F. Zhang, V. H. Crespi and P. Zhang, "Prediction that Uniaxial Tension along 111 Produces a Direct Band Gap in Germanium," Phys. Rev. Lett. 102, 156401 (2009)

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 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.

Comments:
Selected for the April 27, 2009 issue of Virtual Journal of Nanoscale Science & Technology