Antonio B. Catalan

Slater model applied to polarization graded ferroelectrics

Ferroelectric thin films of BaxSr1−xTiO3 with compositional gradients normal to the growth surface have been formed by the successive deposition and annealing of films having step-variable Ba to Sr ratios. By suitably tailoring the magnitude and sense of the gradient in Ba to Sr ratio, directional potentials can be built into the structures yielding a new, but controllable, hysteresis phenomenon. Slater’s empirical model for ferroelectric materials has been extended to also describe thin films with polarization gradients normal to the growth surface, i.e., graded ferroelectric devices. This model accounts for several aspects of these structures, including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset.

Giant effective pyroelectric coefficients from graded ferroelectric devices

Effective pyroelectric coefficients as large as 5 μC/cm2 °C, with peak responsivities at approximately 50 °C, were obtained from compositionally graded barium strontium titanate ferroelectric thin film devices formed on silicon using unbalanced magnetron sputter deposition. These effective pyroelectric coefficients are nearly two orders of magnitude larger than those observed from conventional pyroelectric thin film ferroelectric detectors.

TEMPERATURE DEPENDENCE OF CONVENTIONAL AND EFFECTIVE PYROELECTRIC COEFFICIENTS FOR COMPOSITIONALLY GRADED BAXSR1-XTIO3 FILMS

Ferroelectric thin films (∼1.2 μm) of BaxSr1−xTiO3 with gradients in composition normal to the growth surface were formed on platinum substrates by metalorganic decomposition. Effective (pseudo) pyroelectric coefficients as large as 0.06 μC/cm2 K have been obtained from these active ferroelectric devices under the application of an ac field (charge pumping). In contrast, a value of only −0.003 μC/cm2 K has been measured for the conventional pyroelectric coefficient.

Ferroelectric thin films with polarization gradients normal to the growth surface

Thin film ferroelectrics with polarization gradients normal to the growth surface readily form when gradients in temperature, strain, or composition are coupled to the polarization vector in ferroelectric materials. This letter describes the formation of thin films of potassium tantalum niobate with graded polarizations obtained by grading the tantalum to niobium ratio of the ferroelectric phase. Unlike a simple structure consisting of laminated layers of ferroelectric material, the polarization gradient which forms breaks the natural symmetry of the ferroelectric material at any given plane, resulting in a self‐poling of the structure subsequent to excitation by an oscillatory electric field. Once poled, the devices reveal a measurable potential across the structure which varies with temperature.

Selective laser pyrolysis of metallo-organics as a method of forming patterned thin film superconductors

Fine line superconductors of yttrium-barium-copper were formed on strontium titanate substrates by the completely non-vacuum techniques of metallo-organic deposition in selective laser pyrolysis. Lines 125 micrometers wide were written in a film of metal neodecanoates, using an argon laser, prior to complete pyrolysis. An organic dye which absorbs selective laser light wavelengths, is added to the metallo-organic solution comprising the metal neodecanoates prior to laser exposure, so as to enhance absorption of the laser light at the regions of the metallo-organic film which are subsequently exposed to the laser light. The increased light absorbance at the exposed regions, results in at least partial pyrolysis of the exposed metal neodecanoates. Regions of the metallo-organic film not exposed to laser pyrolysis is developed away using a xylene wash. Subsequent complete pyrolysis of the metal neodecanoates and rapid thermal annealing produced lines having superconducting onsets above 90 K and zero resistance at 40 K.

Preparation of barium strontium titanate thin film capacitors on silicon by metallorganic decomposition

Barium strontium titanate films were deposited on 100‐mm‐diam silicon wafers by metallorganic decomposition. Highly uniform films ranging up to 250 nm in thickness were obtained. Scanning electron beam micrographs revealed dense, crack‐free, low porosity films having grain sizes in the submicron range. Dissipations (tan δ) of less than 0.01 and capacitance densities up to 204 nF/cm2 were measured for 190‐nm‐thick films. These films had dc leakage currents of 0.2 μA/cm2 or less for bias voltages up to 10 V. They also exhibited relatively small temperature coefficients of capacitance. A patterning process was developed that permitted feature resolution down to 5 μm.

Use of ion beams to decompose metalorganics into patterned thin‐film superconductors

Ion implantation of 400 keV O+2 ions, at a dose of 5×1014 ions/cm2, was used to selectively decompose a high Tc superconductor‐forming metalorganic (MO). Implantation rendered the metal carboxylates insoluble in their solvent, xylene, permitting patterning of the MO prior to pyrolysis and annealing. Fine line superconductors were formed having 90 K onset temperatures and zero resistance at 68 K.

Microstructure and ferroelectric properties of fine-grained BaxSr1-xTiO3 thin films prepared by metalorganic decomposition

Thin films of Ba x Sr 1− x TiO 3 ( x = 0.7, 0.8, 0.9, and 1.0) were prepared by metalorganic decomposition (MOD). The relative permittivity, dissipation, polarization, resistivity, and grain size of these films were studied as a function of composition and temperature. Ferroelectric hysteresis loops were observed for all values of x and were found to be independent of measurement temperature though strongly dependent upon grain size.

Characterization of potassium tantalum niobate films formed by metalorganic deposition

Thin films of potassium tantalum niobate, KTa0.6Nb0.4O3, with a Curie temperature of 20 °C were deposited on a variety of substrates by metalorganic deposition. These films had peak relative permittivities of 16 000 at 20 °C. Hysteresis plots of electric displacement as a function of electric field, taken at 0 °C, revealed a coercive field of 800 V/cm, a spontaneous polarization of 3.9 μC/cm2, and a remnant polarization of 0.5 μC/cm2. The hysteresis loops did not change significantly as the temperature was varied down to −100 °C.

Use of electron beam lithography to selectively decompose metalorganics into patterned thin‐film superconductors

Fine line superconductors, approximately 5 μm in width and 260 nm thick, were formed from Y‐Ba‐Cu on 〈100〉SrTiO3 by the combined methods of metalorganic deposition and selective area electron beam exposure. The lines were written in metal neodecanoates using an electron beam having a spot size of 0.25 μm and an energy of 25 kV. The dosage of the exposure was 1200 μC/cm2. Unexposed areas were removed with a 30 s xylene wash. A 500 °C pyrolysis in air for 300 s followed by rapid thermal annealing in oxygen produced lines having superconducting onsets above 90 K and zero resistance at 69 K.