Fabrication of Tin Oxide Thin Film Transistors by RF Magnetron Sputtering Using Sn/SnO Composite Target

Abstract

P-type tin oxide thin film deposited by RF sputtering for transparent thin film transistor (TFT) applications is the subject of this study. P-type tin oxide thin film can be made by doping a cation with a lower valence into n-type SnO2 as an acceptor impurity or by fabrication with tin monoxide (SnO). The later method was investigated in this study by RF magnetron sputtering process, which has a high deposition rate, uniform thickness control, simple stoichiometry control, and reproducibility. A Sn/SnO composite target was used in RF sputtering to utilize the benefits of both metallic and ceramic sputtering targets. A Sn/SnO composite target is expected to provide p-type tin oxide thin films that are easier to manufacture than metallic target or ceramic target. The metallic Sn element provides an excellent control of structural defects, while the ceramic SnO element provides stable stoichiometry. The p-type tin oxide thin film with a Sn/SnO composite target showed very good transparency of ~95%. and excellent electrical properties. The p-type tin oxide thin film of 15nm thickness had a carrier concentration of 8.03X1015cm-3 and a mobility of 15.2cm2/Vs. With these ultrathin tin oxide films, a p-channel TFT was fabricated with a staggered bottom-gate structure, and the effect of different channel thicknesses and different distances between the two electrodes were evaluated. The tin oxide TFT with a thick p-channel layer showed a significant increase in current measured than the tin oxide with a thin p-channel layer. The current measured tended to increase greatly as the distance between electrodes decreased. The IV output curves of the tin oxide TFTs exhibited characteristics of bipolar transistors potentially due to the partial creation of the oxygen-deficient SnO2 like structure, but the mechanism of bipolar transistor characteristics from the p-channel tin oxide TFT is still unclear. The fabrication of highly transparent tin oxide thin film with bipolar transistor characteristics is demonstrated herein.