MBE Growth and Characterization of HgCdTe Heterostructures
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MBE GROWTH AND CHARACTERIZATION OF HgCdTe HETEROSTRUCTURES R.J.KOESTNER, M.W.GOODWIN and H.F.SCHAAKE Texas Instruments, Inc., Central Research Laboratories, Dallas, Tx 75265 ABSTRACT HgCdTe heterostructures consisting of a thin n-type widegap (250 meV or 5 pm cutoff) layer deposited on an n-type narrowgap (100-125 meV or 10-13 pm cutoff) layer offer the promise of very high performance metal-insulator-semiconductor (MIS) photocapacitors for long wavelength infrared (LWIR) detection. Molecular Beam Epitaxy (MBE) is a candidate growth technology for these two layer films due to its fine control in composition, thickness and doping concentration. The critical materials issues are reducing the defect content associated with twins in the grown layers, achieving low net donor concentrations in the widegap layer, and avoiding the formation of misfit dislocations at the HgCdTe heterointerface. This paper will report on our recent progress in these directions. INTRODUCTION We are developing a HgCdTe heterostructure grown by MBE that consists of an n-type widegap layer (with a 3-5 pm cutoff) deposited on an n-type narrowgap layer (with a 10-13 pm cutoff). The n-type heterostructure is then processed to produce a metal-insulator-semiconductor (MIS) photocapacitor for LWIR detection. The critical materials issues that we have addressed to achieve higher performing detectors are reducing the defect content associated with twins in the grown layers, reducing the donor concentration in the widegap layer and avoiding the formation of misfit dislocations at the HgCdTe heterointerface. Single and compound twins are observed in MBE HgCdTe(001) layers. The single twins or twin lamellae nucleate at the substrate interface and propagate through the film. Their origin appears to be impurity adsorption on the substrate surface prior to HgCdTe growth. On the other hand, compound twins, which form surface pyramids on MBE HgCdTe(001) layers, nucleate throughout the 5 pm thick HgCdTe film. Their origin seems to be related to Hg clustering on the growth front. By controlling residual gas adsorption on the substrate surface prior to HgCdTe growth and by modifying our Hg source, we have observed a dramatic reduction in single and compound twin densities in HgCdTe(001) layers, respectively. In addition, compound twins are also observed in MBE HgCdTe(112)Te films with a density nearly equal to that found in our HgCdTe(001) layers. For MBE grown HgCdTe(001) films, net donor densities of 1-2 x 101 cm-3 for the widegap layers (CdTe mole fraction or x = 0.29 at a 5 p~m cutoff) and 2-3 x 10" cm- 3 for the narrowgap layers (x = 0.22 at a 10 pm cutoff) are reproducibly measured in MIS test structures. A further reduction of the net donor density in the widegap layer to 5-10 x 1014 cm-3 is necessary for optimal heterostructure performance. To achieve this, MBE HgCdTe(112)Te widegap layers have been grown which show net donor densities from 2-20 x 1014 cm- 3 .
Mat. Res. Soc. Symp. Proc. Vol. 198. 01990 Materials Research Society
398
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