Pulsed Laser Deposition
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ate processing procedure" to deposit sequential epitaxial layers of high quality materials that possess profoundly different properties. The articles collected in this issue of the
Although PLD is simple, no one understands exactly why the technique works so well. MRS Bulletin highlight some of the rapid advances in this field. We emphasize the historical development of pulsed laser deposition, the physics underlying the process, progress on some thin films and devices currently under investigation, and future trends in the field. In describing recent progress, emphasis is placed on areas that presently have an impact on technology or that are projected to have an impact in the near future. The number of research groups now involved in PLD is large and growing rapidly. Another entire issue could readily be filled with excellent materials-based studies using PLD, but space is limited and our focus is on applications. Many of these studies are referenced in the articles that follow. One may well ask, "What features of pulsed laser deposition set it apart from other deposition methods?" The answer isn't easy and it must be admitted that no one understands exactly why the technique works so well. Conceptually and operationally, PLD is quite simple. A laser pulse enters through a window into a vacuum chamber and impinges on the material to be deposited. The 20-30 nanosecond wide laser pulse is focused to an energy density (—1-10 J/cm2) to vaporize a few hundred Angstroms of surface material (called the "plume") in the form of neutral or ionic atoms and molecules with electron-
volt kinetic energies which then deposit onto the substrate (see this issue's cover). The amount of material deposited per pulse is about 1 A (for -200 mj/pulse), and the laser is pulsed at rates of 1 to 100 Hz. With no ion or evaporation sources that contain hot filaments in the vacuum, it is simple to operate in high pressures (100 mtorr) of reactive gases like oxygen. The most important advantage of PLD is the ability to transfer the composition of the target to the deposited film. Part of the beauty of PLD is the simplicity of the apparatus. However, the process itself involves a complicated interplay among the variables of the ablation laser wavelength and power density delivered to the target, the distance between the laser target and the substrate, the pressure of the background gas, the bias on the substrate, and the temperature of the substrate. Research into the fundamentals is producing exciting new information almost monthly, and while still very incomplete, a picture of the process is emerging. Prior to the rise of PLD, it was generally recognized that film properties are improved if extra energy is added to the atoms arriving at the surface of the growing film, either by heating the sample or bombarding it with particle or electromagnetic energy. For example, in molecular beam epitaxy (MBE), extra energy is added by thermally heating the substrate. The heat provides surface mobility after the atoms are deposited so that they re
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