James Lettieri

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB21 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor2, with a relatively long coherence length3. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap4,5 of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology6,7. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical–chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.

c-axis oriented epitaxial BaTiO3 films on (001) Si

c-axis oriented epitaxial films of the ferroelectric BaTiO3 have been grown on (001) Si by reactive molecular-beam epitaxy. The orientation relationship between the film and substrate is (001) BaTiO3‖(001) Si and [100] BaTiO3‖[110] Si. The uniqueness of this integration is that the entire epitaxial BaTiO3 film on (001) Si is c-axis oriented, unlike any reported so far in the literature. The thermal expansion incompatibility between BaTiO3 and silicon is overcome by introducing a relaxed buffer layer of BaxSr1−xTiO3 between the BaTiO3 film and silicon substrate. The rocking curve widths of the BaTiO3 films are as narrow as 0.4°. X-ray diffraction and second harmonic generation experiments reveal the out-of-plane c-axis orientation of the epitaxial BaTiO3 film. Piezoresponse atomic force microscopy is used to write ferroelectric domains with a spatial resolution of ∼100nm, corroborating the orientation of the ferroelectric film.

Epitaxial growth and properties of metastable BiMnO3 thin films

Epitaxial thin films of BiMnO3 were deposited on single-crystal substrates of (100)-oriented SrTiO3 by pulsed-laser deposition. Structural analysis by x-ray diffraction, electron diffraction, and transmission electron microscopy (TEM) indicated that the films were monoclinic and twinned with two dominant orientation relationships. The first is (111)u200aBiMnO3u200a∥u200a(100)u200aSrTiO3 and ∼[101]u200aBiMnO3u200a∥u200a〈010〉u200aSrTiO3; the second is (101)u200aBiMnO3u200a∥u200a(100)u200aSrTiO3 and ∼[121]u200aBiMnO3u200a∥u200a〈010〉u200aSrTiO3. High-resolution TEM images revealed that there is no reaction or appreciable interdiffusion at the substrate/film interface, despite the high temperature of the substrate during deposition (∼1000 K). Magnetic characterization was carried out (both magnetization versus temperature and hysteresis loops) and the results agree with previous reports of a ferromagnetic transition with TC∼105 K. The actual value of TC in the films is a few degrees lower than the bulk material, the discrepancy being attributed to strain, nonstoichiometr...

Interface structure and thermal stability of epitaxial SrTiO3 thin films on Si (001)

We have used medium energy ion scattering, temperature programmed desorption, and atomic force microscopy to study the interface composition and thermal stability of epitaxial strontium titanate thin films grown by molecular-beam epitaxy on Si (001). The composition of the interface between the film and the substrate was found to be very sensitive to the recrystallization temperature used during growth, varying from a strontium silicate phase when the recrystallization temperature is low to a Ti-rich phase for a higher recrystallization temperature. The films are stable towards annealing in vacuum up to ∼550°C, where SrO desorption begins and the initially flat film starts to roughen. Significant film disintegration occurs at 850°C, and is accompanied by SiO and SrO desorption, pinhole formation, and finally titanium diffusion into the silicon bulk.

Superconducting properties of nanocrystalline MgB2 thin films made by an in situ annealing process

We have studied the structural and superconducting properties of MgB2 thin films made by pulsed-laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature (Tc0) of 34 K and a zero-field critical current density (Jc) of 1.3×106u200aA/cm2 were obtained. The irreversibility field was ∼8 T at low temperatures, although severe pinning instability was observed. The result is a step towards making the in situ deposition process a viable technique for MgB2 Josephson junction technologies.

Probing domain microstructure in ferroelectric Bi4Ti3O12 thin films by optical second harmonic generation

The domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3(001) substrate is studied by second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. A theoretical model is presented that explains the observed symmetries and extracts quantitative information on the nonlinear optical coefficients of the material and statistics of domain variants present in the film area being probed. The following ratios of nonlinear coefficients and birefringence was determined: d12/d11=−3.498±0.171, |d26/d12|=0.365±0.010, |d26/d11|=1.273±0.036, and |nb−na|=0.101±0.018 (at 532 nm).

Large optical nonlinearities in BiMnO3 thin films

Large third-order optical nonlinearities were observed in epitaxial thin films of BiMnO3 grown on SrTiO3 substrates. Using 140 fs laser pulses at 900 nm, a negative nonlinear refractive index (nI∼−0.53u2009cm2/GW) and a nonlinear absorption coefficient (αI∼−0.08u2009cm/kW) for BiMnO3 are measured at room temperature. Large electric-field-induced enhancement of three to four orders of magnitude in the second-harmonic response at 450 nm is observed with effective nonlinear coefficients of deff∼40(115)u2009pm/V at 300 (473) K under applied fields of ∼707 (177) V/mm, respectively, from a 110 nm multivariant thin film.

Domain rearrangement in ferroelectric Bi4Ti3O12 thin films studied by in situ optical second harmonic generation

Electric-field induced rearrangement of domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3 (001) substrate is studied by optical second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. Changes in these symmetries with the application of electric field are experimentally studied at 23°u200aC and 60u200a°C, and theoretically modeled to extract the hysteresis loops that reflect quantitative changes in the fraction of domain variants. A strong correlation is observed between the dc electrical conductance (as distinct from transient currents) and the ferroelectric domain state of the film, which is proposed to arise from the creation and destruction of charged domain walls within the film with applied field.

Microstructure and electrical properties of epitaxial SrBi2Nb2O9 And SrBi2Ta2O9 films

Abstract SrBi2Nb2O9 (and in some cases SrBi2Ta2O9) epitaxial thin films were deposited on (001), (110), and (111) SrTiO3 substrates by pulsed laser deposition (PLD), both with and without epitaxial SrRuO3 bottom electrodes. Films grow epitaxially with the c-axis inclined by 0°, 45°, and 57° from the substrate surface normal, respectively. Greater tilts of the c-axis into the plane of the substrate surface provide a greater component of the polar axis (the a-axis of the orthorhombic unit cell) perpendicular to the substrate surface, leading to increased remanent polarization (P r) values. Portions of the same films used for electrical characterization were examined by transmission electron microscopy (TEM). Films have a single c-axis tilt angle and are fully crystalline with no observable second-phase inclusions. All films are observed to have a high density of out-of-phase boundaries (OPBs).

Out-of-phase Boundary (OPB) Nucleation in Layered Oxides

Out-of-phase boundaries (OPBs), planar faults between regions of a crystal that are misaligned by a fraction of a unit cell dimension, occur frequently in materials of high structural anisotropy. Rarely observed in the bulk, OPBs frequently exist in epitaxial films of layered complex oxides, such as YBa 2 Cu 3 O 7- δ -type, Aurivillius, and Ruddlesden-Popper phases, and frequently propagate through the entire thickness of a film, due to their large offset and the improbability of opposite-sign OPB annihilation. OPBs have previously been demonstrated to have a significant impact upon properties, so it is important to understand their generation. These faults arise through the same few mechanisms in the various layered complex oxides. An effort is made to unify the discussion of nucleation of these defects, common to layered oxide materials. OPBs can nucleate at the film-substrate interface (primary) via steric, chemical, or misfit mechanisms, or post-growth (secondary) through crystallographic shear during decomposition of volatile components. Examples of the mechanisms observed during highresolution transmission electron microscopy (HRTEM) study of Aurivillius and RuddlesdenPopper phases are presented. A method for estimating the relative OPB density in a film from correlation of x-ray diffraction (XRD) θ-2θ data with TEM information on OPBs is presented.