NEAGaN光电阴极第一性原理研究

采用基于密度泛函理论(DFT)框架下广义梯度近似投影缀加平面波方法,在六方结构GaN结构优化的基础上,计算了GaN(0001)A面吸附Cs后功函数变化,指出吸附系统表面形成了一个有效的GaN－Cs电偶极子层,降低了原本的GaN表面势垒,形成更加有利于电子逸出的外光电发射效应特性。接着图示吸附Cs、O后的电子结构,指出吸附原子和衬底之间的键合。六方结构GaN材料的光学性质通过Kramers-Kronig 关系得出。根据GaN的介电函数谱,得出了254nm光波长下以GaN为激活层材料的反射式光电阴极在不同少子扩散长度下的内量子效率。计算结果表明六方结构GaN(0001)A面是可见光盲光电阴极的优良发射表面,且254 nm处的量子效率可达到60%,远大于碱金属卤化物紫外光电阴极。     

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.     

负电子亲和势氮化镓光电阴极

负电子亲和势GaN光电阴极在紫外探测技术领域具有诱人的应用前景.本文在介绍和分析负电子亲和势GaN光电阴极的特点、工作原理及其能带结构的基础上,设计了GaN外延材料的结构和阴极制作工艺.指出负电子亲和势GaN光电阴极制备的关键在于材料的生长、与输入光窗的融焊、衬底的减薄及彻底的去气处理和超高真空状态下的铯、氧激活.     

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.     

A HIGH STABLE LONG LIFE-SPAN REFLECTIVE GaAs(Cs,F)PHOTOCATHODE

A stable surface layer is essential for a high stable long life-span photocathode.Byusing fluorine instead of oxygen in activating the surface of GaAs suhstrate,the strongelectronegativity of fluorine may improve the stability of photocathode.Life-span of reflectiveGaAs(Cs,F)photocathode has been made up to 8,500 hours.Furthermore,Its technologicalreproducibility and anti-contamination ability are better than those of GaAs(Cs,O_2)photocathodes.     

透射式GaAs（Cs,O）光电阴极发射电子横向能量的研究

制备了透射式ＧａＡｓ阴极组件,测量了ＧａＡｓ阴极激活过程中阴极光 敏度和发射电子平均横向能量随激活时间的变化,用扫描电镜观察ＧａＡｓ（Ｃｓ,Ｏ）阴极表面形貌,结果表明：ＧａＡｓ（Ｃｓ,Ｏ）阴极发射电子横向能量取决于ＧａＡｓ晶格温度、阴极表面形貌和电子在能带弯曲区的多次散射,与（Ｃｓ,Ｏ）激活层无关。     

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.     

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.     

A modified model for Si/SiGe MOS-gate delta-doped HEMTs

An analytical model for threshold voltage (V_th) and minimum gate voltage (V_tl) of Si/SiGe MOS-gate delta-doped HEMT is presented in this letter. The model is valid for any width of the delta-doped layer and any distance of the layer from the Si/SiO₂ interface. Using the model, V_th and V_tl of a Si/SiGe MOS-gate delta-doped HEMT of known dimensions are calculated. To investigate the effect of variation of the width of the delta-doped layer, the threshold voltage and the minimum gate voltage have been plotted against the width. Medici™ simulation have been performed on the same device to evaluate V_th and V_tl for different delta-doped layer widths. The simulation results are in good agreement with the results found using the analytical model.     

透射式GaAs（Cs,O）光电阴极稳定性的研究

研究了光照强度和残余气体对阴极稳定性的影响,比较阴极在管壳内和激活室内的阴极稳定性,用俄歇谱仪分析激活的GaAs光电阴极表面和灵敏度衰减到O时的GaAs光电阴极表面。结果表明,真空中有害残余气体与阴极表面的相互作用是引起阴极衰减的主要原因。     

Efficient GaN photocathodes for low-level ultraviolet signaldetection

We report on the properties of GaN-based photocathodes for low light ultraviolet (UV) signal detection. Cesiated Mg-doped p-type GaN layers with 1-μm thickness were used as photocathode materials. Quantum efficiency (QE) as measured on a completed device showed values as high as 30% at 200 nm. A UV/visible rejection ratio of three orders of magnitude at 500 nm was observed. A net increase in the QE was also observed with increasing conductivity of the material     

PROCESS AND PROPERTIES OF THE MODERN MULTIALKALI PHOTOCATHODE USED IN A CASCADE IMAGE INTENSIFIER

A new processing technology and the properties of multialkali antimonide photocathodesare described.The technique requires an initial antimony layer to be deposited on a fiber-optic window at asuitable temperature.After that.the layer is activated by multialkali and antimonide.Thus.the multialkaliphotocathodes named“The Modern Multialkali Photocathode”are obtained.It has been found that aphotocathode thus processed gives a sensitivity of 200 to 300μA/Im for white light at a color temperature of2854K.This paper shows the variation of photosensitivity during processing of a Modern MultialkaliPhotocathode and the variatioon of spectral sensitivity at different wavelengths.The author discusses theresults.gives a supposition of“photoemission center”,and discusses the role of Cs in multialkaliphotocathodes.     

高积分光电灵敏度多层Be浓度掺杂的GaAs负电子亲和势光电阴极

研究了变掺杂浓度结构对GaAs负电子亲和势光电阴极积分光电灵敏度的影响.通过MBE生长了两组GaAs同质外延样品.其中一组采用了均匀掺杂的单层结构,Be掺杂浓度为1×1019cm-3;另一组采用了变掺杂的多层结构,从衬底开始Be的掺杂浓度依次为1×1019,7×1018,4×1018和1×1018cm-3.负电子亲和势光电阴极通过在高真空系统中交替通入Cs和O激活得到.在线光谱响应测试曲线表明,多层Be掺杂结构光阴极的积分光电灵敏度比单层Be掺杂结构光阴极的积分光电灵敏度至少提高了50%.两种结构的GaAs样品表现出不同的表面应力情况.     

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.     

Nearly White-Light Emission From GaN-Based Light-Emitting Diodes Integrated With a Porous SiO 2 Layer

In this letter, we develop a nearly white-light-emitting device by integrating blue/green emission from a GaN-based light-emitting diode with red emission from a porous SiO₂ layer. The porous SiO ₂ layer was fabricated by a novel process procedure to create Si nanocrystals on top of the n-type GaN layer. Red light is generated from the metal-oxide-semiconductor (Ni-Au-SiO₂ oxide-n-type GaN) structure due to the electron-hole recombination in the Si nanocrystals. The device shows a blue light emission at a low biased voltage and nearly white-light emission (green and red colors) at a bias voltage between 14 and 16 V. Our results show the potential of applying such an integrated structure to white-light illumination     

梯度掺杂结构GaN光电阴极的激活工艺研究

利用自行研制的光电阴极多信息量测试评估系统,在线测试了梯度掺杂结构GaN光电阴极在激活过程中的光电流、Cs源电流和O源电流。根据激活后的阴极光谱响应曲线和量子产额曲线,分析了超高真空室真空度、激活前的阴极表面净化程度、激活中首次进Cs量、激活中的Cs/O比以及Cs/O源在激活中的供给方式等对阴极激活的影响,提出了梯度掺...     

高真空系统气体成分对GaAs光电阴极稳定性的影响

GaAs 光电阴极以其量子效率高、光谱可调等优点广泛应用于微光夜视领域,尤其以高积分灵敏度的特性区别于多碱光电阴极,而 GaAs 光电阴极负电子亲合势的特性是通过 Cs,O 激活实现的,但是激活结束后,负电子亲合势的维持受诸多因素影响,如激活源、激活方式、气体氛围等。为了探究超高真空系统中影响 GaAs 光电阴极稳定性的因素,开展了 GaAs 光电阴极的激活实验和稳定性实验,对激活光电流曲线与腔室气体成分进行了监测,实验结果表明,在真空度优于 1×10^-6 Pa 的高真空系统中,影响其稳定性的是腔室中的气体成分,其中对稳定性影响最大的是 H₂O,真空系统中 H₂O分压的增加会导致 GaAs 光电阴极的 Cs,O 激活层迅速破坏,光电发射能力急剧下降。     

GaAs光阴极激活稳定性研究

为了提高Cs-O激活后GaAs光阴极的稳定性,延长像管的使用寿命,从Cs-O激活方面着手进行研究,寻找解决途径。改变GaAs光阴极激活中的Cs过量,并在线监测真空环境下光电流的变化情况,寻找激活对GaAs光阴极稳定性的影响因素。分别进行3组5种比例的激活实验,在光电流下降至Cs峰峰值90%、70%、50%、30%和10%时给O进行交替激活。激活结束后,在低于110-8Pa的真空环境下在线监控30min内的光电流,发现过Cs90%、70%、50%激活的光阴极稳定性好,过Cs30%次之,过Cs10%相对最差。结果表明:GaAs光阴极Cs-O激活时,Cs量越多,表面势垒的建构越完整,光阴极的稳定性就越好,对改善GaAs光阴极稳定性,延长使用寿命具有重要意义。     

Low-/spl kappa/ BCB passivation on AlGaN-GaN HEMT fabrication

Due to the stress-induced polarization effect on the GaN HEMTs, the surface passivation of the device is critical and is deserved to conduct a detailed study. It has been proven that the GaN HEMTs demonstrate nondispersive pulsed current-voltage (I-V) characteristics and better microwave power performances after passivating the Si/sub 3/N/sub 4/ film on the GaN surface. In this letter, we proposed to use the BCB material, a negative photoresist with a low-/spl kappa/ characteristic, as the surface passivation layer on GaN HEMTs fabrication. After comparing the dc I-V, pulsed I-V, RF small-signal, microwave power characteristics, and device reliability, this BCB-passivated GaN HEMT achieved better performance than the Si/sub 3/N/sub 4/ passivated device.     

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。