Prediction that uniaxial tension along 111 produces a direct band gap in germanium

We predict a new way to achieve a direct band gap in germanium, and hence optical emission in this technologically important group-IV element: tensile strain along the 111 direction in Ge nanowires. Although a symmetry-breaking band splitting lowers the conduction band at the corner of the Brillouin zone (at the L point), a direct gap of 0.34 eV in the center of the Brillouin zone (at Γ) can still be achieved at 4.2% longitudinal strain, through an unexpectedly strong nonlinear drop in the conduction band edge at Γ for strain along this axis. These strains are well within the experimentally demonstrated mechanical limits of single-crystal Ge (or GexSi1-x) nanowires, thereby opening a new material system for fundamental optical studies and applications.