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

Abstract

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.

Understanding the Crystallization Behavior of Surface-Oxidized GeTe Thin Films for Phase-Change Memory Application

“…The binding energies of the four peaks coincide with literature values for GeTe. [22][23][24][25]28,29 The binding energies in amorphous and crystalline GeTe are very close 28,29 but from the STEM studies presented above, one can safely conclude that the XPS spectra of Figures 4g−j were measured in the amorphous part of the film, below the limit of the oxygen penetration.…”

“…To explore the nature of chemical environments in the surface-oxidized GeTe film, we performed XPS depth profiling at room temperature (Figure ). We measured the Te 4d and Ge 3d spectra and the Te 3d 3/2 and Te 3d 5/2 spectra, which are relevant for studying GeTe oxidation. − …”

Novel Sn-Based Contact Structure for GeTe Phase Change Materials

“…Bates et al have shown that SnTe is stable against the reaction with Fe. Because the HCl treatment leaves the surface Te-rich, the subsequent SnTe layer formed is thicker in the HCl-treated sample than in the DI H 2 O-treated sample (Ge-rich surface). The thicker SnTe may act as a barrier to the formation of a Fe–Ge layer above SnTe and reduce the diffusion of Fe into GeTe.…”

“…We have recently shown that both DI H 2 O and HCl treatments are effective in removing surface oxides GeO 2 and TeO 2 , though they do not passivate the surface. Ge-rich and Te-rich surfaces are obtained by DI H 2 O and HCl treatments, respectively . Therefore, a greater amount of SnTe or Ge x Sn 1– x Te alloy at the interface might form after the HCl treatment.…”

“…In this study, two different premetallization surface preparations (HCl and DI H 2 O) were used to modify the surface chemistry of GeTe films. We have shown previously that H 2 O and HCl treatments tune the surface to be Ge-rich and Te-rich, respectively . For the first time, a novel contact stack of Sn/Fe/Au was employed to incorporate Sn at the interface and form SnTe or an alloy with GeTe in contact to GeTe.…”