Comparative research on the transmission-mode GaAs photocathodes of exponential-doping structures

Early research has shown that the varied doping structures of the active layer of GaAs photocathodes have been proven to have a higher quantum efficiency than uniform doping structures. On the basis of our early research on the surface photovoltage of GaAs photocathodes, and comparative research before and after activation of reflection-mode GaAs photocathodes, we further the comparative research on transmission-mode GaAs photocathodes. An exponential doping structure is the typical varied doping structure that can form a uniform electric field in the active layer. By solving the one-dimensional diffusion equation for no equilibrium minority carriers of transmission-mode GaAs photocathodes of the exponential doping structure, we can obtain the equations for the surface photovoltage (SPV) curve before activation and the spectral response curve (SRC) after activation. Through experiments and fitting calculations for the designed material, the body-material parameters can be well fitted by the SPV before activation, and proven by the fitting calculation for SRC after activation. Through the comparative research before and after activation, the average surface escape probability (SEP) can also be well fitted. This comparative research method can measure the body parameters and the value of SEP for the transmission-mode GaAs photocathode more exactly than the early method, which only measures the body parameters by SRC after activation. It can also help us to deeply study and exactly measure the parameters of the varied doping structures for transmission-mode GaAs photocathodes, and optimize the Cs-O activation technique in the future.     

Comparative research for transmission-mode GaAs photocathodes of different doping structures on surface photovoltage

In order to well study the internal body performance for transmission-mode GaAs photocathode of different varied doping structures, two GaAs photocathodes of exponential doping structure and gradient doping structure were designed respectively. Because surface photovoltage spectrum has close relation with the internal properties of GaAs photocathodes, the connection between surface photovoltage and internal electronic field was well discussed through deduction and calculation. The difference of two structures and the value of internal electronic energy were exactly calculated and verified by experiments. The internal band bending energy could form an internal electronic field with the same direction, which could help the photo-excited electrons to move toward surface barrier layer. This research shows a better method to well study the varied doping structures for GaAs photocathode materials and will help to improve the growth structure for transmission-mode GaAs photocathode module in the future.     

Comparative research on activation technique for GaAs photocathodes

The properties of GaAs photocathodes mainly depend on the material design and activation technique. In early researches, high-low temperature two-step activation has been proved to get more quantum efficiency than high-temperature single-step activation. But the variations of surface barriers for two activation techniques have not been well studied, thus the best activation temperature, best Cs–O ratio and best activation time for two-step activation technique have not been well found. Because the surface photovoltage spectroscopy (SPS) before activation is only in connection with the body parameters for GaAs photocathode such as electron diffusion length and the spectral response current (SRC) after activation is in connection with not only body parameters but also surface barriers, thus the surface escape probability (SEP) can be well fitted through the comparative research between SPS before activation and SEP after activation. Through deduction for the tunneling process of surface barriers by Schrödinger equation, the width and height for surface barrier I and II can be well fitted through the curves of SEP. The fitting results were well proved and analyzed by quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (ADXPS) which can also study the surface chemical compositions, atomic concentration percentage and layer thickness for GaAs photocathodes. This comparative research method for fitting parameters of surface barriers through SPS before activation and SRC after activation shows a better real-time in system method for the researches of activation techniques.     

发射层厚度对透射式GaAs光电阴极表面光电压谱的影响

通过求解一维稳态少子扩散方程,推导了含有后界面复合速率和发射层厚度的透射式GaAs光电阴极表面光电压谱理论方程.通过对发射层厚度分别为1.6 μm和2.0 μm,掺杂浓度为1×1019 cm－3的GaAs透射式阴极样品测试,理论曲线和实验曲线基本一致.通过引入表面光电压谱积分灵敏度公式,仿真探讨了表面光电压谱在一定体材料参量条件下,积分灵敏度受发射层厚度的影响|发现在体材料参量一定条件下,透射式GaAs光电阴极具有最佳厚度,同时最佳厚度受后界面复合速率的影响更大,同时GaAlAs窗口层也能很好降低发射层后界面复合速率.     

指数掺杂结构对透射式GaAs光电阴极量子效率的影响研究

通过在一维连续性方程光电子产生函数项中加入短波约束因子,修正了指数掺杂和均匀掺杂透射式GaAs光电阴极量子效率公式.利用修正的透射式阴极量子效率公式分别拟合制备的指数掺杂和均匀掺杂透射式阴极量子效率实验曲线,符合得很好.另外拟合得到的阴极性能参数表明,由于内建电场的作用,指数掺杂阴极的性能要好于均匀掺杂阴极,指数掺杂结构能够明显提高透射式阴极的量子效率. 关键词： 透射式光电阴极 指数掺杂 量子效率 内建电场     

透射式变掺杂GaAs光电阴极研究

为了将变掺杂GaAs材料应用于微光像增强器,开展了透射式变掺杂GaAs光电阴极实验研究,制备了2种反转结构透射式变掺杂GaAs光电阴极。测试了玻璃粘接前后GaAs光电阴极载流子浓度变化,发现高温粘接后载流子浓度增加现象。通过测试高温激活的透射式变掺杂GaAs光电阴极发现,在450 nm～550 nm波段内,变掺杂GaAs光电阴极仍然具有较高的光谱响应。     

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.     

指数掺杂GaAs光电阴极量子效率的理论计算

将GaAs光电阴极发射层掺杂浓度由体内到发射表面从高到低的进行指数掺杂，能在发射层形成一个恒定的内建电场，有利于光电子的逸出.在考虑内建电场的作用下，通过建立和求解少数载流子所遵循的一维连续性方程，得到了反射式和透射式指数掺杂阴极的量子效率公式，并利用这些公式对其量子效率进行了理论计算和仿真.计算结果显示发射层指数掺杂能较明显的提高阴极的量子效率，与均匀掺杂阴极相比，能使反射式阴极积分灵敏度提高约20%，透射式阴极提高30%以上.指数掺杂提高阴极量子效率的主要原因与内建电场有关，光电子在内建电场作用下以扩散加漂移的方式到达阴极表面，从而减小了后界面复合速率对阴极的影响，同时提高了阴极的等效电子扩散长度. 关键词： 指数掺杂 内建电场 能带结构 量子效率     

具有e指数内建电场的透射式GaAs光电阴极响应特性的理论分析

设计了具有e指数内建电场的透射式GaAs负电子亲和势阴极,利用数值计算方法研究了它的时间响应特性和量子效率特性。结果表明,当吸收区厚度L~0.2~1.5 μm时,阴极的响应时间和量子效率均随L的增大而增大;尤其当L~1.1 μm时响应时间达到10 ps,量子效率达到12.5%~20%,迄今为止,与其他GaAs光电阴极相比,在相同光谱响应条件下,该响应速度是最高的。另外,在不同L下,获得了平均时间衰减常数τ'的函数分布和能够获得最短响应时间的最优系数因子β分布,为新型高速响应GaAs光电阴极的时间响应和量子效率优化提供了必要的理论基础和数据支持。     

Improvement of photoemission performance of a gradient-doping transmission-mode GaAs photocathode

Two types of transmission-mode GaAs photocathodes grown by molecular beam epitaxy are compared in terms of activation process and spectral response, one has a gradient-doping structure and the other has a uniform-doping structure. The experimental results show that the gradient-doping photocathode can obtain a higher photoemission capability than the uniform-doping one. As a result of the downward graded band-bending structure, the cathode performance parameters, such as the electron average diffusion length and the surface electron escape probability obtained by fitting quantum yield curves, are greater for the gradient-doping photocathode. The electron diffusion length is within a range of from 2.0 to 5.4 μm for doping concentration varying from 10¹⁹ to 10¹⁸ cm^-3 and the electron average diffusion length of the gradient-doping photocathode achieves 3.2 μm.     

Influence of varied doping structure on photoemissive property of photocathode

The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface,thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement,which might be due to the increase in either the number or the energy of electrons reaching the surface,is not clear at present. In this paper,the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed,and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.     

Influence of built-in electric filed on the energy and emergence angle spreads of transmission-mode GaAs photocathode

In order to research the transport characteristic of photoelectrons in different-structure transmission-mode GaAs photocathodes, the energy and emergence angle spreads of photoelectrons reaching the band-bending region are calculated and the photoemission properties are analyzed. Based on the established atomic configuration models and ionized impurity scattering formulas of the uniform-doping and exponential-doping photocathodes, the trajectories of photoelectrons in different GaAs photocathodes have been calculated. The results show that, the emergence angle spread of the exponential-doping photocathode is more centralized than that of the uniform-doping one. The influence of the built-in electric field on the photoemission is obvious in the short-wave region. The built-in electric field not only increases the quantum efficient, but also improves the resolution of photocathode. This research can be propitious to investigate the photoemission mechanism, and to analyze the effect of the excited photoelectrons on the image intensifier performance.     

变组分AlGaAs/GaAs透射式光电阴极分辨力特性分析

建立了变组分AlGaAs/GaAs光电阴极二维载流子输运连续性方程.在一定的边界条件下,利用数值计算方法对此方程进行求解,得到了变组分AlGaAs/GaAs光电阴极调制传递函数(MTF)理论计算模型.利用该模型计算了透射式变组分和均匀组分阴极的理论MTF,分析了分辨力与Al组分变化范围、入射光子波长、AlGaAs和GaAs层厚度的关系.计算结果表明,变组分阴极与均匀组分阴极相比,阴极分辨力显著提高.当空间频率f在100—500 lp·mm-1区间时,分辨力的提高最为明显,如当f=200 lp·mm-1时,一般可提高150%—260%.变组分阴极分辨力的提高是内建电场作用的结果,但内建电场太大时,也会由于Al组分含量过高而影响阴极的长波响应.     

一种提高透射式GaAs光电阴极响应速度的方案

本文提出了一种可提高透射式GaAs光电阴极响应速度的阴极结构方案,并对这种结构的透射式GaAs光电阴极在电场作用下的响应时间进行了理论计算。计算结果表明,这种结构可便透射式GaAs光电阴极的响应时间由1ns左右减少到几个或几十个ps。     

GaAs光电阴极激活后的表面势垒评估研究

根据阴极制备过程中单独Cs激活和Cs-O交替激活两个阶段产生的光电流的峰值比,结合对电子穿过表面单势垒和双势垒后能量分布的理论曲线,提出了一种评估NEA GaAs光阴极表面势垒参数的新方法.利用该方法研究的结果完全符合双偶极层模型理论,并且和对实测阴极电子能量分布曲线拟合的结果非常一致.该方法简单、有效,在不增加测试手段的前提下,丰富了对NEA GaAs光电阴极激活效果及表面特征评价的方法和途径. 关键词： GaAs 光电阴极 表面势垒 能量分布     

GaAs光电阴极激活时Cs的吸附效率研究

从NEA GaAs光电阴极的激活光电流曲线发现,当系统真空度不很高时,在首次Cs激活阶段,表面掺杂浓度较低的阴极材料,其光电流产生需要的时间也较长.同时,随着系统真空度的提高,这种时间上的差异又变得不再明显.该现象表明,Cs原子在阴极表面的吸附效率同表面层掺杂浓度以及系统真空度之间有直接的联系.为定量分析这种关系,本文根据实验数据建立了Cs在阴极表面吸附效率的数学模型,利用该模型仿真的结果同实验现象非常符合.该研究对进一步开展变掺杂阴极结构设计和制备工艺研究具有重要的价值和意义. 关键词： GaAs光电阴极 吸附效率 真空度 表面掺杂浓度     

激活台内透射式GaAs光电阴极的光谱响应特性研究

利用在线光谱响应技术,对透射式GaAs光电阴极在高温激活和低温激活后的光谱响应特性进行了测试,计算并比较了高温激活和低温激活后阴极的积分灵敏度和光谱响应特性参数.结果发现,与高温激活相比,低温激活后GaAs光电阴极的积分灵敏度提高了13％,低温激活后阴极的截止波长和峰值波长均向长波有微小偏移.将已获得的测试曲线与铟封后的三代像增强器的光谱响应曲线进行了比较,结果发现铟封前后的光谱响应曲线存在明显差异.     

GaAs光电阴极表面电子逸出概率与波长关系的研究

GaAs光电阴极量子效率公式中用到的表面电子逸出概率,在阴极工作波段范围内通常视为与入射光子波长无关的常数。应用该结论对反射式GaAs光电阴极激活实验结果进行了拟合分析。实验采用分子束外延GaAs材料,外延发射层厚度为1.6μm、掺杂浓度为1×1019cm-3,分析结果显示理论曲线与实验曲线存在偏差,而在激活台内阴极灵敏度下降后的光谱响应曲线拟合结果偏差更大。这种偏差是由于表面电子逸出概率对入射光子波长的依赖关系造成的,并非通常认为的与波长无关。经过光谱响应曲线的拟合分析得出,反射式阴极表面电子逸出概率与入射光子波长之间近似满足指数关系,两者通过表面势垒因子相联系。高、低温激活后阴极表面势垒因子分别为3.53和1.36。     

高性能透射式GaAs光电阴极量子效率拟合与结构研究

为了探索高性能透射式GaAs光电阴极的特征结构,对光电阴极量子效率公式进行了光谱反射率与短波截止限的修正,并利用修正后的公式对ITT透射式GaAs光电阴极量子效率(≈43%)曲线进行了拟合,得到拟合相对误差小于5%时的结构参数为:窗口层Ga_1-xAl_xAs的厚度介于0.3-0.5 μm,Al组分x值为0.7,发射层GaAs的厚度介于1.1-1.4 μm.另外,根据拟合结果讨论了均匀掺杂透射式GaAs光电阴极的优化结构参数,如果光电阴极具有0.4 μm厚的Ga_1-xAl_xAs(x=0.7)窗口层和1.1-1.5 μm厚的GaAs发射层,则积分灵敏度可以达到2350 μA/lm以上. 关键词： 透射式GaAs光电阴极 量子效率 积分灵敏度 光学性能     

Structural Design and Experiment of Narrow-Band Response GaAlAs Photocathodes

To obtain the peak response at 532 nm,narrow-band response GaAlAs photocathodes with two GaAlAs active layers of different aluminum compositions are designed in consideration of the maximum absorptivity and quantum efficiency.The transmission-mode and the corresponding reflective-mode photocathodes are grown by metalorganic chemical vapor deposition.The results indicate that the peak response and the cut-off wavelength occur at 532 nm for the two kinds of photocathodes respectively.The response of the reflection-mode photocathode is an order of magnitude higher than that of the transmission-mode photocathode,whereas the better growth quality and the thicker second GaAlAs active layer can improve the transmission-mode response.