Influences of grain size and microstructure on optical properties of microcrystalline diamond films

Microcrystalline diamond（MCD） films with different grain sizes ranging from 160 nm to 2200 nm are prepared by using a hot filament chemical vapor deposition（HFCVD） system, and the influences of grain size and structural features on optical properties are investigated. The results show that the film with grain size in a range of 160 nm–310 nm exhibits a higher refractive index in a range of（2.77–2.92）. With grain size increasing to 620±300 nm, the refractive index shows a value between 2.39 and 2.47, approaching to that of natural diamond（2.37–2.55）, and a lower extinction coefficient value between 0.08 and 0.77. When the grain size increases to 2200 nm, the value of refractive index increases to a value between 2.66 and 2.81, and the extinction coefficient increases to a value in a range of 0.22–1.28. Visible Raman spectroscopy measurements show that all samples have distinct diamond peaks located in a range of 1331 cm;–1333 cm;,the content of diamond phase increases gradually as grain size increases, and the amount of trans-polyacetylene（TPA）content decreases. Meanwhile, the sp;carbon clusters content and its full-width-at-half-maximum（FWHM） value are significantly reduced in MCD film with a grain size of 620 nm, which is beneficial to the improvement of the optical properties of the films.