO2 mask. A set of devices are selectively released and transferred to flexible aluminum tape. The transferred LEDs exhibit blue light emission around 435 nm. The approach presented here is scalable on any wafer size, can be applied to other types of nitride‐based devices, and can be compatible with commercial pick‐and‐place handlers for mass production.

AlGaN/GaN distributed Bragg reflectors have been grown on GaN templates by Metal Organic Vapor Phase Epitaxy (MOVPE). To overcome the problem of crack generation and achieve high reflectivity with an optimum number of periods a simple approach has been investigated. Using this approach, a coherently strained 20-pair Al 0.27 Ga 0.73 N/GaN (42 nm/36 nm) DBR has been designed and grown. No cracks were observed over 2” wafer. Around 90% reflectivity at 384 nm and a stop-bandwidth of 18 nm was obtained by having good crystalline quality and abrupt and flat interfaces.

In this paper we report on a spontaneous 2D/3D transition observed in InGaN alloys after 60 nm of growth. This transition is responsible for the formation of a stack of distinct InGaN layers. The driving mechanism is shown to be lateral fluctuations of the indium composition, that arise to accommodate the increasing strain energy of the InGaN layer. Three distinct stages of growth have been identified. First, a homogeneous, 2D InGaN layer forms, pseudomorphically strained on the underlying GaN. Then, at around 30 nm large lateral fluctuations of the indium composition are observed and a second pseudomorphic layer, composed of indium-rich and indium-poor clusters, is formed. Finally induces a 2D/3D transition at 60 nm and a 3D InGaN layer is formed.

Highly relaxed GaN nanodots and submicron ridges have been selectively grown in the NSAG regime using MOVPE on lattice mismatched 6H–SiC and AlN substrates. 2D real space and 3D reciprocal space mapping was performed with a CCD detector using 10.4 keV synchrotron X-ray radiation at the 2-ID-D micro-diffraction beamline at Advanced Photon Source (APS). Calibration procedures have been developed to overcome the unique challenges of analyzing NSAG structures grown on highly mismatched substrates. We studied crystallographic planar bending on the submicron scale and found its correlation with strain relaxation in the NSAG ridges.

Nanodots arrays of GaN have been successfully grown on GaN template substrates using nano-selective area growth (NSAG). The substrates used in NSAG were partially covered by dielectric masks in which nano-apertures have been patterned by electron-beam nanolithography and reactive ion etching. The growths were performed by low pressure metal organic vapour phase epitaxy (MOVPE) using 100% N2 as carrier gas. TMGa and NH3 were used as sources of gallium and nitrogen, respectively. The layers were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Perfect selectivity of GaN on the masked substrate has been obtained. The nanodots grown in the nano-apertures are well shaped and homogenous with smooth surface side walls.

BGaN materials with good structural quality and surface morphology have been successfully grown on GaN template substrates by low pressure metal organic vapour phase epitaxy. TEB and NH3 were used as precursors of boron and nitrogen, respectively. All the growths were performed under 100% N 2 process gas. Boron concentration was estimated by HRXD measurements combined with SIMS analysis. Single-crystal layers BGaN with B content as high as 3.6% have been obtained.

Thin films of GaN were grown on template substrates of 4-μm-thick GaN layers on sapphire substrates by low-pressure metal organic vapour-phase epitaxy (LP-MOVPE) in a new-design reactor with the shape T. Wide range of growth temperature from 520 to 1100 °C was explored. At low temperature growth between 550 and 690 °C, dimethylhydrazine (DMHy) was used as source of atomic nitrogen while ammonia (NH 3 ) was used at high temperature growth (1000–1100 °C). At low temperature micro-Raman spectroscopy revealed a significant relaxation of the selection rules for the scattering by the optical phonons in the films grown at lower temperatures. Variation of the intensity ratio for E 2 H and E 1 phonon modes has been attributed to changes in the structural quality of the films grown at different temperatures. At high temperature, the quality of GaN layers were comparable to that of the substrate before growth.

In this study, Raman scattering measurements are reported on the BxGa1–xN layers grown on AlN-on-sapphire templates by MOVPE. The boron content covered the composition range from 0% (pure GaN) to 1.75%. The Raman spectra were recorded at 300 K with a confocal micro-Raman spectrometer in the backscattering geometries. Raman cartography at a lateral resolution of 1 µm was performed to study the spatial variation of the phonon peaks characteristics. The homogeneity of BGaN layers was studied and the phonon modes characteristics are determined as a function of composition, with respect to the laser polarisation. The E2 and A1(LO) modes frequencies are shifted with respect to the pure GaN. The A1(LO) mode behaviour correlates well with the free carriers concentration measured by Hall Effect method, indicating a phonon-plasmon coupling decreasing with increasing boron content in alloy. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; We report epitaxial growth of GaN layers on z- and x-cut LiNbO3 substrates using MOVPE. GaN layers with the thick- ness of 450 nm were characterized using X-ray diffraction. For both, z- and x-cut orientations of LiNbO3 substrates, the GaN layers have c-axis orientation normal to the substrate plane and the in-plane lattice orientation of GaN layers coin- cides with the primary axes of LiNbO3 substrates. Although GaN layers exhibit almost complete strain relaxation, the re- sidual compressive strain determined with respect to a free- standing GaN is of the order of +0.37% and +0.2% for z- and x-cut substrates, respectively.

Symposium W on Current Trends in Optical and X-Ray Metrology of Advanced Materials for Nanoscale Devices III / Spring Meeting of the European-Materials-Research-Society (E-MRS), Strasbourg, FRANCE, MAY 14-18, 2012; International audience; Submicron beam synchrotron-based X-ray diffraction (XRD) techniques have been developed and used to accurately and nondestructively map chemical composition and material quality of selectively grown group nanowires. GaN, AlGaN, and InGaN multi-quantum-well nanowires have been selectively grown on lattice matched and mismatched substrates, and the challenges associated with obtaining and interpreting submicron beam XRD results are addressed and solved. Nanoscale cathodoluminescence is used to examine exciton behavior, and energy-dispersive X-ray spectroscopy is used to verify chemical composition. Scanning transmission electron microscopy is later used to paint a more complete picture. The advantages of submicron beam XRD over other techniques are discussed in the context of this challenging material system.

GaN thin films were grown on ZnO/c-Al 2 O 3 with excellent uniformity over 2 in. diameter wafers using a low temperature/pressure MOVPE process with N 2 as a carrier and dimethylhydrazine as an N source. 5 mm×5 mm sections of similar GaN layers were direct-fusion-bonded onto soda lime glass substrates after chemical lift-off from the sapphire substrates. X-Ray Diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy confirmed the bonding of crack-free wurtzite GaN films onto a glass substrate with a very good quality of interface, i.e. continuous/uniform adherence and absence of voids or particle inclusions. Using this approach, (In) GaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming the expensive sapphire substrate so it can be utilized again for growth.

InGaN-based p-i-n structures were transferred from sapphire to soda-lime glass substrates using two approaches: (1) laser-lift-off (LLO) and thermo-metallic bonding and (2) chemical lift-off (LLO) by means sacrificial ZnO templates and direct wafer bonding. Both processes were found to function at RT and allow reclaim of the expensive single crystal substrate. Both approaches have also already been demonstrated to work for the wafer-scale transfer of III/V semiconductors. Compared with the industry-standard LLO, the CLO offers the added advantages of a lattice match to InGaN with higher indium contents, no need for an interfacial adhesive layer (which facilitates electrical, optical and thermal coupling), no damaged/contaminated GaN surface layer, simplified sapphire reclaim (GaN residue after LLO may complicate reclaim) and cost savings linked to elimination of the expensive LLO process.

Indium Gallium Nitride (InGaN) based PV have the best fit to the solar spectrum of any alloy system and emerging LED lighting based on InGaN technology and has the potential to reduce energy consumption by nearly one half while enabling significant carbon emission reduction. However, getting the maximum benefit from GaN diode -based PV and LEDs will require wide-scale adoption. A key bottleneck for this is the device cost, which is currently dominated by the substrate (i.e. sapphire) and the epitaxy (i.e. GaN). This work investigates two schemes for reducing such costs. First, we investigated the integration of Zinc Oxide (ZnO) in InGaN-based diodes. (Successful growth of GaN on ZnO template layers (on sapphire) was illustrated. These templates can then be used as sacrificial release layers for chemical lift-off. Such an approach provides an alternative to laser lift-off for the transfer of GaN to substrates with a superior cost-performance profile, plus an added advantage of reclaiming the expensive single-crystal sapphire. It was also illustrated that substitution of low temperature n-type ZnO for n-GaN layers can combat indium leakage from InGaN quantum well active layers in inverted p-n junction structures. The ZnO overlayers can also double as transparent contacts with a nanostructured surface which enhances light in/out coupling. Thus ZnO was confirmed to be an effective GaN substitute which offers added flexibility in device design and can be used in order to simultaneously reduce the epitaxial cost and boost the device performance. Second, we investigated the use of GaN templates on patterned Silicon (100) substrates for reduced substrate cost LED applications. Controlled local metal organic chemical vapor deposition epitaxy of cubic phase GaN with on-axis Si(100) substrates was illustrated. Scanning electron microscopy and transmission electron microscopy techniques were used to investigate uniformity and examine the defect structure in the GaN. Our results suggest that groove structures are very promising for controlled local epitaxy of cubic phase GaN. Overall, it is concluded that there are significant opportunities for cost reduction in novel hybrid diodes based on ZnOInGaN-Si hybridization.

International audience; This paper focuses on a comparative study of optical, morphological, microstructural and microcompositional properties of typical InGaN samples which exhibit V-defects but also two additional surface defects features, referred to as inclusion#1 (Ic1) and inclusion#2 (Ic2). HR-XRD, AFM, SEM, STEM and EDX are used to characterize such defects. Furthermore, hyperspectral mapping, spot mode and depth-resolved CL measurements provided useful informations on the optical emission properties and microstructure. The main characteristic of Ic1 luminescence peak is a decrease in intensity and no obvious shift in the CL peak position when going from the outside to the middle of such defect. More interesting was Ic2 which is shown to be local 3D top surface In-rich InGaN domains embedded in an homogeneous InGaN matrix. In fact, this study pointed out that close to the interface GaN/InGaN, it exists a 30 nm thick fully strained InGaN layer with constant indium incorporation. As the growth proceeds spatial fluctuation of the In content is observed and local In-rich 3D domains are shown to emerge systematically around threading dislocations terminations.

International audience; In this paper, we have used simulations to evaluate the impact of the distribution of electrically active defects on the photovoltaic performances of InGaN-based solar cell. The simulations were carried out using Silvaco's ATLAS software. We have modeled a P-GaN/Grad-InGaN/i-In0.53Ga0.47N/Grad-InGaN/N-ZnO where Grad-InGaN corresponds to an InGaN layer with a graded composition. This layer is inserted to eliminate the band discontinuities at the interface between InGaN and the GaN and ZnO layers. The defects were modeled through the introduction of band tails and a Gaussian distribution of defects in i-InGaN material. We have evaluated the influence of band tail widths as well as the parameters of the Gaussian distribution (i.e. defect density, mean position and standard deviation) on the short-circuit current, the open-circuit voltage and the fill-factor (efficiency) of the solar cell. These results have allowed us to identify key structural parameters useful for the optimization of InGaN solar cells, as well as to give realistic estimates of the performances of such cells.

© 2016 IOP Publishing Ltd. Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.

International audience; AlGaN/GaN materials family is one of the best solution to achieve near-UV high reflective Bragg mirrors on GaN substrates. However, the large lattice mismatch occuring between AlGaN and GaN can lead to relaxed structures by the way of cracks which affect the DBR performances. In this work, asymmetrical designs were investigated for the modeling of fully-strained AlGaN/GaN distributed Bragg Reflectors. Such designs should allow to obtain stacks with crack-free surfaces well-adapted for the regrowth of efficient additional monocrystalline layers. First, the critical thickness of MOVPE-grown AlGaN on GaN templates was experimentally determined and modeled. Then, several AlGaN/GaN mirrors with various Al molar fractions and asymmetry factors were simulated demonstrating that non relaxed DBRs could be obtained with adequate parameters. Finally, it has also been shown that there is a best suited Al molar fraction in AlGaN for each DBR centering wavelength.

GaN was grown on ZnO-buffered c-sapphire (c-Al2O3) substrates by Metal Organic Vapor Phase Epitaxy. The ZnO then served as a sacrificial release layer, allowing chemical lift-off of the GaN from the c-Al2O3 substrate via selective wet etching of the ZnO. The GaN was subsequently direct-wafer-bonded onto a glass substrate. X-Ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray microanalysis, Room Temperature Photoluminescence & optical microscopy confirmed bonding of several mm2 of crack-free wurtzite GaN films onto a soda lime glass microscope slide with no obvious deterioration of the GaN morphology. Using such an approach, InGaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming and reusing the substrate.

Metal-semiconductor-metal solar blind ultraviolet photodetectors have been fabricated using both BGaN-GaN and BGaN-AlN superlattices as active layers. A high internal gain (up to 3 × 10 4 for optical power in the nW range) is obtained with a highly reduced dark current thanks to the boron incorporation. In the high optical power regime (W range), the time response is in the nanosecond range, which is much smaller than that of GaNand ZnO-based ultraviolet photodetectors. Moreover, the boron incorporation in GaN material allows the tuning of the cutoff wavelength.

We study the use of a BGaN back-barrier layer in the GaN buffer of Al y Ga1 −y N/GaN highelectron mobility transistors to improve confinement of carriers in the 2D electron gas region. Unlike InGaN back-barrier designs, whose polarization-induced sheet charges form an electrostatic barrier at the backbarrier/ buffer interface, BGaN back-barrier designs create an electrostatic barrier at the channel/backbarrier interface. This can result in carrier confinement to sub-15 nm thickness, even when the channel is 30 nm wide. Although polarization sheet charges due to the BGaN back-barrier form a secondary well at the back-barrier/buffer interface, increasing the thickness of the back-barrier may move the secondary well so that it no longer interacts with the primary channel.

International audience; Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.

International audience; A GaN/ultrathin BGaN/GaN heterojunction is used in AlGaN/GaN high electron mobility transistors (HEMTs) to provide an electrostatic barrier to electrons and to improve the confinement of the 2-dimensional electron gas. BGaN back-barrier layers limit leakage in the GaN buffer thanks to two effects: a polarization-induced band discontinuity and a resistive barrier originating from excellent insulation properties of BGaN. Compared to conventional AlGaN/GaN HEMTs, structures grown with BGaN back-barrier showed a significant improvement of static performances, transport properties, and trapping effects involving a limited current collapse in dynamic regime. A DC maximum current increase of 58.7% was observed

In this paper we demonstrate a solution to systematically obtain thick, single phase InGaN epilayers by MOVPE. The solution consists in periodically inserting ultra-thin GaN interlayers during InGaN growth. Measurements by HAADF-STEM, X-ray diffraction, cathodoluminescence and photoluminescence demonstrate the effective suppression of the three-dimensional sublayer that is shown to spontaneously form in control InGaN epilayers grown without this method. Simulation predicts that tunneling through the GaN barriers is efficient and that carrier transport through this semi-bulk InGaN/GaN structure is similar to that of bulk InGaN. Such structures may be useful for improving the efficiency of InGaN solar cells by allowing thicker, higher quality InGaN absorption layers.

LGEP 2011 ID = 751; International audience; We report on the crystal quality of metalorganic vapour phase epitaxy-grown InGaN with indium content ranging from 0% to 20%. Absorbance measurements are fit to a model including band tails and a defect represented as a Brendel oscillator (R. Brendel, Appl. Phys. A 50, 587, 1990). Band tail absorbance, corresponding to contorted bonds, increases with increased In content. Above 10% of In, the presence of another defect, the concentration of which increases with In content, has been correlated with x-ray diffraction and Raman. We suggest that this defect corresponds to nitrogen vacancies, in agreement with a reported model for GaN.

In the last decades, development of the (Al,Ga,In)N materials has led to new generations of opto- and micro-electronic devices. More recently, novel B(Al,Ga,In)N alloys have been proposed for optical applications in the UV range. Since material containing boron possesses unique properties, the B(Al,Ga,In)N materials system is expected to permit the design of improved and/or novel devices. To evaluate this potential, an improved knowledge of the physical properties of these new materials will be required, however. In this work, investigation of optical, structural, and compositional properties of low-boron content BGaN and BAlN ternary and BInGaN quaternary materials grown through Metalorganic Vapor Phase Epitaxy (MOVPE) are presented. It is shown that inclusion of a small amount of boron strongly affects the optical properties allowing the fabrication of BGaN-based Distributed Bragg Reflectors (DBRs) or Distributed Bragg Confinement layers (DBCs) with large refractive index contrast. Indeed, 1% of boron in BGaN/GaN multilayer structures gives a refractive index contrast of more than 0.1, which is equivalent to that of AlGaN/GaN containing 22% aluminum. The potential of boron-based material technology is illustrated for visible range solar cells applications through the example of BInGaN with good crystalline quality grown on ZnO buffered silicon substrates. It was found that through boron introduction, reduced lattice mismatch, and thus reduced tensile strain, could be obtained for high In contents.

International audience; Large internal gains that can be obtained in wide band gap semiconductors-based (GaN and ZnO types) Schottky and/or metal-semiconductor-metal photodetectors are generally accompanied by large dark current and time response. We show that, using quasi-alloy of BGaN/GaN superlattices as the active layer, the dark current can be lowered while maintaining high internal gain (up to 3 x 10(4)) for optical power in the nW range and low time response (few tens of ns) for optical power in the W range. Furthermore, the boron incorporation allows the tuning of the cutoff wavelength

LGEP 2011 ID = 754; International audience; BxGa1−xN layers as well as BxGa1−xN/GaN Distributed Bragg Reflector (DBR) structures were grown, for boron compositions between 0% and 1.3%, by Metal-Organic Vapour Phase Epitaxy. Refractive index, absorption coefficient and bandgap bowing of BxGa1−xN are extracted from a two-stage procedure based on spectroscopic ellipsometry and reflection measurements in the 250-850 nm range at room temperature. For all compositions of our BGaN alloys, we have obtained good agreement between experimental and simulated curves. It is shown that a large refractive index contrast between BGaN and GaN can be achieved for only 1% of boron. Moreover, the lattice mismatch between B0.01Ga0.99N and GaN is only 0.2%, which can lead to good structural quality of BGaN/GaN DBRs. Those properties can enable the development of innovative BGaN DBR technologies for Vertical Cavity Surface Emitting Lasers (VCSELs) in the blue-violet spectral range.

International audience; InGaN/GaN layers were grown on ZnO-buffered Si (111) substrates by metalorganic vapour phase epitaxy (MOVPE). The dissociation of ZnO observed during conventional MOVPE growth of InGaN/GaN was combated through the use of a low pressure/temperature MOVPE approach with N2 as a carrier gas and dimethylhydrazine added to the ammonia (nitrogen precursor) in order to enhance the concentration of atomic nitrogen at relatively low temperature. Electron Microscopy of cross-sections, High Resolution X-Ray Diffraction (HR-XRD), secondary ion mass spectroscopy and cathodoluminescence studies suggested that single phase wurtzite InGaN layers with between about 17.5 and 21.5% indium were grown epitaxially, with no evidence of back-etching of the ZnO templates. HR-XRD revealed highly pronounced c-axis texture for both the InGaN/GaN and ZnO. Immersion in dilute nitric acid dissolved the ZnO such that the InGaN/GaN could be lifted-off from the substrate.

LGEP 2010 ID = 608; International audience; BInGaN quaternary alloys with up to 2% boron and 14% of indium have been grown on GaN/sapphire template substrates by metal-organic vapour phase epitaxy (MOVPE). Epitaxial layer composition was determined by secondary ion mass spectroscopy (SIMS), and confirmed by X-ray photoelectron spectroscopy (XPS). Bandgap energies were measured using optical transmission and reflection spectroscopy. We find that boron incorporation in BInGaN reduces the bandgap, causing an effect similar to the increase of indium content in InGaN. However, adding boron has the advantage of decreasing the lattice mismatch with conventional GaN substrates.

International audience; BGaN ternary alloys with 0.7% and 1.7% boron grown on GaN/sapphire template substrates by metalorganic vapour phase epitaxy (MOVPE) have been investigated through HAADF-STEM. At low boron content, 0.7%, no compositional fluctuations were observed with XRD or STEM measurements. Photoluminescence at room temperature was measured. At higher boron content, cubic BGaN nano-sized clusters were identified. The clusters are 3 nm wide, homogeneously distributed in size and in density and coherent with the surrounding wurtzite BGaN matrix. Their boron composition was estimated by EDX.

GaN nanodots and nanostripes with smooth sidewall surfaces have been selectively grown on 6H-SiC substrates by metal organic vapor phase epitaxy. By varying the growth reactor pressure, we have been able to grow either isolated nanostructures or laterally overgrown structures. As confirmed by the Raman scattering and X-ray diffraction techniques, the nanostructures have no influence of step bunching that occurs in the unmasked area of the continuous GaN film. The frequency shift of the E2 optical phonons shows that the residual strain in the nanostripes is relaxed compared to the continuous GaN film. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)