Polarized and electronic Raman scattering in GaN epitaxial layers grown on sapphire

We have carried out a detail Raman scattering studies in GaN layers grown on C-, A-, R-, and M- plane sapphire substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) technique. First-order optical phonons of A(1), E-1, and E-2 symmetries were observed in the Raman spectra of GaN layers grown on sapphire substrates, where the peaks are indicative of the wurtzite crystal structure. Polarized Raman measurements performed on GaN layers grown on various orientations of sapphire exhibit quasi-polar modes of both E-1 and A(1) symmetries. The appearance of these quasi-polar phonons are explained by considering the combined effect of long-range electrostatic and short-range inter atomic forces in the Raman process. Furthermore, the nature of the electronic state is investigated by defect-induced Raman scattering in resonance with yellow luminescence transitions in GaN layers grown on C-plane sapphire.