Semiconductor Photodiodes and Electroabsorption in Far-Infrared Mercury - Cadmium - Telluride.

The far-infrared (FIR) portion (8-14mu m) of the electromagnetic spectrum has become important for many applications. Three factors contribute to the usefulness of this region: (1) a good transmission window in the atmosphere; (2) the peak thermal emission of the earth; (3) the powerful CO_2 laser which emits at 10.6mum. Most FIR systems are limited by the ability of the detector to receive very weak signals among high background radiation. There are circumstances where the sensitive FIR detector of such a system might be subjected to an optical overload, either accidently or purposefully. It would be useful to have an FIR detector which could be protected from such blinding. The Franz-Keldysh effect may be used to electrically shift the absorption of a semiconductor material from nonabsorbing to absorbing states for a given wavelength. We investigated the use of Franz-Keldysh Electroabsorption photodetector as an overload protected device at the 10.6mu m wavelength. This dissertation discusses the FIR spectrum and detection methods, explores mercury-cadmium -telluride (MCT) as an FIR semiconductor material, and analyzes the Franz-Keldysh effect as it applies to MCT. The design, fabrication, and evaluation of a metal-insulator-semiconductor (MIS) device which appears to function as a tunnel diode is presented. The device theory and fabrication of Schottky barrier and pn-junction diodes in small-bandgap MCT is also presented. The MIS-type devices displayed photovoltaic response to optical excitation for wavelengths from 5mu m to beyond 12mum. We used the MCT MIS-type devices as electroabsorption photodiodes and determined the peak wavelength response could be shifted to longer wavelengths by about 0.5mum and the response at the important 10.6mu m wavelength could be increased by a factor of ten with the application of a large reverse bias across the diode, with both effects conforming to theoretical predictions. Our experiments showed that the Franz-Keldysh effect does have significant strength in MCT and may provide a useful method by which FIR MCT photodetectors can be protected from optical overload.