The optimal thickness of a transmission-mode GaN photocathode

A 150-nm-thick GaN photocathode with a Mg doping concentration of 1.6×1017cm-3 is activated by Cs/O in an ultrahigh vacuum chamber,and a quantum efficiency(QE) curve of the negative electron affinity transmission-mode(t-mode) of the GaN photocathode is obtained.The maximum QE reaches 13.0% at 290 nm.According to the t-mode QE equation solved from the diffusion equation,the QE curve is fitted.From the fitting results,the electron escape probability is 0.32,the back-interface recombination velocity is 5×104 cm·s-1,and the electron diffusion length is 116 nm.Based on these parameters,the influence of GaN thickness on t-mode QE is simulated.The simulation shows that the optimal thickness of GaN is 90 nm,which is better than the 150-nm GaN.     

透射式指数掺杂GaAs光电阴极最佳厚度研究

通过研究指数掺杂GaAs光电阴极中光电子扩散漂移长度与均匀掺杂GaAs光电阴极中光电子扩散长度的差异,确定透射式指数掺杂GaAs光电阴极的最佳厚度范围为16—22 μm.利用量子效率公式对透射式指数掺杂GaAs光电阴极最佳厚度进行了仿真分析,发现厚度为20 μm时阴极积分灵敏度最大.外延生长阴极厚度分别为16和20 μm的两种透射式指数掺杂GaAs样品并进行了激活实验,测得样品的积分灵敏度分别为1228和1547 μA/lm,两者的比值为796%. 实验结果与仿真结果符合. 关键词： GaAs光电阴极 透射式 指数掺杂 厚度     

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     

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.     

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     

NEA GaN光电阴极量子产额研究

围绕NEA GaN光电阴极光电发射过程的光谱响应理论,研究了其量子产额问题。分别给出了反射模式和透射模式NEA GaN光电阴极的量子产额计算公式,分析了影响量子产额的因素。分析了国外制备的反射模式和透射模式NEA GaN光电阴极的量子产额曲线。结果表明,目前制备的NEA GaN光电阴极已具有高达30%的量子效率和良好的发射性能。相同条件下,反射模式比透射模式的量子效率要高,而且反射模式不像透射模式那样存在短波限制。     

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 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.     

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     

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     

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     

Research on quantum efficiency of transmission-mode InGaAs photocathode

To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9 μm, the thickness of the window layer is 0.1–0.3 μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch.     

Influence of wet chemical cleaning on quantum efficiency of GaN photocathode

GaN samples 1-3 are cleaned by a 2:2:1 solution of sulfuric acid(98%) to hydrogen peroxide(30%) to de-ionized water;hydrochloric acid(37%);or a 4:1 solution of sulfuric acid(98%) to hydrogen peroxide(30%).The samples are activated by Cs/O after the same annealing process.X-ray photoelectron spectroscopy after the different ways of wet chemical cleaning shows:sample 1 has the largest proportion of Ga,N,and O among the three samples,while its C content is the lowest.After activation the quantum efficiency curves show sample 1 has the best photocathode performance.We think the wet chemical cleaning method is a process which will mainly remove C contamination.     

In-situ multi-information measurement system for preparing gallium nitride photocathode

We introduce the first domestic in-situ multi-information measurement system for a gallium nitride (GaN) photo- cathode. This system can successfully fulfill heat cleaning and activation for GaN in an ultrahigh vacuum environment and produce a GaN photocathode with a negative electron affinity (NEA) status. Information including the heat clean- ing temperature, vacuum degree, photocurrent, electric current of cesium source, oxygen source, and the most important information about the spectral response, or equivalently, the quantum efficiency (QE) can be obtained during prepa- ration. The preparation of a GaN photocathode with this system indicates that the optimal heating temperature in a vacuum is about 700 C. We also develop a method of quickly evaluating the atomically clean surface with the vacuum degree versus wavelength curve to prevent possible secondary contamination when the atomic level cleaning surface is tested with X-ray photoelectron spectroscopy. The photocurrent shows a quick enhancement when the current ratio between the cesium source and oxygen source is 1.025. The spectral response of the GaN photocathode is flat in a wavelength range from 240 nm to 365 nm, and an abrupt decline is observed at 365 nm, which demonstrates that with the in-situ multi-information measurement system the NEA GaN photocathode can be successfully prepared.     

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.     

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. 关键词： 负电子亲和势 GaN光电阴极 量子效率 表面势垒     

Affection of different doping structure on quantum efficiency and spectrum response of reflection-mode GaN photocathode

In order to analyze the effect of varied doping structure on the quantum efficiency and spectrum response of the photocathode, six kinds of reflective varied doping GaN photocathode samples are designed and grown. The quantum efficiency curves and the spectrum response curves are obtained after these samples are activated. Through the analysis of experiment result, the factors and the reasons that can affect the quantum efficiency and spectrum response are concluded. The experiment result shows that the photocathode with exponential-doping structure has the best spectrum response and the highest quantum efficiency. The experiment result provides suggestions for the study on optimizing design of photocathode doping structure.     

利用紫外透射光谱研究透射式GaN光电阴极的材料结构及光学特性

GaN紫外光电阴极是近年发展起来的一种高性能真空紫外探测器件,其中透射式结构作为光电阴极实际应用的工作模式,其多层结构参数及光学特性对阴极的最终光电发射性能有着重要的影响.测试了透射式GaN阴极材料的紫外透射光谱,通过建立透射式GaN阴极样品的透射模型,得到了GaN阴极样品的薄膜厚度、光学吸收系数与透射谱之间的函数关系.计算得到的GaN外延材料的厚度与实际值误差小,吸收系数与已发表数据一致,表明紫外透射光谱法能够准确地实现透射式GaN阴极材料结构及光学特性的评估.     

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

通过调整非故意掺杂氮化镓层的厚度,分析氮化镓基LED外延生长过程中应力的演变行为,以控制外延片表面的翘曲程度,从而获得高均匀性与一致性的外延片。由于衬底与外延层之间的热膨胀系数差别较大,在生长温度不断变化的过程中,外延片的翘曲状态也随之改变。在n型氮化镓生长结束时,外延片处于凹面变形状态。在随后的过程中,外延薄膜"凹面"变形程度不断减小,甚至转变为"凸面"变形,所以n型氮化镓生长结束时外延片的变形程度会直接影响多量子阱沉积时外延片的翘曲状态。当非掺杂氮化镓沉积厚度为3.63μm时,外延片在n型氮化镓层生长结束时变形程度最大,在沉积多量子阱时表面最为平整,这与PLmapping测试所得波长分布以及标准差值最小相一致。通过合理控制非故意掺杂氮化镓层的厚度以调节外延层中的应力状态,可获得均匀性与一致性良好的LED外延片。     

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.