We present the results of room-and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN. ͓S0163-1829͑98͒06840-4͔

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We report on the influence of different pre-etch methods on specific contact parameters of GaN contacts. For these investigations we used ex situ chemically assisted ion beam etching and in situ sputter etching before metal deposition. The electrical contact parameters were determined using the extended circular transmission line model (CTLM). For nitrogen as an etching gas we obtained rectifying character (Schottky) of metal-n-GaN contacts compared with mostly linear (Ohmic) behaviour for conventional etching gases such as Ar or ArCl 2 . We assume that a decrease of N vacancies caused by the N 2 treatment is responsible for the Schottky behaviour of these contacts. Pre-etch sputtering with Ar + ions reduces on the one hand the specific contact resistance ρ C but on the other hand the CTLM model reveals that simultaneously the sheet resistance R sk in the near-surface region increases.

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