Thermal phenomena (laser-target-plasma interactions) in pulsed laser deposition of high-T_c superconducting thin films

The understanding of thermal phenomena is pulsed laser evaporation (PLE) technique is very important for fabrication of films with desired surface morphology and other suitable properties. We have theoretically and experimentally investigated various thermal phenomena during pulsed laser evaporation of high-T_c YBa₂Cu₃O₇ superconductors. Based on the experimental results, a model for laser-target-plasma interactions has been proposed. The interaction of high-powered laser beams on targets results in evaporation plasma formation and expansion, and subsequent deposition of thin films. The thermal effects of pulsed nanosecond lasers on targets were calculated by solving the heat flow equation with appropriate boundary conditions. The optical properties (absorption coefficient, reflectivity), laser parameters (wavelength, power density, and pulse duration), and thermophysical properties (thermal conductivity, heat capacity, latent heat, etc) control the nature of the evaporation process. Depending on these parameters, the laser-target interaction can be classified into different heating and evaporation regimes. These calculations predict increased sub-surface superheating with increasing pulse energy density and absorption depth. This may lead to removal of small particles from the target surface. The evaporating material may itself absorb the laser radiation, resulting in the formation of a high temperature plasma. The laser generated plasma, which is initially at high temperature and pressure, expands anisotropically in vacuum. The three-dimensional anisotropic expansion of this plasma gives rise to spatial thickness and compositional variations observed in PLE films. This model can explain most of the salient features of PLE process including, (i) the presence of particles in PLE films and its dependence on target and laser variables, (ii) the applicability of this technique for deposition of and deposition characteristics, etc.