Raman scattering spectra in C-implanted GaN epilayers

ABSTRACTWe have investigated the Raman scattering spectra in C-implanted GaN epilayers. (a) In as-implanted GaN, new Raman bands at 293, 376, 669, 1027, 1094 and 1053 cm−1 appeared. From phonon-dispersion curves for hexagonal GaN, the 293 cm−1 and 669 cm−1 bands were tentatively assigned to the highest acoustic-phonon branch and the optical-phonon branch at the Brillouin zone boundaries, respectively; the 1027 and 1094 cm−1 peaks might be caused by two-phonon scattering involving the 376 and 669 cm−1 phonons, and the 376 cm−1and A1(LO) phonons respectively. (b) Two sharp bands at ∼1350 and ∼1600 cm−1 were observed in the Raman spectra of carbon- implanted GaN after post-implantation annealing treatments. The intensities of these two bands increased while their full widths at half maximum decreased with increasing annealing temperature. We assigned them, respectively, to the D and G bands of small graphite crystallites arising from C implantation and post-implantation annealing. The observation of these two bands indicates the formation of small graphite crystallites in C-implanted GaN.

EFFECT OF MULTIPLE PHONON BRANCHES ON THE PHASE TRANSITIONAL BEHAVIOR OF A TWO-DIMENSIONAL POLARON GAS

Using an all-coupling variational method and the random phase approximation, it is shown that a two-dimensional polaron gas exhibiting no localization-delocalization transition with a single optical phonon branch may undergo such a transition in the presence of an additional phonon branch. However, if the system already shows a transition, with a single phonon branch, then the effect of an additional branch is just to enhance the low-mobility self-trapped phase of the polaron.