Injection dependent long carrier lifetimes in high quality CVD diamond

In this paper we report an experimental study of photocurrent mobility x lifetime products and free carrier lifetimes in CVD grown polycrystalline diamond of various qualities. The investigated samples are low impurity samples, nitrogen content similar to 10(15) cm(-3), with an average grain size ranging from 25 mum up to 110 mum. This large difference in average grain size makes it possible to distinguish effects due to Lifetime limiting trapping and recombination defect centers inside the grains from effects caused by defect centers at grain boundaries. At low carrier densities, < 10(13) cm(-3) the effective free carrier lifetime is in the sub-nanosecond to nanosecond range in all samples due to intra-grain trapping and recombination centers. At high carrier densities, > 10(13) cm(-3), the intra-grain centers becomes saturated and the effective lifetime becomes predominately given by carrier diffusion to and recombination at the defects related to the grain boundaries. Hence, the effective lifetime at high carrier densities is strongly related to the average grain size and increases up to several tens of nanoseconds, in samples with a large average grain size, whereas it remains in the nanosecond range for samples with small average grain size. In addition, we observe a lower mobility x lifetime product and decay constant with increasing nitrogen content, clearly showing the negative influence of nitrogen and nitrogen-related defects on these important material parameters. (C) 2001 Elsevier Science B.V. All rights reserved.