Photodetection by light-induced barrier modulation in Cu-diffused Au–CdS diodes

Abstract

Modified Schottky barriers of the type Au–CdS : Cu were prepared by diffusing Cu to a depth of 0.1–0.2 μm into single crystals of CdS, prior to evaporating rectifying Au contacts. The electronic and photoelectronic properties of these junctions are adequately described by a simple model in which the Cu acts as an ionized acceptor, resulting in a ``humped'' potential barrier between the Au and the bulk CdS. Hole trapping by the acceptors under band-gap illumination reduces their ionization and, consequently, the hump height. The forward current density in the dark and under illumination can be accurately described by a thermionic emission model as J=A*T2 exp {—e[VB+VH(0)]/kT exp}(eV/βkT), where the hump potential VH (0) is light sensitive and where β (1<β<1.35) arises from a weak voltage dependence of the barrier height. Barrier height determinations, in the dark and under illumination with 5000-Å light, by means of internal photoemission, thermal activation energy, and J-V measurements unequivocally show that under saturating light conditions the hump in the potential barrier is reduced by 0.23±0.03 V. This barrier mechanism of photoconductive response results in steady-state electron/photon gains in excess of 106 at light intensities lower that 5×1011 photons cm−2 sec−1, where response times exceed 1 sec.