Structure and composition of tungsten silicide thermal-field microprotrusions

The thermal-field microprotrusions that grow on the surface of a tungsten tip coated with silicon when the tip is heated in an electric field are investigated by a suite of field emission methods, including electron field emission, ion desorption microscopy, and the atomic-probe method. For Si coatings more than a few monolayers thick, microprotrusions are observed to grow in the field desorption regime when the tip is heated to temperatures T=1100–1200 K in an electric field with initial intensity F=5.7–8.6×10⁷ V/cm. The field at which they evaporate is 1.2–1.8×10⁸ V/cm. The set of moving spots (i.e., microprotrusions) forms rings whose collapse signals the dissolution of the thermal-field growths on the developed faces. The most interesting structures are the sharp microprotrusions that grow on the central facet of a {110} tungsten tip under certain conditions. Atomic-probe analysis of their composition reveals that they consist of tungsten trisilicide WSi₃ with a monolayer surface skin whose composition is close to WSi₂. The intense growth of these formations on an initially smooth closepacked {110} face of tungsten is evidence that reconstruction of the latter takes place under the influence of the strong field and the interaction with silicon. Zh. Tekh. Fiz. 67, 102–109 (September 1997)     

Thermal and field emission effects of laser radiation on metal whisker diodes: Application to infrared detection devices

In a series of recent experiments, research groups have made absolute frequency measurements with laser beams in the infrared region of the spectrum (λ ? 10 μm) using a metal point contact diode for generation, frequency mixing and detection. It has been postulated that the mechanism for the nonlinear current-voltage characteristic of the diode is tunnelling of electrons through an intermediate oxide film from the whisker into the metal base, i.e., the configuration is considered to be a metal-oxide-metal (MOM) tunnelling junction. Several features of the diode's operation create considerable doubt concerning the applicability of the MOM tunnelling mechanism. Analysis of the available experimental data led us to postulate an alternate solid state mechanism, namely a thermally enhanced field emission process. Such emission would be a consequence of the immersion of the whisker tip in the laser radiation resulting in (1) conduction heating which induces thermionic emission and (2) generation of an electric field at the tip necessary for electron tunnelling by field emission. In this paper we calculate rigorously the power absorbed in the metal whisker from the incident radiation. From the power absorbed, the heat conduction equation is solved for model geometries to obtain the laser induced temperature distribution at the whisker surface. Estimates of the electric field are obtained and combined with temperature calculations to obtain the nonlinear I–V characteristics of the thermally enhanced field emission model. Finally some simple experiments are proposed to test the thermal field emission hypothesis as a possible mechanism to explain the nonlinear characteristics of the metal whisker point contact diode.     

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.     

Enhanced electron field emission from dense Si nano-dots prepared by laser crystallization of ultrathin amorphous Si films

Dense Si nano-dots with a surface area density of >10¹⁰ cm^?2 were fabricated by excimer laser induced crystallization of 15 nm-thick amorphous Si thin films. The enhanced electron field emission characteristics were found from laser irradiated samples. The threshold electric field is as low as 9.8V/μm and the field enhancement factor can reach as large as 719, which is compatible with the other good cold cathode materials. The improvements in field emission behavior can be associated with the change in the surface morphology after laser irradiation as well as the enhanced internal electric field due to the formation of Si nano-dots within the films.     

Surface reconstruction of a field electron emitter

The surface and emission images of a metal field’s electron cathode in the form of a tip are simulated. The surface structure is calculated in the thin-shell and broken-bond (local-environment) models for the perfect crystal lattice. The cathode shape and macroscopic electric field are represented by the sphere-on-cone model. The amplification of a local electric field is the adjustable parameter of the model. The method of determination of the emitter tip’s crystal faces based on the analysis of the surface atoms’ environment geometry is proposed. It is shown that it is enough to restrict the consideration of geometric environment by the fifth order of the nearest neighbors for the emitter radius of 100–1000 lattice parameters (31.6–316 nm for the tungsten). The crystallographic model of work function anisotropy in the broken-bond approach is used: the local work function’s value is set in accordance with Miller indices of the face containing this area. The model adequacy is corroborated by the comparison of current-voltage characteristics and emission images with the data of the natural experiment.     

The use of antenna theory to calculate the electric fields in a thermal field emission metal whisker diode

In recent years, research groups have used metal-metal point contact diodes for frequency mixing and detection of infrared laser radiation. It has been postulated that the mechanism for the nonlinear current-voltage characteristic of the diode is the tunneling of electrons through an intermediate oxide film from the whisker tip to the metal base, i.e., the configuration is considered to be a metal-oxide-metal (MOM) tunneling junction. Several features of the diodes' operation create considerable doubt concerning the applicability of the MOM tunneling mechanism. Analysis of the available data led us to postulate an alternate solid state mechanism, namely a thermally enhanced field emission process. Such emission would be a consequence of the immersion of the whisker in the laser radiation resulting in (1) conduction heating which induces thermionic emission and (2) generation of an electric field at the tip necessary for electron tunneling. In an earlier paper, we calculated the power absorbed by the cylindrical shank of a point contact diode in an infrared radiation field. Using the absorbed power as a source, detailed calculations were made of the laser induced temperature distributions on the diode; more approximate treatments were used to obtain the electric fields developed on the tip. Values of the computed temperature and field parameters for tungsten were found to be consistent with a thermal field emission process. In this paper we present a more rigorous calculation of the voltages and fields induced on different metal whisker tips by the incident laser radiation. Linear antenna theory is used to describe the receiving properties of the diode. The actual pointed geometry of the diode tip has been taken into account using Schelkunoff's theory of the conical antenna. The electric fields at the tip are found to be comparable with those necessary for field emission. The highest fields are established on gold tips, consistent with the experiments of Green et al. who found the best responsivity occurs with gold-gold contacts. Finally we discuss the significance of the experimental results of Young et al. on metal-vacuum-metal tunneling characteristics to the MOM tunneling hypothesis.     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Effect of dipole structures on field emission of wide-gap semiconductor emitters

It is shown that dipole structures placed in a thin (less than 1 nm) near-surface layer of a high-resistivity field emitter produce small domains on the emitting surface in which the electric field may exceed 10⁸ V/cm. In these domains, the emitter surface potential is positive, providing effective electron transport from inside the emitter to the emission boundary. Optimal dipole orientations ensuring maximal electric fields at the surface are found. When the surface density of dipoles localized in the near-surface layer is on the order of 10⁶ cm⁻², one can expect an emitter-averaged emission current density of higher than 1 A/cm². The dipole structures in the near-surface layer may persist owing to incorporated impurity molecules having a dipole moment or result from a random combination of positively charged ionized impurities and electrons captured by deep traps. Trap charging/discharging asymmetry accounts for the hysteresis of the emission I–V characteristics.     

The Poole–Frenkel effect in a dielectric under nanosecond irradiation by an electron beam with moderate or high current density

Processes of electron trapping and detrapping determine in many respects intense processes arising in dielectric and delayed by 1–100 ns from the irradiation pulse of a high-power electron beam, such as electron emission, electric discharge in the bulk of the dielectric, flashover, and electric breakdown. A model of charged donor center ionization in a dielectric exposed to a strong electric field is constructed. The model takes into account 1) the energy spectrum of the charged donor center in the dielectric, 2) the semiclassical state density in the donor center, 3) spontaneous emission of phonons by the electron localized in the donor center, 4) increase in the kinetic energy of the electron (heating) in the external electric field, 5) electron tunneling through a potential barrier and its reflection from the barrier depending on the external field intensity, and 6) thermal fluctuations of energy of the electron localized in the donor center. The probability of charged donor center ionization in the dielectric per unit time is calculated. In weak fields, the field dependence of the ionization probability almost coincides with that for the Poole–Frenkel theory. In strong fields, the contribution of electron heating to the external electric field is the deciding factor. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 10–16, December, 2008.     

Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Cross-sectional study of femtosecond laser bulk modification of crystalline α-quartz

Bulk irradiation of crystalline α-quartz was performed with ∼170-fs laser pulses with a wavelength of 800 nm focused below the sample surface. Investigations were carried out using transmission electron microscopy on a cross-sectional specimen prepared using focused ion beam techniques. We observed alternating amorphous–crystalline structures with sharp transitions and associated density changes, surrounded by a highly strained crystalline structure. The alternating sub-surface structures are parallel to the laser’s electric field polarization and exhibit a spacing which is close to the laser wavelength in air. Cracking was also observed in the near proximity of these structures.     

Ultrafast electron pulses from a tungsten tip triggered by low-power femtosecond laser pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.     

空间电荷效应对热场致发射中诺廷汉效应的影响

热场致发射阴极所产生的强流电子束具有很强的空间电荷效应,为研究该效应对热场致发射过程中诺廷汉（Nottingham）效应的影响机理,在理论分析的基础上,用数值方法研究了不同逸出功和多个外加电场条件下考虑空间电荷效应对诺廷汉效应结果的影响,并与不考虑空间电荷效应时的情形进行了对比. 结果表明：空间电荷效应的强弱会显著影响到阴极表面的稳态电场,进而对诺廷汉效应产生不可忽略的影响;当逸出功在3.0–4.52 eV、外加电场在3×10⁹–9×10⁹ V/m范围内时,考虑空间电荷效应的影响后,热场致发射电子所带走的平均能量较不考虑空间电荷效应时增加0–2.5 eV,且温度越高或外加电场越大时,该增加值越大;考虑空间电荷效应对诺廷汉效应的影响后,热场致发射电子从阴极带走的平均能量随外加电场的增加呈非线性下降规律;当阴极表面温度较高时,诺廷汉效应中的冷却效应随二极管间隙距离的变大而增强. 关键词： 热场致发射 诺廷汉效应 空间电荷效应 阴极表面电场     

碳纳米管场致发射中的空间电荷效应

采用微波等离子体化学气相沉积(MWPCVD)方法成功制备以碳纳米管束为单元的场致发射阵列,获得很好的场致发射电流发射特性,在电流密度较大时,发现I-V特性偏离由Fowler-Nordheim公式计算出的结果。采用Electron Beam Simulation(EBS)软件进行模拟分析发现:在电流密度较低时,I-V特性能很好与F-N公式吻合。但碳纳米管尖端电流密度大于106A/cm2时,碳纳米管尖端处的有效电场强度受空间电荷的影响比较明显,进而对碳纳米管的场致发射特性显现出不可忽略的影响,此时碳纳米管的发射电流密度开始受到空间电荷的限制。     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

爆炸电子发射初期阴极表面电场的研究

为了研究二极管爆炸电子发射初始阶段阴极表面复杂的物理现象及规律, 建立了由场致电子发射阴极构成的一维平板真空二极管物理模型,通过自行编程数值求解泊松方程, 考虑了发射出的电子对阴极表面电场的非线性影响,自洽模拟得到了阴极表面电场随时间的变化情况. 模拟结果表明,爆炸电子发射初期,阴极表面电场随时间的增加而呈现出不断振荡的规律, 且振荡幅度越来越小,最终到达一个稳态的值,二极管两极板之间的外加电场越大, 阴极表面稳态电场的绝对值越大;电场增强系数越大,阴极表面稳态电场的绝对值越大. 在整个时间演变过程中,阴极表面的实际电场强度决定着阴极发射的电流密度大小, 反过来阴极发射的电流密度又会影响到阴极表面的电场.     

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.     

Numerical study on the field emission properties of aligned carbon nanotubes using the hybrid field enhancement scheme

The magnitude of the local electric field and the electron emission current density for an array of aligned carbon nanotubes is estimated. For describing in detail the properties of the local electric field in the vicinity of the nanotube tips, a hybrid method allowing for the local determination of the field enhancement factor is introduced. The field factor consists of two parts: an internal factor which describes the structure of the carbon nanotubes and an external factor which represents the field screening effect due to neighboring nanotubes. The current density is obtained using the Fowler–Nordheim equation with the hybrid field enhancement scheme. As a result, the emission properties for an array of nanotubes with a given length are described satisfactorily, and an optimum value for the nanotube spacing is determined. PACS 85.45.Fd; 85.45.Db     

Numerical study of femtosecond laser-assisted atom probe tomography

We investigate the mechanisms of a laser-assisted atom probe tomography technique. In this method, a sub-wavelength tip is subjected both to a very strong static electric field and to a femtosecond laser pulse. As a result, ions are ejected from the tip one by one. By using femtosecond lasers, one can analyze not only metals but also semiconductors and dielectric materials. To better understand the ejection process, a numerical model is developed based on the drift-diffusion approach. The model accounts for such effects as field penetration, hole and electron movement, and laser absorption. For the given value of the dc field, a substantial band bending and an increase in hole density at the surface of the silicon tip are observed. This bending effect changes silicon absorption coefficient at the surface and significantly increases recombination time of laser-induced carriers.     

Investigation of field electron microscope tips bombarded with microparticles at limited field emission currents

Mo, Ta, W, and Re field electron microscope (FEM) tips, bombarded with microparticles at limited field emission currents (400 MΩ resistor in series with the FEM tip) were investigated by means of FEM, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Two groups of tips could be distinguished: One group had a slight tip radius increase to a maximum of 2.5 μm and microcraters were formed along the tip shank. The other group had no detectable tip radius change; however, there was microcrater formation near the tip apex area. FEM patterns showed surface contamination clearly. Heating such contaminated tips and tips where phosphorescent material (Zn:Cd)S had been deposited in less than monolayer concentration by evaporation from a heating coil, resulted in similar sequences of FEM patterns. A new type of microcrater with smooth crater lips could be detected in both groups. These results support the combined microparticle-field emission mechanism [1], which was proposed earlier to be responsible for melting cap and microcrater formation. Excerpts were presented at the 19th Field Emission Symposium, Urbana, Illinois (USA), August 1972.