反射式负电子亲和势GaN光电阴极量子效率衰减机理研究

针对反射式负电子亲和势(NEA) GaN光电阴极量子效率的衰减以及不同波段对应量子效率衰减速度的不同,参照国外给出的NEA GaN光电阴极在反射模式下量子效率曲线随时间的衰减变化情况,利用GaN光电阴极铯氧激活后的表面模型［GaN(Mg):Cs］:O-Cs,结合量子效率衰减过程中表面势垒的变化,研究了反射式NEA GaN光电阴极量子效率的衰减机理. 有效偶极子数量的减小是造成量子效率降低的根本原因,表面I,II势垒形状的变化造成了不同波段对应的量子效率下降速度的不同.     

负电子亲和势GaN光电阴极的研究进展

GaN材料由于其优良的性能,成为紫外探测和真空电子源领域极具发展潜力的材料之一;目前制备的反射式GaN光电阴极的量子效率已达到70%以上,透射式也达到了30%.本文对GaN光电阴极的结构设计、表面清洗和Cs/O激活三大方面进行了综述,分析了影响量子效率的关键因素,并对今后可能的发展方向进行了展望. 关键词： GaN光电阴极 负电子亲和势 量子效率 进展     

负电子亲和势光电阴极量子效率公式的修正

量子效率是表征负电子亲和势（NEA）光电阴极发射性能的最重要的特性参数,准确建立量子效率公式为揭示各个阴极参量对整体性能的影响以及NEA光电阴极的优化设计提供了重要的理论指导.在对NEA光电阴极体内及表面光电发射过程的分析基础上,考虑入射光子能量、表面能带弯曲区以及表面势垒对电子发射的影响,对表面逸出概率和电子衰减长度进行了修正,并利用积分法推导了NEA光电阴极的量子效率公式,其理论预测曲线与实验曲线基本一致,从而验证了修正公式的实用性.为NEA光电阴极的研究提供了有效的理论参考依据. 关键词： 负电子亲和势光电阴极 量子效率 能带弯曲区 衰减长度     

负电子亲和势GaN光电阴极光谱响应特性研究

针对负电子亲和势（NEA） GaN光电阴极成功激活后的量子效率问题,利用自行研制的紫外光谱响应测试仪器,测试了成功激活的NEA GaN光电阴极的光谱响应,给出了230—400 nm波段内反射模式NEA GaN光电阴极的量子效率曲线.测试结果表明：反射模式下NEA GaN光电阴极在230nm具有高达37.4%的量子效率,在GaN光电阴极阈值365 nm处仍有3.75%的量子效率,230 nm和400 nm之间的抑制比率超过2个数量级.文中还结合国外的研究结果,综合分析了影响量子效率的因素. 关键词： 负电子亲和势 GaN光电阴极 光谱响应 反射模式     

负电子亲和势GaN光电阴极激活机理研究

利用自行研制的光电阴极激活评估实验系统,给出了GaN光电阴极Cs激活及Cs/O激活的光电流曲线.针对GaN光电阴极的负电子亲和势（NEA）特性的成因,结合激活过程中光电流变化规律和成功激活后的阴极表面模型,研究了NEA GaN光电阴极激活机理.实验表明：GaN光电阴极在单独导入Cs激活时就可获得明显的NEA特性,Cs/O激活时引入O后光电流的增长幅度不大.用双偶极层模型［GaN（Mg）：Cs］：O—Cs较好地解释了激活成功后GaN光电阴极NEA特性的成因. 关键词： 负电子亲和势 GaN 激活 光电流     

负电子亲和势GaN光电阴极铯吸附机理研究

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.     

反射式GaN光电阴极表面势垒对量子效率衰减的影响

针对反射式GaN光电阴极长波段量子效率衰减较大, 短波段量子效率衰减较小的实验现象, 在考虑谷间散射的情况下, 利用玻尓兹曼分布和基于Airy函数的传递矩阵法, 计算了发射电子能量分布, 分析了表面势垒变化对量子效率衰减的影响, 理论与实验符合较好. 激活层有效偶极子数的减少使表面势垒宽度和高度增加, 引起长波光子激发产生的发射电子能量分布衰减较大, 短波光子激发产生的发射电子能量分布衰减较小, 这是量子效率在长波段衰减较大, 短波段衰减较小的根本原因.     

反射式负电子亲和势GaN光电阴极的光电发射及稳定性研究

利用在线多信息紫外光电阴极激活评估系统,测试了真空室内两个GaN 光电阴极Cs,O激活后及衰减6 h和18 h后补Cs的光谱响应特性曲线和量子效率曲线;并绘制了光纤光源波长为300 nm的光电阴极响应电流衰减变化曲线.实验结果证明,GaN 光电阴极较GaAs阴极具有更好的稳定性,量子效率可保持相对稳定达10 h,然后缓慢衰减,衰减速率较窄禁带半导体材料低得多.补Cs后光电流最大值较刚激活完有16.8%的增长,这充分证明阴极表面量子效率衰减的原因是Cs的脱附,而不是O的吸附.这些现象可由双偶极层模型来解释, 关键词： 光学 光电阴极 量子效率 稳定性     

透射式负电子亲和势GaN光电阴极的光谱响应研究

利用金属有机化合物化学气相淀积(MOCVD)生长了发射层厚度为150 nm、掺杂浓度为1.6×10¹⁷ cm^-3的透射式GaN光电阴极,并在超高真空激活系统中对其进行了激活.通过多信息量测试系统进行了测试,发现透射式负电子亲和势(NEA)GaN光电阴极的量子效率曲线成一个"门"的形状,在255—355 nm波段有较大且平坦的响应,在290 nm处取得最大值为13%,由于AlN缓冲层对短波段光的吸收系数较大,在小于255 nm的波段量子效率出现了下降,当波长大于3 关键词： 透射式 NEA GaN光电阴极 量子效率     

反射式NEA GaN光电阴极量子效率恢复研究

以反射式NEA GaN光电阴极充分激活、衰减以及补Cs后的量子效率曲线为依据,针对阴极量子效率的衰减规律和补Cs后的恢复状况,论述了NEA GaN光电阴极量子效率的衰减和恢复机理.经过重新Cs化处理,反射式NEA GaN光电阴极量子效率在240 nm到300 nm的短波区域恢复到激活后最好状态的94%以上,300 nm到375 nm的长波区域恢复到88%以上.结合反射式NEA GaN光电阴极衰减前后的表面势垒形状和反射式GaN光电阴极量子效率的计算公式,得到了量子效率曲线的衰减规律以及补Cs后的恢复状况与 关键词： 反射式 NEA GaN光电阴极 量子效率     

Quantum efficiency conversion from the reflection-mode GaAs photocathode to the transmission-mode one

In the article, the transmission-mode negative electron affinity GaAs photocathode is compared with the reflection-mode one in structure, working principle and the quantum efficiency equation. The key point to establish relation between the two modes is that the active-layer is in full accord and the biggest difference is that the incident direction of photons is inverse. Based on the relation and difference above, we obtain the electron escape probability P and the spectrum absorption coefficient α of the reflection-mode GaAs photocathode by multilayer film matrix theory. The quantum efficiency of the transmission-mode GaAs photocathode can be estimated through the coefficients P and α of the reflection-mode one. Two groups of samples are analyzed, the one group contains uniform doping reflection-mode and transmission-mode GaAs photocathodes and the other contains exponential doping ones. Through the analysis and curve fitting, we find that the conversion difference of the uniform-doping is about 11.22% while that of the exponential-doping is about 5.46%. Another four groups of samples with exponential-doping active layer are experimentalized and the differences of them are all less than 6%. The results prove the suggested method is feasible and effective and it is more suitable for the conversion of GaAs photocathode with exponential-doping active layer.     

Study on the sensitivity decrease of reflection-mode GaN NEA photocathode

The experiment result discloses that the sensitivity of the reflection-mode NEA GaN photocathodes decrease with time after activation. Through analyzing the experiment result, it is found that the changes of the element content in the surface of the cathodes play an important role in the decrease course of the sensitivity. In order to explain the above result, a surface model of the reflection-mode NEA GaN photocathodes is put forward. By using this model, the inner mechanism leading to this result is that the dipoles direction changes because the harmful gases are adsorbed in the surface of the photocathodes.     

Photoemission of graded-doping GaN photocathode

We study the photoemission process of graded-doping GaN photocathode and find that the built-in electric fields can increase the escape probability and the effective diffusion length of photo-generated electrons,which results in the enhancement of quantum efficiency.The intervalley scattering mechanism and the lattice scattering mechanism in high electric fields are also investigated.To prevent negative differential mobility from appearing,the surface doping concentration needs to be optimized,and it is calculated to be 3.19×10 17 cm 3.The graded-doping GaN photocathode with higher performance can be realized by further optimizing the doping profile.     

Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoemission behaviour, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes have been investigated using a multi-information measurement system. The activation experiment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs-O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.