B. Dwir

Size effect in mesoscopic epitaxial ferroelectric structures: Increase of piezoelectric response with decreasing feature size

An epitaxial 200 nm thick film of Pb(Zr0.40Ti0.60)O3 (PZT) has been deposited by reactive rf magnetron sputtering on conductive Nb-doped SrTiO3 (100) (STO). The patterning process involved electron-beam lithography of polymethylmethacrylate, fabrication of a 75 nm thick Cr hard mask layer by means of a lift-off process, and dry etching of PZT. The smallest PZT features obtained were 100 nm in lateral dimensions. Piezoelectric sensitive scanning force microscopy in the contact mode revealed a strong increase of the piezoelectric response for feature sizes with lateral dimensions below 300 nm. It is proposed that this behavior is mainly due to vanishing a domains.

Focused electron beam induced deposition of gold

Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, we present a new and easy approach to obtain high purity gold lines: the use of inorganic PF3AuCl as a precursor. Electrical resistivities as low as 22 µOhms cm at 295 K (ten times the bulk Au value) were obtained. This is to our knowledge the best value for focused electron beam deposition obtained from the vapor phase so far. No special care was taken to prevent hydrocarbon contamination. The deposited nanostructure consists of gold grains varying in size and percolation with beam parameters.

Evidence for enhancement of critical current by intergrain Ag in YBaCuO‐Ag ceramics

We report the evidence for enhancement of critical current density Jc by ∼50%, which occurs when ∼10 wt. % Ag is added to Y1Ba2Cu3O7−δ ceramics. The maximal Jc (∼700 A/cm2 at T=77 K) appears simultaneously with maximum YBaCuO compactness in the samples. The silver fills the intergranular space (holes) without Cu substitution, and the critical temperature Tc is not reduced from the bulk value (∼91 K). Normal‐state resistivity of Ag‐YBaCuO samples is decreased by an order of magnitude, and samples exhibit improved contact resistance and resistance to water. While the critical density is improved by adding ∼10 wt. % Ag, it decreases at higher Ag concentrations.

Controlled positioning of carbon nanotubes by dielectrophoresis: Insights into the solvent and substrate role

We demonstrate the ability to precisely control the deposition of a defined number of carbon nanotubes (CNTs) from solution onto microfabricated electrodes using dielectrophoresis. The solvation shell around the CNTs, exhibiting a high dielectric constant which is possibly larger than the intrinsic dielectric constant of CNTs, is found to play a crucial role in electrophoretic processes. Substrate resistivity is also very important: The spatial repartition of the electric field between the substrate and the microelectrodes leads to deviations from the precise location of the CNTs. A recipe is given for the dielectrophoresis of CNTs which can be extended to other nanowires or nanotubes.

Wavelength and loss splitting in directly coupled photonic-crystal defect microcavities

Coupling between photonic-crystal defect microcavities is observed to result in a splitting not only of the mode wavelength but also of the modal loss. It is discussed that the characteristics of the loss splitting may have an important impact on the optical energy transfer between the coupled resonators. The loss splitting--given by the imaginary part of the coupling strength--is found to arise from the difference in diffractive out-of-plane radiation losses of the symmetric and the antisymmetric modes of the coupled system. An approach to control the splitting via coupling barrier engineering is presented.

Site-Controlled InGaAs Quantum Dots with Tunable Emission Energy

Semiconductor quantum-dot (QD) systems offering perfect site control and tunable emission energy are essential for numerous nanophotonic device applications involving spatial and spectral matching of dots with optical cavities. Herein, the properties of ordered InGaAs/GaAs QDs grown by organometallic chemical vapor deposition on substrates patterned with pyramidal recesses are reported. The seeded growth of a single QD inside each pyramid results in near-perfect (<10 nm) control of the QD position. Moreover, efficient and uniform photoluminescence (inhomogeneous broadening <10 meV) is observed from ordered arrays of such dots. The QD emission energy can be finely tuned by varying 1) the pyramid size and 2) its position within specific patterns. This tunability is brought about by the patterning of both the chemical properties and the surface curvature features of the substrate, which allows local control of the adatom fluxes that determine the QD thickness and composition.

Dense uniform arrays of site-controlled quantum dots grown in inverted pyramids

We report on the growth and optical properties of homogeneous, dense arrays of site-controlled, single GaAs/AlGaAs quantum dot (QD) heterostructures with periodicities as small as 300 nm. The samples were grown by organometallic chemical vapor deposition on (111)B GaAs substrates containing dense inverted pyramid recess patterns prepared by electron beam lithography and wet chemical etching. Low-temperature microphotoluminescence spectra of the samples show distinct luminescence from the QDs with 1–3 meV linewidth. Low-temperature cathodoluminescence spectrally resolved images reveal uniform emission energy within an ensemble of 900 QDs.

Record-Low Inhomogeneous Broadening of Site-Controlled Quantum Dots for Nanophotonics†

Keywords: epitaxy ; nanotechnology ; photonics ; quantum dots ; self-assembly ; Pyramids ; Arrays ; Energy Reference EPFL-ARTICLE-150247doi:10.1002/smll.201000341View record in Web of Science Record created on 2010-08-04, modified on 2017-05-12

In situ growth of superconducting YBa2Cu3O7-δ thin films on Si with conducting indium-tin-oxide buffer layers

Superconducting YBa2Cu3O7−δ (YBCO) thin films have been grown in situ on Si with conducting indium‐tin‐oxide (ITO) buffer layers. ITO allows YBCO to be electrically connected to the underlying Si substrate. Both the YBCO film and ITO buffer layer, grown by ion beam sputtering, are textured and polycrystalline with a combined room‐ temperature resistivity of about 2 mΩ cm. Superconducting onsets are 92 K with zero resistance at 68 K.

Annular aperture arrays: study in the visible region of the electromagnetic spectrum

Baida and Van Labeke recently proposed a structure that exhibits a supertransmission of light through an array of nanometric coaxial apertures in a metallic film that has been named an annular aperture array (AAA) [Opt. Commun. 209, 17 (2002); Phys. Rev. B 67, 155314 (2003); J. Microsc. 213, 140 (2003)]. We present the first experimental study, to our knowledge, of an AAA structure in the visible region. For technological reasons, the structure under study does not produce a supertransmission of 80% as in Baida and Van Labeke [Opt. Commun. 209, 17 (2002)]. We built the nanostructure and experimentally recorded its far-field spectral response. This transmission shows only one broad band with a maximum around lambda = 700 nm, giving a maximum efficiency around 17%. A finite-difference time-domain simulation reproduces quite well the obtained transmission spectrum.