Strain Mediated Band Gap Engineering of Bent Semiconductor Nanowires from First Principles

Abstract

Semiconductor nanowires (NWs) exhibit both extraordinary mechanical properties and excellent control in the conduction of electrons. NWs present a unique system to explore physical phenomena on a quantum scale and play a critical role in future electronic and optoelectronic devices. The band gaps of semiconductors are commonly tuned to exhibit desired behaviours through means such as alloying or doping. However, a novel method proposed in this work is the use of strain, due to large recoverable elastic strains observed in NWs. Of particular interest is that of strains exhibited in bent NWs; where simultaneous compressive and tensile strains occur along its cross-section. Herein, an ab initio investigation on strain-mediated band gap engineering of bent semiconductor nanowires, was performed using density functional theory (DFT). There are difficulties in simulating bent NWs due to symmetry loss in all directions in the bent structure. By considering that compressive and tensile strain occur simultaneously, this thesis proposes a simplified approach to simulating band gap evolution with strain, by using unit cells under tensions and compressions separately. Furthermore, it is well documented that the minute radial dimension of NWs induces solid state phenomena such as the quantum confinement effect and internal strains; which have considerable contributions to band gap modulation. Separation of the effects of these phenomena with external strain application to band gap evolution can be performed using the proposed simplified unit cell model. Both wurtzite (WZ) and zinc-blende (ZB-(111)) polytypes of III-V and II-VI semiconductors were considered in this study. Simulations implementing DFT were performed in the Vienna ab initio Simulation Package (VASP), using the hybrid HSE06 functional.