Biaxial compressive strain has been used to markedly enhance the ferroelectric properties of BaTiO3 thin films. This strain, imposed by coherent epitaxy, can result in a ferroelectric transition temperature nearly 500 degrees C higher and a remanent polarization at least 250% higher than bulk BaTiO3 single crystals. This work demonstrates a route to a lead-free ferroelectric for nonvolatile memories and electro-optic devices.

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(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO(3) template layer with superior piezoelectric coefficients (e(31,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.

We have grown epitaxial BiFeO 3 thin films with smooth surfaces on ͑001͒, ͑101͒, and ͑111͒ SrTiO 3 substrates using sputtering. Four-circle x-ray diffraction and cross-sectional transmission electron microscopy show that the BiFeO 3 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 BiFeO 3 on high miscut ͑4°͒ ͑001͒ SrTiO 3 , which attributes to a relatively high value of remanent polarization ͑ϳ71 C/cm 2 ͒. Films grown on low miscut ͑0.8°͒ SrTiO 3 have a small amount of impure phase ␣-Fe 2 O 3 which contributes to lower the polarization values ͑ϳ63 C/cm 2 ͒. The BiFeO 3 films grown on ͑101͒ and ͑111͒ SrTiO 3 exhibited remanent polarizations of 86 and 98 C/cm 2 , respectively.

Direct measurement of the remanent polarization of high quality (001)-oriented epitaxial BiFeO3 thin films shows a strong strain dependence, even larger than conventional (001)-oriented PbTiO3 films. Thermodynamic analysis reveals that a strain-induced polarization rotation mechanism is responsible for the large change in the out-of-plane polarization of (001) BiFeO3 with biaxial strain while the spontaneous polarization itself remains almost constant.

Ferroelectric domain structures of epitaxial BiFeO 3 thin films on miscut ͑001͒ SrTiO 3 substrates have been studied by transmission electron microscopy. BiFeO 3 on 0.8°miscut substrates are composed of both 109°and 71°domains; in contrast, only 71°stripe domains are observed in BiFeO 3 on 4°miscut ͑001͒ SrTiO 3 substrates. The domain width in BiFeO 3 on 4°miscut substrates increases as film thickness increases due to a reduction in domain wall energy. The domain configurations of BiFeO 3 thin films affect their ferroelectric switching behavior due to the pinning at the junctions between 109°and 71°domain walls.

Bi Fe O 3 thin films have been deposited on (111) SrTiO3 single crystal substrates by reactive molecular-beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth overpressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry. Four-circle x-ray diffraction reveals phase-pure, untwinned, epitaxial, (0001)-oriented films with rocking curve full width at half maximum values as narrow as 25arcsec (0.007°). Second harmonic generation polar plots combined with diffraction establish the crystallographic point group of these untwinned epitaxial films to be 3m at room temperature.

Antimony-doped p-type ZnO films epitaxially grown on (0001) sapphire substrates were fabricated by pulsed laser deposition at 400–600°C in 5.0×10−2Torr oxygen without postdeposition annealing. The films grown at 600°C have among the highest reported hole concentration of 1.9×1017cm−3 for antimony doping, Hall mobility of 7.7cm2∕Vs, and resistivity of 4.2Ωcm. Transmission electron microscopy reveals that the p-type conductivity closely correlates to the high density of defects which facilitate the formation of acceptor complexes and the compensation of native shallow donors. The thermal activation energy of the acceptor was found to be 115±5meV and the corresponding optical ionization energy is ∼158±7meV.

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