Effect of high temperature annealing on optical and thermal properties of CVD diamond

摘要

Structural changes in diamond films of different qualities caused by annealing in vacuum up to 1600 ℃ have been studied by IR and UV-visible optical absorption, Raman and photoluminescence spectroscopy. An internal degradation of the diamond films and a strong optical absorption enhancement in the whole UV-vis-IR range take place at T > 1300 ℃, and correlate with the loss of bonded hydrogen. At least 25% of the total amount of hydrogen is found to be in the unbound state in some of the as-grown (untreated) films. The diamond darkening is ascribed to appearance of graphite-like phases presumably at grain boundaries. Activation energy of GB transformation process is much lower (250-530 kJ/mol) compared to surface graphitization of single crystal diamond. No evidence of charge transfer altering the concentration of substitutional nitrogen N_S~0 and N_A - N_D (in B-doped films) upon annealing was found. Thermal conductivity measured by laser flash technique remains almost constant (20 W/cmK) even after annealing to 1575 ℃, then catastrophically drops because of crack development in the film.