Suppression of Bulk Defects in Antimonide Superlattice Infrared Photodiodes
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Suppression of Bulk Defects in Antimonide Superlattice Infrared Photodiodes E. H. Aifer, E. M. Jackson1, B. R. Bennett, I. Vurgaftman, J. R. Meyer, and G. G. Jernigan Naval Research Laboratory, Washington, DC 20375-5347, U.S.A. 1 SFA Inc., Largo, MD 20774, U.S.A. ABSTRACT While physical properties of ideal antimonide superlattices (ASL) indicate that they should significantly outperform mercury cadmium telluride (MCT) based infrared photodiodes for low dark current applications in the long and very long wave-infrared (LWIR and VLWIR), this potential has not yet been fully realized. Even though measured Auger and tunneling rates in ASL’s are reduced as predicted, overall carrier lifetimes remain much shorter, and dark currents much higher than expected. The large carrier losses are the result of defects in the ASL structure, with contributions measured from large bulk defects and surface channels along mesa sidewalls, and the remaining component believed to be due to midgap states. In this presentation we report on several studies of epitaxial growth parameters and their influence on defect formation. X-ray photoelectron spectroscopy analysis of oxide desorption from GaSb substrates shows the presence of both antimony and gallium oxides, along with their decomposition and desorption behavior with anneal temperature. A study of buffer growth shows that defect density and size are critically dependent on growth temperature, with an optimal o growth window between 480 and 500 C. . Side-by-side GaSb buffer growths on vicinal ((100) o + 1 (111)) and flat (100) substrates show that while growing on vicinal material can suppress mound formation, it does not yield epilayers as flat as can be obtained on (100) substrates grown under optimal conditons. Finally, the ratio of As to In flux during superlattice growth can be used to tune the lattice parameter both above and below that of GaSb, with strain-related defects appearing when the mismatch reaches roughly 0.1%.
INTRODUCTION Antimonide superlattices (ASLs), composed of binary (GaSb/InAs) or ternary antimonide layers (InxGa1-xSb/InAs), are potentially an important material system for long-wave and verylong-wave infrared (LWIR 8-14 µm & VLWIR 14-24 µm) focal plane arrays. The interest in ASL material is due to key advantages it offers over the current material of choice, Hg1-xCdxTe (MCT). Smith and Mailhiot first proposed that type II antimonide superlattices could be used to realize an artificial direct narrow-gap semiconductor with comparable optical absorption to MCT [1]. The type II band alignment of the ASL (Fig. 1) results in quantum confinement of electrons in the InAs conduction band and holes in the GaSb valence band, producing electron and hole minibands with a positive IR gap. In the binary structure (Fig. 1(a)), the IR gap is tuned by adjusting the thickness of the layers. This is potentially a problem for long wavelength devices since the electron-hole wavefunction overlap gets smaller with increased layer thickness, although high efficiency has been observed i
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