Structural and band alignment properties of Al₂O₃ on epitaxial Ge grown on (100), (110), and (111)A GaAs substrates by molecular beam epitaxy

Structural and band alignment properties of atomic layer Al ₂O₃ oxide film deposited on crystallographically oriented epitaxial Ge grown in-situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers were investigated using cross-sectional transmission microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). High-resolution triple axis x-ray measurement demonstrated pseudomorphic and high-quality Ge epitaxial layer on crystallographically oriented GaAs substrates. The cross-sectional TEM exhibited a sharp interface between the Ge epilayer and each orientation of the GaAs substrate as well as the Al₂O₃ film and the Ge epilayer. The extracted valence band offset, ΔE_v, values of Al₂O₃ relative to (100), (110), and (111) Ge orientations using XPS measurement were 3.17 eV, 3.34 eV, and 3.10 eV, respectively. Using XPS data, variations in ΔE_v related to the crystallographic orientation were Δ E V (110) Ge > Δ E V (100) Ge ≥ Δ E V (111) Ge and the conduction band offset, ΔE_c, related to the crystallographic orientation was Δ E c (111) Ge > Δ E c (110) Ge > Δ E c (100) Ge using the measured ΔE_v, bandgap of Al ₂O₃ in each orientation, and well-known Ge bandgap of 0.67 eV. These band offset parameters are important for future application of Ge-based p- and n-channel metal-oxide field-effect transistor design.