For MCT, the technique of liquid-phase epitaxy (LPE) was demonstrated in the early 1980s and has matured to become a workhorse of the industry. Elements to form the layers are first dissolved in a melt of mercury or tellurium. The substrate is immersed in the melt and the temperature is ramped down, causing the elements to crystallize and form a layer. A second melt is used to form a second layer. N-type and p-type dopants, respectively, are included in the melts, so that the interface between layers becomes a p-n junction. The substrate for nearly all LPE growth is cadmium zinc telluride (CZT), which is chemically and physically compatible with MCT and is transparent in the IR.
Advantages of LPE are that (1) it occurs close to thermodynamic equilibrium, typically near 500°C, causing it to be relatively forgiving of defects; (2) dopants can be incorporated in a very controllable manner; and (3) excellent material quality is routinely achieved. The disadvantages are that detector structures requiring more than two layers are impractical, and it is not possible to maintain sharp interfaces between layers because of interdiffusion during growth. Also, LPE growth cannot be performed on alternative substrates such as silicon.
Molecular beam epitaxy (MBE) has become the preferred growth method for more advanced MCT device structures, such as two-color arrays for third-generation sensors, as well as avalanche photodiodes. It also enables the growth of MCT on silicon and GaAs substrates that are larger and cheaper than CZT. MBE growth is performed in an ultrahigh-vacuum chamber with the elements being emitted by hot effusion cells and depositing on the substrate, which is held at about 200°C. Sharp interfaces can be formed because the molecular beams can be turned on and off abruptly and because interdiffusion is negligible at the low growth temperature. In most cases the substrate is CZT, silicon, or GaAs.
MBE is more challenging than LPE because it is less tolerant of growth defects, and it requires very tight control of the substrate temperature and the beam pressures of the species arriving at the substrate. MBE equipment is more expensive to acquire and maintain than that of LPE. However, MBE technology has matured to the point that multilayer epitaxial structures, in which the MCT alloy composition and the doping are controllably changed several times during the growth run, are produced on a regular basis. This has enabled complex device structures in MCT that would have otherwise not been possible. Also, the ability to grow MCT on silicon has enabled the fabrication of very large focal plane arrays (FPAs). There is a large lattice mismatch between HgCdTe and both GaAs and silicon, which has severe implications for the growth process, in particular the formation of dislocations and other growth defects. Significant progress has been made in learning how to accommodate the lattice mismatch, particularly in the HgCdTe:GaAs system as discussed in Chapter 3. The availability of larger and cheaper substrates for epitaxial growth will have a major impact on the performance and cost of future HgCdTe FPAs.