Ch. Buchal

Resistive switching in metal–ferroelectric–metal junctions

The aim of this work is to investigate the electron transport through metal–ferroelectric–metal (MFM) junctions with ultrathin barriers in order to determine its dependence on the polarization state of the barrier. To that end, heteroepitaxial Pt/Pb(Zr0.52Ti0.48)O3/SrRuO3 junctions have been fabricated on lattice-matched SrTiO3 substrates. The current–voltage (I–V) characteristics of the MFM junctions involving a few-nanometer-thick Pb(Zr0.52Ti0.48)O3 barriers have been recorded at temperatures between 4.2 K and 300 K. Typical I–V curves exhibit reproducible switching events at well-defined electric fields. The mechanism of charge transport through ultrathin barriers and the origin of the observed resistive switching effect are discussed.

Electroluminescence of erbium–oxygen‐doped silicon diodes grown by molecular beam epitaxy

We have fabricated erbium–oxygen‐doped silicon light emitting diodes with molecular beam epitaxy by simultaneously evaporating erbium and silicon and providing a suitable background pressure of oxygen. In reverse bias, the diodes show intense room‐temperature electroluminescence at λ=1.54 μm, originating from the intra‐4f transition of erbium. This luminescence does not show temperature quenching between 4 and 300 K. In forward bias the erbium peak intensity is reduced by a factor of 30 at low temperatures and shows temperature quenching.

Ferroelectric BaTiO3 thin-film optical waveguide modulators

High-quality BaTiO3 epitaxial thin films on MgO substrates have been grown by pulsed-laser deposition. Both, c-axis and a-axis BaTiO3 orientations were studied. Mach–Zehnder optical waveguide modulators with a fork angle of 1.7° have been fabricated by ion-beam etching. The waveguides are of the ridge type, the BaTiO3 thickness is 1 μm, the ridge step 50 nm, and the width 2 μm. Light was coupled into the waveguides from optical fibers. The BaTiO3 waveguide propagation losses are 2–3 dB/cm. Electrodes of 3 mm length were deposited besides the waveguides. Electro-optic modulation has been demonstrated with Vπ=6.3 V at 632 nm wavelength and Vπ=9.5 V at 1550 nm wavelength for the a-axis samples, and with Vπ=8 V at 632 nm wavelength and Vπ=15 V at 1550 nm for the c-axis samples. Theoretical modelling of the Mach–Zehnder modulators for both crystalline orientations of the BaTiO3 films gave the Pockels coefficients r51=80 pm/V for the c-axis film and an effective Pockels coefficient reff=734 pm/V for the a-axis ...

Structural and optical characterization of epitaxial waveguiding BaTiO3 thin films on MgO

Epitaxial waveguide structures of c-axis oriented BaTiO3 thin films on MgO(001) have been grown by pulsed laser deposition. The structural properties of the samples have been characterized by Rutherford backscattering spectrometry/ion channeling (RBS/C), x-ray diffraction, and atomic force microscopy. We found excellent crystalline quality even up to thicknesses of a few microns. This has been confirmed by RBS/C minimum yield values of 2%–3%, a full width at half maximum of 0.36° of the BaTiO3 (002) rocking curve, and a rms roughness of 1.1 nm for a 950 nm BaTiO3 film. The out-of-plane refractive index was measured to be close to the extraordinary bulk value with the birefringence being about one third of the bulk value. Waveguide losses of 2.9 dB/cm have been demonstrated.

Room-temperature photoluminescence from Tb ions implanted in SiO2 on Si

The room-temperature photoluminescence (PL) of Tb3+ ions has been studied. The Tb ions were implanted into 200 nm thick SiO2 on Si wafers. To achieve a uniform Tb distribution, the implantations were performed at 50, 100, and 190 keV to a total dose of 8.8×1014–1.3×1016 ions/cm2, resulting in Tb concentrations of 0.18–2.7 at. %. The PL spectrum consists of sharp lines due to the Tb3+ intra-4f transitions and a broadband due to SiO2 defects. The samples were annealed at temperatures ranging from 600 to 1050 °C. Up to 900 °C, the annealing procedure improves the PL yield; at temperatures higher than 1000 °C, the PL yield drops again at high dose. The PL spectra show noticeable influence of Tb–Tb crossrelaxation, which favors the green PL over the blue PL.

Modeling of refractive index profiles of He+ ion‐implanted KNbO3 waveguides based on the irradiation parameters

Planar optical waveguides were fabricated by He+ ion implantation in KNbO3 crystals with doses from 2.5×1014 to 1×1016 cm−2 and energies from 1 to 3.5 MeV, and the index profiles were analyzed. A comparison with theoretical radiation damage profiles calculated by the trim code indicates that the index change is mainly caused by nuclear collisions. However, at higher implantation doses the influence of the electronic excitation on the refractive index profiles cannot be neglected. A method is derived to predict the index profile of nb from the He+ implantation parameters energy and dose. The measured mode spectra of waveguides produced by single and dual energy implantation are in excellent agreement with the predicted spectra.

OPTICAL AND STRUCTURAL PROPERTIES OF MEV ERBIUM-IMPLANTED LINBO3

LiNbO3 single crystals have been implanted with 2.0 or 3.5 MeV Er ions with fluences between 2.0×1014 and 7.5×1015 cm−2 and annealed at temperatures between 500 and 1060 °C in a wet oxygen atmosphere. Photoluminescence spectroscopy, Rutherford backscattering spectroscopy, and secondary‐ion‐mass spectrometry have been used to study the influence of the annealing treatment on the optical activity of the Er ions, the crystal structure of the implanted LiNbO3 layer, and the depth distribution of the Er ions, respectively. The as‐implanted, amorphized LiNbO3 already emits the characteristic photoluminescence (PL) of Er3+ around 1.53 μm. Annealing for 1 min at 500 °C causes recrystallization of the amorphized layer by columnar solid‐phase epitaxial regrowth from the substrate. The PL intensity increases by more than one order of magnitude on annealing at 500 °C and the PL lifetime rises from 1.65 to 2.85 ms. In contrast, much longer annealing times and a much higher temperature are necessary to remove the colum...

Ferroelectric properties of epitaxial BaTiO3 thin films and heterostructures on different substrates

Ferroelectric thin films of BaTiO3 and BaTiO3∕SrRuO3 epitaxial heterostructures on different single-crystalline substrates were fabricated by pulsed laser deposition. The BaTiO3 films of 100–400nm thickness show high structural perfection and c-axis-oriented growth. For the electrical characterization of the BaTiO3 in a thin-film capacitor structure, Pt top electrodes were deposited by e-beam evaporation. The results are compared to the current experimental and theoretical models. Special consideration is given to the model of charge injection from the electrodes.

High quality epitaxy of YBa2Cu3O7−x on silicon‐on‐sapphire with the multiple buffer layer YSZ/CeO2

High quality YBa2Cu3O7−x films on silicon‐on‐sapphire were grown using a multiple buffer layer, consisting of YSZ and CeO2. Ion channeling reveals the high crystalline perfection of the YBa2Cu3O7−x films. A channeling minimum yield for the Ba signal as low as 3% was measured. The normal state resistivity of 200 μΩ cm at 300 K, critical temperatures above 90 K, with transition widths down to 0.5 K, critical current densities above 2×106 A/cm2 at 77 K and surface resistance values of 1.4 mΩ at 18.9 GHz and 77 K confirm the high quality of the YBa2Cu3O7−x films. These results are very promising for integrated superconductor and semiconductor applications.

Fabrication of ZnO nanoparticles in SiO2 by ion implantation combined with thermal oxidation

Zinc-oxide (ZnO) nanoparticles (NPs) are fabricated in silica glasses (SiO2) by implantation of Zn+ ions of 60 keV up to 1.0×1017ions∕cm2 and following thermal oxidation. After the oxidation at 700 °C for 1 h, the absorption in the visible region due to Zn metallic NPs disappears and a new absorption edge due to ZnO appears at ∼3.25eV. Cross-sectional transmission electron microscopy confirms the formation of ZnO NPs of 5–10 nm in diameter within the near-surface region of ∼80nm thick and larger ZnO NPs on the surface. Under He–Cd laser excitation at λ=325nm, an exciton luminescence peak centered at 375 nm with FWHM of 113 meV was observed at room temperature.