Nirupam Banerjee

Epitaxial Perovskite Oxide Devices Fabricated by Lift-off Technology

Promotion date: September 17.

Promotor: Guus Rijnders

Assistant Promotor: Gertjan Koster

Perovskite oxides are able to display diverse functional properties while retaining similar crystal structures. In the past decades intense research efforts were focused on achieving novel/enhanced functionalities in perovskite films and interfaces, requiring also the development of new patterning technologies for structuring high quality hetero-interfaces and epitaxial multilayers. This thesis demonstrates the development of a novel lithographic technology, termed as "epitaxial lift-off patterning strategy", for in situ patterning of high temperature grown epitaxial complex perovskite oxide multilayers and detailed investigations of various lift-off fabricated functional devices.

Employing the epitaxial lift-off strategy, precise patterns of the conducting heterointerface between LaAlO3 and SrTiO3 (001) were successfully fabricated without using any physical ion etching step and preserving the high quality interfacial transport properties. It was also demonstrated that the lift-off pathway can precisely pattern even more complex interface systems such as SrCuO2-LAO-STO(001), containing high mobility carriers. In addition, the patterning process was modified and integrated with e-beam lithographic process, enabling fabrication of nano-scale devices to show high mobility interface conductivity and interfacial magnetism.

To demonstrate the versatility, the thesis includes detailed discussion on the fabrication of all-oxide freestanding piezo-MEMS devices which are capable of functioning as very high sensitivity mass sensors. The fabrication is enabled by the lift-off pathway, facilitating a single step lift-off patterning of heteroepitaxial oxide multilayers. These freestanding epitaxial devices were studied in detail to characterize different intrinsic (material mediated) and extrinsic mechanisms causing vibrational damping, which were optimized to achieve a high quality factor of electromechanical resonance leading to enhanced mass sensitivity. The lift-off patterning strategy is a potential pathway to accomplish micro and nano-devices incorporating delicate perovskite heterointerfaces and/or functional epitaxial perovskite multilayers.

Your approach is really novel, isn’t it?

The lift-off mask technique is a new way to pattern oxide multilayers and fabricate devices, exploiting the advantages and special properties of these oxides which are preserved well during my process. It is possible to create multilayers in one go, avoiding a built-up of errors which occurs by lithography methods using a multitude of steps. While patterning with my technique, high temperatures are feasible, as inorganic resists are being used.

I came across this fabrication idea reading literature after which I found enthusiastic support from my supervisor, professor Guus Rijnders, who immediately responded the solution found was a very nice and unique one. From there on I was able to show its feasibility, starting from one layer unto multilayers and eventually building nice structures and even devices. Among these were: two insulating layers with a conductive interface, ferroelectric complicate patterned multilayers and nanoscale memory elements. The nanostructures took a lot of effort to fabricate but the final devices were individually addressable and switchable, promising very high density memory applications for the future. The substrate was shown to remain recoverable by my technique - as protected by the mask - making the substrate free for next fabricating steps.

In future, several applications derived from my novel approach might develop. Also, fundamental properties, such as high mobility and magnetoresistance phenomena can be studied from the physics point of view by realizing novel devices.

Employing the lift-off technique, unique features were obtained while fabricating ultra thin cantilevers like unusual shape of the anchor, leading to new properties and enhanced sensitivity as a microbalance. Using frequency and weight features of these new cantilevers, bacteria’s and proteins might even be measurable in a new way because of the geometrical advances and possibilities to ‘catch’ them inside blood samples using gold and chemical binding linkers. This dream has been justified as I have showed the proof of principle within the PhD project.

Can you recall some special moments during your PhD period?

Surely, finding out that the material properties of the complicated oxide systems were not deteriorated using my fabrication method, was a great moment. From that moment on I was sure to continue the work and that it could lead to beautiful results.

In what way did you develop personally, as a researcher and scientist?

I enjoyed the freedom and nice working atmosphere within Mesa+. Together with the excellent guidance one is able to develop toward an independent scientific researcher. All the right expertise is present at Mesa+ concerning oxide materials. The colleagues, from my group as well as from other groups, are approachable and very willing to share their expertise. Because of the valuable collaborations, I was happy to contribute to my colleagues’ work equally. This occurred time and time again at the weekly meetings of the Inorganic Materials Science (IMS) Group.

In what magazines were you able to publish your findings?

Two article appeared in Applied Physics Letters, and one in APL Materials which is an open access journal.

What are your future plans?

I will be working as a post-doc at the IMS Group for a period of a half to one full year from now, when some new topics will be investigated. After that I would like to apply for a job in academics or in industry. I am not sure about that at the moment. However, I hope to be able to stay in the Netherlands for some extra time.

What features of the Mesa+ institution are worthwhile for the future?

Coming to the Netherlands Mesa+ was very helpful for feeling at home right from the start. Also in using unknown equipment I found out I was not alone, meeting helpful technicians such as from the ICE and TST groups.

Visiting the Mesa+ Days was very useful to provide insights on the ongoing research within and to meet and interact with colleagues. The NanoLab is very important for structuring the kind of materials I have worked on. Without the facilities at hand, I would have never succeeded.

In my view, it might be a good idea to form new clusters in which people who work on similar topics or who use similar experimental techniques can meet and discuss to share their knowledge and experience. It can speed up the scientific progress especially by solving experimental issues and challenges.