Abstract
The influence of grain boundaries on the photoconductivity of undoped CVD diamond films has been studied using subgap illumination with photon energy ranging from 2.4 eV to 3.53 eV. It is found that, with the grain size increasing from 0.5 μm to 40 μm, the subgap photoconductivity is decreased by a maximum of five orders of magnitude. Meanwhile, the Rose exponent γ, which is deduced from the power-law dependence of photoconductivity on generation rate, is found to be decreased in the temperature range from 10 K to 300 K. This indicates that with grain size increasing, effective recombination centers are reduced due to a decrease in the density of mid-gap states. These results suggest that the density of the gap states in undoped CVD diamond films is mainly determined by the density of the grain boundaries.
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