First-Principles Study Of Several â…¡-â…£-â…¤₂ Chalcopyrite Semiconductor Materials

Chalcopyrite semiconductors have recently attracted much attention due to their promising technological applications in a variety of fields including nonlinear optics,light emitting diodes, semiconductor lasers, Infrared parametric oscillators as well as solar cells. First-principles calculations of structural phase transitions, elasticity,electric band structure, optical and lattice dynamical properties of several II-IV-V2 chalcopyrite semiconductors have been performed within the frame of density functional theory. The main work and the results obtained are presented as follows:(1) The structural, elastic and lattice dynamical properties of chalcopyrite Be Si V2 and Mg Si V2(V=P, As, Sb) have been systematically investigated using normconserving pseudopotentials and generalized-gradient approximation. The optimized equilibrium structural parameters are in good agreement with available experimental and theoretical values. Single-crystal elastic constants, linear and volume compressibilities, shear anisotropic factors, as well as polycrystalline bulk, shear and Young’s modulus and Poisson’s ratio have been theoretically predicted based on the optimized crystal structures. The results show that, more pronounced tetragonal distortion and larger elastic anisotropy in both compression and shear have been presented in Mg Si V2 ductile compounds compared with Be Si V2 brittle compounds.Phonon dispersion relations, phonon density of states, as well as infrared absorption spectra and Raman scattering spectra have been calculated for the considered compound crystals using the linear response method within the density functional perturbation theory. Besides, the phonon frequencies and atomic displacement patterns for Raman-active(E, B2, B1 and A1), infrared-active(E and B2), and silent(A2)modes at zone-center Γ point have been analyzed in detail. The direct factors affecting phonon vibrational frequencies have been found to be both chemical bonding strengths and masses of bonding atoms. The Si–V bonding is remarkably stronger than the Mg–V bonding in Mg Si V2 chalcopyrite semiconductors according to the analysis of state densities of phonons.(2) The pressure-induced structure phase transition, band structure, optical,elastic, lattice dynamic and thermodynamic properties of Cd Sn V2(V=P, As, Sb)compound semiconductors have been systematically studied using norm-conserving pseudopotentials and generalized-gradient approximation. The optimized ground structural parameters are in good agreement with available experimental and theoretical values. Enthalpy calculations show that Cd Sn P2, Cd Sn As2 and Cd Sn Sb2 compounds undergo a pressure-induced first-order phase transition from chalcopyrite(CH) to rocksalt(RS) phase under hydrostatic pressures with transition pressures 29.2GPa, 20.1 GPa and 15.8 GPa, respectively. Fundermental band-gap energy, singlecrystal elastic constants, polycrystalline bulk modulus, phonon dispersion spectra, IR absorption and Raman scattering spectra, as well as zone-center optical phonon frequencies evolution with hydrostatic pressure have been discussed in detail. Lattice dynamic and elastic stablities of these compounds under high pressures have been analysed. The obtained results show that the underlying physical origin of CHâ†'RS phase transition might be in close relation with the pressure-induced low-frequency phonon modes and the softening shear elastic modulus.