Ohmic Contact at Al/TiO_2/n-Ge Interface with TiO_2 Deposited at Extremely Low Temperature

TiO_2 deposited at extremely low temperature of 120^\circ\!C by atomic layer deposition is inserted between metal and n-Ge to relieve the Fermi level pinning. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy indicate that the lower deposition temperature tends to effectively eliminate the formation of GeO_x to reduce the tunneling resistance. Compared with TiO_2 deposited at higher temperature of 250^\circ\!C, there are more oxygen vacancies in lower-temperature-deposited TiO_2, which will dope TiO_2 contributing to the lower tunneling resistance. Al/TiO_2/n-Ge metal-insulator-semiconductor diodes with 2 nm 120^\circ\!C deposited TiO_2 achieves 2496 times of current density at -0.1 V compared with the device without the TiO_2 interface layer case, and is 8.85 times larger than that with 250^\circ\!C deposited TiO_2. Thus inserting extremely low temperature deposited TiO_2 to depin the Fermi level for n-Ge may be a better choice.