Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

摘要

The effects of CH_4/C_2H_4 flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/ VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65-2.5 eV by manipulating CH_4/C_2H_4 flow ratio and a wide range, 2.5-0.95 by thermal annealing. The ESR spin density varied between 10~(19) to 10~(17) spins/cm~3 depending on the CH_4/C_2H_4 flow ratio (1:3 to 3:1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp~3/sp~2 carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness <0.5 nm).