thin films

Thin films in materials systems are essentially crystalline solid layers on a substrate, and having a thickness ranging from a few nanometres to some mikrons. In technological applications thin films fulfil a number of functions, and they cannot be substituted by other elements in high-tech products. Thin films serve as isolating, dielectric, magnetic or electronic components e.g. for integrated antennas and transmitters and radio frequency filters in mobile phones, or for semiconductor elements in integrated circuits of microelectronic products (computer).

The quality of a film essentially determines the quality of its function. We differentiate between simple films with amorphous or polycrystalline Structure and films of higher quality, which are constituted of crystals ranging from the film/substrate interface to the film surface and where the crystals may have a preferred crystallographic orientation (texture). A film may even consist of one thin single crystal which is then named epitactic. Epitactic films can only be produced on a single crystalline substrate, which by its crystal structure serves as a kind of template for the epitactic film growth. Epitactic films grow best in case of good fit in atomic distances and angles at the interface between the crystals structures of substrate and film. For technological applications epitactic films are mostly produced by routes involving gassy phases such as molecular beam epitaxy (MBE) or chemical vapour deposition (CVD). However, epitactic films can also be produced by sputter methods or laser ablation. All these methods require complex inert gas or vacuum techniques, which leads to long times for the preparation of deposition and to high costs.

Alternatively, the more simple and low-cost procedures of chemical solution deposition (CSD) are studied in our group. The procedure can be described as follows: solutions containing molecular precursors of the film material are deposited on a substrate e.g. by spin coating to produce a homogeneously distributed film of the solution.

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© WA

During heat treatment the deposited precursor solution is decomposed to the desired composition of the solid film. The initially developed film consists of very small crystallites which may grow in a subsequent heat treatment at higher temperatures to a single crystalline film.

The effect of the parameters for film production to the mechanisms of film growth are little understood. There has much more research to be performed in order to clarify the relationship between microscopic aspects (interfaces, structure, bonding) and macroscopic features and properties (crystallinity, texture, etc.). The understanding of such connections is essential for establishing defined and reproducible processes.


The CSD approach is already used and established in the technological production of piezoelectric oxide films based on led titanate and lead zircon titanate (PZT). Research in our group is focussed on oxide materials with unusual optical and electrooptical properties such as zinc oxide, spinels (MgAl2O3, Co3O4), and rare earth hexaaluminates, where the growth is studies on various substrate materials. Additionally, we look for and try other alternative chemical routes for thin film preparation.

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© WA

Interface between sapphire (bottom) and lanthanum hexaaluminate (top).
Reconstructed high resolution phase contrast image of the interface between sapphire substrate and lanthanum hexaaluminate film grown by chemical solution deposition with interface structure model and simulated image (left). Due to closely spaced La and Al ions across interface, every third La site is not occupied (arrows in image). This is also shown in the intensity profile of La ions at the boundary (bottom right, red) compared to the uniform profile in bulk lanthanum hexaaluminate (top right, blue).

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