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 us ing sub-band-gap illumination with photon energy ranging from 2.4 to 3.53 e V. It was found that, with the grain size increased from 0.5 to 40 mu m, th e sub-band-gap photoconductivity measured under the constant photocurrent m ode was lowered by up to a maximum of five orders of magnitude, and accompa nied by a decrease in the Rose exponent gamma. This is indicative of a chan ge in the recombination mechanism, likely due to fewer mid-gap states actin g as recombination centers. These results suggest that the gap states in th e undoped CVD diamond films are mainly determined by the density of grain b oundaries.