A luminescence resulting from the bombardment of a substance with an electron (cathode-ray) beam. The principal applications of cathodoluminescence are in television, computer, radar, and oscilloscope displays. In these a thin layer of luminescent powder (phosphor) is evenly deposited on the transparent glass faceplate of a cathode-ray tube. After undergoing acceleration, focusing, and deflection by various electrodes in the tube, the electron beam originating in the cathode impinges on the phosphor. The resulting emission of light is observed through the glass faceplate, that is, from the unbombarded side of the phosphor coating. See also: Cathode-ray tube
The luminescence of most phosphors comes from a few sites (activator centers) occupied by selected chemical impurities which have been incorporated into the matrix or host solid. The interaction of cathode rays with the phosphor involves a collective excitation of all the atoms of the host rather than a selective excitation of the luminescent centers, a condition that allows the dissipation of beam energy by competing nonluminescent processes. An appreciable energy loss occurs as soon as the primary cathode-ray beam strikes the phosphor; 25–35% of the electrons are immediately reflected (backscattered) due to coulombic repulsion. The electrons that actually penetrate the phosphor give rise to a combination of several processes that can be described only qualitatively. Some x-rays are produced, but in the main the high-energy electron beam ionizes the solid, producing a plasma of many lower-energy (secondary) electrons. These electrons are multiply scattered, successively losing more and more energy to the solid by various nonradiative paths. Although the bombarded phosphor is in the complex excited condition described above, a small part of the excitation energy is transferred by various mechanisms to the activator centers, causing them to luminesce. Because of the complex mode of interaction of cathode rays with phosphors, the energy efficiency of light production by cathodoluminescence is lower than the best efficiencies obtainable with photoluminescence. Conversion efficiencies of currently used display phosphors are between 2 and 23%.
The brightness B of a phosphor under cathode-ray excitation depends on the accelerating voltage V and the current density j. Many phosphors exhibit a dead voltage V0 below which they show diminished output, presumably due to radiationless dissipation of the energy by poison centers, which are present only at the surface where this low voltage excitation occurs. Above V0, the brightness is proportional to (V − V0)q, where q is between 1 and 3. At a given voltage, the brightness initially varies linearly with the current density, and then may increase more slowly with increasing j (saturation). The blue-emitting zinc sulfide (ZnS) and green-emitting zinc-cadmium sulfide [(Zn,Cd)S] phosphors used in color television exhibit this saturation effect, as well as a color shift and shorter afterglow at high current density, but the red-emitting europium-activated phosphor and other rare earth–activated phosphors do not. Linearity at high current density is an important requirement for phosphors to be used in projection television or aircraft pilot displays.
The blue and green phosphors for television are broadband emitters with dominant wavelengths of 464 and 556 nanometers, respectively; the red europium-activated yttrium oxysulfide phosphor is a line emitter at 605 nm. Their luminous efficiencies are 7.5, 65, and 17 lumens per input watt, respectively, and all have persistences of less than 10−4 s to reduce the smearing of fast-moving objects in television pictures. For easy viewing of slowly scanned radar screens, on the other hand, phosphors with persistences of up to 0.55 are used.
The activators in zinc sulfide and zinc-cadmium sulfide are parts-per-million traces of donor-acceptor pairs of impurities, chlorine and silver in the former and aluminum and copper in the latter. Luminescence in these phosphors is produced when electrons trapped at donors recombine with holes trapped at acceptors. The broad emission band is a complex of emissions of slightly different wavelengths from pairs having a variety of separations in the host lattice. This mechanism can also explain phosphor saturation, shortened persistence, and color shifts in the sulfides at high current density. In the rare earth–activated phosphor, the emitting center Eu3+ is present at a concentration of 4 mole %. The deep-lying atomic orbits of Eu3+ are not appreciably affected by interaction with the other constituent atoms of the phosphor, and the red light therefore appears in a very narrow range of wavelengths. See also: Hole states in solids; Semiconductor
An important requirement for a good cathodoluminescent phosphor is the possession of good secondary electron emission properties; otherwise, it charges up negatively and reduces the effective potential of the bombarding beam. In most cases the secondary electron emission coefficient R is less than 1, and the screen must be coated with a film of aluminum to provide conductance to the power supply in order to prevent charge buildup. The film provides two additional benefits. Its action as an optical mirror nearly doubles the display brightness, and it shields the phosphor from bombardment by residual gas ions that remain or are generated in the tube (ion burn). The phosphor host crystal structure can be disrupted by ion bombardment, and to a lesser extent by prolonged electron bombardment, creating absorbing centers (discoloration), poison centers, and other lattice defects which can reduce the efficiency of the luminescence. See also: Luminescence; Secondary emission