The role of structural defects and texture variability in the performance of poly-Si thin film transistors

High-performance poly-Si Thin Film Transistors (TFTs) with champion mobility and threshold voltage characteristics have been reported by several groups in the literature. Performance improvements have been especially spectacular after the wide acceptance of laser-based, lateral crystallization technology and its numerous variants. Despite the noted improvements in average transistor performance, variation in TFT characteristics still presents substantial difficulties to control. Speculation on the cause of the variation has been provided by various sources but, so far, no systematic study has been performed to that effect. In this work we have conducted an extensive study of characteristics of poly-Si TFTs, fabricated by laser crystallization, and their relationship to the microstructural details of the active layer. Our study clarifies the effect of a single grain boundary inclusion in the device channel and pinpoints the key causes of variation in performance even in the absence of "hard" boundaries. The existence of structural defects, as well as texture variation, is found to correlate well with the observed variability in TFT mobility and threshold voltage. These findings have also been confirmed by a simple quantitative model. Our work points to the importance in achieving consistent microstructure and the difficulty in doing so by employing blanket crystallization technologies. To that end, crystallization technology emphasizing location control appears to be preferable. (C) 2007 Published by Elsevier B.V.