Metal Induced Growth of Thin Si Films for Photovoltaics: New Approaches and Applications
Kozarsky, Eric S.
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The metal induced growth (MIG) process has been explored as an approach to producing microcrystalline Si at reduced temperatures. In this study, Pd was explored as an alternative catalyst for the MIG process. The process was optimized in order to maximize the photovoltaic response without raising the temperature too far above 600°C. Optimal conditions were found by depositing a 5 μm thick Si film at a temperature of 625°C with subsequent annealing at 700°C for 2 hours in forming gas. The photo I-V measurements provided a JSC of 5.39 mA/cm 2a VOC of 245 mV, and a fill factor of 0.447. Catalyst combinations were explored to combine Pd with other metals such as Cr, Co, and Ni. Initially, a device structure of Co/Ni/Pd was found to produce the best result. A later study found an improved stability with the removal of Ni from the device structure. This Co/Pd structure allowed for a reduction in deposition temperature to 575°C without sacrificing performance or stability. An annealing study was performed on these devices. It was found that the optimal condition for these devices involved annealing at 800°C for 12 hours. As this time is too extensive, annealing at 800°C for 2 hours was chosen as an alternative which sacrifices very little in terms of performance. A new process known as metal induced lateral growth (MILG) was introduced. This process utilized adjacent metal pads to laterally crystallize Si which is deposited on the sample. This metal free microcrystalline region allows for additional approaches to analysis and applications. Contacts were preserved prior to Si deposition allowing for easy analysis. As these bulk measurements produced inconsistent results, an alternative process was developed. The alternative process involved patterning much smaller catalyst pads followed by the MILG process. A secondary alignment process allowed for a top contact to be deposited. The top contacts allowed electrical measurements to be performed across metal free small gaps. This offered improved electrical measurements using the transmission line model. This process was extended into the area of thin film transistors. For this purpose, the source and drain metals are deposited and Si is deposited and crystallized in-situ. A secondary patterning deposits the source and drain contacts followed by gate oxide and gate metal depositions. Unfortunately, these devices suffer from large leakage currents which limit saturation. The field effect mobility was found to be between 0.3-0.6 cm 2 /Vs. This low field effect mobility is primarily due to the MILG/MILG grain boundary present in the center of the channel. A final study involves the formation of a heterojunction device using Al doped ZnO deposited on top of the MIG Si. A buffer layer of SiO 2 was introduced between the Al-ZnO and the MIG Si. Simulations suggest this device without the buffer layer can achieve a potential JSC of 20.79 mA/cm 2a VOC 670 mV, and an efficiency of 11.675%. Although the fabricated device showed good rectification, the SiO 2 buffer layer proved to be too thick and blocked any photovoltaic response.