Strain-modulated electronic properties of Ge nanowires: A first-principles study

Abstract

We used density-functional theory based first principles simulations to study the effects of uniaxial strain and quantum confinement on the electronic properties of germanium nanowires along the [110] direction, such as the energy gap and the effective masses of the electron and hole. The diameters of the nanowires being studied are up to 50 Å. As shown in our calculations, the Ge [110] nanowires possess a direct band gap, in contrast to the nature of an indirect band gap in bulk. We discovered that the band gap and the effective masses of charge carries can be modulated by applying uniaxial strain to the nanowires.These strain modulations are size-dependent. For a smaller wire (~ 12 Å), the band gap is almost a linear function of strain; compressive strain increases the gap while tensile strain reduces the gap. For a larger wire (20 Å -50 Å), the variation of the band gap with respect to strain shows nearly parabolic behavior: compressive strain beyond -1% also reduces the gap. In addition, our studies showed that strain affects effective masses of the electron and hole very differently. The effective mass of the hole increases with a tensile strain while the effective mass of the electron increases with a compressive strain. Our results suggested both strain and size can be used to tune the band structures of nanowires, which may help in design of future nano-electronic devices. We also discussed our results by applying the tight-binding model.

First Principles Study of Si/Ge Core-Shell Nanowires ---- Structural and Electronic Properties

“…2). This is consistent with the findings in pure Si and Ge nanowires with H passivation (Logan & Peng, 2009;Peng et al, 2009), in which small Si or Ge nanowires along the [110] direction were expanded, comparing to their bulk lattices.…”

“…It is interesting to note that the band gap of the core-shell nanowires is smaller than that of both pure Si and Ge nanowires, at a given diameter. For example, the DFT gap for Si and Ge wires with the diameter 2.5 nm are 1.02 eV and 0.73 eV Logan & Peng, 2009), respectively. However the gap for the Si/Ge core-shell wires are 0.58 eV(Ge-core) and 0.54 eV (Si-core), respectively, which are both smaller than that of Si and Ge wires.…”

Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain

“…In addition to size, strain has become a routine factor to engineer band gaps of semiconductors in the field of microelectronics. Researchers have theoretically demonstrated the modulated band gap by external strains in a variety of systems such as pure Si 17 and Ge 18 and Si/Ge Core-shell nanowires. 19 It would be very interesting to investigate strain effects on the band structure of WZ GaAs nanowires and examine if the direct-indirect band gap transition can be engineered for applications.…”

Mechanical and electronic properties of strained Ge nanowires usingab initioreal-space pseudopotentials

“…These results differ from previous nanowire studies that show a lattice expansion with decreasing size. 4,20,21 The mechanism behind the lattice expansion is thought to be a compressive stress on the wire surface that causes axial expansion by the Poisson effect. 36 The nanowires in our simulation were found to be consistent with the Poisson effect; that is, axial tensile stress causes the cross-sectional area to contract slightly, and compressive stress causes the area to expand.…”

“…16 While numerous ab initio computational studies have been performed on Si nanowires, few have been done for Ge, [17][18][19] only some of which have examined the effects of strain. [20][21][22] Here we examine previously unstudied growth directions and further analyze the effects of axial strain on the mechanical and electronic properties of Ge nanowires.…”