T. Sands

Ferroelectric La-Sr-Co-O/Pb-Zr-Ti-O/La-Sr-Co-O heterostructures on silicon via template growth

Ferroelectric Pb0.9La0.1Zr0.2Ti0.8O3 thin film capacitors with a symmetrical La‐Sr‐Co‐O top and bottom electrodes have been grown on [001] Si with yttria stabilized zirconia (YSZ) buffer layer. A layered perovskite ‘‘template’’ layer (200–300 A thick), grown between the YSZ buffer layer and the bottom La‐Sr‐Co‐O electrode, is critical for obtaining the required orientation of the subsequent layers. When compared to the capacitors grown with the Y‐Ba‐Cu‐O top and bottom electrodes, these structures possess two advantages: (i) the growth temperatures are lower by 60–150 °C; (ii) the capacitors show a larger remnant polarization ΔP (ΔP=switched polarization–nonswitched polarization), 25–30 μC/cm2, for an applied voltage of only 2 V (applied field of 70 kV/cm). The fatigue, retention, and aging characteristics of these new structures are excellent.

Fatigue and retention in ferroelectric Y‐Ba‐Cu‐O/Pb‐Zr‐Ti‐O/Y‐Ba‐Cu‐O heterostructures

Fatigue and retention characteristics of ferroelectric lead zirconate titanate thin films grown with Y‐Ba‐Cu‐O(YBCO) thin‐film top and bottom electrodes are found to be far superior to those obtained with conventional Pt top electrodes. The heterostructures reported here have been grown in situ by pulsed laser deposition on yttria‐stabilized ZrO2 buffer [100] Si and on [001] LaAlO3. Both the a‐ and c‐axis orientations of the YBCO lattice have been used as electrodes. They were prepared using suitable changes in growth conditions.

Damage-free separation of GaN thin films from sapphire substrates

Gallium nitride thin films grown on sapphire substrates were successfully separated and transferred onto Si substrates using single 38 ns KrF excimer laser pulses directed through the transparent substrate at fluences in the range of 400–600 mJ/cm2. The absorption of the 248 nm radiation by the GaN at the interface induces rapid thermal decomposition of the interfacial layer, yielding metallic Ga and N2 gas. The substrate is easily removed by heating above the Ga melting point of 30 °C. Scanning electron microscopy and x-ray diffraction of the GaN films before and after lift-off demonstrate that the structural quality of the GaN films is not altered by the separation and transfer process.

Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off

Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.

Equilibrium limits of coherency in strained nanowire heterostructures

Due to their unique boundary conditions, nanowire heterostructures may exhibit defect-free interfaces even for systems with large lattice mismatch. Heteroepitaxial material integration is limited by lattice mismatches in planar systems, but we use a variational approach to show that nanowire heterostructures are more effective at relieving mismatch strain coherently. This is an equilibrium model based on the Matthews critical thickness in which the lattice mismatch strain is shared by the nanowire overlayer and underlayer, and could as well be partially accomodated by the

Nanoscale design to enable the revolution in renewable energy

The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2–5 years), mid-term (5–10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum of energy conversion, storage, and conservation technologies. We anticipate that the conclusions and recommendations herein will be of use not only to the technical community, but also to policy makers and the broader public, occasionally with an admitted emphasis on the US perspective.

InxGa1−xN light emitting diodes on Si substrates fabricated by Pd–In metal bonding and laser lift-off

Indium–gallium nitride (InxGa1−xN) single-quantum-well (SQW) light emitting diodes (LEDs), grown by metalorganic chemical vapor deposition on sapphire, were transferred onto Si substrates. The thin-film InxGa1−xN SQW LED structures were first bonded onto a n+-Si substrate using a transient-liquid-phase Pd–In wafer-bonding process followed by a laser lift-off technique to remove the sapphire growth substrate. Individual, 250×250 μm2, LEDs with a backside contact through the n+-Si substrate were then fabricated. The LEDs had a typical turn-on voltage of 2.5 V and a forward current of 100 mA at 5.4 V. The room-temperature emission peak for the InxGa1−xN SQW LEDs was centered at 455 nm with a full width at half maximum of 19 nm.

Insights into the Electrodeposition of Bi2Te3

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 + + 2Bi 3+ + 18e - + 9H + → Bi 2 Te 3 + 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in I M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO + 2 to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3+ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO + 2 to H 2 Te followed by the chemical interaction with Bi 3+ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range -0.1 < E < -0.52 V vs. Ag/AgCl (3 M NaCI) on Pt substrates as determined by powder X-ray diffraction (XRD). Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.

Epitaxial growth of ferromagnetic ultrathin MnGa films with perpendicular magnetization on GaAs

We have successfully grown ferromagnetic MnGa ultrathin films on GaAs substrates by molecular beam epitaxy. Reflection high energy electron diffraction and cross‐sectional transmission electron microscopy show that monocrystalline MnGa films are grown with the c axis of the tetragonal unit cell normal to the (001) GaAs substrates. Both magnetization measurements by vibrating sample magnetometer and extraordinary Hall effect (EHE) measurements indicate perpendicular magnetization, with the remnant magnetization of 225 emu/cm3 and EHE resistivity in the range of 0.5–4 μΩ cm at room temperature. The material possesses properties ideal for certain nonvolatile magnetic memory coupled with underlying III‐V circuitry.

Stable and epitaxial metal/III-V semiconductor heterostructures

Long before the advent of nanofabrication and quantum-effect devices, the technological limitations imposed by polycrystalline, multiphase and thermally unstable contacts to III-V semiconductors were of concern to forward-looking materials scientists. In the early 1980s, efforts to elucidate the complex behaviour of reactive metal/III-V systems were initiated. These early efforts evolved slowly and culminated in the recent achievement of stable and epitaxial metallizations to III-V semiconductors. In this review, we first describe the criteria that must be met for the fabrication of metal/III-V heterostructures. Bulk phase equilibria are useful guides for selecting metal/semiconductor combinations which will not react during growth at moderate temperatures or during subsequent processing steps. We show, however, that phase stability is not sufficient for the fabrication of ultrathin metal overlayers or buried metal heterostructures. Growth conditions must be carefully optimized and combined with the appropriate selection of metallic phases with high melting points in order to suppress the strong tendency for island formation during growth and film agglomeration during overgrowth or processing. In our discussion of metal/semiconductor hetero-structures we highlight the relationship between symmetry differences and defects (domain boundaries) with particular emphasis on semiconductor overlayers grown on high-symmetry metals. Our work and that of others has shown that stable and epitaxial metallizations to III-V semiconductors as well as more complex metal/III-V heterostructures can be achieved with two classes of metallic materials; the transition-metal gallides and aluminides with the CsCl structure (TM-III) and the rare-earth monopnictides with the NaCl structure (RE-V). We discuss and compare the growth of these III-V/TM-III/III-V and III-V/RE-V/III-V heterostructures by molecular beam epitaxy, focusing special attention to the initial stages of growth of metallic films on III-V substrates and III-V overlayers on metallic films. Going beyond the strictly materials issues, we describe the electrical properties of such heterostructures, including stable enhanced-barrier Schottky contacts and semiconductor-clad metallic quantum wells, structures which may be the basis for exciting and novel electronic, photonic and magnetic devices.