Effects of air annealing on microstructure and optical properties of HFCVD grown microcrystalline diamond thin films

In this report, we investigate the annealing effects on microcrystalline diamond thin films of 1 µm thickness grown on silicon substrate by hot filament chemical vapor deposition technique. Annealing is important from the perspective of etching away the non-diamond/graphitic phase situated mainly at the grain boundaries to yield a high-quality continuous diamond film with higher sp3 %. From the Raman spectroscopy and also from the morphology of the films, we could see that with annealing in air, the degradation of the sp2 component sets in at around 500 °C leaving behind the sp3 component in the films, however the films are completely oxidised with annealing at 700 °C. Our results show that annealing at 500 °C has been able to etch away the sp2 phase efficiently as evident from the Raman spectroscopic analysis making the characteristic diamond peak more intense compared to graphitic peaks and also led to the decrease in FWHM of the diamond peak. The photoluminescence properties reveal the presence of silicon-vacancy centers in the film whose zero-phonon line (ZPL) shows a drastic decrease in the FWHM as we anneal at higher temperatures of more than 700 °C.