Dominant free-exciton recombination radiation has been obtained from chemical vapor deposited diamonds. The crystals are formed in a microwave plasma using the mixtures CO and CO2 diluted with hydrogen. The concentration of CO2 and the position of the samples in the plasma affect the crystallinity of the samples. The peak emission intensity ratio between the free-exciton recombination radiation and the band A emission is more than 20 in the samples formed at the edge of discharging area and with the optimum concentration of CO2. The value is the highest ever reported in diamond.
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