Heterojunction-Assisted Impact Ionization and Other Free Carrier Dynamics in Si, ZnS/Si, and ZnSe/Si
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With increasing global energy demand and diminishing fossil fuel supplies, the development of clean and affordable renewable energy technology is more important than ever. Photovoltaic devices harvest the sun’s energy to produce electricity and produce very little pollution compared to nonrenewable sources. In order to make these devices affordable, however, technological advances are required. In this dissertation a novel photovoltaic device architecture that is designed to enhance sunlight-to-electricity conversion efficiency of photovoltaics is proposed and demonstrated. The increase in efficiency arises due to enhancement of the internal quantum efficiency of photoexcitation in the semiconductor absorber. In other words, the probability that the absorption of a single photon will produce two or more electron-hole pairs, instead of just one, is increased. This occurs through the process of impact ionization, by which a highly excited charge carrier (via absorption of a high energy photon) relaxes by excitation of a second electron-hole pair. The result is an increased photocurrent, and efficiency, of the photovoltaic device. Using thin films of ZnS on Si substrates, we demonstrate that the probability of impact ionization is enhanced at the (unbiased) heterojunction between these layers. The magnitude of enhancement depends on material properties, including crystallinity of the ZnS film as well as concentration of oxygen (impurity) at the interface. Thin films of ZnSe on Si substrates do not exhibit heterojunction-assisted impact ionization, but they do display promising characteristics that make them an intriguing system for future work. The same is true for ZnS/Si materials fabricated by O2-free chemical bath deposition. For the analysis of plain Si as well as ZnS/Si and ZnSe/Si heterostructures, we employ a novel pump-probe transient transmission and reflection spectroscopy technique. A method is demonstrated for using this technique to quantify internal quantum efficiency as well as interface recombination velocity in each of these materials. In bulk silicon, a free carrier absorption cross section that depends on free carrier concentration (above 1018 cm-3) is observed and the relationship is quantified. This dissertation includes unpublished and previously published co-authored material.