Self-consistent linear muffin-tin-orbital band-structure calculations are used to investigate the optical and structural properties of III-V semiconducting nitrides under hydrostatic pressure. The pressure behavior of the energy band structures is discussed in the context of the postulated chemical trends in III-V semiconductors. The regions in k space of dominant interband contributions to the elements of structure in the dielectric functions are identified. The total-energy calculations suggest that all the nitrides under pressure transform to the semiconducting rocksalt phase. The calculated transition pressures are 21.6 GPA (InN), 51.8 GPa (GaN), 16.6 GPa (AlN), and 850 GPa (BN). Experimental values that agree well with this have been found for the first three compounds. The fact that GaN and AlN have such different transition pressures in spite of their very similar ionicities is explained by the presence of 3d states on Ga.

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.

Total-energy calculations within the local-density approximation (LDA) to the density-functional theory are used to study the properties of AlN under pressure. The free energy (at zero temperature) is calculated as a function of pressure for AlN in the wurtzite, zinc-blende, rocksalt, nickel arsenide, anti-NiAs, and β-tin structures. According to the calculations the wurtzite structure is stable from zero pressure (P) up to ≊12.5 GPa, where AlN transforms into the rocksalt phase. The volume reduction is 19%. The volume change agrees with recent experiments, but the transition pressure is lower than observed, presumably due to the LDA. It is argued that the transition pressure in reality is close to 17 GPa. Another transformation is predicted at higher pressures, and the calculations suggest that this structure could be similar to that of NiAs, but with c/a=1.72. The transition to the NiAs structure would occur between 30 and 40 GPa, and it is accompanied by a very small volume change. The internal parameters (c/a and u) of the hexagonal phases are optimized by total-energy minimization, and their variation with pressure is derived. At P=0 the internal parameters of wurtzite-AlN are (c/a,u)=(1.596, 0.3820). When P is increased c/a decreases monotonically, and u increases, but the interrelation between u and c/a differs from ideality (defined as the case where all bonds would have the same lengths).

The electronic band structures of ${\text{In}}_x{\text{Ga}}_{1-x}\text{N}$, ${\text{In}}_x{\text{Al}}_{1-x}\text{N}$, and ${\text{In}}_x{\text{Ga}}_y{\text{Al}}_{1-x-y}\text{N}$ alloys are calculated by ab initio methods using a supercell geometry, and the effects of varying the composition and atomic arrangements are examined. Particular attention is paid to the magnitude of and trends in bowing of the band gaps. Indium composition fluctuation (clustering) is simulated by different distributions of In atoms and it is shown that it strongly influences the band gaps. The gaps are considerably smaller when the In atoms are clustered than when they are uniformly distributed. An explanation of this phenomenon is proposed on the basis of an analysis of the density of states and the bond lengths, performed in detail for ternary alloys. Results for the band gaps of ${\text{In}}_x{\text{Ga}}_y{\text{Al}}_{1-x-y}\text{N}$ quaternary alloys show a similar trend. It is suggested that the large variation in the band gaps determined on samples grown in different laboratories is caused by different degrees of In clustering.

A comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented. A large set of the x-ray emission and absorption spectra was collected at the several synchrotron radiation facilities at installations offering the highest possible energy resolution. By taking advantage of the linear polarization of the synchrotron radiation and making careful crystallographic orientation of the samples, the bonds along c axis $\left(\pi \right)$ and “in plane” $\left(\sigma \right)$ in the wurtzite structure could be separately examined. Particularly for AlN we found pronounced anisotropy of the studied bonds. The experimental spectra are compared directly with ab initio calculations of the partial density of states projected on the cation and anion atomic sites. For the GaN, AlN, and InN the agreement between structures observed in the calculated density of states (DOS) and structures observed in the experimental spectra is very good. In the case of hexagonal BN we have found an important influence of insufficient core screening in the x-ray spectra that influences the DOS distribution. The ionicity of the considered nitrides is also discussed.

Native defects and some common dopants in cubic GaN, AlN, and BN are examined by means of ab initio calculations using a supercell approach in connection with the full potential linear muffin-tin orbital method. The atomic positions, the electronic structure, and the defect formation energies are calculated. In particular, the vacancies are calculated to be abundant defects. The high-pressure behavior of the defect states is also studied, and it is found that the pressure coefficients of the defect states depend mainly on their position in the energy gap.