A close look at "Schottky barrier" lead salt infrared detectors

Summary form only given, as follows. The primary purpose of this work has been to understand the underlying physics of the metal/lead salt "Schottky" devices currently being developed as detectors sensitive in the 3-5 or 8-14 micrometer spectral region. This note reports on a process developed at the Night Vision Laboratory for producing "transparent electrode" or "Schottky barrier" photodiodes on lead-tin telluride. In addition we shall report evidence which, for the first time, indicates that most, if not all, lead salt "Schottky barrier" infrared detectors made to date are in fact, planar impurity diffused detectors. We have mage infrared photodiodes sensitive in the 8-14 micrometer spectral region by evaporating "1000A" of indium onto Pb 8 Sn 2 Te annealed to carrier concentrations of 2-6 x l0 l6 cm 3 (p-type). This was followed by a vacuum "bake out" of 130°C for 15 minutes at 10 -6 torr normally given such diodes when about to be tested or packaged as detectors. These procedures have given us detectors with junction resistance-area products of 0.4 ohm-cm 2 and quantum efficiencies of 20%. It is widely felt that this "bake out" procedure improves the device performance by either driving off adsorbed water vapor which can cause diode surface leakage or thermally forcing the "Schottky" metal through a metal/semiconductor interface layer into intimate electrical contact with the semiconductor. We have found that such "baked" devices in effect, have diffused junctions. This was proved by sputtering all of the thin (l000A) indium off of the detectors' active areas. Responsivity contour plots made of the devices' active areas showed the total junction areas to be unchanged after the above mentioned sputtering, thus strongly indicating that the indium has