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.

We analyze optical absorption, transmission, luminescence, and Raman scattering in $n$-type GaN at hydrostatic pressures up to 30 GPa. The results show freeze-out of free electrons above $\sim 20\mathrm{GPa}$ due to trapping at levels that are resonant at ambient pressure but become gap states at high pressures. Ab initio calculations show that both the N vacancy and the Ga interstitial undergo this transition at $\sim 20\mathrm{GPa}$, but the vacancy should be more abundant. The pressure dependence of the yellow luminescence indicates that a transition between a shallow donor and a deep acceptor is involved.

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.

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.

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.

Results of experimental and theoretical studies of the fundamental electronic properties of GaN are presented. Single crystals of GaN grown by means of an equilibrium high-pressure technique have been used for the determination of the energy gap ${\mathit{E}}_{\mathit{g}}$, refractive index n, and their pressure dependences. The electronic band structure of GaN has been calculated by means of the self-consistent linear-muffin-tin-orbital method. It has allowed us to determine the pressure variation of the measured electronic properties of GaN and to reexamine the character of ${\mathit{E}}_{\mathit{g}}$ (direct or indirect). A satisfactory agreement between the experimental and theoretical results has been obtained, and it is found that GaN is a direct-band-gap semiconductor. The pressure dependences of ${\mathit{E}}_{\mathit{g}}$ and n have been used also for comparison of GaN with other semiconductors and to examine the validity of some semiempirical schemes used for the classification of semiconductor compounds.