I. Grzegory

ELASTIC CONSTANTS OF GALLIUM NITRIDE

The elastic constants of GaN have been determined using Brillouin scattering; in GPa they are: C11=390,  C33=398,  C44=105,  C66=123,  C12=145,  and   C13=106. Our values differ substantially from those quoted in the literature which were obtained from the determination of mean square displacement of atoms measured by x‐ray diffraction.

Lattice parameters of gallium nitride

Lattice parameters of gallium nitride were measured using high‐resolution x‐ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.

Chemical polishing of bulk and epitaxial GaN

Abstract Bulk single crystals of GaN and heteroepitaxial GaN layers were subjected to free-etching and mechano-chemical polishing in aqueous solutions (10-1N) of KOH and NaOH. It has been established that free-etching of bulk crystals is kinetically controlled and strongly anisotropic, resulting in the formation of numerous stable pyramids. Etching is terminated when the {0 0 0 1}-oriented surface of GaN is completely covered by these pyramids. On the other hand, when a soft polishing pad and pressure above 2 kg/cm2 are employed, all surface irregularities (etch pyramids, roughness after mechanical polishing, growth hillocks on epitaxial layers) are removed. The procedure is very effective: removal of a few tenths of a micron from the surface are usually sufficient to polish out irregularities 200 nm in height. When optimized mechano-chemical polishing is used, atomically flat surfaces of bulk GaN have been reproducibly obtained (RMS= 0.1nm), as measured by Atomic Force Microscopy.

Investigation of longitudinal‐optical phonon‐plasmon coupled modes in highly conducting bulk GaN

We report a cross‐correlated investigation, performed by means of Raman scattering and infrared spectroscopy, of coupled LO phonon‐plasmon modes in bulk GaN. Using different samples with different (high) residual concentrations of free carriers, we find that the high‐energy Raman mode follows closely the plasma frequency resolved from the infrared data. On the opposite, the low‐frequency modes appears down shifted, with respect to the standard TO phonon frequency, by about 11 cm−1. Both findings agree satisfactorily with predictions of the linear response theory for undamped phonon‐plasmon modes and establish Raman scattering as a powerful and nondestructive tool to investigate the residual doping level of GaN up to about 1020 cm−3 .

Thermodynamical properties of III–V nitrides and crystal growth of GaN at high N2 pressure

Abstract In this paper, thermodynamical properties of AIN, GaN and InN are considered. It is shown that significant differences in melting conditions, thermal stability and solubilities in liquid group III metals lead to different possibilities of growing crystals from high temperature solutions, at N 2 pressure up to 20 kbar. It is shown that the best conditions for crystal growth at available pressure and temperature conditions can be achieved for GaN. High quality 6–10 mm single crystals of GaN have been grown at high N 2 pressure in 60–150 h processes. The mechanisms of nucleation and growth of GaN crystals are discussed on the basis of the experimental results. The crystallization of AlN is less efficient due to relatively low solubility of AlN in liquid Al. Possibility for the growth of InN crystals is strongly limited since this compound loses its stability at T > 600°C, even at 2 GPa N 2 pressure. The crystals of GaN grown at high pressure are the first crystals of this material used for homoepitaxial layer deposition. Both MOCVD and MBE methods have been successfully applied. Structural, electrical and optical properties of both GaN single crystals and homoepitaxial layers are reviewed.

Mechanism of yellow luminescence in GaN

We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.

Thermal expansion of gallium nitride

Lattice constants of gallium nitride (wurzite structure) have been measured at temperatures 294–753 K. The measurements were performed by using x‐ray diffractometry. Two kinds of samples were used: (1) bulk monocrystal grown at pressure of 15 kbar, (2) epitaxial layer grown on a sapphire substrate. The latter had a smaller lattice constant in a direction parallel to the interface plane by about 0.03%. This difference was induced by a higher thermal expansion of the sapphire with respect to the GaN layer. However, this thermal strain was created mainly at temperatures below 500–600 K. Above these temperatures the lattice mismatch in parallel direction diminished to zero at a temperature of about 800 K.

Temperature dependence of the energy gap in GaN bulk single crystals and epitaxial layer

We performed optical‐absorption studies of the energy gap in various GaN samples in the temperature range from 10 up to 600 K. We investigated both bulk single crystals of GaN and an epitaxial layer grown on a sapphire substrate. The observed positions of the absorption edge vary for different samples of GaN (from 3.45 to 3.6 eV at T=20 K). We attribute this effect to different free‐electron concentrations (Burstein–Moss effect) characterizing the employed samples. For the sample for which the Burstein shift is zero (low free‐electron concentration) we could deduce the value of the energy gap as equal to 3.427 eV at 20 K. Samples with a different free‐electron concentration exhibit differences in the temperature dependence of the absorption edge. We explain the origin of these differences by the temperature dependence of the Burstein–Moss effect.

High pressure growth of bulk GaN from solutions in gallium

In this paper, the growth of GaN single crystals from solutions of atomic nitrogen in liquid gallium under high N2 pressure is described. GaN single crystals obtained by the high nitrogen pressure solution method, without an intentional seeding, show strong growth anisotropy, which results in their platelet shape. The attempts to enhance the growth into (0001) directions by the increase of the integral supercooling in the solution often lead to growth instabilities on both N-polar and Ga-polar (0001) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface. The results of the seeded growth in directions parallel and perpendicular to the c-axis of GaN are discussed. In particular, it is shown that dominating mechanisms of the unstable growth on (0001) polar surfaces such as the cellular growth or edge nucleation can be suppressed and the crystal can be grown in a much more stable way. Physical properties most relevant for understanding the growth of GaN crystals are reviewed. The most important feature of GaN crystals grown by the HNPS method is that they are almost free of dislocations and therefore used as substrates give a possibility to grow perfect epitaxial structures.

Luminescence and reflectivity in the exciton region of homoepitaxial GaN layers grown on GaN substrates

Abstract In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: E A = 3.4780eV, E B = 3.4835eV and E C = 3.502eV.