We report on highly resolved photoluminescence (PL) and reflectance (RF) spectra of a homoepitaxial GaN layer grown by metal-organic vapor phase epitaxy. This sample exhibits narrow linewidths of several PL emission peaks, down to $\approx 100\mu \mathrm{eV}$ full width at half maximum for some bound exciton transitions. As a consequence, we have detected new PL features as, e.g., a fivefold fine structure of the donor-bound exciton line at $\approx 3.471\mathrm{eV},$ and other known PL transitions could be determined with high precision. In RF measurements, the extraordinary quality of the epitaxial layer allowed observation of weakly damped excitonic ground-state transitions and of narrow excited exciton transitions with high signal-to-noise ratio.

We have investigated the lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering. Several dispersion branches and phonons at high-symmetry points have been measured, including the two zone-center Raman- and infrared-inactive silent modes. The experiments have been complemented by ab initio calculations. They are in very good agreement with our measurements, not only for phonon energies, but also for scattering intensities, thus validating the correctness of the eigenvectors. Other phenomenological and ab initio theories exhibit significant differences.

The major obstacle to the production of a blue laser is posed by difficulties with the preparation of defect-free GaN layers. A considerable amount of empirical work is presently being undertaken to achieve this goal. However, there is a lack of basic research on the reduction of residual stress and defects in these epilayers since the mechanical characteristics of GaN have not been measured yet. This is due to difficulties with experimental examination of thin films. This work addresses the mechanical properties of bulk GaN obtained by a high-pressure method. Young’s modulus (295 GPa), hardness (20 GPa), yield strength (15 GPa), and the stress–strain curve of GaN have been evaluated using nano-indentation. The cause of the sudden depth excursions during indentation of GaN epilayers has been clarified.

Visible light communications using a Gallium-nitride (GaN) laser diode is reported. Devices, which are cased in TO packages, show modulation bandwidths of up to 1.4 GHz. We demonstrate error-free data transmission, defined as transmission of $1\times {10}^{-9}$ bits without any errors, at $2.5\mathrm{Gbit}/\mathrm{s}$ with a sensitivity of 11.5 dBm.

We have applied a low-energy positron beam and secondary ion mass spectrometry to study defects in homoepitaxial and heteroepitaxial GaN layers. Positron experiments reveal high concentrations of Ga vacancies in nominally undoped n-type GaN, where the conductivity is due to unintentional oxygen incorporation. Ga vacancies are observed in both homoepitaxial and heteroepitaxial layers, indicating that their formation is independent of the dislocation density. No Ga vacancies are detected in p-type or semi-insulating samples doped with Mg, as predicted by the theoretical formation energies. In samples where n-type conductivity is due to Si doping and the incorporation of oxygen impurities is suppressed, the concentration of Ga vacancies is much lower than in n-type samples containing oxygen. This indicates that the presence of oxygen donor in GaN promotes the formation of Ga vacancy. We suggest that this effect is due to the creation of ${\mathrm{V}}_{\mathrm{Ga}}{-\mathrm{O}}_{\mathrm{N}}$ complexes during the epitaxial growth.

We report on AlGaN/GaN heterostructures and heterostructure field-effect transistors (HFETs) fabricated on high-pressure-grown bulk GaN substrates. The $\text{2d}$ electron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as $\text{μ=1650\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V s}$ combined with a very large electron sheet density $n_s{\text{≈1.4×10}}^{13} {\mathrm{cm}}^{\mathrm{-2}}.$ The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs ${\mathrm{(n}}_s{\text{μ>2×10}}^{16} {\mathrm{V}}^{\mathrm{-1}}{\mathrm{ s}}^{\mathrm{-1}}\mathrm{)}$ grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior.