Infrared reflectance (IR) of GaN grown on sapphire and silicon substrates has been studied both theoretically and experimentally. The theoretical calculation of the IR spectra is based on the transfer matrix method. The IR spectral characteristics influenced by several factors, such as film thickness, incident angle, free carriers, are systematically examined. Combined with experimental results, surface scattering and interface layer effects are also studied. For GaN epilayers grown on sapphire, carrier concentrations and mobility are determined by fitting to the IR reststrahlen band and compared with the Hall measurement. The interface effect is demonstrated to cause a damping behaviour of the interference fringes away from the reststrahlen band. For GaN grown on Si, the IR spectra predicted the large surface roughness of the epilayers. A variation of IR reststrahlen band is correlated to the microstructures of the films, i.e. their polycrystalline nature of the GaN films grown on Si. A three-component effective medium model is proposed to calculate the IR spectra for polycrystalline GaN, and a qualitative correlation between the IR spectra and structure of the film is established. All results show that IR, as a non-destructive method, is efficient for characterising GaN epilayers in semiconductor processing.
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