Broad band spectroscopic ellipsometry for the characterization of photovoltaic materials

Abstract

The availability of commercial spectroscopic ellipsometers (SE) has been restricted to the UV-visible range from 250–900 nm. Although this is useful for many applications, it must be extended to the near IR region (up to 1700 nm) for the study of the optical behavior of most photovoltaic materials. This paper discusses the development of a broad band (300–1700 nm) SE which has been used to measure the optical characteristics of various materials. Among these are the polycrystalline thin film materials, CuInSe₂ and CdTe (for which single crystal samples have also been investigated), and materials for high efficiency cascade solar cells including InP, InGaAs and InGaAsP. Most of these data are not presently available over such a wide spectral range.

Experimentally, a rotating polarizer-fixed analyzer ellipsometer with an a.c. detection system has been developed for accurate measurement of ψ and Δ, the relevant ellipsometric parameters, in the near IR. This approach has certain advantages over the rotating analyzer-fixed polarizer systems including reduced sensitivity to room light. The analytical methods include the use of a specially developed computer modeling program which gives ψ and Δ for a given set of values related to the film thickness (which may be finite or zero) and to the optical properties of the substrate.