The present invention relates to semiconductor materials, and, more particularly, to the annealing of mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) and related materials.
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation detection. Indeed, Hg.sub..8 Cd.sub..2 Te has a bandgap of about 0.1 eV which corresponds responds to a photon wavelength of 12 .mu.m and Hg.sub..73 Cd.sub..27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, p-n junction Hg.sub.1-x Cd.sub.x Te photodiode arrays have long been used (see, for example, Lorenze, U.S. Pat. No. 4,286,278), and extrinsic p-type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane MIS detector arrays operating in the 10-12 .mu.m wavelength window. (Note that intrinsic p-type Hg.sub.1-x Cd.sub.x Te, whose doping is presumably dominated by mercury vacancies, was recently found to have midgap recombination centers proportional in concentration to the shallow acceptors; see C. Jones et al., 3 J. Vac. Sci. Tech. A 131 (1985). And these recombination centers shorten minority carrier lifetimes and are sources of recombination-generation noise; thus extrinsic p-type Hg.sub.1-x Cd.sub.x Te is preferred to intrinsic p-type.) And these detectors are fabricated in large area Hg.sub.1-x Cd.sub.x Te that may be grown by LPE. MOCVD, MBE or bulk techniques.
The usual manufacture of bulk Hg.sub.1-x Cd.sub.x Te includes recrystallization and homogenization at a high temperature (650.degree. C.) followed by a low temperature (50.degree.-300.degree. C.) anneal in mercury vapor for extended times to reduce the concentration of metal vacancies; this processing yields an n-type skin free of excess tellurium and a p-type core of condensed metal vacancies and precipitated tellurium. Similarly, Hg.sub.1-x Cd.sub.x Te grown by LPE, MOCVD, and MBE is typically annealed at low temperatures in mercury vapor to adjust the intrinsic metal vacancy concentration; see, generally, H. Schaake et al, The Effect of Low Temperature Annealing on Defects, Impurities, and Electrical Properties of (Hg,Cd)Te, 3 J. Vac. Sci. Tech. A 143 (1985). The metal vacancy concentration depends upon temperature, mercury partial pressure, and composition (the x in Hg.sub.1-x Cd.sub.x Te), and appears to have an exponential dependence on the negative reciprocal of temperature; see. H. Vydyanath. 128 J.Electrochem.Soc. 2609 (1981).
The usual method of performing the anneal to adjust the metal vacancy concentration is to encapsulate the Hg.sub.1-x Cd.sub.x Te in a quartz ampoule with excess mercury for an isothermal anneal at 50.degree. to 300.degree. C. of from several hours to more than sixty days depending upon the material and the desired skin thickness. However, for large area slices and epitaxial films whose width may exceed one inch, the vacuum encapsulation method is cumbersome and inefficient, and it is a problem to encapsulate large area material and remove them without expense and risk of damage upon opening the ampoule.