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 Alo.7Gao.3Aso.9Po.1/GaAso.9Po.1 photocathode grown by molecular beam epitaxy is tentatively fabricated. A comparison of photoelectric property, spectral characteristic and performance parameter be- tween 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.

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 5.4μm for doping concentration varying from 10^19 to 10^18 cm^-3 and the electron average diffusion length of the gradient-doping photocathode achieves 3.2 μm.

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

A transmission-mode GaAs photocathode includes four layers of glass, Si3N4, Gal-xAlxAs and GaAs. A gradientdoping photocathode sample was obtained by molecular beam epitaxy and its transmittance was measured by spectrophotometer 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 73. The presence of glass layer roughness, layer interface effects and surface oxides is interpreted on the modification.

Photoemission of graded-doping GaN photocathode

We study the photoemission process of graded-doping CaN 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.

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.

NEA GaN光电阴极表面模型研究

针对目前NEAGaN光电阴极研究中Cs激活或Cs/O激活后表面状态的形成过程还不清楚的问题,围绕NEAGaN光电阴极的光电发射机理,结合GaN光电阴极激活过程中出现的现象及成功激活的最终效果,给出了GaN光电阴极铯氧激活后的表面模型[GaN(Mg):Cs]:O-Cs。利用该模型可很好地解释单独用Cs激活时约-1.0eV的有效电子亲和势和Cs/O共同激活时-1.2eV的有效电子亲和势的成因,也较好地解释了表面吸附原子的组合形式,即Cs/O激活后激活层的化学结构由Cs2O2和CsO2构成。

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.

NEA GaN光电阴极的制备与评估

对GaN的结构性质进行了介绍,研究了NEA GaN光电阴极的制备方法,利用MOCVD生长了p型掺杂浓度为1.6×1017 cm-3发射层厚度150 nm的GaN样品,在进行了GaN表面净化处理得到原子级清洁表面后,在超高真空系统中对GaN光电阴极进行了Cs/O激活,获得了NEA GaN光电阴极。利用实验室制备的多信息量测试系统对成功激活后的NEA GaN光电阴极进行了评估,结果显示:NEA GaN光电阴极的反射式量子效率达到了30%,透射式量子效率达到了12%。

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。

反射式变掺杂负电子亲和势GaN光电阴极量子效率研究

从变掺杂负电子亲和势(NEA)Ga N光电阴极材料的光电发射机理入手,给出了反射式变掺杂NEA Ga N光电阴极内建电场和量子效率的计算公式.利用初步设计的变掺杂NEA Ga N光电阴极,介绍了变掺杂NEA Ga N阴极的激活过程和激活光电流的变化特点.结合国内外典型的变掺杂NEA Ga N阴极的量子效率曲线,分析了Ga N光电阴极量子效率曲线的特点.结果显示:由于内建电场的存在,反射式变掺杂NEA Ga N光电阴极量子效率在240 nm处即可达到56%,在较宽的入射光波长范围内,阴极具有相对平稳的量子效率,量子效率值随入射光子能量的增加而增加,并且量子效率曲线在阈值附近表现出了明显的锐截止特性.

GaN光电阴极激活后的光谱响应分析

由于GaN光电阴极的突出性能，其在紫外探测方面有着广泛的应用。文章在超高真空激活系统中，对GaN样品进行了Cs／O激活实验，并分析了激活后反射式的量子效率：在240－350nm的紫外波段内，量子效率约在30％～10％之间，曲线较为平坦，在240rim处达到30％的最大值，与国外结果对比后发现，本文获得的GaN光电阴极量子效率在短波段尚有不足。研究了GaN（0001）表面的原子排列，利用3D模拟了表面原子排列模型，并推测了Cs在其表面的吸附情况。

透射式GaN光电阴极在正面光照和背面光照下的量子效率特性研究

研究了透射式GaN光电阴极在正面光照和背面光照两种工作模式下的光电发射特性差异及影响因素,并推导了这两种工作模式下的量子效率理论公式,利用所推导的公式对实验获得的GaN光电阴极量子效率曲线进行了分析。研究结果表明,两种工作模式下光电子的表面逸出几率与光子能量具有不同的依赖关系,背面光照下,光电子将以一个一致的表面逸出几率从表面发射出去,并且短波响应会受到缓冲层及界面特性的影响,总体光电发射效率依赖于阴极表面势垒所决定的平均电子逸出几率。利用所推导的量子效率公式对实验曲线进行拟合,可实现多个阴极参量的评估,这为透射式GaN阴极材料与制备工艺水平的分析提供了重要依据。

负电子亲和势GaN阴极光电发射机理研究

GaN阴极中光电子的发射分为价带电子的激发、光生载流子的输运和载流子的发射3个阶段。分析了激发阶段GaN光电阴极中电子吸收入射光产生从价带到导带的跃迁过程和输运阶段导带中的光电子从体内到表面的扩散过程及发射阶段GaN阴极表面光电子的逸出过程;推导了Cs激活过程中到达阴极表面光激发电子的逸出几率公式;比较了仅用Cs激活和共用Cs/O激活过程中到达阴极表面光激发电子逸出几率的变化情况;结果表明:GaN阴极的光电发射为直接跃迁激发,输运阶段仅遭受电子-声子散射,表面光激发电子的逸出几率取决于激活程度,引入Cs是激活的必需因素,O的引入仅可小幅度提升光电发射效率;最后利用实验证实了Cs激活的充分性。

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,梯度掺杂结构阴极在激活过程中光电流增速较慢,激活时间相对较长,激活成功后量子效率较高。采用场助光电阴极发射模型可以很好地解释二者间存在的差异,梯度掺杂结构中内建电场的存在增加了电子向阴极表面的漂移运动,提高了电子到达阴极表面的几率。

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。

表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响

研究表面势垒对梯度掺杂GaN光电阴极电子逸出几率的影响.计算梯度掺杂透射式GaN光电阴极的电子能量分布及逸出几率,结果显示梯度掺杂与均匀掺杂相比,可以获得更大的电子逸出几率;I势垒对电子逸出几率的影响显著,而II势垒影响较小.利用GaN光电阴极多信息量测试系统,测试两种GaN阴极样品的光电流.实验结果表明,梯度掺杂GaN样品比均匀掺杂电子逸出几率更大;单独进行Cs激活形成的I势垒对电子逸出几率有显著影响,而Cs/O共同激活形成的II势垒对其影响较小.

GaN光电阴极测试与评估技术研究

GaN光电阴极的制备成功与否需用科学手段加以评估。在GaAs光电阴极多信息量测试系统的基础上,增设紫外光源,并对评估软件重新加以编写,设计出GaN光电阴极测试评估系统。利用该系统测试了GaN光电阴极在制备过程中的Cs源电流、O源电流、光电流和激活室真空度等多个信息量,并利用激活后的光谱响应对阴极进行了评估,实现了对GaN光电阴极性能的客观评价。

NEA GaN和GaAs光电阴极的比较

针对GaN基光电阴极激活过程中Cs-O交替存在的光电流的增幅问题,本文主要比较了GaN和GaAs材料性质、表面结构以及激活过程中光电阴极的光电流。发现GaN的熔点高于GaAs,在制备GaN基光电阴极时则需要更高的热清洗温度;如果用双偶极子模型描述GaN(1000)和GaAs(100)表面的光电发射机理,GaN(1000)表面Cs原子与O原子形成第二偶极矩O-Cs,几乎"平躺"在表面,对光电发射贡献不大;GaAs(100)表面Cs原子与O原子形成第二偶极矩O-Cs几乎"垂直"于表面,降低了表面功函数,对光电发射贡献很大;Cs-O激活过程中,对于GaAs光电阴极,Cs、O交替过程形成的光电流与单纯Cs激活时的光电流相比,有几倍甚至上百倍的增长;GaN只提高了20%左右。通过第一性原理计算,与现在的GaN基(1000)面相比,GaN基的(11 2 0)和(10 1 0)面是极具潜力的光电发射面;预计闪锌矿GaN基(100)面会取得更好的结果。

GaN nanowires/Si photocathodes for CO_(2) reduction towards solar fuels and chemicals: advances, challenges, and prospects

Photoelectrocatalytic(PEC)production of fuels and chemicals by using solar energy,water,and CO_(2) paves a promising avenue toward carbon neutrality.Over the past decades,for accelerating this process,a variety of photocathodes have been explored.Among them,the hybrid of GaN nanowires(NWs)and planar silicon has appeared as a disruptive platform for this grand topic,owing to their distinctive structural,optoelectronic,and catalytic properties.This review illustrates the most recent advances in GaN NWs/Si-based photocathodes for CO_(2) reduction reactions powered by simulated sunlight,beginning with a discussion of the critical requirements and fundamental challenges of PEC CO_(2) reduction.The characteristics of GaN NWs/Si are then discussed,showing its great potential in precisely controlling the behavior of photons,charges,and chemical species.As the focus of this review,the progress on the PEC CO_(2) reduction reactions toward different products over GaN NWs/Si-based photocathodes is highlighted.In the end,the challenges and prospects of GaN NWs/Si-based photocathodes for the practical synthesis of solarfuels and chemicals are proposed.

NEA GaN光电阴极材料光学特性研究

针对NEA GaN光电阴极结构设计和制备工艺需进一步优化的问题,结合阴极量子效率表达式和影响量子效率的因素,采用理论和实验相结合的方法,分别研究了GaN光电阴极材料的表面反射率、光学折射率、光谱吸收系数以及透射光谱等光学参数。结果表明在250 nm到365 nm的波长范围内,表面反射率相对平稳,是影响量子效率的直接因素,而光学折射率则通过电子表面逸出几率间接影响着量子效率。给出了均匀掺杂GaN光电阴极的光谱吸收系数的特点,根据变掺杂NEA GaN光电阴极的结构特点,给出了光谱平均吸收系数的概念和等价计算公式,并对均匀掺杂与变掺杂NEA GaN光电阴极光谱吸收系数进行了对比。