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Anomalous Threshold Reduction from <100> Uniaxial Strain for a Low-Threshold Ge Laser

We theoretically investigate the effect of <100> uniaxial strain on a Ge-on-Si laser using deformation potentials. We predict a sudden and dramatic ~200x threshold reduction upon applying sufficient uniaxial tensile strain to the Ge gain medium. This anomalous reduction is accompanied by an abrupt jump in the emission wavelength and is explained by how the light-hole band raises relative to the heavy-hole band due to uniaxial strain. Approximately 3.2% uniaxial strain is required to achieve this anomalous threshold reduction for 1x1019 cm-3 n-type doping, and a complex interaction between uniaxial strain and n-type doping is observed. This anomalous threshold reduction represents a substantial performance advantage for uniaxially strained Ge lasers relative to other forms of Ge band engineering such as biaxial strain or tin alloying. Achieving this critical combination of uniaxial strain and doping for the anomalous threshold reduction is dramatically more relevant to practical devices than realizing a direct band gap.