Faculty Sponsor: Meng-ju Renee Sher
Live Poster Session: Zoom Link
Abstract: Silicon (Si) semiconductors has been used in a broad range of fields including photodetection devices and solar cells. Nevertheless, intrinsic Si solar cells can only absorb light with wavelengths shorter than near-infrared wavelengths. Adding deep level impurities into intrinsic Si will help facilitate electron excitation for low energy photons and increasing the efficiency of solar cells. Tellurium has been shown as a potential dopant for Si semiconductors. It has a low diffusion rate in Si substrate, allowing higher doping concentrations, and shows thermal stability up to 400 Celsius. Previous study found that, under decreasing temperature, Te-hyperdoped silicon showed increasing spectral responsivity. Hence, it is important to study how temperature variation influences carrier recombination and carrier lifetime. To prepare for temperature dependent measurements, we conducted experiment under room temperature to decide the dopant concentration and pump power for low temperature measurements. We studied the carrier decay dynamics for Te-hyperdoped Si with concentrations at 0.25%, 0.5%, 1%, 1.5%, 2%, and 2.5% under 400 nm laser with varying intensities at room temperature. We measured the change in material conductivity before and after laser excitation through THz spectroscopy. Carrier lifetime can then be deducted by fitting the data with a bi-exponential decay curve. Comparing the initial conductivity after excitation for different concentrations. We found that the initial increase in conductivities for dopant concentrations smaller than 1.5% are one magnitude larger than that for dopants with concentrations larger or equal to 1.5%. This can be explained by insulator to metal transition (IMT) effect which happens between 1% to 1.5% for Te-hyperdopped Si. In addition, we observed 1.5% and 0.25% samples with significantly smaller carrier lifetime. We then measured change in conductivity for 1% and 2% sample with pump power varying from 0.6 to 6mW. By fitting with the bi-exponential decay model, we find that, for 1% sample, carrier lifetime increases with pump power, showing a potential saturation effect. For 2% sample, a higher dopant concentration allows faster carrier recombination, and the saturation effect was not observed. Furthermore, we found a non-linear relationship between initial conductivity and pump power. This showed that conductivity has fluence dependency, which can be explained by change in carrier mobility as carrier density increases. Further work includes testing samples crystallinity to explain whether 1.5% and 0.25% sample behave differently due to crystal structure. Perform temperature dependent measurement for 1% and 2% sample under 1 mw to observe IMT and prevent saturation effect.
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