Semiconductors are typically classified either as direct-bandgap materials, known for their efficient light-emission properties, or as indirect-bandgap materials, often used in light-harvesting applications and photo-detectors. The less known pseudodirect bandgap configuration can be found in Wurtzite semiconductors: in this case, electron and hole wavefunctions overlap strongly but optical transitions between these states are impaired by symmetry. Switching a material between bandgap configurations would enable novel photonic applications but large anisotropic strain is needed to induce such band structure transitions.[3-5]
Here we show that Wurtzite GaAs nanowires can be switched reversibly between direct and pseudodirect bandgap configuration under the influence of a small uniaxial stress. When tensile stress is applied, the direct configuration can be obtained and the nanowires emit light efficiently; upon compression, the pseudodirect configuration is achieved and light emission can be reduced by more than three orders of magnitude. We demonstrate a remarkable energy shift of the PL due to transitions between the bright conduction band state and the heavy hole band (345meV) or the light hole band (257meV), by varying the strain over a range of ±2%. The splitting between the dark and bright conduction band states could also be tuned continuously over a range of more than 230meV.
Using Raman scattering spectra as a relative strain gauge and fitting the optical transition energies to a kp model, we were able to determine all bandstructure parameters of the Wurtzite GaAs nanowire in unstrained conditions, i.e. the bandgap (1.41eV±8meV), the crystal field (197meV±50meV) and spin-orbit splitting (293meV±129meV) and, most importantly, the splitting between the bright and the dark conduction bands (33meV±47meV). These results provide, for the first time, a conclusive picture of the energy and symmetry of the valence and conduction band states in Wurtzite GaAs and constitute a solid foundation to the understanding of strain effects on the optical and electronic properties of III-V nanowires.
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