Atomic Hydrogen Effects on Chemical Bonding of Diamond-Like Carbon Films Prepared by Pulsed Laser Deposition

<b>1. Introduction</b><br/>Tetrahedral amorphous carbon (ta-C) films have been prepared by pulsed laser deposition (PLD) [1-4]. Stock et al. [1] reported that the sp³ carbon ratio [sp³ C/(sp³C + sp²C)] in the ta-C film prepared using an ArF excimer laser (198 nm) was 80% or more, which was higher than the sp³ ratio (~60%) in the film deposited using a KrF excimer laser (248 nm). Yoshitake et al. [2] reported that the ta-C film prepared in a hydrogen atmosphere had a higher sp³ ratio than the film deposited in an oxygen atmosphere. Cheng et al. [3,4] performed postdeposition-annealing of ta-C with atomic hydrogen irradiation and found that atomic hydrogen etching accelerated crystallization of micro/nano graphite as the annealing temperature rose. In this study, we have deposited diamond-like carbon (DLC) films by PLD using a KrF excimer laser and investigated atomic hydrogen effects on the chemical bonding of the DLC films. We clarified the optimum deposition conditions to promote sp³-bonded carbon atoms in the films. The novelty of this study is that the hydrogen effects were investigated by varying the deposition parameters systematically.<br/><b>2. Experimental methods</b><br/>DLC films were prepared on Si(100) substrates by PLD using a KrF excimer laser with a wavelength of 248 nm. The DLC film was deposited at a substrate temperature of RT, a laser repetition rate of 20 Hz, a laser intensity of 0.25 J/pulse using an atomic hydrogen source (hydrogen pressure: 0.01 Pa, tungsten filament current: 4 A) for 60 min as a standard sample. One of the substrate temperature, laser repetition rate, hydrogen pressure, and filament current was varied with the other parameters fixed. The sp³ ratio [sp³ C/(sp³C + sp²C)] was determined from C 1s core-level spectra obtained by X-ray photoelectron spectroscopy (XPS).<br/><b>3. Results and discussion</b><br/>The DLC films prepared with atomic hydrogen irradiation had higher sp³ ratios than those prepared in a vacuum. Especially, the DLC film deposited at a filament current of 4 A exhibited the highest sp³ ratio. The hydrogen pressure at which the sp³ ratio reached a maximum was found to be 0.01 Pa. When the substrate temperature was increased from RT to 700 °C, the sp³ ratio decreased. It was found that the sp³ ratio in the film deposited at a repetition rate of 20 Hz was higher than that in the film deposited at 10 Hz. The maximum sp³ ratio of 57% was obtained in this study, which was close to that reported in Ref. 1, in which PLD using a KrF excimer laser was employed.<br/>Yasui et al. [5] have reported that the amount of atomic hydrogen generated using a tungsten filament source increases with the filament temperature. C.-L. Cheng et al. [3] have shown that hydrogenated, disordered carbon films are deposited at RT, whereas the graphitization of the films occurs at substrate temperatures above 500 °C, which is facilitated by hydrogen etching. When the filament temperature and hydrogen pressure increase, the amount of atomic hydrogen increases, leading to the acceleration of the formation of C–H bonds and selective etching of sp²-bonded carbons by atomic hydrogen. When the substrate temperature decreases or the repetition rate (the flux of C atoms) increases, the surface diffusion length of C atoms decreases. In this case, C atoms adsorbed on the surfaces are not able to arrive at stable sites, and the film becomes a metastable form of amorphous carbon.<br/><b>4. Summary</b><br/>The maximum sp³ ratio of 57% was obtained when the hydrogen pressure, substrate temperature, laser intensity, and laser repetition rate were 0.01 Pa, RT, 0.25 J/pulse, and 20 Hz, respectively.<br/><b>References</b><br/>[1] F. Stock et.al, Appl. Phys. A <b>123</b> (2017) 590.<br/>[2] T. Yoshitake et al., Diam. Relat. Mater. <b>9</b> (2000) 689.<br/>[3] C.-L. Cheng et al., Appl. Surf. Sci. <b>174</b> (2001) 251.<br/>[4] C.-L. Cheng et al., Diam. Relat. Mater. <b>11</b> (2002) 262.<br/>[5] K Yasui et al., Jpn. J. Appl. Phys. <b>44</b> (2005) 1361.