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 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.     

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.     

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.     

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。     

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     

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.     

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.     

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 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.     

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     

均匀掺杂和梯度掺杂结构GaN光电阴极性能对比研究

高温退火与Cs/O激活是形成负电子亲和势GaN光电阴极的外来诱因,GaN材料本身性能是影响阴极形成的内在因素.针对均匀掺杂和梯度掺杂GaN光电阴极在结构上的不同,结合阴极在激活过程中光电流的变化规律和激活后的量子产额,分析了均匀掺杂和梯度掺杂负电子亲和势GaN光电阴极性能的异同.实验表明,与均匀掺杂结构阴极相比,梯度掺...     

Distribution of carriers in gradient-doping transmission-mode GaAs photocathodes grown by molecular beam epitaxy

The gradient-doping structure is first applied to prepare the transmission-mode GaAs photocathode and the integral sensitivity of the sealed image tube achieves 1420~μ A/lm. This paper studies the inner carrier concentration distribution of the gradient-doping transmission-mode GaAs photocathode after molecular beam epitaxy (MBE) growth using the electrochemical capacitance-voltage profiling. The results show that an ideal gradient-doping structure can be obtained by using MBE growth. The total band-bending energy in the gradient-doping GaAs active-layer with doping concentration ranging from 1× 10^19~cm^-3 to 1×10^18~cm^-3 is calculated to be 46.3 meV, which helps to improve the photoexcited electrons movement toward surface for the thin epilayer. In addition, by analysis of the band offsets, it is found that the worse carrier concentration discrepancy between GaAs and GaAlAs causes a lower back interface electron potential barrier which decreases the amount of high-energy photoelectrons and affects the short-wave response.     

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.     

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.     

大梯度指数掺杂透射式GaAs光电阴极响应特性的理论分析

讨论了一种具有超快时间响应特性的新光电阴极, 即大梯度指数掺杂透射式GaAs 负电子亲和势 (NEA) 光电阴极, 模拟了它的量子效率、时间分辨和空间分辨能力等特性. 理论分析结果表明, 由于大梯度指数掺杂设计方式, 在吸收层内形成较大的内建电场, 因此光生电子在GaAsNEA阴极内的渡越时间大大缩短, 当GaAs吸收层厚度～0.9 μm时, 其响应时间达到～ 10 ps, 说明这种新NEA阴极具有远优于传统均匀掺杂NEA阴极的超快响应特性. 同时在整个光谱响应范围内, 量子效率达到约10%-20%, 空间分辨力显著高于以往的计算结果. 分析结果表明,在保证较高的量子效率条件下, 这种新阴极能够突破常规GaAsNEA阴极的时间分辨率极限, 提高GaAsNEA阴极本身的分辨力, 有望用于超快摄影、电子加速器和自由电子激光器的电子源等领域, 进一步扩展NEA光电阴极的应用范围.     

反射式变掺杂GaAs光电阴极量子效率模型研究

借助指数掺杂与均匀掺杂光电阴极的量子效率公式,提出了采用加权平均法表示的反射式变掺杂光电阴极量子效率理论模型.使用该模型对某掺杂结构的反射式GaAs光电阴极量子效率曲率进行了拟合,获得了最佳的拟合效果,且模型的拟合结果能够定量地揭示材料的变掺杂结构,在不同入射光波段对阴极量子效率的贡献不相同这一现象.针对此现象进行了深入地分析和探讨. 关键词： GaAs 变掺杂 光电阴极 量子效率     

均匀掺杂GaAs材料光电子的输运性能研究

通过建立原子结构的理论模型和电离杂质散射理论公式,研究了光电子在透射式均匀掺杂GaAs光电阴极体内的输运过程,分析了光电阴极的掺杂浓度、发射层厚度、电子扩散长度等相关因素对阴极出射面的弥散圆斑以及到达阴极出射面的光电子数与激发光电子总数之比的影响.计算结果表明,当透射式均匀掺杂GaAs光电阴极发射层厚度为2 μm、电子扩散长度为3.6 μm、掺杂浓度为1×10¹⁹ cm^-3时,其极限线分辨率为769 mm^-1.此GaAs材料光电子的输运性能 关键词： GaAs 光电阴极 光电子输运 弥散圆斑 分辨率     

指数掺杂反射式GaAlAs和GaAs光电阴极比较研究

采用指数掺杂技术, 通过金属有机化学气相沉积法外延生长了反射式GaAlAs和GaAs光电阴极, GaAlAs发射层的Al组分设计为0.63. 在超高真空系统中分别对两种阴极进行激活实验, 得到激活后的光谱响应曲线. 利用指数掺杂反射式光电阴极量子效率公式对实验曲线进行拟合并分析了电子漂移扩散长度、后界面复合速率、表面电子逸出几率等性能参数对光电发射性能的影响. 结果表明, 与GaAs光电阴极相比, GaAlAs光电阴极的Al组分虽然在一定程度上不利于光电发射, 但是解决了GaAs光电阴极由于响应波段宽而不能很好地用于窄波段可见光探测领域的问题, 制备出只对蓝绿光响应的反射式GaAlAs光电阴极.