Reactivity in metal-Ge-Te systems: Thermodynamic predictions and experimental observations

Abstract

Thermal stability of neighboring layers in a thin film structure is critical to the device endurance and reliability. The geometry of GeTe-based devices for radio frequency switches and nonvolatile memory technologies often places GeTe thin films in contact with metal thin films. Despite the potential effect of metal/GeTe reactions on device performance, few studies have addressed the reactivity between elemental metals and GeTe or outlined the thermal stability of GeTe with metals. In this work, the authors use literature or estimated values of thermodynamic data to calculate metal-Ge-Te condensed ternary phase diagrams for a series of metals (Ag, Al, Au, Cd, Co, Cu, Fe, Hf, Ir, Mn, Mo, Os, Pd, Pt, Re, Rh, Ru, Sc, Ta, Ti, W, Y, and Zn). If present, the dominant phase of each metal-Ge-Te system is identified so that the system is classified as GeTe dominant, metal telluride or germanide dominant, or ternary phase dominant, and the authors predict whether or not there is a thermodynamic driving force for a metal to react with GeTe at room temperature. In addition to comparing the calculated work to the literature, they confirm the predictions of reactivity for a select group of metals (Ag, Al, Cu, Fe, Mn, Mo, Pd, Re, Ru, and Ti) using cross-sectional transmission electron microscopy (TEM) and/or plan view selected area electron diffraction of metal/GeTe thin film structures both after metal deposition and again after the samples are annealed for 12 h at 200 °C. TEM imaging and elemental mapping are also used to identify metal/GeTe reaction products and to observe the extent of metal diffusion into the GeTe film. Nine of the 24 studied metals are not reactive with GeTe (Au, Ir, Mo, Os, Re, Ru, Ta, W, and Zn), according to experiments or thermodynamic calculations while 15 metals are thermodynamically favored to react with GeTe at room temperature (Ag, Al, Cd, Co, Cu, Fe, Hf, Mn, Ni, Pd, Pt, Rh, Sc, Ti, and Y). Interestingly, the calculations demonstrate that most of the unreactive metals, with the exception of Au and Zn, are not necessarily in thermodynamic equilibrium with GeTe at room temperature. These metals are refractory, and the lack of reactivity is ascribed to kinetic limitations. The authors also observed diffusion and solubility of certain metals (Cu, Fe, Mn, and Pd) in the GeTe film that extended beyond the metal/GeTe reaction layer. Understanding the reactivity and extent of diffusion between metals and GeTe should be valuable for the design of future phase change material devices, where reactions could either affect reliability or be used to engineer improved interfacial behavior.