Transport properties of hydrogenated nanocrystalline germanium and germanium carbide and modeling of trap conversion instability in hydrogenated amorphous silicon
Booher, Jeremy (2004) Transport properties of hydrogenated nanocrystalline germanium and germanium carbide and modeling of trap conversion instability in hydrogenated amorphous silicon. Masters thesis, Iowa State University.
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This work is in two parts. The first part describes the measurement of mobility and carrier concentration in nanocrystalline semiconductors, specifically nanocrystalline Ge:H and (Ge,C):H alloys. These are new materials with significant technological applications in the fields of solar energy conversion and thin film electronic devices. The materials were grown using electron-cyclotron-resonance plasma deposition techniques from a mixture of germane, hydrogen and methane. The crystallinity and grain size were determined using combinations of Raman spectroscopy and x-ray diffraction. The carrier concentration and mobility were measured using the Hall effect. To measure the small Hall mobilities in highly resistive materials, a special apparatus with shielded cables was set up to minimize noise. It was found that Hall mobility increased as the grain size increased. Mobilities of the order of 5 cm2/V-sec were measured in nc-Ge:H, which are among the highest values ever obtained in nanocrystalline materials. Hall mobility and carrier concentrations were also measured as a function of temperature. It was found that mobility and carrier concentration both increased with increasing temperature. The increase in mobility could be explained by postulating that transport was governed by grain boundaries. The increase in carrier concentration implies that there are deeper defects in nc-Ge, and that electrons are excited from these defects into the conduction band at higher temperatures. In the second part of the thesis, I carried out numerical simulation of the trap-to-dangling bond conversion model for instability in a-Si:H . This model was originally postulated by Adler and then quantified by Dalal. There are a number of parameters in the model, such as the ratio of capture cross-sections between charged and neutral defects, the ratio of initial charged to neutral dangling bonds etc. A numerical simulation of the model was carried out and matched to the experimental data on specially made samples where the influence of charged defects was likely to be large. An excellent fit was obtained between the experimental data and the model, and from this fit, the various parameters of the model were estimated.
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