Characterization of temperature profile in furnace and solubility of iron in silicon

Abstract

A better understanding of the behavior of impurities, such as iron, in silicon can lead to increases in solar cell efficiency. The purpose of this thesis was to study the behavior of iron in silicon via three sub-tasks: (a) to determine an appropriate dwell time, characterize the temperature profile within a vertical high-temperature tube furnace as a function of depth and flow rate, (b) to use this information to diffuse iron into silicon at a known temperature, and (c) to measure the in-diffused iron concentration by comparing minority carrier lifetime measurements before and after iron-boron pair (Fei-Bs) dissociation. For the thermocouple used, a furnace dwell time of 15 minutes was determined to be sufficient to reach a stable temperature. At a forming gas flow rate of 1.0 SCFM, the ratio of the temperature in the furnace to the set temperature, [theta], as a function of depth in inches, d, is described by the equation [theta]= -7.29x10 4 d2 + 0.0177-d + 0.897. The effect of varying the flow rate on the temperature profile was minor, and, depending on the application, negligible. There was a great deal of variance in the measured iron concentrations in the samples, suggesting that improvements to the procedure would yield more consistent results. Possible sources of error included presence of other metastable defects (in addition to Fei-Bs / Fei), surface recombination, and high sensitivity of the calculated iron concentration to the measured minority carrier lifetimes.