Ge diffusion and bonding state change in metal/high-k/Ge gate stacks and its impact on electrical properties

Abstract

Graphical abstractDisplay Omitted Plasma-assisted oxidation is preferable to form HfO2/GeOx/Ge structures.Al on HfO2/GeOx/Ge stack induces the reduction of GeOx and Al germanide formation.Hf germanide was formed in Pt/HfO2/GeOx/Ge stacks by annealing.Germanide formation should be suppressed for high-quality metal/high-k/Ge stack. We systematically investigated Ge diffusion and chemical bonding states in metal/high-k/Ge gate stacks by synchrotron radiation photoemission spectroscopy (SR-PES) to understand their impact on electrical properties. Although Hf germanide was found in both Ge 3s and Hf 4f spectra for HfO2/GeOx/Ge gate stacks formed by ultrathin metal Hf deposition and subsequent in situ low-pressure thermal oxidation, such germanide could be fully oxidized when using plasma-assisted oxidation. However, Al electrode deposition on HfO2/GeOx/Ge stacks was found to reduce interfacial GeOx layer, resulting in the formation of Al germanide at the Al/HfO2 interface even at room temperature. No germanide was formed in the stacks with inert Pt electrode. This indicates that the Al layer may promote upward diffusion of GeO molecules through the HfO2 layer. The thermal stability of metal/HfO2/GeOx/Ge gate stacks was also evaluated by in situ SR-PES. Hf germanide was observed to form near the HfO2/GeOx interface probably due to Ge atoms intermixing in the HfO2 layer in the Pt-gate stacks, in contrast to the enhanced formation of Al germanide in Al-gate stacks. Electrical characterization revealed that the formation of metal germanide led to severe degradation of insulating properties in metal/high-k/Ge stacks.