טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEtinger-Geller Yael
SubjectStructural Aspects of Nanometer Size Amorphous Materials
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Boaz Pokroy


Abstract

Amorphous materials, in contrast to crystalline ones, lack long-range order. Its order decays rapidly with the distance and while the local environment for a particular type of atom is quite similar, it is not identical; these fine changes in the atomistic structure of the materials lead to new and very interesting phenomena which are unique for amorphous materials. Although many aspects of science and technology rely on amorphous materials, much less research is conducted on their structure than on their crystalline counterparts.

The inspiration for this research comes from nature, where there are many organisms that use crystallization via an amorphous precursor phase in order to achieve controlled mineralization. One of the main advantages of this method is that it enables the organism to exert control over the resulting polymorph, which is not necessarily the thermodynamically-stable one, by first controlling the short-range order in the amorphous phase.

In this research we draw inspiration from nature and study the ability to control various structural aspects of amorphous materials via nanometer size effects. We chose atomic layer deposition as our material deposition method, since it is a technique that can provide extremely precise, sub-nanometric, thickness control and can deposit conformal and pinhole-free amorphous films of various materials.

Amorphous thin films of several oxides, deposited by atomic layer deposition method, were found to vary structurally as a function of size; thinner films exhibited a different structure, in comparison to thicker ones. According to different analyses that were performed, these changes arise from the surface of the film. Theoretical investigation revealed that these atomistic alterations are expected to change the amorphous thin film’s average density, and indeed it was found to vary with the layers' thickness. This effect is explained in terms of the deposition process, where each newly deposited layer is a new surface layer that ‘remembers’ its structure, resulting in thin films of substantially lower density. This further encouraged us to study the effect of size on different density-dependent properties and it was indeed found that the refractive index and dielectric constant of these layers also change with the thin films’ thickness. We believe that the ability to tune one property or another solely by size, according to a specific requirement, can open new possibilities for materials selections and applications, in science and technology.