Internal photoemission mechanisms at interfaces between germanium and thin metal films

The quantum efficiency associated with the internal photoemission of electrons over the Schottky barrier (of height φ
B
) at the metal-Ge interface has been studied experimentally for several metals (Au, Cu, Ag, Pb, and Ni). A theoretical description of this mechanism has been developed in which we take into account the front and back optical absorptance, hot electron scattering, and multiple reflections of excited electrons from the surfaces of the thin electrode film. We have found it necessary to impose a modification of the Fowler theory of photoemission when applied to internal photoemission from thin metal films over a Schottky barrier. This modification relates to an enhanced photoexcitation within the metal films which is attributed in the present theory to a density of states which exhibits a peaked distribution in energy rather than the simple parabolic bands assumed by Fowler. It is clear from the present study that the majority of photoelectron excitation occurs from a small region of energy of the order of a fraction of an electron volt near the Fermi energy. The theoretical model presented here defines two important parameters: a hot-electron mean free path (L
e
) and an energy (E
ef
) given by the difference between the Fermi level and the effective conduction hand minimum associated with the region of energy in the metal near the Fermi level where the electron distribution is strongly peaked. Values of L
e
for Au is 550 Å, Ag is 570 Å, Cu is 450 Å, and Pb is 55 Å. E
ef
for Au is 0.1 eV, Ag is 0.152 eV, Cu is 0.11 eV, Pb is 0.1 eV. The validity of this model is confirmed by the experimental finding that the parameters L
e
and E
ef
are independent of metal thickness.