体空位缺陷对氧化铝二次电子发射特性的影响分析

Analysis of effect of bulk vacancy defect on secondary electron emission characteristics of Al_(2)O_(3)

基于第一性原理和蒙特卡罗模拟方法,系统地研究了氧化铝晶体内部O空位缺陷和Al空位缺陷对二次电子发射特性的影响.密度泛函计算结果表明,空位缺陷会导致能带结构发生改变,其中Al空位缺陷的存在使得禁带宽度变窄,费米能级降低至价带内部.在此基础之上,获得了不同晶体结构下的弹性和非弹性平均自由程.氧化铝中存在Al空位缺陷时的弹性平均自由程最大,而存在O空位缺陷时的非弹性平均自由程最大.为了分析不同缺陷浓度下的二次电子发射特性,对已有蒙特卡罗模拟算法进一步优化.模拟结果表明,随着O空位和Al空位缺陷占比的增加,最大二次电子发射系数随之而下降.相比于Al空位缺陷,相同缺陷占比下O空位缺陷导致二次电子发射系数降低更多.

Based on the combination of the first-principles and Monte Carlo method,the effect of vacancy defect on secondary electron characteristic of Al_(2)O_(3) is studied in this work.The density functional theory(DFT)calculation results show that the band structure changes when the vacancy defects exist.The existence of Al vacancy defects results in a decrease in band gap from 5.88 to 5.28 eV,and in Fermi level below the energy of the valence band maximum as well.Besides,the elastic mean free paths and inelastic mean free paths of electrons in different crystal structures are also obtained.The comparison shows that the inelastic mean free path of electrons in Al_(2)O_(3) with O vacancy defects is much larger than those of Al_(2)O_(3) without defects and Al_(2)O_(3) with Al vacancy defects.When the energy of electrons is smaller than 50 eV,the inelastic mean free path of electrons in Al_(2)O_(3) without defects is longer than that in Al_(2)O_(3) with Al vacancy defects.The elastic mean free path of electrons slightly increases when the vacancy defects exist,and the elastic mean free path of electrons in Al_(2)O_(3) with Al vacancy defects is the largest.In order to investigate the secondary electron emission characteristics under different vacancy defect ratios,an optimized Monte Carlo algorithm is proposed.When the ratio between O vacancy defect and Al vacancy defect increases,the simulation results show that the maximum value of secondary electron yield decreases with the ratio of vacancy defect increasing.The existence of O vacancy defects increases the probability of inelastic scattering of electrons,so electrons are difficult to emit from the surface.As a result,comparing with Al vacancy defect,the SEY of Al_(2)O_(3) decreases greatly under the same ratio of O vacancy defect.