Effect of a deep electron trap with strong lattice relaxation and of interface states on the C-2=ƒ( V) characteristics of Au-CdS Schottky diodes

Abstract

The occurrence of strong lattice relaxation during the electron capture at a bulk deep localised defect in CdS is demonstrated by TSCAP, DLTS and DLOS experiments. We measure its apparent thermal activation energy E a=0.925 eV and its capture cross section S a=5×10 -15 cm 2 as well as its photoionization cross section σ 0 n and its optical ionization energy E 0 n= 2.5 eV. An interpretation is given in a one dimensional configuration coordinate diagram on the basis of a large lattice relaxation. The volume density of this trap, hereafter named X centre, in undoped CdS samples is usually larger than the free electron density and its state of charge has a drastic influence on the C -2=ƒ( V) characteristic of the Schottky diodes. When the electron emission and capture are quenched at a sufficiently low temperature, the C -2=ƒ( V) is linear, either when the X centres are in the empty or in the filled state, but its threshold V 0 is shifted by a few volts by the electron release. The inefficiency of the electron capture at low temperatures gives rise to large persistant photocapacitance effects. In addition the role of interface states in Au-CdS diodes, which exhibit either a non-linear C -2=ƒ( V) characteristic, or an abnormally large threshold value, V 0, is directly demonstrated by the fact that these interface states are detected by DLTS and that they can be eliminated by a thermal treatment of the crystal surface.