负电子亲和势GaN光电阴极铯吸附机理研究

Cs Adsorption Mechanism for Negative Electron Affinity GaN Photocathode

为了得到铯吸附与阴极电子亲和势变化之间的定量关系,利用NEA光电阴极激活评估实验系统对GaN光电阴极进行了铯激活.根据半导体光电发射理论和双偶极层模型,通过对电子亲和势随铯覆盖度变化的实验结果进行拟合运算,得到电子亲和势与铯覆盖度之间的函数关系式.分析了铯的吸附机理,得到激活过程中铯的吸附过程与GaN材料有效电子亲和势下降之间的关系.实验表明:负电子亲和势GaN光电阴极材料在铯激活时光电流随着铯覆盖度的增加而从本底值增为极大值,激活过程中GaN电子能量分布曲线低动能截止点的位置决定于铯的覆盖度.当铯的覆盖度从0、1/2、2/3到1个单层变化时,低动能截止点依次向左移动,当覆盖度从0增加到1个单层时,低动能截止点向左移动了约3eV的距离.研究表明,低动能截止点左移本质上是由于对电子逸出起促进作用的有效偶极子[GaN(Mg):Cs]数量的增多造成的,有效偶极子数量的增多带来了材料表面真空能级的下降.

In order to get the quantitative relationship between Cs adsorption and cathode electron affinity variation,GaN photocathode was activated with Cs using the experimental system for activating and evaluating NEA photocathode.According to the theory of semiconductor photoemission and the double dipole layer model,the electron affinity changes with the Cs coverage degree,the function relation between electron affinity and Cs coverage degree were gotten.Cs adsorption mechanism was analyzed,the relationship between the Cs adsorption course and the decrease of effective electron affinity was gotten.The experiment results show：during the course of activation with Cs for negative electron affinity GaN photocathode material,the photocurrent increases from the background value to the maximum according to Cs coverage degree.The position of low kinetic energy cutoff in the electron energy distribution curve is decided by Cs coverage degree during the activation process.When Cs coverage degree varies from 0,1/2,2/3to 1monolayer,the position of low kinetic energy cutoff shifts to the left in turn.The position of low kinetic energy cutoff will shift to the left about 3eV when Cs coverage degree increases from 0to 1monolayer.The reason of the left shifting is the quantity increase of the dipole[GaN（Mg）：Cs]that help the electrons to escape into the vacuum.The quantity increase of the dipole results in the decrease of vacuum energy level of the surface.