New materials via combinatorial atomic layer deposition
University College Cork
In recent years, hybrid nanolaminate materials have generated great interest due to the physical and chemical properties that 2D nanosheets possess that their bulk counterparts do not. Thin films and nanostructured materials offer unique and potential benefits that could positively impact the areas of communications, electronics and energy arising from the fact that the properties of atoms, groups of atoms and ultra-thin layers are exploited rather than bulk materials. The applications of hybrid films are almost endless, ranging from use as capacitors, dielectrics and insulators, encapsulation layers/ protective coatings, lubricants and optical waveguides. By definition, nanolaminates are multicomponent systems consisting of alternating layers of materials such as metals and metal oxides. Each individual layer has an associated thickness on the nanometre scale. By mixing a wide variety of materials, it may be possible to fine-tune a desirable material in terms of its composition, properties and structure. In this thesis, the growth of nanolaminate structures of ZnO and TiO2 were studied using a C-ALD (combinatorial atomic layer deposition) approach. The specific aim of this study was to combine the photocatalytic properties and robustness of TiO2 with the transparent conducting behaviour and photoactivity of ZnO. Mixed nanolaminate structures containing ZnO/TiO2 layers were fabricated by deliberately under-dosing during the metal precursor parts of the ALD cycle. The ZnO/TiO2 nanolaminate films were deposited at 200 °C. Building upon this nanolaminate study, a fundamental study of ZnO deposited by both thermal and PE-ALD (plasma- enhanced ALD) was employed, focusing on the crystallinity and morphology of the films as well as optical and electrical performance. The same reactor, a Picosun™ R200 ALD reactor, was used for both sets of experiments so as to minimise the influence of reactor design on the nature of the deposited films. Next, doped ZnO films were deposited via thermal and PE-ALD with the intention of improving the electrical properties. ZnO thin films were doped with Al, Ga, Sn and Ti dopants. The introduction of dopants into a host material allows for the manipulation of the composition and structure that can impact both electrical and optical properties of the material. Lastly, ZnO: Ga2O3: Al2O3, ZnO: SnO2: Al2O3 and ZnO: TiO2: Al2O3 ternaries were deposited via PE-ALD at 150 °C in order to study the electrical, optical and structural properties. Ternaries are inherently more complex which provides more opportunities to fine tune the composition and both the atomic and electronic structures.
Atomic layer deposition , Plasma enhanced atomic layer deposition , Nanolaminates , TiO2 , ZnO , Doped ZnO , Thin film deposition
Doyle, S. 2022. New materials via combinatorial atomic layer deposition. PhD Thesis, University College Cork.