ZnO增强MgO薄膜的次级电子发射性能实验和理论研究

Enhanced Secondary Electron Emission of ZnO-Doped MgO Coatings:An Experimental and Theoretical Study

本研究采用热蒸镀结合低氧压热活化技术,在FTO玻璃表面沉积制备了厚度为200~300 nm的MgO薄膜和MgO-ZnO复合薄膜,比较测试了薄膜的次级电子发射性能。采用扫描电镜、能谱仪、X射线衍射、X射线光电子能谱仪、俄歇电子谱仪表征了薄膜材料的显微形貌、厚度、结构及元素深度分布。测试了薄膜的次级发射系数(δ)随一次电子能量(Ep)和持续电子束轰击时间的变化情况。采用基于密度泛函理论的第一性原理计算分析了掺杂Zn2+的MgO薄膜的电子态密度分布。研究结果表明,相对于MgO薄膜,MgO-ZnO复合薄膜具有更优异的δ和耐电子束轰击稳定性。其在氧压5 Pa、500℃活化30 min后,在Ep为800 eV轰击时最大次级发射系数(δm)高达12.1,且经过Ep为600 eV、一次电子束流20μA持续轰击120 h后δm仍大于11。次级发射性能提升的主要原因是薄膜中ZnO的掺杂降低了MgO的禁带宽度,以及部分Zn单质的存在有效抵制了表面荷电效应。

The microstructures,electronic structures,and secondary electron emission properties of vacuum evaporated MgO and MgO-ZnO films were characterized with conventional surface probes.The influence of the primary electron energy(EP)and e-beam bombardment time on the maximum secondary electron yield(δm)and its stability was evaluated;and the band-gaps and densities of states were calculated in first-principles density functional theory(DFT).The results show that the MgO-ZnO film outperforms MgO film.To be specific,MgO-ZnO film has higherδand better stability under long-time heavy e-beam bombardment than MgO film.For example,bombarded at Ep of 800 eV,the MgO-ZnO film,activated at 5 Pa of O2 and 500℃,had aδm of 12.1,and the bombardment by 20μA e-beam for 120 h merely decreasedδm to^11.The fact that the ZnO-doping reduces the band-gap and Zn-atoms prevent surface charge accumulation may explain the higher and more stableδm.