Molecular beam epitaxy grown In1−xAlxSb∕InSb structures for infrared detectors

Abstract

The core of SCD’s present day focal plane array (FPA) technology is a two-dimensional array of InSb photodiodes. It is produced by ion implantation and is attached to a silicon focal plane processor by indium bumps. We have extended this technology with In1−xAlxSb (0<x<0.03) alloys grown by molecular beam epitaxy on InSb substrates, utilizing a p-i-n structure. The advantages of the grown structures over those fabricated by implantation are already demonstrated in 320×256 element FPAs based on epitaxial InSb mesa diodes. These FPAs have dark currents at 100K comparable to those of implanted FPAs at 77K, with an operability (fraction of working diodes) in excess of 99.5%, and with a residual non-uniformity (RNU) at 100K of less than 0.03% (standard deviation/dynamic range) after a two-point non-uniformity correction (NUC). The RNU remains very low (<0.1%) even when the FPA temperature is shifted by as much as 10K from the NUC temperature. The average dark current at 100K and also the width of its distribution are further reduced by increasing the Al concentration in the FPAs by up to 3%, in accordance with the increase in the alloy energy gap. This has enabled us to achieve high quality thermal images at even higher temperatures than in the binary InSb FPAs. A key issue in achieving highly uniform FPAs is the degree of control of the InAlSb composition, misfit strain relaxation, and defect density. A series of In1−xAlxSb layers with x up to 6% was grown in order to study the effect of the growth conditions on these features. We discuss our results in terms of the misfit strain relaxation mechanisms in the In1−xAlxSb structures including dislocations, tilt, microcracks, and morphological crosshatch patterns.