Electron Transport In Thin Films Research Articles

Production of and Electronic Transport in Thin Films of Strontium Iridate

Production of and Electronic Transport in Thin Films of Strontium Iridate

Electron Transport in Thin Films of Polymer and Small-Molecule Acceptors Visualized by Conductive Atomic Force Microscopy

Electron Transport in Thin Films of Polymer and Small-Molecule Acceptors Visualized by Conductive Atomic Force Microscopy

Electronic transport in thin films of BaPbO3 : Unraveling two-dimensional quantum effects

Recently, perovskite related BaPbO$_3$ has attracted attention due to its hidden topological properties and, moreover, has been used as a thin layer in heterostructures to induce two-dimensional superconductivity. Here we investigate the normal state electronic transport properties of thin films of BaPbO$_3$. Temperature and magnetic field dependent sheet resistances are strongly affected by two-dimensional quantum effects. Our analysis decodes the interplay of spin--orbit coupling, disorder, and electron--electron interaction in this compound. Similar to recently discussed topological materials, we find that weak antilocalization is the dominant protagonist in magnetotransport, whereas electron--electron interactions play a pronounced role in the temperature dependence. A systematic understanding of these quantum effects is essential to allow for an accurate control of properties not only of thin films of BaPbO$_3$, but also of topological heterostructures.

Logarithmic Conductivity of Cr-C nanocomposites

The evolution of the electron transport in thin films of chromium-carbon nanocomposites is studied over the temperature range 4.2-300K by chromium concentration variation from 0.2 to 0.4. It is shown that conductivity of the nanocomposites demonstrates the power-like behavior in the two temperature intervals with a separating temperature about 20-25K. Within each temperature interval the power exponent varies from the values close to 0 up to 0.6 and decreases with increase in Cr concentration from metal-insulator transition point. The concentration dependence of the power exponent in the low-temperature interval is characterized by the existence of the expressed minimum at Cr content close to 0.28 – 0.3. The experimental data are discussed and simulated in the model of inelastic tunneling of the electrons in amorphous dielectrics in the framework of the effective medium approximation.

Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance, and its topological overtones in full-Heusler Co2MnSi thin films

We report the evolution of crystallographic structure, magnetic ordering, and electronic transport in thin films of full-Heusler alloy Co2MnSi deposited on (001) MgO with annealing temperatures (TA). By increasing the TA from 300 °C to 600 °C, the film goes from a disordered nanocrystalline phase to B2 ordered and finally to the L21 ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of ≈5.0μB per formula unit for TA=600 °C. At this stage, the films are soft magnets with coercive and saturation fields as low as ≈7 mT and 350 mT, respectively. Remarkable effects of improved structural order are also seen in longitudinal resistivity (ρxx) and residual resistivity ratio. A model based upon electronic transparency of grain boundaries illucidates the transition from a state of negative dρ/dT to positive dρ/dT with improved structural order. The Hall resistivity (ρxy) derives contribution from the normal scattering of charge carriers in external magnetic field, the anomalous effect originating from built-in magnetization, and a small but distinct topological Hall effect in the disordered phase. The carrier concentration (n) and mobility (μ) have been extracted from the high field ρxy data. The highly ordered films are characterized by n and μ of 1.19 × 1029 m−3 and 0.4 cm2V−1s−1 at room temperature. The dependence of ρxy on ρxx indicates the dominance of skew scattering in our films, which shows a monotonic drop on raising the TA. The topological Hall effect is analyzed for the films annealed at 300 °C. We find maximum topological contribution to Hall resistivity around 0.5 T while it approach to zero with increasing magnetic field. The anisotropic magnetoresistance changes its sign from positive to negative as we go from as deposited to 600 °C annealed film, suggesting a gradual increase in the half-metallic character.

Size- and Temperature-Dependent Charge Transport in PbSe Nanocrystal Thin Films

We report the size- and temperature-dependence of electron transport in thin films of PbSe nanocrystals. Upon increasing temperature over the range 28-200 K, the electron transport underwent a transition in mechanism from Efros-Shklovskii-variable-range-hopping (ES-VRH) to nearest-neighbor-hopping (NNH). The transition occurred at higher temperatures for films with smaller particles. The electron localization length, estimated from the ES-VRH model, was comparable to the nanocrystal size and scaled systematically with nanocrystal diameter. The activation energy from the NNH regime was also size-dependent, which is attributed both to size-dependent Coulomb effects and the size-distribution of nanocrystals.

Electronic transport in silicon nanocrystals and nanochains

Si nanocrystals and nanochains, prepared by material synthesis, provide a means to define nanoscale devices using growth rather than lithographic techniques. Electronic transport in thin films of Si nanocrystals is influenced strongly by single-electron charging and quantum-confinement effects, and by the grain boundary regions between nanocrystals. This paper reviews electronic transport mechanisms in Si nanocrystal materials. These include thermionic emission of electrons across grain boundaries, space charge limited current, hopping transport, and single-electron charging effects. The fabrication of single-electron devices in Si nanocrystal thin films and nanochains is considered, particularly with regards to their operation at room temperature.

Semiclassical electronic transport calculations in multilayered granular alloys

We have calculated the electrical conductivity in the current-in-plane geometry of multilayered granular alloys composed of Co clusters embedded in Ag alternating with pure Ag layers. In particular, we have paid attention to the conductivity behavior as a function of Ag layer thickness, Co clusters’ size, and degree of percolation. The electronic structure is self-consistently calculated within the unrestricted Hartree–Fock approximation using a parametrized tight binding Hamiltonian which includes a Hubbard-like term. The conductivity tensor is obtained by using the semiclassical Boltzmann equation in the anisotropic relaxation time approximation. We have used a s−d Mott-like scattering model for the electronic mean free path taking into account the Sondheimer’s picture for electronic transport in thin films. We find that the experimental conductivity behavior at coalescence can be explained through the electronic band contribution. The conductivity behavior of continuous multilayers is already attained in the very early stage of percolation, as in the experiments.

Lorentz magnetoresistance of thin films in the presence of surface scattering and domain structures

A modeling of ballistic electronic transport in thin films with diffuse surface scattering in the presence of a magnetic domain structure is performed. The conductance is obtained from the diffusive mean-square displacement of the conductance electrons. Lorentz magnetoresistance (LMR) effects for several domain structures are simulated and compared to corresponding experiments. We conclude that there is no reason to assume an intrinsic domain-wall resistance (DWR) in pure iron and cobalt films in order to explain experimental results. More generally, simulation results indicate that additional LMR related to the domain structure is best understood in terms of three separate effects, which all scale with the LMR of the homogenously magnetized film.

The current potential of Co2MnSi Heusler alloy electrodes in magnetic tunnel junctions

Heusler alloys are ferromagnetic half-metals and candidates for materials with enhanced tunnel magnetoresistance (TMR). In the article [1], Co2MnSi has been used as one electrode in a magnetic tunnel junction. The cover figure shows, together with the unit cell of Co2MnSi, the TMR characteristics at 20 K at 1 mV (red) and 10 mV (blue), respectively. Large spin polarizations of 61% and 66%, respectively, were achieved. The first author, Andreas Hütten, is working in the group of Günter Reiss at the University of Bielefeld. Their research is devoted to magnetism and spin-dependent electronic transport in thin films and nanostructures, the development of optimized magneto- and spin-electronic devices as well as of preparation methods of magnetic nanostructures. This paper is an invited presentation from the Second Seeheim Conference on Magnetism (SCM2004), held in Seeheim, Germany, 27 June–1 July 2004. The full Proceedings of this conference are published in phys. stat. sol. (c) 1, No. 12 (2004).

NONEQUILIBRIUM PHONON AND ELECTRON TRANSPORT IN HETEROSTRUCTURES AND SUPERLATTICES

Electron and phonon transport in nanostructures is characterized by interface effects and can deviate significantly from the assumption of local thermal equilibrium underlying the transport equations at macroscale. In this article, we elucidate some basic nonequilibrium characteristics for phonon and electron transport in thin films and superlattices. Based on a discussion of the thermal boundary resistance at a single interface, the nonequilibrium nature of phonon transport and the consistency of temperature definition are emphasized. Using a consistent definition for temperature, we obtain simplified expressions for phonon transport in thin films and superlattices, and give examples to illustrate the effectiveness of the approximation by comparing the thermal conductivity of thin films and superlattices obtained from solving the Boltzmann equation with the current approximation. Similar nonequilibrium processes occur for the electron transport in nanostructures. An example is given on concurrent electron and phonon transport in double heterojunction structures, considering the nonequilibrium transport processes both within the electron and phonon subsystems and between them. Finally, the ballistic-diffusive equations are introduced as an approximation to the Boltzmann equation. These equations should be applicable to transport problems from nano- to mascroscales, as well as for fast temporal processes.

Transient electroluminescence measurements on organic heterolayer light emitting diodes

We have investigated the field dependence of the electron and hole drift mobility in tris-(8-hydroxyquinoline) aluminium (Alq 3) in organic multi layer light emitting diodes (LEDs) using transient electroluminescence measurements. The electron transport in thin films of Alq 3 can be understood in terms of hopping processes. We obtain a full set of parameters for a modified Poole–Frenkel equation describing the temperature and field dependence of the electron drift mobility. For the measurement of the hole drift mobility, a special device structure was designed. We were able to determine the hole drift mobility in Alq 3 and found good agreement with the data obtained from previous time-of-flight (TOF) measurements.

Conductivity-type anisotropy in molecular solids

Thin polycrystalline films of perylenetetracarboxylic dianyhydride (PTCDA), an organic molecular solid, exhibits substantial anisotropies in its electronic transport properties. Only electrons transport in the directions along molecular planes, while mainly holes transport in the direction normal to molecular planes. A series of measurements on both field effect transistors with PTCDA active layers and light emitting diodes with PTCDA transport layers documents the anisotropy seen in the electronic transport in thin films of PTCDA.

Optical Detection of Electron Transport in Thin Films of Prussian Blue

AbstractPotassium‐iron(III)‐hexacyanoferrate(II) (Prussian blue) is deposited electrochemically as a thin film (thickness: 300 nm) on a Pt‐disk substrate electrode. The relative change in optical reflectivity of this electrode is detected time‐resolved upon galvanostatic reduction of Prussian blue to Everitt's salt and subsequent reoxidation. Considering different transport properties of electrons and cations within the film, the experimental result is fitted by numerical modelling. The specific diffusion coefficient for electrons is deduced as 5·10−11 cm2/s.

The influence of surface roughness on electronic transport in thin films

In thin films, the structure of the surfaces considerably influences the transport of conduction electrons. For mesoscopic roughnesses in the range of a few nm, this is due to the varying film thickness, which gives rise to a spatially fluctuating conductance. Moreover, microscopic roughnesses can contribute to the scattering of the electrons and therefore additionally enhance the thin-film resistivity. For a quantitative understanding of the transport in these systems, a detailed investigation of the surface roughness combined with measurements of the electronic properties are necessary. Here, we discuss STM imaging of various metal films and the application of these results to the interpretation of electronic thin-film properties. Provided reasonable resolution of STM in the nm range, a good correspondence of STM results with the electrical behaviour of growing metal films can be established. Furthermore, a detailed two-dimensional analysis allows for a calculation of the potential on current-carrying thin films. On the other hand, this method supplies reliable values for the electronic transport parameters.