Infrared Reflectivity Spectroscopy of Short Period AlGaN/GaN Superlattice Optical Phonons

Extensive studies of GaN and AlN compounds led to several microelectronic and optoelectronic applications for these bulk materials. Nonetheless, many characteristics of AlGaN/GaN superlattices are not well known. In particular, scientists have not determined all the properties of phonons in these materials. In order to more fully understand phonon characteristics, this study reports infrared (IR) reflectivity measurements of AlGaN/GaN short period superlattices. Reflectivity measurements of each sample were taken using a Fourier Transform IR vacuum spectrometer. Additionally, a method for measuring low temperature IR spectrum was developed along with a program to analyze spectra. Comparing 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, characterization of phonon energies versus material composition were obtained. Furthermore, new phonons, not present in bulk AlN and GaN, were discovered. Phonon characteristics were additionally measured as a function of temperature, confirming the expectation that phonon energies decrease with increasing temperature.