Electronic structure and phase stability of MgTe, ZnTe, CdTe, and their alloys in the B3, B4, and B8 structures Ji-Hui Yang, Shiyou Chen, Wan-Jian Yin, and X. G. Gong Department of Physics and MOE Laboratory for Computational Physical Sciences, Fudan University, Shanghai 200433, People’s Republic of China Aron Walsh and Su-Huai Wei National Renewable Energy Laboratory, Golden, Colorado 80401, USA Received 25 March 2009; published 2 June 2009 The electronic structure and phase stability of MgTe, ZnTe, and CdTe were examined in the zinc-blende B3, wurtzite B4, and NiAs-type B8 crystal structures using a first-principles method. Both the band-gap and valence-band maximum VBM 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,CdTe compositions, as expected from the preferences of ZnTe and CdTe. However, the Mg,ZnTe alloy undergoes a B3 to B4 transition above 88% Mg concentration and a B4 to B8 transition above 95% Mg concentration. For Mg,CdTe, 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,ZnTe Mg,CdTe alloys are predicted to have accessible direct band gaps in the range 2.391.48–3.253.02 eV, suitable for photovoltaic absorbers. DOI: 10.1103/PhysRevB.79.245202 PACS numbers: 71.20.Nr, 71.23.An, 71.55.Gs, 78.55.Et I. INTRODUCTION Multiternary semiconductor alloys are essential compo- nents in both existing and next-generation optoelectronic de- vices such as solar cells, as they offer great flexibility in tuning emission and absorption wavelengths and controlling lattice constants. 1–8 Experimentally, MgTe, ZnTe, and CdTe are found to have room-temperature direct band gaps of 3.5, 2.4, and 1.5 eV, respectively. 9 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. 10–12 Their potential for high efficiency solid-state light-emission devices has also been noted. 13,14 Despite the small lattice mismatch between MgTe and CdTe less than 1%, and the relatively small atomic size and chemical mismatch between Mg and Zn, alloy formation in this system is expected to be structurally complex. This originates from the ground-state structural preferences of the binary tellurides. ZnTe and CdTe adopt tetrahedral coordina- tion in the cubic zinc-blende B3 structure F4 ¯ 3m. Experi- mentally MgTe is reported to favor the wurtzite B4 struc- ture  P6 3 mc, 15–17 while theoretically MgTe is predicted to be more stable in the NiAs-type B8 structure  P6 3 mmc. 18,19 However, experimentally at relatively low pressures 1–3.5 GPa,a B4 to B8 transition is observed for MgTe, and it has been suggested that the B4 structure may be a high-temperature metastable phase, with the B8 structure being the true thermodynamic ground state, in agreement with theory. 20 In the B3 crystal structure, the anions form an ideal fcc array, with cation occupying half of the tetrahedral holes. In the B4 structure, the anion stacking becomes a hcp array, with reduced C 3v site symmetry. However, the fourfold local coordination in both the B3 and B4 polymorphs is similar. For MgTe the B3 structure lies slightly higher in energy due to its smaller Madelung constant. In contrast, the hexagonal B8 structure features a hcp anion sublattice with cations oc- cupying the octahedral holes the hexagonal analog of the cubic NaCl B1 crystal structure, Fig. 1. 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 pre- dicted at various alloy compositions. Knowledge of the elec- tronic structure and band alignment of these binary tellurides in each polymorph is therefore very important for under- standing the alloy properties and addressing their potential for solar-cell applications. Furthermore, as MgTe, ZnTe, and CdTe in the B8 structure all possess indirect band gaps, for- mation of NiAs-type alloys will be highly undesirable for optoelectronics use and should be avoided. To obtain a rigorous understanding of the relationship be- tween the geometric and electronic structure, we have per- formed first-principles calculations and detailed electronic- structure analysis of MgTe, ZnTe, and CdTe in the binary B3, B3 B4 B8 FIG. 1. Color online Conventional unit cells of the zinc-blende B3, wurtzite B4, and NiAs-type B8 structures. PHYSICAL REVIEW B 79, 245202 2009 1098-0121/2009/7924/2452027 ©2009 The American Physical Society 245202-1