Improvement of crystallization in CVD diamond coating induced by femtosecond laser irradiation

Abstract Herein, structural modification of a chemical vapor deposit (CVD) diamond film by a low fluence femtosecond laser irradiation has been studied with Raman spectroscopy, X-ray adsorption fine structure (XAFS) spectroscopy, and X-ray diffraction (XRD). Polycrystalline CVD diamond films of approximately 20 μm thickness were irradiated with femtosecond laser of varied defocus distances corresponding to the laser fluence range of 0.1–127.3 J/cm2. At fluences below 1.27 J/cm2, although there was no observable change in the appearance of the film surface, however, the FWHM of the diamond peak in the Raman spectra was observed to reduce considerably from 17 to 7 cm−1. Furthermore, the higher fraction of diamond phase with the increase in laser fluence strongly suggests that the microstructure of the diamond film has been modified by the femtosecond laser. By employing XAFS measurement, it can be found that after irradiation, the weak π* fine structure in the pre-edge almost vanished in response to the significant increase in σ* adsorption; further, a distinct diamond crystal characteristic peak was observed, which further confirms the femtosecond laser-induced modification, and its occurrence inside the bulk film. XRD results also pointed out that the improved diamond crystallinity with the integral intensities increased by approximately 20% after the femtosecond irradiation. Based on the above-mentioned results, an improvement in the crystallinity of CVD diamond under the low-fluence femtosecond laser irradiation has been confirmed in this study; this is attributed to a photo-induced phase transition from amorphous carbon to diamond.