Phase and Optical Characterizations of Annealed SnO Thin Films and Their p-Type TFT Application

Amorphous SnO thin films were prepared on quartz and 190 nm substrates by electron beam evaporation. X-ray diffraction results reveal that amorphous SnO transforms into polycrystalline (tetragonal litharge structure) after rapid thermal annealing in Ar ambient at and starts decomposing into (orthorhombic structure) with the expulsion of Sn atoms at . The optical properties were characterized via spectroscopic ellipsometry. The polycrystalline SnO thin films have a higher refractive index and a narrower bandgap than the amorphous ones, which is due to the polarizability enhancement in the crystallization process. Moreover, the relationship between and of the amorphous and polycrystalline SnO thin films can be explained by the "Moss rule" law, and a decreasing trend in was verified with the transformation from SnO to . Bottom-gate-type thin film transistors (TFTs) employing polycrystalline SnO channels on the substrates exhibit p-type field-effect transistor characteristics. The optimum field-effect mobilities and are 0.46 and , respectively, which are the same order of magnitude as those reported for epitaxial SnO TFTs.