SrTiO_3薄膜氧缺陷的第一性原理研究

First-principle study on SrTiO_3 film oxygen imperfection

在广义梯度近似(GGA)下,利用超软赝势对真空条件下SrTiO3超晶胞的体系能量、原子间电子云重叠布局数和电子态密度等进行了自恰计算.结果显示,对有氧缺陷的超晶胞优化后,晶格参数的几何平均值增大了2.711%,这表明在高温条件下外延生长STO薄膜时,产生了氧缺陷,并且氧空位易处于薄膜表层;另外,表层氧缺陷使表层Ti原子明显的发生偏心位移,两个Ti原子的核间距由0.3905 nm增大至0.4234 nm,b轴上的氧原子则向中心靠近了0.0108 nm、并沿c轴方向上突了0.0027 nm,使STO晶体产生自发极化,氧缺陷还使STO的电子态密度的能隙增宽了1.75 eV,达到2.48 eV,从而使STO晶体由顺电相转向铁电相.

Based on General Gradient Approximation （GGA）, ultrasoft pseudopentials plane wave method has been applied to calculate the system energy, electron density of state, overlap populations of electron cloud between atoms of SrTiO3 under vacuum. The self-consistent calculation＇s results show that the geometry average value of crystal lattice parameter increased by 2.711 percent, after optimization of SrTiO3 having oxygen vacancy. It demonstrates that oxygen vacancy is produced and it lies in the film＇s surface easily, when STO film grows by molecular beam epitaxy （MBE） under high temperature. The oxygen vacancy results in the obvious off-center displacement of Ti atoms lying in the film surface, the nuclear distance between two Ti atoms increasing from 0. 3905 nm to 0. 4234 nm, the oxygen atoms along b axis closing up to the center by 0. 0108 nm and sticking up by 0. 0027 nm along c axis, which makes STO crystal occur spontaneous polarization. Moreover, the oxygen imperfection makes the energy gap of STO＇s electron density of state reach 2.48 eV with 1.75 eV augment, which results in the STO crystal＇s phase transformation from paraelectric phase to ferroelectric phase.