反射式变掺杂负电子亲和势GaN光电阴极量子效率研究

Quantum efficiency for reflection-mode varied doping negative-electron-affinity GaN photocathode

从变掺杂负电子亲和势(NEA)Ga N光电阴极材料的光电发射机理入手,给出了反射式变掺杂NEA Ga N光电阴极内建电场和量子效率的计算公式.利用初步设计的变掺杂NEA Ga N光电阴极,介绍了变掺杂NEA Ga N阴极的激活过程和激活光电流的变化特点.结合国内外典型的变掺杂NEA Ga N阴极的量子效率曲线,分析了Ga N光电阴极量子效率曲线的特点.结果显示:由于内建电场的存在,反射式变掺杂NEA Ga N光电阴极量子效率在240 nm处即可达到56%,在较宽的入射光波长范围内,阴极具有相对平稳的量子效率,量子效率值随入射光子能量的增加而增加,并且量子效率曲线在阈值附近表现出了明显的锐截止特性.

As a new kind of ultraviolet photocathode material,the negative-electron-affinity（NEA） GaN photocathode needs to further improve its photoemission performance and the stable performance in practical applications.Under the limit of GaN photocathode material growth level,how to further improve the quantum efficiency of cathode is an important problem.The varied doping technology can help to solve the problem under such circumstances.According to the photoemission mechanism of varying doping NEA GaN photocathode material,the built-in electric field formulas and the quantum efficiency formulas for reflection-mode varied doping NEA GaN photocathode are given.The preliminary structure of varied doping NEA GaN photocathode is designed.The varied doping material sample is divided into four layers according to the doping concentration.Using the self-developed experimental equipment,the varied doping GaN photocathode sample is activated with Cs/O.The activation process and the change characteristics of photocurrent for varied doping NEA GaN photocathode are discussed.At the beginning,the photocurrent is increased steady with the introduction of Cs,then the ＂Cs kill＂ phenomenon appears in the presence of excessive Cs.After the introduction of O,the photocurrent value starts to rise again.The spectral response of varied doping GaN photocathode is tested in situ after activation,and the quantum efficiency values ranging from 240 nm to 354 nm are obtained.On the basis of the obtained experimental results of quantum efficiency,combining to the typical quantum efficiency curve from University of California,the characteristics of quantum efficiency curves are analyzed.The results show that the quantum efficiency value for reflection-mode varied doping NEA GaN photocathode can reach 56% at 240 nm because of the built-in electric field,yet the quantum efficiency maximum value for uniform doping GaN photocathode is only37% at 230 nm.The tested quantum efficiency maximum value of varied doping NEA GaN photocathode is improved much more than that of the uniform doping GaN photocathode.In a wider range of the incident light wavelength,the quantum efficiency of varied doping NEA GaN photocathode is relatively stable,and the excellent properties of varied doping GaN photocathode are confirmed.The reason why the value of quantum efficiency decreases with the increase of incident light wavelength is given.First,the photon energy decreases with the increase of incident light wavelength.Second,the incident light is absorbed from the front surface of cathode for reflection mode.In addition,the quantum efficiency curves of varied doping GaN photocathode show obvious sharp cut-off characteristics near the threshold,and the sharp cut-off characteristic is necessary for high detection sensitivity.The property of negative electron affinity for varied doping GaN cathode material after successful activation is also proved by the sharp cut-off feature.