Directional dependence of AlN intrinsic complex dielectric function, optical phonon lifetimes, and decay channels measured by polarized infrared reflectivity

The directional dependence of AlN intrinsic complex dielectric function, the phonon lifetimes, and decay channels are investigated by means of polarized infrared reflectivity measurements on several facets of self-nucleated wurtzite AlN crystal of high crystalline quality. The measurement technique and the AlN single crystal used have been selected with the purpose to reduce, as much as possible, any instrumental-based effects as well as phonon scattering mechanisms due to defects. The experimental arrangements necessary to detect well-defined crystallographic orientations and phonons are detailed. The dielectric parameters and the phonon lifetimes are precisely determined as functions of the crystallographic direction from a careful Kramers-Kronig and damped Lorentz oscillator analysis. The ordinary (epsilon(infinity perpendicular to)) and extraordinary (epsilon(infinity parallel to)) high frequency dielectric constants for high quality AlN crystal are found to be 3.93 and 4.05, respectively, and the ordinary (epsilon(theta perpendicular to)) and extraordinary (epsilon(theta parallel to)) static dielectric constants are found to be 7.37 and 8.60, respectively. Our values obtained for the pure character phonon lifetimes are in good agreement with Raman measurements when these are obtained with the necessary care to eliminate linewidth broadening due to the finite slit width. The lifetime of a transversal phonon is found to increase with increasing the phonon energy, while that of a longitudinal phonon is found to decrease with increasing the phonon energy. Based on these observations, preferential decay channels for the AlN phonons are estimated. The results show that in the case of AlN, the widely assumed symmetric optical phonon decay into two phonons of lower energy cannot be justified for the zone center longitudinal phonons. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3177323]