Raman studies of GaN/sapphire thin film heterostructures

Using Raman spectroscopy, we have studied the optical phonon modes of GaN nucleation layers with the thicknesses of 7 and 45 nm, grown on sapphire (0001) substrates by metal organic chemical vapor deposition at low temperatures (500-600 degrees C). These layers consisted of mixed hexagonal and cubic phases. The Raman results from mixed phases were compared with those from pure hexagonal layers which were grown at higher temperatures over 1000 degrees C. The E-2(Pi) and A(1)(LO) phonon modes are observed at 548 and 733 cm(-1) for 45 nm thick nucleation layer, while the silent low-frequency B-1 mode which is forbidden in good quality hexagonal GaN is observed at 314 cm(-1). The presence of the strong hexagonal modes for GaN nucleation layers of 45 nm thick confirms the crystalline nature of the GaN nucleation layer and dominant hexagonal phase in this mixed cubic-hexagonal nucleation layer. The observed frequencies are shifted with respect to the corresponding A(1) and E-2 phonon modes in hexagonal GaN. The decrease in mode frequency implies the presence of in-plane tensile strain in these GaN nucleation layers of 45 nm thick. The Raman scattering spectra taken from different positions on the sample show similar spectral features, indicating that the GaN nucleation layers of 45 nm thick are homogeneous in micron scale.