Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part I. Optical emission spectroscopy

A comparative study of ablation plasma plumes originated from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C⁺, C²⁺, and C³⁺ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (E_L) increase for both cases. The C₂ emission intensity from the SCG plasma plume changes drastically with E_L, while that from the a-C plasma plume is almost constant. The C₂/C emission intensity ratio for the a-C case decreases with E_L increase. As for the SCG case, the C₂/C ratio decreases with E_L increase up to 3.0 J/cm², and increases slightly with further E_L increase. Nanohardness of the deposited films decreases with the increase of the C₂/C emission intensity ratio. It is suggested that for both the SCG and a-C target cases, the C₂ molecule in the ablated plasma plume may not play an important role in producing high quality DLC films. It is further proposed that the threshold of laser fluence for the formation of diamond-like character film using KrF excimer PLD is 2.1 J/cm²(0.84 × 10⁸ W/cm²) for the a-C target and 3.0 J/cm²(1.2 × 10⁸ W/cm²) for the SCG target. The C₂ vibrational temperature of the SCG and the a-C plasma plumes show different features on both the laser energy density and nitrogen pressure dependencies. Through optical emission spectroscopy and Langmuir probe measurements in vacuum and nitrogen background, it is concluded that there are many particles with higher mass in the SCG plasma plume, especially at relatively lower laser energy density below 3.0 J/cm².