Piezo- and Ferroelectric A+B5+O3 Thin Films

Abstract

This thesis summarizes atomic layer deposition (ALD) of thin films of a set of alkali metal niobates and tantalates with technologically important physical properties. ALD of LiNbO3 is presented first, where we showcase the possibility of epitaxial integration of complex oxide thin films containing alkali metals. LiNbO3 is a high performance ferroelectric, and these properties are studied to show that ALD can be utilized to obtain highly oriented films with piezo- and ferroelectric activity. The thesis continues with investigating the feasibility of growing sodium- and potassium containing materials with ALD. Prior to the work leading to this thesis, no reports of Na/K-deposition by ALD have been available. Several precursors are screened for self-limiting growth under typical ALD-conditions, and two optimal precursors are used to carry out deposition of sodium- and potassium aluminate. These precursors, both alkali metal t-butoxides, offer the possibility to grow sodium- and potassium containing materials, with water as the co-reactant, at temperatures between 250 and 300 °C. Using the results from general deposition of Na/K-containing materials and the experience from LiNbO3- deposition, four alkali metal niobates and tantalates; NaNbO3, NaTaO3, KNbO3 and KTaO3, were deposited. These materials have interesting intrinsic properties, such as ferroelectricity (KNbO3) and photocatalytic activity (NaNbO3). More technologically interesting, however, are the solid solutions KxNa1-xNbO3 and KTaxNb1-xO3, which are desirable for their strong ferroelectric and electrooptical responses, respectively. Proof-of-concept on the intermixing of these solid solutions by ALD is presented, displaying remarkable compositional control and reproducibility. KxNa1-xNbO3 thin films are deposited for investigation of its electric properties, and piezoelectric activity is studied using piezoelectric force microscopy. The films are epitaxially integrated on a range of substrates, thereby controlling the orientation and direction of the polarity. Finally, some general notes on atomic layer deposition of alkali metal containing complex oxides are made, in light of what has been achieved in this work. Three main papers form the basis for this thesis. One on deposition of LiNbO3, a second on Na/Kdeposition and a third on alkali metal niobate/tantalate-deposition.