Ag-Cr共掺LiZnP新型稀磁半导体的光电性质

采用基于密度泛函理论的第一性原理平面波超软赝势法,对纯Li Zn P, Ag/Cr单掺和Ag-Cr共掺Li Zn P新型稀磁半导体进行了结构优化,计算并分析了掺杂体系的电子结构、磁性、形成能、差分电荷密度和光学性质.结果表明:非磁性元素Ag单掺后,材料表现为金属顺磁性;磁性元素Cr单掺后, sp-d杂化使态密度峰出现劈裂,体系变成金属铁磁性;而Ag-Cr共掺后,其性质与Ag和Cr单掺完全不同,变为半金属铁磁性,带隙值略微减小,导电能力增强,同时形成能降低,原子间的相互作用和键强度增强,晶胞的稳定性增强.通过比较光学性质发现,掺杂体系的介电函数虚部和光吸收谱在低能区均出现新的峰值,且当Ag-Cr共掺时介电峰峰值最高,同时复折射率函数在低能区发生明显变化,吸收边向低能方向延展,体系对低频电磁波吸收加强.

Photoelectric properties of Ag and Cr co-doped LiZnP new diluted magnetic semiconductors

Spintronic devices utilize the electron charge and spin degree of freedom to achieve novel quantum functionalities. Diluted magnetic semiconductors (DMS) constitute an important category of spintronic materials that have the potential to be successfully incorporated into the existing semiconductor industry. The prototypical DMS (Ga,Mn) As, discovered in the 1990s, accomplishes spin and charge doping simultaneously through the heterovalent substitution of the magnetic ion Mn²⁺ for Ga³⁺. Two challenges have presented themselves in this material. First, the heterovalent nature of this “integrated spin/charge” doping results in severely limited chemical solubility in (Ga,Mn) As, restricting specimen fabrication to metastable thin films by molecular beam epitaxy; second, the simultaneous spin and charge doping precludes the possibility of individually tuning the spin and charge degree of freedom. A new type of ferromagnetic DMS based on I-Ⅱ-V group can overcome both of these challenges. Li(Zn,Mn) As utilizes excess Li concentration to introduce hole carriers, while independently making the isovalent substitution of Mn²⁺ for Zn²⁺ in order to achieve local spin doping. With no heterovalent substitution to restrict chemical solubility, bulk samples of Li(Zn,Mn) As are successfully fabricated. However, one drawback of Li(Zn,Mn) As is its use of the toxic element As. The isostructural direct-gap semiconductor LiZnP also undergoes a ferromagnetic transition upon Mn doping, and its bulk magnetic properties are very similar to those of LiZnAs. In this paper, the geometric structure of pure LiZnP, Ag doped, Cr doped, and Ag-Cr co-doped LiZnP new diluted magnetic semiconductor are optimized by using the first-principles plane wave ultra-soft pseudo-potential technology based on the density function theory. Then we calculate the electronic structure, magnetism, formation energy, differential charge density, and optical properties of the doped systems. The results show that the material is a paramagnetic metal after single doping of the nonmagnetic element Ag. When magnetic element Cr is doped with LiZnP, sp-d orbital hybridization makes the peak of density of state nearly E_F-split, leading the system to become metallic ferromagnetism. However, Ag-Cr co-doped LiZnP changes into half-metallic ferromagnetism, which is completely different from the single doping system. The band gap decreases slightly, and the electrical conductivity is enhanced. Meanwhile, the formation energy of the system becomes lower, the bond between atoms strengthens, and the stability of the unit cell becomes stronger. A comparison of the optical properties indicate that the imaginary part of dielectric function and the optical absorption spectrum both present new peaks in low energy region in the doped systems. Ag-Cr co-doped LiZnP has the highest dielectric peak. Meanwhile, the complex refractive index function changes obviously in a low energy region, and the absorption edge extends to the low energy direction. The system enhances the absorption of low-frequency electromagnetic waves.