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 $\Gamma$) can still be achieved at 4.2% longitudinal strain, through an unexpectedly strong nonlinear drop in the conduction band edge at $\Gamma$ for strain along this axis. These strains are well within the experimentally demonstrated mechanical limits of single-crystal Ge (or ${\mathrm{Ge}}_x{\mathrm{Si}}_{1-x}$) nanowires, thereby opening a new material system for fundamental optical studies and applications.

• Received 20 May 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.156401