Pulsed Laser Deposition: Surface Processes and Structures

Growing thin or ultrathin films by pulsed laser deposition (PLD) has many advantages over conventional thermal deposition. The project should provide a better knowledge of the processes occurring and the structures resulting when metal films are grown by PLD, thereby providing a better foundation for future applications of PLD, and also lead to a better understanding of other deposition techniques such as sputter deposition. Furthermore, the project should provide valuable insight into the physics of ultrathin films on the nanometer and sub-nanometer scale. The main analysis technique employed in our project will be scanning tunneling microscopy (STM) with atomic resolution and chemical contrast, allowing us to determine the geometrical and chemical structure of surfaces on the atomic scale. Using this technique, we will be able to study microscopic processes such as insertion of deposited atoms into the surface or mixing between substrate and film in detail. By a combination of these detailed investigations on the atomic scale and measurements of the energy distribution of the deposited atoms and ions, we expect to provide a detailed answer to the question why films produced by PLD are smoother than films grown by thermal deposition, i.e., why PLD results in layer-by-layer growth. High-resolution STM will also allow us to determine the differences between the structures of films grown by thermal deposition and PLD, a prerequisite for understanding the different properties of these films, especially concerning iron films on copper, where structure and magnetic properties are intimately interwoven.

Pulsed laser deposition (PLD) is a versatile technique for growing thin films on solid surfaces, with advantages over conventional evaporation (also known as thermal deposition, TD). In PLD, short pulses of a high-power laser are used to ablate material from a target; the ablated material is deposited on the substrate. In contrast to thermal deposition, the ablated particles arrive in pulses and have high energies, typically hundreds of times higher than thermally evaporated atoms or molecules. In this research project, it could be shown that both of these features of PLD are important and detailed processes how they influence growth of ultra-thin films were determined. The results have implications on understanding the elemental processes in the widely used technique of sputter deposition. In contrast to previous work, we could directly and quantitatively measure the neutral atoms emitted from the target by the laser pulses. We could show that these are only a minor fraction of the ablated material, and low-energy neutrals are very scarce. Most of the material ablated is in the form of ions, which impinge on the substrate with high energy. Nevertheless, for growth of platinum on a well-defined platinum surface, we could show that the initial stages of growth are not influenced by the ion energies, except for very high ion energies (200 eV and above). It is important however, that the many particles (ions) arrive within very short periods of time, which makes it much easier for them to find partners on the surface for forming stable agglomerates. Only for the highest ion energies observed we find that impact on the surface is strong enough to create enough damage to modify subsequent growth of thin films. In any case, the films created by PLD show much smaller structures than those obtained by TD, and we could show that this can lead to the creation of smoother surfaces. When depositing a material different from the substrate, the energy of the ions in PLD plays a more important role. Replacement of a surface atom by an incoming energetic ion is a rather frequent process, leading to a mixed (chemically inhomogeneous) surface. Such a surface provides a variety of different binding sites for further growth of the film, which results in different growth processes and properties of the film than in TD. For ultra-thin films of iron grown on two different types of copper surfaces by PLD, we could also observe details of the crystallographic structure of these films. The differences between PLD- and TD-grown films are important for the magnetic properties of these films.