Comparison of high-k Y2O3/TiO2 bilayer and Y-doped TiO2 thin films on Ge substrate

Abstract Y 2 O 3 /TiO 2 bilayer thin films and Y-doped TiO 2 (YTO) thin films were deposited on a Ge substrate by atomic layer deposition at a substrate temperature of 250 °C. They were used as gate insulators to examine the electrical properties of Pt/TiN/TiO 2 /Y 2 O 3 /p-Ge and Pt/TiN/YTO/p-Ge metal–oxide–semiconductor capacitors. A 7 nm thick bilayer thin film showed a lower leakage current density by more than one order of magnitude compared to a YTO thin film with the same thickness due to the high conduction band offset between the Y 2 O 3 layer and Ge substrate. However, the bilayer thin film showed a large hysteresis of 950 mV. On the other hand, the YTO thin film showed significantly reduced hysteresis of 120 mV due to the smaller slow trap density. The voltage acceleration factors of the bilayer thin film and YTO thin film were 1.12 and 1.25, respectively, higher in the YTO thin film. The interfacial trap density of the 7 nm thick bilayer and YTO thin films were 3.5 × 10 11 cm −2 eV −1 and 2.7 × 10 11 cm −2 eV −1 , respectively. The equivalent oxide thickness of the YTO film could be scaled down to 0.9 nm, and a leakage current density of 1.4 × 10 −4 A cm −2 at flat band voltage −1 V was achieved. This study confirmed that the YTO film can be used as a promising ternary high- k oxide for a Ge-based field-effect-transistor.