Infrared reflectance and cathodoluminescence of AlN-GaN short period superlattice films

The room temperature infrared reflectance of A1N-GaN short period superlattice films has been measured in the region of the reststrah1 bands of A1N and GaN. These superlattice films were deposited by switched atomic layer metal organic chemical vapor deposition onto GaN or AW buffer layers deposited on basal plane sapphire substrates. The measured reflectance spectra are compared to calculated spectra using an effective medium theory to model the dielectric function of the superlattice. The optical properties of the individual materials making up the samples are modeled with Lorentz oscillators using only bulk input parameters. The effects of film and substrate anisotropy and off normal incidence are included in the calculation. Using this modeling technique, it is possible to obtain thickness estimates for the total superlattice film and the buffer layer. Low temperature cathodoluminescence was also measured on these samples. An ultraviolet peak located above the band gap energy of GaN is present in all samples. Using the film and buffer thicknesses determined by the reflectance measurement the observed uv peak can be identified as originating from the superlattice layer.