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

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 Al₂O ₃ films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al ₂O₃ dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al ₂O₃ 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/Al₂O ₃ 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.