PHOTOEMISSION FROM Au AND Cu INTO CdS

Abstract

Many metal-semiconductor surface barrier rectifiers
\nshow photosensitivity for photon energies (hv) less than the semiconductor energy gap (E_g).
\nCases in the literature include metals evaporated
\nor electrodeposited on elemental and III-V
\ncompound semiconductor surfaces. In these studies
\nthe source of the low-energy photocurrent, when
\nhv < E_g, was shown to be the photoemission of
\ncarriers over the Schottky barrier between the metal
\nfilm and the semiconductor. An extensive investigation
\nhas been reported for a series of metals,
\nparticularly Cu and Au, electroplated on n-type CdS
\nwith the conclusion that here also photoemission
\nfrom the metal is responsible for most of the low-energy photovoltage. However, recent studies have
\nquestioned this conclusion for the CdS case. One
\nstudy proposed that the origin of the low-energy
\nphotovoltaic response is electron photoexcitation
\nfrom Cu impurities located in the CdS and within a
\ndiffusion length of the space charge region. Hole
\nconduction probably in the 3d Cu levels was postulated
\nfor these samples, which had ≈ 30-ppm Cu. A second study interpreted the results as a p·n junction photovoltaic effect.