Thermal stability of pulsed laser deposited diamond-like carbon films

Pulsed laser deposited diamond-like cabron (PLD DLC) films were prepared with the visible 532 nm Nd:YAG pulsed laser beam of power density Φ from 2.7 × 109to 2.0 × 1010W cm-2, below the Nagel criterion (5 × 1010W cm-2for 1064 nm Nd:YAG laser). The temperature dependence of the electrical conductivity, σ(T), the imaginary part of the refractive index, k, and the surface morphology of the films were studied as functions of Φ and post-annealing temperature Ta. Particulates appear on the surface of all samples, with their size increasing with increasing φ. For Φ between 2.7 × 109and 5.9 × 109W cm-2, the room temperature electrical conductivity σRand k decrease from 1.28 × 10-4to 3.37 × 10-5Ω-1cm-1, and from 0.2 to 0.18, respectively. These results indicate that higher Φ increases the diamond-like content in the films. For Φ from 5.9 × 109to 2.0 × 1010W cm-2, σRand k increase to reach 3.69 × 10-4Ω-1cm-1and 0.22, respectively. In this power density range the surface graphitic particulates dominate the film properties. Higher Φ generates larger graphite particulates and degrades the film quality, thus leading to increases in σRand k. The as-deposited samples were annealed at temperatures Tabetween 100 and 900 °C. In this temperature range, ΦRincreases by a factor of 106, while k rises from about 0.2 to 0.5, indicating that a diamond-like-to-graphitic transition has occurred. The degradation process was observable at Taas low as 100 °C, and became significant for higher Ta. Finally, the films reach a graphitic state when Ta≥ 750 °C. Since the surface density and size of the particulates are not influenced significantly by annealing, we suggest that the variation of the films properties arises from changes in the matrix in which the graphitic particulates are embedded.