Effect of neutron irradiation on the hydrogen state in CVD diamond films

The effects of high temperature, up to 1680 °C, and annealing in vacuum on the optical properties of fast neutron irradiated chemical vapour (CVD) deposited polycrystalline diamond films (CVD DFs) are examined. It is shown that at least fifteen relatively narrow (full width at half-height from 10 to 25 cm−1) CHx stretching vibration bands with maxima close to 2800, 2818, 2834, 2849, 2855, 2875, 2899, 2906, 2924, 2936, 2950, 2966, 2985, 3012 and 3034 cm−1, as well as a 3123 cm−1 band due to the NVH0 centers, are observed in the IR absorption spectra of the films. The transformation of the CHx stretching vibration bands in the absorption spectra of fast neutron irradiated (with a fluence of F = 2×1019 cm−2) diamond films is investigated with successive isochronous annealing runs. It is found that the neutron irradiation significantly (by ∼200°) increases the temperature above which the graphitization (darkening) of the samples starts. That threshold is determined by the temperature of graphitization of intercrystallite (grain) boundaries due to the break of C-H bonds. The phenomena observed are explained by the effect of radiation damage on the structure of grain boundaries in the diamond films.