Observations of Electron Emission sites on Alumina Surface by using a Photoelectron Emission Microscope

用光电子发射显微镜（PEEM）观察了氧化铝表面的电子发射点。沿面闪络是电力设备小型化的一个障碍，因为绝缘子表面的沿面闪络电压远低于相同间隙长度的间隙击穿电压。沿面闪络主要是由电子发射从三结和随后的电子发射从绝缘子表面。绝缘体表面的电子发射特性至今还没有得到很好的研究。在本项目中，观察到了紫外光激发的光电子发射点。紫外光辐照的最大能量约为11 eV，大于氧化铝的禁带宽度，观察到氧化铝表面的光电子发射点是局域的（直径<几μm），发射不是连续的，而是间歇的。这种电子发射模式与金属的电子发射模式完全不同，因为电子发射位置在激发光照射的区域上扩展，并且电子连续发射。观察还发现，比氧化铝带隙能量低的激发光也能激发光生驻极体发射。该结果确认了在带隙中形成了由晶体缺陷产生的能带，但需要根据带电性、定域位置的元素分析和电子状态等，进一步进行实验来解释绝缘体表面的电子发射特性。

Electron emission sites on alumina surfaces have been observed by a photoelectron emission microscope (PEEM). Surface flashover is an obstacle to minimize the size of electric power devices, since surface flashover voltage on an insulator surface is fairy lower than that of gap breakdown voltage of the same gap lemgth. Surface flashover is dominated by electron emission from triple junctions and subsequent electron emission from the insulator surface. Electron emission properties from insulator surface have not been clarified yet. In this project, photoelectron emissionsites excited by ultraviolet light were observed. Maximum energy of the irradiated ultraviolet light was about 11 eV, which is greater than the band gap of alumina.Observations revealed that photoelectron emission sites on alumina surface were localized (diameter < few μm) and the emission took place not continuously, but intermittently. This electron emission schem is quite different from that from metals, since the electron emission site expands over the who ; e area where excitation light was irradiated and electrons are emitted continuously. Also revealed by the observations was that lower energy of excitation light that that of the band gap of alumina could excite phoelectreon emission. This result confirms that energy bands created by crystal defects are formed in the band gap.Further experiments to explain these electron emission properties from an insulator surface should be performed, on the basis of charging property, elemental analysis at the localized sites, and electronic states.