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Nano- and mesoscale morphology evolution of metal films on weakly-interacting surfaces
Linköping University, Department of Physics, Chemistry and Biology, Nanoscale engineering. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-0908-7187
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thin films are structures consisting of one or several nanoscale atomic layers of material that are used to either functionalize a surface or constitute components in more complex devices. Many properties of a film are closely related to its microstructure, which allows films to be tailored to meet specific technological requirements. Atom-by-atom film growth from the vapor phase involves a multitude of atomic processes that may not be easily studied experimentally in real-time because they occur in small length- (≤ Å) and timescales (≤ ns). Therefore, different types of computer simulation methods have been developed in order to test theoretical models of thin film growth and unravel what experiments cannot show. In order to compare simulated and experimental results, the simulations must be able to model events on experimental time-scales, i.e. on the order of microseconds to seconds. This is achievable with the kinetic Monte Carlo (kMC) method.

In this work, the initial growth stages of metal deposition on weakly-interacting substrates is studied using both kMC simulations as well as experiments whereby growth was monitored using in situ probes. Such film/substrate material combinations are widely encountered in technological applications including low-emissivity window coatings to parts of microelectronics components. In the first part of this work, a kMC algorithm was developed to model the growth processes of island nucleation, growth and coalescence when these are functions of deposition parameters such as the vapor deposition rate and substrate temperature. The dynamic interplay between these growth processes was studied in terms of the scaling behavior of the film thickness at the elongation transition, for both continuous and pulsed deposition fluxes, and revealed in both cases two distinct growth regimes in which coalescence is either active or frozen out during deposition. These growth regimes were subsequently confirmed in growth experiments of Ag on SiO2, again for both pulsed and continuous deposition, by measuring the percolation thickness as well as the continuous film formation thickness. However, quantitative agreement with regards to scaling exponents in the two growth regimes was not found between simulations and experiments, and this prompted the development of a method to determine the elongation transition thickness experimentally. Using this method, the elongation transition of Ag on SiO2 was measured, with scaling exponents found in much better agreement with the simulation results. Further, these measurement data also allowed the calculation of surface properties such as the terrace diffusion barrier of Ag on SiO2 and the average island coalescence rate.

In the second part of this thesis, pioneering work is done to develop a fully atomistic, on-lattice model which describes the growth of Ag on weakly-interacting substrates. Simulations performed using this model revealed several key atomic-scale processes occurring at the film/substrate interface and on islands which govern island shape evolution, thereby contributing to a better understanding of how 3D island growth occurs at the atomic scale for a wide class of materials. The latter provides insights into the directed growth of metal nanostructures with controlled shapes on weakly-interacting substrates, including twodimensional crystals for use in catalytic and nano-electronic applications.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 68
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1835
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-144217DOI: 10.3384/diss.diva-144217ISBN: 9789176855706 (print)OAI: oai:DiVA.org:liu-144217DiVA, id: diva2:1172862
Public defence
2018-02-02, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilÅForsk (Ångpanneföreningen's Foundation for Research and Development)
Note

I den tryckta versionen saknades den populärvetenskapliga sammanfattningen på svenska. I den elektroniska versionen är den tillagd mellan Abstract (sida II) och Preface (sida III).

Available from: 2018-01-11 Created: 2018-01-11 Last updated: 2018-01-12Bibliographically approved
List of papers
1. Unravelling the Physical Mechanisms that Determine Microstructural Evolution of Ultrathin Volmer-Weber Films
Open this publication in new window or tab >>Unravelling the Physical Mechanisms that Determine Microstructural Evolution of Ultrathin Volmer-Weber Films
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 4, p. 044302-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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-103920 (URN)10.1063/1.4890522 (DOI)000340710700078 ()
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2018-01-11
2. Dynamic competition between island growth and coalescence in metal-on-insulator deposition
Open this publication in new window or tab >>Dynamic competition between island growth and coalescence in metal-on-insulator deposition
2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 16, p. 163107-1-163107-5Article in journal (Refereed) Published
Abstract [en]

The morphology of thin metal films and nanostructures synthesized from the vapor phase on insulating substrates is strongly influenced by the coalescence of islands. Here, we derive analytically the quantitative criterion for coalescence suppression by combining atomistic nucleation theory and a classical model of coalescence. Growth simulations show that using this criterion, a coalescence-free growth regime can be reached in which morphological evolution is solely determined by island nucleation, growth, and impingement. Experimental validation for the ability to control the rate of coalescence using this criterion and navigate between different growth regimes is provided by in situ monitoring of Ag deposition on SiO2. Our findings pave the way for creating thin films and nanostructures that exhibit a wide range of morphologies and physical attributes in a knowledge-based manner.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-112133 (URN)10.1063/1.4900575 (DOI)000344363000073 ()
Available from: 2014-11-17 Created: 2014-11-17 Last updated: 2018-01-11Bibliographically approved
3. Coalescence-controlled and coalescence-free growth regimes during deposition of pulsed metal vapor fluxes on insulating surfaces
Open this publication in new window or tab >>Coalescence-controlled and coalescence-free growth regimes during deposition of pulsed metal vapor fluxes on insulating surfaces
2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 13, article id 134304Article in journal (Refereed) Published
Abstract [en]

The morphology and physical properties of thin films deposited by vapor condensation on solid surfaces are predominantly set by the processes of island nucleation, growth, and coalescence. When deposition is performed using pulsed vapor fluxes, three distinct nucleation regimes are known to exist depending on the temporal profile of the flux. These regimes can be accessed by tuning deposition conditions; however, their effect on film microstructure becomes marginal when coalescence sets in and erases morphological features obtained during nucleation. By preventing coalescence from being completed, these nucleation regimes can be used to control microstructure evolution and thus access a larger palette of film morphological features. Recently, we derived the quantitative criterion to stop coalescence during continuous metal vapor flux deposition on insulating surfaceswhich typically yields 3-dimensional growthby describing analytically the competition between island growth by atomic incorporation and the coalescence rate of islands [Lu et al., Appl. Phys. Lett. 105, 163107 (2014)]. Here, we develop the analytical framework for entering a coalescence-free growth regime for metal vapor deposition on insulating substrates using pulsed vapor fluxes, showing that there exist three distinct criteria for suppressing coalescence that correspond to the three nucleation regimes of pulsed vapor flux deposition. The theoretical framework developed herein is substantiated by kinetic Monte Carlo growth simulations. Our findings highlight the possibility of using atomistic nucleation theory for pulsed vapor deposition to control morphology of thin films beyond the point of island density saturation. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-117792 (URN)10.1063/1.4916983 (DOI)000352645100033 ()
Note

Funding Agencies|Linkoping University via the "LiU Research Fellows" program; Swedish Research Council [VR 621-2011-5312]; AForsk through the project "Towards Next Generation Energy Saving Windows"

Available from: 2015-05-11 Created: 2015-05-08 Last updated: 2018-03-13
4. Scaling of elongation transition thickness during thin-film growth on weakly interacting substrates
Open this publication in new window or tab >>Scaling of elongation transition thickness during thin-film growth on weakly interacting substrates
2017 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 111, no 8, article id 084101Article in journal (Refereed) Published
Abstract [en]

The elongation transition thickness (hElong) is a central concept in the theoretical description of thin-film growth dynamics on weakly interacting substrates via scaling relations of hElong with respect to rates of key atomistic film-forming processes. To date, these scaling laws have only been confirmed quantitatively by simulations, while experimental proof has been left ambiguous as it has not been possible to measure hElong. Here, we present a method for determining experimentally hElong for Ag films growing on amorphous SiO2: an archetypical weakly interacting film/substrate system. Our results confirm the theoretically predicted hElong scaling behavior, which then allow us to calculate the rates of adatom diffusion and island coalescence completion, in good agreement with the literature. The methodology presented herein casts the foundation for studying growth dynamics and cataloging atomistic-process rates for a wide range of weakly interacting film/substrate systems. This may provide insights into directed growth of metal films with a well-controlled morphology and interfacial structure on 2D crystals-including graphene and MoS2-for catalytic and nanoelectronic applications. Published by AIP Publishing.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-140966 (URN)10.1063/1.4993252 (DOI)000408570000044 ()2-s2.0-85028308625 (Scopus ID)
Note

Funding Agencies|Linkoping University (LiU) [Dnr-LiU-2015-01510]; Swedish research council [VR-2011-5312, VR-2015-04630]; Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC)

Available from: 2017-09-19 Created: 2017-09-19 Last updated: 2018-01-11Bibliographically approved

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