GaAs真空电子源衰减模型研究

利用X射线光电子能谱(XPS)仪对激活后的GaAs真空电子源进行了随时间衰减变化的XPS分析,分析发现了电子源阴极表面各元素百分含量随时间的变化,揭示了杂质气体吸附造成的偶极矩方向的改变是电子源灵敏度显著下降的主要原因.基于上述结论,通过分析真空系统中杂质气体的吸附过程,推导并得到了GaAs电子源衰减模型.该模型从理论上揭示了GaAs电子源的指数衰减规律以及寿命与真空度的反比关系,与实验现象完全一致. 关键词： 电子源 X射线光电子能谱 衰减模型 真空度     

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.     

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.     

GaN光电阴极表面势垒对电子逸出几率的影响

针对GaN光电阴极表面势垒对电子逸出几率的影响问题,应用玻尓兹曼分布和基于Airy函数的传递矩阵法计算了GaN光电阴极的电子逸出几率,发现电子逸出几率主要由I势垒决定,II势垒对电子逸出几率的影响有限.利用自行研制的GaN光电阴极激活评估实验系统,测试了透射式GaN光电阴极样品的激活光电流.实验发现,Cs单独激活引起电子逸出几率的显著增加,而Cs单独充分激活后的Cs/O交替激活对电子逸出几率的影响有限.理论计算结果与激活光电流测试结果一致,其原因是Cs单独激活对降低真空能级的贡献远大于Cs/O共同激活.     

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.     

GaAs光电阴极制备工艺中表面污染的微区分析

基于超高真空光电阴极制备与表面分析互联装置开展了反射式GaAs光电阴极激活实验,并利用扫描聚焦X射线光电子能谱对化学清洗后及Cs/O激活后的阴极表面进行了微区分析.通过X射线激发样品产生二次电子图像定位需要分析的微小区域,更加准确地检测了阴极表面存在的杂质.检测发现化学清洗后的GaAs阴极样品会受到金属压片的二次污染,出现钠、铯污染.表面分析和激活实验表明,高温加热和激活并不能去除钠污染,且此污染会影响表面砷的脱附,阻碍激活过程中Cs、O的吸附,降低阴极的光电发射性能.采用扫描聚焦X射线成像技术对阴极表面进行微区选点分析,有助于更加准确地分析阴极激活前后的表面成分变化.     

透射式蓝延伸GaAs光电阴极光学结构对比

用金属有机物化学气相沉积法外延制备了一个透射式蓝延伸GaAs光电阴极,积分灵敏度达到1980 μA/lm,同时与美国ITT公司的一条蓝延伸阴极光谱响应曲线对比,分别对两者进行了光学结构拟合. 结果表明,国内阴极在Ga_1-xAl_xAs层厚度、Al组分、电子扩散长度和后界面复合速率上与国外 存在差距,这导致国内阴极的蓝延伸性能不及国外.国内蓝延伸阴极的表面电子逸出几率、发射层厚度与 国外阴极拟合结果一致,这使得两者长波响应性能差别远小于短波部分的差别.另外响应波段全谱的吸收率 小于国外阴极,导致国内透射式蓝延伸GaAs光电阴极光谱响应、积分灵敏度尚不及国外.     

梯度掺杂与均匀掺杂GaN光电阴极的对比研究

为了提高负电子亲和势(NEA)GaN光电阴极的量子效率,利用金属有机化合物化学气相淀积(MOCVD)外延生长了梯度掺杂反射式GaN光电阴极,其掺杂浓度由体内到表面依次为1×10¹⁸ cm^-3,4×10¹⁷ cm^-3,2×10¹⁷ cm^-3和6×10¹⁶ cm^-3,每个掺杂浓度区域的厚度约为45 nm,总的厚度为180 nm.在超高真 关键词： NEA GaN光电阴极 梯度掺杂 量子效率 能带结构     

Comparison of blue–green response between transmission-mode GaAsP-and GaAs-based photocathodes grown by molecular beam epitaxy

In order to develop the photodetector for effective blue–green response, the 18-mm-diameter vacuum image tube combined with the transmission-mode Al_(0.7)Ga_(0.3)As_(0.9)P_(0.1)/GaAs_(0.9)P_(0.1) photocathode grown by molecular beam epitaxy is tentatively fabricated. A comparison of photoelectric property, spectral characteristic and performance parameter between the transmission-mode GaAsP-based and blue-extended GaAs-based photocathodes shows that the GaAsP-based photocathode possesses better absorption and higher quantum efficiency in the blue–green waveband, combined with a larger surface electron escape probability. Especially, the quantum efficiency at 532 nm for the GaAsP-based photocathode achieves as high as 59%, nearly twice that for the blue-extended GaAs-based one, which would be more conducive to the underwater range-gated imaging based on laser illumination. Moreover, the simulation results show that the favorable blue–green response can be achieved by optimizing the emission-layer thickness in a range of 0.4 μm–0.6 μm.     

Spectral transmittance and module structure fitting for transmission-mode GaAs photocathodes

A transmission-mode GaAs photocathode includes four layers of glass,Si 3 N 4 ,Ga 1x Al x As and GaAs. A gradient-doping photocathode sample was obtained by molecular beam epitaxy and its transmittance was measured by spec-trophotometer from 600 nm to 1100 nm. The theoretical transmittance is derived and simulated based on the matrix formula for thin film optics. The simulation results indicate the influence of the transition layers and the three thin-film layers except glass on the transmittance spectra. In addition,a fitting coefficient needed for error modification enters into the fitted formula. The fitting results show that the relative error in the full spectrum reduces from 19.51% to 4.35% after the formula is modified. The coefficient and the thicknesses are gained corresponding to the minimum relative error,meanwhile each layer and total thin-film thickness deviation in the module can be controlled within 7%. The presence of glass layer roughness,layer interface effects and surface oxides is interpreted on the modification.