Band Offset Enhancement of a-Al2O3/Tensile-Ge for High Mobility Nanoscale pMOS Devices

The band alignment properties of amorphous Al
₂
O
₃
on strain-engineered biaxial tensile-strained epitaxial Ge, grown in situ by molecular beam epitaxy on In
_x
Ga
_1-x
As virtual substrates, are presented. X-ray photoelectron spectroscopy investigation demonstrated an increase in the valence band offset of the Al
₂
O
₃
/strained Ge system with increasing tensile strain. For Ge strain-states of 1.14%, 1.6%, and 1.94%, the corresponding valence band offsets were found to be 4.43 ± 0.1 eV, 3.95 ± 0.1 eV, and 4.55 ± 0.1 eV, respectively, demonstrating a ~0.8 eV increase as compared with Ge grown on GaAs. The observed enhancement in the valence band discontinuity between tensile-strained Ge and Al
₂
O
₃
offers a unique and novel path for the simultaneous improvement of hole mobility (via strain) and hole confinement (via a larger valence band offset) in future low-power and high-performance Ge-based nanoscale pMOS devices.