Further studies of electroforming effects in mim structures

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Thin film sandwich structures of metal-SiO/B203- metal with the dielectric thickness in the range 400 to 4000 A have been prepared by thermal evaporation in a vacuum of less than 10-5 torr. The metal electrodes used were Cu,, Ag and Al. After the electroforming process the devices showed a voltage-controlled negative resistance (VCNR) and emission of electrons into a vacuum. Devices with Cu electrodes were found to give the most suitable characteristics. The effect of air pressure and temperature on VCNR were studied. The vacuum-electroformed samples exhibited a pressure-voltage memory effect when operated in air at atmospheric pressure and the highimpedance state induced then could be erased by reducing the air pressure and by applying a bias voltage exceeding the threshold voltage, VT. Thermal-voltage memory effects were investigated in devices carrying Cu electrodes and the time-dependent transitions of the high-impedance memory states to a normal low impedance state at low temperatures were measured. The results have been explained employing the filamentary model of Dearnaley, Morgan and Stoneham. The localised defect regions on the surfaces of the samples, produced during the electro-forming process and electrical operation of the device, were investigated by means of a scanning electron microscope. The defects are believed to be mainly due to the formation of conducting filaments across the insulator and to the consequential Joule heating and electrolytic processes occurring along the filaments. A memory switching effect was observed in devices having Ag and Cu electrodes. The samples were formed initially in the atmosphere by the application of a direct voltage in series with a large resistance. The formed sample then could switch between a low conductivity 'off' state and a high-conductivity 'on' state, which has an ohmic characteristic. The observed switching behaviour is explained in terms of a monofilamentary model based on the formation of a metallic filament across the insulator during the forming process of the sample

ELECTRONIC AND THERMOELECTRIC PROPERTIES OF PURE AND ALLOYS In 2 O 3 TRANSPARENT CONDUCTORS

Electronic and thermoelectric properties of pure In 2 O 3 and In 1.5 T 0.5 O 3 (T = Sc, Y) alloys including the band gap, the electrical and thermal conductivity, Seebeck coefficient and figure of merit have been investigated using semi-classical Boltzmann transport theory. The calculated results indicated that substituting indium atoms by these dopants have a significant influence on the electronic properties of alloyed In 2 O 3 crystals. Substitution of Sc and Y atoms for In atoms increases the band gaps and Seebeck coefficient. The intrinsic relations between electronic structures and the transport performances of In 2 O 3 and its alloys with Sc and Y are also discussed

First-principles study of the optical properties of PbTiO

The optical properties of PbTiO3 were studied from first principles using the density functional theory. The dielectric functions and optical constants are calculated using the full potential–linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA). The theoretical calculated optical properties and energy loss (EEL) spectrum yield a static refractive index of 2.83 and a plasmon energy of 23.1 eV for cubic phase. The effective electron number at low energy saturates near 20 eV with the value of 18.1 for the effective electron number. In the tetragonal phase the static refractive index decreases to 2.59 and yields a plasmon energy of 22.7 eV

First principles study of CaTiO3 crystal in paraelectric and ferroelectric phases

  Electronic properties of CaTiO3 crystal in paraelectric and ferroelectric phases have been studied by first principles, using Hohenberg-Kohn-Sham density functional theory (DFT). In paraelectric phase the results show an indirect band gap of about 2eV at -R direction in the Brilluoin zone and a strong hybridization between Ti-3d an O-2P orbital. In ferroelectric phase a direct band gap of about 1eV is seen at point. Up to our knowledge no data has been reported on the ferroelectric phase so far, therefore our results might be useful for the future works

Effect of ferroelectric substrate on carrier mobility in graphene field-effect transistors

Effect of LiNbO3 ferroelectric substrate on the carrier mobility in top gated graphene field-effecttransistors (G-FETs) is demonstrated. It is shown that, at the same residual concentration of thecharge carriers, the mobility in the G-FETs on the LiNbO3 substrate is higher than that on the SiO2/Si substrate. The effect is associated with reduction of Coulomb scattering via screening thecharged impurity field by the field induced in the ferroelectric substrate, but significant only formobilities below 1000 cm2/V s. Raman spectra analysis and correlations established between mobilityand microwave loss tangent of the Al2O3 gate dielectric indicate that the charged impuritiesare located predominantly at the gate dielectric and/or at the gate dielectric/graphene interface andare likely associated with oxygen vacancies