Interpretation of the structure in the current versus reciprocal field curves in photofield emission

The structure observed in the logarithm of the current versus reciprocal field curves in photofield emission is interpreted by a simple analytic model.     

Oscillatory photocurrent in photofield emission

Several groups have reported an oscillatory field-dependent component in the photocurrent detected in photofield emission. New measurements have been performed in the experimental configuration used by one of these groups. It is demonstrated that the oscillations observed in this configuration are due to the magnetic deflection of the electron beam across the retarding mesh in the electron optic system, which modulates the detected electron current. No evidence is found for an oscillatory component inherent in the electric field dependence of the total photofield emission current from tungsten.     

Emission photoélectrique d'une pointe EN W OU W-Ba Soumise à un champ électrique

The photoelectric emission from a pure or barium-coated single crystal tungsten tip submitted to a high electric field is studied under illumination by light of energy smaller than the work fonction of the substrate. The results are discussed according to the range of the electric field applied. It is found that the average values of the work function for barium coated tips as obtained by photofield emission are different from those obtained by field emission. The photofield emission and field emission patterns are compared.     

Atomic structure and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces

We present results of theoretical and experimental studies of the structural and electronic properties of Ta(1 1 2) and W(1 1 2) surfaces. Atomic geometries of these surfaces and their electronic structures have been obtained from the first-principles pseudo-potential calculations based on the density functional theory and the use of the plane wave basis set. In the experimental part of our studies we have performed measurements of the photofield emission spectroscopy. The calculated density-of-states distributions are compared and discussed with energy distributions of electrons extracted from photofield emission characteristics. The photofield emission spectra measured for the surface regions of considered systems present very dense structures of features, while the corresponding dependences obtained for the bulk have much more smooth shape. Experimental results correspond very well with calculated density-of-states distributions.     

Photofield emission: An explanation of the rapid oscillations with applied field

We argue that emission from a band of surface states (these states being final states in the photo-excitation process) on a small-area-plane of the field emission tip, may be the cause of the observed rapid oscillations with the applied field of the photofield current.     

The shape of the total energy distribution from W(110) in sub-threshold photoemission

Measurements of the final state energy distribution of electrons photoemitted from W(110) at energies less than the vacuum level are presented. A 127° velocity selector analyzer was used to energy resolve the photoemitted current. A significant improvement in both energy resolution and signal-to-noise ratio over previous studies resulted. The energy distributions obtained from W(110) are compared the theoretical predictions based on a recent model of photofield emission. The sensitivity of this theoretical model to various parameters of interest are investigated. The reasonable agreement between theory and experiment indicates that the final states probed in these measurements are free electron-like and not strongly representative of the final bulk density of states of the emitting material.     

Irradiance dependence of photofield emission from tungsten

Measurements of the irradiance dependence of photofield emission from tungsten have been extended to 1.0 GW m⁻². The photoyield (the photocurrent per unit irradiance) from the (111) crystal plane is found to be almost independent of irradiance. The (211) and (310) crystal planes are found to exhibit small nonlinearities which are attributed to changes in the work function of the emitting surface caused by laser heating. Even at the highest irradiance, no measurable current of photoelectrons arising from multiphoton excitation is observed.     

Densities of states of low-index surfaces of tantalum by photofield emission

The total energy distributions (TED) of the true photofield emission current from clean (1 0 0), (1 1 0) and (1 1 1) facets of Ta field emitters have been measured using a new method to remove the thermocurrent associated with laser heating. Each TED exhibits one or more prominent peaks that are interpreted by comparing them with ab initio calculations of the TED of the emission current based on density functional theory. The generally good agreement with experiment indicates that the same method can be used for accurate calculations of the densities of states of low-index surfaces of Ta. Each of the experimental TEDs shows, in addition to the prominent peaks, a set of weaker peaks that are not predicted by the calculation and whose spacing depends on the sharpness of the field emitter. These weaker peaks are interpreted as arising from size-effect resonances in the microcrystal at the apex of the field emitter.     

Polarization dependence of photoexcitation in photofield emission

The mechanisms of one-photon photoexcitation in photofield emission have been studied experimentally by measuring the photocurrent from selected crystallographic planes of a tungsten field emitter as a function of the polarization of the incident light. Two distinct components of the photocurrent were identified. The corresponding excitation mechanisms were deduced from the results of the polarization measurements supplemented by band structure considerations. Surface effect emission dominates at large angles of light incidence, and is observed even from within the geometric shadow of the tip. It depends on the light polarization inside the metal in a characteristic way. Indirect bulk photoemission is prominent at small angles of light incidence and is independent of the polarization inside the metal. It is concluded that bulk and surface excitations in photofield emission can be distinguished by appropriately selecting the direction of polarization and the angle of incidence of the incident light.     

Structural and electronic properties of the Ti/W(1 1 1) adsorption system

We present theoretical and experimental study of the structural and electronic properties of the Ti/W(1 1 1) adsorption system. Atomic arrangements of the considered surfaces and their electronic structures have been obtained from first-principles pseudopotential calculations based on the density functional theory in a plane-wave-basis implementation. The corresponding experimental data have been provided by photofield emission spectroscopy. Investigations of the clean and Ti-covered W(1 1 1) surfaces indicate substantial structural relaxations deep into the substrate, and a noticeable modification of the surface electronic properties of the system induced already by a thin film of titanium. Configuration with adatoms positioned in substrate-lattice-continuation (i.e., deep-hollow) sites is found to be energetically most favorable. A good agreement between the measured photofield emission spectra and the computed local-density-of-states distributions confirms our theoretical predictions for a clean W(1 1 1) substrate as well as Ti coverages of 0.25 and 1 ML.     

Field emission tip as a nanometer source of free electron femtosecond pulses

We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.     

Thermal field emission and thermal photofield emission from tungsten (110)

The total energy distibution in field emission from the (110) surface of tungsten has been measured at temperatures in the range from 300 to 1400 K, which are sufficiently high to cause significant emission from thermally populated electronic states. Above the Fermi level, a striking departure from the prediction of free electron theory is observed. A similar anomaly is observed in the total energy distribution in photofield emission from thermally populated initial states on tungsten (110). In s-polarized light at normal incidence the anomaly vanishes when the polarization vector is parallel to the [001] direction. The anomalous behaviour is attributed to emission from the p-like energy band that is responsible for the ellipsoidal sheet of the Fermi surface centred at the symmetry point N. A comparison between the enhancement factors measured in sand p-polarized light yields no evidence for surface resonances close to the upper limit of the p-like energy band.     

Photofield emission spectroscopy of electronic states near the fermi level of 111 tungsten

The two closely situated electronic bands for the 111 tungsten direction between the Fermi level and the vacuum level have been observed using photofield emission spectroscopy and s-polarized light in the visible range. Measurements with p-polarized light reveal surface states below the Fermi level. Optical transitions have been found from the shoulders of the photocurrent characteristics. The observed excitations are in good agreement with the bulk band structure and the surface density of states of tungsten calculated by theory.     

Aggregates of chemisorbed copper on the (110) and (100) surfaces of tungsten: electronic structure

In a certain range of temperature and coverage, copper atoms chemisorbed on low-index facets of a thermally annealed tungsten field emitter aggregate to form stable islands surrounded by regions of extremely low adsorbate density, yielding a spatially inhomogeneous overlayer. Spatially resolved field emission and photofield emission measurements of the surface density of electronic states have been carried out for copper aggregates on the (110) and (100) facets. Features detected in the surface density of states near the centre of an aggregate yield clues to its structure. On the (110) facet, the measurements reveal striking differences between the surface density of states near the edge of an aggregate and that near the centre. It is argued that the adsorbate configuration in the region between the aggregates might approximate single-atom adsorption.     

Experimental study of quantum size effects in very thin metal films

The influence of the quantum size effect (QSE) in very thin Pd and Au films (0.5<d<10 [nm]) deposited on glass tips is studied by means of field emission at various anode potentials. The thickness-dependent field emission current characteristics show short periodic variations with a period of about 0.3 nm for both Pd and Au. The measured QSE oscillations and their amplitudes differ strongly from those expected qualitatively from model calculations for the smallest thicknesses. A model for the metal film based on a modification of the uniform background model is presented, and the thickness dependence of the conductance and the field emission current density are qualitatively discussed. Possible explanations of some discrepancies between calculations and the measured field emission current characteristics are proposed.     

场致发射的温度效应对微波管爆炸电子发射的影响

为研究场致发射的温度效应对微波管中爆炸电子发射过程的影响,在对比分析低温条件下的场致发射电流密度Fowler-Nordheim（FN）和一般的电子发射电流密度积分公式的基础上,利用细长圆柱形微凸起模型,重点考虑焦耳加热和热传导两个因素,编程计算得到了微凸起内部的温度分布和不同位置处温度随时间的变化。结果表明:场致发射的温度效应是一个重要影响因素,考虑温度对场致发射的影响后,微凸起内部各点的温度随时间呈非线性增长,且增长速率越来越大;在微波电场强度较弱时,若不考虑场致发射的温度效应而直接用FN公式表示的电流密度代入计算,会使爆炸发射延迟时间变短;当微波电场很强时,温度效应对爆炸发射延迟时间的影响则较小。     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

Emission properties of a silicon-polymer-vacuum planar structure

Self-sustained electron emission from a silicon-polymer planar structure is studied experimentally. It is found that emission can be initiated by a short-term action (e.g., a vacuum microdischarge). The I–V characteristics and time dependence of the emission current are recorded. It is shown that the value and stability of the emission current depend on the thickness of the polymer film. A model explaining the origin of and conditions for self-sustained electron emission is proposed. It is based on today’s concepts of electron emission from insulating layers (Malter effect) with regard to the structure and properties of polymer films.     

Photofield migration and desorption of impurity ions

Models which permit the theoretical study of diffusion-like processes under the simultaneous application of a pulsed laser field and a spatially nonuniform, quasiconstant electric field are proposed. The mean rates of photofield desorption are found. Evaluations show that it is comparatively easy to observe these effects experimentally and that they can find application in solving various problems in nanotechnologies. Zh. Tekh. Fiz. 67, 59–61 (December 1997)     

空间电荷效应对阴极微凸起热不稳定性的影响

 由于金属微凸起爆炸电子发射的预发射电流密度一般都超过108 A/cm2,因此必须考虑其空间电荷效应的影响。基于金属微凸起爆炸电子发射起始过程模型,通过理论分析和数值模拟,给出了考虑预发射电流空间电荷效应的微凸起爆炸发射延迟时间随二极管平均电场的变化关系。与不考虑预发射电流空间电荷效应的结果进行对比表明,预发射电流的空间电荷效应可以显著增加金属微凸起的爆炸发射延迟时间。