Vibrating Kelvin Probe Measurements of a Silicon Surface with the Underside Exposed to Light
Dukic, Megan Marie
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This thesis addresses the use of a vibrating Kelvin probe to monitor the change in the front surface potential of a silicon wafer while the rear surface is illuminated with monochromatic, visible light. Two tests were run to verify the change in surface potential. One test increased the intensity of the light and the other increased the wavelength while recording the front surface potential. The change in the surface potential for a range of intensities of incident light was recorded and analyzed. The results show that the change in surface potential increased with increasing intensity. For each wafer, the smallest change in surface potential occurred at the lowest intensity, 3.77 mW. In the same respect, the largest change in surface potential occurred at the highest intensity, 17.8 mW. For all wafers, the change in surface potential ranged from approximately 8 mV at 3.77 mW to approximately 80 mV at 17.8 mW. The change in the surface potential for a range of wavelengths of incident light was also recorded and analyzed. The results showed that the change in surface potential formed a skewed bell curve with increasing wavelength of incident light. For each wafer, the largest change in surface potential occurred at mid-range wavelengths, between 600 nm and 700 nm. The smallest change in surface potential occurred at 450 nm, the shortest wavelength, and 800 nm, the longest wavelength. For all wafers, the change in surface potential ranged from approximately 8 mV at 800 nm to approximately 165 mV at 700 nm. A model based on excess electron diffusion within the silicon wafer was used to predict material properties. After curve fitting the model with experimental results, an excess electron lifetime of ôN = 17 µs and surface recombination rates of sFRONT = sREAR = 18,000cm/s were predicted. These values suggest poor silicon wafer quality relative to commercial silicon devices. Regardless of the quality, the results show that the front surface potential of a silicon wafer is affected by incident light on the rear surface. The quantitative effect of the light is dependent on the properties of the light and the material properties of the silicon wafer.