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Unravelling the Physical Mechanisms that Determine Microstructural Evolution of Ultrathin Volmer-Weber Films
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0003-4811-478X
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0003-0099-5469
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0003-2864-9509
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 4, 044302- p.Article in journal (Refereed) Published
Abstract [en]

The initial formation stages (i.e., island nucleation, island growth, and island coalescence) set characteristic length scales during growth of thin films from the vapour phase. They are, thus, decisive for morphological and microstructural features of films and nanostructures. Each of the initial formation stages has previously been well-investigated separately for the case of Volmer-Weber growth, but knowledge on how and to what extent each stage individually and all together affect the microstructural evolution is still lacking. Here we address this question using growth of Ag on SiO2 from pulsed vapour fluxes as a case study. By combining in situ growth monitoring, ex situ imaging and growth simulations we systematically study the growth evolution all the way from nucleation to formation of a continuous film and establish the effect of the vapour flux time domain on the scaling behaviour of characteristic growth transitions (elongation transition, percolation and continuous film formation). Our data reveal a pulsing frequency dependence for the characteristic film growth transitions, where the nominal transition thickness decreases with increasing pulsing frequency up to a certain value after which a steady-state behaviour is observed. The scaling behaviour is shown to result from differences in island sizes and densities, as dictated by the initial film formation stages. These differences are determined solely by the interplay between the characteristics of the vapour flux and time required for island coalescence to be completed. In particular, our data provide evidence that the steady-state scaling regime of the characteristic growth transitions is caused by island growth that hinders coalescence from being completed, leading to a coalescence-free growth regime.

Place, publisher, year, edition, pages
2014. Vol. 116, no 4, 044302- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-103920DOI: 10.1063/1.4890522ISI: 000340710700078OAI: oai:DiVA.org:liu-103920DiVA: diva2:693011
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2017-12-06
In thesis
1. Thin Film Growth using Pulsed and Highly Ionized Vapor Fluxes
Open this publication in new window or tab >>Thin Film Growth using Pulsed and Highly Ionized Vapor Fluxes
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Microstructure and morphology of thin films are decisive for many of their resulting properties. To be able to tailor these properties, and thus the film functionality, a fundamental understanding of thin film growth needs to be acquired. Film growth is commonly performed using continuous vapor fluxes with low energy, but additional handles to control growth can be obtained by instead using pulsed and energetic ion fluxes. In this licentiate thesis the physical processes that determine microstructure and morphology of thin films grown using pulsed and highly ionized vapor fluxes are investigated.

The underlying physics that determines the initial film growth stages (i.e., island nucleation, island growth and island coalescence) and how they can be manipulated individually when using pulsed vapor fluxes have previously been investigated. Their combined effect on film growth is, however, paramount to tailor film properties. In the thesis, a route to generate pulsed vapor fluxes using the vapor-based technique high power impulse magnetron sputtering (HiPIMS) is established. These fluxes are then used to grow Ag films on SiO2 substrates. For fluxes with constant energy and deposition rate per pulse it is demonstrated that the growth evolution is solely determined by the characteristics of the vapor flux, as set by the pulsing frequency, and the average time required for coalescence to be completed.

Highly ionized vapor fluxes have previously been used to manipulate film growth when deposition is performed both normal and off-normal to the substrate. For the latter case, the physical mechanisms that determine film microstructure and morphology are, however, not fully understood. Here it is shown that the tilted columnar microstructure obtained during  off-normal film growth is positioned closer to the substrate normal as the ionization degree of the flux increases, but only if certain nucleation characteristics are present.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 53 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1641
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-103921 (URN)10.3384/lic.diva-103921 (DOI)978-91-7519-426-4 (ISBN)
Presentation
2014-02-28, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note

The series name "Linköping studies in science and technology. Licentiate Thesis" is incorrect. The correct series name is "Linköping studies in science and technology. Thesis".

Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2014-02-04Bibliographically approved
2. Fundamental processes in thin film growth: The origin of compressive stress and the dynamics of the early growth stages
Open this publication in new window or tab >>Fundamental processes in thin film growth: The origin of compressive stress and the dynamics of the early growth stages
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Fundamentala processer under tunnfilmstillväxt : Tryckspänningars ursprung och dynamiska processer i de tidiga tillväxtstegen
Abstract [en]

The fundamental mechanisms behind the generation of compressive stresses in polycrystalline thin films, the effects of pulsed deposition fluxes on the dynamics of the early growth stages as well as the generation of energetic Ar+ ions in high power impulse magnetron sputtering (HiPIMS) discharges has been studied in this thesis.

It was found that compressive film stresses in Mo films deposited using energetic vapor fluxes are correlated with high film densities while only a slight lattice expansion compared to relaxed Mo was found. This implies that the stress is caused by grain boundary densification and not defect creation in the grain bulk. The compressive stress magnitude should scale with the grain boundary length per unit area, or the inverse grain size, if the stress originates in the grain boundaries. This was found to be the case for dense Mo films confirming that the observed compressive stresses originate in the grain boundaries. Similarly to what has been suggested for conditions where adatoms are highly mobile we suggest that atom insertion into grain boundaries is the cause of the compressive stresses observed in the Mo films.

Island nucleation, growth and coalescence are the dynamic processes that decide the initial microstructure of thin films growing in a three dimensional fashion. Using Ag on SiO2 as a model system and estimations of adatom life times and coalescence time it was shown that the time scales of island nucleation and coalescence are in the same range as the time scale of the vapor flux modulation in HiPIMS and other pulsed deposition methods. In situ real time measurements were used to demonstrate that it is possible to decrease the thickness at which a continuous film is formed from 21 to 15 nm by increasing the flux modulation frequency. A more in depth study where in situ real time monitoring was coupled with ex situ imaging and kinetic Monte Carlo simulations showed that this behavior is due to the interplay of the pulsed deposition flux and island coalescence where island coalescence is hindered at high pulsing frequencies.

The generation of energetic Ar+ ions was investigated by ion mass spectrometry and Monte Carlo simulations of gas transport. It was shown that the energetic Ar+ ions originate from Ar atoms backscattered from the target that are ionized in the plasma by correlating the length of the high energy tail in the ion energy distribution functions with the atomic mass of the Cr, Mo and W sputtering targets. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 116 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1592
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-105791 (URN)19.3384/diss.diva-105791 (DOI)978-91-7519-352-6 (ISBN)
Public defence
2014-05-16, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-04-08 Created: 2014-04-07 Last updated: 2017-01-16Bibliographically approved
3. Dynamics of the Early Stages in Metal-on-Insulator Thin Film Deposition
Open this publication in new window or tab >>Dynamics of the Early Stages in Metal-on-Insulator Thin Film Deposition
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thin films consist of nanoscale layers of material that are used in many technological applications to either functionalize a surface or serve as parts in miniaturized devices. The properties of a film are closely related to its microstructure, which in turn can be tuned during film preparation. Thin film growth involves a multitude of atomic-scale processes that cannot always be easily studied experimentally. Therefore, different types of computer simulations have been developed in order to test theoretical models of thin film growth in a highly controlled way. To be able to compare simulation and experimental results, the simulations must be able to model events on experimental time-scales, i.e. several seconds or minutes. This is achievable with the kinetic Monte Carlo method.

In this work, kinetic Monte Carlo simulations are used to model the initial growth stages of metal films on insulating, amorphous substrates. This includes the processes of island nucleation, three-dimensional island growth and island coalescence. Both continuous and pulsed vapor fluxes are investigated as deposition sources, and relations between deposition parameters and film morphology are formulated. Specifically, the film thickness at what is known as the “elongation transition” is studied as a function of the temporal profile of the vapor flux, adatom diffusivity and the coalescence rate. Since the elongation transition occurs due to hindrance of coalescence completion, two separate scaling behaviors of the elongation transition film thickness are found: one where coalescence occurs frequently and one where coalescence occurs infrequently. In the latter case, known nucleation behaviors can be used favorably to control the morphology of thin films, as these behaviors are not erased by island coalescence. Experimental results of Ag growth on amorphous SiO2 that confirm the existence of these two “growth regimes” are also presented for both pulsed and continuous deposition by magnetron sputtering. Knowledge of how to avoid coalescence for different deposition conditions allows nucleation for metal-on-insulator material systems to be studied and relevant physical quantities to be determined in a way not previously possible. This work also aids understanding of the growth evolution of polycrystalline films, which in conjunction with advanced deposition techniques allows thin films to be tailored to specific applications.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 54 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1687
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-112136 (URN)10.3384/lic.diva-112136 (DOI)978-91-7519-192-8 (ISBN)
Supervisors
Available from: 2014-11-17 Created: 2014-11-17 Last updated: 2014-11-18Bibliographically approved
4. Nanoscale structure forming processes: Metal thin films grown far-from-equilibrium
Open this publication in new window or tab >>Nanoscale structure forming processes: Metal thin films grown far-from-equilibrium
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thin film growth from the vapor phase has for a long time intrigued researchers endeavouring to unravel and understand atomistic surface processes that govern film formation. Their motivation has not been purely scientific, but also driven by numerous applications where this understanding is paramount to knowledge-based design of novel film materials with tailored properties.

Within the above framework, this thesis investigates growth of metal films on weakly bonding substrates, a combination of great relevance for applications concerning e.g., catalysis, graphene metallization and architectural glazing. When metal vapor condenses on weakly bonding substrates three dimensional islands nucleate, grow and coalesce prior to forming a continuous film. The combined effect of these initial growth stages on film formation and morphology evolution is studied using pulsed vapor fluxes for the model system Ag/SiO2. It is shown that the competition between island growth and coalescence completion determines structure evolution. The effect of the initial growth stages on film formation is also examined for the tilted columnar microstructure obtained when vapor arrives at an angle that deviates from the substrate surface normal. This is done using two metals with distinctly different nucleation behaviour, and the findings suggest that the column tilt angle is set by nucleation conditions in conjunction with shadowing of the vapor flux by adjacent islands. Vapor arriving at an angle can in addition result in films that exhibit preferred crystallographic orientations, both out-of-plane and in-plane. Their emergence is commonly described by an evolutionary growth model, which for some materials predict a double in-plane alignment that has not been observed experimentally. Here, an experiment is designed to replicate the model’s growth conditions, confirming the existence of double in-plane alignment.

New and added film functionalities can further be unlocked by alloying. Properties are then largely set by chemistry and atomic arrangement, where the latter can be affected by thermodynamics, kinetics and vapor flux modulation. Their combined effect on atomic arrangement is here unravelled by presenting a research methodology that encompasses high resolution vapor flux modulation, nanoscale structure v vi probes and growth simulations. The methodology is deployed to study the immiscible Ag-Cu and miscible Ag-Au model systems, for which it is shown that capping of Cu by Ag atoms via near surface diffusion processes and rough morphology of the Ag-Au growth front are the decisive structure forming processes in each respective system.

The results generated in this thesis are of relevance for tuning structure of metal films grown on weakly bonding substrates. They also indicate that improved growth models are required to accurately describe structure evolution and emergence of a preferred in-plane orientation in films where vapor arrives at an angle that deviates from the substrate surface normal. In addition, this thesis presents a methodology that can be used to identify and understand structure forming processes in multicomponent films, which may enable tailoring of atomic arrangement and related properties in technologically relevant material systems.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 71 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1804
National Category
Inorganic Chemistry Other Materials Engineering Other Physics Topics Materials Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-132895 (URN)10.3384/diss.diva-132895 (DOI)9789176856390 (ISBN)
Public defence
2017-01-20, Planck, Fysikhuset, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2016-11-30 Created: 2016-11-30 Last updated: 2016-11-30Bibliographically approved

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Elofsson, ViktorMagnfält, DanielMünger, PeterSarakinos, Kostas

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