secondary emission

Secondary emission

The emission of electrons from the surface of a solid into vacuum caused by bombardment with charged particles, in particular with electrons. The mechanism of secondary emission under ion bombardment is quite different from that under electron bombardment; it is only in the latter case that the term secondary emission is generally used.

The bombarding electrons and the emitted electrons are referred to, respectively, as primaries and secondaries. Secondary emission has important practical applications because the secondary yield, that is, the number of secondaries emitted per incident primary, may exceed unity. Thus, secondary emitters are used in electron multipliers, especially in photomultipliers, and in other electronic devices such as television pickup tubes, storage tubes for electronic computers, and so on.

The emission of secondary electrons can be described as the result of three processes: (1) excitation of electrons in the solid into high-energy states by the impact of high-energy primary electrons, (2) transport of these secondary electrons to the solid-vacuum interface, and (3) escape of the electrons over the surface barrier into the vacuum. The efficiency of each of these three processes, and hence the magnitude of the secondary emission yield δ, varies greatly for different materials.

Most of the materials used in practical devices are semiconductors or insulators whose band-gap energies are much larger than their electron affinities. Examples are magnesium oxide (MgO), beryllium oxide (BeO), cesium antimonide (Cs₃Sb), and potassium chloride (KCl). Maximum δ values in the 8–15 range are typically obtained at primary energies of several hundred volts.

In certain semiconductors the bands are bent downward to such an extent that the vacuum level lies below the bottom of the conduction band in the bulk. A material with this characteristic is said to have negative effective electron affinity. The most important material in this category is cesium-activated gallium phosphide, GaP(Cs). The illustration shows the curve of yield δ versus primary energy E_p for GaP(Cs) by comparison with MgO. Values of δ exceeding 100 are readily obtained, with maximum yields at energies in the 5–10-kV region. See Band theory of solids, Semiconductor

Secondary emission yield versus primary energy for GaP(Cs)