Strain-Engineered Biaxial Tensile Epitaxial Germanium for High-Performance Ge/InGaAs Tunnel Field-Effect Transistors

The structural, morphological, and energy band alignment properties of biaxial tensile-strained germanium epilayers, grown in-situ on GaAs via a linearly graded In_xGa_1-xAs buffer architecture and utilizing dual chamber molecular beam epitaxy, were investigated. Precise control over the growth conditions yielded a tunable in-plane biaxial tensile strain within the Ge thin films that was modulated by the underlying In_xGa_1-xAs “virtual substrate” composition. In-plane tensile strains up to 1.94% were achieved without Ge relaxation for layer thicknesses of 15 to 30 nm. High-resolution x-ray diffraction supported the pseudomorphic nature of the Ge/In_xGa_1-xAs interface, indicating a quasi-ideal stress transfer to the Ge lattice. High-resolution transmission electron microscopy revealed defect-free Ge epitaxy and a sharp, coherent interface at the Ge/In_xGa_1-xAs heterojunction. Surface morphology characterization using atomic force microscopy exhibited symmetric, 2-D cross-hatch patterns with root mean square roughness less than 4.5 nm. X-ray photoelectron spectroscopic analysis revealed a positive, monotonic trend in band offsets for increasing tensile strain. The superior structural and band alignment properties of strain-engineered epitaxial Ge suggest that tensile-strained Ge/In_xGa_1-xAs heterostructures show great potential for future high-performance tunnel field-effect transistor architectures requiring flexible device design criteria while maintaining low power, energy-efficient device operation.