Applications Of Multichannel Spectroscopic Ellipsometry For Cdte Photovoltaics

Spectroscopic ellipsometry (SE) is a powerful tool for non-destructive evaluation of thin films consisting of single layers or multilayers on substrates. For such thin films, SE can provide structural parameters and component material optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of the materials. Installation of the SE instrument on a deposition chamber enables monitoring of the thin film during deposition. This experiment, referred to as in-situ real time SE (RTSE), enables high surface sensitivity, fast data acquisition, and non-invasive probing which lead to unique insights into the dynamics of film growth. In this dissertation research, RTSE was applied for analysis of the structural evolution of oxygenated CdS (CdS:O) films deposited on c-Si substrates using different [O2 ]/{[Ar] + [O2]} gas flow ratios. The analysis of RTSE data provides valuable information including the initial film growth mode (e.g. layer-by-layer or clustering), subsequent bulk layer and surface roughness thickness evolution, the growth rate in terms of effective thickness (or volume/area), and the final film optical properties. As an additional application of SE, ex situ through-the-glass mapping SE (TG-M-SE) has been used to study the structural properties and area uniformity of CdS/CdTe solar cells over large areas. The mapping results can be correlated with the efficiency of solar cells fabricated over the same area, exploiting inevitable non-uniformities in the process to identify the optimum structural parameters for highest efficiency solar cells. In a second component of this research, RTSE has proven to be very powerful for the development and optimization of thin film FeS2 deposited by a novel hybrid sputtering/co-evaporation method. This effort started with the sputtering of elemental Fe metal thin films and proceeded to the evaporation of sulfur simultaneously with Fe deposition using different overall rates and substrate temperatures. The effects of these deposition parameters on the resulting iron sulfide film properties were explored. The nucleation, growth evolution, and optical properties of thin film FeS2 deposited by this hybrid method are key outcomes of this study that relies on in-situ RTSE. Other characterization techniques including X-ray diffractometry, scanning electron microscopy SEM, and energy-dispersive X-ray spectroscopy (EDS) were used in the dissertation research to determine the crystal structure, surface microstructure, and the S:Fe atomic ratio of the iron sulfide films to complement the RTSE analysis. In the final component of this dissertation research, CdTe/CdS solar cells in the substrate configuration were fabricated and studied. Development of CdTe/CdS solar cells in this non-standard configuration provides an alternative approach for exploring CdTe back contacts that may require high-temperature vacuum processing. The experimental research on this subject started with a study of the effect of CdS bath temperature on the CdS/CdTe solar cell performance in the superstrate configuration in an attempt to advance the process for use in the substrate configuration. Materials including MoO3, copper indium selenide (CIS), and nitrogen doped CIS (CIS:N) were studied as p-type back contact interlayers between Mo and CdTe in the CdTe/CdS substrate configuration solar cells. The performance of the solar cells incorporating these p-type back contacts are compared and discussed in the dissertation.