"Epitaxial Lift-off" - a novel in-situ patterning approach for epitaxial perovskite micro/nano structures
In my research project we have developed a lift-off patterning strategy suitable for in-situ patterning of epitaxial single/multi layers of functional perovskite oxides. These materials are traditionally grown at higher temperatures and hence use of organic resists is not suitable. Instead we have utilized a sacrificial layer of high temperature stable oxide (AlOx) as lift-off resist mask. The fabrication strategy and its subsequent modifications enabled us to achieve in-situ patterned micro and nano structured devices incorporating heteroepitaxial perovskite multilayers with unaltered functional properties. Two specific examples are given below.
Direct Patterning of Functional Interfaces with 2DEG without using Physical Etching
The hetero interface between two wide band-gap perovskite insulators LaAlO3 (LAO) and SrTiO3 (STO) has been found to exhibit a plethora of exceptionally interesting properties like interface conduction with highly mobile 2DEG, magnetism, interface-superconductivity and even co-existence of both simultaneously. To develop these unique interfaces into useful technologies, high-quality devices have to be fabricated from thin films by reproducible structuration techniques. Reactive Ar-ion etching has been used extensively to produce controlled structures in various materials. However, the implementation of this technique for LAO-STO interfaces is hampered due to the formation of oxygen vacancies, which would result in a conducting surface layer at the STO substrate. Utilizing sacrificial mask mediated lift-off structuration we demonstrated direct patterning of high-quality structures incorporating the LAO-STO interface without performing any reactive ion beam etching. The patterned structures incorporating the LAO-STO interface exhibited high quality interfacial properties which have previously been measured only in thin film samples.
Figure : Epitaxial Lift-off patterning of LAO-STO interface and interfacial resistance as function of temperature.
For details see : Appl. Phys. Lett. 100, 041601 (2012)
In-situ structured Nano-Ferroelectrics : A new insight in ferroelectric size effect
Ferroelectrics are amongst most investigated perovskite oxides for their intrigued multifunctional properties, promising for several device applications. For their use in very high density non-volatile RAMs and NEMS devices, downscaling of material size is an key step. Moreover low-dimensional downscaling also promises the possibility of achieving better properties owing to reduction in number of grains (and grain-boundaries) and stabilization of new ferroelectric domains. We have utilized epitaxial lift-off technique for congruent structuration of ferroelectric PbZr0.52Ti0.48O3 (mPZT) together with epitaxial top and bottom electrodes of SrRuO3 without affecting ferroelectric and piezoelectric properties. The fabrication strategy was modified to be integrated with e-beam lithography and extended to nano-scale which fabricated PZT structures with dimensions down to ~100 nm. This high density nano-structures were probed with piezo-response force microscopy (PFM) when they were found to retain ferroelectric properties and were individually addressable without any cross-talk. Interestingly high resolution X-ray diffraction revealed presence of extended ferroelectric domains in such in-situ patterned structures grown in nano-confined environment which can put new insight into the ferroelectric size effect.
Figure: SEM and PFM images of PZT nano-structures patterned via modified epitaxial lift off strategy. In the PFM image, the nano-dot at center was switched to opposite polarization state by application of an external DC bias.
For details see : Appl. Phys. Lett. 102, 142909 (2013)