W. K. Simon

Rutgers, The State University of New Jersey, New Brunswick, NJ, United States

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Publications (16)21.42 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Morphotropic phase boundary composition KNaNbO<sub>3</sub>-LiTaO<sub>3</sub>-LiSbO<sub>3</sub> (KNN-LT-LS) thin films were grown by pulsed laser deposition on niobium doped conductive SrTiO<sub>3</sub> substrates. Using X-ray diffractometry, the phase evolution, stoichiometry, and growth orientation were monitored for various deposition conditions including temperature and ambient oxygen partial pressure. The results indicate that the scattering of the cations such as potassium, sodium, and lithium by the oxygen molecules is a very critical factor in the formation of the secondary phases in the films. We report thin films with a good crystallinity grown at 650degC and in ambient oxygen partial pressure. On 1 micron films, a longitudinal relative permittivity of 500, along with a loss tangent of 0.5% at 1 MHz has been obtained.
    Applications of ferroelectrics, 2006. isaf '06. 15th ieee international symposium on the; 01/2008
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    ABSTRACT: We investigate the nonlinear dielectric properties of 0.9 Pb ( Mg 1/3 , Nb 2/3 ) O 3 ∙0.1 Pb Ti O 3 (PMN-PT) and Ba [ Ti 0.85 , Sn 0.15 ] O 3 (BTS) paraelectrics experimentally and theoretically. We measure the nonlinear dielectric response in the parallel plate capacitor configuration, whereby we obtain the low frequency linear permittivity (ε 33 ) , and the higher order permittivities (ε 3333 ,ε 333333 ) at 298  K as ε 33 <sup> PMN - PT </sup>=2.1×10<sup>-7</sup> and ε 33 <sup> BTS </sup>=4.1×10<sup>-8</sup>  F / m , ε 3333 <sup> PMN - PT </sup>=-4.9×10<sup>-20</sup> and ε 3333 <sup> BTS </sup>=-7.3×10<sup>-21</sup>  F <sup>3</sup>  m / C <sup>2</sup> , and ε 333333 <sup> PMN - PT </sup>=7.6×10<sup>-33</sup> and ε 333333 <sup> BTS </sup>=9.85×10<sup>-34</sup>  F <sup>5</sup>  m <sup>3</sup>/ C <sup>4</sup> . By using a self-consistent thermodynamic theory in conjunction with the experimental data, we compute the E 3 dependence of electrostatic free energy ΔG , the field-induced polarization P 3 , and the t- hermodynamic tunability ∂<sup>2</sup>P 3 /∂E 3 <sup>2</sup> , and prove that electrostatic free energy has to be expanded at least up to the sixth order in the electric field to define the critical field |E 3 <sup>*</sup>| at which maximum tunability is attained. We also show that |E 3 <sup>*</sup>| is a function on |ε 3333 |/ε 333333 only. Consequently, we find |E 3 <sup>*</sup>| PMN - PT =8.0×10<sup>5</sup>  V / m and |E 3 <sup>*</sup>| BTS =8.6×10<sup>5</sup>  V / m . We compute the engineering tunabilities as Γ PMN - PT =65% and Γ BTS =55% , and then define a normalized tunability ξ to take into account the |E 3 <sup>*</sup>| parameter. Thereof, we determine |ξ| PMT - PT =8.1×10<sup>-5</sup>%/ V   m <sup>-1</sup> and |ξ| BTS =6.4×10<sup>-5</sup>%/ V   m <sup>-1</sup> . Our results reveal that |E 3 <sup>*</sup>| BTS ≫|E 3 <sup>*</sup>| PMN - PT although Γ BTS ≪Γ PMN - PT , unequivocally showing the need for defining a critical field parameter in evaluating the nonlinear dielectric response and tunability, in particular, and in nonlinear dielectrics in gene
    Journal of Applied Physics 02/2007; · 2.21 Impact Factor
  • E Koray Akdogan, William K Simon, Ahmad Safari
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    ABSTRACT: Epitaxial and (110) oriented paraelectric thin films of Ba0.60Sro.40TiO3 were grown on (100) oriented NdGaO3 orthorhombic substrates, and the nonlinear dielectric properties were studied at 10 GHz along selected in-plane crystallographic directions in the film thickness range of 25-1200 nm. The measured dielectric properties show strong residual strain and in-plane directional dependence. For instance, the in-plane relative permittivity is found to vary from as much as 500 to 150 along [110] and [001], respectively, in the 600 nm film. Tunability was found to vary from as much as 54% to 20% in all films and directions. In a given film, the best tunability is observed along the compressed axis in a mixed strain state, 54% along [110] in the 600 nm film. It is shown that, by nanoscale manipulation of epitaxy and planar anisotropy, the return loss and phase shift in a paraelectric can be tuned over a rather wide range. The approach presented herein opens avenues for obtaining various degrees of phase shift on the same film, enabling one with an additional degree of freedom in device design and fabrication as well as multifunctionality.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 01/2007; 53(12):2323-32. · 1.82 Impact Factor
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    ABSTRACT: Polycrystalline Ba <sub>0.6</sub> Sr <sub>0.4</sub> Ti O <sub>3</sub> (BST) films grown on r -plane sapphire exhibit strong variation of in-plane strain over the thickness range of 25–400 nm . At a critical thickness of ∼200 nm , the films are strain relieved; in thinner films, the strain is tensile, while compressive strain was observed in the 400 nm film. Microwave properties of the films were measured from 1 to 20 GHz by the interdigital capacitor method. A capacitance tunability of 64% was observed in the 200 nm film, while thinner films showed improved Q factor. These results demonstrate the possibility of incorporating frequency agile BST-based devices into the silicon on sapphire process.
    Applied Physics Letters 11/2006; · 3.79 Impact Factor
  • W. K. Simon, E. K. Akdogan, A. Safari
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    ABSTRACT: Strain relaxation in (Ba0.60Sr0.40)TiO3 (BST) thin films on 〈110〉 orthorhombic NdGaO3 substrates is investigated by x-ray diffractometry. Pole figure analysis indicates a [010]BST∥[10]NGO and [001]BST∥[001]NGO in-plane and [100]BST∥[100]NGO out-of-plane epitaxial relationship. The residual strains are relaxed at h ∼ 200 nm, and for h>600 nm, films are essentially strain free. Two independent dislocations mechanisms operate to relieve the anisotropic misfit strains along the principal directions. The critical thickness for misfit dislocation formation along [001] and [010] are 11 and 15 nm, respectively. Stress analysis indicates deviation from linear elasticity for h<200. The films with 10<h<25 nm are of monoclinic symmetry due to a finite principal shear stress along [110] of the initial orthorhombic cell.
    Applied Physics Letters 07/2006; 89(2):022902-022902-3. · 3.79 Impact Factor
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    ABSTRACT: The in-plane dielectric response of [110] oriented Ba0.60Sr0.40TiO3 epitaxial films grown on [100] NdGaO3 is used to determine the field induced polarization at 10 GHz. The nonlinear polarization curve is used to determine the linear and nonlinear permittivity terms for the in-plane principal directions, [001] and [10]. Studied films are in the thickness range of 75–1200 nm, and clearly show the influences that drive tunability down with increasing residual strain. The variation of the tunability, along the [001] direction, proves to be less sensitive to residual strain then the [10] direction, although [10] is capable of greater tunability at low residual strains.
    Applied Physics Letters 03/2006; 88(13):132902-132902-3. · 3.79 Impact Factor
  • E. K. Akdogan, W. K. Simon, A. Safari
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    ABSTRACT: Nonlinear dielectric response of (Ba0.60 Sr0.40)TiO3 epitaxial films on ≪100≫ and ≪110≫-oriented NdGaO3 substrates were investigated as a function of film thickness. The second, fourth and sixth order permittivities were determined at 10 GHz and at room temperature from which the so-called critical field for maximum tunability was computed using a thermodynamic formalism recently developed by the authors along the principal residual misfit strain directions. It is shown that the critical field is anisotropic in the plane of the film and its magnitudes are governed by the film thickness.
    01/2006;
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    ABSTRACT: The effects of isotropic and anisotropic epitaxy on the nonlinear dielectric response in (Ba0.6,Sr0.4TiO3) thin films were investigated experimentally. Under isotropic epitaxy, it is shown that the misfit is detrimental to the non-linear response of the films with decreasing film thickness. Nonlinear dielectric properties of films with in-plane compression exhibit more pronounced strain dependence, while the same does not set-in in film under tension down to 200 nm in thickness. In the case of anisotropic epitaxy, tunability exhibits directional dependence in the plane of the film and possesses orthorhombic symmetry in accordance to the Curie principle. The nonlinear dielectric response was found to be greater along the compressed directions.
    Ferroelectrics. 01/2006; 333(1):185-191.
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    ABSTRACT: This paper presents the microwave properties of barium strontium titanate (BST) thin films on r-plane sapphire substrates. A series of films with thickness 25-400 nm was prepared by pulsed laser deposition (PLD). Microwave properties of the films, including capacitance tunability and loss tangent, were extracted by patterning interdigitated capacitors (IDCs) on the film surface. The highest tunability of 64% was observed in the 200 nm film. These results demonstrate the possibility of integrating BST into the silicon on sapphire process.
    01/2006;
  • E.K. Akdogan, W.K. Simon, A. Safari
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    ABSTRACT: Nonlinear dielectric response of (Ba<sub>0.60</sub> Sr<sub>0.40</sub>)TiO<sub>3</sub> epitaxial films on <100> and <110>-oriented NdGaO<sub>3</sub> substrates were investigated as a function of film thickness. The second, fourth and sixth order permittivities were determined at 10 GHz and at room temperature from which the so-called critical field for maximum tunability was computed using a thermodynamic formalism recently developed by the authors along the principal residual misfit strain directions. It is shown that the critical field is anisotropic in the plane of the film and its magnitudes are governed by the film thickness.
    Applications of ferroelectrics, 2006. isaf '06. 15th ieee international symposium on the; 01/2006
  • W.K. Simon, E.K. Akdogan, A. Safari
    [show abstract] [hide abstract]
    ABSTRACT: Strain relaxation in (Ba<sub>0.60</sub> Sr<sub>0.40</sub>)TiO<sub>3</sub> epitaxial films on <110>-oriented NdGaO<sub>3</sub> substrates is investigated in the thickness range, h= 25-1200 nm. The BST films prepared by PLD show that residual strains mostly relax by h~200 nm, and for h>600 nm films are essentially strain free. Two independent dislocation mechanisms operate to relax anisotropic strains along the principal directions. The critical thickness for misfit dislocation formation along [001] and [010] were found to be 11 and 15 nm, respectively. Deviation from linear elasticity for h<200 was observed, and increased as thickness decreased. Films with h<25 nm are monoclinic due to a finite principal shear stress along [110] of the BST cell. The effects of misfit strain relaxation on the nonlinear dielectric response and tunability will be discussed as well. The in plane dielectric response demonstrates a directional dependence that increases with the magnitude of the residual strain.
    Applications of ferroelectrics, 2006. isaf '06. 15th ieee international symposium on the; 01/2006
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    ABSTRACT: In-plane dielectric properties of ⟨110⟩ oriented epitaxial (Ba0.60Sr0.40)TiO3 thin films in the thickness range from 25–1200 nm have been investigated under the influence of anisotropic epitaxial strains from ⟨100⟩ NdGaO3 substrates. The measured dielectric properties show strong residual strain and in-plane directional dependence. Below 150 nm film thickness, there appears to be a phase transition due to the anisotropic nature of the misfit strain relaxation. In-plane relative permittivity is found to vary from as much as 500–150 along [10] and [001] respectively, in 600 nm thick films, and from 75 to 500 overall. Tunability was found to vary from as much as 54% to 20% in all films and directions, and in a given film the best tunability is observed along the compressed axis in a mixed strain state, 54% along [10] in the 600 nm film for example.
    Applied Physics Letters 08/2005; 87(8):082906-082906-3. · 3.79 Impact Factor
  • W. K. Simon, E. K. Akdogan, A. Safari
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    ABSTRACT: We have studied the evolution of anisotropic epitaxial strains in ⟨110⟩-oriented (Ba0.60Sr0.40)TiO3 paraelectric (m3m) thin films grown on orthorhombic (mm2) ⟨100⟩-oriented NdGaO3 by high-resolution x-ray diffractometry. All the six independent components of the three-dimensional strain tensor were measured in films with 25–1200-nm thickness, from which the principal stresses and strains were obtained. Pole figure analysis indicated that the epitaxial relations are [001]m3m‖[001]mm2 and [10]m3m‖[010]mm2 in the plane of the film, and [110]m3m‖[100]mm2 along the growth direction. The dislocation system responsible for strain relief along [001] has been determined to be ∣b∣(001) = 3/4∣b∣. Strain relief along the [10] direction, on the other hand, has been determined to be due to a coupled mechanism given by = ∣b∣ and = /4∣b∣. Critical thicknesses, as determined from nonlinear regression using the Matthews–Blakeslee equation, for misfit dislocation formation along [001] and [10] direction were found to be 5 and 7 nm, respectively. The residual strain energy density was calculated as ∼ 2.9×106 J/m3 at 25 nm, which was found to relax an order of magnitude by 200 nm. At 200 nm, the linear dislocation density along [001] and [10] are ∼ 6.5×105 and ∼ 6×105 cm−1, respectively. For films thicker than 600 nm, additional strain relief occurred through surface undulations, indicating that this secondary strain-relief mechanism is a volume effect that sets in upon cooling from the growth temperature.
    Journal of Applied Physics 05/2005; 97(10). · 2.21 Impact Factor
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    ABSTRACT: MicroPen™ direct-write technique has been used to fabricate polycrystalline paraelectric BST 60/40 11-140 μm thick films on alumina substrates. The dielectric properties of the thick films were studied as a function of film thickness, microstructure, and the state of residual stresses. Sin<sup>2</sup>Ψ X-ray analyses indicated a compressive state of stress in the films which decreased from ∼40 MPa at 36 μm to ∼15 MPa at 140 μm. The cubic→tetragonal transition temperature was found to be -10°C. No thickness dependence of the transition temperature was observed. Field induced polarization curves also indicate no apparent nonlinear behavior up to 4 kV/mm.
    Applications of Ferroelectrics, 2004. ISAF-04. 2004 14th IEEE International Symposium on; 09/2004
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    ABSTRACT: This work investigates the effects of anisotropic epitaxial strains on the microwave frequency dielectric response of BST (60/40) thin films. Dielectric properties such as permittivity and tunability along the <-110>, <-111> and <001> crystallographic directions correlate well with the variation of elastic strain energy as a function of film thickness. We demonstrate unequivocally that the maximum permittivity and tunability is obtained along a crystallographic direction where the strain is a minimum.
    Applications of Ferroelectrics, 2004. ISAF-04. 2004 14th IEEE International Symposium on; 09/2004
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    ABSTRACT: Ba0.60Sr0.40TiO3 thin films were deposited on orthorhombic <100> oriented NdGaO3 substrates by pulsed-laser deposition. Film thickness ranged from 25 nm to 1200 nm. X-ray pole figures have shown consistent <110> textured films with good alignement to the substrate. X-ray strain analysis indicates up to 0.5% compressive strain along the (001) direction, and weaker tensile strain along (-110). Dislocation densities, as computed from strain data, were found to be in the range 5–6×105 cm-1 along both directions. The critical thickness for dislocation formation along (001) and (-110) were found to be 5 and 7 nm, respectively. Permittivity and tunability were investigated using interdigitated capacitors in the 45 MHz-20 GHz range. Dielectric properties and tunability in the <110> oriented films exhibited strong strain and directional properties. Tunability up to 54% was observed at moderate field levels (∼ 5 kV/mm).
    MRS Proceedings. 12/2003; 833.