Spectroscopic ellipsometry on GaN: Comparison between hetero-epitaxial layers and bulk crystals

Abstract

In the present study spectroscopic ellipsometry was used for characterising GaN bulk crystals obtained at high pressure and thin films grown on sapphire. The undoped GaN films grown by MOCVD show interference fringes below the fundamental gap. The ellipsometric data (Δ and ψ), measured in the wavelength range between 500 nm and 680 nm, were analysed using a multilayer description in the transfer matrix formalism. For the thin films grown on sapphire the model includes a buffer layer, a GaN epilayer, and a hypothetical overlayer. Furthermore, both, the real and imaginary part of the complex refractive index were taken into account. The real part of the refractive index n1 was found to follow a Cauchy-type of dispersion n1 = 2.290 + 0.06 (280/λ-280)2. For bulk crystals n1 was found to be 2.337 ± 0.010 at the wavelength of 632.8 nm. This value compares well with MOCVD grown GaN where n1(X = 632.8 nm) = 2.328 ± 0.003.