Impact of Premetallization Surface Preparation on Nickel-based Ohmic Contacts to Germanium Telluride: An X-ray...

Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) after different treatments in an effort to understand the effect of premetallization surface treatments on the resistance of Ni-based contacts to GeTe. UV-O₃ is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV-O₃ treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeO_x (x < 2) and TeO₂, respectively. Then the oxides were removed by deionized (DI) H₂O, (NH₄)₂S, and HCl treatments. Additionally, in situ Ar⁺ ion etching was used to clean the GeTe surface without prior UV-O₃ treatment. Ar⁺ ion etching, H₂O, and (NH₄)₂S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH₄)₂S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH₄)₂S does not passivate the GeTe surface. The refined transfer length method (RTLM) was used to measure the contact resistance (R_c) of as-deposited Ni-based contacts to GeTe as a function of premetallization surface preparation. HCl-treated samples had the highest R_c (0.036 ± 0.002 Ω·mm), which was more than twice that of the other surface treatments. This increase in R_c is attributed to formation of the Ni_1.29Te phase at the Ni/GeTe interface due to an abundance of Te at the surface after HCl treatment. In general, treatments that resulted in Ge-rich surfaces offered lower R_c.