Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.

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Articles you may be interested inStructural and electrical properties of c-axis epitaxial homologous Sr m−3 Bi 4 Ti m O 3m+3 (m=3, 4, 5, and 6) thin films J. Appl. Phys. 94, 544 (2003); 10.1063/1.1579864 Microstructure and growth mode at early growth stage of laser-ablated epitaxial Pb(Zr 0.52 Ti 0.48 ) O 3 films on a SrTiO 3 substrate J. Appl. Phys. 89, 4497 (2001); 10.1063/1.1356426Effects of precursors and substrate materials on microstructure, dielectric properties, and step coverage of (Ba, Sr)TiO 3 films grown by metalorganic chemical vapor depositionWe have investigated the optoelectronic characteristics of bulk single-crystal SrTiO 3 (STO) and epitaxial STO on Si by photoluminescence and cathodoluminescence (CL) techniques. In particular, we have explored to what extent these techniques can offer information about crystal quality. We have complemented these observations with atomic force microscopy, transmission electron microscopy (TEM), and micro-Raman measurements. Panchromatic CL imaging of bulk STO revealed contrast features associated with growth-related striations, extended defects, and mechanical damage. CL imaging of undoped high-resistivity substrates was limited by beam charging effects. The weak nature of the CL signal from epitaxial STO (relative to bulk material) made it very difficult to visualize any features by analog detection. On the other hand, spectrally resolved CL measurements of epitaxial STO using single-photon counting techniques, revealed sensitivity to the defect content and film quality across a 3-in wafer. Preliminary results indicate a qualitative correlation in the room-temperature near band-edge luminescence properties ͑3.2-3.5 eV͒ and crystalline quality as determined by micro-Raman spectroscopy and TEM.