Pulsed Laser Deposition History and Laser-Target Interactions
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natural evolution led by theoretical studies. Shortly after the demonstration of the first laser, the most intensely studied theoretical topics dealt with laser beam-solid interactions. Experiments were undertaken to verify different theoretical models for this process. Later, these experiments became the pillars of many applications. Figure 1 illustrates the history of laser development from its initial discovery to practical applications. In this tree of evolution, Pulsed Laser Deposition (PLD) is only a small branch. It remained relatively obscure for a long time. Only in the last few years has
Figure 1. Evolution of laser technology and its applications.
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this branch started to blossom and bear fruits in thin film deposition. Conceptually and experimentally, PLD1 is extremely simple, probably the simplest among all thin film growth techniques. Figure 2 shows a schematic diagram of this technique. It uses pulsed laser radiation to vaporize materials and to deposit thin films in a vacuum chamber. However, the beamsolid interaction that leads to evaporation/ ablation is a very complex physical phenomenon. The theoretical description of the mechanism is multidisciplinary and combines equilibrium and nonequilibrium processes. The impact of a laser beam on the surface of a solid material, electromagnetic energy is converted first into electronic excitation and then into thermal, chemical, and even mechanical energy to cause evaporation, ablation, excitation, and plasma formation. Evaporants leave the surface in a hydrodynamic flow in which a large number of molecular collisions take place. The beam-solid interaction is also wavelength dependent. However, with lasers available in a broad spectrum of wavelengths, pulse energies, and pulse widths, it is envisioned that with the choice of an appropriate laser, PLD can be used to deposit thin films of almost any material. The versatility of this technique is reflected in the fact that at the most recent counting, close to 128 different materials have been deposited in thin film form by PLD.2 It has always been a puzzle why such a powerful technique remained relatively obscure for so long. A contributing factor to the slow development of PLD was the development of other techniques, namely sputtering and molecular beam epitaxy (MBE). Sputtering is regarded as the workhorse for depositing metal and dielectric thin films, and MBE has become a process for producing high-quality semiconductor films for electronic and optoelectronic devices. Robust and well developed, these two techniques meet most of the requirements set by the electronics industry. Because they absorbed a great deal of research and development resources, the available resources and attention devoted to PLD were diluted. The versatility of PLD should also take part of the blame for its slow development. The absence of a focal point (i.e., a material for which the technique provided obvious advantages) was needed in order for PLD to be coupled into the mainstream of materials science. In the last few years,
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