Thin Films of TiO2 and Related Oxides by ALD/MLD:Tailoring of Transport Properties

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Janne-Petteri Niemelä Name of the doctoral dissertation Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties Publisher School of Chemical Technology Unit Department of Chemistry Series Aalto University publication series DOCTORAL DISSERTATIONS 136/2015 Field of research Inorganic Chemistry Manuscript submitted 10 September 2015 Date of the defence 8 January 2016 Permission to publish granted (date) 28 October 2015 Language English Monograph Article dissertation (summary + original articles) Abstract The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive postdeposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 oC showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the asdeposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phononboundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.The key for improving the performance of energy conversion devices is to develop new functional materials – a challenging task that often requires engineering of the material structures in nanoscale. This challenge can be met by employing atomic and molecular layer deposition (ALD and MLD) techniques that allow for fabrication of layered structures, doped and hybrid inorganic-organic materials in a conformal manner on nanostructured substrate surfaces. One interesting multifunctional material is TiO2 that has recently, in addition to its traditional use as a photocatalyst, arisen interest as a transparent conductor and a thermoelectric material for conversion of waste heat into electricity, particularly when doped with Nb. In this dissertation, first, an introductory background is presented regarding the properties of and requirements for the materials for energy applications relevant to this work. Second, a brief introduction to the experimental methods used and their application in this work is given. Third, the results of the experimental work communicated via the chemistry and physics journals of the research field are summarized. In the experimental part of this dissertation an ALD route employing TiCl4, Nb(OEt)5 and H2O as precursors for fabrication of Nb-doped TiO2 thin films was developed. Niobium was found to readily incorporate as pentavalent in anatase-structured films upon a reductive postdeposition annealing treatment such that the materials could be identified as degenerate semiconductors with metal-like transport properties. Initial crystallinity in the as-deposited films heavily affected the final transport properties of the films; in the initially amorphous films the intra-grain properties were found to govern the electron transport, while any crystallinity present prior to the annealing resulted in films where grain boundaries substantially suppressed electron mobility. The Ti0.93Nb0.07O2 films deposited at 160-175 oC showed particularly promising transparent conducting oxide properties. In particular, the ALD precursors TiCl4 and H2O for TiO2 were combined with the MLD precursor hydroquinone in order to fabricate inorganic-organic superlattices. First, in the asdeposited films the organic component was found to sensitize TiO2 to visible light – a fact that could potentially lead to applications in the fields of photocatalysis and solar cells. Second, post-deposition annealing enabled conversion of the as-deposited TiO2:HQ superlattice films into TiO2:C films, new type of inorganic-organic thin film structures where graphitic carbon layers were periodically confined between TiO2 layers. Remarkably, incoherent phononboundary scattering enabled ultra-low thermal conductivities in both TiO2:HQ and TiO2:C superlattices, interesting for thermal barrier and thermoelectric applications.