Abstract

The far infrared optical properties of a selection of thin GaN epilayers on GaP substrates have been investigated at room temperature by oblique incidence reflection spectroscopy in p- and s-polarization. The GaN layers were deposited in the cubic, wurtzite, or mixed (cubic and wurtzite) phase by molecular beam epitaxy. The GaN and GaP reststrahlen regions are well separated in frequency and in each case a good theoretical fit to the measured spectrum is obtained. The model uses damped simple harmonic oscillators to describe the dielectric response of the phonons in the two materials, and the frequencies of TO and LO phonons in the GaN layers are determined from the model fit. The frequency of an interference feature in the GaP reststrahl region provides a useful probe of the film thickness, and the feature is found to change after cooling the sample to 77 K, indicating that the films are not stable. A weak GaAs-like feature, which correlates well with the use of As during growth, is seen in the mixed phase sample.

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