Investigation of Ga Diffusion in (001) and (111) CdTe Layers Grown on (001) GaAs
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INVESTIGATION OF Ga DIFFUSION IN (001) AND (111) CdTe LAYERS GROWN ON (001) GaAs J.J. DUBOWSKI*, J.M. WROBEL', J.A. JACKMAN" AND P. BECLA÷ Laboratory for Microstructural Sciences, NRCC, 100 Sussex Dr., Ottawa, Ont., Canada K1A 0R6 Metals Technology Laboratories, CANMET, 568 Booth St., Ottawa, Ont., Canada KIA OGI + Francis Bitter National Magnet Laboratory, MIT, Cambridge, Ma 02139, USA
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ABSTRACT A secondary ion mass spectroscopy study of Ga diffusion in CdTe grown on (001) GaAs was carried out. The layers were grown by pulsed laser evaporation and epitaxy. Two characteristic regions with increased Ga concentration were found. The first was the CdTe/GaAs interface where the concentration of Ga decreases rapidly to the detection limit of - 8 x 1014 cm"3. This region was usually less than 300 nm wide. The second was a surface region with a Ga accumulation of up to - 1017 cm"3. Ion imaging revealed that in the (001) CdTe layers, Ga accumulates near the surface at localized spots, up to about 8 pm in diameter. This feature is less apparent in the (111) CdTe layers. Annealing at 500 °C for 1 h increased the Ga concentration in the whole layer to above 1016 cm"3. We also observe thermal annealing induced precipitation of Ga at the surface of bulk CdTe samples which were originally uniformly doped with Ga. INTRODUCTION Significant attention has been paid to the use of Si, GaAs and InSb substrates in the growth of CdTe and HgCdTe thin films. This interest has been stimulated by the considerable cost of commercial CdTe wafers. Also, the size and concentration of defects, together with their tendency to continue in the grown epilayers, make CdTe wafers difficult to handle, for example, in the growth of planar detector structures. GaAs substrates are an attractive alternative in the growth of CdTe films. Depending on the substrate treatment and growth conditions, it is possible to produce epitaxial layers of both (001) and (111) CdTe on (001) GaAs, as well as (111) CdTe on (111) GaAs. However, there have been reports of Ga migration into CdTe layers 1 ". Such migration may strongly affects the electronic properties of CdTe, leading to limited application of GaAs substrates in CdTe technology. Thus, the problem of Ga migration in CdTe/GaAs heterojunctions should be addressed in more detail. In this paper, we describe a secondary ion mass spectroscopy ( SIMS ) study of Ga behaviour in (001) and (111) CdTe layers grown on (001) GaAs by pulsed laser evaporation and epitaxy ( PLEE )'.
EXPERIMENTAL DETAILS
The epitaxial layers of (001) and (111) CdTe were grown on Si doped (001) GaAs substrates ( Sumitomo Electric, Inc. ). The growth temperature for most of the samples studied was 300 °C and in some cases it was 260 °C. One sample was deposited on a substrate held at 25 °C and it had a polycrystalline structure. The growth of (111) oriented layers was carried out on Ne÷ ion cleaned substrates, and (001) layers were grown on chemically etched and vacuum baked substrates6- 7.The structural quality of the (001) CdTe layers was st
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