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

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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 ($\mu_{\text{FE}}$) of 4.3 cm2 V-1 s-1, a subthreshold slope ($\mathit{SS}$) of 1.3 V/dec, and an on/off current ratio of $9\times 10^{7}$. On the other hand, ZnO TFTs using the channel grown with a higher VI/II ratio of 100,000 exhibit a $\mu_{\text{FE}}$ of 2.1 cm2 V-1 s-1, an $\mathit{SS}$ of 0.64 V/dec, and an on/off current ratio of $1\times 10^{8}$. The improvement in $\mathit{SS}$ and the reduction in $\mu_{\text{FE}}$ 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 $\mu_{\text{FE}}$, $\mathit{SS}$, and on/off current ratio are 8.9 cm2 V-1 s-1, 0.42 V/dec, and $3\times 10^{8}$, 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.
2011-04-25