Origin of Berreman effect in GaN layers on sapphire substrates

Oblique incidence polarized IR reflectivity measurements of Metal Organic Chemical Vapor Deposition (MOCVD) grown GaN epitaxial layers on sapphire are discussed in the context of recent literature on Berreman effect. The dependence of the p-polarized reflectivity spectrum on incidence angle and thickness of the GaN films is analyzed theoretically and the results are compared with experiment. The "Berreman minimum" that is the reflectivity minimum near the longitudinal-optical (LO) phonon frequency is shown to be due to optical interference. Our calculations show that in GaN layers with thickness greater than 0.1 μm and for high incidence angles, the frequency of "Berreman minimum" does not correspond to the A1 LO phonon frequency.