Mobility enhancement limit of excimer-laser-crystallized polycrystalline silicon thin film transistors

Abstract

The effects of various carrier scattering mechanisms on excimer-laser-crystallized polycrystalline silicon (poly-Si) thin film transistors (TFTs) fabricated using 450 °C processes on a glass substrate were studied. Good performance of a separated by ion implanted oxygen (SIMOX) metal–oxide–semiconductor field-effect transistor (MOSFET) with field-effect mobility of 670 cm2/V s and a subthreshold swing value of 0.087 V/dec was obtained using these 450 °C processes. The results showed the formation of a good silicon/silicon dioxide (SiO2) interface that is comparable to that of thermal oxide, as well as the high capability of 450 °C processes. The performance of the above SIMOX-MOSFET is superior to that of excimer-laser-crystallized poly-Si TFTs fabricated using the same 450 °C processes. This shows that poorer performance of poly-Si TFTs is caused by the poor crystalline quality of the poly-Si film. The field-effect mobility is affected little by the in-grain microdefects and surface morphology of the excimer-laser-crystallized poly-Si film, but it is highly sensitive to the grain size. A field-effect mobility of 320 cm2/V s was obtained for an average grain size of 700 nm. The increase in field-effect mobility began to saturate with grain sizes of approximately 1000 nm. It is not necessary to enlarge the grain size beyond the saturation point of the field-effect mobility to improve performance, because the field-effect mobility of an average grain size of 700 nm is limited by phonon scattering, but not by the grain boundary.