T. Suski

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.

Large, nitrogen-induced increase of the electron effective mass in InyGa1−yNxAs1−x

A dramatic increase of the conduction band electron mass in a nitrogen-containing III–V alloy is reported. The mass is found to be strongly dependent on the nitrogen content and the electron concentration with a value as large as 0.4m0 in In0.08Ga0.92As0.967N0.033 with 6×1019 cm−3 free electrons. This mass is more than five times larger than the electron effective mass in GaAs and comparable to typical heavy hole masses in III–V compounds. The results provide a critical test and fully confirm the predictions of the recently proposed band anticrossing model of the electronic structure of the III–N–V alloys.

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 .

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.

Effect of Si doping on the dislocation structure of GaN grown on the A‐face of sapphire

Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.

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.

Determination of the effective mass of GaN from infrared reflectivity and Hall effect

Infrared reflectivity and Hall effect measurements were performed on highly conducting n‐type GaN (n≊6×1019 cm−3) bulk crystals grown by the high‐pressure high‐temperature method. Values of electron‐plasma frequency and free‐electron concentration were determined for each sample of the set of seven crystals. It enabled us to calculate the perpendicular effective mass of electrons in the wurtzite structure of GaN as m*=0.22±0.02 m0. Effects of nonparabolicity and a difference between parallel and perpendicular components of the effective mass are small and do not exceed the experimental error.

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

Hardness and fracture toughness of bulk single crystal gallium 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.