Al2O3 buffer in a ZnO thin film transistor with poly-4-vinylphenol dielectric

Abstract

We compared the characteristics of bottom-gate ZnO-thin film transistors using poly-4-vinylphenol (PVP) and PVP/Al2O3 dielectrics. The PVP dielectric is more hydrophobic than the PVP/Al2O3 dielectric and is not useful for TFT devices because of its high leakage current density, but this leakage current density can be significantly reduced by inserting Al2O3. We deposited ZnO and Al2O3 films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al2O3 dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al2O3 exhibits clear pinch-off and excellent saturation with an enhanced mode operation. The on/off ratio of 7.9 × 104 for the device containing the hybrid dielectric is about three orders of magnitude higher than the ratio of 47 for the device containing PVP. The subthreshold gate swings are 12 V/decade for the TFT containing PVP and 1.2 V/decade for the TFT containing PVP/Al2O3. The density of the interface trap state is significantly lower in the device containing PVP/Al2O3 than in the ZnO-TFT containing PVP. The saturation mobility was 0.05 and 0.8 cm2 V−1 s−1, respectively, in the TFTs containing PVP and PVP/Al2O3.