Mechanism of mobility enhancement in Ge p-channel metal-oxide-semiconductor field-effect transistor due to introduction of Al atoms into SiO2/GeO2 gate stack

In this paper, we present comprehensive results on Al-postmetallization annealing (Al-PMA) effect for the SiO2/GeO2 gate stack on a Ge substrate, which were fabricated by a physical vapor deposition method. The effective oxide thickness of metal-oxide-semiconductor (MOS) capacitor (CAP) was ~7 nm, and the Al-PMA was performed at a temperature in the range of 300-400 °C. The flat band voltage (VFB), the hysteresis (HT), the interfacial states density (Dit), and the border traps density (Dbt) for MOSCAPs were characterized by a capacitance-voltage method and a constant-temperature deep-level transient spectroscopy method. The MOSCAP without Al-PMA had an electrical dipole of ~−0.8 eV at a SiO2/GeO2 interface, which was disappeared after Al-PMA at 300 °C. The HT, Dit, and Dbt were decreased after Al-PMA at 300 °C and were maintained in the temperature range of 300-400 °C. On the other hand, the VFB was monotonically shifted in the positive direction with an increase in PMA temperature, suggesting the generation of negatively charged atoms. Structural analyses for MOSCAPs without and with Al-PMA were performed by a time-of-flight secondary ion mass spectroscopy method and an X-ray photoelectron spectroscopy method. It was confirmed that Al atoms diffused from an Al electrode to a SiO2 film and reacted with GeO2. The dipole disappearance after Al-PMA at 300 °C is likely to be associated with the structural change at the SiO2/GeO2 interface. We also present the device performances of Al-gated p-channel MOS field-effect transistors (FET) with PMA treatments, which were fabricated using PtGe/Ge contacts as source/drain. The peak field-effect mobility (μh) of the p-MOSFET was reached a value of 468 cm2/Vs after Al-PMA at 325 °C. The μh enhancement was explained by a decrease in the total charge densities at/near the GeO2/Ge interface.