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Deposition of Al-doped ZnO films by high power impulse magnetron sputtering
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, Faculty of Science & Engineering.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Transparent conducting oxides (TCOs) are an important class of materials with many applications such as low emissivity coatings, or transparent electrodes for photovoltaics and flat panel displays. Among the possible TCO materials, Al-doped ZnO (AZO) is studied due to its relatively low cost and abundance of the raw materials. Thin films of AZO are commonly produced using physical vapour deposition techniques such as magnetron sputtering. However, there is a problem with the homogeneity of the films using reactive direct current magnetron sputtering (DCMS). This homogeneity problem can be related to the bombardment of the growing film with negative oxygen ions, that can cause additional acceptor defects and the formation of insulating secondary phases. In this work AZO films are deposited by high power impulse magnetron sputtering (HiPIMS), a technique in which high instantaneous current densities are achieved by short pulses of low duty cycle.

In the first part of this thesis, the possibility to improve the homogeneity of the deposited AZO films by using HiPIMS is demonstrated. This improvement can be related to the high instantaneous sputtering rate during the HiPIMS pulses, so the process can take place in the metal mode. This allows for a lower oxygen ion bombardment of the growing film, which can help to avoid the formation of secondary phases. Another problem of AZO is the stability of the properties in humid environments. To assess this problem, the degradation of the electrical properties after an aging procedure was investigated for films deposited by both DCMS and by HiPIMS. A method was proposed, to restore the properties of the films, using a low temperature annealing under N2 atmosphere. The improvement of the electrical properties of the films could be related to a diffusion process, where water is diffusing out of the films. Then, the influence of the substrate temperature on the properties of AZO films deposited by HiPIMS was studied. The electrical, optical and structural properties were found to improve with increasing substrate temperature up to 600 C. This improvement can be mostly explained by the increase in crystalline quality and the annealing of defects. Finally, the deposition of AZO films on flexible PET substrates was investigated. The films are growing as a thick porous layer of preferentially c-axis oriented columns on top of a thin dense seed layer. The evolution of the sheet resistance of the films after bending the films with different radii was studied. There is an increase in the sheet resistance of the films with decreasing bending radius, that is less pronounced for thicker films.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. , 85 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1889
National Category
Condensed Matter Physics Materials Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-142925DOI: 10.3384/diss.diva-142925ISBN: 9789176854181 (print)OAI: oai:DiVA.org:liu-142925DiVA: diva2:1156146
Public defence
2017-12-12, Planck, Fysikhuset, Campus Valla, Linköping, 10:00 (English)
Opponent
Supervisors
Available from: 2017-11-10 Created: 2017-11-10 Last updated: 2017-12-08Bibliographically approved
List of papers
1. Room temperature deposition of homogeneous, highly transparent and conductive Al-doped ZnO films by reactive high power impulse magnetron sputtering
Open this publication in new window or tab >>Room temperature deposition of homogeneous, highly transparent and conductive Al-doped ZnO films by reactive high power impulse magnetron sputtering
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2016 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 157, 742-749 p.Article in journal (Refereed) Published
Abstract [en]

Aluminum doped zinc oxide (AZO) films have been deposited using reactive high power impulse magnetron sputtering (HiPIMS) and reactive direct current (DC) magnetron sputtering from an alloyed target without thermal assistance. These films have been compared in terms of their optical, electrical and structural properties. While both DC and HiPIMS deposited films show comparable transmittance, their electrical properties are significantly improved by the HiPIMS process. The HiPIMS deposited films show a low resistivity down to the order of 10(-4) Omega cm with a good homogeneity across the substrate, making them potential candidates for electrodes in solar cells. The density of electrons reached up to 11 x 10(20) cm(-3), making ionized impurities the main scattering defects. This improvement of the film properties can be related to the specific plasma/target interactions in a HiPIMS discharge. This allows the process to take place in the transition mode and to deposit highly conductive, transparent AZO films on large surfaces at low temperature. While the overall oxygen content is above that of stoichiometric ZnO, higher localization of oxygen is found at the interfaces between crystalline domains with substoichiometric composition. (C) 2016 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2016
Keyword
Transparent conducting oxide; AZO; Thin films; Electronic properties; HiPIMS
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-132201 (URN)10.1016/j.solmat.2016.07.020 (DOI)000384391700088 ()
Note

Funding Agencies|European Commission within the DocMASE project

Available from: 2016-11-01 Created: 2016-10-21 Last updated: 2017-11-29
2. Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air
Open this publication in new window or tab >>Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air
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2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 27, 14426-14433 p.Article in journal (Refereed) Published
Abstract [en]

The properties of Al-doped ZnO (AZO) films are known to degrade with exposure to humidity. Different AZO films deposited using reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) have been aged in ambient laboratory conditions and annealed at temperatures between 160 and 180 degrees C in a N-2 atmosphere. Their electrical and optical properties, which have been investigated both ex situ and in situ during the annealing, are improved. The results of the in situ measurements are interpreted in terms of a diffusion process, where hydroxyl groups are decomposed and water is diffusing out of the films. As hydroxyl groups are known to act as a trap for charge carriers in ZnO, their removal from the film can explain the improvement of the electrical properties by the annealing.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2017
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-139560 (URN)10.1021/acs.jpcc.7b03020 (DOI)000405761600006 ()
Note

Funding Agencies|European Commission

Available from: 2017-08-08 Created: 2017-08-08 Last updated: 2017-11-10

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Mickan, Martin

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