Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in w urtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs ( 111)A, and GaAs (111)B oriented substrates by means of molecular beam epita xy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon freq uencies are in agreement with other studies and with lattice dynamic calcul ations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrat es than in samples grown on GaAs (001) substrates. The observation of disor der-activated modes gives information about sample quality. Comparison of t he spectra from different thickness epilayers shows that the GaN is more hi ghly disordered close to the substrate, particularly for the (001) substrat es.