Plasma Dynamics During Pulsed Laser Evaporation of High T c Superconductors
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PLASMA DYNAMICS DURING PULSED LASER EVAPORATION OF HIGH Tc SUPERCONDUCTORS RAJIV K. SINGH *, R. NEIFELD** and J. NARAYAN *** * Materials Science and Engineering Department, University of Florida, Gainesville, FL 326112066 ** US Army SLCET-ED LABCOM, ETDL, Fort Monmouth, NJ. *** Department of Materials Science and Engineering, North Carolina State University, Raleigh NC 27695-7916 ABSTRACT We have theoretically and experimentally investigated the dynamics of the evaporated material generated by nanosecond excimer laser irradiation of YBa 2 Cu 3O 7 targets in vacuum. The velocity distribution and the ionization of the plasma were determined by the ion time of flight measurements. The excimer laser ablated species possessed very high velocities (> 106 cm/sec) which increased non-linearly with energy density. The ionization/ volume of the evaporated material exhibited a weak dependence on energy density, thereby suggesting the role of nonthermal mechanisms in the ionization process. These experimental results have been correlated with the theoretical model analyzing the plasma dynamics during pulsed laser evaporation of materials. A new modification to the earlier theoretical model is developed which accurately predicts the terminal velocities and the effect of ionization on these velocities. Various factors including, evaporation rates, degree of ionization, and laser wavelength which affect the plasma velocities will also be discussed. INTRODUCTION There has been an extensive interest in the ultraviolet excimer laser deposition of thin films of high-Tc superconductors and other materials 11-3]. The incident laser parameters including the wavelength, pulse duration and energy density determines the nature of the evaporation characteristics of the target and the deposition characteristics of the film. In the regime applicable for pulsed laser deposition of thin films, the evaporated material interacts with the incoming laser beam and forms a high temperature plasma. The composition and temperature of the plasma and the kinetic energy of its species are the fundamental parameters which determine the nature of thin film deposition 14-71. In this paper, we will briefly summarize some of the theoretical and experimental results on the dynamics of the plasma species during excimer laser deposition of superconducting YBa 2 Cu 3 0 7 films. The evaporation characteristics have been considered separately in another paper.181 THEORY The nature of the plasma dynamics controls the deposition characteristics of films fabricated by the pulsed laser evaporation technique (PLE). The plasma velocities are strong affected both by the laser parameters (energy density, pulse duration, laser wavelength,etc.) and the target parameters including the type and preparation of the target, nature of the evaporated species, irradiation spot shape and dimensions. In our previous papers 14,51 we had outlined a theoretical Mat. Res. Soc. Symp. Proc. Vol. 201.
1991 Materials Research Society
502
model to explain most of the deposition characterist
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