Uniaxial strain-modulated electronic structures of CdX (X=S, Se, Te) from first-principles calculations: A comparison between bulk and nanowires

Using first-principles calculations based on density functional theory, we systematically study the structural deformation and electronic properties of wurtzite CdX (X=S, Se, Te) bulk and nanowires (NWs) under uniaxial [0001] strain. Due to the intrinsic shrinking strain induced by surface contraction, large NWs with {1010} facets have heavy hole (HH)-like valence band maximum (VBM) states, while NWs with {1120} facets have crystal hole (CH)-like VBM states. The external uniaxial strain induces an HH-CH band crossing at a critical strain for both bulk and NWs, resulting in nonlinear variations in band gap and hole effective mass at VBM. Unlike the bulk phase, the critical strain of NWs highly depends on the character of the VBM state in the unstrained case, which is closely related to the size and facet of NWs. The critical strain of bulk is at compressive range, while the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile strain, respectively. Due to the HH-CH band crossing, the charge distribution of the VBM state in NWs can also be tuned by the external uniaxial strain. Despite the complication of the VBM state, the electron effective mass at conduction band minimum (CBM) of NWs shows a linear relation with the CBM-HH energy difference, the same as the bulk material.