Recent Progress in The Omvpe Growth of HgCdTe
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RECENT PROGRESS IN THE OMVPE GROWTH OF HgCdTe
SORAB K. GHANDHI Electrical, Computer and Systems Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180
ABSTRACT The last two years has seen rapid development in the growth of mercury cadmium telluride material for use in far infrared detectors. This paper will briefly review the progress before this period, and will focus on recent developments in these materials. The emphasis will be on the direct alloy growth of HgCdTe material by organometallic vapor phase epitaxy (OMVPE). TECHNICAL The epitaxial growth of Hgl_•Cd.Te (MCT) has received considerable attention during the past several years, because of its potential for producing active layers of higher electrical quality than those grown by bulk methods. Several methods have been reported for the growth of these layers; of these, organometallic vapor phase epitaxy (OMVPE) has emerged as the most promising candidate [1-8] for the commercialization of this material. To this end, a major effort is underway at the present time. Compositional Uniformity: The driving force for these developments is the need for focal plane array detectors, operating in the 10.6 um range. Here, the main requirements are layers of 10-15 lm thickness, with an z value around 0.2. A high degree of compositional uniformity (±0.005) is required for z (the Cd-fraction), and devices as large as 1 cm x 1 cm are envisioned. This composition uniformity is relatively easy to obtain in mass transport limited systems such as AlGaAs and InGaAs. In these systems, OMVPE has a unique advantage since source reactants can be mixed prior to entry into the reactor. As a result, compositional uniformity over the substrate is achieved under all flow conditions and independent reactor details or layer thickness. The situation with HgCdTe is, unfortunately, more complex, since growth is limited by surface kinetics. One approach around this problem is to grow the binaries, HgTe and CdTe, in a sequential manner, and then homogenize them in a final heat treatment. This method, known as the Interdiffused Multilayer Process (IMP), can be used [9, 101 to readily achieve the compositional uniformity goal. However, it involves an interdiffusion process which requires a rather high temperature, which offsets the advantage of low temperature growth. In addition, the crystallinity of interdiffused HgCdTe layers has been shown to be poorer than that of alloy grown HgCdTe, as determined by double-crystal x-ray diffraction [11]. This has been attributed to defects generated due to the lattice mismatch between HgTe and CdTe, and to incomplete diffusion. Additional problems with the IMP process come about because of the large differences in incorporation rate for dopants in HgTe and CdTe. Together with the fact that diffusion rates of the preferred dopants are extremely slow compared to the homogenization rate, this leads to striations in doping rather than uniformly doped layers. It is for these reasons that we have adopted the conventional approach of di
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