ABSTRACT
One of the first photoconductors which gave a response in the near infrared region to approximately 1.1 jim was thallium sulfide, investigated in about 1920 by Case.1 The next group of materials to be studied were the lead salts (PbS, PbSe and PbTe) which extended the wavelength response to 7 jim. PbS photo conductors were originally fabricated by Kutzscher2 at the University of Berlin in the 1930s from natural galena found in Sardinia. However, for any practical applications it was necessary to develop a technique for producing synthetic crystals. PbS thin film photoconductors were first produced in Germany and next in the United States at Northwestern University in 1944 and, in 1945, at the Admiralty Research Laboratory in England.3 During World War II the Germans produced systems using PbS detectors which were able to detect hot aircraft engines. Immediately after the war, communications, fire-control, and search systems began to stimulate a strong development effort that has extended to the present day. The Sidewinder heat-seeking infrared-guided missiles received a great deal of public attention. After 50 years, low-cost, versatile PbS and PbSe polycrystalline thin films remain the photoconductive detectors of choice for many applications in the 1-3 jim and 3-5 \xm spectral range. At present, the commercial market of lead salt detectors is still amongst the largest.4 As examples, they have been used in automotive-pollution-monitoring devices, missile seekers, and missile-guidance systems. Current development with lead salts is in focal plane arrays (FPAs) configuration and lead salt-Si heterojunc tions. Such multi-elements are applicable toward the problem of spectrally selective detection.
