Study of Band-Gap States in CVD Diamond Using Sub-Band-Gap Illumination

Band-gap states in nominally undoped CVD diamond films have been studied using sub-band-gap illumination with photon energy ranging from 2.4 to 3.53 eV. It was found that, with the grain size increased from 0.5 to 40 μm, the sub-band-gap photoconductivity measured under the constant photocurrent mode was lowered by up to a maximum of five orders of magnitude, and accompanied by a decrease in the Rose exponent γ. This is indicative of a change in the recombination mechanism, likely due to fewer mid-gap states acting as recombination centers. These results suggest that the gap states in the undoped CVD diamond films are mainly determined by the density of grain boundaries.