Seeing in the Dark

Abstract

A tiny band of wavelengths has enabled a quite remarkable global network of communications. Most of them depend on the use of semiconductors, either as sources or as detectors. The first attempts to apply IR techniques to solving practical problems can be traced to the First World War, when serious efforts were made to develop military systems. The most significant development in this context was the use of photoconductivity, in the semiconductor material thallium sulfide, to detect radiation in the near-IR part of the spectrum. Photoemission has played an important part in the development of quantum theory. The effect can be seen as an important step towards the development of light detectors. It demonstrates the possibility of making efficient photocathodes from epitaxially grown GaAs, which ultimately proved essential to the manufacture of transmission cathodes. The growth of bulk crystals proved extremely difficult from the point of view of obtaining adequate control over composition, uniformity and impurity concentration, while an additional complication manifested itself in that small departures from stoichiometry served to dope the material, thus introducing unwanted free carriers.