Das Anish, Kumar Aparabal, Banerji P
Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India.
J Phys Condens Matter. 2020 Jun 17;32(26):265501. doi: 10.1088/1361-648X/ab7ad8.
The electronic structure and thermoelectric transport in SnSe and its alloy with CuSe have been studied using the first principles technique and semi classical Boltzmann transport theory. Our study reveals that SnSe is p-type with indirect band gap of 0.66 eV, while the alloy is phase separated and n-type with negligible indirect band gap of 0.064 eV. In both cases, two fold degeneracy in band extrema have been observed within the range of 25 meV. Delocalization of Se lone pair has been observed due to Cu substitution in Sn sites, which is supposed to lower its lattice thermal conductivity. A chemical potential map has been generated obeying thermodynamic restrictions to predict the possible existence of secondary phases. Our study shows the existence of SnSe as a secondary phase, while the possibility of CuSe as a secondary phase is negligible due to its higher formation energy. We calculated the transport coefficients as a function of carrier concentration and temperature to understand the range of optimized thermoelectric performance. The transport coefficients are similar along in plane direction whereas significant deviation is observed along the cross plane direction due to anisotropy in effective masses in SnSe. The effective masses are more isotropic in alloy than SnSe, thus transport properties show less anisotropy along three directions. Significant contribution of bipolar transport is observed in SnSe, while that is not noticed in the alloy. The behaviors of the Seebeck coefficients in both cases are discussed in terms of Mott's theory and density of states modification near Fermi energy. Electron mobilities limited by acoustic phonon, ionized impurities, alloy scattering and inter carrier scattering have been examined relying on deformation potential approach and effective mass theory. The results indicate that acoustic phonon scattering is dominant scattering mechanism in SnSe over inter carrier scattering, whereas for the alloy the former contribute very weakly. Ionized impurity scattering and inter carrier scattering are most dominant in the alloy. Alloy scattering with U = 2 eV also contribute significantly.
利用第一性原理技术和半经典玻尔兹曼输运理论，对SnSe及其与CuSe的合金的电子结构和热电输运进行了研究。我们的研究表明，SnSe为p型，间接带隙为0.66 eV，而该合金发生相分离，为n型，间接带隙可忽略不计，为0.064 eV。在这两种情况下，在25 meV范围内均观察到能带极值处的二度简并。由于Cu替代了Sn位点，观察到Se孤对的离域化，这被认为会降低其晶格热导率。根据热力学限制生成了化学势图，以预测次生相的可能存在。我们的研究表明，SnSe作为次生相存在，而由于其形成能较高，CuSe作为次生相的可能性可忽略不计。我们计算了作为载流子浓度和温度函数的输运系数，以了解优化热电性能的范围。沿平面内方向的输运系数相似，而由于SnSe中有效质量的各向异性，沿平面外方向观察到显著偏差。合金中的有效质量比SnSe更具各向同性，因此沿三个方向的输运性质表现出较小的各向异性。在SnSe中观察到双极输运的显著贡献，而在合金中未观察到。根据莫特理论和费米能级附近的态密度修正，讨论了两种情况下塞贝克系数的行为。依靠形变势方法和有效质量理论，研究了受声学声子、电离杂质、合金散射和载流子间散射限制的电子迁移率。结果表明，在SnSe中，声学声子散射是主导散射机制，超过了载流子间散射，而对于合金，前者的贡献非常微弱。电离杂质散射和载流子间散射在合金中最为主导。U = 2 eV的合金散射也有显著贡献。
