S. K. Streiffer

Room-temperature ferroelectricity in strained SrTiO3

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (Tc) is traditionally accomplished by chemical substitution—as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (εr) 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 Tc in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc 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 Tc 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 εr at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

Alternative dielectrics to silicon dioxide for memory and logic devices.

The silicon-based microelectronics industry is rapidly approaching a point where device fabrication can no longer be simply scaled to progressively smaller sizes. Technological decisions must now be made that will substantially alter the directions along which silicon devices continue to develop. One such challenge is the need for higher permittivity dielectrics to replace silicon dioxide, the properties of which have hitherto been instrumental to the industrys success. Considerable efforts have already been made to develop replacement dielectrics for dynamic random-access memories. These developments serve to illustrate the magnitude of the now urgent problem of identifying alternatives to silicon dioxide for the gate dielectric in logic devices, such as the ubiquitous field-effect transistor.

Composition-control of magnetron-sputter-deposited (BaxSr1 - X)Ti1 + yO3 + z thin films for voltage tunable devices

Precise control of composition and microstructure is critical for the production of (BaxSr1−x)Ti1+yO3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high-frequency devices. Here, we present results on composition-microstructure-electrical property relationships for polycrystalline BST films produced by magnetron-sputter deposition, that are appropriate for microwave and millimeter-wave applications such as varactors and frequency triplers. Films with controlled compositions were grown from a stoichiometric Ba0.5Sr0.5TiO3 target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O2) process pressure, while the O2/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties ...

Ferroelectricity in thin films: The dielectric response of fiber-textured (BaxSr1−x)Ti1+yO3+z thin films grown by chemical vapor deposition

We have investigated the dielectric response of a series of {100} fiber-textured (BaxSr1−x)Ti1+yO3+z samples deposited by liquid-source metalorganic chemical vapor deposition onto Pt/SiO2/Si, as a function of the two most commonly varied microstructural parameters: film thickness and Ti nonstoichiometry y. We find that the overall behavior of these samples is adequately described by mean-field, Landau–Ginzburg–Devonshire theory as for bulk ferroelectrics. However, we quantify the impact of three separable factors for these films that greatly alter the dielectric susceptibility as a function of temperature, compared to that found for bulk ceramic samples at the same Ba/Sr ratio of 70/30: (i) Ti nonstoichiometry; (ii) the apparent interface effect; and (iii) the plane equibiaxial stress state resulting from thermal expansion mismatch strains. When these factors are properly taken into consideration, we show that these fine grained thin films behave in a manner entirely consistent with expectations based on ...

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.

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

Evaluation of intrinsic and extrinsic contributions to the piezoelectric properties of Pb(Zr1−XTX)O3 thin films as a function of composition

The piezoelectric, dielectric, and ferroelectric properties of highly (111)-textured, 200-nm-thick polycrystalline lead zirconate titanate (PZT) films have been investigated as a function of Zr/Ti ratio. The distinct peak in piezoelectric coefficient at the morphotropic phase boundary found in bulk PZT ceramics is not observed in thin film PZTs. Measurements of the temperature dependence of relative permittivity and the nonlinear behavior of relative permittivity and piezoelectric coefficient suggest that non-180° domain wall motion in these films is negligible, indicating that the extrinsic contribution to the room temperature permittivity is dominated by only 180° domain wall motion. The semiempirical phenomenological equation relating the piezoelectric coefficient to measured polarization and permittivity values is demonstrated to give an excellent description of the piezoelectric behavior in these films, assuming bulk electrostrictive and elastic coefficients. The small deviation between calculated an...

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.

Grazing-incidence small angle x-ray scattering studies of phase separation in hafnium silicate films

Grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution transmission electron microscopy (HRTEM) were used to investigate phase separation in hafnium silicate films after rapid thermal annealing between 700 and 1000 °C. 4-nm-thick Hf–silicate films with 80 and 40 mol % HfO2, respectively, were prepared by metalorganic vapor deposition. Films of the two compositions showed distinctly different phase-separated microstructures, consistent with two limiting cases of microstructural evolution: nucleation/growth and spinodal decomposition. Films with 40 mol % HfO2 phase separated in the amorphous by spinodal decomposition and exhibited a characteristic wavelength in the plane of the film. Decomposition with a wavelength of ∼3 nm could be detected at 800 °C. At 1000 °C the films rapidly demixed with a wavelength of 5 nm. In contrast, films with 80 mol % HfO2 phase separated by nucleation and growth of crystallites, and showed a more random microstructure. The factors determining specific film...

Relaxor ferroelectricity in strained epitaxial SrTiO3 thin films on DyScO3 substrates

The ferroelectric properties of 500A thick strained, epitaxial SrTiO3 films grown on DyScO3 substrates by reactive molecular-beam epitaxy are reported. Despite the near 1% biaxial tensile strain, the x-ray rocking curve full widths at half maximum in ω are as narrow as 7arcsec (0.002°). The films show a frequency-dependent permittivity maximum near 250K that is well fit by the Vogel-Fulcher equation. A clear polarization hysteresis is observed below the permittivity maximum, with an in-plane remanent polarization of 10μC∕cm2 at 77K. The high Tmax is consistent with the biaxial tensile strain state, while the superimposed relaxor behavior is likely due to defects.