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The ejection of electrons from a solid (or less commonly, a liquid) by incident electromagnetic radiation. Photoemission is also called the external photoelectric effect. The visible and ultraviolet regions of the electromagnetic spectrum are most often involved, although the infrared and x-ray regions are also of interest.
The salient experimental features of photoemission are the following: (1) There is no detectable time lag between irradiation of an emitter and the ejection of photoelectrons. (2) At a given frequency the number of photoelectrons ejected per second is proportional to the intensity of the incident radiation. (3) The photoelectrons have kinetic energies ranging from zero up to a well-defined maximum, which is proportional to the frequency of the incident radiation and independent of the intensity.
In 1905 Albert Einstein made the clarifying assumption that electromagnetic radiation had characteristics like those of particles when it delivered energy to electrons in the emitter. In Einstein's approach the light beam behaves like a stream of photons, each of energy hν, where h is Planck's constant, and ν is the frequency of the photon. The energy required to eject an electron from the emitter has a well-defined minimum value &phgr; called the photoelectric threshold energy. When a photon interacts with an electron, the latter absorbs the entire photon energy. See Photon
For hν values below the threshold, photoelectrons are not ejected. Even though the electrons absorb photon energy, they do not receive enough to surmount the potential barrier at the surface, which normally holds the electrons in the solid. For photon energies above &phgr;, the kinetic energies of photo-electrons range from zero up to a maximum value, E= hν - &phgr;. This is the Einstein photoelectric law, and E is commonly termed the Einstein maximum energy. See Heat radiation, Schottky effect
(also the external photoelectric effect), the ejection of electrons from solids or liquids into a vacuum or some other medium by the action of electromagnetic radiation, or photons. Photoemission from solids—that is, from metals, semiconductors, and dielectrics—into a vacuum is of practical value in most cases.
The main regularities of photoemission consist in the following: (1) the number of ejected electrons, or photoelectrons, is proportional to the intensity of the incident radiation; (2) for every substance, a photoelectric threshold—that is, a minimum radiation frequency ω0 or a maximum wavelength λ0 beyond which photoemission does not occur—exists for a specified surface state at a temperature T → 0°K; and (3) the maximum kinetic energy of the photoelectrons increases linearly with radiation frequency and is independent of radiation intensity.
Photoemission results from the following three successive processes: the absorption of a photon and the appearance of an electron with an energy that is high in comparison with the mean electron energy; the movement of the high-energy electron toward the surface, during which some of the energy may be dissipated; and the escape of the electron across an interface into another medium. The photoelectric yield Y, which is the number of electrons ejected per photon incident on the surface of the solid, is a quantitative characteristic of photoemission. The value of Y depends on the properties of the solid, the state of solid’s surface, and the photon energy.
Photoemission from metals occurs if the photon energy ℏω, where ℏ is Planck’s constant and ω is the frequency of the radiation, exceeds the work function eɸ of the metal. The work function for pure metal surfaces is greater than 2 electron volts (eV); for most metals, it is greater than 3 eV. Therefore, if the work function is not reduced by a special surface coating, photoemission from metals may be observed in the visible and ultraviolet regions of the spectrum (for alkali metals and barium) or only in the ultraviolet region (for all other metals). For most metals, Y ~ 10–4 electron/photon near the photoelectric threshold. The value of Y is low because the surfaces of metals strongly reflect visible and near-ultraviolet radiation (the reflection coefficient R > 0.9), so that only a small fraction of the incident radiation penetrates the metal. Furthermore, as photoelectrons move toward the surface, they strongly interact with conduction electrons, which are numerous (~ 1022 cm–3) in metals, and rapidly dissipate the energy gained from the radiation. Only photoelectrons that are produced near the surface, at a depth of not more than a few nanometers (nm), conserve enough energy to achieve the work function (see Figure 1,a). Less energetic photoelectrons may trave.se a path tens of times longer in a metal without losing energy, but the energy of such electrons is not sufficient to surmount the potential barrier at the surface and escape into a vacuum.
As the photon ℏω increases, the value of Y for metals increases slowly at first. At ℏω = 12 eV, the value of Y for pure metallic films, which are obtained by evaporating the metal in a high vacuum, is 0.04 electron/photon for Al and 0.015 electron/photon for Bi. At ℏω > 15 eV, R decreases sharply to 0.05 and Y increases, reaching 0.1–0.2 electron/photon in certain metals, such as Pt, W, Sn, Ta, In, Be, and Bi. Accidental contamination may reduce ɸ considerably; as a result, the photoelectric threshold is shifted toward longer wavelengths and Y may increase greatly in the new spectral region. A sharp increase in Y and a shift of the photoelectric threshold into the visible region of the spectrum are accomplished by coating the pure surface of a metal with a monatomic film of electrically positive atoms or molecules, such as Cs, Rb, or Cs2O; such particles form an electric dipole layer on the surface. For example, a Cs film reduces ɸ and shifts the photoelectric threshold from 5.05 to 1.7 eV for W, from 4.62 to 1.65 eV for Ag, from 4.52 to 1.55 eV for Cu, and from 4.74 to 1.42 eV for Ni.
In semiconductors and dielectrics, the strong absorption of electromagnetic radiation begins at photon energies ℏω equal to the width Δℰ of the forbidden band in the case of direct optical transitions. At ℏω ≈ Δℰ the absorption coefficient K ≈ 104 cm–1, increasing to 105 cm–1 as ω increases. The photoelectric threshold ℏω0 = Δℰ + X, where X is the electron affinity, that is, the height of the potential barrier for conduction electrons (see Figure 1,b). In lightly doped semiconductors, there are few conduction electrons; therefore, in contrast to metals, the dissipation of photoelectron energy by conduction electrons does not play a role. In such materials, a photoelectron loses energy by interacting with valence electrons—that is, by impact ionization—or by interacting with thermal vibrations of the crystal lattice—that is, by phonon production. The rate of energy dissipation and the depth from which photoelectrons can escape into a vacuum depend on the value of X and on the relation of X and Δℰ. If X > 2 Δℰ, a photoelectron with an initial kinetic energy that is equal to or greater than X produces an electron-hole pair. The mean free path for energy dissipation in such an event (1–2 nm) is many times smaller than the depth to which radiation penetrates in the crystal (0.1–1 micrometers [μm]). In this case, most photoelectrons lose energy on the way to the surface and do not escape into the vacuum. This picture applies to Si (Δℰ = 1.1 eV, X = 4.05 eV), Ge (Δℰ = 0.7 eV, X = 4.2 eV), GaAs (Δℰ = 1.4 eV, X = 4.07 eV), and other semiconductors. In these materials, Y ~ 10–6 electron/photon near the photoelectric threshold and does not exceed 10–4 electron/photon even at relatively large distances from the threshold, that is, even when ℏω = ℏω0 + 1 eV. If X is less than Δℰ but greater than the energy of an optical phonon (10–2eV), photoelectrons lose energy when optical phonons are produced. In this loss mechanism, the photoelectron energy is dissipated over a mean free path of just 10–30 nm in semiconductors. Therefore, if the value of X for a semiconductor is reduced, say, from 4 to 1 eV, the photoemission near the threshold remains small. In crystals of alkali metal-halogen compounds, the mean free path is greater than 50–100 nm and eɸ is small. Therefore, the value of Y for such crystals sharply increases above the photoelectric threshold and becomes large. For example, in Csl, Δℰ = 6.4 eV, X = 0.1 eV, and Y = 0.1 electron/photon even when ℏω = 7 eV, which is just 0.6 eV above the threshold; Y remains virtually constant as ℏω increases.
Because of the large values of Δε, the photoelectric threshold for alkali metal-halogen crystals lies in the ultraviolet region, for which the crystals, in the form of a thin film on a conducting substrate, are good photocathodes. Materials that have a high Y for visible and near-infrared radiation at small Δℰ and X are important for most technical applications. Semiconductor materials based on elements from groups I and V of the periodic system (for example, Cs3Sb and Na2KSb), often in combination with oxygen, are used the most as photocathodes and are the most appropriate materials for this purpse. For such materials, Δℰ ≤ 2 eV, X < 2 eV, and Y in the visible region attains a value of ~ 0.1 electron/photon.
The improvement of the technique of cleaning the surfaces of semiconductors in an ultrahigh vacuum has made it possible to reduce sharply the value of eɸ for III-V semiconductors and p-type Si to less than that of Δℰ and simultaneously to generate a strong intrinsic electric field, which accelerates photoelectrons, in the thin surface layer of the semiconductor. In this case, the work function eɸ > Δℰ, and the height X of the potential barrier at the surface is below the level of the bottom of the conduction band in the bulk of the crystal. As a result, a substantial number of thermalized electrons—that is, electrons with thermal energies—can escape into the vacuum from a great depth, of the order of the minority-carrier diffusion length (~ 10–4 cm). Such photocathodes are called negative-electron-affinity (NEA) photo-cathodes (see Figure 1,c). NEA photocathodes have the highest photoelectric yield in the near-infrared region; a yield of 0.09 electron/photon is attained at λ = 1.06 μm.
Photoemission is widely used in the study of the energy structure of substances; in chemical analysis, for example, in photoelectron spectroscopy; in measuring equipment; in sound motion-picture reproducing equipment; in automation devices, such as phototubes and multiplier phototubes; in television camera tubes, for example, the image iconoscope and image orthicon; in infrared equipment, such as image converters; and in other devices designed to detect X-ray, ultraviolet, visible, or near-infrared radiation.
REFERENCESSoboleva, N. A. Fotoelektronnyepribory. Moscow, 1965.
Sommer, A. Fotoemissionnye materialy. Moscow, 1973. (Translated from English.)
Soboleva, N. A. “Novyi klass elektronnykh emitterov.” Uspekhi fizicheskikh nauk, 1973, vol. 111, issue 2, pp. 331–53.
Nenakalivaemye katody. Moscow, 1974.
T. M. LIFSHITS