Chen-Fong Tsai

Extremely High Tunability and Low Loss in Nanoscaffold Ferroelectric Films

There are numerous radio frequency and microwave device applications which require materials with high electrical tunability and low dielectric loss. For phased array antenna applications there is also a need for materials which can operate above room temperature and which have a low temperature coefficient of capacitance. We have created a nanoscaffold composite ferroelectric material containing Ba(0.6)Sr(0.4)TiO(3) and Sm(2)O(3) which has a very high tunability which scales inversely with loss. This behavior is opposite to what has been demonstrated in any previous report. Furthermore, the materials operate from room temperature to above 150 °C, while maintaining high tunability and low temperature coefficient of tunability. This new paradigm in dielectric property control comes about because of a vertical strain control mechanism which leads to high tetragonality (c/a ratio of 1.0126) in the BSTO. Tunability values of 75% (200 kV/cm field) were achieved at room temperature in micrometer thick films, the value remaining to >50% at 160 °C. Low dielectric loss values of <0.01 were also achieved, significantly lower than reference pure films.

Integration of Self-Assembled Vertically Aligned Nanocomposite (La0.7Sr0.3MnO3)1–x:(ZnO)x Thin Films on Silicon Substrates

Epitaxial (La0.7Sr0.3MnO3)(1-x):(ZnO)x (LSMO:ZnO) in vertically aligned nanocomposite (VAN) form was integrated on STO/TiN-buffered silicon substrates by pulsed-laser deposition. Their magnetotransport properties have been investigated and are systematically tuned through controlling the ZnO concentration. The composite film with 70% ZnO molar ratio exhibits a maximum magnetoresistance (MR) value of 55% at 70 K and 1 T. The enhanced tunable low-field MR properties are attributed to structural and magnetic disorders and spin-polarized tunneling through the secondary ZnO phase. The integration of LSMO:ZnO VAN films on silicon substrates is a critical step enabling the application of VAN films in future spintronic devices.

Growth-controlled surface roughness in Al-doped ZnO as transparent conducting oxide

The surface morphology of Al(2)O(3)-doped ZnO (AZO, 2 wt%) thin films varies from a uniform layer to nanorod structure by simply controlling oxygen pressure during growth. All AZO films were deposited on sapphire(0001) substrates using a pulsed laser deposition (PLD) technique. In the low oxygen pressure regime (vacuum approximately 50 mTorr), AZO films grow as a smooth and uniform layer. In the high oxygen pressure regime (100-250 mTorr) AZO thin films with nanorods have formed. Detailed cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) studies reveal that, besides the obvious variation in the film morphology, the in-plane d spacing of AZO film increases and the out-of-plane d spacing decreases, as oxygen pressure increases. A bilayer AZO film with a nanorod structure on top of a uniform layer was demonstrated by controlling the oxygen pressure for the two layers. Electrical resistivity and optical transmittance measurements were carried out to correlate with the microstructures obtained under different oxygen pressures. The bilayer AZO films could find applications as a transparent conducting oxide (TCO) with a unique light trapping function in thin film solar cells.

Enhanced critical current in YBa2Cu3O7?? thin films through pinning by ferromagnetic YFeO3 nanoparticles

Nanoscale ferromagnetic inclusions of YFeO3 have been incorporated into pulsed laser deposited YBa2Cu3O7 ? ? (YBCO) thin films. The poisoning of the YBCO through the addition of the magnetic material is minor, with 1?mol% doping resulting in an unsuppressed superconducting transition temperature of 90?K. The critical current density of the magnetically doped films is enhanced both in field and at self-field, and values of 3.0?MA?cm ? 2 have been achieved at 77?K, self-field in films 1??m thick, compared to 1.5?MA?cm ? 2 in an undoped film prepared by the same process. Such an enhancement in critical current at such low dopant levels is suggestive of an additional contribution to the flux pinning from the magnetic constituent.

Enhanced flux pinning in YBa2Cu3O7-δ thin films using Nb-based double perovskite additions

The addition of a new niobate double perovskite pinning phase to YBa2Cu3O7?? thin films grown by pulsed laser deposition is reported. The YBa2NbO6 phase self-assembles into stacks of ~10?nm second phase particles, aligned with the c-axis of the YBa2Cu3O7??. The YBa2Cu3O7??/YBa2NbO6 composite thin films have enhanced critical current, by a factor of 2, at 1?T () over the pure YBa2Cu3O7??, whilst maintaining a high transition temperature. Niobium does not substitute in the YBa2Cu3O7?? matrix. This, together with the high stability of the second phase formed, makes it an ideal pinning additive.

Magnetotransport properties of quasi-one-dimensionally channeled vertically aligned heteroepitaxial nanomazes

A unique quasi-one-dimensionally channeled nanomaze structure has been self-assembled in the (La0.7Sr0.3MnO3)1−x:(ZnO)x vertically aligned nanocomposites (VANs). Significantly enhanced magnetotransport properties have been achieved by tuning the ZnO composition x. The heteroepitaxial VAN thin films, free of large angle grain boundaries, exhibit a maximum low-field magnetoresistance (LFMR) of 75% (20 K and 1 T). The enhanced LFMR close to the percolation threshold is attributed to the spin-polarized tunneling through the ferromagnetic/insulating/ferromagnetic vertical sandwiches in the nanomazes. This study suggests that the phase boundary in the nanomaze structure is an alternative approach to produce decoupled ferromagnetic domains and thus to achieve enhanced magnetoresistance.

Understanding nanoparticle self-assembly for a strong improvement in functionality in thin film nanocomposites

The striking influence of the growth kinetics and substrate enhanced surface mobility on the control of the self-assembly of rare earth tantalate particles (1.5 mol% of nanoparticles in YBa(2)Cu(3)O(7) thin films) is demonstrated. Strongly enhanced flux pinning, control of the anisotropy property and superior critical current densities were achieved. Owing to the unique ability to probe nanoparticle self-assembly through determination of the nature and extent of the anisotropy of the superconducting properties, this system serves as the perfect model system for understanding how to tune and control functional nanocomposite nanostructures for a wide range of multifunctional applications.

Optimizing Flux Pinning of YBCO Superconductor With

Addition of nanophase defects to YBa₂Cu₃O₇ superconductor thin films enhances flux pinning, resulting in an increase in transport current densities (<i>J</i>_ct). While previous studies focused on single-phase additions, the addition of several phases simultaneously has shown strong improvements by combining different flux pinning mechanisms. This paper further explores the effect of mixed phase nanoparticle pinning, with the addition of insulating, nonreactive phases of BaSnO₃ and Y₂O₃. Processing parameters vary the BaSnO₃ concentration of 3, 5, and 10 vol. %, while maintaining Y₂O₃ constant at 3 vol.%. Pulsed laser deposition produces films on LaAlO₃ and SrTiO₃ substrates at deposition temperatures of 750-815°C. Current density is measured for fields ranging from <i>H</i> = 0 to 9 T with <i>H</i> // c, and temperatures from 5 to 77 K, providing a detailed picture of pinning effects. Optimized results of flux pinning, magnetic current densities <i>J</i>_cm (<i>H, T</i>), critical transition temperatures (<i>T</i>_c), lattice parameters, and microstructures are presented.

\hbox{BaSnO}_{3} + \hbox{Y}_{2}\hbox{O}_{3}

Vertically aligned nanocomposites (VAN) combined ferrimagnetic CoFe2O4 with non-magnetic CeO2 ((CoFe2O4)x:(CeO2)1−x) in different phase ratios (x = 10%, 30% to 50%) have been grown by a pulsed laser deposition technique. Various unique magnetic domain structures form based on the VAN compositions and growth conditions. Anisotropic and tunable ferrimagnetic properties have been demonstrated. These ordered ferrimagnetic nanostructures have been incorporated into YBa2Cu3O7−δ thin films as both cap and buffer layers to enhance the flux pinning properties of the superconducting thin films. The results suggest that the ordered magnetic VAN provides effective pinning centers by both defect and magnetic nanoinclusions.

Dual Mixed Phase Additions

A thin layer of a vertically aligned nanocomposite (VAN) with separated phases of ferromagnetic Fe2O3 and non-magnetic CeO2, arranged as alternating nanopillars, is introduced in YBa2Cu3O7?? (YBCO) thin films as either a cap or a buffer layer using a pulsed laser deposition method. Detailed microstructural characterization including XRD, high resolution XTEM and STEM is conducted and correlated with the superconducting properties to investigate the flux pinning properties introduced by the magnetic VAN layers. The Tc values of both doped samples are above 89?K and the measured at 65?K increased to 150% of that of the reference YBCO prepared under the same conditions. As the measurement temperature decreases, the magnetic pinning effect increases and the field dependent Jc(H???c) is further improved to more than 200% of the Jc value of the reference YBCO sample. This suggests that the Fe2O3:CeO2 VAN can provide both ordered magnetic pillars and controlled defect density. Furthermore, the magnetic pillars are very effective pinning centers especially in the high field and low temperature regime.