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     

梯度掺杂对n型异质结太阳能电池性能的影响

通过仿真软件AFORS-HET对a-Si:H(p)/i-a-Si:H/c-Si(n)异质结太阳能电池的光伏特性进行分析及优化,主要对比了a-Si:H(p)层的均匀掺杂和表面掺杂浓度D₁=1×10²⁰ cm^-3>界面掺杂浓度D₂=4×10¹⁹ cm^-3的梯度掺杂情况时的光伏特性,实现了在梯度掺杂时22.32%的光电转换效率。与均匀梯度掺杂相比,发射层的梯度掺杂除了引入一个附加电场,还优化了能带结构、光谱响应、表面复合速率。结果表明,梯度掺杂可以有效地改善电池的光电转换性能。     

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     

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     

负电子亲和势GaN光电阴极光谱响应特性研究

针对负电子亲和势（NEA） GaN光电阴极成功激活后的量子效率问题,利用自行研制的紫外光谱响应测试仪器,测试了成功激活的NEA GaN光电阴极的光谱响应,给出了230—400 nm波段内反射模式NEA GaN光电阴极的量子效率曲线.测试结果表明：反射模式下NEA GaN光电阴极在230nm具有高达37.4%的量子效率,在GaN光电阴极阈值365 nm处仍有3.75%的量子效率,230 nm和400 nm之间的抑制比率超过2个数量级.文中还结合国外的研究结果,综合分析了影响量子效率的因素. 关键词： 负电子亲和势 GaN光电阴极 光谱响应 反射模式     

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.     

近紫外波段NEA GaN阴极响应特性的研究

为了深入理解近紫外波段NEA GaN阴极的光谱响应特性, 在超高真空系统中对MOCVD生长的不同发射层厚度和掺杂浓度的三个样品进行激活实验, 并在线测试样品光谱响应. 利用反射式GaN阴极量子效率公式和最小二乘法对入射光波长为0.25—0.35 μ之间的 阴极响应量子效率实验数据进行拟合, 分别得到后界面复合速率和拟合直线L的斜率, 并使用量子效率公式对入射光波长为0.35 μ时的反射式GaN阴极光谱响应量子效率进行仿真. 结果表明, 后界面复合速率和直线v的斜率都能很好地反映GaN阴极的响应性能, 当GaN阴极后界面复合速率小于10⁵ cm/s, 发射层的厚度取0.174—0.212 μ时, 阴极光谱响应性能最好. 关键词： 反射式GaN 势垒 最小二乘法 后界面缺陷     

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     

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.     

势垒硅掺杂对GaN基LED极化电场及其光电性能的影响

势垒硅掺杂对InGaN量子阱中的电场及LED器件的光电性能有着重要的影响。采用6×6 K·P方法计算了不同势垒硅掺杂浓度对量子阱中电场的变化,研究表明当势垒硅掺杂浓度>1e¹⁸ cm^-3时,阱垒界面处的电场强度会变大,这主要是由于硅掺杂浓度过高导致量子阱中界面电荷的聚集。进一步发现随着势垒掺杂浓度的升高,总非辐射复合随之增加,其中俄歇复合增加,而肖克莱-霍尔-里德复合随之减少,这是由于点陷阱的增大形成了缺陷能级。电流电压曲线表明势垒掺杂可有效改善GaN基LED的工作电压,这归于掺杂浓度的提高改善了载流子的传输特性。当掺杂浓度为1e¹⁸ cm^-3时,获得了较高的内量子效率,这主要是由于适当的势垒掺杂降低了量子阱中界面电荷的损耗。     

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.     

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...     

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.     

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.     

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.     

Negative affinity 3–5 photocathodes: Their physics and technology

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed. The threshold of response of a NEA photocathode is set by the semiconductor bandgap. By alloying to form ternary or quaternary 3–5 compounds (3–5 compounds are formed from elements of the 3rd and 5th columns of the periodic table), the bandgap (and thus the threshold) can be placed at any desired photon energy within certain limits. The most important limit is that at about 1.1 eV which is the lowest limit achieved for NEA cathodes. This limit is set by the point at which the bandgap of the 3–5 material becomes less than the surface potential barrier. Fundamental work aimed at understanding the 3–5: cesium oxide “interfacial” barrier which sets this limitation is briefly discussed. Because of the “interfacial” barrier, the quantum yield of NEA cathodes decreases as the threshold of response moves to lower photon energy. Field assisted photocathodes provide a means of extending the threshold of response beyond 1.1 eV. Two different approaches to field assisted photocathodes and recent achievements are discussed. A major advancement has been the achievement of semi-transparent NEA photocathodes by sealing GaAs to glass. This makes possible practical NEA image tubes. The thermionic emission from 3-5 NEA cathodes can be orders of magnitude lower than that from conventional photocathodes. The reasons for this are discussed. Yield and dark current data are given on 3-5 NEA cathodes in operating photomultipliers. Work supported in part by the Advanced Research Projects Agency of the Department of Defense and monitored by Night Vision Laboratory, U.S Army Electronics Command, under Contract No. DAAK 02-74-C-0069.     

The optimal thickness of transmission-mode GaN photocathode

A 150-nm-thick GaN photocathode with an Mg doping concentration of 1.6× 10¹⁷cm^-3 is activated by Cs/O in ultrahigh vacuum chamber, and quantum efficiency (QE) curve of negative electron affinity transmission-mode (t-mode) 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.     

Oscillations in the threshold photoemission spectra of GaN(0001) with submonolayer Cs coverages

It is found that Cs adsorption on the n-type GaN(0001) surface generates an unusual change in the electronic properties of the surface and the near-surface space-charge layer, which leads to the appearance of photoelectron emission upon excitation in the transparent region of GaN. It is established that the photoemission is due to the formation of quasimetallic states induced by Cs adsorption in the band-bending region near the surface. The behavior of the photoemission threshold upon excitation by s-polarized light is studied as a function of the Cs coverage. It is found that the minimum value of the threshold corresponds to ～1.4 eV at a concentration of Cs atoms of ～4.5×10¹⁴ atom/cm² in the submonolayer coverage. A new effect is revealed, namely, the appearance of oscillations in the spectral curves of threshold photoemission. A model is proposed for photocurrent oscillations that takes into account the formation of quasimetallic states in the near-surface layer of GaN band bending and the occurrence of interference in the GaN slab upon light irradiation in the transparent region.