MBE growth and Raman studies of cubic and hexagonal GaN films on (001)-oriented GaAs substrates

Using different nucleation layers, we have grown cubic and hexagonal GaN ep ilayers on (001)-oriented GaAs substrates by radio-frequency plasma-assiste d molecular beam epitaxy. First- and second-order Raman spectra are taken f rom these epilayers in back-scattering configurations at room temperature. For the first-order Raman spectra, transverse-optical (TO) phonons and long itudinal-optical (LO) phonons of cubic GaN are found to be at 533 and 739 c m(-1), as well as the E-2 and A(1)(LO) frequency of hexagonal GaN are found to be at 568 and 733 cm(-1), and the polarized spectra are in good agreeme nt with Raman selection rules for cubic and hexagonal GaN, respectively. Th ese results confirm the single crystalline nature of the samples. As for th e second-order Raman spectra, the bands show a continuum, at about twice th e energy of the LO phonon of cubic GaN and A(1)(LO) phonon of hexagonal GaN , which are nearly 10 times weaker than the first-order scattering. Two-pho non spectra are dominated by contributions due to longitudinal optical phon ons. (C) 2000 Elsevier Science B.V. All rights reserved.