Infrared Reflectivity Spectroscopy of Optical Phonons in Short-period AlGaN/GaN Superlattices

GaN and AlN compounds have been proven useful in wide bandgap microelectronics and optoelectronics. Also properties of bulk GaN and AlN have been studied extensively. However, many characteristics of AlGaN/GaN superlattices are not well known. In particular, the properties of phonons have not been determined. In order to determine phonon properties, this study measured infrared reflectivity spectra on short period superlattices, which were grown by high quality molecular beam epitaxy. The superlattices consisted of 300 periods of alternating layers of GaN and AlGaN, each containing between 1 and 8 monolayers. Next, the reflectivity of each sample was measured using a Bruker IFS-66V spectrometer. From these experimental spectra the dielectric function, and hence the optical phonon properties (namely phonon frequency and phonon damping), were determined. Mapping the experimental spectra with theoretical calculations determined the longitudinal and transverse optical phonon energies present in the AlGaN/GaN superlattices. Through the examination of different AlGaN/GaN superlattice combinations, plots of phonon energies versus material composition were obtained. Furthermore, new phonons, that were not present in bulk AlN and GaN, were discovered. Finally, phonon characteristics were measured as a function of temperature, confirming that phonon energies decrease with increasing temperature.