A New Route Toward Atomically Flat and Defect-Free Ge/SiGe Planar Heterostructures

Germanium-based planar heterostructures are emerging as versatile platforms for next-generation quantum devices. In particular, Ge/SiGe quantum wells (QWs) enable confinement of holes with exceptionally high mobilities and highly tunable spin–orbit interaction, offering full electrical control for scalable quantum information systems. Achieving macroscopic, atomically flat, and defect-free Ge QW structures on reverse-graded SiGe buffers and commercial Ge substrates remains challenging, as minimizing defect density typically requires high-temperature epitaxy, which limits compatibility with CMOS processing.

Here, we report a low-temperature approach for fabricating high-quality planar SiGe/Ge heterostructures via a combination of thermal and electron-beam evaporation. We systematically investigate the effects of ex-situ and in-situ substrate preparation, growth temperature, and post-deposition annealing on surface roughness and defect formation. We find that optimized in-situ oxide desorption and controlled post-growth annealing dramatically improve surface morphology and crystallinity. Under optimized conditions, we obtain atomically smooth surfaces (σ_rms ≤ 10 Å) with negligible defect densities. We further observe that thermally evaporated Ge initially forms oriented triangular crystallites, which elongate upon annealing under high Ge vapor pressure.

These results demonstrate that low-temperature physical deposition, combined with tailored thermal processing, provides a viable and CMOS-compatible route to defect-free Ge QW heterostructures, enabling scalable platforms for quantum computing and quantum sensing applications.