Optical characterisation of interface properties for hexagonal GaN grown by MBE on GaAs

A new optical method for the characterisation of substrate/film interface properties of GaN grown on GaAs is presented. It is based on reflectivity measurements over an extended spectral range. The influence of surface roughness is separated by a comparison of the experimental data with calculations for a smooth film above the band gap of GaN, while at lower energies, the deviations from the calculated one-layer behaviour (without interface) are strongly correlated to the effective refractive index of the interface nint and its thickness. It is shown that for GaAs substrates under certain MBE growth conditions, nint is mainly determined by the void fraction.