Optical study of the -GaN/GaAs interface properties as a function of MBE growth conditions

Undoped hexagonal GaN films have been grown by molecular beam epitaxy on (111)B GaAs substrates. Both Ga flux and nitrogen plasma conditions were fixed, while the growth temperature was varied in the range from 650 up to 720 C. Reflectivity measurements below and above the GaN band gap were employed in order to determine optical parameters characterizing the substrate/film interface properties. The refractive index of the interface layer was found to decrease with increasing growth temperature indicating the formation of a mixed medium in this region. This effect is related to the columnar growth of GaN and outdiffusion of substrate atoms via channels between the columns which promotes the formation of voids in the substrate/film interface. Low temperature initiated growth or nitridation of the substrate were observed to reduce the influence of elevated growth temperatures.