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 asfabricated ZnO TFTs exhibited depletion-type device characteristics with a drain current of about 24 μA 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 cm² V^-1 s^-1, 4.70 V/decade, 0.6 nA, and 10⁶, respectively. The postfabrication N²O plasma treatment on the asfabricated 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 N₂Oplasma-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.