Dual Ion Beam Sputter Deposition of Cdte, Hgte and Hgcdte Films
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DUAL ION BEAM SPUTTER DEPOSITION OF CdTe, HgTe AND HgCdTe FILMS S. V. KRISHNASWAMY, J. H. RIEGER, N. J. DOYLE, AND M. H. FRANCOMBE Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, PA 15235 ABSTRACT Experiments have been performed to assess the feasibility of using ionbeam sputter deposition for the growth of CdTe, HgTe and HgCdTe films. Some simple cryogenically cooled dual-target configurations have been employed in an investigation of epitaxial growth on CdTe substrates. Good-quality epitaxy was achieved for CdTe at temperatures down to 140"C, and for HgTe and HgCdTe at temperatures extending to below 50"C. Based upon compositional and phase analyses, and upon IR absorption measurements, we conclude that, using an excess Hg flux, stoichiometric transfer of the HgCdTe target composition to the substrate is approximately obtained. However, some departure from stoichiometry is produced at higher substrate temperatures (> 150"C) due to thermal re-evaporation of Hg, and under high sputtered Hg fluxes due to selective re-sputtering of HgTe. The good structural quality and excellent compositional uniformity of the films indicate that ion-beam sputter deposition may be suitable for low-temperature processing of IR detector structures. INTRODUCTION In recent years considerable efforts have been devoted to the development of Hg1 Cd Te as multi-spectral, high-performance IR detector material. Growth Ey epitaxy of heterojunction [1] IR detectors embodying precise thickness, composition and doping profiles has required particular attention. There is also strong interest in HgTe/CdTe superlattice [2] as a promising candidate for IR detection at wavelengths beyond 18 rum. In order to achieve the desired composition and doping profiles in such layered structures, growth temperatures should be below 250"C. In fact, recent results [3] for superlattices suggest that measurable interdiffusion can occur at temperatures even as low as 110*C. The only growth techniques that appear capable of satisfying these conditions are MBE [4], laserassisted deposition (LADA) [5], energy-assisted MOCVD [6], and sputtering [7]. In general, all these techniques are at the research or exploratory technology stage, and although preliminary detector (and even array) fabrication has been achieved, the full potential and relative merits of these approaches have yet to be demonstrated. In general for MBE, LADA, and MOCVD growth methods, the deposition rate at the substrate surface, and hence the film composition, is controlled by thermal effects characterized by rather slow time response. On the other hand, sputtering, and in particular ion-beam sputtering, is unique in that thermal influences can be virtually eliminated. The ion-beam current and rate of sputtering can be modulated instantaneously by electronic means, and hence the deposition of sputtering material is amenable to precise control. In an earlier report [8] we demonstrated that ion-beam sputtering from cryogenically cooled targets of HgTe and HgCdTe results in stoichiometric transfer of
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