Strain and Defect Dynamics in (Si)GeSn Alloys Epitaxially Grown Around Free Standing Ge Nanowires for CMOS Compatible Optical Interconnects

The scaling of electrical interconnects in integrated circuits (ICs) has reached a performance bottleneck, limiting bandwidth, increasing latency, and significantly contributing to power consumption in modern microprocessors. As transistors continue to scale, the limitations of electrical interconnects become more prominent, motivating the exploration of optical interconnects to address these challenges. Optical interconnects, which have already revolutionized long-haul communications, provide promising solutions for on-chip applications due to their potential to reduce energy consumption and improve data transmission speeds. However, the lack of a direct bandgap in Group IV materials hinders the integration of optical devices, especially in realizing an efficient, CMOS-compatible light source. (Si)GeSn alloys have emerged as promising candidates for CMOS-compatible optical interconnects, offering a tunable direct bandgap that overcomes the inherent limitations of the indirect bandgaps in pure silicon and germanium.

In-plane nanostructures offer several advantages for the integration of optical devices with CMOS technology. Their compatibility with lithographic techniques ensures precise patterning and alignment, making them highly suitable for large-scale fabrication without the need for complex catalyzed growth processes. Additionally, the absence of metal catalysts like gold, which can introduce deep-level traps, helps maintain the purity of the material and reduces the risk of contamination in CMOS processing. However, the in-plane growth of (Si)GeSn alloys also presents significant challenges, particularly in controlling defect formation. Threading dislocations and other crystallographic defects can arise due to strain relaxation and lattice mismatch, which can severely degrade optical performance by increasing non-radiative recombination. In this work, we present the fabrication and characterization of (Si)GeSn alloys grown epitaxially around free-standing Ge nanowires, leveraging the nanowire geometry to achieve enhanced strain relaxation. The effects of Sn content, strain dynamics, quantum confinement and strategies to mitigate defect formation are examined in detail.

High-quality epitaxial germanium virtual substrate (Ge-VS) is grown directly on silicon (001) demonstrating 0.21% biaxial tensile strain. An ultra-thin germanium-on-insulator (GOI) structure is fabricated by transferring the Ge-VS film onto an oxide layer, followed by chemical mechanical polishing (CMP), resulting in an atomically flat surface with a roughness of just 0.2 nm. The GOI layer, thinned to a range of 50–98 nm, is ideal for quantum confinement, enabling a broad spectral range due to the thin nanowire geometry. Raman spectroscopy confirms that the biaxial tensile strain from the Ge-VS persists on the GOI. In-plane nanowires are patterned on the GOI using e-beam lithography, and the underlying oxide is etched, resulting in suspended nanowires demonstrating 2% uniaxial strain. (Si)GeSn alloys are epitaxially grown on the nanowires, with a focus on understanding growth kinetics, including Sn segregation, strain effects, and defect formation. Detailed analysis via STEM/EDS and dark-field TEM of longitudinal and transverse cross-sections, prepared by focused ion beam (FIB) techniques, provide insights into the strain distribution and Sn composition. By combining biaxial tensile strain from the Ge-VS, uniaxial strain from nanowire lithography, and triaxial strain from the (Si)GeSn alloying, we create a highly strained system with a pathway to realizing both a direct Ge band gap for efficient light emission and CMOS compatibility.

Acknowledgement: This work is supported by the U.S. Department of Energy, under Award No. DEAC02-76SF00515, FWP100786. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-2026822.