Low-pressure chemical vapor deposition by thermolysis of disilane for low-temperature fabrication of pn junction solar cells
A low-pressure chemical vapor deposition (LPCVD) reactor was built in order to implement a low-temperature process to deposit thin-films of silicon and fabricate pn junction photovoltaic devices using disilane as the source gas. This work represents the first reported work on using disilane for fabrication of photovoltaic devices. Films doped with diborane showed high growth rates of approximately 45-150 Å/min for temperatures ranging from 450 to 550 °C. Undoped films were grown and found to have significantly lower growth rates and were not practical at temperatures less than 500 °C. The films were completely amorphous for growth temperatures of less than 500 °C, and crystallinity increased sharply above 500 °C. The optical properties of the films exhibited low optical bandgaps of approximately 1.4-1.1 eV. The conductivity of the doped films was found to be on the order of 10⁻³ S/cm. Devices were fabricated by depositing p-type layers on n-type crystalline silicon substrates to form pn junctions. Diodes and pn junction photovoltaic devices were fabricated, exhibiting modest but promising performance, and were limited by parasitic series resistance. This research represents the first reported work on fabricating pn junction photovoltaic devices in a low-temperature LPCVD process using disilane, and serves as a solid foundation for future work to improve the process and fabricate novel device structures.