Direct bandgap germanium-on-silicon inferred from 5.7% <100> uniaxial tensile strain [Invited]

We report uniaxial tensile strains up to 5.7%along h100i in suspended germanium(Ge)wires on a silicon substrate,measured using Raman spectroscopy.This strain is sufficient to make Ge a direct bandgap semiconductor.Theoretical calculations show that a significant fraction of electrons remain in the indirect conduction valley despite the direct bandgap due to the much larger density of states;however,recombination can nevertheless be dominated by radiative direct bandgap transitions if defects are minimized.We then calculate the theoretical efficiency of direct bandgap Ge LEDs and lasers.These strained Ge wires represent a direct bandgap Group IV semiconductor integrated directly on a silicon platform.

Optically pumped low-threshold microdisk lasers on a GeSn-on-insulator substrate with reduced defect density

Despite the recent success of GeSn infrared lasers,the high lasing threshold currently limits their integration into practical applications.While structural defects in epitaxial GeSn layers have been identified as one of the major bottlenecks towards low-threshold GeSn lasers,the effect of defects on the lasing threshold has not been well studied yet.Herein,we experimentally demonstrate that the reduced defect density in a GeSn-on-insulator substrate improves the lasing threshold significantly.We first present a method of obtaining high-quality GeSn-oninsulator layers using low-temperature direct bonding and chemical–mechanical polishing.Low-temperature photoluminescence measurements reveal that the reduced defect density in GeSn-on-insulator leads to enhanced spontaneous emission and a reduced lasing threshold by0 times andtimes,respectively.Our result presents a new path towards pushing the performance of GeSn lasers to the limit.