Dynamics Of MoS 2 Photoablation
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DYNAMICS OF MoS2 PHOTOABLATION
P. T. MURRAY*, V. J. DYHOUSE*, L GRAZULIS*, and D. R. THOMAS** *Research Institute, University of Dayton, Dayton, OH 45469. **DCA 10 NET. 7887 Washington Village Dr., Dayton, OH 45459
ABSTRACT Speed distributions were determined for the ions ejected from MoS
2
by excimer laser ablation.
It was found that the majority of Ionic species were atomic and were ablated with speeds in excess of 6 km/s. The speed distributions were found to be non-Maxwellian and were best described by a supersonic expansion model. The differences in ArF and KrF excimer laser ablation are discussed. INTRODUCTION Laser ablative deposition is an emerging film growth process which possesses a number of advantages over more conventional techniques. Among these advantages are the capability of growing high purity films, the potential for evaporating refractory materials, and, under the appropriate conditions, congruent target removal. Laser-induced material removal has been the subject of a number of investigations, and it has been found that the process can occur by at least three mechanisms. The first is laser desorption. In this process, the laser provides only enough energy to desorb weakly-bound species from the solid surface. In the second mechanism, laser evaporation, chemically bonded species are removed from the target, and the laser serves as a thermal source, with the irradiated area in approximate local thermodynamic equilibrium. The third mechanism is laser ablation which occurs by directly severing chemical bonds. Garrison and Srinivasan [1) have carried out molecular dynamics calculations on the latter two mechanisms. Their calculations indicate that evaporation results in distortion, or melting, of the solid and that the evaporated material is ejected in a broad angular distribution. Their calculations suggest that ablation is accompanied by the formation of well-defined pits in the solid target, that the ablated material is ejected in a narrow angular distribution, and that ablation occurs layer-by-layer. Furthermore, the average perpendicular velocity of the ablated material was predicted to be on the order of 1-2 km/s. The purpose of the work presented here was to investigate the dynamics of excimer laser ablation of MoS2. Thin films of MoS2 have use as solid lubricants in a variety of spaceborne applications. It has been shown [2,3] that stoichiometric thin films of MoS2 can be grown by pulsed laser evaporation, using frequency-doubled (X-532 nm) Nd:YAG laser radiation. It was found [41 that MoS2 irradiation under these conditions resulted primarily in the ejection of atomic Mo and S, with mixed MoSSy clusters present at much lower concentration. The speed distribution of neutral S was found to be well-described by a Maxwell-Boltzmann distribution, with a translational temperature of approximately 1500K. It was also found [41 that the films grown by Nd:YAG radiation contained a number of spherical and oblate particles. These were attributed to target splashing [51, a phenomenon consistent wi
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