Ce和S共掺杂SnO_(2)材料导电性能的第一性原理研究

First Principle Study of Electrical Properties of Ce and S Co-Doped SnO_(2)

本文利用密度泛函理论的第一性原理平面波超软赝势法计算了Ce和不同浓度S元素共掺杂后SnO_(2)的电子结构,定性分析了各掺杂模型的晶体结构、能带结构、态密度和电荷布局,定量计算了相对电导率。分析结果表明:随着S原子掺杂浓度的增大,掺杂体系的形成能不断增大,稳定性也随之降低;与Ce单掺相比,共掺后能带整体向费米能级方向移动,能带曲线更紧密,禁带宽度进一步减小,载流子跃迁所需的能量进一步降低;共掺后费米能级附近的态密度有所增大,这是因为S原子轨道和Sn和Ce原子轨道发生杂化,电子转移加剧,价带顶部被S-3p轨道占据,提供了更多载流子;同时共掺使材料的电荷密度分布改变,共价性减弱,离子性增强;共掺后,自由电子浓度明显增大,电子迁移率明显提高,电导率也明显优于单掺体系;与Ce-2S掺杂结构相比,Ce-S掺杂结构的自由电子迁移率更高,电导率也更高,说明Ce-S共掺结构的导电性能更佳。

AgSnO_(2)was a new type of contact material,which not only had the characteristics of non-toxic and harmless,but also had good anti-welding and anti-wear properties.However,under the action of long-term arc erosion,SnO_(2)enriched areas would be generated on the surface of the material.Since SnO_(2)was a wide-bandgap semiconductor with poor electrical conductivity,the poor electrical conductivity of the enriched region led to an increase in the contact resistance of the composite material and a decrease in the overall electrical conductivity.At present,many studies had confirmed that Ce or S doping SnO_(2)could effectively improve its electrical conductivity,but there was no report on the co-doping of SnO_(2)with Ce and S.Therefore,the first-principles approach based on density functional theory was used to study the electronic structure and electrical conductivity of SnO_(2)co-doped with Ce and S.1×2×2 SnO_(2)supercell was built by Materials Studio software,which contained 24 atoms,including 8 Sn atoms and 16 O atoms.Doping adopted the method of atomic substitution.The Sn atom at the center of the body was replaced by a Ce atom,and O atoms closest to Ce atom was replaced by S atoms.The doping ratio of S in Ce-S model was 6.25%,and the doping ratio of S in Ce-2S model was 12.5%.All doping systems were geometrically optimized to make the supercell reach the most stable state.The electronic structure of SnO_(2)after co-doping of Ce and S with different concentrations was calculated by CASTEP module.The crystal structure,energy band structure,density of states and charge layout of each doping model were qualitatively analyzed,and the relative conductivity was quantitatively calculated.The results showed that as the concentration of S atoms increased,the formation energy of the doping system increased gradually,the difficulty of the impurity atoms entering the unit cell increased,and the overall stability became worse.In intrinsic SnO_(2),the highest point of the valence band and the lowest point of the conduction band were both in G point direction of Brillouin zone,which indicated that the transition mode of electrons was direct transition.The band gap of the energy band was narrowed after doping,which indicated that the energy required for the electron transition was lower.Compared with Ce single doping,the overall energy band was closer to the Fermi level after co-doping,and the energy band curve was tighter,the forbidden band width was further reduced,and the energy required for carrier transition was lower.This was affected by S-3p state.In the co-doped system,when the doping concentration of S was 6.25%,the energy band gap value was 0.527 eV,which was lower than that of Ce single-doped system.When the doping concentration of S was 12.5%,the band gap value was reduced to 0.512 eV,and the reduction range was significantly reduced.This showed that the incorporation of S atoms improved the ability of Ce atoms to reduce the band gap,but this improvement was limited.Because the doping atoms introduced more valence electrons into the system,the overall density of states of the doped system increased.The conduction band moved toward Fermi level,which narrowed the band gap,reduced the energy required for carrier transition,and improved the electrical conductivity.The density of states near Fermi level increased after co-doping.This was because S atomic orbitals were hybridized with Sn and Ce atomic orbitals,and the electron transfer was intensified.The top of the valence band was occupied by S-3p orbital,which provided more carriers.In the co-doped system,ionic covalent Ce-S bonds were generated.With the increase of the doping ratio of S atoms,the charge overlaps between Ce and S atoms intensified,and the number of charges overlap layouts between Sn-O bonds,Ce-O bonds and Sn-S bonds decreased.This showed that the ionicity of the system was enhanced,and the covalency of the system was weakened,so the metallicity of the co-doped system was stronger and the conductivity was further improved.Compared with Ce single-doped structure,the free electron concentration,relative mobility and relative conductivity were significantly improved after co-doping.However,as the concentration of S atoms increased,the conductivity showed a downward trend.This showed that although the co-doping of Ce and S could improve the electrical conductivity of SnO_(2),the ability to improve was limited.When the concentration of S was too large,it would inhibit the electrical conductivity of the system.In conclusion,the electrical conductivity of Ce-S co-doped structure was better.It provided a new idea and theoretical basis for the doping research of AgSnO_(2)contact materials.