First-principles study on the effective masses of zinc-blend-derived Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se)

The electron and hole effective masses of kesterite (KS) and stannite (ST) structured Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se) semiconductors are systematically studied using first-principles calculations. We find that the electron effective masses are almost isotropic, while strong anisotropy is observed for the hole effective mass. The electron effective masses are typically much smaller than the hole effective masses for all studied compounds. The ordering of the topmost three valence bands and the corresponding hole effective masses of the KS and ST structures are different due to the different sign of the crystal-field splitting. The electron and hole effective masses of Se-based compounds are significantly smaller compared to the corresponding S-based compounds. They also decrease as the atomic number of the group IV elements (Si, Ge, Sn) increases, but the decrease is less notable than that caused by the substitution of S by Se.Comment: 14 pages, 6 figures, 2 table

Alloy Stabilized Wurtzite Ground State Structures of Zinc-Blende Semiconducting Compounds

The ground state structures of the A$_x$B$_{1-x}$C wurtzite (WZ) alloys with $x=$0.25, 0.5, and 0.75 are revealed by a ground state search using the valence-force field model and density-functional theory total energy calculations. It is shown that the ground state WZ alloy always has a lower strain energy and formation enthalpy than the corresponding zinc-blende (ZB) alloy. Therefore, we propose that the WZ phase can be stabilized through alloying. This novel idea is supported by the fact that the WZ AlP$_{0.5}$Sb$_{0.5}$, AlP$_{0.75}$Sb$_{0.25}$, ZnS$_{0.5}$Te$_{0.5}$, and ZnS$_{0.75}$Te$_{0.25}$ alloys in the lowest energy structures are more stable than the corresponding ZB alloys. To our best knowledge, this is the first example where the alloy adopts a structure distinct from both parent phases

First-principles study of defect properties of zinc blende MgTe

We studied the general chemical trends of defect formation in MgTe using first-principles band structure methods. The formation energies and transition energy levels of intrinsic defects and extrinsic impurities and some defect complexes in zinc blende MgTe were calculated systematically using a new hybrid scheme. The limiting factors for p-and n-type doping in MgTe were investigated. Possible solutions to overcome the doping limitation of MgTe are proposed. The best p-type dopant is suggested to be N with nonequilibrium growth process and the best n-type dopant is suggested to be I with its doping complex V Mg + 4I Te

Electronic structure and phase stability of MgTe, ZnTe, CdTe, and their alloys in the B3, B4, and B8 structures

The electronic structure and phase stability of MgTe, ZnTe, and CdTe were examined in the zinc-blende ͑B3͒, wurtzite ͑B4͒, and NiAs-type ͑B8͒ crystal structures using a first-principles method. Both the band-gap and valence-band maximum ͑VBM͒ deformation potentials of MgTe, ZnTe, and CdTe in the B3 structure were analyzed, revealing a less negative band-gap deformation potential from ZnTe to MgTe to CdTe, with a VBM deformation potential increase from CdTe to ZnTe to MgTe. The natural band offsets were calculated taking into account the core-level deformation. Ternary alloy formation was explored through application of the special quasirandom structure method. The B3 structure is found to be stable over all ͑Zn,Cd͒Te compositions, as expected from the preferences of ZnTe and CdTe. However, the ͑Mg,Zn͒Te alloy undergoes a B3 to B4 transition above 88% Mg concentration and a B4 to B8 transition above 95% Mg concentration. For ͑Mg,Cd͒Te, a B3 to B4 transition is predicted above 80% Mg content and a B4 to B8 transition above 90% Mg concentration. Using the calculated band-gap bowing parameters, the B3 ͑Mg,Zn͒Te ͓͑Mg,Cd͒Te͔ alloys are predicted to have accessible direct band gaps in the range 2.39͑1.48͒-3.25͑3.02͒ eV, suitable for photovoltaic absorbers

Oxygen-defect structure of non-superconducting La_1.85Sr_1.15Cu₂O_6.25:excess oxygen in the interlayer site

The structure of the metallic, but nonsuperconducting, double-layer phase, La1.85Sr1.15Cu2O6.25 has been refined from neutron powder diffraction data; space group 14/mmm, a=3.8530(1) A ̊, c=20.0833 (3) A ̊. Sr2+ ions are ordered predominantly into the nine-coordinated site in the T-type layer of the structure. Occupation of the interlayer site by the relatively large La3+/Sr2+ ions allows intercalation of the excess oxygen at the (0, 0, case1 2) site between the CuO2 layers. In addition, vacancies are introduced in the O(1) sites of the CuO2 planes. This partial destruction of the two-dimensional nature of the structure may be responsible for the lack of superconductivity in this phase, in contrast to the recent observation of a Tc of 60 K in the similarly-doped calcium phase La1.6Sr0.4CaCu2O6.</p

Excess oxygen defects in layered cuprates

Neutron powder diffraction has been used to study the oxygen defect chemistry of two non-superconducting layered cuprates, La1.25Dy0.75CuO3.75F0.5, having a T*-related structure, and La1.85Sr1.15Cu2O6.25, having a structure related to the that of the newly discovered double-layer superconductor La2-xSrxCaCu2O6. The role played by oxygen defects in determining the superconducting properties of layered cuprates is discussed.</p

Excess oxygen defects in layered cuprates

Neutron powder diffraction has been used to study the oxygen defect chemistry of two non-superconducting layered cuprates, La1.25Dy0.75CuO3.75F0.5, having a T*-related structure, and La1.85Sr1.15Cu2O6.25, having a structure related to the that of the newly discovered double-layer superconductor La2-xSrxCaCu2O6. The role played by oxygen defects in determining the superconducting properties of layered cuprates is discussed.</p