Prediction of HgCdTe spectral inhomogeneities based on optical and material properties

Abstract

HgCdTe infrared detectors have been extensively developed over the past few decades. They owe their popularity to the many advantages of HgCdTe material, which has a tunable energy bandgap over the 1–30μm range and high quantum efficiency. They are increasingly used to perform quantitative spectral imaging, which implies a high control of their spectral response. In order to optically model the spectral behavior of this material, it was mandatory to know its refractive index dispersion. The optical properties of this ternary alloy have been particularly studied, as they contribute to detector performance. To the best of our knowledge, however, there are no published measurements of the real part of the refractive index of HgCdTe at low temperatures, above the bandgap energy. We therefore undertook a thorough study of this material at 80K, where we estimated the real part of the refractive index for several alloy compositions in the spectral range of [1800cm-1,6000cm-1]. We used a modified Kramers–Kronig relation, combined with published experimentally measured points. First, we proved the high accuracy of this technique at room temperature, by comparing results with previously published models. We then simulated an HgCdTe bulk layer, in order to extract the dependence of the cut-off wavenumbers on cadmium composition and the layer thickness at low temperature. The influence of these macroscopic parameters on the spectral nonuniformities is discussed.