Electron Emission from Surfaces

Type of physical science: Condensed matter physics

Field of study: Solids

Certain solids emit electrons from their surfaces when subjected to heat (thermionic emission), electromagnetic radiation (photoemission), and/or an electric field (field emission). Much of electronics, including vacuum tubes, cathode-ray tubes in their various manifestations, and electron microscopes, depend on the emission of electrons from metals and the manipulation of these electrons to perform various tasks.

Overview

The electrons within a metal can be visualized and modeled as a form of "electron gas" in which individual outer-shell electrons are capable of moving freely under the influence of an electric field; this movement of electrons is responsible for the function of electrical circuits. At the surface of a metal, a potential barrier exists which prevents electrons from escaping unless certain conditions are met, whereupon the metal emits electrons into the surrounding vacuum or gas. This emission produces a beam of free electrons which carries current and is capable of being manipulated in many of the same ways that light is manipulated. Both the current-carrying and optical properties of electrical beams have unique aspects that make such beams indispensable in electronics.

The behavior of electrons at the surface of a metal is a quantum effect. Electrons bound to atoms exist in discrete energy states. An electron may exist in the ground state, corresponding to absolute zero temperature, or it may absorb energy and be raised to a discrete higher energy level by heat or irradiation by electromagnetic radiation. In metals and crystalline solids, the shared electrons occupy energy bands rather than discrete energy levels.

Electrons are unable to escape from the surface of the solid because the energy of an outer-shell, ground-state electron in the solid is less than the energy of a free electron in a vacuum. In order for an electron to be ejected from the surface, it must either surmount the surface-potential barrier by having an energy equal to or greater than that of a free electron in the surrounding medium, or tunnel through the barrier. The phenomenon of tunneling is considered to be in the context of field emission, to which it is specific.

The ejection of electrons from a heated conductor is known as thermionic emission. In its most basic form, a thermionic device consists of the heated, negatively charged cathode (which serves as the electron emitter) and a positively charged anode to draw off emitted electrons, both of which are enclosed in a vacuum--typically a glass tube. A vacuum is required because electrons traveling through a gas are scattered and dissipate their energy in heating the gas. In addition, chemical reactions between the cathode and any substances present in the tube poison the cathode, decreasing emission. Unless an anode is present to draw off emitted electrons, they build up in a space charge around the cathode, increasing the energy required for electron emission.

The current density J of a thermionic emitter is described by the equation J=AT²e-(φ/kT), where A is a constant, φ is the electronic work function (a property of the specific emitter), k is Boltzmann's constant, and T is the absolute temperature.

The current density is thus highly temperature-dependent, being related both to the square of the temperature and the negative exponent of the reciprocal of the temperature. For tungsten, the classic emitter, J is 10^-15;A/cm² at 1,000 Kelvins versus 15 A/cm² at 3,000 Kelvins. At temperatures above absolute zero, some electrons have energies above the Fermi energy; these electrons in the upper tail of the distribution are responsible for thermionic emission. Because of the requirement for potential difference to draw off space charge, some field emission takes place in thermionic-emission devices.

Photoemission relies on the interaction between an incident photon of electromagnetic radiation and an electron near the surface of a metal or electromagnetic semiconductor. The energy of the incoming photon (E = hv, where v is frequency and h is Planck's constant) is entirely absorbed by the electron with which it interacts. Electron emission occurs when the photon energy exceeds the difference φ between the Fermi energy level and the energy of a free electron in a vacuum.

The quantity φ is known as the photoelectric threshold energy and is identical to the electronic work function in a metal. A graphic representation of photoemission versus wavelength shows that photoemission does not occur below a threshold value (which depends on the composition of the emitter) and rises steeply thereafter. Above the threshold value, the current level is dependent on the intensity of light but not on its wavelength. As temperature rises, the steepness of the rise in current around the threshold value is less marked because some electrons are already at higher energy levels and require less added energy for emission.

Field emission or cold emission occurs when a high-voltage difference exists between the cathode and the anode. The presence of an electric field causes a deformation and thinning of the potential barrier at the surface of a metal. The electron behaves as a wave rather than as a particle. The wave function does not disappear at the potential barrier but decays into it, with a finite probability that the electron will be located outside the barrier. The electron is said to have tunnelled through the barrier since it did not acquire the energy to overcome it.

The actual emitter in a field-emission device consists of an exceedingly fine wire with a rounded tip. The tip, which may be a submicroscopic 100 angstroms in diameter, acts as a point source of electrons radiating in a cone. In field emission and field ionization microscopes, the radial pattern of radiation propagation serves as its own magnifying system; in other electron-beam applications, such as electron microscopes and cathode-ray tubes (CRTs), the beam is focused by means of electromagnetic lenses.

Electrons at or near the Fermi energy level, where the barrier is thinnest, contributed most to field emission. Field emission is only weakly temperature-dependent.

Secondary emission occurs when electrons from an electron beam are used to liberate electrons from a target to form another electron beam. Since one electron is capable of producing from three to thirty secondary electrons, secondary emission results in amplification and is used in photomultiplier tubes.

Applications

Thermionic emission forms the basis for tube circuit elements, X-ray tubes, and thermionic energy-conversion systems. In 1904, John Ambrose Fleming invented the two-electrode rectifying tube (diode), and in 1907, Lee de Forest placed a current-carrying grid between the cathode and anode in a thermionic emitter tube, creating the triode. Diodes and triodes are the basic building blocks of tube-based electronic devices. In the triode, a weak current on the grid controls the modulation of a stronger current passing as an electron beam between the cathode and anode, the whole system acting as a switch and amplifier. In a tube radio, for example, the modulations of a weak signal picked up by the antenna and propagated across the grid control a stronger household current which runs the actual sound-producing apparatus. A thermionic tube needs to "warm up" and requires energy for heating; this energy requirement and the relatively large size, cost, and complexity of thermionic tubes led to their replacement by transistors in most applications after 1960.

X rays are produced when high-energy electrons in an electron beam strike a solid target; they were discovered accidentally by Wilhelm Conrad Rontgen during experiments with cathode-ray tubes. A gas X-ray tube employs emitted electrons to create ions, which in turn collide with a target to produce rays. In a high-vacuum X-ray tube, electrons produced by thermionic emission produce X rays by striking a metallic anode. Potential differences that are great enough to cause direct field emission are undesirable in an X-ray apparatus. Therefore, a series of electromagnetic accelerators is used in X-ray tubes, producing very high-energy X rays.

X rays can also be an unwanted by-product of any electron-beam apparatus, such as a welding device, which relies on the deacceleration of high-energy electrons.

Thermionic energy conversion involves creating a sufficiently high temperature differential between circuit terminals of suitable composition to cause current to flow. It has been investigated as a method of converting nuclear and solar energy to electrical energy witout moving parts such as turbines. As an alternative to the semiconductor-based photovoltaic cell, thermionic energy converters have never achieved significant commercial success.

Photoemission formed the basis for V. K. Zworykin's ionoscope and its successor, the image orthocon, which was an early version of the television camera. The image orthocon operated by focusing an image onto a photocathode plate placed parallel to a positively charged metal grid. Electrons emitted from the photocathode impinged on the grid, creating an electrostatic image which was reinforced by photomultiplication at a glass plate immediately behind the grid. An electron beam scanned the glass plate. Variations in the plate charge caused variations in the absorption of electrons from the beam, and hence variations in the reflected beam signal. Variations in the return beam translated the image into an electronic signal. Modern television cameras are simpler in design and employ semiconductors.

Photoemission also formed the basis for the photomultiplier, a device used beginning in the 1930's to detect low-level electromagnetic radiation. In a photomultiplier tube, light impinging on a photocathode produces electrons, which in turn produce secondary electrons through interactions with a series of metal oxide dynodes. Photomultiplier tubes are capable of detecting a candle at a distance of 10 kilometers and of magnifying the signal with very little added "noise" (inference). They have been used as scintillation counters and as detectors of faint light in the region of the visible spectrum in astronomy. The wavelength sensitivity of a photomultiplier tube is dependent on the substance from which the cathode is constructed.

Field emission is the principal means of generating electron beams for use in television receivers, CRT terminals, and transmission and scanning electron microscopes. An electron gun based on field emission consists of a fine wire cathode which emits electrons from its tip and a plane or concave anode with a hole in its center through which the electron beam passes. A focusing electrode between the cathode and anode adjusts the radiated electrons into a narrow-diameter beam.

The choice of an emitter for an electron microscope is more critical than for a CRT device. An electron microscope requires maximum brightness to achieve adequate magnfication and a precise emitter which emits uniformly in all directions in order for the electronic lens to perform without aberration. The stability of emitters is of importance to electron microscopy to ensure both the longevity of instrument components and the reproducibility of results. The lifetime of emitters is compromised by the poisoning of emitter centers by foreign substances in an imperfect vacuum and by the evaporation of the cathode material itself. Tungsten and tungsten coated with cesium are favored emitter substances; carbon sheathed in tungsten, and lanthanum and calcium boride have been used in electron microscopes.

Field-emission microscopy and field-ion microscopy employ field emission to study the structure of the emitter directly. In field-emission microscopy, the anode incorporates a phosphor screen, which converts variations in the radiation cone into a visible pattern. Because of their small diameter, emitter tips typically consist of a single crystal. The normal orientation of atoms within the crystal lattice leads to small variations in the work function at the surface of the crystal, which translate into light and dark spots in the visible image. Lattice defects and impurities also become visible. The field-ion microscope, which is capable of a considerably higher level of resolution, also relies on field emission but produces an image from a gas ionized near the surface by emitted electrons rather than from the electrons themselves. The field-ion microscope is the only microscope that allows scientists to view individual atoms directly. The field-ion microscope is useful for studying spatial relationships at the surface of metallic solids, and it is a valuable tool for designing more effective emitters.

Context

Electron emission from solids is a fundamental process underlying electrical transmission in a gas or vacuum, and as such, was among the earliest of electrical phenomena to be observed scientifically. In the mid-eighteenth century, Jean-Antoine Nollet and William Morgan conducted experiments showing that the passage of electrical discharge in partially evacuated tubes produced a glow between the electrodes. In the nineteenth century, Johann Hittorf and Sir William Crookes independently investigated the radiation produced by a cathode in a vacuum tube, demonstrating that an invisible "light" was produced which caused glass to fluoresce and cast shadows. In 1897, Joseph John Thomson demonstrated that these cathodic rays were actually beams of negatively charged particles (that is, electrons). He measured their charge-to-mass ratio and noted their behavior in electric and magnetic fields. Thomas Edison obtained a patent in 1884 for a thermionic-emission device, consisting of an incandescent wire in an electric field within an evacuated envelope, which was the forerunner of amplifier tubes.

Early investigators were puzzled by the sharp threshold value for photoemission with respect to wavelength, an observation which could not be explained by classical electrical theory but which was correctly described as a quantum process by Albert Einstein in 1905.

The cathode-ray tube (CRT), equipped with a phosphorescent screen, became the basic device for translating electronic signals into visual displays--initially in scientific instrumentation, in such devices as oscilloscopes, and later in television and computer screens.

Modern CRT devices rely on electron guns based on field emission, which provide greater life, brightness, and focusing ability than the thermionic or photoemission sources. The electron source in a CRT acts as an amplifier of a weak signal (from an antenna or the logic circuit of a computer) in a manner which is analogous to the triode. This electron source also incorporates a magnetic deflector, which moves the electron beam across the phosphorescent background at a constant rate to create a two-dimensional image from an essentially one-dimensional electronic signal.

The ionoscope, invented by Zworykin in 1923, and the multiplier phototube, invented by Zworykin and others in 1935, set the stage for television and video cameras. A television camera is a device for converting photons into electrical signals which operates on the principle of photoemission. Early versions incorporated tubes, but later cameras used semiconductor technology.

Transmission and scanning electron microscopes employ the optical properties of electrons, notably the short wavelength, to produce images of very high resolution. Crude models were developed in the 1930's by Max Knoll and Ernst Ruska, but it was not until after World War II that practical electron microscopes became available. Transmisson electron microscopes require an electron beam of maximum brilliance, minimum divergence, and high focusability, and they continue to place demands on the development of increasingly sophisticated electron-beam technology. Electron-emission microscopy and ion-emission microscopy came into prominence in the 1960's as powerful tools for examining the surface structure of matter at the atomic level.

Electron beams are capable of delivering high levels of energy with great precision. In the 1950's and early 1960's, considerable research was devoted to the use of electron beams in welding, machining, and metal refining on a miniature scale. These devices were cumbersome because of the need to operate in a vacuum, and this technology has largely been supplanted by lasers. The development of a very powerful and flexible laser based on magnetic manipulation of an electron beam brought electron-beam technology back into the forefront of instrumentation.

Principal terms

CATHODE RAYS: radiation consisting of electrons

FERMI ENERGY: the energy level associated with a ground-state, outer-shell electron (an energy band in a metal or crystalline solid)

ION: a charged particle consisting of an atomic nucleus with fewer or more electrons than necessary to balance the charge on the nucleus

VOLTAGE: a measure of difference in electrical potential; conceptually, voltage is a measure of the amount of energy per unit electrical particle, and current is a measure of the number of particles

WORK FUNCTION: the amount of energy, measured in electronvolts, necessary to raise a conducting electron in a metal to the energy level of a free electron in a vacuum

Bibliography

Bakish, Robert A., ed. INTRODUCTION TO ELECTRON BEAM TECHNOLOGY. New York: John Wiley & Sons, 1962. This textbook is designed for engineering students. Includes a useful history of devices based on electron emission from the eighteenth to mid-twentieth century and chapters on the design of electron guns. These chapters discuss the types of emission and their applicability to specific designs, the design and function of electron microscopes, and research and industrial applications of electron-beam devices. Many of these applications have been supplanted by laser technology. Since each chapter prefaces technical and mathematical discussion with a general discussion of the subject, this text is relatively accessible to the nonspecialist.

Cardona, Manuel, and Lothar Ley, eds. PHOTOEMISSION IN SOLIDS. 2 vols. New York: Springer-Verlag, 1978-1979. An advanced technical treatise, the last half of which is devoted to X-ray photoemission spectroscopy (XPS). The introductory chapter presents a history of scientific developments in the field of photoemission.

Dummler, G. W. A. ELECTRONICS INVENTIONS, 1745-1976. Oxford, England: Pergamon Press, 1976. A chronological table, with brief (one- to two-paragraph) descriptions of each individual invention, emphasizing consumer electronics. Shows graphically how technlogy based on emission from solids--cathode-ray tubes, rectifying and amplifying tubes, electron microscopes--played a prominent role in consumer electronics in the first half the twentieth century but has tended to be supplanted by semiconductor-based techology.

Dyke, W. P. "Advances in Field Emission." SCIENTIFIC AMERICAN 210 (January, 1964): 108-118. Contains a general description of the theory of field emission; informative diagrams visualizing work functions, barrier thinning, and related concepts; and schematic diagrams of an electron gun. Followed by a description of the use of field emission to produce short bursts of X rays for high-speed photography.

Hren, John J., and Srinivasa Ranganathan. FIELD-ION MICROSCOPY. New York: Plenum Press, 1968. Includes a general discussion of field emission and field-emission microscopy, as well as the relationship between field emission and field-ionization microscopes. Paired illustrations of field-ion images and diagrams of a crystal lattice give a good impression of how field-ion images operate as a research tool.

Nixon, W. C., ed. SCANNING ELECTRON MICROSCOPY: SYSTEMS AND APPLICATIONS. Bristol, England: Institute of Physics, 1973. This collection of conference papers includes several chapters on the design of emitters for electron guns and the applicability of various materials as electron sources.

Owen, George E., and P. W. Keaton. FUNDAMENTALS OF ELECTRONICS. Vol. 2. New York: Harper & Row, 1967. A basic engineering textbook which is old enough to give equal weight to vacuum tubes and semiconductors. Chapter 3, on vaccum tubes, discusses electron emission and the theory and design of diodes and triodes.

Radio and Television

Electrical Properties of Solids