Optical Properties and Defect Structure of CVD Diamond Films Annealed at 900-1600 �C

Abstract

Optical absorption evolution is studied for 150–800 μm thick diamond films grown by microwave plasma enhanced CVD technique and annealed in vacuum at temperatures 900–1600 °C. A noticeable internal degradation (darkening) of the films takes place at T > 1300 °C. Changes in the concentration of different CHx groups are revealed from the IR spectra starting at T > 1000 °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-like phase (being under a high stress) presumably at grain boundaries, that is revealed by the appearance of a specific absorption peak in UV spectra.