Physical operating principles of scandate cathodes for microwave devices

The electronic structure of barium oxide crystallites determining the emission properties of both dispenser and scandate cathodes has been studied using electron spectroscopy for chemical analysis, electron energy loss spectroscopy, and optical spectroscopy. It has been established that the other elements (calcium, aluminum, scandium, and tungsten) contained in cathode materials are diluted in barium oxide and significantly affect its electronic structure and, consequently, emission properties. The obtained results give an idea about the physical and physicochemical mechanisms of the effect of scandium on the reduction of the work function of scandate cathodes relative to that of the cathodes of other types.     

单晶CeB_6发射性能及磁电阻各向异性研究

采用X射线劳厄定向法对单晶CeB_6的(110),(111),(210)和(310)晶面进行了定向.系统研究了不同晶面热发射性能及磁场对电阻率的影响规律.结果表明,当阴极温度为1873 K时(110),(111),(210)和(310)晶面最大发射电流密度分别为38.4,11.54,50.4和20.8 A/cm~2,表现出了发射性能的"各向异性".RichardsonDushman公式计算逸出功结果表明,上述晶面中(210)晶面具有最低的逸出功,为2.4 eV.从实际应用来看,该晶面有望替代商业化的钨灯丝成为新一代的场发射阴极材料.磁电阻率测量结果显示,当晶体从[001]方向旋转至[011]方向时电阻率从73μ?·cm变化至69μ?·cm,表明电阻率在磁场中沿不同方向同样具有"各向异性"的特点.     

关于热电子发射理论的评述(Ⅲ)——动态表面发射中心

本文提出能解释热阴极中各种现象的动态表面发射中心模型。电子发射来源于一个原子集团。在这原子集团中,当运动着的电子具有瞬时的最高能量,或集团的原子核吸引力达到最小时,便产生电子发射。超额钡是提供高能电子的根源。氧则按其在集团中的相对数量和相对位置而分别起有益或有害的作用。虽然锶、钙、铝酸盐、钨酸盐等对发射只起次要的作用,但它们对蒸发、徙动、传递电子等却有影响。实用热阴极的发射不均匀性证明发射中心的存在,而发射的不稳定性,例如闪变噪声,显示出发射中心的动态特性。     

关于热电子发射理论的评述(Ⅱ)——单原子层和偶极子理论

本文对热电子发射中应用单原子层和偶极子理论作了分析和批判。我们认为:1.单原子层的存在不是无条件的,它与基底的温度、结构、纯度和外界真空度都有关系;2.关于时而能、时而又不能观察到的电子发射峰值,是一个有待于进一步明确的现象,不能简单地用单原子层和偶极子理论来解释;3.吸附了外界原子或分子使电子发射增加的事实,并不是由于偶极矩降低了基底的逸出功,发射的电子来源于被吸附物质的价电子;4.实验证明“L”阴极是Ba-O-W系统,它既不是单原子层,也不是单分子层,也不适用偶极子理论。     

LaB6与TaC混合涂层热阴极

研究了一种LaB6与TaC混合涂层热阴极,其发射层通过均匀混合LaB6与TaC粉末得到。比较了该阴极在LaB6与TaC混合比例为1∶1, 2∶1和3∶1下的发射特性,发现混合型LaB6阴极的功函数与多晶LaB6阴极的非常接近,在3∶1的混合比例下,电极的发射电流稳定,得到的电流密度为30 A/cm2。该电极可被应用于需要大电流密度和超大面积阴极的真空电子器件及动态真空电子束设备中。     

InAs(001)表面脱氧动力学分析

利用反射式高能电子衍射(RHEED)实时监控对InAs衬底进行两步完全脱氧的过程, 对比了有低(高)砷等效束流压强保护下采用两步法对InAs衬底缓慢长时间的高温脱氧过程. InAs衬底两步完全脱氧法的第一步为传统的缓慢升温脱氧方法, 第二步为高温In束流辅助脱氧方法. 衬底高温脱氧的RHEED衍射图样说明了高温In束流辅助脱氧最终完全清除传统的缓慢升温法无法去掉的残留氧化物, 通过脱氧完成同质外延生长后的扫描隧道显微镜图像, 说明高砷等效束流压强保护下的脱氧方法是可行的; 分析了高温In束流能完全清除衬底表面残余In氧化物的原理. 关键词： 热分解 Ⅲ-V族半导体 反射式高能电子衍射     

The study of structure and work function of barium oxide films and the BaO double system on the (0001) face of rhenium: I. Barium oxide on rhenium (0001) face

The structure of barium oxide films on the (0001) face of rhenium has been investigated by the techniques of low energy electron diffraction (LEED) and contact potential difference method in a wide range of coverages. The changes in surface structure of the BaO films have been observed after annealing at various temperatures. It was shown that the structure and orientation of the films depended essentially on the interaction of barium oxide with the substrate. The result of this interaction was equivalent to the oxidation of the (0001) face of rhenium. This fact was confirmed by experiments with the adsorption of BaO films on an oxidized substrate. The structures observed after heating to high temperatures (900°–1000°C) seem to be formed by the products of chemical interaction of barium oxide with rhenium (rhenates).     

Adsorption studies of aluminum oxide on rhenium by means of field emission microscopy

Aluminum oxide deposited on a clean rhenium field emission microscope tip at room temperature starts to migrate on the metal surface at about 600° C. Various face specific layer formations occur above this temperature. They either increase or decrease the local electron emission. Complete desorption of the aluminum oxide is obtained at about 1350°C. The remaining oxygen can be removed by heating at 1900°C. Average work function changes are discussed together with the corresponding FEM patterns of the various adsorption stages. Comparisons are made with the adsorption system aluminum oxide on tungsten.     

New mechanism for electron emission from planar cold cathodes: the solid-state field-controlled electron emitter

A new mechanism for electron emission from planar cathodes is described. The theoretical analysis shows that, with an ultrathin wide band-gap semiconductor layer (UTSC) on a metal, the surface barrier is lowered to approximately 0.1 eV due to the creation of a space charge induced by the electrons injected from the metal. The barrier height depends mostly on the UTSC thickness and not on the state of the surface, as in thermionic and field emissions. This mechanism explains the measured stable emission at 300 K and 10(-7) Torr, with a threshold field of only approximately 50 V/&mgr;m, from these solid-state field-controlled emitters.     

Regularities in the formation of barium oxide films on tungsten tips by field methods

The effects of increasing quantities of barium oxide deposited on the surface of a tungsten tip are monitored by analyzing field-emission images, I-V characteristics and electron energy spectra. Changes are observed in the emission activity of the system throughout the entire adsorption time, as well as the appearance of nonlinear I-V characteristics and a shift in the position of the spectra relative to the Fermi level of the tungsten substrate after the amount of adsorbed barium oxide on the W surface reaches a level that depends on the substrate face. The experimental data set forms the basis of a model for the creation of multilayer films of BaO on W. Fiz. Tverd. Tela (St. Petersburg) 39, 1476–1478 (August 1996)     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

Surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes during accelerated life testing

A study has been made of the surface composition and barium evaporation rate of “pedigreed” impregnated tungsten dispenser cathodes. The effect of air exposure on coated cathodes was examined and was found to have no significant effect on barium evaporation rate although in some cases longer reactivation times were required. No changes in surface topography were apparent following air exposure and reactivation. Life testing was done at 100°C above the typical operating temperature for the cathode, where the typical operating temperature was taken to be 950°C for coated cathodes and 1050°C for uncoated cathodes. The cathodes were examined at different stages of life testing, up to 1200 h. Significant decreases in barium evaporation rates were found after as few as 500 h of life testing. After 1000 h the evaporation rate had decreased more than an order of magnitude. Changes in surface composition were also found. The effects of tungsten particle size, used in manufacture of the billet, on barium evaporation rate were also studied but no correlation was found.     

Investigation of local work function distribution of field emission cathode from carbon materials doped by Ba with the help of AFM/SEM/X-ray

The joint application of atomic force microscopy (AFM) and SEM/X-ray micro-analyzer for investigation of electron work function (EWF) distribution over the surface of field emission cathode (FEC) is described in the paper. This method was used to investigate the graphite cathodes surface after barium doping. The possibility of defining the average value of EWF over the surface of FEC is shown. The results of AFM measurements are compared with results of field emission tests. The reduction of work function down to 3–3.2 eV was found after barium doping.     

Field electron emission from an aluminum oxide covered tungsten {112} plane at temperatures up to 1700°K

Field electron emission from an aluminum oxide mono-layer, face specifically adsorbed on a tungsten {112} plane, was measured between 1670°K and room temperature and at local field strength ranging from 17 MV/cm to 35 MV/cm. In a Fowler-Nordheim plot straight lines were obtained up to about 820°K. The work function at 650°K was determined to be 3.2 eV. The experimental values were compared with those determined by Christov's unified theory of field and thermionic emission. Deviations are attributed to the change in the shape of the potential barrier, which is caused by the aluminum oxide adsorption.     

Chemisorption and rotational epitaxy of lithium on Ru(001)

Rotational epitaxy of an overlayer along a non-symmetry direction of the substrate has been observed as a mechanism for the accommodation of lattice mismatch in rigid thin films. We have examined the adsorption and rotational epitaxy of Li on the Ru(001) surface using low energy electron diffraction and thermal desorption spectroscopy. At 80 K, commensurate (2 × 2) and (√3 × √3)R30° phases were observed at coverages below 0.33 monolayer. At higher coverages, the Li layer orientation rotates relative to the substrate as the interatomic spacing of the layer is compressed. This behavior is qualitatively similar to that observed for Na on Ru(001), but differences were observed which may suggest that Li overlayers are less rigid than Na overlayers.     

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I-V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm² at 2.5 V/μm.     

Computer controlled techniques for high emission density mapping of thermionic cathodes

Some of the techniques commonly used (e.g. SLEEP and thermionic emission microscope) for measuring emission or work function uniformity of thermionic cathode surfaces require the use of very low or near zero current densities, thus the cathode is characterized at current densities and temperatures much lower than that of a normally operating cathode. The system reported on here uses a high voltage pulse technique and is capable of measuring emission densities in the range 1 to 80 A/cm² at normal cathode operating temperatures. The cathode surface is scanned with an anode having a 0.025 mm aperture whose position is controlled by computer operated stepping motors. The current through the aperture to a collector electrode is measured using a sample-and-hold amplifier. Pulsing and sampling are computer synchronized with the scanning, and data for each pulse are accumulated and can be processed and displayed in several ways using the computer, including a detailed “three-dimensional” map of either the electron emission density or work function variations. The entire surface of the cathode or any portion of it can be mapped in steps as small as 0.001 mm (1μm), but typically steps of 5–100 μm were used. Measurements are presented illustrating the uniformity or nonuniformity of the electron emission densities and work functions for type-B and type-M cathodes.     

Ga束流低温辅助清理GaAs衬底表面氧化物的研究

采用低流量Ⅲ族Ga束流在分子束外延中低温辅助清理GaAs衬底表面氧化物.采用高能电子衍射监控衬底表面氧化物清理过程并得出氧化物清理速率及氧化层对高能电子的吸收系数.根据清理速率对衬底温度依赖的Arrhenius关系,估算了Ga辅助清理过程中发生化学反应所需要的活化能.Ga束流辅助清理衬底表面氧化物技术在外延生长中具有实...     

金属基底对石墨烯薄膜阴极气体击穿稳定性影响

利用石墨烯二维材料极好的场发射能力和发射稳定性,提出了石墨烯阴极提高气体开关击穿稳定性的技术路线。采用化学气相沉积法和基底腐蚀转移法两种方法制备金属基底石墨烯薄膜阴极。利用扫描电子显微镜和拉曼光谱表征了石墨烯薄膜阴极质量,确认了石墨烯层数和均匀性。实验研究了两种石墨烯薄膜阴极气体开关,在微秒脉冲均匀电场作用下的击穿特性,获得了击穿电压幅值和分散性的变化规律。结果表明:当气体为0.6 MPa N2、电极间距为5 mm时,铜基底石墨烯薄膜阴极平均击穿电压为85.9 kV,相对标准差为3.2%;不锈钢基底石墨烯薄膜阴极平均击穿电压仅为59.8 kV,相对标准差为2.4%。当两种阴极击穿电压均为80 kV时,相对标准差比较,不锈钢基底仅为铜基底的44%。分析认为,不锈钢基底石墨烯薄膜质量优于铜基底,石墨烯薄膜导致阴极表面微观场增强因子更高,表面分布更均匀,在电场作用下场致发射产生均匀稳定的大量初始电子流,降低了气体开关击穿电压,有效提高了击穿稳定性。     

Ru(0001)表面氮分子和钡原子的相互作用

本文采用密度泛函理论方法研究了Ru(0001)表面氮分子和钡原子的相互作用。计算结果表明,钡原子的作用弱化了氮分子键。氮分子键长从Ru(001)-N2表面的0.113 nm伸长至Ru(001)-N2/Ba表面的0.120 nm;分子的拉伸振动频率从2221 cm-1减小到1746 cm-1;氮分子得到的电荷数从清洁表面的0.3 e增加到1.1 e。电荷从钡原子6s轨道向钌原子4d轨道转移,转移电荷增强了氮分子 空轨道和钌原子4d轨道间的杂化作用,导致 分子轨道和 杂化轨道发生极化。轨道极化使分子电偶极矩增加了约-0.136 eÅ。金属钡在Ru(0001)表面氮分子活化过程中具备电子型助催剂的特征。