Transport Properties in Nanocrystalline Silicon and Silicon Germanium
Saripalli, Satyalakshmi (2008) Transport Properties in Nanocrystalline Silicon and Silicon Germanium. PhD thesis, Iowa State University.
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Nanocrystalline(nc-) Si:H and (Si,Ge):H films are important electronic and optical materials. They consist of crystallite grains surrounded by amorphous tissues. To successfully apply these materials for solar cell applications we need to understand their fabrication procedures and develop ways to characterize the material. Transport properties determine whether the generated photons in the device effectively reach the terminal and contribute to the output current. While there are several reports in literature about growth and the carrier transport mechanism in nc-Si, nc-SiGe is a fairly new material. Motivation to study nc-SiGe comes from its lower bandgap which makes it an alternative material for lower cell in tandem devices. This thesis deals with measurement of fundamental transport properties in these materials like minority carrier lifetime and diffusion length. While there have been previous reports of diffusion length, lifetime was never measured before in nc-Si or nc-(Si,Ge). We adopt the reverse recovery method to calculate lifetime in device type structures. The typical values of lifetime in nc-Si and nc-SiGe were found to be in the range of 300-600ns and 150-250ns respectively. Systematic measurement of lifetimes and defect densities in the same solar cell devices enabled us to prove that Shockley Read-Hall (SRH) recombination holds true in these nanocrystalline materials. With respect to temperature we found that the lifetime follows a U shape, with a decreasing trend till a certain temperature and increasing thereafter. Both the effects could be explained by understanding the actual recombination process. We could get an estimate of the trap locations from lifetime versus temperature data. By examining the reverse recovery waveforms in different types of device structures, we deduced that this method is appropriate when the carrier transport is diffusion dominated. Lifetime and diffusion length were measured in p(+)-n-n(+) devices where the base layer(n) was either nc-Si or nc-SiGe fabricated using VHF plasma deposition, at a frequency of 46MHz using mixtures of SiH4 and GeH4 with H2. From Raman spectroscopy it was observed that nc-SiGe requires significantly higher Hydrogen dilution ratios compared to nc-Si. Graded TMB doping was used to enhance carrier collection in the base layer. Defect densities were measured using capacitance spectroscopy. In SiGe devices it was observed that extreme care has to be taken at the p+/ n and n+/ n interfaces to ensure efficient carrier collection. Even though the actual process of grain growth is still unclear in nc-SiGe, from the lifetime versus defect density data, we do conclude that the carrier transport is primarily controlled by recombination at grain boundaries.
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