‘Epitaxial Oxides on Silicon by Pulsed Laser Deposition’
In this thesis the integration of perovskite oxides on silicon is central. ‘These oxides offer a wide range of physical properties, making them interesting for use in new types of microelectronic devices,’ David Dubbink explains. ‘However, the integration should be epitaxial because the functional properties depend on its crystalline quality.’
Epitaxial growth on silicon is not straightforward. A buffer layer should be incorporated between silicon and the perovskites. ‘In this thesis various strategies to do so, were investigated,’ says David. ‘Our approach is based on pulsed laser deposition (PLD). The Inorganic Materials Science Group is widely known, as having available the most actual and advanced PLD equipment possible.’
In this thesis project, first the growth of the buffer layer yttria-stabilized zirconia (YSZ) was studied. ‘Here we learned to control all growth parameters in order to obtain high quality reproducible buffer layer growth,’ David says. ‘With our top-down growth strategy, we hope to bring functional materials towards industrial applications more quickly, to be ultimately reproducible onto 8-inch wafers. PLD is a very suitable industrial technique for that.’
Also studied was the growth of SrRuO3, as part of a double buffer layer. ‘We learned a lot on growing such extra layers, to come to smooth and well-defined layer films,’ David says.
As a key moment in his PhD project, David Dubbink mentions the alternative method he studied to obtain SrTiO3 (STO) on silicon.
‘In fact this novel approach – in which the necessity for ultra-high vacuum conditions is avoided – has come to light as a nice coincidence. In studying SrO assisted silicon deoxidation, crystallization of a silicate occurred in one of the process steps. This silicate allowed epitaxial crystallization of the perovskite SrTiO3.’
Equipment and experiments
In the PhD thesis David concludes that by controlling temperature and process time, epitaxial crystallization on the silicon is feasible. ‘Optimization of the silicate template formation and perovskite crystallization process, may further improve the morphology and crystallinity of the STO film,’ he writes as a recommendation.
The approach is a nice step forward, though certain aspects need further research before being of practical importance, thus David. ‘Worldwide the IMS/Mesa+ research is renowned for its PLD equipment and knowledge about its use. We are able to analyze all process steps in time, and improve from there. Using this knowledge, we also study the growth processes in industrial scale systems.’
David believes, in the years to come, exciting developments lay ahead regarding perovskite oxides for the microelectronic industry. ‘As for myself, I am slightly changing my topic of research during the post-doc year, in which I will continue to work at the IMS Group,’ he says. ‘Here again PLD process analyzes will be central, but not on silicon substrates.’
Systematically planning a career is not yet on David’s mind. ‘I believe in industry as well as in academics I can contribute in R&D projects,’ he says. ‘During the PhD project I have learnt to perform experiments in a much more structured and efficient way. As a character trait, I tend to go to the laboratory sometimes too quickly. I now resist this temptation, and first think carefully on the experimentation strategy to follow. It is important to collect the data that really matter, and to interpret them in a scientifically appropriate way. Then progress will be much faster and to the point.’