Relaxation in undoped polycrystalline CVD diamond films under red illumination

A long-term decay of the photocurrent under red light has been observed and analysed in polycrystalline CVD diamond films. The kinetics of this photocurrent decay is non-exponential and can rather be described by a combination of several exponential functions or by a stretched-exponential function. This relaxation is only observed if the samples are preliminary exposed to band-to-band UV light excitation during a few minutes and it is perfectly reproducible. This long-term relaxation of the below-gap photocurrent following UV exposure is not observed on a homo-epitaxial crystalline CVD layer, where the photocurrent remains almost constant. Therefore, it is related to the disorder in the CVD polycrystalline diamond films and to the broad distribution of localised gap states, which are mainly created by the grain boundaries. The results are discussed in terms of defect influence in a range of diamonds in relation with their nature and synthesis process. (C) 2002 Elsevier Science B.V. All rights reserved.