Transport Properties in Band-Tails of High Mobility Poly-Si TFTs

Abstract

Electrical properties of high-mobility polycrystalline silicon thin film transistors (poly-Si TFTs) are measured over the wide temperature range from 300 K to 1.5 K in order to clarify the role of band-tail states in the grain boundaries (GBs). Two distinct regimes of weak and strong localization of electrical transport have been identified below and above the carrier trap state density N st≈1.0×1012 cm-2 in the GBs for high-mobility poly-Si TFTs. Poly-Si TFTs with a smaller carrier trap state density N st (≈6×1011 cm-2) show metallic behavior where the mobility increases as temperature decreases. Poly-Si TFTs with larger N st (≈1.5×1012 cm-2) show semiconducting behavior of mobility. At low temperatures, on the former poly-Si TFT the resistance changes due to the weak-localization effect and on the latter poly-Si TFT due to the variable-range hopping (VRH) in the regime of strong localization. In both regimes, negative magnetoresistances have been observed and interpreted by the quantum interference effect. All of these features are attributed to the band-tail states decaying exponentially from the band-edge, based on the observations of irregular GB structure in the poly-Si film from transmission electron microscope images and the roughness at the SiO2/poly-Si interface seen on atomic force microscope images.