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Publication Date
May 31, 2017
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Wurtzite CdX(X=S, Se, Te) semiconductors and nanostructures have attracted a lot of attention because of the direct band gaps well-matched to the solar spectrum, good light absorption and emission. Strain engineering is an effective way to tune the properties of semiconductors and nanostructures. Due to surface effect and quantum confinement effect, the variations of properties of CdX nanostructures under strain may be different from that of bulk materials. In this thesis, using first-principles calculations based on density functional theory, we systematically study and compare the influence of uniaxial [0001] strain on the structural deformation and electronic structures of CdX bulk and nanowires (NWs). We find that, for CdX bulk materials, the switch of the axial bondlength and the nonaixal bondlength induced by the uniaxial strain leads to the heavy hole (HH) and crystal hole (CH) band crossing at a critical strain. Consequently, the band gap and hole effective mass under uniaxial strain are highly nonlinear. However, the electron effective mass at conduction band minimum (CBM) is linear to the CBM-HH energy difference. <div>For the unstrained pristine CdX NWs along [0001] direction, the surface contraction can influence the character of the VBM state by changing the relation between the averaged axial bondlength and the averaged nonaxial bondlength. Large NWs with {10-10} facets have HH-like VBM state, while NWs with {11-20} facets or smaller diameters have CH-like VBM state. Under external uniaxial strain, due to the HH-CH band crossing at the critical strain induced by the switch of the averaged axial bandlength and the averaged bondlength, the band gap and hole effective mass show highly nonlinear variations, similar to the bulk. Unlike the bulk, the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile range, respectively. Because of the different charge distributions of the HH and CH states, the charge distribution of the VBM state and the overlap between electron and hole states can be tuned by the external uniaxial strain. However, the electron effective mass of all NWs shows a good linear relation with the CBM-HH energy difference, the same as bulk.</div><div>In addition, we analysis the atomic structure and band gap of the core/shell NWs composed by CdX semiconductors. We consider the CdSe/CdS and CdSe/CdTe core/shell NWs with type-I and type-II band offsets, respectively. CdSe core region undergoes a spacially homogeneous but anisotropic compression and expansion, respectively. However, strain in the shell region is spacially nonhomogeneous and highly anisotropic. The electronic structure of core/shell NWs can be modified by the intrisic strain due to lattice mismatch and quantum confinement effect.</div><div>This study reveals the differences and similarities of the properties of CdX bulk materials and NWs under strain, would help to understand the strain effect on the electronic properties of nanostructures, and would provide a theoretical guide to synthesize nanostructures with desirable properies using strain engineering.<br /><p style="text-indent: 24pt" class="MsoNormal"><font face="times new roman, times" size="2"> </font></p></div>

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