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Applied Surface Science (v.251, #1-4)
Preface
by Georg Gaertner (Guest Editor); Yiman Wang (Guest Editor); David Barratt (Guest Editor); Daniel den Engelsen (Guest Editor); Fujiang Liao (Guest Editor); Jinjun Feng (Guest Editor); Ji Li (Guest Editor) (pp. 1-3).
Electron emission and work function—Past, present and future by Shigehiko Yamamoto (pp. 4-13).
The history of electron emission is reviewed from a standpoint of the work function and the applications. For years, in the field of thermionic emission, a great deal of efforts have been devoted to search for low work function materials with a high melting temperature, while the reduction of the local change in time of the work function rather than the work function itself has been the main issue of field emission investigations. High brightness and long life are the central targets of the emission material investigations for the scientific instrument application, while high current density and low power consumption are the guiding principles for the display application. In both fields, field emission has recently become dominant in research and development. In all above cases, the main issue in the future research works will be to analyze the work function in atomic level and thereby to understand the mechanism of the work function reduction by atom adsorption, the change in time of the local work function leading to the current fluctuation, and the relationship between microscopic and macroscopic work functions. Our attempt is discussed, where the work function in atomic level is measured by utilizing the STM technique and it is made clear how far the work function in atomic level extends its influence over the neighboring sites. As a result, a simple relationship is established between microscopic and macroscopic work functions.
Keywords: Thermionic emission; Field emission; Local barrier height; Work function
A work function study of ultra-thin alumina formation on NiAl(110) surface by Weijie Song; Michiko Yoshitake (pp. 14-18).
We have investigated the oxidation of NiAl(110) surface at 1020 and 670K using ultra-violet photoelectron spectroscopy, Kelvin probe, X-ray photoelectron spectroscopy and low-energy electron diffraction. The work function change during oxidation was monitored in situ as a function of oxygen exposure. It was observed that the work function decreased by 0.6eV after 7.9Å of well-ordered Al2O3 formation on NiAl(110) at 1020K. The formation of the interfacial dipole layer was the main factor that determined the work function and XPS binding energy shifts of Al2O3 energy levels. The work function decreased by 0.8eV after 5.1Å of amorphous Al2O3 formation at 670K. The oxide layer structure was one of the key factors that determined the work function of the Al2O3/NiAl(110) system.
Keywords: PACS; 81.65.Mq; 61.14.Hg; 68.43.−hNiAl(1; 1; 0); Work function; UPS; Kelvin probe
Two-dimensional Child–Langmuir law of planar diode with finite-radius emitter by Yongdong Li; Hongguang Wang; Chunliang Liu; Jian Sun (pp. 19-23).
A planar diode with finite-radius emitter is studied with particle-in-cell simulations to extend the classical one-dimensional (1D) Child–Langmuir law to two-dimensions. It is found that the 2D space–charge-limited current density in units of the classical 1D value follows a monotonically decreasing function of the dimensionless ratio of emitter radius ( R) to gap separation ( D), and the function can be extended to relativistic regime without significant modification. It is also verified that Lau's analytical prediction of the 2D Child–Langmuir law is in better agreement with the simulation results in comparison with Rokhlenko's.
Keywords: PACS; 85.45.−w; 52.59.MvChild–Langmuir law; Particle-in-cell; Planar diode; Relativistic regime
Hundred years anniversary of the oxide cathode—A historical review by Georg Gaertner; Daniel den Engelsen (pp. 24-30).
In this year, we celebrate the 100 years anniversary of Arthur Wehnelts discovery of the oxide cathode. In his famous article in “Annalen der Physik� in 1904 Wehnelt showed that coating of noble metals with alkaline earth oxides, such as BaO and CaO lowered cathode fall and work function drastically. This was the start of the oxide cathode, which has become one of the most commonly used electron sources in vacuum tube technology and also in low pressure gas discharge lamps. In the past century, a vast amount of literature has been generated on the technology and emission mechanisms of the oxide cathode. This paper reviews the evolution of technology, applications and theoretical models of the oxide cathode from 1904 to the present status.
Keywords: PACS; 79.40Oxide cathode; Arthur Wehnelt; Thermionic emission; Vacuum tube
The base metal of the oxide-coated cathode by F. Poret; J.M. Roquais (pp. 31-41).
The oxide-coated cathode has been the most widely used electron emitter in vacuum electronic devices. From one manufacturing company to another the emissive oxide is either a double—Ba, Sr—or a triple—Ba, Sr, Ca—oxide, having always the same respective compositions. Conversely, the base metal composition is very often proprietary because of its importance in the cathode emission performances.The present paper aims at explaining the operation of the base metal through a review. After a brief introduction, the notion of activator is detailed along with their diffusivities and their associated interfacial compounds. Then, the different cathode life models are described prior to few comments on the composition choice of a base metal. Finally, the specificities of the RCA/Thomson “bimetal� base metal are presented with a discussion on the optimized composition choice illustrated by a long-term life-test of five different melts.
Keywords: Oxide-coated cathode; Base metal; Nickel alloys; Thermochemistry; Diffusion; Interface compounds
Analytical interfacial studies of double carbonate thermionic oxide cathodes over accelerated operational life by D.K. Barber; S.N. Jenkins; M.J. Whiting; M.A. Baker (pp. 42-49).
Interfacial interactions between the nickel cap and emissive oxide in double carbonate thermionic oxide cathodes have been studied as a function of operational lifetime by SEM, AES, EDX and TEM. The surface of the interface region has been studied by stripping away the oxide from the cap. In cross-section, the interface has been examined by (a) preparing 10° taper cross-sections and (b) cutting FIB sections. Cracks are observed to develop in the bulk Ni and become more pronounced as a function of operational lifetime. Cracks are found both at grain boundaries and close to the surface, parallel to the interface. It is proposed that the cracks develop from voids, formed as a result of the high diffusivity of Mg and Al in the Ni matrix. EDX shows the presence of Al, Mg, Ba, Sr and O within the cracks and it is proposed that MgO and (Ba, Sr)Al2O4 are two major reaction products. A thin (skin) layer containing Al, Ba, Sr and O forms at the Ni cap/emissive layer interface, attributed primarily to (Ba, Sr)Al2O4 formation.
Keywords: Interface; Life-test; FIB; TEM; SEM; AES
Model of dopant action in oxide cathodes by Daniel den Engelsen; Georg Gaertner (pp. 50-58).
The paper describes an electrochemical model, which largely explains the formation of Ba in the oxide cathode at activation and normal operation. In a non-doped oxide cathode electrolysis of BaO is, besides the exchange reaction from the activators in the cathode nickel, an important source of Ba. By doping with rare earth oxides the conductivity of the oxide layer increases, which implies that the potential difference during current drawing over the oxide layer becomes lower and electrolysis of BaO is suppressed. This implies that the part of the electronic conductivity of the (Ba,Sr)O layer induced by the dopants also controls the sensitivity for poisoning: the higher the dopant level, the larger the sensitivity for poisoning. Furthermore, the suppression of electrolysis during normal operation largely explains why doped oxide cathodes have a better life performance than non-doped cathodes. Finally a hypothesis on the enhancement of sintering upon doping is presented.
Keywords: Oxide cathode; Electrical conductivity; Electrolysis; Diffusion; Doping; Semi-conductor impurity levels; Poisoning
Stretched exponential degradation of oxide cathodes by Byung Mook Weon; Jung Ho Je (pp. 59-63).
In this study, the degradation behavior of oxide cathodes for cathode ray tubes (CRTs) is described using the stretched exponential model, which has been successfully used to describe the dynamics of complex systems characterized by heterogeneity. We derive a longevity equation from the two parameters: (i) characteristic life and (ii) heterogeneity parameter, which characterize the stretched exponential model. From the temperature dependences of the two parameters in the longevity equation, we reveal that the longevity follows the Arrhenius relation in oxide cathodes. The longevity equation and the Arrhenius relation enable us to predict the longevity in early life. The stretched exponential degradation is explained based on the heterogeneity of oxide cathodes.
Keywords: PACS; 79.40.+z; 84.47.+wOxide cathodes; Stretched exponential; Longevity; Heterogeneity
Development of new types of oxide cathodes by Xianheng Liao; Xiaoxia Wang; Qinglan Zhao; Mingfen Meng (pp. 64-68).
High reliability, long lifetime and low temperature, high emission current density oxide cathodes are presented in this paper. The characteristics of these cathodes, such as dc and pulse emission, evaporation, resistance to poisoning and lifetime, are discussed. They have been widely applied to TWTs and klystrons with high power, long pulse and high duty ratio. The results showed that high emission current density and long lifetime can be realized.
Keywords: High reliability; Long lifetime; High emission current density; Oxide cathode
An improved reservoir oxide cathode by Xiaoxia Wang; Xianheng Liao; Jirun Luo; Qinglan Zhao (pp. 69-72).
A new type of reservoir oxide cathode has been developed in IECAS. The emission characteristics of the cathode are tested. The results show the new cathode has higher emission current density and better resistance to poisoning at same operating condition compared with those of conventional reservoir oxide cathode.
Keywords: Emission current; Resistance to poisoning; Reservoir oxide cathode
Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes by G. Gaertner; D. Barratt (pp. 73-79).
Ba-oxide, Ba-dispenser and Ba-Scandate cathodes have been continuously improved in their emission performance in the past decades. Ba-oxide and Ba-dispenser cathodes are also the dominant types of thermionic cathodes used in most vacuum tube applications. When improvements in emissive properties are introduced, their impact on cathode life – where several years in a vacuum tube environment are typically required – also needs to be known. Hence, the investigation of cathode life-limiting effects is the basis of accelerated life predictions and of further cathode improvement. In this contribution, the main effects limiting the operating life of Ba/BaO-based thermionic cathodes are discussed, especially related to intrinsic dispensation and resupply to the emissive surface. Emission poisoning induced by adsorption of poisonous gases will not be addressed here. We will stress common points and point out the differences between the three types.
Keywords: PACS; 79.40Oxide cathode; Ba-dispenser cathode; Scandate cathode; Thermionic emission; Cathode life; Ion bombardment
Operating model for scandia doped matrix scandate cathodes by Wei Liu; Ke Zhang; Yiman Wang; Kexin Pan; Xin Gu; Jinshu Wang; Ji Li; Meiling Zhou (pp. 80-88).
In this paper, scandate cathodes with scandia doped tungsten matrices are studied. Attempts are made to clarify some interesting phenomena presented in the whole process. The sub-micron porous tungsten matrix with uniformly distributed Sc2O3 was formed by quasi-spherical scandia doped W particles. During impregnation the reaction between Sc2O3 and barium aluminates impregnant results in the formation of compounds, preferentially of Ba2ScAlO5. But after the subsequent ultrasonic water cleaning, Ba2ScAlO5 is dissolved and mostly removed from the surface region. In the activation process, Ba and Sc re-aggregate to the surface. A uniformly distributed Sc and Ba layer on surface of tungsten substrate after proper activation has been verified by the results of high resolution scanning Auger microscopy (SAM) and Auger electron spectroscopy (AES) depth profile. For such cathodes typical emission of more than 30A/cm2 space charge limited current density at 850°Cb has been achieved. However, the uniform distribution of Sc and Ba on surface still seems not be able to remedy the abnormal Schottky effect. We propose that a surface multi-layer containing Ba, Sc and O, formed after proper activation on the W base, is the reason for copious emission and abnormal Schottky effect of such cathodes.
Keywords: Scandate cathodes; Mixed matrix cathodes; Impregnated Ba dispenser cathodes; Emission properties; Surface analysis; Sub-micron structure
Emission efficiency optimization of RE2O3 doped molybdenum thermionic cathode by application of pattern recognition method by Jinshu Wang; Wei Liu; Yanqin Liu; Meiling Zhou (pp. 89-96).
As an alternative for thoriated tungsten thermionic cathodes, molybdenum doped with either a single rare earth oxide such as La2O3, Y2O3 and Sc2O3 or a mixture thereof has been produced by powder metallurgy. It is shown that carbonization can greatly improve the emission properties (i.e. emission capability and stability) of RE2O3 doped molybdenum due to the formation of a (metallic) rare earth atomic layer on the surface of the cathode by the reduction reaction of molybdenum carbide and rare earth oxide. Among all the carbonized samples, La2O3 and Y2O3 co-doped molybdenum cathode showed the best performance in emission. In addition, computer pattern recognition technique has been used to optimize the composition of the material and of the cathode preparation technique. We derive the equation of the emission efficiency as a function of cathode composition and carbonization degree. Based on the projecting coordinates obtained from the equation, the optimum projection region was identified, which can serve as guide for the composition and carbonization degree design.
Keywords: PACS; 79.40Thermionic emission; Thoriated tungsten; Rare earth oxide (RE; 2; O; 3; ); Molybdenum; Carbonized cathode; Pattern recognition method
Preparation and “in situ� analysis of La coated Mo cathodes by Shiming Hao; Zuoren Nie; Jiancan Yang; Xiaoli Xi (pp. 97-100).
This paper describes the surface analyses of “in situ� prepared Mo cathodes with a La coating. It was found that the electron emission depends strongly on the atomic La/O ratio at the surface: the higher this ratio, the better the emission. The emission current density increases sharply when the La/O ratio starts to deviate from the stoichiometric ratio of La2O3. A model, which describes the emission of La coated Mo cathodes qualitatively, is presented.
Keywords: Thermionic emission; Pulsed laser deposition; Cathode; Surface
Weld techniques for reservoir cathodes by Bernard K. Vancil; Edwin G. Wintucky (pp. 101-105).
We report improvements in weld technology in support of our efforts to produce a commercial, long-life, miniature reservoir cathode. We have found that the laser weld between sintered tungsten pellet and reservoir sleeve can be made reliably hermetic if we apply a subsequent coating of molybdenum–ruthenium brazing alloy to the sides of the pellet and especially to the weld region. The emissive material is then inserted in the open end of the reservoir sleeve, which is then closed by a secondary weld. This secondary weld is reliably hermetic without the Mo–Ru coating, because it does not involve the porous tungsten matrix. Moreover, we have discovered an inspection procedure that allows us to check weld hermeticity at each assembly step. Thermal cycling tests indicate good weld integrity. Emission life testing is proceeding.
Keywords: Thermionic cathodes; Reservoir cathodes; Dispenser cathodes
Characteristics of scandate-impregnated cathodes with sub-micron scandia-doped matrices by Haiqing Yuan; Xin Gu; Kexin Pan; Yiman Wang; Wei Liu; Ke Zhang; Jinshu Wang; Meiling Zhou; Ji Li (pp. 106-113).
We describe in this paper scandate-impregnated cathodes with sub-micron scandia-doped tungsten matrices having an improved uniformity of the Sc distribution. The scandia-doped tungsten powders were made by both liquid–solid doping and liquid–liquid doping methods on the basis of previous research. By improving pressing, sintering and impregnating procedures, we have obtained scandate-impregnated cathodes with a good uniformity of the Sc2O3- distribution. The porosity of the sub-micron structure matrix and content of impregnants inside the matrix are similar to those of conventionally impregnated cathodes. Space charge limited current densities of more than 30A/cm2 at 850°Cb have been obtained in a reproducible way. The current density continuously increases during the first 2000h life test at 950°Cb with a dc load of 2A/cm2 and are stable for at least 3000h.
Keywords: Thermionic emission; Scandate-impregnated cathode; Nano-sized scandia; Sub-micron structure matrix; I; –; V; curve
Scandate cathode for TWT by Wang Shuguang (pp. 114-119).
Because of its low work function, a scandate cathode can have a high emission current density at a low operating temperature. However, this cathode has got limited application because of its lack of emission uniformity and poor stability in operation. By modifying both the fabrication of the tungsten sponge and the cathode structure, the emission uniformity and the emission current density of these cathodes have been improved at well-chosen operating temperatures. If the variation of the heater power is kept within ±5%, then the corresponding change of cathode temperature will be less than 25–30°C, and the fluctuation in the emission current will be less than ±2% in the application of traveling wave tubes (TWTs) in our company.
Keywords: Scandate cathodes; Traveling wave tube (TWT); Emission current; Work function
Correlation of cathode parameters of high power grid tubes with material characteristics of cathode-grid units by Irina P. Melnikova; Igor V. Polyakov; Dmitry A. Usanov (pp. 120-125).
One way to increase the longevity of dispenser cathodes is based on reducing the Barium evaporation. This can be achieved by the decrease of the reaction “activity� of the emitter impregnant with the porous tungsten (W) body, which supplies free Barium from the interior of the porous cathode to its surface.
Keywords: PACS; 79.40Dispenser cathodes; Cathode longevity; Ba evaporation; Pore size; Powder “activity�
A new dispenser cathode with dual-layer by Yutao Li; Honglai Zhang; Pukun Liu; Mingchen Zhang (pp. 126-129).
The emission and surface characteristics of the dispenser cathode coated with Os–W alloy and that coated with Os–W/Re are studied and compared. The dispenser cathode coated with Os–W/Re has been applied in electron gun measurement system for making measurement of higher emission current and life test. We called the cathode coated with Os–W/Re as the cathode with dual-layer. It is found that the dispenser cathode coated with dual-layer has higher current density than that coated only with Os–W alloy. After being activated, the cathode coated with dual-layer presents ternary composition on the surface of it. The W surface composition does not rise with time comparing with that of the cathode coated with Os–W alloy. In electron gun, the dispenser cathode coated with dual-layer has pulse current density of 30A/cm2 and life of more than 800h.
Keywords: Dispenser cathode; Os–W alloy; Os–W/Re; Dual-layer
Emission and surface characteristic of ternary alloy Ir/Re/W-coated impregnated tungsten cathodes by Honglai Zhang; Yanwen Liu; Mingchen Zhang; Yutao Li (pp. 130-133).
In order to improve the activation characteristics and emission ability of the conventional Ir-coated impregnated tungsten cathodes, a new type of dispenser cathode with ternary alloy Ir/Re/W coating was developed. The improved cathodes show higher emission current density and faster activation characteristics than that of the conventional pure Ir-coated impregnated tungsten cathodes. X-ray photoelectron spectroscopy (XPS) was used to analyze the element compositions on the surface of the cathodes coated with pure Ir and Ir/Re/W alloy. The results show that for pure Ir coating cathode, binary alloy (Ir/W) is formed. The surface atom concentration is near 50/50 after full activation. For ternary alloy coating cathode, the surface atom concentration has changed from 35%Ir–25%Re–40%W to 33%Ir–19%Re–48%W before and after activation.
Keywords: Thermionic electron emission; Cathode; Impregnated cathode; Dispenser cathode
Study on preparation and emission properties of nano-composite W–La2O3 material by Xiaoli Xi; Zuoren Nie www.bjut.edu.cn; Wei Wang; Jiancan Yang; Shiming Hao; Yanqun Guo; Tieyong Zuo (pp. 134-138).
This paper describes the preparation of nano-composite W–La2O3 material by freeze-drying and spark plasma sintering (SPS). The thermionic electron emission capability and surface properties of this material have been investigated. The surface compositions and depth profiles of the prepared material were checked “in situ� by means of AES from 298K to 1573K. Nano-composite W–La2O3 material exhibits good electron emission performance: a saturated emission current density of 1.46Acm−2 can be reached at 1673K with a corresponding work function of 2.79eV. The surface analysis results showed that the good emission capability might be related to an about 10nm surface layer contained excess La caused by diffusion of La from bulk material to the surface.
Keywords: Nano-composite; Thermionic emission; Freeze-drying; Spark plasma sintering; Surface properties
The decline of impregnated cathodes in CRTs by Daniel den Engelsen; Linsu Tong (pp. 139-145).
In the 1990s various CRT-companies introduced impregnated cathodes (I-cathodes) in color picture tubes (CPTs) in order to meet the requirements of good life performance at high beam currents.The initially successful introduction of I-cathodes in mass production did not result in a complete change over; on the contrary, I-cathodes are gradually phased out from CRTs. The reasons are the recent improvements of oxide cathodes, rather high cost of I-cathodes, sensitivity of I-cathodes for ion bombardment and electron optic considerations.
Keywords: Current density; Cathode load; Electron optics; Ion-surface impact; Field emission; Electrolysis
Aging process of I-cathode with magnetic ion trap by Xiaobing Zhang; Wei Lei; Niangen Feng; Jos Havekes; Linsu Tong; Daniel den Engelsen (pp. 146-150).
An aging process, which applies a high frequency (HF) magnetic field on the electron gun during the aging process, is introduced to solve the unbalanced I-cathode emission slump. The effect is that the scanning electron beam and the HF magnetic field heat up the gun parts by electron bombarding and eddy current heating. In this way, the grids are effectively degassed. A part of the desorbed gases is pumped by the Ba-getter in the tube, whereas another part is ionized by electron collision. These ionized gas molecules, notably Ar+, are partially trapped in gun parts. Therefore, a lower residual gas pressure and emission slump can be achieved.
Keywords: PACS; 41.90+e; 51.50+vAging; Ion trap; Residual gas
High current density M-type cathodes for vacuum electron devices by Ji Li; Zhiqiang Yu; Wensheng Shao; Ke Zhang; Yujuan Gao; Haiqing Yuan; Hui Wang; Kaizhi Huang; Qilue Chen; Suqiu Yan; Shaolun Cai (pp. 151-158).
We investigated high current density emission capabilities of M-type cathodes used for vacuum electron devices (VEDs). The experimental results of emission and lifetime evaluating in both close-spaced diode structure and electron gun testing vehicles are given. Emission current densities measured in the diode structure at 1020°CBr in the CW mode were above 10A/cm2; while in electron gun testing vehicles, emission current densities were above 8A/cm2 in CW mode and above 32A/cm2 in pulsed mode, respectively. The current density above 94A/cm2 has been acquired in no. 0306 electron gun vehicle while the practical temperature is 1060°CBr. For a comparison some of the data from I-scandate cathodes are presented. Finally, several application examples in practical travelling wave tubes (TWTs) and multi beam klystrons (MBKs) are also reported.
Keywords: M-type cathodes; High current density; Lifetime; Emission characteristics; Multi beam klystrons
Application of vitreous and graphitic large-area carbon surfaces as field-emission cathodes by Charles E. Hunt; Yu Wang (pp. 159-163).
Numerous carbon bulk or thin-film materials have been used as field-emission cathodes. Most of these can be made into large-area and high-current field-emission cathodes without the use of complex IC fabrication techniques. Some of these exhibit low-extraction field, low work-function, high ruggedness, chemical stability, uniform emission, and low-cost manufacturability. A comparison of all of these materials is presented. Two viable cathode materials, reticulated vitreous carbon (RVC) and graphite paste are examined here and compared.
Keywords: PACS; 73.01AGraphite; Reticulated vitreous carbon (RVC); Field emission; Cathode; Ion radiation
Special features of electron sources with CNT field emitter and micro grid by Wolfram Knapp www.uni-magdeburg.de/avp; Detlef Schleußner (pp. 164-169).
A micro-sized electron source plays an important role for new vacuum triode applications. For these applications, an electron source with CNT field emitter and micro grid for 1mA was developed and investigated. The miniaturisation of the electron source was achieved by the use of a carbon nanotube (CNT) field emitter and a micro grid, with a distance of only a few micrometers. Because of the threshold field strength for field emission of CNTs being in the range 1–5V/μm, the grid voltage can be lower than 100V. In our contribution, we discuss the influence of the micro grid on electron source properties, especially anode-current hysteresis, anode-field penetration through the micro grid and micro-lensing effect.
Keywords: PACS; 07.77.KElectron source; Electron field emitter; Carbon nanotube field emitter
Characteristics of a cold cathode electron source combined with secondary electron emission in a FED by Wei Lei; Xiaobing Zhang; Xuedong Zhou; Zuoya Zhu; Chaogang Lou; Hongping Zhao (pp. 170-176).
In electron beam devices, the voltage applied to the cathode (w.r.t. grid voltage) provides the initial energy for the electrons. Based on the type of electron emission, the electron sources are (mainly) classified into thermionic cathodes and cold cathodes. The power consumption of a cold cathode is smaller than that of a thermionic cathode. The delay time of the electron emission from a cold cathode following the voltage rise is also smaller. In cathode ray tubes, field emission display (=FED) panels and other devices, the electron current emitted from the cathode needs to be modulated. Since the strong electric field, which is required to extract electrons from the cold cathode, accelerates the electrons to a high velocity near the gate electrode, the required voltage swing for the current modulation is also high. The design of the driving circuit becomes quite difficult and expensive for a high driving voltage.In this paper, an insulator plate with holes is placed in front of a cold cathode. When the primary electrons hit the surface of the insulator tunnels, secondary electrons are generated. In this paper, the characteristics of the secondary electrons emitted from the gate structure are studied. Because the energies of the secondary electrons are smaller than that of the primary electron, the driving voltage for the current modulation is decreased by the introduction of the insulator tunnels, resulting in an improved energy uniformity of the electron beam. Triode structures with inclined insulator tunnels and with double insulator plates are also fabricated and lead to further improvements in the energy uniformity. The improved energy uniformity predicted by the simulation calculations is demonstrated by the improved brightness uniformity in the screen display images.
Keywords: PACS; 79.20.Rf; 34.50.FaCold cathode; Secondary electron emission; Insulator tunnel; Triode structure; Field emission display
Theoretical analysis of the field enhancement in a two-dimensional triple junction by Moon S. Chung; Tae S. Choi; Byung-G. Yoon (pp. 177-181).
The field enhancement at a triple junction is theoretically investigated. For a long metal–dielectric–vacuum junction, the field behavior in the three media is described as a function of the geometry and the dielectric constant. It is found that the key factor determining the field enhancement is the ratio of the angles subtended by the dielectric and vacuum portions. This phenomenon can account for the field emission enhancement and the vacuum breakdown observed in experiments. The current result implies that the triple junction can be a new type of field emission source.
Keywords: Triple junction; Field enhancement; Metal–dielectric–vacuum
Analysis of the transverse energy distribution of hopping electrons through a glass funnel by Xiaobing Zhang; Wei Lei; Min Liu; Laibin Zhang; Daniel den Engelsen; Xuedong Zhou; Qilong Wang (pp. 182-190).
The transverse energy distribution of electrons leaving the exit hole of a glass funnel in a hopping field emission display (HopFED) has been analyzed. These electrons are accelerated in a uniform field onto the anode screen. The luminance distribution of the spot on the screen is largely determined by the transverse energy distribution of the electron beam. The analysis of the luminance distribution shows that the transverse energy is rather low and that a HopFED will have good beam directionality.
Keywords: PACS; 85.45.FdField emission display; Hopping electron; Transverse energy distribution
Analysis of the carrier concentration for field emission from Al xGa1− xN by Tae S. Choi; Moon S. Chung (pp. 191-195).
The field emission current density j from the ternary alloy Al xGa1− xN is theoretically calculated as a function of composition x for 0≤ x≤1. By considering the doping and background concentrations and the internal field emission as sources of the carrier concentration n, we use a fully exact scheme to calculate j from Al xGa1− xN. It is found that the exact x-dependence of n yields theoretical j in agreement with experiment.
Keywords: Field emission; AlGaN; Schottky barrier; Electron affinity; Carrier concentration
Field emission cathodes based on milled carbon fibers by E.P. Sheshin; A.S. Baturin; K.N. Nikolskiy; R.G. Tchesov; V.B. Sharov (pp. 196-200).
In this paper, the production technology of field emission cathodes based on ball-milled carbon fibers is presented. The method is to use carbon fiber powder with optimal particle size. For the powder deposition on the cathode base, a screen-printing technique has been applied. The cathode structure obtained can provide significant electric field enhancement. The field emission properties of these cathodes are reported.
Keywords: PACS; 79.70Field emission; Carbon fibers; Cold cathodes; Screen-printing
Fabrications of Spindt-type cathodes with aligned carbon nanotube emitters by Ming Q. Ding; Xinghui Li; Guodong Bai; Jing J. Feng; Fuquan Zhang; Fujiang Liao (pp. 201-204).
In this paper, we present a technique for fabricating carbon nanotubes (CNTs) field emission arrays in an integrated gate structure. Two kinds of such Spindt-type cathodes with vertically aligned carbon nanotubes emitters were fabricated. One process involved direct growth of CNTs from the bottom of micro-sized cells and the other process was to grow CNTs on pre-deposited Mo tips in Spindt-type arrays. The former had fairly good, consistent emitters with a number of CNTs self-“shaped� in a tip-like form, whereas the latter showed that the emitters comprised only a few or single CNTs on Mo tips. Preliminary current–voltage measurements of a 20×20 array with CNTs grown from the bottom of each cell showed an onset voltage of ∼30V with a relatively large gate leakage current.
Keywords: PACS; 07.77.KSpindt-type cathodes; dc plasma; Aligned carbon nanotubes; Emitters
Atomic-scale field emitter with self-repairable function and thermodynamically stable structure: FEM study on Pd-covered nanopyramids on W<111> tips by E. Rokuta; T. Itagaki; D. Miura; T. Moriyama; T. Ishikawa; B.-L. Cho; T.Y. Fu; T.T. Tsong; C. Oshima (pp. 205-209).
We have fabricated nanometer-scaled pyramids on the W tip via Pd deposition and post-deposition annealing, and the field electron emission (FE) properties of the nanopyramids were investigated. Owing to the pyramid formation, the emission angles of the FE beams were drastically confined toward the forward direction. Further, the FE characteristics were reproduced via the self-repairing function inherently possessed by the nanopyramids. In the FEM observations, two characteristic features were seen. One is a rectangular triangle composed of three distinct spots, which is attributable to the trimer tip consisting of three atoms. The other pattern is a single bright spot, which is attributable to FE from a single atom tip. Semicone angle of the single-spot beam was 3.5°, which is comparable to those of the previous W nanotips.
Keywords: Field electron nanotips; Self-repairing function; Thermodynamically stable structures; Coherent electron beams; Single-atom tips; Trimer tips
Effect of ageing process on performance of molybdenum field emission arrays by Xinghui Li; Chongfeng Yang; Jinjun Feng; Jun Cai; Guodong Bai; Mingqing Ding; Fuquan Zhang; Fujiang Liao (pp. 210-214).
The paper describes the beneficial effect of application of ballast resistors and an ageing process on the stability of field emission from Mo-tips. High-resistant silicon substrates were used as ballast resistor and better emission uniformity was obtained as compared with low-resistant silicon. The emission current of the field emitter arrays (FEAs) increased by 43% and 50% by DC and pulse mode ageing, respectively, and the emission current fluctuation as a function of time was decreased to 3.7%.
Keywords: PACS; 07.77.KField emission arrays; Ballast resistor; Spindt tip; Emission uniformity; Ageing process
Field emission characteristics of oriented-AlN thin film on tungsten tip by S.L. Yue; C.Z. Gu; C.Y. Shi; C.Y. Zhi (pp. 215-219).
(002) Oriented-aluminum nitride (AlN) films with different thicknesses were deposited on tungsten (W) tips by using radio frequency magnetron reactive sputtering system. Compared studies of field emission (FE) characteristics were performed between the bare and AlN coated W tips. The results showed that enhanced electron emission could be obtained from oriented-AlN film on W tip. The hysteresis behaviors shown in Current–electric field ( I–E) curves during downward electric field sweeps were observed, and the extent of hysteresis in I–E curves strongly depended on the thickness of the AlN film. The stability measurement of FE current presented that the hysteresis could be attributed to the charging in AlN film as an insulator.
Keywords: PACS; 77.84.B; 81.10.A; 79.70.+qAluminum nitride; Orientation; Field emission
Synthesis and field emission properties of aluminum nitride nanocones by Chun Liu; Zheng Hu; Qiang Wu; Xizhang Wang; Yi Chen; Weiwei Lin; Hai Sang; Shaozhi Deng; Ningsheng Xu (pp. 220-224).
One-dimensional aluminum nitride nanostructures have displayed superior field emission due to the combination of small or negative electron affinity and one-dimensional quantum confinement effect. Herein we report on the synthesis of quasi-aligned AlN nanocones via chemical vapor deposition on the Ni-coated silicon wafer at 750°C through the reaction between AlCl3 vapor and NH3/N2 gas. The as-prepared hexagonal AlN nanocones grow preferentially along c-axis with the tips’ sizes of about 60nm and the lengths up to several microns. The field emission measurement exhibits a notable electron emission with the apparent turn-on field of 17.8V/μm, indicating their potential applications as the field emitters. Due to space charge effect, the corresponding Fowler–Nordheim plot shows a two-sectional characteristic with the field enhancement factors of 1450 and 340 at low and high electric fields, respectively.
Keywords: PACS; 79.70.+qAlN nanocones; Chemical vapor deposition; Field emission; Two-sectional Fowler–Nordheim plot; Space charge effect
Enhanced electron emission from diamond film deposited on pre-seeded Si substrate with nanosized diamond power by C.Z. Gu (pp. 225-229).
Diamond film was synthesized by microwave plasma chemical vapor deposition (MWPCVD) method. The deposition process of diamond film on 4-in. pre-seeded mirror polished silicon wafer was divided into four steps: (a) seeding nano-diamond powder on Si surface; (b) annealing for increasing the adsorbed strength between diamond powder and substrate; (c) diamond film growth; (d) bombarding with H+ ion for decreasing film stress and obtaining planar field emission from diamond film with large area. Scanning electron microscopy (SEM), Raman spectroscopy and stress measurement system were used to characterize the structure and property of diamond film. The electron emission from large area diamond film on seeded substrate was described and compared with that from diamond film deposited on Si substrate scratched by diamond powder. The results suggested that low-field electron emission and high emission current could be obtained from diamond film deposited on seeded substrate due to the reduction of interface energy barrier for electron tunneling. A threshold field of 3.0V/μm and emission current density of 1mA/cm2 at 30V/μm were achieved.
Keywords: Diamond film; Field emission; Seeded substrate; Chemical vapor deposition
Cathode units with a carbon fiber field emitter by M. Chupina; O. Ivanov; O. Maslennikov; E. Orekhov (pp. 230-235).
We have developed a carbon fiber field emitter (CFFE) with a diameter of 7μm and a bullet shape at the top. We found that a micro-pit on the top of the fiber can focus the emitted electron beam, so that the beam is inside a small solid angle around the axis of symmetry. Our investigations show that due to this effect the fiber emitter with optimized dimensions of the micro-pit has good emission stability, good reproducibility for different cathode samples and high electron optical brightness. The CFFE with micro-pit on the top has been successfully used for 14 years in the “Cwikscan-50A� electron microscope instead of the conventional tungsten field emitter.
Keywords: Carbon fiber field emitter; Micro-pit; Electron microscope; Beam focusing
Field emission properties of diamond-like carbon films annealed at different temperatures by J.J. Li; C.Z. Gu; H.Y. Peng; H.H. Wu; W.T. Zheng; Z.S. Jin (pp. 236-241).
Effect of annealing on the field emission properties of diamond-like carbon (DLC) films deposited by high power excime laser technique was studied. Raman spectrometry and X-ray photoelectron spectrometry (XPS) were used to determine the chemical bonding structural change of the annealed DLC films. It was found that the annealing transformed gradually DLC films into a graphitic structure as annealing temperature was increased to more than 600°C, simultaneously field emission properties of DLC films were enhanced markedly. Graphitization of DLC films induced by annealing caused the formation of a large fraction of sp2 C in the DLC films, which played an important role in the enhancement of field emission properties for DLC films. Rich sp2 C cluster acted as a conduction part in the films could increase the conductivity of DLC films and lower the effective work function of DLC films, consequently improve field emission properties of DLC films.
Keywords: PACS; 81.15.Fg; 79.70. +q; 79Diamond-like carbon films; Field emission; Annealing
Electrophoretic deposition and field emission properties of patterned carbon nanotubes by Haifeng Zhao; Hang Song; Zhiming Li; Guang Yuan; Yixin Jin (pp. 242-244).
Patterned carbon nanotubes on silicon substrates were obtained using electrophoretic method. The carbon nanotubes migrated towards the patterned silicon electrode in the electrophoresis suspension under the applied voltage. The carbon nanotubes arrays adhered well on the silicon substrates. The surface images of carbon nanotubes were observed by scanning electron microscopy. The field emission properties of the patterned carbon nanotubes were tested in a diode structure under a vacuum pressure below 5×10−4Pa. The measured emission area was about 1.0mm2. The emission current density up to 30mA/cm2 at an electric field of 8V/μm has been obtained. The deposition of patterned carbon nanotubes by electrophoresis is an alternative method to prepare field emission arrays.
Keywords: PACS; 61.46.+w; 82.45.−h; 79.70.+qCarbon nanotubes; Electrophoresis; Field emission; Field emission arrays
Field electron emission from branched nanotubes film by Baoqing Zeng; Shikai Tian; Zhonghai Yang (pp. 245-248).
We describe the preparation and analyses of films composed of branched carbon nanotubes (CNTs). The CNTs were grown on a Ni catalyst film using chemical vapor deposition from a gas containing acetylene. From scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses, the branched structure of the CNTs was determined; the field emission characteristics in a vacuum chamber indicated a lower turn on field for branched CNTs than normal CNTs.
Keywords: PACS; 79.70Carbon nanotubes (CNTs); Chemical vapor deposition; Ni catalyst; Field enhancement
Fabrication and characterization of high-current-density carbon-nanotube cold cathodes by Chunhui Zhu; Chaogang Lou; Wei Lei; Xiaobing Zhang (pp. 249-253).
In this paper, we report a recent experimental observation of high field-emission current density (6.5A/cm2) from carbon nanotube emitters. Carbon nanotubes are grown on Ni catalyst coated on Si substrates using chemical vapor deposition of C2H2 gas at 700°C. Our research shows that it is beneficial for getting the high-current density in our experiment that Au film was deposited onto Si substrate before evaporating a Ni catalyst dot onto the substrate. We compared the field emission characteristics of CNTs cathodes grown on the Ni films with different thickness of 50, 70, 100nm. The thickness of the Ni film is very important for the growth of CNTs, which will affect the field emission properties very much. The Fowler-Nordheim plot showed a good linear fit, indicating that the emission current of carbon nanotubes follows Fowler-Nordheim behavior. The calculated field enhancement factor was 2370.
Keywords: PACS; 79.20.Rf; 34.50.FaHigh field-emission current density; Carbon nanotube emitter
New method to fabricate field-emission cathode of carbon nanotubes by Chaogang Lou; Xiaobing Zhang; Wei Lei; Chen Qi (pp. 254-257).
This paper presents a novel method to fabricate the field-emission cathode of carbon nanotubes (CNTs) to improve the stability of field emission against ion bombardment. The traditional processing of powder metallurgy is used to fabricate carbon nanotube cathodes, which have stronger adhesion of CNTs to substrates. In this method, metal powder and carbon nanotubes are mixed, pressed and sintered to form a bulk material. The surface of the sintered sample is etched by acid to remove a thin surface layer of metal, thus the top of CNTs form the tips for field emission, and their roots are embedded into the metal to enhance the adhesion of CNTs. The cathode fabricated by this method has typical I– V curve and F– N curve, and demonstrates more stable field emission than that made through chemical vapor deposition (CVD).
Keywords: PACS; 79.70.+q; 81.20.Ev; 81.07.DeCarbon nanotubes; Field emission; Powder metallurgy
Electron field emission properties of carbon nanotubes-deposited flexible film by Huizhong Ma; Lan Zhang; Junjie Zhang; Liwei Zhang; Ning Yao; Binglin Zhang (pp. 258-261).
A carbon nanotubes-deposited flexible film was prepared by electrophoretic method. A polyimide film coated with titanium was used as a cathode and a stainless steel plate as an anode, respectively. The two electrodes were placed into a bath with solvent consisting of grinded carbon nanotubes powder, Mg(NO3)2·6H2O and isopropyl alcohol (IPA). Mixed solution was stirred for about 2–4h to assure the complete dissolution of Mg(NO3)2·6H2O and the dispersion of carbon nanotubes powder, dc electric field was applied to the two electrodes. The anode voltage was about 100V. After 1–2min, the carbon nanotubes powders were deposited on the polyimide film coated with titanium. The electron field emission properties of the prepared thin film were tested by a diode assembly. The turn-on field of the flexible film was about 2.96V/μm. The current density of the flexible film was about 200μA/cm2 at an electric field of 5.9V/μm. The morphology and structure of the carbon nanotubes-deposited flexible film were analyzed by scanning electron microscope (SEM) and Raman spectroscopy. The experimental results indicate that this film could be a promising material applicable to cold cathodes.
Keywords: PACS; 79.70 +q; 81.15.Gh; 73.40.GKElectron field emission; Carbon nanotubes; Electrophoretic deposition; Polyimide; Diode assembly; Fowler–Nordheim plot
Investigations of the multi-pulsed emission characteristics of velvet by Liansheng Xia; Kaizhi Zhang; Jinshui Shi; Linwen Zhang (pp. 262-266).
Some efforts were made to measure the electron emission characteristics of velvet under multi-pulse mode. The voltages across the diode were 1MV for two-pulse mode and 500kV for four-pulse mode. Pulse duration was 100ns and pulse to pulse separation was 400ns. The research indicates that under multi-pulse mode the emission of velvet is not uniform and that beam degradation is caused by cathode plasma motion. The factors that determine the plasma motion are analyzed and the experimental results of plasma velocity and its distribution are presented.
Keywords: PACS; 51.50; 52.80Multi-pulsed diode; Field emission; Velvet; Cathode plasma
Angle-dependent XPS study of the mechanisms of “high–low temperature� activation of GaAs photocathode by Xiaoqing Du; Benkang Chang (pp. 267-272).
The surface chemical compositions, atomic concentration percentage and layer thickness after “high-temperature� single-step activation and “high–low temperature� two-step activation were obtained using quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (XPS). It was found that compared to single-step activation, the thickness of GaAs–O interface barrier had a remarkable decrease, the degree of As–O bond became much smaller and the Ga–O bond became dominating, and at the same time the thickness of (Cs, O) layer also had a deduction while the ratio of Cs to O had no change after two-step activation. The measured spectral response curves showed that a increase of 29% of sensitivity had been obtained after two-step activation. To explore the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on photoemission, surface electric barrier models based on the experimental results were built. By calculation of electron escape probability it was found that the decrease of thickness of GaAs–O interface barrier and (Cs, O) layer is the main reasons, which explained why higher sensitivity is achieved after two-step activation than single-step activation.
Keywords: Angle-dependent X-ray photoelectron spectroscopy (XPS); GaAs; Photocathode; Activation; Escape probability
The variation of spectral response of transmission-type GaAs photocathode in the seal process by Liu Lei; Chang BenKang; Du YuJie; Qian YunSheng; Gao Pin (pp. 273-277).
In this paper, firstly the spectral response of transmission-type GaAs photocathode is measured online by the spectral response-testing instrument. Then the cathode is sealed in the third generation intensifier and put into the instrument again to get another spectral response curve. The variation of spectral response curves was compared. The results show that through the seal process, the spectral response in the long wavelength decrease. Based on these curves, the spectral matching factors of GaAs photocathode for green vegetation and rough concrete are calculated. The calculated performance parameters show that the variation of the spectral response in the seal process is an important influence factor on the performance of the intensifier in the use of night vision.
Keywords: GaAs photocathode; Spectral response; Spectral matching factor; Visual range