Laser-Target Interactions and its Effect on Surface Morphology of Laser Deposited thin films
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LASER-TARGET INTERACTIONS AND ITS EFFECT ON SURFACE MORPHOLOGY OF LASER DEPOSITED THIN FILMS R. K. SINGH* and D. BHATTACHARYA* and J. NARAYAN** *Materials Science and Engineering Department, University of Florida, Gainesville, FL 3261 12066 ** Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695-7916 ABSTRACT The laser-target interactions during pulsed laser evaporation (PLE) of materials have been investigated in detail. Sub-surface temperatures have been calculated to be higher than the surface temperatures during planar surface evaporation of the target material. While the evaporating surface is being cooled due to the latent heat of vaporization, sub-surface superheating occurs due to the finite absorption depth of the laser beam. Detailed computer simulations have been carried out to understand the evaporation characteristics of different targets (Si, YBa 2 Cu 3O 7 ) as a function of laser and target variables. For silicon targets irradiated with 25 nanosecond laser pulses (energy density of 1OJ/cm 2 , absorption coefficient of 105 cm- 1), the sub-surface temperatures were found to be more than 2000'C above the surface evaporation temperatures. The sub-surface superheating increased with increasing energy density and absorption depth. This internal superheating effects may lead to volume evaporation of the target where a solid material in form of particles may be ejected from the target surface. Based on the above understanding, parameters required to reduce the particle density in PLE films are predicted. INTRODUCTION The pulsed laser evaporation technique has attracted a great interest in recent years because of its many unique properties, including stoichiometric evaporation in multicomponent systems, deposition, formation and rapid expansion of a high temperature plasma, and the forward directed nature of the deposited material [ 1-3]. This technique has been particularly successful in the deposition of high-Tc superconducting thin films which possess very high critical current densities (in zero magnetic fields) in the range of 5.0 -6.5 x 106 A/cm 2 at 77 K [4-5]. The versatility of this technique has been clearly shown in the in-situ multilayer deposition of superconducting superlattice structures [6], and buffer layers between superconducting films and substrates like Si and GaAs. However, the main drawback of the PLE technique is the surface morphology which is characterized by the presence of particles whose dimensions vary from 0.1 pim to 2 jim. The number density and the size of the particles are dependent on the nature and preparation of the target material, and the laser beam characteristics (wavelength, pulse duration, energy density,etc.). These particles have been speculated [7] to arise from the explosive interaction of laser beam with the target material i.e microexplosions inside the target. An understanding of the laser-target interactions is essential to control and reduce the particle density in PLE films. In this paper, we will analyze the ther
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