The need for large area HgCdTe material of good crystalline quality for IR detectors and advanced focal plane arrays has prompted research in thin film HgCdTe. Although several techniques are currently being explored, liquid phase epitaxy (LPE) appears to be the most promising.
A HgCdTe LPE approach is defined by the growth solution or melt, the growth mode, and the mechanics of bringing the substrate, usually CdTe, in contact with the melt. Growth of narrow bandgap Hgl_xCdxTe alloys is obtained from both Hg-rich and Te-rich melts. An additional post-anneal step in a Hg atomsphere is required for n-type conversion. Equilibrium cooling and isothermal growth are the most frequently used growth modes. Each mode requires careful determination of the liquidus temperature to obtain films with reproducible characteristics. Isothermal growth from a large melt offers the inherent advantage of a more uniform alloy composition in the depth of the film. The three customary methods of bringing the melt and substrate into contact are: 1) dipping the substrate into the melt, 2) sliding the melt on and off the substrate, and 3) tipping the melt on and off the substrate. Of the three, tipping appears to be least suited for a production process.
In their present state of development, Hgl_xCdxTe epitaxial films with carrier concentrations typically in the range n = 5-10 x 1015 cm-3 are routinely obtained. In some cases, values in the high 1014 cm-3 range have been reported. Advanced focal plane array structures for long wavelength applications will require material with n < 4 x 1014 cm-3. Judging from the present state of the art, this goal will likely be attained within the next few years.