Direct bandgap transformation in silicon and germanium nanowires and the effect of oxygen doping

We present a computational study on the band structures of silicon (Si) nanowires grown along th [001] direction and germanium (Ge) nanowires grown along the [111]_ᅩ (perpendicular to [111] direction), in which various diameters of nanowires and oxygen (O) doping bonds on the surface are considered. The calculation results show that a direct band gap can be obtained on the Si [001] nanowires or the Ge [111]_ᅩ nanowires, which is attributed to the conduction band valley shifting from X to Γ point for Si [001] nanowires and shifting from L to Γ point for Ge [111]_ᅩ nanowires. In the calculation investigation, the quantum confinement (QC) effect and the Heisenberg principle related to ⊿k ~ 1/⊿x on the quantum nanowires are explored for transforming from indirect bandgap to direct bandgap. Surprisingly, the electron localized states are built from Si = O double bond and Si–O–Si bridge bond on nanowire surface at conduction band valley, in which the three energy levels’ system is built for lasing, including the opening states due to the QC effect on nanowire structures as pumping levels and the electron localized states originated from impurities on surface as emission with lasing levels. The mechanism and the model of the direct bandgap transformation for emission with lasing are built in Si and Ge nanowires doped with oxygen. These interesting results have a good application in optoelectronics devices and large-scale integration.