S. D. Raevskii

Gan films grown by vapor-phase epitaxy in a hydride-chloride system on Si(111) substrates with AlN buffer sublayers

Oriented GaN layers with a thickness of about 10 μm have been grown by hydride-chloride vaporphase epitaxy (HVPE) on Si(111) substrates with AlN buffer layers. The best samples are characterized by a halfwidth (FWHM) of the X-ray rocking curve of ωθ = 3–4 mrad. The level of residual mechanical stresses in AlN buffer layers decreases with increasing temperature of epitaxial growth. The growth at 1080°C is accompanied by virtually complete relaxation of stresses caused by the lattice mismatch between AlN and Si.

Microstructure of bulk GaN layer grown on sapphire substrates with amorphous buffer

The structure of bulk GaN layers grown on (0001) sapphire substrates by vapor-phase epitaxy has been studied by x-ray diffraction and transmission electron microscopy (TEM). It is found that these layers contain grown-in and screw dislocations. The dislocation density decreases away from the interface. The effect of an amorphous buffer layer on the formation of the initial GaN layer and, thus, on the degree of perfection of gallium nitride layers is elucidated. A model of generating grown-in dislocations and the relaxation mechanism of misfit stresses are proposed.

Photoelectric properties of GaN/GaP heterostructures

Thin interference layers of n-GaN were grown on n-and p-type GaP substrates with (100) and (111) orientations by vapor phase epitaxy in an open chloride system. Photosensitivity spectra of isotypic and anisotypic heterojunctions under linear polarized light from the side of the wideband component (GaN) at oblique incidence were investigated. Induced polarized photosensitivity was observed, and peculiarities of this photosensitivity caused by interference in the GaN layers are discussed.

High-temperature annealing of bulk GaN layers

The influence of high-temperature (1010°C) annealing in a flow of gaseous ammonia on the properties of bulk GaN layers, grown by chloride-hydride VPE and then separated from SiO2 substrates, was studied using atomic force microscopy. The bulk (∼360-μm-thick) epitaxial GaN layers were synthesized in two steps: a first stage of nucleation and growth at a low temperature (530°C) followed by epitaxy at a high temperature (970°C). It was found that the annealing increases the nanorelief height and activates the donor-acceptor recombination on the surface of a GaN layer grown at the lower temperature and decreases the intensity of photoluminescence from the layer grown at the higher temperature.

Initial stages of the GaN growth on oxidized silicon

Initial stages of the GaN layer growth during an HVPE process at 520°C on an oxidized silicon substrate were studied by atomic force microscopy. It was established that (i) the growth of GaN islands is controlled by the surface diffusion and (ii) the nucleus size distribution on the surface significantly changes when the growth time increases from 10 to 200 min: during this period of time, the average nucleus size increases from 15 to 400 nm and their size scatter becomes 20 times as large as the initial size distribution width. The experimentally determined growth rate of the GaN nuclei and the nucleus size distribution are in good agreement with theoretical calculations.

Bulk GaN layers grown on oxidized silicon by vapor-phase epitaxy in a hydride-chloride system

Bulk GaN layers with a thickness of up to 1.5 mm and a lateral size of 50 mm were grown by hydride-chloride vapor-phase epitaxy (HVPE). The best samples show a half-width (FWHM) of the X-ray rocking curve of ωθ=27′, a charge carrier density of ∼8 × 1019 cm−3, and a mobility of ∼50 cm2/(V s). The photoluminescence spectra of the obtained epitaxial GaN layers exhibit an edge emission band at 348 eV. The HVPE layers are characterized by a high mechanical strength: HV = 14 GPa.

Bulk large-area GaN layers

Mirror-smooth, transparent bulk GaN layers with an area of 2×3 cm2 and a thickness of up to 1 mm were grown by hydrochloride vapor-phase epitaxy. Cracking of the material was eliminated by using a two-stage growth process; separation from a substrate was provided by a low-temperature buffer layer of preset thickness. For the best samples, FWHM of the X-ray rocking curve was ωθ=3.5′ and the dislocation density amounted to 107–108 cm−2.

Absorption spectra of GaN:Er3+ crystals

We have studied the f-f absorption spectra of Er3+ in GaN crystal grown by gas phase epitaxy in chlorine system. A detailed study of 4I15/2 yields 2H11/2 transitions in the region 19000-19260 cm-1, has been carried out at 293, 77 and 2K. At T equals 2K the 4I15/2 yields 2H11/2 spectrum contains 6 lines in accordance with theoretical predictions for Er3+ in noncubic crystal field. The levels of 2H11/2 multiplet are situated at: 19041, 19049, 19074, 19108, 19191, 19246 cm-1. The number of lines observed at T equals 2K and its small width indicate that in our samples Er3+ ions occupy predominantly a unique position in GaN lattice. Most probably, Er3+ substitutes Ga atoms or enters in interstitial position.

The absorption spectra of gallium nitride crystals doped with erbium ions

We have measured the absorption spectra of Er3+ ions introduced by diffusion into bulk GaN crystals grown by vapor phase epitaxy in a chloride system. The wavelength interval of 518–527 nm corresponding to the region of the 4I15/2→2H11/2 transition was studied in detail at 293, 77, and 2 K. At 2 K, the spectrum of this transition displays six lines, which correspond to the theoretically possible number of sublevels of the 2H11/2 state of Er3+ ion occurring in a noncubic crystal field. The positions of levels in the 2H11/2 multiplet correspond to 2.360, 2.361, 2.365, 2.369, 2.379, and 2.386 eV. Both the number and narrow width of lines observed at 2 K indicate that Er3+ ions predominantly occupy the same regular position in GaN crystals. Most probably, erbium ions occupy gallium sites in the crystal lattice.

Photoluminescence of n-GaN: Influence of chemical treatment of the surface using sulfide solutions

Results are presented of the photoluminescence of n-GaN (T=300 K) after chemical treatment of the surface using solutions of inorganic sulfides (Na2S and (NH4)2S) in water or isopropanol. It is shown that the maximum intensity of the photoluminescence spectrum of n-GaN increases after chemical treatment of the surface using alcohol solutions of sulfides and this increase is greater for solutions of the strong-base sulfide Na2S compared with the weak-base sulfide (NH4)2S.