Optical properties and defect structure of CVD diamond films annealed at 900-1600 degrees C

Optical absorption evolution is studied for 150-800 mu m thick diamond film s grown by microwave plasma enhanced CVD technique and annealed in vacuum a t temperatures 900-1600 degrees C. A noticeable internal degradation (darke ning) of the films takes place at T > 1300 degrees C. Changes in the concen tration of different CH, groups are revealed from the IR spectra starting a t T > 1000 degrees C. Up to 25% of the total amount of hydrogen in as-grown films is found to be unbonded. An interesting effect of reduced absorption in UV range due to the formation of new C-H bonds by reaction of primarily unbonded hydrogen is observed. The diamond darkening correlating with the bonded hydrogen loss process, is ascribed to the appearance of a graphite-l ike phase (being under a high stress) presumably at grain boundaries, that is revealed by the appearance of a specific absorption peak in UV spectra.