Improved Characteristics of Metal Organic Chemical Vapor Deposition-Grown ZnO Thin-Film Transistors by Controlling VI/II Ratio of ZnO Film Growth and Using a Modified Thin-Film Transistor Layer Structure

Thin-film transistors (TFTs) were fabricated on glass substrates using ZnO channel layers grown with two VI/II (oxygen/diethylzinc) ratios of 25,000 and 100,000 by metal organic chemical vapor deposition (MOCVD). The ZnO TFTs employing the channel grown with a VI/II ratio of 25,000 exhibit a field-effect mobility (µ FE ) of 4.3 cm 2 V -1 s -1 , a subthreshold slope ( SS ) of 1.3 V/dec, and an on/off current ratio of 9 × 10 7 . On the other hand, ZnO TFTs using the channel grown with a higher VI/II ratio of 100,000 exhibit a µ FE of 2.1 cm 2 V -1 s -1 , an SS of 0.64 V/dec, and an on/off current ratio of 1 × 10 8 . The improvement in SS and the reduction in µ FE are respectively attributed to the lower growth rate and smaller grain size of the ZnO film. The ZnO TFTs fabricated by using the higher-VI/II-ratio-grown ZnO layer together with a thin MOCVD-grown MgZnO layer at the channel/gate insulator interface exhibit high performance, and their µ FE , SS , and on/off current ratio are 8.9 cm 2 V -1 s -1 , 0.42 V/dec, and 3 × 10 8 , respectively. This is the best performance reported to date for ZnO TFTs that are realized on glass substrates with MOCVD-grown channel layers. The superior performance of ZnO TFTs with the MgZnO layer is ascribed to the larger grain size of the ZnO film and the better channel/gate insulator interface.