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Scandium Nitride Thin Films for Thermoelectrics
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermoelectric devices are one of the promising energy harvesting technologies, since they can convert heat (i.e. a temperature gradient) to electricity. This result leads us to use them to harvest waste heat from heat engines or in power plants to generate usable electricity. Moreover, thermoelectric devices can also perform cooling. The conversion process is clean, with no emission of greenhouse gases during the process. However, the converting efficiency of thermoelectrics is very low because of the materials limitations of the thermoelectric figure of merit (ZTm). Thus, there is high demand to maximize the ZTm.

I have discovered that ScN has high power factor 2.5 mW/(mK2) at 800 K, due to low metalliclike electrical resistivity (∼3.0 μΩm) with retained relatively large Seebeck coefficient of -86 μV/K. The ScN thin films were grown by reactive dc magnetron sputtering from Sc targets. For ScN, X-ray diffraction, supported by transmission electron microscopy, show that we can obtain epitaxial ScN(111) on Al2O3(0001). We also reported effects on thermoelectric properties of ScN with small changes in the composition with the power factor changing one order of magnitude depending on e.g. oxygen, carbon and fluorine content which were determined by elastic recoil detection analysis. The presence of impurities may influence the electronic density of states or Fermi level (EF) which could yield enhancement of power factor.

Therefore, the effects of defects and impurities on the electronic density of states of scandium nitride were investigated using first-principles calculations with general gradient approximation and hybrid functionals for the exchange correlation energy. Our results show that for Sc and N vacancies can introduce asymmetric peaks in the density of states close to the Fermi level. We also find that the N vacancy states are sensitive to total electron concentration of the system due to their possibility for spin polarization. Substitutional point defects shift the Fermi level in the electronic band according to their valence but do not introduce sharp features. The energetics and electronic structure of defect pairs are also studied. By using hybrid functionals, a correct description of the open band gap of scandium nitride is obtained, in contrast to regular general gradient approximation. Our results envisage ways for improving the thermoelectric figure of merit of ScN by electronic structure engineering through stoichiometry tuning and doping.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 72 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1559
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-85917Local ID: LIU-TEK-LIC-2012:44ISBN: 978-91-7519-733-3 (print)OAI: oai:DiVA.org:liu-85917DiVA: diva2:573765
Presentation
2012-12-11, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Anomalously high thermoelectric power factor in epitaxial ScN thin films
Open this publication in new window or tab >>Anomalously high thermoelectric power factor in epitaxial ScN thin films
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 23, 232113- p.Article in journal (Refereed) Published
Abstract [en]

Thermoelectric properties of ScN thin films grown by reactive magnetron sputtering on Al2O3(0001) wafers are reported. X-ray diffraction and elastic recoil detection analyses show that the composition of the films is close to stoichiometry with trace amounts (similar to 1 at. % in total) of C, O, and F. We found that the ScN thin-film exhibits a rather low electrical resistivity of similar to 2.94 mu Omega m, while its Seebeck coefficient is approximately similar to-86 mu V/K at 800 K, yielding a power factor of similar to 2.5 x 10(-3) W/mK(2). This value is anomalously high for common transition-metal nitrides.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-75290 (URN)10.1063/1.3665945 (DOI)000298006100041 ()
Note
Funding Agencies|Swedish Research Council (VR)|621-2009-5258|Available from: 2012-02-27 Created: 2012-02-24 Last updated: 2017-12-07
2. Effect of point defects on the electronic density of states of ScN studied by first-principles calculations and implications for thermoelectric properties
Open this publication in new window or tab >>Effect of point defects on the electronic density of states of ScN studied by first-principles calculations and implications for thermoelectric properties
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 19Article in journal (Refereed) Published
Abstract [en]

We have investigated the effect of defects and impurities on the electronic density of states of scandium nitride using first-principles calculations with the generalized gradient approximation and hybrid functionals for the exchange correlation energy. Our results show that Sc and N vacancies can introduce asymmetric peaks in the density of states close to the Fermi level. We also find that the N vacancy states are sensitive to total electron concentration of the system due to their possibility for spin polarization. Substitutional point defects shift the Fermi level in the electronic band according to their valence but do not introduce sharp features. The energetics and electronic structure of defect pairs are also studied. By using hybrid functional calculations, a correct description of the band gap of scandium nitride is obtained. Our results envisage ways for improving the thermoelectric figure of merit of ScN by electronic structure engineering through stoichiometry tuning and doping.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87213 (URN)10.1103/PhysRevB.86.195140 (DOI)000311694200001 ()
Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06

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