M. Bockowski

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.

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.

Thermal conductivity of GaN crystals in 4.2- 300 K range

Results of measurements of thermal conductivity of bulk GaN crystals in the temperature interval 4.2 – 300 K are reported. Experiments were performed on two types of single GaN crystals grown under high-pressure: highly conducting n-type sample and on a highly resistive sample compensated by magnesium doping. For n-GaN crystals, the highest thermal conductivity kmax is equal to 1600 W/m K at Tmax ¼ 45 K; and k . 220 W/m K at 300 K. Our analysis indicates that for the best n-GaN crystal and for T

Thermal properties of indium nitride

Tmax; the contribution of Umklapp phonon scattering processes dominate whereas for other samples scattering of phonons by point mass defects represents the main contribution. The dependence of kðTÞ is used to reveal possible mechanisms of thermal resistance of GaN crystals at temperatures Tmax: Our thermal conductivity measurements yields Debye’s temperature uD < 400 K: q 2003 Elsevier Ltd. All rights reserved.

High mobility two-dimensional electron gas in AlGaN∕GaN heterostructures grown on bulk GaN by plasma assisted molecular beam epitaxy

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

The results on growth and magnetotransport characterization of AlGaN∕GaN heterostructures obtained by plasma assisted molecular beam epitaxy on dislocation-free (below 100cm−2) GaN high pressure synthesized bulk substrates are presented. The record mobilities of the two dimensional electron gas (2DEG) exceeding 100000cm2∕Vs at liquid helium temperature and 2500cm2∕Vs at room temperature are reported. An analysis of the high field conductivity tensor components allowed us to discuss the main electron scattering mechanisms and to confirm unambiguously the 2DEG room temperature mobility values.

Lattice constants, thermal expansion and compressibility of gallium nitride

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.

Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing

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.

High pressure phase transition in aluminium nitride

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 °C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values ρ and the ρ(T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, EA≈0.15 eV), and the second one much more deep, E2≈0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ 106 Ω cm at ambient temperature) in samples with dominant E2 states. For the first type of samples, annealing at Tann<500 °C leads to a decrease of their resistivity and is associated with a...