Effective strain relaxation of GeSn single crystal with Sn content of 16.5% on Ge grown by high-temperature sputtering

In this work, epitaxy of GeSn with high-Sn content of 16.5% on Ge substrate by sputtering at a relative high temperature of 259 degrees C is reported. The strain relaxation in constant-Sn content (CSC) and step-increased-Sn content (SSC) GeSn is systematically investigated. For CSC GeSn, strain mainly relaxes through generation of 60 degrees dislocations and few 90 degrees dislocations at Ge/GeSn interface. A defect-rich region is formed above the Ge/GeSn interface due to interaction of 60 degrees and/or 90 degrees dislocations, which results in associated stacking faults (SFs). For SSC GeSn, the strain relaxes preferentially through GeSn/GeSn interfaces by generation of 60 degrees dislocations rather than through Ge/GeSn interface. The gradual strain relaxation at GeSn/GeSn interfaces eliminates bunching of dislocations and helps improve the material quality. Benefiting from the high-temperature deposition process, large strain relaxation of 85% and 65% is obtained in 120 nm CSC GeSn and 230 nm SSC GeSn, respectively. The results suggest that high-temperature sputtering epitaxy is a promising approach to prepare strain-relaxed GeSn buffer layers and promote understanding of strain relaxation mechanisms in physical vapor deposited GeSn alloys.