Strain and Interface Effects in Polar Oxides

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Riku Oja Name of the doctoral dissertation Strain and Interface Effects in Polar Oxides Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 66/2013 Field of research Engineering Physics, Theoretical and Computational Physics Manuscript submitted 12 February 2013 Date of the defence 10 May 2013 Permission to publish granted (date) 26 March 2013 Language English Monograph Article dissertation (summary + original articles) Abstract Ferroelectric materials, i.e. materials with a permanent electric dipole, have numerous applications in electronics. Their electric polarization is very sensitive to external stimuli, which means they can be used as sensors. They have a very high electric permittivity, and they are therefore excellent insulators in capacitors. Because the direction of polarization can be switched with an electric field, ferroelectric materials can be used as memories.Ferroelectric materials, i.e. materials with a permanent electric dipole, have numerous applications in electronics. Their electric polarization is very sensitive to external stimuli, which means they can be used as sensors. They have a very high electric permittivity, and they are therefore excellent insulators in capacitors. Because the direction of polarization can be switched with an electric field, ferroelectric materials can be used as memories. In electronic applications, such materials are employed as very thin films. In these cases, the properties of the materials change. The substrate on which the materials are grown strains the film, changing its polarization. This is an effect thus far not used in applications. Further, combining polar materials results in interfaces which can have completely novel electronic properties. New physical phenomena can be found at the interfaces of well-known polar oxides. Indeed, modern theory of polarization in oxides and interfaces has only recently been established. Such interfaces could be employed in future electronics. Predicting the effects that strain and interfaces will have in polar oxides requires careful quantum mechanical calculations, because the new phenomena result from a delicate interplay between different electronic states when the ionic environment is slightly changed. Calculating the electronic properties of such structures is possible with density-functional theory (DFT), which can yield close to accurate results for quantum mechanical systems of many electrons. In this thesis, I first report DFT calculations on strain in well-known polar oxides. Collaborating with experimental researchers, I establish that it is possible to trigger ferroelectricity in a non-ferroelectric oxide by external strain. Further, I find out that the crystalline direction and symmetry of the substrate will have a crucial effect on the polarization of a ferroelectric oxide. In (111) crystalline direction, changing strain can switch the direction of one polarization component, or even prevent ferroelectric polarization. Second, I report on my DFT studies on interfaces between certain polar oxides. So-called charge-imbalanced interfaces are found to be metallic, i.e. they give rise to a highly localized two-dimensional (2D) electron or hole gas. The interface type affects the ionic displacements in the structure. Further, in collaboration with experimental researchers, I suggest that one type of interface will become metallic and ferromagnetic. This is a new type of ferromagnetism, emerging at a 2D metallic interface between materials which are insulating and non-magnetic.