Physical properties of the deep X electron trap responsible for long time persistent photocapacitance in CdS

Abstract

We describe the main physical properties of a very deep electron trap which is responsible for the long time persistent photocapacitance of Schottky diodes on undoped CdS single crystals. TSCAP and DLTS have been used to determine the apparent thermal activation energy Ea = 0.925 eV and the apparent electron capture cross section σ∞ = 5.5. 10-15cm2 of these centers. Photocapacitance measurements have been performed at various temperatures to allow the determination of the spectral dependence of the photoionisation cross section. The deduced optical threshold En° of the “bound state to the free conduction band state” transition is 2.5 eV. This transition gives a long tail absorption near the fundamental edge which is unambigously identified by the thermal emptying of the X centres. The large difference between Ea = 0.925 eV and En° = 2.5 eV is explained in the large lattice relaxation model. These values are consistent with the results of experiments which locate the X centres, under quasi-equilibrium conditions, at 0.87 eV below the conduction band. They also agree with the results of direct measurement of the thermal activation energy of the electron capture cross section EB = 0.2 eV. The low temperature quenching of both electron emission and capture is responsible for very large persistent photocapacitance effects which suggest that these X centres are acceptor like and probably correlated with cadmium vacancies.