CCMGCCMGCCMGCCMGCCMGCCMGCCMGCCMG Ji-Hui Yang, Shiyou Chen, Wan-Jian Yin, and X.G. Gong Department of Physics and MOE laboratory for computational physical.

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Presentation transcript:

CCMGCCMGCCMGCCMGCCMGCCMGCCMGCCMG Ji-Hui Yang, Shiyou Chen, Wan-Jian Yin, and X.G. Gong Department of Physics and MOE laboratory for computational physical sciences, Fudan University, Aron Walsh, and Su-Huai Wei National Renewable Energy Laboratory, Golden, Colorado 80401, USA Electronic structure and phase stability of MgTe, ZnTe, CdTe, and their alloys in the B3, B4, and B8 structures Introduction Experimentally, MgTe, ZnTe and CdTe are found to have room temperature direct band gaps of 3.5 eV, 2.4 eV and 1.5 eV, respectively. This makes them excellent candidates for low-cost thin film or high efficiency multi-junction solar cell materials, to complement existing CdTe and Cu(In,Ga)Se 2 technologies. However, the ground structures of the three binaries are not the same. While ZnTe and CdTe adopt tetrahedral coordination in the cubic zinc-blende (B3) structure, MgTe is reported to favor the wurtzite (B4) structure experimentally and NiAs-type (B8) structure theoretically. As a result of this mismatch in cation coordination preferences, alloys formed from MgTe, ZnTe and CdTe exhibit a sensitive structure-composition dependence, with B3, B4 and B8 crystals predicted at various alloy compositions. Calculation methods Calculations were performed using density functional theory (DFT) based on the local density (LDA), as implemented in the VASP code. The electron and core interactions are included using the frozen-core projector augmented wave (PAW) approach. The shallow-core Zn 3d and Cd 4d states are explicitly treated as valence electrons. The cut-off kinetic energy for the plane wave basis wave functions is chosen as 300 eV for all the calculations. The Monkhorst- Pack 9x9x9 for the B3 binary structure and 12x12x6 for the B4 and B8 structures were employed. The bulk binary structures were each optimized to their equilibrium volume through minimization of the total energy and stress. The natural band offsets of the three tellurides were calculated taking into account core-level deformation. The ternary random alloys A x B 1-x Te were modeled within 64-atom (32 mixed cation) supercells using the special quasirandom structure (SQS) approach to determine the cation site occupancies. The cubic B3 structural alloys are constructed with ideal lattice constants 2a and the B4 and B8 structured alloys are represented through orthorhombic cells with ideal lattice constants of 2a, a, and a. The internal atomic positions inside the SQS supercells were fully relaxed through minimization of the quantum mechanical force on each atom to be below 0.02 eV/A. For the B3 structure, 3x3x3 k-point meshes and for the B4 and B8 structures, 4x2x2 k-point meshes were employed. The total energy was converged to within 0.5 meV/2-atom for B3 and B4 supercells and 1 meV/2- atom for the B8 supercells. Results We found B3 MgTe will turn to an indirect semiconductor when compressed hydrostatically by 4.5%. B. Band-gap deformation potentials A. Structural property C. Absolute deformation potentials Results The ternary random alloys A x B 1-x Te formed by MgTe, ZnTe and CdTe were studied in the B3, B4 and B8 structures. In our calculations, we constructed five SQS with x=0.125, 0.25, 0.50, 0.75 and for each alloy. D. Ternary alloy formation The VBM states increase from Mg to Zn to Cd. As there are shallow core d levels in Zn and Cd, the anion p-cation d coupling will push their VBM high and this explains why Zn and Cd compounds have higher VBMs. Although Cd has deeper d states and weaker p-d coupling than Zn, the larger lattice constants of Cd compounds results in their higher VBM compared to the Zn compounds The interaction parameter in the same structure decreases from (Mg,Zn)Te to (Zn,Cd)Te to (Mg,Cd)Te, indicating that both strain and chemical property (electronegativity) contribute to the formation energy but strain dominates in this system. b g of (Mg,Zn)Te alloys or (Mg,Cd)Te alloys arise mainly from b CE, as Mg has a very different electronegativity from Zn or Cd and b g of (Zn,Cd)Te alloys are largely determined by b VD due to the large lattice mismatch between ZnTe and CdTe. Conclusion The stability of ternary alloys were studied, and with the calculated bowing parameters, the zinc-blende (Mg,Zn)Te ((Mg,Cd)Te) alloys are predicted to be stable with less than 88%(80%) Mg content with direct alloy band gaps between 2.39 (1.48) eV and 3.25 (3.02) eV. The (Zn,Cd)Te zinc-blende alloy is stable over the full compositional range. The work at Fudan University is partially supported by the National Science Foundation of China, the Special Funds for Major State Basic Research, and the project of MOE and Shanghai Municipality. The work at NREL is funded by the U.S Department of Energy (DOE), under Contract No. DE-AC36-08GO28308, which employed computing resources of the National Energy Research Scientific Computing Center supported by DOE under Contract No. DE-AC02-05CH ACKNOWLEDGEMRNT For detail information, see Physical Review B 79,245202(2009)