Spectroscopic ellipsometry measurement and analysis of the optical constants of InAs/InAsSb and InGaAs/InAsSb superlattices and their bulk constituents

Abstract

The optical constants of strain-balanced InAs/InAsSb and InGaAs/InAsSb superlattices and their constituents GaAs, GaSb, InAs, InSb, and InAsSb are measured using spectroscopic ellipsometry. An optical constant model is developed that accurately describes the index of refraction and absorption coefficient in the vicinity of the fundamental bandgap of these III–V materials. The model describes the spectral shape of the absorption edge in terms of bandgap energy, below-bandgap Urbach absorption tail, and above-bandgap power law. The index of refraction in the vicinity of the bandgap is described using Kramers–Kronig integration over the absorption edge. In the analysis, an optical structure model that comprises the sample layer structure and optical constants is constructed to simulate the reflection of polarized light from the sample surface. The optical constant model is implemented within the optical structure model and fit to the measured ellipsometric data with the optical constants of the layer of interest as best-fit parameters. The superlattice measurements exhibit spurious periodic peaks in the optical constants extracted using the point-by-point fit method. Multi-sample analysis of identical midwave superlattices grown at different thicknesses significantly reduces the presence of the spurious peaks and, in general, improves the optical constant fit and the extraction of the absorption edge parameters. In comparing superlattice results, the InGaAs/InAsSb superlattices have a shorter period, demonstrate stronger absorption due to greater electron–hole wavefunction overlap, and exhibit broader absorption tails as a result of greater alloy and interface disorder.