Modeling of HgCdTe Heterojunction Devices
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MODELING OF HgCdTe HETEROJUNCTION DEVICES KEN ZANIO AND KEN HAY Ford Aerospace & Communications Corporation Ford Road, Newport Beach, CA 92658-9983 ABSTRACT A model for generating the composition and doping profiles from growth and Poisson's diffusion parameters was developed for heterostructure devices. equation was applied to these structures to predict barriers in the conduction band to minority carrier flow for long wavelength HgCdTe infrared detectors prepared by LPE techniques. Spectral response and quantum efficiency measurements illustrate the presence of these barriers and support the use of this model in predicting barrier formation. A. INTRODUCTION Heterojunction technology has significantly increased the resistance area products (RA) of long wavelength HgCdTe infrared detectors prepared by LPE techniques. Often increases in the RA of these heterojunction devices are with decreases in the quantum efficiencies. Electronic band associte modelsLI-,] have predicted potential barriers in HgCdTe which block minority carrier flow to the electrical junction and decrease the quantum efficiency. Thesemodels use the complementary error function to determine the composition profiles. This approach prevents the calculation of band diagrams for material having more complex composition profiles such as those obtained in the LPE growth of HgCdTe by the tipping technique. Even for the simple case of cap layers with flat composition profiles, previous models arbitrarily place the relative position of the metallurgical and electrical junctions, assume abrupt shapes of the doping profiles, and assume the interdiffusion coefficient in the transition region between the cap and base layers to be independent of composition. Because the resolution of the measurement of the actual composition and doping profiles is limited at best to a few thousand angstroms in LPE HgCdTe, further modeling is, necessary to realistically predict the actual shape of the band diagrams, especially at these reduced dimensions. In this paper we incorporate growth and diffusion in an iterative model to predict the shape of the composition and doping profiles. We then use these profiles to calculate the band diagrams and the height of the barriers to minority carrier flow. A range of practical crystal growth conditions for low barrier heights are obtained when transition and depletion widths are comparable and doping levels in the n-type caps are either higher than or comparable to the doping levels in the p-type absorber layers. Our device results support this model.
Mat. Res. Soc. Symp. Proc. Vol. 90. t 1987 Materials Research Society
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B. GROWTH AND DIFFUSION MODEL In the AlGaAs heterostructure system the composition profiles at the interfaces are abrupt because the interdiffusion coefficients are small. However in HgCdTe rapid interdiffusion occurs between Cd and Hg at LPE growth temperatures (375°C - 475°C) and must be considered in any electronic band structure model. The profiles are further complicated by a sign~ficant increase in the interd
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