Enhancement-Mode Metal Organic Chemical Vapor Deposition-Grown ZnO Thin-Film Transistors on Glass Substrates Using N2O Plasma Treatment

Thin-film transistors (TFTs) were fabricated on a glass substrate with a metal organic chemical vapor deposition (MOCVD)-grown undoped zinc oxide (ZnO) film as a channel layer and plasma-enhanced chemical vapor deposition (PECVD)-grown silicon nitride as a gate dielectric. The as-fabricated ZnO TFTs exhibited depletion-type device characteristics with a drain current of about 24 mu A at zero gate voltage, a turn-on voltage (V-on) of -24 V, and a threshold voltage (V-T) of -4 V. The field-effect mobility, subthreshold slope, off-current, and on/off current ratio of the as-fabricated TFTs were 5 cm(2) V-1 s(-1), 4.70 V/decade, 0.6 nA, and 10(6), respectively. The postfabrication N2O plasma treatment on the as-fabricated ZnO TFTs changed their device operation to enhancement-mode, and these N2O-treated ZnO TFTs exhibited a drain current of only 15 pA at zero gate voltage, a V-on of -1.5 V, and a V-T of 11 V. Compared with the as-fabricated ZnO TFTs, the off-current was about 3 orders of magnitude lower, the subthreshold slope was nearly 7 times lower, and the on/off current ratio was 2 orders of magnitude higher for the N2O-plasma- treated ZnO TFTs. X-ray phtotoelectron spectroscopy analysis showed that the N2O-plasma-treated ZnO films had fewer oxygen vacancies than the as-grown films. The enhancement-mode device behavior as well as the improved performance of the N2O-treated ZnO TFTs can be attributed to the reduced number of oxygen vacancies in the channel region. (C) 2010 The Japan Society of Applied Physics