J. Jun

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.

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.

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.

III-V nitrides : thermodynamics and crystal growth at high N2 pressure

Abstract In this paper, thermodynamical properties of AIN, GaN and InN, melting, thermal stability and solubility in liquid Al, Ga and In at N 2 pressures up to 20 kbar are considered. It is shown that significant differences in the thermodynamical properties of AlN, GaN and InN are caused mainly by different bonding energy in the solid phase. These differences lead to different results in the crystal growth of AlN, GaN and InN from the solutions in liquid Al, Ga and In, at high nitrogen pressure. High quality, 1-mm single crystals of GaN can be grown in a 5–24 h processes. The crystallization of AlN is less efficient due to the relatively low solubility of AlN in liquid Al, in the experimentally accessible temperature range. Possibility for the growth of InN crystals is strongly limited since this compound loses its stability at T > 600 °C, even at 20 kbar N 2 pressure. The mechanisms of nucleation and growth of GaN crystals is discussed on the basis of the experimental results. The quality of the 1-mm and 1-cm GaN single crystals is compared and discussed in terms of growth stability, which is the necessary condition for obtaining high quality, large single crystals of GaN. The physical properties of pressure grown crystals are reviewed briefly.

Homoepitaxy of GaN on polished bulk single crystals by metalorganic chemical vapor deposition

Bulk single crystals of GaN were used for epitaxial growth of GaN by metalorganic chemical vapor deposition. Photoluminescence (at 2 K) from polished substrates yields a broad near‐band‐edge emission band centered at 3.32 eV and the commonly observed yellow luminescence band. In contrast, the epitaxial layer displays a strong, sharp bound exciton line at 3.458 eV and a weak yellow band. Transmission electron microscopy reveals a sharp, planar interface between substrate and epilayer: The substrate contains small Ga inclusions, and the epilayer consists of less than 108 dislocations per cm2, mostly in the form of dislocation loops, which originate at the interface.Bulk single crystals of GaN were used for epitaxial growth of GaN by metalorganic chemical vapor deposition. Photoluminescence (at 2 K) from polished substrates yields a broad near‐band‐edge emission band centered at 3.32 eV and the commonly observed yellow luminescence band. In contrast, the epitaxial layer displays a strong, sharp bound exciton line at 3.458 eV and a weak yellow band. Transmission electron microscopy reveals a sharp, planar interface between substrate and epilayer: The substrate contains small Ga inclusions, and the epilayer consists of less than 108 dislocations per cm2, mostly in the form of dislocation loops, which originate at the interface.

Thermal properties of indium nitride

Abstract Indium nitride single crystals, grown by the nitrogen microwave plasma method have been used in the determination of thermal properties of InN. Specific heat of InN was measured in the temperature interval between 150 and 300 K. InN Debye temperature and Gruneisen parameter calculated from these data are: Θ = 660 K and γ = 0.77. Thermal conductivity of InN has been measured by the laser-flash method. The InN thermal conductivity, obtained from measurement of InN ceramics, was 45 W/(m·K) This is much below 176 W/(m·K), the ideal lattice estimate based on phonon-phonon inelastic scattering, indicating a large contribution from point defects and grain boundaries to phonon scattering. InN vs. In + N 2 stability has been studied by ultra-high-pressure X-ray measurements: for nitrogen pressure p = 60 kbar, InN has been found to be stable up to T = 710 ± 10 °C. It has been also demonstrated that the decomposition of InN at temperatures below 660 °C is kinetically controlled.

The microstructure of gallium nitride monocrystals grown at high pressure

Abstract This work shows the results of X-ray diffractometric measurements performed on gallium nitride (wurtzite structure, (00.1) oriented plates) bulk crystals grown using the high-pressure (12–15 kbar)-high-temperature (about 1800 K) method. The examinations included: rocking-curve analysis, reciprocal lattice mapping, topography and measurements of lattice parameters. Monocrystals of size up to about 1 mm exhibit a very high crystallographic perfection (rocking curves of 20–30 arc sec). Bigger crystals possess a mosaic structure (0.1–1 mm crystallites separated by 1–3 arc min angle boundaries) visualised by X-ray topography. A model of the creation of those low-angle boundaries is proposed. It is based on the following observations: (i) the mosaic crystals are dome-shaped; (ii) the concave side is a “nitrogen-terminating” one, which grows faster; (iii) this side possesses smaller lattice parameters with respect to the “gallium-terminating” side. We have related the decrease of the lattice parameters to the gallium precipitation (observed in electron microscopy) beneath the “nitrogen-terminating” side. The difference between the lattice parameters on the two sides of the crystal causes its bending, which is then relaxed by emission of the low-angle boundaries.

Lattice constants, thermal expansion and compressibility of gallium nitride

High-resolution X-ray diffraction measurements can be performed at variable temperatures and pressures. The usefulness of such experiments is shown when taking gallium nitride, which is a wide-band semiconductor, as an example. The GaN samples were grown at high pressures (bulk crystals) and as epitaxial layers on silicon carbide and sapphire. The X-ray examinations were done at temperatures of 293-750 K and at pressures of up to 8 kbar. The results served for an evaluation of the basic physical properties of gallium nitride; namely lattice constants, thermal expansion and compressibility. The comparison of monocrystals with epitaxial layers grown on highly mismatched substrates provided important information about the influence of the substrate on the crystallographic perfection of the layers.