On the evaluation of optical parameters of a thin semiconductor film from transmission spectra, and application to GaN films

摘要

The analysis of the optical transmission spectra of semiconductor thin films on a transparent substrate is revisited. A new equation for the optical transmission is derived. Based on this equation and on the spacing of the interference pattern, a method is described and analyzed in detail for the determination of the film thickness, refractive index and absorption coefficient. As a notable feature, small values of the absorption coefficient can be determined as a fitting parameter at all wavelengths, regardless of strong interference. The method is compared to the frequently encountered envelope method of Manifacier et al and Swanepoel. The suitability of the method is illustrated on two GaN thin films grown on sapphire. Below the band gap, the index of refraction of GaN can be described by the equation n(lambda(nm)) velence 2.19 + 2.57(lambda - 345)~(-1/2).