Structural studies of HgCdTe grown by MOCVD on lattice-matched substrates
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I. INTRODUCTION
The growth of Hgi ,Cd x Te by metal organic chemical vapor deposition (MOCVD) is now a wellestablished technique, and both direct ternary alloy growth1"5 and the interdiffused multilayer process (IMP)5"8 have been utilized for deposition above 300 °C. IMP takes advantage of the high interdiffusion coefficients by growing alternate layers of CdTe and HgTe under near-optimized flow conditions and, provided the combined layers are thin enough (0.1 to 0.3 ju,m), they completely interdiffuse and homogenize during growth and a short post-anneal at the same temperature.7'9 IMP offers improved compositional uniformity both laterally and with depth,10 and growth rates between 10 and 20 /u,m h"1 can be maintained compared to 3 to 4 fim h"1 for alloy growth.5'9 The use of diisopropyltelluride in place of diethyltelluride has reduced the deposition temperature from 400 to 350 °C, and use of further alternative tellurium precursors is allowing growth of HgCdTe below 300 °C. With these developments, MOCVD is a prime candidate for growing multilayer HgCdTe structures in a production environment. A variety of substrate materials has been used for MOCVD of Hg!_,Cd,Te. Even CdTe with 0.3% mismatch with Hgi-jCdxTe (x = 0.2) generates misfit dislocations in epilayers, and lattice-matched substrates are preferred. Candidates include CdTeZ, where Z = 4.4% Zn, 4.2% Se, or 10.7% Mn. Both Cdo.96Zno.o4Te and CdTeo.96Se0.o4 have been used for LPE11'12 and MOCVD growth of HgCdTe13 and have reduced the interfacial dislocation density, improving the structural quality of the epitaxial films. Ghandhi et al. grew simultaneously 12 ^m thick Hgi-^Cd^Te (x = 0.183) layers at 380 °C with diisopropyltelluride on CdTe and CdTeSe substrates.13 The FWHM values of the doublecrystal rocking curves were 151 and 47 arcsec, respecJ. Mater. Res., Vol. 5, No. 7, Jul 1990
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tively. The improvement in crystal quality was assigned to better lattice matching to the substrate material. Ternary alloy growth had been used with a growth rate of 3.5 fim h"1. This paper reports the results of structural assessment of HgCdTe films on CdZnTe substrates using the IMP technique, and both cross-sectional transmission electron microscopy (XTEM) and double-crystal x-ray diffraction measurements are utilized in the characterization. The correlation of double-crystal rocking curve widths with the microstructural data derived by XTEM was explored to use the former technique for nondestructive and routine screening purposes. The techniques confirm the improved crystal quality of HgCdTe films on these lattice-matched substrates. II. EXPERIMENTAL APPROACH
HgCdTe layers were grown by the interdiffused technique in a system similar to that previously described.7'9 Particular attention has been given to pressure balancing between the reactor and exhaust lines, the switching manifold for vent/run operation, and the reactor design. The precursors were diethyltelluride, dimethylcadmium (both supplied by Epichem Ltd.),
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