The Crystal Structure Evolution Of Semiconductors In The Hexagonal Crystal System

The structure evolution, band structure, elastic properties and phonon frequencies of typical semiconductors under uniaxial and biaxial strains in the hexagonal crystal system are systematically investigated via first principles calculations. The previous first-principle studies were mainly conducted with either the local density approximation(LDA) or the generalized gradient approximation(GGA). In standard density functional theory(DFT), both the Eg are always severely underestimated. In this work, our aim is to accurately compare the effects of uniaxial and biaxial strains on the structural stability and band structure for wurtzite Semiconductors using the Heyd-Scuseria-Ernzerhof(HSE) hybrid functional. The main findings are summarized as follows:(1) The newly predicted graphite-like phase is observed at large compressive uniaxial strains, not at large tensile biaxial strains, which is attributed to the different elastic responses to uniaxial and biaxial strains. The direct band structures are obtained in wurtzite Zn S under uniaxial and biaxial strains, whereas the indirect band gaps are only observed in graphite-like Zn S under large uniaxial strain. Therefore, the uniaxial strain along the [0001] direction and biaxial strain in the(0001) plane are nonequivalent for the structural and electronic properties of wurtzite Zn S. Our results are different from the widely accepted conclusion but are in good agreement with the available experimental data.(2) Although the wurtzite structure transforms to an intermediate graphite-like structure for Al0.5Ga0.5N alloy both uniaxial and biaxial strains, the second-order phase transition is found for uniaxial strain and the first-order transition for biaxial strain. The transition is driven by the mechanical and dynamical instabilities for uniaxial strain, and by the mechanical instability for biaxial strain. The wurtzite phase always remains the direct band structure, whereas the band gap of graphite-like phase is always indirect. The band gaps of wurtzite and graphite-like phases are greatly reduced by internal strains.