Final Report Summary - NOVOX (Perfectly interfaced nanomaterials for next generation oxide electronics)
The aim of the project was to learn more about oxide thin films interfaces for potential future electronics. Over the last 30 years, the discovery of the many exotic and amazing properties in a wide range of transition metal oxides e.g. magnetics, ferroelectrics, multiferroics, semiconductors, transparent conductors, catalysts, ionic conductors, and the recent emergent effects at interfaces, and topological states at surfaces, has been nothing short of remarkable. Much work is motivated by the search for a replacement for silicon as we near the end of Moore’s law. But as very few of these applications have come about from oxide thin films. Epitaxial thin films of high perfection and controlled interfaces are needed for electronic applications. There has been insufficient materials science and engineering targeted at this area. A clear understanding of what limits properties needed, as thin films and interfaces have properties which are far inferior to high temperature-grown single crystals. In the ERC project, we devised new structures to understand oxide interfaces much better and to improve their properties. We probed these interfaces using a range of x-ray, neutron and electrical measurements.
The new thin film structure we developed (vertical epitaxial nanocomposites) enabled much new understanding about interfaces (and near-interfaces). The vertical interfaces in the structures form by self assembly, orders of magnitude more slowly (for the same overall film growth rate) than standard planar film/substrate interfaces in standard films. As a consequence, kinetics are sufficient for a high density of perfectly-spaced interface misfit dislocations to form, which then leads to very high crystalline perfection, much greater perfection than in standard thin films and heterostructures of equivalent dimension. Hence, we were able to remove/reduce the interface ‘dead layer’ which is common to nearly all oxide thin film systems and forms when the strain cannot be relaxed by misfit dislocations. Normally, many other disordering effects occur. From a practical point of view, we showed that the perfect interfaces allowed radical property improvements to be induced in ferroelectrics, multiferroics, magnetics and ionics. A further boost to properties was made by inducing strain using thermal expansion effects in composite films. This is much more ‘clean’ than inducing strain using interface heteroepitaxy because it eliminates the defects which are induced through interface straining.
The above information for composite systems was then translated to standard planar heterostructures. To make them much more perfect and to relieve strain, they were simply grown much more slowly. This enabled strongly enhanced properties (e.g. above room temperature ferroelectricity and ferrimagnetism in the Bi,(Fe,Mn)O3) system) to be achieved, as never before.
The new thin film structure we developed (vertical epitaxial nanocomposites) enabled much new understanding about interfaces (and near-interfaces). The vertical interfaces in the structures form by self assembly, orders of magnitude more slowly (for the same overall film growth rate) than standard planar film/substrate interfaces in standard films. As a consequence, kinetics are sufficient for a high density of perfectly-spaced interface misfit dislocations to form, which then leads to very high crystalline perfection, much greater perfection than in standard thin films and heterostructures of equivalent dimension. Hence, we were able to remove/reduce the interface ‘dead layer’ which is common to nearly all oxide thin film systems and forms when the strain cannot be relaxed by misfit dislocations. Normally, many other disordering effects occur. From a practical point of view, we showed that the perfect interfaces allowed radical property improvements to be induced in ferroelectrics, multiferroics, magnetics and ionics. A further boost to properties was made by inducing strain using thermal expansion effects in composite films. This is much more ‘clean’ than inducing strain using interface heteroepitaxy because it eliminates the defects which are induced through interface straining.
The above information for composite systems was then translated to standard planar heterostructures. To make them much more perfect and to relieve strain, they were simply grown much more slowly. This enabled strongly enhanced properties (e.g. above room temperature ferroelectricity and ferrimagnetism in the Bi,(Fe,Mn)O3) system) to be achieved, as never before.