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Germanium material holds great potentials for application in low-power, high-mobility field-effect transistors with the advantages of high electron and hole mobility, narrow bandgap, and compatibility with silicon CMOS technologies. The development of high-quality gate oxide processes is crucial in the fabrication of high-mobility Ge-based transistors, especially those with high dielectric constant for superior gate control and preferable gate stability. Rare-earth oxides typified with LaLuO3, featuring high dielectric constants and high crystallization temperatures, are potential candidates for Ge-based MOSFET gate technology. This paper fabricates a germanium (Ge)-based oxide dielectric LaLuO3 utilizing a p-type Ge substrate with a (111) crystal orientation and a doping concentration of 1×1016 cm-3, and radio-frequency (RF) co-sputtering with 2-inch 99.9% La2O3 and Lu2O3 targets. Systematical investigations have been conducted to evaluate the effects of annealing process conditions on the characteristics of the LaLuO3/Ge MOS gate structure under three specifically designed annealing atmospheres, i.e. nitrogen, oxygen, and a nitrogen-oxygen mixed gas with N2:O2 ratio of 0.999:0.001. Meanwhile, the influence of annealing pressure has also been explored. It has been revealed that annealing in pure oxygen ambient at atmospheric pressure can diminish the hysteresis of gate capacitance but leads to a formation of interface layer. Correspondingly, annealing technique based on high-pressure and low-oxygen-content (0.1% O2) atmosphere has been developed, which not only improves the LaLuO3/Ge interface quality and suppresses the oxygen vacancy generation, but also achieves an extremely low equivalent oxide thickness (EOT) of 1.8 nm and a hysteresis voltage of only 40 mV, resulting in an ideal LaLuO3/Ge MOS structure. This work thus provides a high-performance LaLuO3/Ge gate process solution for Ge MOSFETs.
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Keywords:
- Ge MOSFET /
- gate stack process /
- rare-earth oxide /
- high-k dielectric constant
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