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The electronic structure near the band gap of the corundumlike a phase of Ga2O3 has been investigated by means of optical absorption and spectroscopic ellipsometry measurements in the ultraviolet (UV) range (400-190 nm). The absorption coefficient in the UV region and the imaginary part of the dielectric function exhibit two prominent absorption thresholds with wide but well-defined structures at 5.6 and 6.3 eV which have been ascribed to allowed direct transitions from crystal-field split valence bands to the conduction band. Excitonic effects with large Gaussian broadening are taken into account through the Elliott-Toyozawa model, which yields an exciton binding energy of 110 meV and direct band gaps of 5.61 and 6.44 eV. The large broadening of the absorption onset is related to the slightly indirect character of the material., This work has been supported by the Spanish MINECO/FEDER under Contracts No. MAT2015-71035-R and No. MAT2016-75586-C4-1-P. We acknowledge T. Yamaguchi from the Department of Electrical Engineering and Electronics of Kogakuin University, Tokyo, Japan, for supplying us with the samples used in this study.

Vertical oriented GaNmicrorods were grown by metal organic vapor phase epitaxy with four different n type carrier concentration sections above 1019 cm amp; 8722;3 along the c axis. In cathodoluminescence investigations carried out on each section of the microrod, whispering gallery modes can be observed due to the hexagonal symmetry. Comparisons of the spectral positions of the modes from each section show the presence of an energy dependent mode shift, which suggest a carrier induced refractive index change. The shift of the high energy edge of the near band edge emission points out that the band gap parameter in the analytical expression of the refractive index has to be modified. A proper adjustment of the band gap parameter explains the observed whispering gallery mode shift

In this work we systematically study the structural, optical and electrical properties of AlGaN/GaN heterostructures grown by MOCVD 3-inch on SiC substrates Thefinally developed HEMTs demonstrate excellent highvoltage stability, high power performance and large power added efficiencies. For a drain bias of 100 V an output-power-density around 26 W/mm with 25 dB linear gain is obtained. On 36 mm gate width devices an output power beyond 100 W is achieved with a power added efficiency above 60% and a linear gain around 17 dB. Ruggedness on these large devices is proven by successfully passing harsh intentional device mismatch tests during operation at 50 V. Reliability is tested at a drain bias of 50 V. Under DC conditions a drain-current degradation below 20% after 20 years is extrapolated. Under RF stress the observed change in output power is well below 0.1 dB after a test duration of more than 500h.

International audience; A comprehensive characterization of the optical properties of wurtzite InN films grown by molecular beam epitaxy on Si(111) substrates is presented. Two types of films are investigated in this work: InN on AlN/Si(111) and InN on GaN/AlN/Si(111). Their properties are compared to a layer deposited on GaN/ sapphire substrate. The dielectric function (DF) is obtained from spectroscopic ellipsometry (SE). The infrared studies yield the plasma frequency and thus the electron density, while the interband absorption is probed between 0.56 and 9.8 eV. For InN grown on Si(111) substrate, the absorption onset is slightly shifted to higher energies with respect to the InN film grown on GaN/sapphire which can be attributed to higher electron concentrations. Despite this, strongly pronounced optical transitions due to critical points of the band structure are found in the high-energy part of the DF. It emphasizes the already promising quality of the InN films on silicon. Band-gap renormalization (BGR), band filling, and strain are taken into account in order to estimate the intrinsic band gap of wurtzite InN. For the InN layers on silicon, we get a band gap between 0.66 and 0.685 eV.

The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped Al(x)Ga(1-x)N/GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6x10(12) to 1.2x10(13) cm(-2). A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26+-0.04) and (0.61+-0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-content is the same for samples grown with different techniques. Thus, the interplay between epitaxy and process technology defines the threshold voltage.

Ga-face GaN/AlGaN/GaN heterostructures with different cap thicknesses are investigated by electroreflectance spectroscopy (ER). The voltage dependent electric field strengths of the barrier and cap layers are determined. The AlGaN electric field amounts of up to -2.6 MV/cm, whereas the GaN electric field is always below 700 kV/cm. The two electric fields have opposite signs. Characteristic features in the voltage maps of the ER spectra are assigned to the formation/depletion of a two-dimensional electron gas below and a two-dimensional hole gas above the AlGaN barrier. Between -6.5 and 0 V, both carrier gases coexist.

The MBE grown anti-modulation doped GaAs/AIGaAs structures with near surface single quantum wells (QWs) were exposed to a DC hydrogen plasma (~400eV) and investigated using PL, PLE and PR spectroscopy at 5 K. Strong acceptor related free to bound transition (FB) dominates for QW related PL but excitonic features are still observed in PLE spectra. After hydrogen plasma treatment the PL intensity of FB transition from QW was strongly increased for above AIGaAs band gap excitation and was unchanged for below AIGaAs one. These results are consistent with atomic hydrogen passivation of deep defects in Al-GaAs barriers. At the same time radiative excitonic recombination was quenched by hydrogenation. PLE and PR spectra indicate on a strong increase of electric field in subsurface region of the structure after hydrogenation. The increase of electric field GaAs barriers. At the same time radiative excitonic recombination was quenched by hydrogenation. PLE and PR spectra indicate on a strong increase of electric field in subsurface region of the structure after hydrogenation. The increase of electric fieldin anti-modulation doped structure after hydrogen plasma treatment is supposed to be due to passivation by atomic hydrogen of surface states that leads to unpinning of Fermi level from mid gap to carbon acceptor level position in GaAs cap layer. It causes the further band bending and surface electric field increase that strongly suppress excitonic recombination in near surface QWs.