Cs、O激活对GaAsP光阴极光谱响应特性的影响

为了探究Cs、O激活电流对透射式GaAsP光阴极光谱响应特性的影响,通过光谱响应测试仪测试不同Cs、O电流激活后的透射式GaAsP光阴极光谱响应曲线,结果表明,随着铯氧蒸发电流比的减小,GaAsP光阴极光谱曲线的形状会发生一定的改变,长波响应能力逐渐降低、短波响应能力逐渐提高。利用双偶极层模型理论,分析认为铯氧蒸发电流比的改变影响了GaAsP光阴极表面势垒的形状,使得不同激发能量的光电子通过隧道效应穿越表面势垒宽度发生变化,从而影响GaAsP光阴极光谱响应特性。根据此现象对进一步提高GaAsP光阴极在530 nm特征波长的量子效率具有积极的意义。     

均匀掺杂GaAs光阴极表面势垒特性研究

根据激活过程中光电流变化规律及原位光谱响应测试,模拟了GaAs光阴极表面势垒的形成过程,在光阴极表面双偶极子模型的基础上作了修正,建立了三偶极子模型。新模型认为,光阴极表面势垒由三个偶极层套构而成,第一偶极层由GaAs（Zn）-Cs+偶极子组成,第二偶极层由Cs2O偶极子组成,第三偶极层由GaAs-O-Cs偶极子组成,第二、三偶极层嵌入第一偶极层中。根据隧道效应与量子效率测试结果,确立了势垒中分段均匀的电势分布,计算得出势垒宽度为1.65 nm,有效电子亲和势为-0.44 eV。新模型的建立对理解光电阴极表面发射机理具有重要意义。     

GaN与GaAs NEA光电阴极稳定性的比较

利用自行研制的光电阴极激活评估实验系统,对激活后的反射式GaN及GaAs光电阴极进行了稳定性测试,获得了Cs/O激活一段时间后阴极随时间变化的光谱响应,通过计算得到量子效率曲线.结果表明:激活结束后GaN灵敏度可以在较长时间内保持稳定,而后缓慢衰减.而GaAs光电阴极的光电流随时间近似呈指数衰减.结合阴极表面双偶极层结构以及表面化学成分,分析原因主要是:两种阴极表面进行Cs/O激活后形成的双偶极子的结构不同、衰减过程中双偶极层化学成分变化方式不同决定.GaN光电阴极激活后cs以复杂氧化物存在,更加稳定,灵敏度的衰减主要是由未分解的氧引起,而GaAs灵敏度下降的原因主要是表面双偶极层中的Cs极易脱附,影响其稳定性.     

高温退火对GaAs光阴极表面状态的影响

利用XPS分析了GaAs光阴极高温退火、激活前后表面组分的变化,结合光阴极退火过程中通过四级质谱仪获取的CO2、H2O、O、As、Cs等分压曲线,比较、讨论了两次退火机理的差异。提出第二次加热的目的不仅在于清洁光阴极表面,更重要的是促使第一次激活在光阴极表面形成的偶极层向更稳定的结构转化,Cs2O偶极子在高温下与GaAs本底反应生成了可在光阴极表面稳定存在的GaAs-O-Cs偶极子,形成主要由键合强的GaAs（Zn）--Cs+、GaAs-O-Cs偶极子及靠范德瓦尔斯力附着在光阴极表面的Cs2O偶极子构成的偶极层。根据这一结论,解释了光阴极两次激活过程中光电流变化规律的差异。对理解光阴极激活及表面光电发射模型具有重要意义。     

GaAs：Gs—O NEA光电阴极电子表面逸出几率的计算

介绍了计算CaAs:Cs-O NEA光电阴极电子表面逸出几率的方法，选用双偶极层表面模型，计算了两个极层形成的界面势垒各自对电子表面逸出几率的影响，提出了增大电子表面逸出几率的条件，并粗略推测了第一偶极层的厚度。     

GaAs∶Cs-O NEA光电阴极电子表面逸出几率的计算

介绍了计算CaAs∶Cs-O NEA光电阴极电子表面逸出几率的方法,选用双偶极层表面模型,计算了两个偶极层形成的界面势垒各自对电子表面逸出几率的影响,提出了增大电子表面逸出几率的条件,并粗略推测了第一偶极层的厚度.     

NEA GaN光电阴极光电发射机理研究

围绕GaN光电阴极NEA特性的成因,结合激活过程中光电流变化规律和成功激活后阴极表面模型,研究了NEA GaN光电阴极光电发射机理.实验表明:NEA GaN激活过程中光电流在约1 min之内就可达到峰值,Cs/O激活时引入O后光电流的增长幅度不大.根据Spicer光电发射理论,给出了反射模式NEA GaN光电阴极量子产额理论公式,激活成功后GaN光电阴极NEA特性的成因可以用双偶极层模型[GaN(Mg):Cs]:Cs-O解释.     

高稳定长寿命反射式GaAs(Cs,F)光阴极

一个稳定的光阴极表面层是获得一个稳定的长寿命光阴极的必不可少的条件。由于反射式GaAs(Cs,F)光阴极用氟代替氧进行表面激活,氟的强烈的电负性使得表面层的稳定性得到改善。反射式GaAs(Cs,F)光阴极的寿命达到8500小时。除此之外,它的工艺重复性和抗污染能力也比GaAs(Cs,O2)光阴极的好。     

短波红外阈场助式光电阴极p-InGaAs/p-InP异质结设计与仿真

针对一种短波红外阈透射式光阴极,简要介绍其光子吸收层和电子发射层分离式异质结结构后,结合异质结两侧对红外光子显著吸收效应参数和对电子高效输运特性参数进行建模,详细研究了场助电压对p-p型异质结能带结构和对短波红外阈透射式光电阴极的影响。研究结果显示:Schottky势垒偏压至少要达到8 V才能较好消除p-In Ga As/p-In P异质结的势垒影响。此时,为达到较小的漏电流,In0.53Ga0.47As光吸收层厚度2μm,In P发射层厚度1μm,掺杂浓度均为1×1016 cm-3。     

用异质结构控制GaN阴极电子发射

从自洽求解薛定谔方程和泊松方程出发,研究了GaN异质结构上偏压变化时异质结电场的变化。发现异质结量子阱能把外电场屏蔽在异质界面以外。利用这种异质结量子阱的屏蔽效应,可以使外电场都降落在异质结表面来控制表面势。为了把表面电势剪裁成半导体阴极所需的陡直下降的电势结构,进一步深入研究了双势垒异质结的电场结构,发现外面的异质结能屏蔽里面异质结的势垒。利用这种双势垒异质结的屏蔽效应设计出可由偏压直接控制电子亲合势的异质结构,从而为半导体阴极开辟出一条新的研究途径。     

Theoretical study of cesium and oxygen activation processes on GaN (0001) surface

The structure properties and work function of reflection-mode GaN photocathodes are investigated by using first-principles calculation within density-function theory (DFT). Seven different GaN (0001) (1×1) surface models are used in this paper to simulate cesium and oxygen activation process. Before starting the surface models calculations, clean bulk GaN surface is first optimized, when compared with the models before optimization, the correctness of the calculations method is verified. Next, change of work function of different surface models caused by adsorption is calculated and analyzed, the results show that over-cesiuminizd atmosphere is not only benefit for work function declining but also conductive to the formation of negative electron affinity (NEA), the optimal ratio of Cs to O for activation is between 3:1 and 4:1, excess Cs or O modules increase the work function and undermine the photocathode. Then a series of experiments are performed to verify the calculation results. First of all, NEA GaN photocathode activation and evaluation system is established, “yo-yo” activation method is verified. Then, an activation experiment is performed on high quality p-type Mg-doped reflection-mode GaN substrate grown by metal organic chemical vapor deposition (MOCVD), the photocurrent is controlled and monitored by multi-information on-line monitoring system, the results are consistent with our calculation models. Finally, combined with the data provided by Stanford University, schematic energy band variation of the GaN photocathode after only-Cs and Cs/O activations is given. Our result opens the possibility to engineer the activation properties of GaN photocathode.     

Growth of InGaN and double heterojunction structure with InGaN back barrier

We study the growth of an InGaN and AIGaN/GaN/InGaN/GaN double heterojunction structure by metalorganic chemical vapor deposition (MOCVD).It is found that the crystal quality of the InGaN back barrier layer significantly affects the electronic property of the AIGaN/GaN/InGaN/GaN double heterojunction.A high crystal quality InGaN layer is obtained by optimizing the growth pressure and temperature.Due to the InGaN layer polarization field opposite to that in the AIGaN layer,an additional potential barrier is formed between the GaN and the InGaN layer,which enhances carrier confinement of the 2DEG and reduces the buffer leakage current of devices.The double heterojunction high-electron-mobility transistors with an InGaN back barrier yield a drain induced barrier lowering of 1.5 mⅤ/Ⅴ and the off-sate source-drain leakage current is as low as 2.6μA/mm at VDs = 10 Ⅴ.     

Charge selectivity in polymer:Fullerene-based organic solar cells with a chemically linked polyethylenimine interlayer

The power conversion efficiency of solar cells can be optimized via an efficient charge collection by electrodes. In this study, a simple linear polyethylenimine (LPEI), which is an insulating polymer, was adopted as the cathode interfacial layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)-based bulk-heterojunction organic solar cells (OSCs) with a non-inverted configuration. All photovoltaic parameters of the OSCs were significantly enhanced by depositing LPEI onto the oxygen plasma-treated P3HT:PCBM active layers. The causes of performance enhancement in OSCs were studied. Results revealed that the microstructure and morphology of the P3HT:PCBM layer were almost unaffected by the oxygen plasma treatment and the subsequent LPEI deposition. The X-ray photoelectron spectra of the specimens demonstrated that with the aid of oxygen plasma treatment, the linked LPEI molecules formed a well-aligned dipole layer on top of the P3HT:PCBM layer through the bonding of nitrogen (N) with oxygen (O). The results from quantum chemical calculations showed that the LPEI molecule with an N–O bond had a larger dipole moment at an appropriate direction than that without an N–O bond. By contrast, the LPEI molecules can form a dipole layer with a random orientation in the absence of N–O bonds. The conductive atomic force microscopy images of the specimens showed that the well-aligned dipole layer could facilitate electron transfer and could block hole transfer from the P3HT:PCBM to the cathodes. The well-aligned and augmented interface dipoles improved the charge selectivity at the cathodes and the photovoltaic performance of the devices.     

基于Al2O3中间层的InP/SOI晶片键合技术研究

研究了基于Al2O3中间层的InP/SOI晶片键合技术.该方案利用原子层沉积技术在SOI晶片表面形成Al2O3作为InP/SOI键合中间层,同时采用氧等离子体工艺对晶片表面进行活化处理.原子力显微镜和接触角测试结果表明,氧等离子体处理使得晶片的表面特性更适于实现键合.透射电子显微镜和X射线能谱仪测试结果证实,采用Al2O3中间层可以实现InP晶片与SOI晶片的可靠键合.     

A note on the evaluation of Schottky diode parameters in the presence of an interfacial layer

Metal-semiconductor barrier height and semiconductor doping concentration evaluated from room temperature current-voltage and capacitance-voltage data of Schottky barrier diodes exhibit significant error in the presence of an interfacial oxide layer of appreciable thickness (?50 ?). The effective Richardson constant obtained from an activation energy plot of saturation current is shown to provide a sensitive indication of the presence of the interfacial layer.     

Roles of cesium and oxides in the processing of gallium aluminum arsenide photocathodes

The roles of cesium (Cs) and oxides in the processing of gallium aluminum arsenide (GaAlAs) photocathodes have been investigated. The Cs/O activation, Cs/O reactivation and degradation experiments are performed on GaAlAs photocathode. The activated photocurrent, degraded photocurrent and quantum efficiency curves are measured using a self-developed multi-information measurement system. We use the quantum efficiency formula to fit the experimental quantum efficiency curves, and obtain the performance parameters of the photocathode. The results show that the oxide gases play an important role in the degradation of the photocathode, and the Cs atoms could react with the oxides on the degraded surface and the residual oxide gases. Besides, the quantum efficiency of the degraded GaAlAs photocathode could be restored to a good level after Cs/O reactivation.     

Influence of Al2O3 barrier on the interfacial electronic structure of Au/Ti/n-GaAs structures

The Au/Ti/n-GaAs structures with and without Al₂O₃ interfacial layer have been fabricated.The Al₂O₃ interfacial layer has been formed on the GaAs substrate by atomic layer deposition.The effects of the interfacial layer on the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the devices have been investigated in the temperature range of 60-300 K.It has been seen that the carrier concentration from C-V characteristics for the MIS (metal/insulating layer/semiconductor) diode with Al₂O₃ interfacial layer has a higher value than that for the reference diode without the Al₂O₃ interfacial layer (MS).Such a difference in the doping concentration has been attributed not to doping variation in the semiconductor bulk but to the presence of the Al₂O₃ interfacial layer because both diodes have been made on the pieces cut from the same n-type GaAs wafer.The temperaturedependent I-V characteristics of the MIS diode do not obey the thermionic emission current theory because of the presence of the Al₂O₃ layer.An electron tunneling factor,aδ(χ)^1/2,value of 20.64 has been found from the I-V-T data of the MIS diode.An average value of 0.627 eV for the mean tunneling barrier height,χ,presented by the Al₂O₃ layer has been obtained.