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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 (µFE) of 4.3 cm2 V-1 s-1, a subthreshold slope (SS) of 1.3 V/dec, and an on/off current ratio of 9×107. On the other hand, ZnO TFTs using the channel grown with a higher VI/II ratio of 100,000 exhibit a µFE of 2.1 cm2 V-1 s-1, an SS of 0.64 V/dec, and an on/off current ratio of 1×108. The improvement in SS and the reduction in µFE 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 µFE, SS, and on/off current ratio are 8.9 cm2 V-1 s-1, 0.42 V/dec, and 3×108, 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.