Electronic Conduction Mechanism Research Articles

Conduction Mechanisms in SrTiO3 Thin Films on Silicon

ABSTRACTNew materials with high dielectric constant are currently being explored to replace silicon dioxide as gate dielectric for device scaling below 0.1 μm. With respect to conventional SiO2, these high permittivity dielectrics provide the required equivalent oxide thickness (EOT) without of further reduction of the insulator physical thickness, which is a key issue to limit gate leakage current and to maintain comparable MOSFET operation and reliability. The present paper presents preliminary results on conduction mechanisms in thin epitaxial SrTiO3 films grown by MBE on Si (100). I-V measurements were performed on Al/STO/Si structures at temperatures ranging from 40 K to 290 K. At temperatures lower than 100 K the conduction mechanism of electrons from gate electrode across the oxide barrier neither Schottky emission nor tunneling. For temperatures higher than 100 K Schottky emission occurs and barrier heights were extracted, showing an approximate linearly increase with temperature. In case of Si/STO interface, a Fowler-Nordheim tunneling mechanism was detected at 40 K and at intermediate fields. The extracted barrier heights are: 0.07 eV at Si/STO interface and from 0.26 eV (130 K) to 0.53 eV (290 K) at Al/STO interface.

Electronic conduction in multi-walled carbon nanotubes: role of intershell coupling and incommensurability

Geometry incommensurability between weakly coupled shells in multi-walled carbon nanotubes is shown to be the origin of unconventional electronic conduction mechanism, power-law scaling of the conductance, and remarkable magnetotransport and low temperature dependent conductivity when the dephasing mechanism is dominated by weak electron–electron coupling.

On an anomaly of the thermal filamentation mechanism in a high-power electromagnetic field of a strongly collisional plasma

It is shown that, under the conditions when the mean free path of electrons in a fully ionized plasma is small compared with the London plasma length, the thermal mechanism of inverse bremsstrahlung absorption and electronic conductivity has a stabilizing action on filamentation instability of high-power electromagnetic radiation, in contrast to the known case of comparatively low-intensity radiation. This new nonlinear property of a plasma is shown to be determined by a decrease in the effectiveness of plasma heating in a heating pumping field with increasing the intensity of radiation if electron oscillations have above-thermal rates.

Scanning tunnelling spectroscopic imaging of Photosystem II

Scanning probe microscopy is a useful alternative to crystallographic methods in the study of PS II structure. In particular, scanning tunnelling microscopy (STM) and spectroscopy (STS) allow high-resolution (2 - 4 A) structure determination with high-resolution (< 1 meV) electronic spectroscopy on isolated single molecules of various photosynthetic proteins including PS II, LHC II, antennae complexes and chlorophyll binding proteins. The pros and cons of STM/STS are discussed and PS II structural results using STM are compared with corresponding crystallographic results. The suggested protein organization is supported by corresponding STM of CP43-depleted cores. This work is extended to two-dimensional arrays of PS II and amorphous high-density films of small photosynthetic proteins. We also observe evidence for transmolecular electron tunnelling in these 2D PS II arrays. The main electronic conduction mechanisms in single PS II molecules are determined and a simple model described which includes all these mechanisms. This model is consistent with a time-averaged theory based on the known electron transfer processes in PS II. Single-molecule electronic properties of PS II are compared with those for other organic photovoltaics thereby demonstrating the importance of PS II as a model in biomimetic applications. Finally, the potential of STM in imaging other biomolecular components in plants is demonstrated by STM of DNA with 0.5 A resolution.

The electronic and magnetic properties of LaCrO4 and Nd1 − xCaxCrO4 (x = 0–0.2) and the conduction mechanism

The electric conductivity of monazite-type LaCrO4 and zircon-type Nd1 − xCaxCrO4 (x = 0–0.2) were characterized by measurements of dc electric conductivity and the Seebeck coefficient in the 300–600 K range. The LaCrO4 and NdCrO4 were found to be n-type semiconductors, while commonly known monazite- or zircon-type oxides are insulators. In mixed valence compounds of CrV and CrVI, Nd1 − xCaxCrO4 (x = 0.1, 0.2), hole hopping conduction arising from the mixed valency was also observed. The Nd1 − xCaxCrO4 (x = 0.1, 0.2) compounds obeyed Curie–Weiss behavior above the Neel temperature, and the observed magnetic moments for the CrV ions were in good agreement with the theoretical values. Experimental results and the calculated spin densities by the UHF method indicated that most of the unpaired electrons from CrV were localized on Cr atoms. The ab initio MO calculations for CrO43− clusters in NdCrO4 revealed that the SOMO is the degenerate dπ* state and that the LUMO is the pσ* state (O 2p origin): the SOMO forms the degenerate states at the top of the valence band and the LUMO forms a wide conduction band. For NdCrO4 and LaCrO4 the electronic conduction mechanism as semiconductors was explained by the band model. For Nd1 − xCaxCrO4 (x = 0.1, 0.2) electronic conduction was described by the band model combined with hopping conduction of holes in degenerate dπ* states. It is concluded that the electronic conductivity of these compounds is caused by an intermixing of the ligand-to-metal charge-transfer (LMCT) state into the ionic configuration.

Highly conductive electrostatically assembled porphyrazine films

The electrochemical and photoelectrochemical response of molecular films formed by (3,4-pyridyl)porphyrazine coordinated to four peripheral [Ru(bipy) 2Cl] + groups (TRPz) has been investigated, in comparison with the corresponding electrostatic assembled bilayer films generated in the presence of the tetrasulfonate phthalocyaninatecopper(II) complex (CuTSPc). Both types of films are redox conductors, but an additional electron conduction mechanism through the stacked rings π system has been observed in the bilayer films, based on impedance spectroscopy. Their combined action promotes an enhanced photoaction response for the bilayer films in the presence of dissolved dioxygen.

Study of the electronic conduction mechanism in Nb-doped SrTiO3 thin films with Ir and Pt electrodes

Strontium titanate (ST) is an attractive material for dynamic random access memory applications. Doping ST with other elements such as Nb has been shown to reduce problems such as frequency dispersion and resistance degradation associated with this material. In this report, we study the electronic conduction mechanism in Nb-doped ST films [Sr(Ti1−xNbxO3)] with Ir and Pt electrodes. Film thicknesses were of the order of 35–40 nm. It was found that the “true” leakage current satisfied both Schottky and Frenkel–Poole conduction mechanism equations for both positive and negative polarities. The effect of Nb content (x=0, 0.001, 0.01, and 0.05) on the barrier heights using both the mechanisms were determined. It was found that the barrier height decreased with increased Nb content, which was correlated with the increase in leakage current.

Electronic conduction in electropolymerized carbazole thinfilms

The electronic conduction mechanism in polycarbazole (PCz) films wasinvestigated. Electropolymerization was used to fabricate PCz thin films. Thecurrent density versus the electric field J(E) indicated an ionicpolarization. Ions were (1) quenched by the effect of a high hydrostaticpressure or (2) eliminated by dipping PCz in de-ionized water. The electronicconduction was characterized by J(E) measurements at different temperaturesand by impedance spectroscopy, and was modelled as a Schottky-type injection.A comparison of the results obtained following treatments 1 and 2 indicated alarge difference in the model parameters. This disparity was interpreted interms of differences in the effective mass due to interfacemodification.

The electronic conductivity mechanism in Bi-Sr-Ca-Cu-O glass-ceramics

The results of electrical conductivity measurements in Bi 2 O 3 -SrO-CaO-CuO glass ceramics are reported. It is shown that a sharp increase in the conductivity observed at 460°C is attributed to the crystallization of the Bi 2 Sr 2 CuO 6 (Bi-2201) superconducting phase in the glass matrix. The mechanism of electrical conductivity in this glass ceramic can he explained as a result of discontinuous metallic structures of the Bi-2201 phase which appear in the glass matrix during annealing. The low-temperature conductivity obeys the law σ(T) = σ 0 exp[(-T 0 /T) n ] with n changing from 1/4 to 1/2 with decrease in the temperature. This cross-over is related to a granular structure in which a parabolic Coulomb gap exists in the density of states N(E) near E F . We also show that the superconductivity effect should be taken into account for T 1/2 conductivity behaviour. It is shown that the Bi-Sr-Ca-Cu-O glass after a very short heat treatment at 820 C exhibits the superconducting transition. In the normal state the conductivity is typical for granular metals and the activation energy continuously decreases with decreasing temperature.

The electronic conductivity mechanism in Bi-Sr-Ca-Cu-O glass-ceramics

The electronic conductivity mechanism in Bi-Sr-Ca-Cu-O glass-ceramics

Direct observation of semiconduction and photovoltaic behaviour in single molecules of the Photosystem II reaction centre

Scanning tunnelling spectroscopy of single intact Photosystem II molecules was used to directly show that the reaction centre behaves as an organic semiconductor and photovoltaic. The main electron conduction mechanisms observed were (a) semiconduction associated with the normal step-wise linear forward electron transfer pathway OEC–Y z–P680–pheo–Q A–Q B, (b) a photovoltaic effect, (c) tunnelling to surface vibronic and delocalized states, and (d) hopping conduction between polypeptide residues. Parameters for these conduction processes were obtained for both dark and illuminated conditions. The photovoltaic behaviour of the reaction centre contrasts with the photoconductive behaviour of the associated light-harvesting complex LHC-II.

ELECTRONIC CONDUCTION IN ION IMPLANATED AMORPHOUS CARBON THIN FILMS

The electronic conduction mechanism occuring in amorphous thin films is quite complicated. In amorphous carbon films it is further exacerbated by the rich diversity of its microstructure as well as the large number of gap states present in the films. One of the main reasons for the tunability of the optical band gaps of these films not being exploited in active devices, has been the inability firstly to reduce the gap states to an acceptable level, and secondly finding suitable dopants that are electrically active at room temperature. The large number of gap states in the films further exacerbates its problems by not allowing suitable barriers (eg. Schottky) to be formed on to the amorphous carbon films. In this paper we hope to first divide the amorphous carbon thin films into two main categories. Namely, diamond-like carbon films which have a high precentage of C-C sp 3 bonding, and polymer-like carbon films that also have a high percentage of sp 3 bonding which is a mixture of C-C and C-H bonds, and have a high percentage of hydrogen as well as large optical band gaps. Recent results based on ion implantation using ions such as N, B, C will be contrasted to in-situ dopant incorporation via a gaseous source., and is shown to be a very powerful technique of modifying the conduction properties. It will be shown that in the diamond-like films the conduction properties are usually controlled via Poole-Frenkel type defect conduction, while for the polymer-like films it is more a space charge based bulk and possibly, barrier controlled process. But, due to the large band gaps of these films it is difficult to distinguish between the bulk effects and the barrier effects.

The electrical conductivity of the lower mantle phase magnesiowüstite at high temperatures and pressures

Experimental measurements of magnesiowüstite electrical conductivity at Fe/Fe + Mg 0.05 to 0.2 and Fe3+/Fe3+ + Fe2+ 0.01 to 0.7 at high pressure and high‐temperature are presented. Below 1000 K, conduction occurs by a small‐polaron process of electron hopping between ferric and ferrous sites, but above 1000 K there is a change in mechanism. This high‐temperature mechanism is postulated to be a large‐polaron process in which holes are promoted in the oxygen valence band via the reactions: 1/2 O2 = OOx + VMg″ + 2h• and FeFe• = FeFex + h•. The hole and its associated polarization field are free to move in the valence band until trapped by a ferrous ion. Activation energies for the low‐temperature, small‐polaron regime are ∼0.3 eV across the range of Fe/Fe + Mg and Fe3+/Fe3+ + Fe2+ studied, in agreement with previous studies. The high‐temperature, large‐polaron activation energy decreases with increasing Fe/Fe+Mg and decreasing Fe3+/Fe3+ + Fe2+, ranging, from 0.4 to 1.1 eV. Both regions show a small, negative activation volume (ΔVlt = −0.33 (19) cm3 mol−1; ΔVht = −0.26(69) cm3 mol−1), consistent with previous high pressure studies of electronic conduction mechanisms. A compilation of the available data shows a discrepancy between measurements at low and high‐temperatures, consistent with the new results presented here. At the temperature of the lower mantle, the dominant conduction mechanism in magnesiowüstite will be the more mobile large‐polaron process. This is less sensitive to iron content than small‐polaron conduction at Fe/Fe+Mg &lt; 0.17 (the likely compositional range of lower mantle magnesiowüstite) and has a different temperature dependence from the low‐temperature process.

Classification of Icosahedral Quasicrystals and their Approximants by the Electronic Conduction Mechanisms

AbstractIn this paper, two independent factors, electronic structure at the Fermi energy and electron scattering, both of which determine the electrical resistivity, are clearly separated by using a plot of ρ4K versus RRR (RRR = ρ4K/ρ300K) for icosahedral quasicrystals and their approximants. Each contribution of the electronic structure and the electron scattering on the electrical resistivity was systematically revealed, and the origin for the high resistivities observed in the quasicrystals and approximants is well understood by taking each effect into account. We found that the quasicrystals and approximants are classified into three groups in terms of the electron scattering mechanism, which dominates the temperature dependence of the resistivity. The temperature dependence of the electrical resistivity in the first group is well understood in terms of the Boltzmann transport mechanism, and those in the second and the third groups are in terms of the weak localization and the Anderson localization, respectively.

Novel Solid Nano Diamond/Pyrocarbon Semiconductor Materials

AbstractPorous solids made of nano diamonds cemented by pyrocarbon show a semiconductor behavior where the properties are controlled by the factor γ = mass ratio of sp2/sp3 phases. The volume electrical conductivity μ (T) was measured in the temperature range 77 - 290 K as a function of γ. By controlling the ratio γ from 0 to 50 %, the electrical conductivity will change by 12 orders of magnitude. The semiconductor activation energy depends on γ-ratio and has values between 0.03 - 0.3 eV. The variable-range hopping conduction was checked by measuring μ against T and to look for a T1/4 dependence. A positive thermoelectric power showed that NDC was p-semiconductor with a narrow band gap. The paramagnetic nanodiamonds, g = 2.0027(1) and δH = 0.86(2) mT, will still have this property in the NDC material and both the g-value and line width are not dependent on γ-ratio and temperature. The concentration of unpaired electrons is 41019 spin/g that equals to one dangling bond per nanodiamond particle. Absences of magnetoresistance and Hall potential were observed for all NDC materials. This was interpreted as resulting from a Tamm - Lifshitz - Pekar electronic conduction mechanism within onedimensional band of surface states.

Deep levels in low temperature GaAs probed by field effect deep level transient spectroscopy

Low temperature (LT) GaAs grown on semi-insulating GaAs by molecular beam epitaxy has been investigated by field effect deep level transient spectroscopy at several reverse bias fields ranging from −1×105 to −4×105 V/cm. The activation energy, capture cross section, and electron–phonon coupling parameter all have been found to be modulated by the applied field. The theories of the Schottky effect and the electron–phonon coupling effect have been considered to interpret the electron conduction mechanisms in LT GaAs.

The electronic conduction mechanism in barium strontium titanate thin films

In the literature, the Schottky emission equation is widely used to describe the conduction mechanism in perovskite-type titanate thin films. Though the equation provides a good fit to the leakage current data, the extracted values of the Richardson and dielectric constants are inconsistent with their experimental values. In this work, a modified Schottky equation is applied. This equation resolves the difficulties associated with the standard Schottky equation. Also, the electronic mobility in thin films of barium strontium titanate is reported.

Single-molecule high-resolution structure and electron conduction of Photosystem II from scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy (STM) and spectroscopy (STS) were used to obtain the first direct high resolution (∼0.3 nm) images of single isolated Photosystem II (PS II) molecules, and to determine the supramolecular organization of oxygen-evolving PS II core complexes and PS II membrane fragments including the identification, assignment, location and dimensions of the polypeptide units. Our results predict a unique structural model which we then compare with alternative models. We show that the combination of quasi-constant-height mode STM operation, STS and suitable choice of sample–substrate preparations can be used to enable investigation of the structure and function of single PS II particles under normal thermodynamic and hydration conditions without the requirement and complications of ordered PS II arrays or crystals. STS was also used to characterize single-molecule electron conduction and tunneling mechanisms in PS II including the semiconduction and photoconduction behavior of the reaction center and photoexcitation effects in the light-harvesting complex LHC II.

Electronic characteristics of the SrBi2Ta2O9–Pt junction

The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to the temperature and voltage dependencies of leakage currents in Bi-excess SBT; this is the first application of such theories to ferroelectric films and results in a generalization of the Child’s Law approximation for space-charge-limited currents. A single fitting parameter T*=315±20 K satisfied data at the voltages 3&amp;lt;V&amp;lt;4 and temperatures 300 K&amp;lt;T&amp;lt;400 K most useful for engineering device applications.

Thermoelectric Power and Hall Effect in Co–Al–O Granular Films

We have measured the thermoelectric power and Hall effect on three Co–Al–O granular films whose conduction mechanism varies from metallic to tunneling type depending on O content. We obtained a finite thermoelectric power (TEP) which decreases with increasing resistivity. In the tunneling type sample, a large field dependence of TEP correlated with the magnetoresistance has been observed. The Hall coefficient of Co–Al–O decreases with increasing sample resistivity, in contrast with the giant Hall effect reported for Ni–SiO 2 granular sample with a similar resistivity. The dominant electronic conduction mechanisms are found to be different between the giant magnetoresistance and the giant Hall effect in ferromagnetic-metal-oxide granular systems.