Electrical, optical, and vibrational properties of oxygen-related defects in Czochralski-grown silicon for solar cells
Abstract
This thesis investigates the electrical, optical, and vibrational properties of oxygen-related defects in high-efficiency silicon solar cells, and their evolution during annealing at temperatures from 450 Degrees C to 850 Degrees C, in commercially available Czochralski-grown silicon (Cz - Si) wafers. The thesis is divided into 3 main sections. In the first part, we investigate the luminescence, vibrational and electronic characteristics of thermal donors generated at 450 Degrees C. We identify six sub-bandgap luminescence peaks and link a peak centered at 1206nm with thermal donors. We suggest that five other luminescence peaks are related to oxygen precipitates, or their nuclei. In the second part of the thesis, we estimate the capture cross sections of slow traps, induced by thermal donors, using novel photoconductance measurements together with a pulse-filling technique. We measure minority carrier cross sections of two distinct trap states on the order of 10 - 18 cm2. We show the majority carrier cross sections to be orders of magnitude smaller than minority carrier capture cross sections, suggesting trapping is dominated by minority carriers. Moving on from low-temperature thermal donors, we then investigate the vibrational and optical properties of new thermal donors and other defects generated at 650 Degrees C, 750 Degrees C and 850 Degrees C using photoluminescence and FTIR. We show that not all oxygen related defects are optically active. Overall, we demonstrate the power of luminescence spectroscopy and infrared spectroscopy to differentiate various species of oxygen-related defects. These results have implications for the design and processing of high-efficiency silicon solar cells.