Final report

Abstract

This grant was a continuation of research conducted at the University of Florida under Grant No. DE-FG05-91ER45462 in which we investigated the energy bandgap shifts produced in semiconductor quantum dots of sizes between 1.5 and 40 nm. The investigated semiconductors consisted of a series of Column 2-6 compounds (CdS, CdSe, CdTe) and pure Column IV elements (Si and Ge). It is well-known of course that the 2-6 semiconductors possess a direct-gap electronic structure, while the Column IV elements possess an indirect-gap structure. The investigation showed a major difference in quantum confinement behavior between the two sets of semiconductors. This difference is essentially associated with the change in bandgap energy resulting from size confinement. In the direct-gap semiconductors, the change in energy (blue shift) saturates when the crystals approach 2-3 nm in diameter. This limits the observed shift in energy to less than 1 eV above the bulk value. In the indirect-gap semiconductors, the energy shift does not show any sign of saturation and in fact, we produced Si and Ge nanocrystals with absorption edges in the UV. The reason for this difference has not been determined and will require additional experimental and theoretical studies. In our work, we suggest, but do not prove that mixing of conduction band side valleys with the central valley under conditions of size confinement may be responsible for the saturation in the blue-shift of direct-gap semiconductors. The discovery of large bandgap energy shifts with crystal size prompted us to suggest that these materials may be used to form photovoltaic cells with multi-gap layers for high efficiency in a U.S. Patent issued in 1998. However, this possibility depends strongly on the ability to collect photoexcited carriers from energy-confined crystals. The research conducted at the University of Arizona under the subject grant had a major goal of testing an indirect gap semiconductor in size-confined structures to determine if photocarriers could be collected. Thus, we tested a variety of semiconductor-glass nano-composite structures for photoconductivity. Tests were conducted in collaboration with the Laser Physics Division at Sandia National Laboratories. Nano-composite samples were formed consisting of Ge nanocrystals embedded in an indium-tin-oxide matrix. Photoconductivity measurements were conducted with exposure of the films to sub-bandgap and super-bandgap light. The results showed a clear photoconductivity effect arising from exposure to super-bandgap light only. These results suggest that the high-efficiency photovoltaic cell structure proposed in DOE sponsored U.S. Patent 5,720,827 is viable. The results of fabrication studies, structural characterization studies and photovoltaic measurements are presented in the report. This report is taken from a PhD dissertation of Tracie J. Bukowski submitted to the University of Florida in May 2002. ''The optical and photoconductive response in germanium quantum dots and indium tin oxide composite thin film structures,'' Dr. Bukowski conducted her PhD study under this grant at the University of Arizona and under Grant No DE-FG05-91ER45462 at the University of Florida, as well as during a two-year fellowship at Sandia National Laboratories.