Ab initiophonon dispersions of wurtzite AlN, GaN, and InN

Abstract

Phonon excitations play an important role in electronic transport, nonradiative electron-relaxation processes, and other properties of interest for materials characterization, device engineering, and design. We have calculated the phonon dispersions and density of states for wurtzite AlN, GaN, and InN using state-of-the-art density-functional perturbation theory. The calculations are in good agreement with the existing experimental data for zone-center modes and predict the full phonon dispersions throughout the Brillouin zone. In particular, it is found that the three-phonon decay of the LO phonon in two acoustic phonons is not allowed in GaN and InN, since the LO frequency is much larger than the acoustic frequencies over the entire spectrum. The substantial potential of the group-III nitrides and their alloys for applications in optoelectronic and high speed devices has attracted a great deal of interest. AlN, GaN, and InN have direct energy gaps which span a substantial range, from the visible to the ultraviolet region of the spectrum. Consequently, their alloys have direct gaps that can be tuned to any value within this range simply by varying the alloy composition. This tunability offers many possibilities for device engineering. In particular, InGaN alloys were used in the realization of light emitting diodes and laser diodes operating in the blue and UV spectral region. Under ambient conditions, the III nitrides crystallize in a hexagonal, wurtzite ~2H! structure, although thin films having a cubic, zincblende ~3C! structure have also been grown. 1 Properties of interest for device engineering and design, such as electronic transport, nonradiative electron relaxation processes, lattice specific heat, etc., are strongly influenced by phonon excitations. Furthermore, a number of nondestructive experimental techniques of sample characterization, for instance, Raman spectroscopy or IR reflectivity, involve phonon measurements. A characterization of the phonon dispersions and densities of states for the group-III nitrides is therefore desirable. However, since it is very challenging to grow single crystals of suitable size for neutron-scattering experiments, there are no experimental data for the phonon dispersions of these compounds. Only very recently have the phonon density of states for AlN and GaN been obtained from time-of-flight neutron spectroscopy using bulk powders. 2,3 In addition, numerous studies of the zone-center phonons in GaN and AlN films have been conducted using Raman and IR spectroscopy. Due to the lack of latticematched substrates, these samples are affected by the high density of defects and strain present in the films and it is therefore unclear how well these data represent the true bulk values. The least studied of the three nitrides is InN for which there are only few Raman studies. 4,5