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.

We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature ( T c ) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO 3 layers in BaTiO 3 /SrTiO 3 superlattices are not only ferroelectric (with T c as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO 3 layers adjacent to them. T c was tuned by ∼500 kelvin by varying the thicknesses of the BaTiO 3 and SrTiO 3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity.

Room-Temperature Ferroelectricity in Strained SrTiO 3 . -Single crystals of DyScO3 are prepared and used as a new substrate material for the growth of SrTiO3 films under uniform biaxial tensile strain. The epitaxial strain increases the ferroelectric transition temperature TC by hundreds of degrees and produces room temperature ferroelectricity in SrTiO3, a material that is not normally ferroelectric at any temperature. The high room temperature dielectric constant ε r in these films (≈7000 at 10 GHz) and its sharp dependence on electric field are promising for device applications. -(HAENI, J. H.; et al.; Nature (London, UK) 430 (2004) 7001, 758-761; Dep. Mater.

A far ultraviolet ͑UV͒ spectroscopic ellipsometer system working up to 9 eV has been developed, and applied to characterize high-K-dielectric materials. These materials have been gaining greater attention as possible substitutes for SiO 2 as gate dielectrics in aggressively scaled silicon devices. The optical properties of four representative high-K bulk crystalline dielectrics, LaAlO 3 , Y 2 O 3-stabilized HfO 2 (Y 2 O 3) 0.15-(HfO 2) 0.85 , GdScO 3 , and SmScO 3 , were investigated with far UV spectroscopic ellipsometry and visible-near UV optical transmission measurements. Optical dielectric functions and optical band gap energies for these materials are obtained from these studies. The spectroscopic data have been interpreted in terms of a universal electronic structure energy scheme developed form ab initio quantum chemical calculations. The spectroscopic data and results provide information that is needed to select viable alternative dielectric candidate materials with adequate band gaps, and conduction and valence band offset energies for this application, and additionally to provide an optical metrology for gate dielectric films on silicon substrates.

As a first step in the identification of suitable alternative gate dielectrics for metal oxide semiconductor field-effect transistors (MOSFETs), we have used tabulated thermodynamic data to comprehensively assess the thermodynamic stability of binary oxides and nitrides in contact with silicon at temperatures from 300 K to 1600 K. Sufficient data exist to conclude that the vast majority of binary oxides and nitrides are thermodynamically unstable in contact with silicon. The dielectrics that remain are candidate materials for alternative gate dielectrics. Of these remaining candidates, the oxides have a significantly higher dielectric constant (ĸ) than the nitrides. We then extend this thermodynamic approach to multicomponent oxides comprising the candidate binary oxides. The result is a relatively small number of silicon-compatible gate dielectric materials with ĸ values substantially greater than that of SiO2 and optical bandgaps ≥ eV.