R. R. Das

Giant piezoelectricity on Si for hyperactive MEMS.

High-quality piezoelectric thin films are grown and exhibit superior properties for microelectromechanical systems. Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offer integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO3 template layer with superior piezoelectric coefficients (e31,f = –27 ± 3 coulombs per square meter) and figures of merit for piezoelectric energy-harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, microfluidic control, mechanical sensing, and energy harvesting.

Synthesis and ferroelectric properties of epitaxial BiFeO3 thin films grown by sputtering

We have grown epitaxial BiFeO3 thin films with smooth surfaces on (001), (101), and (111) SrTiO3 substrates using sputtering. Four-circle x-ray diffraction and cross-sectional transmission electron microscopy show that the BiFeO3 thin films have rhombohedral symmetry although small monoclinic distortions have not been ruled out. Stripe ferroelectric domains oriented perpendicular to the substrate miscut direction and free of impurity phase are observed in BiFeO3 on high miscut (4°) (001) SrTiO3, which attributes to a relatively high value of remanent polarization (∼71μC∕cm2). Films grown on low miscut (0.8°) SrTiO3 have a small amount of impure phase α-Fe2O3 which contributes to lower the polarization values (∼63μC∕cm2). The BiFeO3 films grown on (101) and (111) SrTiO3 exhibited remanent polarizations of 86 and 98μC∕cm2, respectively.

Polarization switching in epitaxial BiFeO3 films

Ferroelectric domain structure and polarization switching in an epitaxial BiFeO3 film grown on a 0.8° miscut SrTiO3(001) substrate were studied by piezoelectric force microscopy. The film shows a two-domain stripe pattern, with the polarization vectors oriented along two ⟨111⟩ axes which form an angle of 71°. Polarization switching was investigated by locally poling the film. By combining the perpendicular and in-plane piezoresponse data we found that polarization rotates by 71° and 109°, while 180° switching is mainly observed at low fields.

Ferroelectric domain structure in epitaxial BiFeO3 films

Piezoelectric force microscopy is employed to study the ferroelectric domain structure in a 600nm thick epitaxial BiFeO3 film. In the as-grown film, a mosaic-like domain structure is observed. Scans taken with the cantilever pointing along the principal crystallographic directions enabled us to reconstruct the polarization direction. By combining the perpendicular and in-plane piezoresponse data, we found that the ferroelectric domain structure is mainly described by four polarization directions. These directions point oppositely along two body diagonals, which form an angle of ∼71°. The other variants are also occasionally observed.

Ferroelectric domain structures of epitaxial (001) BiFeO3 thin films

Ferroelectric domain structures of epitaxial BiFeO3 thin films on miscut (001) SrTiO3 substrates have been studied by transmission electron microscopy. BiFeO3 on 0.8° miscut substrates are composed of both 109° and 71° domains; in contrast, only 71° stripe domains are observed in BiFeO3 on 4° miscut (001) SrTiO3 substrates. The domain width in BiFeO3 on 4° miscut substrates increases as film thickness increases due to a reduction in domain wall energy. The domain configurations of BiFeO3 thin films affect their ferroelectric switching behavior due to the pinning at the junctions between 109° and 71° domain walls.

Synthesis and ferroelectric properties of epitaxial BiFeO{sub 3} thin films grown by sputtering.

We have grown epitaxial BiFeO3 thin films with smooth surfaces on (001), (101), and (111) SrTiO3 substrates using sputtering. Four-circle x-ray diffraction and cross-sectional transmission electron microscopy show that the BiFeO3 thin films have rhombohedral symmetry although small monoclinic distortions have not been ruled out. Stripe ferroelectric domains oriented perpendicular to the substrate miscut direction and free of impurity phase are observed in BiFeO3 on high miscut (4°) (001) SrTiO3, which attributes to a relatively high value of remanent polarization (∼71μC∕cm2). Films grown on low miscut (0.8°) SrTiO3 have a small amount of impure phase α-Fe2O3 which contributes to lower the polarization values (∼63μC∕cm2). The BiFeO3 films grown on (101) and (111) SrTiO3 exhibited remanent polarizations of 86 and 98μC∕cm2, respectively.

Single domain strain relaxed PrScO3 template on miscut substrates

The authors have grown strain relaxed epitaxial template of a rare-earth scandate, PrScO3, on miscut (001) SrTiO3 and (001) (LaAlO3)0.3–(Sr2AlTaO3)0.7 substrates by pulsed laser deposition of PrScO3 buffer layers followed by postannealing and overlayer growth. X-ray diffraction exhibits that the PrScO3 is a single domain with bulk lattice parameters and the out-of-plane crystalline quality is comparable with SrTiO3 single crystals. Cross-sectional transmission electron microscopy micrographs show dislocation-free overlayers containing boundaries with very small in-plane misalignment. The growth of strain relaxed rare-earth scandate templates with controlled lattice parameters offers strain and domain engineering of epitaxial multifunctional oxide thin films.

Microstructure and strain relaxation of epitaxial PrScO3 thin films grown on (001) SrTiO3 substrates

We have studied the microstructure and strain relaxation of epitaxial PrScO3 films grown on miscut (001) SrTiO3 substrates by transmission electron microscopy. PrScO3 films grown on highly miscut (>1°) SrTiO3 substrates are single domain films, fully strain relaxed via interfacial misfit dislocations, small angle tilt boundaries, and antiphase boundaries bounded by partial dislocation. In contrast, strain in PrScO3 films on low miscut (<0.2°) SrTiO3 substrates is relaxed by misfit dislocation as well as the formation of six different crystallographic domains. The formation of single domain PrScO3 films on high angle miscut substrates could be due to interfacial strain-energy minimization.

Strain tunability of spontaneous polarization and enhanced ferroelectric properties in epitaxial (001) BiFeO3 thin films

We report the strain dependence of remanent polarization and coercive field of epitaxial (001)p BiFeO3 films. Our measurements reveal that the large spontanoues polarization of BiFeO3 is indeed intrinsic, the remanent polarization of (001)p BiFeO3 thin films has a strong strain dependence, even stronger than (001) PbTiO3 films, and the coercive field of BiFeO3 films is also tunable. In addition, the low coercive filed and the reduced leakage current in (001)p BiFeO3 membranes allows us to achieve a fatigue-free switching behavior to 1010 cycles. This experimental result strongly suggests that epitaxial (001)p BiFeO3 thin films are very promising materials for non-volatile memories and magnetoelectric devices.