Ab-initio DFT-FP-LAPW/TB-mBJ/LDA-GGA investigation of structural and electronic properties of Mg_xZn_(1−x)O alloys in Würtzite, Rocksalt and Zinc-Blende phases

摘要

We report on ab-initio DFT/FP-LAPW/TB-mBJ/GGA and LDA investigation of structural and electronic properties of Mg_xZn_(1−x)O alloys in Würtzite (WZ), Rocksalt (RS) and Zinc-Blende (ZB) phases. TB-mBJ corrections of electronic exchange and correlation interactions make it possible to improve considerably the computational results which we have found very close to the experimental data. As expected, our results show that at 0.375 < x < 0.5, Mg_xZn_(1−x)O undergoes a phase transition from the tetrahedrally coordinated non-centrosymmetric covalent WZ phase to the octahedrally coordinated ionic RS phase, and the ZB phase remains metastable in the whole 0 ≤ x≤1 range. Our structural properties results show that the lattice parameters vary nonlinearly with x in all three phases. In ZB phase, a_(ZB) increases with x very slightly (Δa_(ZB)/a_(ZB)≤+0.6%) while in RS phase, a_(RS) decreases with x more strongly (Δa_(RS)/a_(RS) ≈ −1.7%). The strongest lattice parameters variations with x are found in WZ phase where increasing x results in increasing a_(WZ) (Δa_(WZ)/a_(WZ)≈+1.4%), decreasing c_(WZ) (Δc_(WZ)/c_(WZ) ≈ −2.9%), decreasing c_(WZ)/a_(WZ) ratio (Δ(c_(WZ)/a_(WZ))/(c_(WZ)/a_(WZ))≈-3.31%), and increasing u internal parameter (Δu/u≈+2.8%). Our electronic properties results show that the fundamental energy bandgap E_G is Γ-direct in WZ and ZB Mg_xZn_(1−x)O in the whole 0 ≤ x≤1 range. In RS Mg_xZn_(1−x)O, E_G is indirect throughout 0 ≤ x < 1 range including RS ZnO and excluding RS MgO where E_G is Γ-direct. In all three phases, E_G increases with x first linearly in the low x range (x < 0.5), then nonlinearly at higher x (x≥0.5). At x < 0.5, E_G ~(ZB)(x) = 2.609 + 2.861× eV, E_G ~(WZ)(x) = 2.863 + 1.990× eV, and E_G ~(RS)(x) = 3.190 + 1.508× eV. At x≥0.5, nonlinear E_G(x) variations show strong bowing parameters depending on the crystal phase: b_(ZB) = 2.709eV, b_(WZ) = 4.079eV, and b_(RS) = 16.83eV. These strong E_G(x) variations which are confined to a narrow high x range, are tightly correlated with the addition of Zn to MgO and are the strongest in RS Mg_xZn_(1−x)O. Densities of states and energy band structure data analysis show dominant influence of strong exchange and correlation interactions between hybridized O:2p-Zn:3d orbitals which take place as soon as Zn is introduced into MgO. Our results confirm early findings that O:2p-Zn:3d interactions do play a leading role in overall Mg_xZn_(1−x)O physical properties whatever the phase, with strongest effects in RS phase, especially in huge E_G bowings and in RS-WZ phase transition.