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Anisotype heterojunctions involving wide band gap II-VI semiconductors Taneja, Narayan Dass


This thesis is concerned with (i) the theory of anisotype hetero-junctions and (ii) the fabrication and study of heterojunctions of wide band, gap II-VI compounds with other semiconductors. The theory of anisotype heterojunctions without interface states is discussed on the basis of minority carrier injection. Expressions for the built-in voltage and current voltage relations are derived. The recombination- generation of the carriers in heterojunctions with interface states is considered, using Sah-Shockley-Noyce. theory. The expressions for the recombination-generation rate of the carriers for nine different cases are derived considering three carrier transport mechanisms. ZnSe films were deposited on Ge, GaAs and mica by sublimation under near-equilibrium conditions in an ultra high vacuum. The crystallite size in the films deposited on mica at 500°c was about 15μ. Epitaxy of ZnSe on GaAs was observed around 380°C. The conductivity and Hall mobility of the films on mica were measured under illumination and the results are explained on the basis of a mosaic film model. The photoconductivity of the films was barrier limited at low intensities but carrier limited at high intensities. The nInAs-pZnTe heterojunctions were fabricated by the interface alloying technique. The current voltage and capacitance-voltage characteristics of the devices were studied, and their theoretical relations are derived. Light emission was observed in these heterojunctions under reverse bias conditions. The spectrum of the emitted light was in the energy range of 1.69 eV to 3.5 eV. A number, of peaks were observed in the spectrum at low currents while at high currents only one peak at 2.04 eV was observed. An energy band diagram of ZnTe with different impurity centers is developed to explain the electroluminescence characteristics. The pGe-CdS (photosensitive) heterojunctions were made by a simple alloying technique. The current-voltage and capacitance-voltage characteristics of the heterojunctions were studied under different illumination intensities. The I-V characteristics were non-rectifying at low intensities but rectifying at high intensities. Energy band diagrams, equivalent circuits and the theory of the prepared devices are discussed.

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