Mapping of the Optical Breakdown Threshold in CVD Diamond

A technique of two-stage three-dimensional mapping of the optical breakdown threshold in diamond crystals is proposed. In the first stage diamond is irradiated by high-energy ultrashort laser pulses, which initiate multiple microbreakdowns in the probed volume of material; these breakdowns cause formation of graphite microinclusions. A sharp decrease in the spatial density of microinclusions in some crystal zone indicates a significant increase in the average value of breakdown threshold in this zone as compared with the neighborhood. The second stage of mapping implies measurement of the absolute values of breakdown threshold by choosing the minimum laser pulse energy necessary for the formation of a graphite microinclusion at a point studied. The “abnormal” crystal zones revealed in the first stage are investigated especially thoroughly, with optimal spatial resolution. Application of this technique to synthetic diamond single crystals from different manufacturers, grown by chemical vapor deposition (CVD), revealed zones in these crystals where the breakdown threshold changes by a factor of more than 10. The boundaries of these zones are located parallel to the (100) growth face; the layer thickness and position vary unpredictably.