Electrical Studies on IIVI Compound Semiconductors for Device Applications

Abstract

II-VI compounds are vital materials for high performance optoelectronic devices such as photovoltaic, light detecting, light emitting diodes and laser diodes in the blue-green to ultraviolet spectral range. The direct band gap ranging over entire visible region and high absorption coefficient of these materials are main features that make them very attractive for such applications. Studies carried out so far on these materials have provided valuable insight into both fundamental and application aspects. In thin film form, these compounds are of interests for optoelectronic applications. However, the improvement in the efficiency of these devices was rather slow, due to the difficulty in doping and lack of control over the defects. These compounds are known to exhibit high electrical resistivity with low intrinsic carrier concentration and low carrier mobility in the thin film form. In addition, the selfcompensation effects involving defects restrict the conductivity of the films. An effective utilization of these compound semiconducting thin films can only be possible by improving the carrier density and/or mobility of the carriers. Detailed investigations are needed to achieve these improvements. In this thesis, an attempt was made to understand the electrical transport properties in some of the technologically important II-VI compound heterojunctions. As a first step, a detailed study including structural, compositional, and electrical characterizations were carried out on CdTe, CdSe, ZnTe, and ZnSe films grown by vacuum evaporation. Further, the effect of substrate temperature on the properties of these compounds was assessed. It was found that as the substrate temperature increased above room temperature, the composition and crystalline quality improved and hence, the electrical conductivity. Among the four compounds studied, CdTe had high resistivity; therefore, more attention was paid to improve its electrical conductivity through doping with indium and by adding excess Te. The CdTe films were further annealed in air and vacuum to study the effect of annealing on electrical properties of the films. Indium doped films showed n type conductivity and tellurium rich films showed p-type conductivity. The CdTe films showed improvement in electrical conductivity with increasing dopant concentration.Further, four different combinations of heterojunctions (p-CdTe/n-ZnSe, nCdSe/p-ZnTe, n-CdTe/p-Si, and p-CdTe/n-Si) were fabricated using the condition obtained in the first step. The heterojunctions were evaluated using current-voltage (IV) and capacitance-voltage (C-V) characterizations. To identify the dominant conduction mechanism in the heterojunctions, I-V curves were fitted to various models. Series resistance and leakage current were found to affect the characteristics of the junctions. Further, C-V measurements showed that the interfaces had large defect density and contributed to the measured capacitance along with space charges. The results of all the above mentioned studies are presented and analyzed in the present thesis. The doping studies were successful to certain extent, but did not result in drastic improvement in the device characteristics. However, the studies gave a good insight into the behavior of these compounds, which will help in improving the device suitability of the compound semiconductor thin films. Further research is required to improve electrical properties of the films by minimizing the defects, which control the film characteristics, by suitable passivation steps.