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

Structural changes in diamond films of different qualities caused by anneal ing in vacuum up to 1600 degreesC have been studied by IR and UV-visible op tical absorption, Raman and photoluminescence spectroscopy. An internal deg radation of the diamond films and a strong optical absorption enhancement i n the whole W-vis-IR range take place at T > 1300 degreesC, and correlate w ith the loss of bonded hydrogen. At least 25% of the total amount of hydrog en 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 pha ses presumably at grain boundaries, Activation energy of GB transformation process is much lower (250-530 kJ/mol) compared to surface graphitization o f single crystal diamond, No evidence of charge transfer altering the conce ntration 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 tec hnique remains almost constant (20 W/cmK) even after annealing to 1575 degr eesC, then catastrophically drops because of crack development in the film. (C) 2001 Elsevier Science B,V, All rights reserved.