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The pressure behavior of phonon modes of the hexagonal and cubic modifications of GaN and hexagonal AlN was investigated experimentally. The mode pressure coefficients were determined from Raman measurements at hydrostatic pressures up to 6 GPa. The low-frequency E 2 phonon in GaN exhibits a weak softening which is qualitatively similar to that of zone-boundary transverse acoustic modes of zinc-blende III-V semiconductors. In AlN the E 2 ͑low͒ phonon frequency is essentially constant under pressure. For both materials an increase of the LO-TO splitting is observed, which results from the interplay between the pressure dependence of the high-frequency dielectric constant and Born's transverse dynamical effective charge. The latter turns out to be nearly constant under pressure, a behavior deviating from that of other III-V semiconductors. The experimental findings are compared to results of ab initio calculations.

Local vibrational modes in the region of the acoustic and optical phonons are reported for Mg-doped GaN grown by molecular beam epitaxy. The modes, studied by Raman spectroscopy, appear in addition to the known modes in the high-energy region around 2200 cm Ϫ1. We suggest disorder-activated scattering and scattering from Mg-related lattice vibrations to be the origin of the low-energy modes. Our assignment is supported by calculations based on a modified valence-force model of Kane. Temperature-dependent measurements between 4 and 300 K exclude an electronic Raman-scattering mechanism. We also report a new line at 2129 cm Ϫ1 and discuss the origin of all five observed high-energy modes.

We present a selection of our contributions to basic research on the lattice dynamical properties of group-III nitrides and their alloys. We used first-order Raman scattering to determine the zone-center phonons and their dependence on structural attributes such as stress, chemical composition, impurities, and doping. Results on the angular dispersion of the polar modes, strain distribution, coupled LO-phonon plasmon modes, multi-mode behavior in Al x Ga 1-x N, and the quantitative determination of the phase purity of cubic and hexagonal GaN are shown. Second-order Raman-scattering experiments on GaN and AlN provide information on the vibrational states throughout the entire Brillouin zone. Based on a comparison of experimental data and calculated phonon-dispersion curves we assigned the observed structures to particular phonon branches and points in the Brillouin zone. We also discuss the behavior of the optical modes under large hydrostatic pressure. 1 Introduction During the last decade group-III nitrides have been an object of increasing research because of their interesting physical properties such as an adjustable band gap (1.9 eV to 6.2 eV) by varying the composition of the alloys [1] and application as basic materials for optoelectronic devices working in the blue and ultraviolet spectral region [2][3][4]. In particular GaN and AlN can withstand high temperatures and have large piezoelectric constants which makes them suitable for applications in highfrequency devices, sensors and low-dimensional structures [5,6].Although basic information about group-III nitrides e.g. crystal-and band structure have been available for a decade several details still remained unclear. In the following review of our work we present results of first-and second-order Raman investigations on various GaN, AlN, and Al x Ga 1-x N samples and point out some remarkable details of the structural and optical properties of these materials.

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.