FAR-INFRARED SPECTROSCOPY OF THIN EPITAXIAL LAYERS OF GAN DEPOSITED BY MOLECULAR-BEAM EPITAXY ON GAP SUBSTRATES

The far infrared optical properties of a selection of thin GaN epilaye rs on GaP substrates have been investigated at room temperature by obl ique incidence reflection spectroscopy in p- and s-polarization. The G aN layers were deposited in the cubic, wurtzite, or mixed (cubic and w urtzite) phase by molecular beam epitaxy. The GaN and Gap reststrahlen regions are well separated in frequency and in each case a good theor etical fit to the measured spectrum is obtained. The model uses damped simple harmonic oscillators to describe the dielectric response of th e phonons in the two materials, and the frequencies of TO and LO phono ns in the GaN layers are determined from the model fit. The frequency of an interference feature in the GaP reststrahl region provides a use ful probe of the him thickness, and the feature is found to change aft er cooling the sample to 77 K, indicating that the films are not stabl e. A weak GaAs-like feature, which correlates well with the use of As during growth, is seen in the mixed phase sample.