|M.Sc Student||Leonid Bloch|
|Subject||Characterization of Structural Aspects of Nanometric Scale|
Thickness ALD Deposited Films
|Department||Department of Materials Science and Engineering||Supervisor||Professor Pokroy Boaz|
|Full Thesis text|
The atomic layer deposition (ALD) is an advanced thin film deposition technique that excels in growth of highly uniform and conformal films, the thickness of which can be controlled on the atomic level. Due to its versatility, new use cases and new ALD processes are constantly being reported in various fields of study. The amount of scientific publications concerning the ALD is constantly on the rise.
This research successfully tackled a basic scientific question regarding the size effect on the short range order in amorphous materials, and the influence of size on the structural variations during the amorphous to crystalline transformation. The ALD was used as a tool to grow highly controlled and consistent samples, but the implications of the obtained results go beyond the realm of ALD.
The inspiration for this research was drawn from nature, where some organisms can control the short range order in amorphous minerals, and by doing so they can direct the subsequent crystallization, or acquisition of long range order, of these minerals to obtain a specific crystalline polymorph needed for its biological function. This approach enables a fine control over the shape of the resulting crystal, and is lowering the energy needed for its formation. Even thermoydynamically unstable polymorphs, and single crystals with extremely high curvature surfaces, that would not have been formed spontaneously, can be grown this way.
In this work, surface and size induced variations of the short range order in ALD-deposited amorphous alumina thin films are demonstrated experimentally using electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). It is shown that the short range order in such films is strongly influenced by size. This phenomenon is similar to the well-known size effect on lattice parameters in crystalline materials. In particular, it became evident that the 4-coordinated Al sites are more abundant in thinner amorphous alumina films, and their abundance decreases with depth in thicker amorphous films.
Another important finding is that the evolution of the short range order during the process of amorphous to crystalline transition differs considerably between films of different thickness, and a short range order of the succeeding crystalline polymorph is achieved in the amorphous phase before crystallization.
Additionally, direct measurements of crystallization temperature have shown a dramatic increase in amorphous to crystalline transition temperature for thinner amorphous alumina films.
As side-projects of this research, different innovative ALD processes and techniques were studied.