Properties of HgTe:ZnTe Strained Layer Superlattices Grown by MOVPE
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Properties of HgTe:ZnTe Strained Layer Superlattices Grown by MOVPE J.T. Mullins, P.A. Clifton, P.D. Brown, D.O. Hall and A.W. Brinkman. Applied Physics Group, S.E.A.S. University of Durham, South Road, Durham DH1 3LE, U.K. Abstract HgTe:ZnTe superlattices have been grown by thermal MOVPE at temperatures down to 325°C . At this temperature, interdiffusion is sufficiently low to make superlattice periods as low as 45A practicable. The results of theoretical calculations of the electronic structure of these materials are also reported. These show that the electronic structure may be significantly different to that found for the HgTe:CdTe system due to the large biaxial compression present in the HgTe well layers. Introduction HgTe:ZnTe strained layer superlattices have been proposed as an alternative to Hgl-,.CdTe for long wavelength infra-red devicesEl] . In contrast to the more widely studied HgTe:CdTe superlattice systemf 23 , interdiffusion coefficients are expected to be about an order of magnitude lower. This makes them suitable for growth by thermal Metal Organic Vapour Phase Epitaxy (MOVPE) at currently attainable temperatures. In a previous paper we reported the development of a process for the growth of HgTe:ZnTe superlattices by thermal MOVPE. In that work, growth was carried out at 395°C using di-methyl zinc (DMZ), di-ethyl telluride (DET), and elemental mercury and some interdiffusion was observed. Here we report growth at the reduced temperature of 325°C which has been made possible by the substitution of di-isopropyl telluride (DIPT) for DET as the tellurium precursor. Transmission electron (T.E.) microscopy has been used to characterise the resulting structures and provide information on the total thickness and hence period of the superlattices. EDX was used to determine the composition and hence to provide an estimate of the relative well and barrier thicknesses. Infra-red transmission measurements were performed at room temperature and for one superlattice indicated an energy gap of 0.2 eV. Also reported are the results of some theoretical calculations of the electronic structure of these materials. The HgTe wells may be under considerable biaxial compression due to the 5.2% mismatch between HgTe and ZnTe. This is in contrast to the case of HgTe:CdTe superlattices where the wells are under a small biaxial tension. This change in the sense of the strain can lead to significant differences in the electronic structure of the two systems. Structure and Interdiffusion A T.E. micrograph of a HgTe:ZnTe superlattice with a period of ; 115A grown on {100O GaAs at 395'C using DET is shown in Fig.1(a). Fringe contrast can be observed for approximately 20 periods at the top of the layer. By assuming that the lower portion of
Mat. Res. Soc. Symp. Proc. Vol. 161. ©1990 Materials Research Society
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the layer had interdiffused, and from a knowledge of the growth time and superlattice period, it was possible to estimate a value of , X 1615 cm2s- 1 for the interdiffusion coefficient between HgTe and ZnTe at 395
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