Determination of individual layer composition and thickness in multilayer HgCdTe structures

Abstract

The reproducible molecular-beam epitaxy (MBE) growth of dual-band Hg1−xCdxTe (MCT) heterostructures requires routine post-growth wafer analysis for constituent layer thickness and alloy composition, therefore, demanding nondestructive characterization techniques that offer quick data feedback. This paper reports a multilayer structure model, which can be least-square fit directly to either Fourier transform infrared (FTIR) transmission or reflection spectra to provide individual layer thickness, alloy composition, and grading information for various complex structures. The model, we developed, is based on an accurate representation of both the real and imaginary parts of the MCT dielectric function across and above Eg as a function of alloy composition. The parametric, MCT optical-dielectric function for compositions varying between x=0.17 to x=0.5 was developed in the range from 400 cm−1 to 4,000 cm−1, based on a semi-empirical model for the absorption coefficient and extrapolation of the refractive index across Eg. The model parameters were refined through simultaneous fits to multiple reflection and transmission data sets from as-grown, double-layer planar heterostructure (DLPH) structures of variable thickness. The multilayer model was tested on a variety of simple DLPH structures with thick absorber layers (>8 µm) and was compared against traditional FTIR analysis and cross-section optical microscopy and showed good agreement in both composition and thickness. Model fits to dual-color MCT data and subsequent analysis of the internal parameter correlation have demonstrated that error bars on absorber layer composition and thickness could be as low as ∼0.0005 and ∼0.02 µm, correspondingly.