The Capture of Hot Electrons by Gold Centres in n-type Germanium

Abstract

Measurements of current density against electric field for specimens of n-type germanium containing gold, at 77 °K indicate an increase of the capture rate with field for electrons at singly negatively charged gold centres. The effect of this is a rapid break-away from the ohmic line as soon as the electron temperature rises. Pulse-field measurements show the effect directly. Assuming the capture cross section to be of the form AS(E) · EpTq where A is a constant, S(E) the Coulomb barrier tunnelling factor, E, electron energy, T, lattice temperature, and taking the electron distribution function to be Maxwellian at high fields we obtain p = -1. Comparison with work on thermal equilibrium capture cross sections leads to ½ ⩽ q ⩽ 1. Although at 77 °K the Coulomb barrier effect is dominant, the inverse-power dependence on electron energy of the energy-loss part of the capture cross section precludes the appearance of any distributed differential negative resistance. A good fit to experiment is obtained by taking the `tunnelling' Coulomb effective mass of the electron to be 0.084 m0. This indicates that preferential capture of electrons travelling in {111} transverse mass planes occurs. Differences in capture effects shown by specimens with the field in a ⟨100⟩ direction and those with the field in a ⟨211⟩ direction can be explained in terms of the usual hot electron anisotropy. The absolute magnitude of the capture rate at 77 °K is 6 × 10-10 cm3 sec-1, corresponding to a capture cross section of 4 × 10-17 cm2. At fields from 2-3 kv cm-1 a rapid rise in current density is observed. This is attributed to impact ionization of the gold centre having an energy level 0.2 ev below the conduction-band edge.