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 °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 °C. The E2H and A1(LO) phonon modes are observed at 548 and 733 cm-1 for 45 nm thick nucleation layer, while the silent low-frequency B1 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 A1 and E2 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. [ABSTRACT FROM AUTHOR]