We investigate lattice ordering phenomena for the heterovalent ternaries that are based on the wurtzite lattice, under the constraint that the octet rule be preserved. We show that, with the single exception of a highly symmetric twinned structure, all allowed lattice orderings can be described by a pseudospin model corresponding to the two different stackings of ABAB rows of atoms in the basal plane that occur in the P na21 and P mc21 crystal structures. First-principles calculations show that the difference in the energies of formation between these two structures is 13±3 meV/fu (formula unit) for ZnSnN2 and is an order of magnitude larger for ZnGeN2, and that for both materials the P m31 structure, which contains only octet-rule-violating tetrahedra, has a significantly higher energy of formation and a signficantly lower band gap. We predict almost random stacking and wurtzite-like x-ray diffraction spectra in the case of ZnSnN2, consistent with reported measurements. The octet-rule-preserving model of disorder proposed here predicts a band gap that for ZnSnN2 is relatively insensitive to ordering, in contrast to the prevailing model, which invokes the random placement of atoms on the cation sublattice. The violations of the octet rule in the latter model lead to significant narrowing of the band gap. The Raman and photoluminescence spectra of ZnSnN2 are interpreted in light of the ordering model developed here. The observation that ZnGeN2 orders in the P na21 structure under appropriate growth conditions is consistent with the larger difference in the energies of formation of the P na21 and P mc21 structures for this material. The ordering model presented here has important implications for the optical, electronic and lattice properties of all wurtzite-based heterovalent ternaries.
We report the synthesis of a direct gap semiconductor, ZnSnN 2 , by a plasma-assisted vapor-liquid-solid technique. Powder X-ray diffraction measurements of polycrystalline material yielded lattice parameters in good agreement with predicted values. Photoluminescence efficiency at room temperature was observed to be independent of excitation intensity between 10 3 and 10 8 W/cm 2 . The band gap was measured by photoluminescence excitation spectroscopy to be 1.7 ± 0.1 eV. The range of direct band gaps for the Zn(Si,Ge,Sn)N 2 alloys is now predicted to extend from 4.5 to 1.7 eV, opening up this little-studied family of materials to a host of important applications.
The role of exchange defects on the band structure of ZnGeN2 is investigated. Exchange defects are defined through the exchange of cations Zn and Ge starting from the ideal P na21 crystal structure, which obeys the local octet rule. Each such exchange creates several nitrogen-centered tetrahedra which violate the local octet rule although overall charge neutrality is preserved. We study several distributions of exchange defects, some with all antisites making up the exchange defect close to each other and with increasing numbers of exchange defects, and others where the two types of antisites ZnGe and GeZn are kept separated from each other. We also compare the results for these models with a fully random distribution of Zn and Ge on the cation sites. We show that for a single-nearest-neighbor exchange defect, the band gap is narrowed by about 0.5 eV due to two effects: (1) the ZnGe antisites form filled acceptor states just above and merging with the valence band maximum (VBM) of perfect crystal ZnGeN2 and (2) the GeZn antisites form a resonance in the conduction band which lowers the conduction band minimum (CBM). When more exchange defects are created, these acceptor states broaden into bands which can lower the gap further. When tetrahedra occur surrounded completely by four Zn atoms, states even deeper in the gap are found localized all near these tetrahedra, forming a separate intermediate band. Finally, for phase segregated ZnGe and GeZn the gap is significantly more reduced, but no separate band is found to occur. The ZnGe acceptor-like states now form a percolating defect band which is significantly wider and hence reaches deeper into the gap. In all cases, the wave functions near the top of the new VBM remain to some extent localized near the ZnGe sites. For a fully random case, the gap is even more severely reduced by almost 3 eV. The total energy of the system increases with the number of octet-rule-violating tetrahedra and the energy cost per exchange defect of order 2 eV is quite high.
Characterizing the crystalline disorder properties of heterovalent ternary semiconductors continues to challenge solid-state theory. Here, a Landau theory is developed for the wurtzite-based ternary semiconductor ZnSnN 2 . It is shown that the symmetry properties of two nearly co-stable phases, with space groups Pmc2 1 and Pbn2 1 , imply that a reconstructive phase transition is the source of crystal structure disorder via a mixture of phase domains. The site exchange defect, which consists of two adjacent antisite defects, is identified as the nucleation mechanism of the transition. A Landau potential based on the spacegroup symmetries of the Pmc2 1 and Pbn2 1 phases is constructed from the online databases in the ISOTROPY software suite and this potential is consistent with a system that undergoes a paraelectric to antiferroelectric phase transition. It is hypothesized that the low-temperature Pbn2 1 phase is antiferroelectric within the c-axis basal plane. The dipole arrangements within the Pbn2 1 basal plane yield a nonpolar spontaneous polarization and the electrical susceptibility derived from the Landau potential exhibits a singularity at the Né el temperature characteristic of antiferroelectric behavior. These results inform the study of disorder in the broad class of heterovalent ternary semiconductors, including those based on the zincblende structure, and open the door to the application of the ternaries in new technology spaces.
Single crystals of Sb2Te3 doped with Cr (cCr=0–6×1020 cm−3) were prepared by the Bridgman method. The measurements of the Hall coefficient reveal a nonmonotonous dependence of hole concentrations on the Cr content in the crystal. The hole concentration decreases at low content of Cr, while at higher content of Cr it increases again. However, according to magnetic measurements, Cr atoms enter the structure and form uncharged substitutional defects CrSb×, which cannot affect the free carrier concentration directly. The observed dependence can be elucidated by means of a point defect model. The model is based on an assumption that defect structure of Sb2Te3 can be treated as hybrid Schottky and antisite defect disorder. Thus, we assume an interaction of CrSb× with the most populated native defects in the structure—antisite defects SbTe−1 and vacancies in the Te sublattice VTe+2.
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