A3.2 Raman and IR studies of GaN

Infrared reflection and Raman spectroscopies have been employed to derive zone center and some zone boundary phonon energies in wurtzite and in zincblende GaN (Table 1.) [1-10]. Phonon and coupled modes have been employed to characterize stress conditions and carrier densities in thin films and device structures. Due to the wide bandgap non-resonant Raman scattering is easily performed using visible lines of an Ar laser. The strong bond in GaN and the light N atom result in high phonon energies limiting the range of possibly observable local vibrational modes of impurities to even lighter elements at higher frequencies. Energies of optical phonons modes in Γ are very similar in wurtzite and cubic GaN. Selection rules in wurtzite allow a splitting of longitudinal and transversal modes into A1 and E1 modes and the occurrence of additional Raman active modes E2. Phonon modes have been calculated in first principles pseudo potential calculations [11].