Low Al-content n-type AlxGa1-xN layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic\ua0chemical vapor deposition\ua0to deliver high-quality\ua0n-AlxGa1−xN (x\ua0= 0\ua0–\ua00.12, [Si] = 1 71017\ua0cm−3)\ua0epitaxial layers\ua0on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13\ua0–\ua00.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5 71015\ua0cm−3\ua0and carbon of 2 71016\ua0cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while\ua0screw dislocations\ua0only raise for\ua0x\ua0above 0.077. The room temperature\ua0electron mobility\ua0of the\ua0n-AlxGa1−xN remain in the range of 400\ua0–\ua0470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties

Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 113, 203513 (2013) and may be found at https://doi.org/10.1063/1.4807581.We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104 cm−2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures

Ion implantation in β-Ga2O3 : Physics and technology

Gallium oxide, and in particular its thermodynamically stable β-Ga2O3 phase, is within the most exciting materials in research and technology nowadays due to its unique properties. The very high breakdown electric field and the figure of merit rivaled only by diamond have tremendous potential for the next generation “green” electronics enabling efficient distribution, use, and conversion of electrical energy. Ion implantation is a traditional technological method used in these fields, and its well-known advantages can contribute greatly to the rapid development of physics and technology of Ga2O3-based materials and devices. Here, the status of ion implantation in β-Ga2O3 nowadays is reviewed. Attention is mainly paid to the results of experimental study of damage under ion irradiation and the properties of Ga2O3 layers doped by ion implantation. The results of ab initio theoretical calculations of the impurities and defect parameters are briefly presented, and the physical principles of a number of analytical methods used to study implanted gallium oxide layers are highlighted. The use of ion implantation in the development of Ga2O3-based devices, such as metal oxide field-effect transistors, Schottky barrier diodes, and solar-blind UV detectors, is described together with systematical analysis of the achieved values of their characteristics. Finally, the most important challenges to be overcome in this field of science and technology are discussed

Anisotropy, Phonon Modes, and Free Charge Carrier Parameters in Monoclinic β-Gallium Oxide Single Crystals

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic β-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and (2̅01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent β-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with Au and Bu symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in β-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in β-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes.We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for β-Ga2O3

Long-lived excitons in thermally annealed hydrothermal ZnO

Applying thermal annealing to hydrothermal ZnO crystals an enhancement of exciton lifetime from 80 ps to 40 ns was achieved boosting PL quantum efficiency of the UV luminescence up to 70 %. The lifetime improvement is related to the reduced density of carrier traps by a few orders of magnitude as revealed by the reduction of the slow decay tail in pump probe decays coupled with weaker defects-related PL. The diffusion coefficient was determined to be 0.5 cm2/s, providing a large exciton diffusion length of 1.4 μm. The UV PL lifetime drop at the lowest exciton densities was explained by capture to traps. Release of holes from acceptor traps provided delayed exciton luminescence with ∼200 μs day time and 390 meV thermal activation energy. Pump-probe decays provided exciton absorption cross-section of 9 × 10−18 cm2 at 1550 nm wavelength and verified the PL decay times of excitons. Amplitudes and decay times of the microsecond slow decay tails have been correlated with the trap densities and their photoluminescence. A surface recombination velocity of 500 cm/s and the bimolecular free carrier recombination coefficient 0.7 × 10−11 cm3/s were calculated. Therefore, the properly annealed hydrothermally grown ZnO can be a viable and integral part of many functional devices as light-emitting diodes and lasers