J. Lettieri

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.

Oxide nano-engineering using MBE

Abstract Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer level; its use for the integration of oxides with similar nanoscale customization appears promising. This paper describes the use of reactive MBE to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides SrO, SrTiO 3 , BaTiO 3 , PbTiO 3 , and Bi 4 Ti 3 O 12 . The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the dielectric and ferroelectric properties of materials. Oxide nano-engineering is accomplished by supplying the incident species in the desired layering sequence with submonolayer composition control. Comparisons between the growth of compound semiconductors and oxides by MBE are made.

Critical issues in the heteroepitaxial growth of alkaline-earth oxides on silicon

The critical aspects of the epitaxial growth of alkaline-earth oxides on silicon are described in detail. The step by step transition from the silicon to the alkaline-earth oxide as shown through reflection high energy electron diffraction is presented, with emphasis placed on the favorable interface stability, oxidation, structural, and strain considerations for each stage of the growth via molecular beam epitaxy.

Superconducting MgB2 thin films on silicon carbide substrates by hybrid physical–chemical vapor deposition

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD). The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB2 using HPCVD. The MgB2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have Tc above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB2 thin films.

Epitaxial growth of (001)-oriented and (110)-oriented SrBi2Ta2O9 thin films

Epitaxial SrBi2Ta2O9 thin films have been grown with (001) and (110) orientations by pulsed laser deposition on (001) LaAlO3–Sr2AlTaO6 and (100) LaSrAlO4 substrates, respectively. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films. Minimization of surface mesh mismatch between the film and substrate (i.e., choice of appropriate substrate material and orientation) was used to stabilize the desired orientations and achieve epitaxial growth.

Epitaxial growth of non-c-oriented SrBi2Nb2O9 on (111) SrTiO3

Epitaxial SrBi2Nb2O9 thin films have been grown with a (103) orientation on (111) SrTiO3 substrates by pulsed-laser deposition. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase-pure epitaxial films. Epitaxial (111) SrRuO3 electrodes enabled the electrical properties of these (103)-oriented SrBi2Nb2O9 films to be measured. The low-field relative permittivity was 185, the remanent polarization was 15.7 μC/cm2, and the dielectric loss was 2.5% for a 0.5-μm-thick film.

Synthesis and properties of c-axis oriented epitaxial MgB2 thin films

We report the growth and properties of epitaxial MgB2 thin films on (0001) Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films via radio-frequency (rf) magnetron sputtering, followed by a postdeposition anneal at 850 °C in magnesium vapor. X-ray diffraction and cross-sectional transmission electron microscopy reveal that the epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3 substrate with a 30° rotation in the (0001) plane with respect to the substrate. The critical temperature was found to be 35 K and the anisotropy ratio, Hc2∥/Hc2⊥, was about 3 at 25 K. The critical current densities at 4.2 and 20 K (at 1 T perpendicular magnetic field) are 5×106 and 1×106 A/cm2, respectively. The controlled growth of epitaxial MgB2 thin films opens a new avenue in both understanding superconductivity in MgB2 and technological applications.

Epitaxial La-doped SrTiO3 on silicon: A conductive template for epitaxial ferroelectrics on silicon

Use of an epitaxial conducting template has enabled the integration of epitaxial ferroelectric perovskites on silicon. The conducting template layer, LaxSr1−xTiO3 (LSTO), deposited onto (001) silicon wafers by molecular-beam epitaxy is then used to seed {001}-oriented epitaxial perovskite layers. We illustrate the viability of this approach using PbZr0.4Ti0.6O3 (PZT) as the ferroelectric layer contacted with conducting perovskite La0.5Sr0.5CoO3 (LSCO) electrodes. An important innovation that further facilitates this approach is the use of a low-temperature (450 °C) sol–gel process to crystallize the entire ferroelectric stack. Both transmission electron microscopy and x-ray diffraction analysis indicate the LSCO/PZT/LSCO/LSTO/Si heterostructures are epitaxial. The electrical response of ferroelectric capacitors (for pulse widths down to 1 μs) measured via the underlying silicon substrate is identical to measurements made using conventional capacitive coupling method, indicating the viability of this approach.

Growth of (103) fiber-textured SrBi2Nb2O9 films on Pt-coated silicon

(103) fiber-textured SrBi2Nb2O9 thin films have been grown on Pt-coated Si substrates using a SrRuO3 buffer layer. High-resolution transmission electron microscopy reveals that the fiber texture arises from the local epitaxial growth of (111) SrRuO3 grains on (111) Pt grains and in turn (103) SrBi2Nb2O9 grains on (111) SrRuO3 grains. The films exhibit remanent polarization values of 9 μC/cm2. The uniform grain orientation (fiber texture) should minimize grain-to-grain variations in the remanent polarization, which is important to continued scaling of ferroelectric memory device structures.

Morphology, structure, and nucleation of out-of-phase boundaries (OPBs) in epitaxial films of layered oxides

Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.