H. Helava

Deep hole traps in undoped n-GaN films grown by hydride vapor phase epitaxy

Deep hole traps were studied in bulk free-standing GaN crystals and in thinner (10–20u2009μm) GaN films prepared by hydride vapor phase epitaxy (HVPE) on sapphire. Six hole traps in different combinations were detected in these crystals, H1 (activation energy 0.92–0.94u2009eV), H2 (0.55u2009eV), H3 (0.65–0.7u2009eV), H4 (0.85–0.9u2009eV), H5 (1.1–1.2u2009eV), and H6 (0.95–1.05u2009eV). The dominant traps in all samples were the H5 and H6 traps that were attributed, respectively, to gallium vacancy complexes with oxygen (VGa-O) and substitutional carbon related centers. We associate the H5 hole traps with the red luminescence bands, the H4 hole traps with the green luminescence bands, and the H6 hole traps with the yellow luminescence bands often observed in HVPE GaN. These attributions are based on the low energy thresholds of the deep traps optical excitation spectra and the depth of the respective trap levels.

Short-period superlattices of AlN/Al0.08Ga0.92N grown on AlN substrates

High-quality short-period superlattices of AlN∕Al0.08Ga0.92N have been grown by gas-source molecular-beam epitaxy with ammonia on Al face of AlN (0001) substrates. A significant reduction was achieved in the dislocation density, down to 3×108cm−2. Complete removal of residual Al2O3 surface oxide is needed in order to obtain low dislocation density in homoepitaxy on AlN. We show that the presence of Al2O3 islands with the surface coverage as low as 0.2% results in increased dislocation density.

Optical phonon decay in bulk aluminum nitride

We report Raman studies of the E22 and A1(LO) symmetry phonons of bulk AlN from 13to375K. Based on observed dependences of the phonon energies and linewidths, and accounting for the temperature-dependent thermal expansion, we determine that both phonon lifetimes are limited by two-phonon decay. The E22 decays symmetrically. The A1(LO) asymmetrically decays into vibrations with energies 586 and 309cm−1 which we attribute to A1(TO) and LA phonons from the M point of the Brillouin zone.

Growth of AlN Bulk Crystals by Sublimation Sandwich Method

We report on new results in AlN bulk crystal growth by sublimati on sandwich method. The single crystals of 12 mm in diameter and up to 5 mm long have bee n o tained with the growth rate up to 0.5 mm/h. Growth was carried out in a nitrogen environment at temperatures of 19002300°C and atmospheric pressure. 4Hand 6H-SiC both of (0001)C and (0001)Si orienta ti ns were used as seeds. Growth kinetics and operating conditions favorable for impr oving the long-term process stability and crystal quality are discussed on the basis of both experimental and theoretical studies. The grown AlN crystals are characterized by X-ray diffraction and R man scattering.

Growth of Faceted Free-Spreading SiC Bulk Crystals by Sublimation

We report on a new method of free-spreading SiC bulk crystal grow th by sublimation, where the crystal is kept out of touch with the crucible side wall and SiC polycrystalline deposits. The free-spreading SiC tends to form a crystal of pronounced hexagonal sha pe during the growth. The 6Hand 4H-SiC crystals up to 35 mm in diameter with the quality compar able or better than that of the Lely crystals are obtained. The growth conditions favorable for the f ree-spreading crystal growth are discussed. Different efficiencies of donor (N) and acceptor (Al) i ncorporation are observed on singular facets.

Evaluation of the concentration of point defects in GaN

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016u2009cm−3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.

Sublimation Growth of AlN and GaN Bulk Crystals on SiC Seeds

Growth techniques of high quality AlN and GaN bulk crystals on SiC seeds by sublimation sandwich method are presented. GaN crystals were grown in the temperature range of 1100-1250 °C and with addition of ammonia (NH3) to prevent GaN decomposition. GaN powder or metallic Ga was used as the source. AlN crystals up to 2 inch diameter have been grown on SiC seeds in the temperature range of 1950 -2050 0С. Kinetic mechanisms and transport features in the sandwich cell are discussed. The achieved high crystal quality has allowed producing semiconductor devices on their basis, in particular, ultraviolet LEDs

Structural and electric properties of AlN substrates used for LED Heterostructures’ growth

The structural characteristics and electrical properties of bulk aluminum nitride crystals grown by sublimation and used as substrates for light emitting diode (LED) structures and AlGaN/GaN field effect transistors were studied. The crystalline perfection was assessed by selective chemical etching and by X-ray diffraction techniques. Electrical and optical properties were investigated using the temperature dependence of conductivity, admittance spectroscopy, high-temperature/low-frequency capacitance voltage measurements and by photoinduced transient current spectroscopy (PICTS), microcathodoluminescence (MCL) spectra and MCL imaging techniques. It was established that the studied samples were single crystals with a large grain substructure, with characteristic grain size of several hundred microns and a dislocation density of 102–104 cm−2 inside the grains. The electrical characteristics of the crystals were governed by the compensation of residual donors with a level near Ec—0.3 eV by deep centers with activation energy of 0.7 eV, both centers manifesting themselves in the temperature dependence of conductivity and in admittance spectra. In addition, deep centers responsible for the luminescence band with the peak energy of 3.3 eV and associated with low-angle grain boundaries were also observed.

Modeling Analysis of Free-Spreading Sublimation Growth of SiC Crystals

Recently, an advanced technique for growing free-spreading SiC bulk crystals by sublimation has been demonstrated. This method was used to grow 6H- and 4H-SiC boules free of polycrystalline deposits on the crystal periphery, up to 35 mm in diameter with the micropipe density less than 20 cm -2 and the dislocation density about 10 2 -10 3 cm -2 . In this paper, we report on the numerical modeling of free-spreading crystal growth. We consider the global heat transfer in an inductively heated growth system, species transport in the growth cell and in the powder charge, and thermoelastic stress, focusing on the crystallization front dynamics, poly-SiC deposition, and powder source evolution. Special attention was given to the validation of the simulations. The computed thermal field and evolution of the powder and crystal shape were found to agree qualitatively with observations.

Photo-Electrochemical Etching in the Process of Direct H2 Generation by Illumination of GaN-Based Material Structures Immersed in Water

Hydrogen is considered a promising candidate for energy storage. We investigated the cleanest way for hydrogen production by direct photo electrolysis of water with GaN/AlGaN based p-n structures used as working electrodes. Besides the H2 production rate, an important consideration is the material etching (corrosion) that accompanies the photo-electrochemical process. The GaN-based structures were grown on sapphire substrates by the chloride hydride vapor phase epitaxy and used as a photo anode immersed into an aqueous electrolyte. For a p-n GaN/AlGaN structure we observed a H2 production rate of 0.6 mL/cm2×h. Corrosion of the electrode proceeds in two steps. First, there is a near vertical etching process, which is associated with defects in the material and penetrates deep into the structure. Subsequently, the process involves etching of n-type layers in lateral direction resulting in the formation of voids and cavities. The lateral etching is due to net positive charges arising from the spontaneous and...