Ge interface engineering using ultra-thin La₂O₃ and Y₂O₃ films: A study into the effect of deposition temperature

A study into the optimal deposition temperature for ultra-thin La₂O₃/Ge and Y₂O₃/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides (La₂O₃ and Y₂O₃) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La₂O₃ has been found to be more reactive to Ge than Y₂O₃, forming LaGeO_x and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400 °C. In contrast, Y₂O₃/Ge deposited at 400 °C allows for an ultra-thin GeO₂ layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeO_x/Ge interface. The Y₂O₃/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y₂O₃/Ge stacks has been estimated to be 5.7 ± 0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y₂O₃/Ge stack exhibits a higher conduction band offset (>2.3 eV) than the La₂O₃/Ge (∼2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (∼10⁻⁷ A/cm² at 1 V) five orders of magnitude lower than the respective La₂O₃/Ge stack. Our study strongly points to the superiority of the Y₂O₃/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology.