REFLECTIVITY STUDY OF HEXAGONAL GAN FILMS GROWN ON GAAS - SURFACE-ROUGHNESS, INTERFACE LAYER, AND REFRACTIVE-INDEX

Detailed reflectivity studies of hexagonal GaN films grown by molecula r beam epitaxy on GaAs substrates have been carried out in the energy range from 1.4 to 3.8 eV at room temperature. Measurements using ambie nt media with different refractive indexes verify that the reflectivit y is strongly influenced by a surface roughness. Furthermore, the opti cal data give a clear evidence for the formation of an interface layer (mixture of GaAs with voids) between the film and the substrate which agrees well with transmission electron microscopy observations. A qua ntitative analysis has been performed by correcting the reflectivity s pectra for the surface roughness and making use of a two-layer model t o take into account the interface layer. This procedure yields for all samples the same energy dependent refractive index of GaN despite the differing surface and interface layer properties. The obtained values of the root mean squared roughness are close to the atomic force micr oscopy data. Growth on (001) substrates has been found to be accompani ed by the formation of a thicker interface layer than that for (111)B orientation. (C) 1998 American Institute of Physics. [S0021-8979(98)01 415-7].