Al掺杂对Mg_2Si介电性质影响的第一性原理研究

First principles study on dielectric properties of Al-doped Mg_2Si

采用基于密度泛函理论的第一性原理平面波超软赝势方法,计算了本征Mg_2Si和Al掺杂Mg_2Si的形成能、电子结构和介电函数.结果表明,Al掺杂Mg_2Si后,Al以替位杂质(Al替Mg位,即AlMg)或填隙杂质(Al位于晶胞间隙位,即Ali)的形式进入Mg_2Si晶格.费米能级进入导带,体系呈n型导电.掺杂后体系的介电函数的实部和虚部在低频时比未掺杂时均增大,主要是由于晶格中的施主杂质Al离子束缚着附近的过剩电子,在外加交变电场的作用下,束缚在Al离子周围的电子要克服一定的势垒不断地往复运动造成松弛极化和损耗.另外,掺杂体系相对于未掺杂体系,介电函数的虚部在0.5 eV附近出现了一个额外的介电峰,该峰主要是由电子从价带跃迁到Al杂质能级引起的.计算结果为Mg_2Si基光电子器件的设计和应用提供了理论依据.

The electronic structures and dielectric properties of intrinsic and Al-doped Mg2Si have been investigated by the first principle plane-wave pseudopotential based on the density function theory.The calculated formation energy differences indicate that the most probable location of Al dopant is in magnesium sublattice,but one cannot rule out the interstitial position,which is only slightly less probable.The calculations of density of states(DOS)show that the Fermi level of Al-doped Mg2Si shifts into the conduction band and the system is n-type,which is consistent with the experimental observation.The calculated dielectric functions reveal that both the real part and the imaginary part of permittivity of the systems are increased with Al-doping,which is attributed that the electrons bounded to Al ions need to overcome a certain barriers during the reciprocating motion in alternating electric field.Besides the main dielectric imaginary peak(at2.71eV),an additional small peak(at0.5eV)emerges after Al-doping,which is ascribed to the transition of electrons from valence band to impurity level related to Al.The results provide the theoretical reference for the design and application of the Mg2Si-based optoelectronic devices.