Molecular beam epitaxy grown In_1-xAl_x SbInSb structures for infrared detectors

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-x Alx Sb (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 100 K comparable to those of implanted FPAs at 77 K, with an operability (fraction of working diodes) in excess of 99.5%, and with a residual non-uniformity (RNU) at 100 K 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 10 K from the NUC temperature. The average dark current at 100 K 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-x Alx Sb 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-x Alx Sb structures including dislocations, tilt, microcracks, and morphological crosshatch patterns.