Effects of interfacial oxide layers of the electrode metals on the electrical characteristics of organic thin-film transistors with HfO2 gate dielectric

Abstract

HfO2-based devices employing various electrode metals (Al, W, Pt, Cu, and Cr) were fabricated and characterized in order to examine the importance of the choice of metal electrode when sputter-depositing HfO2 films. It was found that metal-insulator-metal capacitors with an Al bottom electrode exhibit a significantly smaller leakage current and a larger breakdown field strength than devices using W, Pt, Cu, and Cr electrodes. By examining HfO2/metal interfaces with x-ray photoelectron spectroscopy, it was found that metal electrodes are oxidized during the deposition of HfO2, resulting in interfacial oxide layers of the electrode metals (Al2O3, WO3, PtO, CuO, and Cr2O3 + CrO3) between the metal electrode and the HfO2 layer. The formation of a metal oxide interlayer is a consequence of the high-energy oxygen ions generated during HfO2 sputtering. The difference in the device performance was attributed to the electronic properties of the oxide interlayers. It was found that when the oxide interlayers are semiconducting (e.g., WO3, PtO, CuO, or Cr2O3 + CrO3), devices have high leakage currents and low breakdown field strengths, and an insulating oxide interlayer (Al2O3) enhances these device characteristics. Organic thin-film transistors (OTFTs) using a HfO2 dielectric layer showed that an Al gate has a much lower off-state current, larger on/off ratio, and smaller sub-threshold slope compared with OTFTs using a Cr electrode. These results demonstrate the importance of the metal gate electrode and the electrical characteristics of its oxide when using the sputter-deposition of HfO2 to fabricate thin-film transistors.