The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Modern computer microprocessor chips are marvels of engineering complexity. For the current 45 nm technology node, there may be nearly a billion transistors on a chip barely 1 cm2 and more than 10 000 m of wiring connecting and powering these devices distributed over 9-10 wiring levels. This represents quite an advance from the first INTEL 4004B microprocessor chip introduced in 1971 with 10 μm minimum dimensions and 2 300 transistors on the chip! It has been disclosed that advanced microprocessor chips at the 32 nm node will have more than 2 billion transistors.1 For instance, Figure 1 shows a sectional 3D image of a 90 nm IBM microprocessor, containing several hundred million integrated devices and 10 levels of interconnect wiring, designated as the back-end-of-the-line (BEOL). Since the invention of microprocessors, the number of active devices on a chip has been exponentially increasing, approximately doubling every two years. This trend was first described in 1965 by Gordon Moore,2 although the original discussion suggested doubling the number of devices every year, and the phenomenon became popularly known as Moore’s Law. This progress has proven remarkably resilient and has persisted for more than 50 years. The enabler that has permitted these advances is known as scaling, that is, the reduction of minimum device dimensions by lithographic advances (photoresists, tooling, and process integration optimization) by ∼30% for each device generation.3 It allowed more active devices to be incorporated in a given area and improved the operating characteristics of the individual transistors. It should be emphasized that the earlier improvements in chip performance were achieved with very few changes in the materials used in the construction of the chips themselves. The increase of performance with scaling * Corresponding author. E-mail: gdubois@us.ibm.com. † IBM Almaden Research Center. ‡ Stanford University. Willi Volksen received his B.S. in Chemistry (magna cum laude) from New Mexico Institute of Mining and Technology in 1972 and his Ph.D. in Chemistry/Polymer Science from the University of Massachusetts, Lowell, in 1975. He then joined the research group of Prof. Harry Gray/Dr. Alan Rembaum at the California Institute of Technology as a postdoctoral fellow and upon completion of the one-year appointment joined Dr. Rembaum at the Jet Propulsion Laboratory as a Senior Chemist in 1976. In 1977 Dr. Volksen joined the IBM Research Division at the IBM Almaden Research Center in San Jose, CA, where he is an active research staff member in the Advanced Materials Group of the Science and Technology function.

Low dielectric constant materials.

Poisson's ratio is, for specified directions, the ratio of a lateral contraction to the longitudinal extension during the stretching of a material Although a negative Poisson's ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, this property is generally believed to be rare in crystalline solids1 In contrast to this belief, 69% of the cubic elemental metals have a negative Poisson's ratio when stretched along the [110] direction For these metals, we find that correlations exist between the work function and the extremal values of Poisson's ratio for this stretch direction, which we explain using a simple electron-gas model Moreover, these negative Poisson's ratios permit the existence, in the orthogonal lateral direction, of positive Poisson's ratios up to the stability limit of 2 for cubic crystals Such metals having negative Poisson's ratios may find application as electrodes that amplify the response of piezoelectric sensors

Negative Poisson's ratios as a common feature of cubic metals

We have applied the perturbation theory for calculating the piezoelectric potential distribution in a nanowire (NW) as pushed by a lateral force at the tip. The analytical solution given under the first-order approximation produces a result that is within 6% from the full numerically calculated result using the finite element method. The calculation shows that the piezoelectric potential in the NW almost does not depend on the z-coordinate along the NW unless very close to the two ends, meaning that the NW can be approximately taken as a “parallel plated capacitor”. This is entirely consistent to the model established for nanopiezotronics, in which the potential drop across the nanowire serves as the gate voltage for the piezoelectric field effect transistor. The maximum potential at the surface of the NW is directly proportional to the lateral displacement of the NW and inversely proportional to the cube of its length-to-diameter aspect ratio. The magnitude of piezoelectric potential for a NW of diameter 50 nm and length 600 nm is 0.3 V. This voltage is much larger than the thermal voltage (25 mV) and is high enough to drive the metal-semiconductor Schottky diode at the interface between atomic force microscope tip and the ZnO NW, as assumed in our original mechanism for the nanogenerators. Developing novel technologies for wireless nanodevices and nanosystems is of critical importance for applications in biomedical sensing, environmental monitoring, and even personal electronics. Miniaturization of a power package and self-powering of these tiny devices are some key challenges for their applications. Various approaches have been developed for harvesting energy from the environment based on approaches such as thermoelectricity and piezoelectricity. Innovative nanotechnologies are being developed for converting mechanical energy (such as body movement, muscle stretching), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as body fluid and blood flow) into electric energy that will be used to power nanodevices that operate at low power. Recently, using piezoelectric ZnO nanowire (NW) arrays, a novel approach has been demonstrated for converting nanoscale mechanical energy into electric energy. 1-3 The single nanowire nanogenerator (NG) relies on the bending of a NW by a conductive atomic force microscope (AFM) tip, which transfers the displacement energy from the tip to the elastic bending energy of the NW. The coupled piezoelectric and semiconducting properties of the NW perform a charge creation, accumulation, and discharge process. Most recently, this approach has been extensively developed to produce continuous direct-current output with the use of aligned NWs that were covered by a zigzag top electrode, and the nanogenerator was driven by ultrasonic wave, establishing the platform of producing usable power output for nanodevices by harvesting energy from the environment. 4 Furthermore, based on the coupled piezoelectric and semiconducting properties of the NW, a new field of nanopiezotronics has been created, 5,6 which is the basis for fabricating piezoelectric field effect transistors, 7 piezoelectric diode, 8 piezoelectric force/humidity/chemical sensors, 9 and more. The theoretical background for the nanogenerator and nanopiezotronics is based on a voltage drop created across the cross section of the NW when it is laterally deflected, with the tensile side surface in positive voltage and compressive side in negative voltage. 1,5 It is essential to quantitatively calculate and even develop analytical equations that can give a direct calculation of the voltage at the two side surfaces of the NW, which is important to calculating the efficiency of the nanogenerator and the operation voltage of the nanopiezotronics. In the literature, numerous theories for onedimensional (1D) nanostructure piezoelectricity have been proposed, including first-principles calculations, 10,11 molecular dynamics (MD) simulations, 12 and continuum models. 13

Electrostatic potential in a bent piezoelectric nanowire. The fundamental theory of nanogenerator and nanopiezotronics

Research efforts addressing spin waves (magnons) in micro- and nanostructured ferromagnetic materials have increased tremendously in recent years. Corresponding experimental and theoretical work in magnonics faces significant challenges in that spin-wave dispersion relations are highly anisotropic and different magnetic states might be realized via, for example, the magnetic field history. At the same time, these features offer novel opportunities for wave control in solids going beyond photonics and plasmonics. In this topical review we address materials with a periodic modulation of magnetic parameters that give rise to artificially tailored band structures and allow unprecedented control of spin waves. In particular, we discuss recent achievements and perspectives of reconfigurable magnonic devices for which band structures can be reprogrammed during operation. Such characteristics might be useful for multifunctional microwave and logic devices operating over a broad frequency regime on either the macro- or nanoscale.

https://iopscience.iop.org/article/10.1088/0953-8984/26/12/123202/pdf

Review and prospects of magnonic crystals and devices with reprogrammable band structure

The elastic properties of ZnO films deposited by rf magnetron sputtering on Al2O3 substrates have been analyzed by means of an acoustic investigation technique. The phase velocities of a spectrum of acoustic modes propagating along the layered structure have been measured and the results exploited for determining the complete set of elastic constants of the film. The effective constants of the film are lower than those of the bulk material by amounts which depend on the elastic constant considered and range from −1.2% for c33 to −24.8% for c11. The values obtained were used for determining the dispersion curves of acoustic modes propagating along ZnO layers deposited on fused quartz and silicon and showed good agreement with experimental results.

Acoustic investigation of the elastic properties of ZnO films

In this work it is shown that the Brillouin light scattering technique can be successfully applied for determining the five effective elastic constants of a single transparent film of hexagonal symmetry, in the micron range of thicknesses. Measurements have been performed on a polycrystalline ZnO film, about 1.3 mu m thick, supported by a Si substrate. A major result of this work is that the elastic constant c66 is selectively determined for the first time from detection of the shear horizontal mode travelling parallel to the film surface. Similarly, a selective determination of c11 is attained from observation of the longitudinal mode guided by the film. The three remaining elastic constants, namely c13, c33, and c44, can be then obtained from detection of the Rayleigh surface mode and of the longitudinal bulk wave propagating at different angles from the surface normal.

https://iopscience.iop.org/article/10.1088/0953-8984/7/48/006/pdf

Brillouin scattering determination of the whole set of elastic constants of a single transparent film of hexagonal symmetry

Epitaxial ultrathin Cu/Ni/Cu films with Ni thickness in the range 1.5–6 nm have been grown by UHV evaporation on the Si(111)-737 surface. In situ characterization made by low-energy electron diffraction and Kikuchi electron diffraction revealed that both Ni and Cu films grow epitaxially on Si, with a ~111! orientation and with their ~110! axis parallel to the ~121! axis of the Si substrate. Magneto-optical Kerr effect measurements performed at room temperature have shown that the preferential direction of magnetization lies in the film plane for Ni thickness above 3 nm, while it is perpendicular to the film plane for lower thickness. Brillouin light scattering was then exploited to study the spin-wave dispersion as a function of both the applied magnetic field and the wave vector direction on the surface plane. In order to interpret the Brillouin data, we have used a macroscopic model which takes into account both dipolar and exchange interactions, as well as bulk and interface anisotropy for the ~111! plane of a cubic crystal. This enabled us to determine, in addition to the other magnetic parameters, both the in-plane and the out-of-plane anisotropy constants. The observed dependence of these constants on the film thickness indicates that the magnetic anisotropy is mainly of magnetoelastic origin. @S0163-1829~97!07838-7#

Perpendicular and in-plane magnetic anisotropy in epitaxial Cu/Ni/Cu/Si(111) ultrathin films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO/sub 2/ substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates. >

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

The spectrum of spin wave excitations on a nanometric two-dimensional periodical array of circular holes in a magnetic film was measured using the Brillouin light scattering technique. Two modes with positive group velocity in the frequency range between 4 and 7GHz were observed. Our calculations show that these correspond to the two lowest modes propagating along the edges of an effective stripe waveguide, perpendicular to the applied field, whose width is equal to the interhole distance. Moreover, a number of higher-frequency modes has been measured and identified as volume excitations of the same effective stripe.

G. Socino

Papers

Acoustic investigation of the elastic properties of ZnO films

Brillouin scattering determination of the whole set of elastic constants of a single transparent film of hexagonal symmetry

Perpendicular and in-plane magnetic anisotropy in epitaxial Cu/Ni/Cu/Si(111) ultrathin films

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Propagating volume and localized spin wave modes on a lattice of circular magnetic antidots