H,Cl和F原子钝化Cu_2ZnSnS_4(112)表面态的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法系统研究了Cu_2ZnSnS_4体相的晶格结构、能带、态密度及表面重构与H,Cl和F原子在Cu_2ZnSnS_4(112)表面上的吸附和钝化机理.计算结果表明:表面重构出现在以金属原子Cu-Zn-Sn终止的Cu_2ZnSnS_4(112)表面上,并且表面重构使表面发生自钝化;当单个H,Cl或F原子吸附在S原子终止的Cu_2ZnSnS_4(112)表面上时,相比于桥位(bridge)、六方密排(hcp)位和面心立方(fcc)位点,三种原子均在特定的顶位(top)吸附位点表现出最佳稳定性.当覆盖度为0.5 ML时,无论H,Cl还是F原子占据Cu_2ZnSnS_4(112)表面的2个顶位均具有最低的吸附能.以S原子终止的Cu_2ZnSnS_4(112)表面在费米能级附近的电子态主要由价带顶部Cu-3d轨道和S-3p轨道电子贡献,此即表面态.当H,Cl或F原子在表面的覆盖度达0.5 ML时,费米能级附近的表面态降低,其中H原子钝化表面态的效果最佳,Cl原子的效果次之,F原子的效果最差.表面态降低的主要原因在于吸附原子从S原子获得电子致使表面Cu原子和S原子在费米能级处的态密度峰几乎完全消失.

First-principles study of H,Cl and F passivation for Cu2ZnSnS4(112) surface states

The first-principles calculation method is used to systematically investigate the lattice structure, energy band, density of states of the bulk Cu₂ZnSnS₄, surface reconstruction, and mechanism of adsorption and passivation of F, Cl and H atoms on Cu₂ZnSnS₄ (112) surface. We find that the surface reconstruction occurs on the Cu-Zn-Sn-terminated Cu₂ZnSnS₄ (112) surface and this reconstruction introduces surface self-passivation. By analyzing the partial density of states of the atoms on the S-terminated Cu₂ZnSnS₄ (112) surface, it can be seen that surface states near the Fermi level are mainly contributed by 3d orbitals of Cu atoms and 3p orbits of S atoms at the top of the valence band. When a single F, Cl or H atom is adsorbed on the S-terminated Cu₂ZnSnS₄ (112) surface, all three kinds of atoms exhibit an optimal stability at a specific top adsorption site in comparison with at the bridge, hcp and fcc sites. And this top position is also the position of the S atom that has the greatest influence on the surface states. When two atoms of the same kind are adsorbed on the surface, H, Cl or F atoms occupy the top sites of two S atoms that cause surface states on the Cu₂ZnSnS₄ (112) surface, which have the lowest adsorption energy. And the surface states near the Fermi level are partially reduced. Therefore, two S atoms that cause the surface states are the main targets of S-terminated Cu₂ZnSnS₄ (112) surface passivation. It has also been found that the passivation effect of H atom for surface states is the most significant and the effect of Cl atom is better than that of F atom. Comparing the partial density of states, the Bader charge and the differential charge of the atoms before and after adsorption, we find that the main reason for the decrease of the surface states is that the adsorption atoms obtain electrons from the S atoms, and the state density peaks of the Cu and S atoms at the Fermi level almost disappear completely. In the surface model, the F atom obtains the same number of electrons from the two S atoms, while the two S atoms have different effects on the surface states. And the H and Cl atoms obtain fewer electrons from the S atoms, that have less influence on the surface states. It may be the reason why the passivation effect of F atom is slightly less than that of H and Cl atoms.