Nanocrystalline Germanium and Germanium Carbide Films and Devices
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Nanocrystalline Germanium and Germanium Carbide Films and Devices Xuejun Niu, Jeremy Booher and Vikram L. Dalal Dept. of Electrical and Computer Engr. and Microelectronics Research Center Iowa State University Ames, Iowa 50011,USA ABSTRACT Nanocrystalline Ge and its alloys with C are potentially useful materials for solar cells, thin film transistors and image sensors. In this paper, we discuss the growth and properties of these materials using remote , low pressure ECR plasma deposition. The materials and devices were grown from mixtures of germane, methane and hydrogen. It was found that high hydrogen dilutions (>40:1) were needed to crystallize the films. Studies of x-ray spectra revealed that the grains were primarily oriented. The grain size was a strong function of hydrogen dilution and growth temperature. Higher growth temperatures resulted in larger grain size. High hydrogen dilution tended to reduce grain size. These results can be explained by recognizing that excessive amounts of bonded H can inhibit the growth of grain, which is the thermodynamically favorable direction for grain growth. Grain sizes as large as 80 nm were obtained in nc-Ge. Addition of C reduced the crystallinity. Mobility and carrier concentrations in nc-Ge were measured using Hall effect. Mobility values of ~5 cm2/V-s and carrier concentrations of ~1x1016/cm3 were obtained in larger grains. p+nn+ devices were fabricated on stainless steel substrates and compared with similar devices deposited in nc-Si:H. It was found that the voltage decreased and current increased in nc-Ge devices, in comparison with devices in nc-Si:H. Addition of C to Ge devices increased the open circuit voltage and shifted the quantum efficiency to larger photon energies, as expected. INTRODUCTION Nanocrystalline Si:H is now a well-established material for solar energy conversion and for TFT devices [1-5]. Crystalline silicon, unfortunately, suffers from the disadvantages of a very indirect bandgap, with low absorption for energies near the bandgap. In contrast, crystalline Ge is almost a direct gap material with only a small (~0.15 eV) separation between the central Γ valley and the lower L valley. Therefore, Ge has a much higher absorption coefficient at energies near its bandgap than is the case for Si. In this paper, we study the growth of nc-Ge:H films and devices, and also study the properties of nc-(Ge,C):H films and devices. The purpose of adding C to Ge is to shift the bandgap to higher energies [6-8] while maintaining the favorable optical properties of nc-Ge. In this study, we systematically investigate the influence of deposition conditions such as hydrogen dilution and growth temperature on the growth and structure of nc-Ge:H, and then study the electronic properties of these films. Measurements of some of the relevant electronic properties are made in both films and in devices by making Hall samples in films, and simple p+nn+ devices on stainless steel substrates.
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EXPERIMENTAL TECHNIQUES The films and devices were both fabricated
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