500-period epitaxial Ge/Si0.18Ge0.82 multi-quantum wells on silicon

Abstract

Ge/SiGe multi-quantum well heterostructures are highly sought-after for silicon-integrated optoelectronic devices operating in the broad range of the electromagnetic spectrum covering infrared to terahertz wavelengths. However, the epitaxial growth of these heterostructures at a thickness of a few micrometers has been a challenging task due to the lattice mismatch and its associated instabilities resulting from the formation of growth defects. To elucidate these limits, we outline herein a process for the strain-balanced growth on silicon of 11.1/21.5 nm Ge/Si0.18Ge0.82 superlattices (SLs) with a total thickness of 16 μm corresponding to 500 periods. Composition, thickness, and interface width are preserved across the entire SL heterostructure, which is an indication of limited Si–Ge intermixing. High crystallinity and low defect density are obtained in the Ge/Si0.18Ge0.82 layers; however, the dislocation pileup at the interface with the growth substrate induces micrometer-long cracks on the surface. This eventually leads to significant layer tilt in the strain-balanced SL and in the formation of millimeter-long, free-standing flakes. These results confirm the local uniformity of structural properties and highlight the critical importance of threading dislocations in shaping the wafer-level stability of thick multi-quantum well heterostructures required to implement effective silicon-compatible Ge/SiGe photonic devices.