High intensity femtosecond laser deposition of diamond-like carbon thin films

Abstract

Hydrogen-free diamond-like carbon (DLC) films have been deposited with a 100 fs (FWHM) Ti:sapphire laser beam at intensities I in the 1014–1015 W/cm2 range. The films were studied with scanning probe microscopy, variable angle spectroscopic ellipsometry, Raman spectroscopy, and electron energy loss spectroscopy. DLC films with good scratch resistance, excellent chemical inertness, and high optical transparency in the visible and near infrared range were deposited at room temperature. As the laser intensity was increased from 3×1014 to 6×1015 W/cm2, the films showed an increased surface particle density, a decreased optical transparency (85%→60%), and Tauc band gap (1.4→0.8 eV), as well as a lower sp3 content (60%→50%). The time-of-flight spectra recorded from the laser plume exhibited a double-peak distribution, with a high energy suprathermal ion peak preceding a slower thermal component. The most probable ion kinetic energy showed an I0.55 dependence, increasing from 300 to 2000 eV, when the laser intensity was varied from 3×1014 to 6×1015 W/cm2, while the kinetic energy of suprathermal ions increased from 3 to over 20 keV and showed an I0.33 dependence. These high energy ions are believed to have originated from an electrostatic acceleration field established by suprathermal electrons which were formed by resonant absorption of the intense laser beams.