We report on the emission properties of nonpolar $a$ -plane GaN layers grown on $r$ -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the $3.0–3.5\mathrm{eV}$ spectral range. In the near band-edge region the emission lines of the donor-bound excitons $\left(3.472\mathrm{eV}\right)$ and free excitons $\left(3.478\mathrm{eV}\right)$ are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The $3.42\mathrm{eV}$ emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and $30\mathrm{meV}$ ) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at $3.29\mathrm{eV}$ is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The $3.29\mathrm{eV}$ emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.

We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the “free-to-bound” recombination model in degenerate semiconductors. For carrier concentrations above $n>\mathrm{}5\mathrm{}\times {10}^{18}{\mathrm{cm}}^{-3}$ the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is $E_g=692\mathrm{}\pm 2\mathrm{meV}$ for an effective mass at the conduction-band minimum $m_{n0\mathrm{}}{=0.042m}_0.$

The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to $[\mathrm{Mg}]\approx 2\times {10}^{19}{\mathrm{cm}}^{-3}$. Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simultaneously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in $p$-GaN.

The thermal conductivity of undoped, Sn-doped, and Fe-doped β-Ga₂O₃ bulk crystals was measured by the 3ω technique in the temperature range of 295–410 K. A unique approach for extracting the thermal conductivity along the lateral and transverse heat flow directions was used in order to determine the thermal conductivity along different crystallographic directions. The data analysis at room temperature confirmed the expected anisotropy of the thermal conductivity of β-Ga₂O₃, revealing the highest value of ∼29 W/m K in the [010] direction. The thermal conductivity of the Sn-doped and Fe-doped β-Ga₂O₃ samples was found to be lower than that of the undoped samples due to the enhanced phonon-impurity scattering contribution, which reduces the thermal conductivity. This tendency was maintained for the thermal conductivity at elevated temperatures. The thermal conductivity in all samples decreased with increasing temperature, but the slope of the temperature dependence was found to depend on both the doping and the crystallographic orientation.

We report optical phonon frequency studies in anisotropically strained $c$-plane- and $a$-plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN $E_1\left(\mathrm{TO}\right)$, $E_1\left(\mathrm{LO}\right)$, and $E_2$ phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials $c_{E_1\left(\mathit{TO}\right)}$, $c_{E_1\left(\mathit{LO}\right)}$, and $c_{E_2}$ are determined. A distinct correlation between anisotropic strain and the $A_1\left(\mathrm{TO}\right)$ and $E_1\left(\mathrm{LO}\right)$ frequencies of $a$-plane GaN films reveals the $a_{A_1\left(\mathit{TO}\right)}$, $b_{A_1\left(\mathit{TO}\right)}$, $a_{E_1\left(\mathit{LO}\right)}$, and $b_{E_1\left(\mathit{LO}\right)}$ phonon deformation potentials. The $a_{A_1\left(\mathit{TO}\right)}$ and $b_{A_1\left(\mathit{TO}\right)}$ are found to be in very good agreement with previous results from Raman experiments [V. Yu. Davydov et al., J. Appl. Phys. 82, 5097 (1997)]. Our $a_{A_1\left(\mathit{TO}\right)}$ and $a_{E_1\left(\mathit{LO}\right)}$ phonon deformation potentials agree well with recently reported theoretical estimations [J.-M. Wagner and F. Bechstedt, Phys. Rev. B 66, 115202 (2002)], while $b_{A_1\left(\mathit{TO}\right)}$ and $b_{E_1\left(\mathit{LO}\right)}$ are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented.