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

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-O is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV-O treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeO (x < 2) and TeO, respectively. Then the oxides were removed by deionized (DI) HO, (NH)S, and HCl treatments. Additionally, in situ Ar ion etching was used to clean the GeTe surface without prior UV-O treatment. Ar ion etching, HO, and (NH)S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH)S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH)S does not passivate the GeTe surface. The refined transfer length method (RTLM) was used to measure the contact resistance (R) of as-deposited Ni-based contacts to GeTe as a function of premetallization surface preparation. HCl-treated samples had the highest R (0.036 ± 0.002 Ω·mm), which was more than twice that of the other surface treatments. This increase in R is attributed to formation of the NiTe 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.