Tunneling Of Charge Carriers Research Articles

The properties of structures based on oxidized porous silicon under the effect of illumination and a gas environment

The effect of illumination and adsorption of polar molecules on the properties of structures based on oxidized porous silicon has been investigated. The specific features of these structures are fluctuations and a change in the sign of open-circuit voltage under the influence of long-term photoexcitation, time-related instabilities of the current in media with a high content of hydrogen sulfide, long periods of photocurrent decay after switching off the illumination, and the occurrence of negative photoconductivity in structures with space-charge-limited currents. The results obtained are explained by the presence of multiply charged traps of different types in oxidized porous silicon, their charge exchange, and the tunneling of charge carriers between silicon nanocrystallites.

Effect of a cluster structure on the interaction of electron and magnetic subsystems in LaCa(Sr)MnO epilayers

The structure, electrical, and magnetic properties of epitaxial LaCa(Sr)MnO single crystal films with a clustered structure have been studied. In films with a “metallic” phase content C m 0 ≤0.15, the electric conductivity is determined by the spin-dependent tunneling of charge carriers between “ metallic” clusters, and the magnetoresistance is maximum at T = 4.2 K. The correlated motion of carriers over the system of tunneling-linked clusters leads to the formation of a window in the Coulomb blockade. The interactions between atomic, magnetic, and electron subsystems increase in the vicinity of the dielectric-metal percolation transition (T = 200–210 K), where the metal phase content C m in the samples with C m 0 ≥0.2 reaches a maximum (C m crit = 0.5) due to an increase in the cluster size upon cooling. In this case, the magnetoresistance exhibits a maximum at T = 260 K, on the dielectric side of the percolation transition. Due to the presence of space charge regions at the periphery of the clusters, the content of a ferromagnetic phase is 1.5–2 times that of the “metallic” phase. For this reason, the calculations are performed using a model combining the tunneling conductivity mechanism with the percolation approximation for the description of magnetization. Allowance for the Coulomb interaction between charge carriers and clusters improves the agreement of theory and experiment.

Tunneling of spin-polarized charge carriers in La0.8Ag0.1MnO3+δ film with variant structure: Magnetotransport and magnetooptical data

Epitaxial La0.8Ag0.1MnO3+δ film with variant structure on ZrO2(Y2O3) substrate was grown for the first time, and the optical, magnetooptical, and transport properties of this film were studied in order to elucidate special features in the mechanism of conductivity and magnetoresistance (MR) in this film. An original method has been developed for separating the MR contributions related to the colossal MR near TC and the tunneling MR component. This method is based on the comparative analysis of data on the MR and the IR magnetotransmission of the film. It is established that the tunneling MR component is related to the large-angle boundaries of structural domains and its temperature dependence is described by the function \(\Delta \rho /\rho \sim (a + b/\sqrt T )\). The degree of spin polarization of charge carriers in the La0.8Ag0.1MnO3+δ film with variant structure amounts to P ∼ 0.5.

Transport mechanism in mica and SiO2 dielectrics

AbstractThe current–voltage (I–V) characteristics in mica have been measured at different temperatures. The I–V characteristics possessed a temperature dependence, which was more clearly evident at lower fields. A comparison of the experimental results with the Frenkel thermo‐emission theory and the theory of multiphonon‐assisted tunnelling of charge carriers from the impurity centers, has been performed. It is shown that the experimental data better agree with the phonon‐stimulated tunnelling theory than the Frenkel emission theory. Temperature‐dependent I–V characteristics of M–SiO2–Si structures measured by Waters and Van Zeghbroeck [Appl. Phys. Lett. 76(8), 1039 (2000)] are also reinterpreted in terms of the phonon‐assisted tunnelling processes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical conductivity and low field magnetoresistance in polycrystalline La 1− xK xMnO 3 pellets prepared by pyrophoric method

Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La 1− x K x MnO 3 polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature ( T C) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature ( T> T MI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50< T< T MI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below T C is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.

Nature of electrical conduction in potassium-substituted lanthanum manganites between 10 and 300 K

The temperature dependence of the electrical resistivity in potassium-substituted LaMnO 3 prepared by a novel pyrophoric method has been investigated between 10 and 300 K for H = 0 and 0.8 T. K substitution enhances the conductivity of this system. Curie temperature ( T C) also increases from 260 to 309K with increasing K content. Above the metal-insulator transition temperature ( T MI), the electrical resistivity is well represented by adiabatic polaronic model, while in the ferromagnetic region at low temperatures, the resistivity data show a nearly perfect fit for all the samples to an expression containing the residual resistivity, a two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Based on a scenario that the doped manganites consist of phase-separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K could be described very well. All the K-doped samples clearly reveal the existence of a resistivity minimum, which is strongly influenced by an applied magnetic field ( H = 0.8 T ). The results have been analyzed in terms of the model-based on charge carrier tunneling between antiferromagnetically coupled grains, and an excellent agreement between the experimental data and those calculated from the model is obtained for all the samples.

Transport in tunneling recombination junctions: A combined computer simulation study

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (μc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si∕a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gill’s law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (μc-Si) are the most promising of all examined structures.

Determination of optimal insulator thickness for MISiC hydrogen sensors

Response mechanisms of hydrogen sensor based on a metal–insulator–SiC (MISiC) Schottky-barrier diode are analyzed. A physical model is established for the hydrogen sensor by combining thermionic emission with quantum-mechanical tunneling of charge carriers, and considering hydrogen-induced barrier-height modulation. Simulated results are in good agreement with experimental data. Relation between device performance and insulator thickness is investigated using the proposed model, and the optimal range of insulator thickness can be determined by taking into account the tradeoff between device sensitivity, reliability and resolution for high-temperature applications.

Entangled photons from small quantum dots

We discuss level schemes of small quantum-dot turnstiles and their applicability in the production of entanglement in two-photon emission. Due to the large energy splitting of the single-electron levels, only one single electron level and one single hole level can be made resonant with the levels in the conduction band and valence band. This results in a model with nine distinct levels, which are split by the Coulomb interactions. We show that the optical selection rules are different for flat and tall cylindrically symmetric dots, and how this affects the quality of the entanglement generated in the decay of the biexciton state. The effect of charge carrier tunneling and of a resonant cavity is included in the model.

Many-electron Coulomb correlations in hopping transport along layers of quantum dots

Experimental data are analyzed on the hopping transport of holes in two-dimensional layers of Ge/Si(001) quantum dots (QDs) under conditions of the long-range Coulomb interaction of charge carriers localized in QDs, when the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the parameters of hopping conduction significantly deviate from the predictions of the model of one-electron excitations in “Coulomb glasses.” Many-particle Coulomb correlations associated with the motion of holes localized in QDs play a decisive role in the processes of hopping charge transfer between QDs. These correlations lead to a substantial decrease in the Coulomb barriers for the tunneling of charge carriers.

Addition of Nb2O5 on the electrical properties of buried resistors in low-temperature cofired ceramics

Nb2O5 was added to buried resistors for low-temperature cofired ceramics, and the electrical properties of the resultant resistors were examined. Remarkable increases in electrical resistivity and attractive decreases in the temperature coefficient of resistance (TCR) were observed by the addition of Nb2O5, which was attributed to the high solubility of Nb2O5 into the PbO-SiO2-Al2O3 matrix glass. With higher dissolved contents of Nb2O5 into the glass, the resistivity of buried resistors increased by approximately six-fold magnitude, while TCR decreased substantially toward zero. It was indicated that the conductance for these buried resistors was limited by tunneling of charge carriers through the thin glass layer penetrating into the ruthenium-oxide agglomerates. A larger separation observed between RuO2 particles due to high solubility of Nb2O5 in the glass increased the charging energy (E) and lump term (Rbo), which in turn gave rise to a higher resistivity and a lower TCR value.

Time dependence of tunnel statistics and the energy resolution of superconducting tunnel junctions

Multiple tunneling of quasiparticle charge carriers in a superconducting tunnel junction (STJ) enhances the signal generated by a photon absorption event. It is also an additional source of noise, responsible for a substantial degradation of the energy resolution. Although tunneling is a binomial chance process, governed by a constant tunneling probability, the resulting cumulative statistics of tunnelled quasiparticles depend on time. In particular, the variance of the total number of tunneled quasiparticles reaches a minimum after a finite integration time, corresponding to a minimum in the spectral linewidth. Since the intrinsic energy resolution of the present generation of STJs is mainly limited by the scatter on the number of tunneled quasiparticles, the improvement of the tunnel noise can be experimentally tested by variation of the pulse integration time. An analytical theory is developed that describes the relation between the tunnel noise and the transfer function of the pulse integration hardware for an STJ characterized by a quasiparticle tunnel and loss time in each electrode. We present experiments that demonstrate that the noise contribution from multiple tunnelling is not constant during the time that the quasiparticles are present in the STJ, and that by proper filtering of the STJ pulses the tunnel noise can be optimized at a level which lies well below the canonical tunnel limit.

Intrinsic tunneling in cuprates and manganites

The most anisotropic high temperature superconductors like Bi 2Sr 2CaCu 2O 8 (BSCCO), as well as the recently discovered layered manganite La 1.4Sr 1.6Mn 2O 7 (LSMO) are layered metallic systems where the interlayer current transport occurs via sequential tunneling of charge carriers. As a consequence, in BSCCO adjacent CuO 2 double layers form an intrinsic Josephson tunnel junction while in LSMO tunneling of spin polarized charge carriers between adjacent MnO 2 layers leads to an intrinsic spin valve effect. We present and discuss interlayer transport experiments for both systems. To perform the experiments small sized mesa structures were patterned on top of single crystals of the above materials defining stacks of a small number of intrinsic Josephson junctions and intrinsic spin valves, respectively.

The nature of the low-temperature minimum of resistivity in ceramic manganites

Electrical transport measurements were carried out on a ceramic samples of La 0.5Pb 0.5MnO 3 containing 10 at.% of Ag in a dispersed form and La 0.8Sr 0.2MnO 3. Their resistivity at zero applied magnetic field exhibits a shallow minimum at the interval of T∼25–30 K. This minimum shifts towards lower T upon applying a magnetic field ( H) and disappears at a certain field H cr, while residual resistivity decreases notably with increasing of H. It is found that the bulk-scattering model is not adequate for describing of such behavior of ρ in the presence of H. It seems that the proposed model of charge-carrier tunneling between antiferromagnetically coupled grains accounts for the results obtained in spite of significant difference in the crystallinity (grain size) of the samples.

Pressure-induced variation of the magnetic structure of the surface of La0.6Sr0.4MnO3 granules

The transport properties of textured films and tunneling junctions of La0.6Sr0.4MnO3, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La0.6Sr0.4MnO3-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.

Processing, microstructure, and electric properties of buried resistors in low-temperature co-fired ceramics

The electrical properties of ruthenium oxide based devitrifiable resistors embedded within low-temperature co-fired ceramics were investigated from −100 °C to 100 °C. Special attention was given to the processing conditions and their effects on resistance and temperature coefficient of resistance (TCR). Results indicate that within this temperature range the conductance for these buried resistors is limited by tunneling of charge carriers through the thin glass layer between ruthenium oxide particles. A modified version of the tunneling barrier model is proposed to account for the microstructure ripening observed during thermal processing. The model parameters determined from curve fitting show that charging energy (i.e., the energy required for a charge carrier to tunnel through the glass barrier) is strongly dependent on particle size and particle–particle separation between ruthenium oxide grains. Initial coarsening of ruthenium oxide grains was found to reduce the charging energy and lower the resistance. However, when extended ripening occurs, the increase in particle–particle separation increases the charging energy, reduces the tunneling probability and gives rise to a higher resistance. The tradeoff between these two effects results in an optimum microstructure with a minimum resistance and TCR. Furthermore, the TCR of these buried resistors has been shown to be governed by the magnitude of the charging energy. Model parameters determined by our analysis appear to provide quantitative physical interpretations to the microstructural changes in the resistor, which in turn, are controlled by the processing conditions.

Time-Resolved Spectroscopy of Single Quantum Dots: Evidence for Phonon-Assisted Carrier Feeding

Interactions of semiconductor quantum dots (QDs) with their nano-environment can significantly alter their electronic properties. In this work, we present results of time-resolved cathodoluminescence for single ZnCdSe QDs identifying a slow carrier feeding mechanism which leads to long living excitonic luminescence. The feeding is attributed to a phonon-assisted tunneling of charge carriers trapped in the vicinity of the QDs.

Numerical modelling of trap-assisted tunnelling mechanism in a-Si:H and μc-Si n/p structures and tandem solar cells

The trap-assisted tunnelling theory was developed to describe the tunnelling of charge carriers via bandgap energy levels in structures based on hydrogenated amorphous silicon and microcrystalline silicon. Its implementation into ASPIN numerical simulator is explained. Models that were verified on n/p single junctions were applied in the tunnel recombination junction area of a tandem solar cell. Thus, it is possible to study a multi-layer solar cell without separately simulating any of its components.

Modeling charge-carrier transport and generation–recombination mechanisms in p +n + a-Si tunnel junctions

Electrical contacts between subcells in a-Si tandem solar cells are n +p + junctions in which a highly conductive path is provided by charge-carrier tunneling transitions between extended states and the states in the gap of a-Si. A tunneling-assisted capture-emission model of carriers is developed in n +p + a-Si junctions with a high built-in electrical field in the depletion region. The pure thermal and the tunneling-assisted thermal capture and emission of carriers determine the occupancy function in the mobility gap and generation–recombination rates in the space-charge region. The developed model is applied to calculate the current–voltage characteristics of an n +p + a-Si junction, and a good agreement with measured characteristics is obtained.

Charge carrier interference in one-dimensional semiconductor rings

Interference of the ballistic charge carriers in one-dimensional (1D) rings formed by two quantum wires in the self-ordered silicon quantum wells was investigated for the first time. The charge carrier transmission coefficient, which is dependent on the carrier energy, is calculated as a function of the length and modulation depth of the parallel quantum wires. The wires can be linked to the two-dimensional reservoirs either by the common source-drain system or by the quantum point contacts. It is predicted that the conductance of a 1D ring in the first case is four times higher than in the second due to the carrier interference. The calculated dependences manifest themselves in the conductance oscillations observed in the 1D silicon rings upon varying the source-drain voltage or the external magnetic field. The results obtained made it possible to design an Aharonov-Bohm interferometer based on a 1D silicon ring in the weak localization mode; its characteristics are demonstrated in the studies of the phase coherence in the tunneling of single charge carriers through the quantum point contact.