Main Charge Transport Mechanism Research Articles

Mechanism of In-Plane and Out-of-Plane Tribovoltaic Direct-Current Transport with a Metal/Oxide/Metal Dynamic Heterojunction.

Interfacial layer engineering has been demonstrated as an effective strategy for boosting power output in semiconductor-based dynamic direct-current (DC) generators, although the underlying mechanism of power enhancement remains obscure. Here, such ambiguity has been elucidated by comparing fundamental tribovoltaic DC output characteristics of prototypical metal-oxide-metal heterojunctions prepared by atomic-layer deposition (ALD) with a vertical (out-of-plane carrier transport through the interfacial layer) and a horizontal (in-plane carrier transport along the interfacial layer) configuration such that the influences from nonequilibrium electronic excitation and interfacial capacitive amplification can be individually tuned and investigated. It is found in the case of Al/TiO2/Ti vertical configurations that the open-circuit voltage (VOC) increases linearly from -0.03 to -0.52 V as the thickness of titanium oxide (tTiO2) increases from 0 to 200 nm with a linear amplification coefficient of -2.31 mV nm-1, which is validated by a parallel-capacitor theoretical model with tribovoltaic electronic excitation. In contrast, the VOC output with the horizontal configuration is ∼55 mV, where the potential difference is merely associated with the accumulation of surface charges and the subsequent charge rearrangement in the depletion region. Meanwhile, it is measured that the short-circuit current density (JSC) shows an initial increasing trend when tTiO2 increases, reaches its peak value at 0.21 A m-2 at tTiO2 = 20 nm, and then decreases as tTiO2 increases further. From current-voltage (I-V) characterization, it is proposed that such DC output variation with an optimal interfacial layer thickness stems from the competition of amplified voltage and increased resistance with increasing interfacial layer thickness, with the main charge transport mechanism switching from quantum tunneling to thermionic emission/trap-assisted transport. In contrast, tribovoltaic excitation is proven to be significantly weaker when a wide band-gap insulator (Al2O3) is involved. The elucidation of the fundamental mechanism of power enhancement by the interfacial layer in this work is of great significance in providing instructional direction for the development and optimization of high-performance DC nanogenerators.

Charge transport mechanism in the metal–nitride–oxide–silicon forming-free memristor structure

Abstract Metal-nitride-oxide-silicon structures that exhibit memristor properties were obtained using the low-pressure chemical vapor deposition at 700° C. The fabricated metal-nitride-oxide-silicon memristor structure does not require a forming procedure. In addition, the metal-nitride-oxide-silicon memristor has a memory window of about 3 orders of magnitude. In our work, the charge transport of high and low resistive states in a metal-nitride-oxide-silicon memristor is analyzed with two contact-limited models and six bulk-limited charge transport models. It is established that the Schottky effect model, thermally assisted tunneling model, Frenkel model of Coulomb traps ionization, Hill-Adachi model of overlapping Coulomb traps, Shklovskii-Efros percolation model, Makram-Ebeid and Lannoo model of multiphonon isolated traps ionization and the Nasyrov-Gritsenko model of phonon-assisted tunneling between traps, quantitatively, do not describe the charge transport of metal-nitride-oxide-silicon memristor. We found that the main charge transport mechanism in the metal-nitride-oxide-silicon memristor in a high resistive state is the model of space-charge-limited current with traps. In a low resistive state, the charge transport mechanism is described by the space-charge-limited current model with filled traps.

Charge transport mechanism in the metal–nitride–oxide–silicon forming-free memristor structure

Silicon oxide and silicon nitride are two key dielectrics in silicon devices. The advantage of Si3N4 over other dielectrics is that silicon nitride is compatible with silicon technology. It is required to study in detail the charge transport mechanism in a Si3N4-based memristor to further improve the cell element and to create a matrix of these elements. Despite many research activities carried out, the charge transport mechanism in Si3N4-based memristors is still unclear. Metal–nitride–oxide–silicon structures that exhibit memristor properties were obtained using low-pressure chemical vapor deposition at 700 °C. The fabricated metal–nitride–oxide–silicon memristor structure does not require a forming procedure. In addition, the metal–nitride–oxide–silicon memristor has a memory window of about five orders of magnitude. We found that the main charge transport mechanism in the metal–nitride–oxide–silicon memristor in a high resistive state is the model of space-charge-limited current with traps. In a low resistive state, the charge transport mechanism is described by the space-charge-limited current model with filled traps. Trap parameters were determined in the Si3N4-based memristor in the high resistive state.

Charge transport in a system of cholesterol molecules deposited on graphene oxide using current sensing atomic force microscope

We have made a system consisting of cholesterol (Ch) molecules transferred onto graphene oxide (GO) by Langmuir-Blodgett technique on highly ordered pyrolytic graphite substrate. We have used current sensing atomic force microscope to study the current (I) - voltage (V) characteristics of Ch-GO system. To prepare Langmuir film of GO we have tuned its amphiphilicity by adjusting pH of the subphase. We find quantum tunneling to be the main mechanism of charge transport and it changes from direct to injection tunneling with increase in the applied bias voltage. Our finding can be useful for fabrication of molecular electronic devices.

Experimental insight into the magnetic and electrical properties of amorphous Ge1-xMnx

We present a study of the electrical and magnetic properties of the amorphous Ge1-xMnx.DMS, with 2% ≤ x ≤ 17%, by means of SQUID magnetometry and low temperature DC measurements. The thin films were grown by physical vapour deposition at 50°C in ultrahigh vacuum. The DC electrical characterizations show that variable range hopping is the main mechanism of charge transport below room temperature. Magnetic characterization reveals that a unique and smooth magnetic transition is present in our samples, which can be attributed to ferromagnetic percolation of bound magnetic polarons.

Leakage currents in porous PZT films

ABSTRACTCharge transport phenomena in porous ceramic PZT films are discussed. The leakage current has been found to be in PZT porous films is 1–1.5 orders higher than for the dense film throughout the whole bias voltage. In the low fields region (∼40–160 kV/cm) the main charge transport mechanism is the ohmic conductivity. Static (steady state) conductivity of porous PZT films is 1–2 orders higher than of dense film. The main possible charge transport mechanism in the high fields region (≥160–180 kV/cm) is the space-charge-limited conduction.

Conduction mechanisms in P(VDF-TrFE)/gold nanowire composites: tunnelling and thermally-activated hopping process near the percolation threshold

High-aspect ratio gold nanowires were dispersed in a P(VDF-TrFE) matrix to form conducting polymer composites. The composites were found to follow a percolation law, with a low percolation threshold of 2.2%vol and attaining a conductivity value of 100 S m−1. The temperature and frequency dependence of the composites were studied using broadband dielectric spectroscopy. Tunnelling was found to be the main charge transport mechanism at temperatures below −100 °C and a thermally-activated hopping mechanism was determined to be responsible for conduction at temperatures above −100 °C. The correlated barrier hopping model was found to be the best fit to explain the conduction mechanisms in the composites.

Some electrical and photoelectric characteristics that are promising for photoconverters p-Cu2ZnSnS4/n-Si heterostructures

The electrical and photoelectric characteristics of the p-Cu2ZnSnS4/n-Si heterojunctions that are promising for the creation of solar cells, were studied. The conclusion about the current flow in such heterostructures was made based on the temperature dependence of the CVC. At low direct voltage, the dominant mechanism of the current transfer is tunnel-recombination processes, and at high voltages (3kT/e < U < 0.6 V), the dominant mechanism is tunneling one. In the case of a reverse bias, the main mechanism of charge transport is single-step tunneling through a potential barrier. A characteristic feature of the photo-CVC characteristic is the presence of the photoresponses in both branches of the characteristics.

Porous Si Based Al Schottky Structures on p+-Si: A Possible Way for Nano Schottky Fabrication

This paper presents the results of a comparison research between the Al/p-Si/Al, Al/p+-Si/Al and Al/PS/p+-Si/Al devices. The current transport mechanism of an Al/por-Si/p+-Si/Al heterojunction device with a porous silicon layer fabricated by the electrochemical anodization process has been investigated using temperature dependent current–voltage measurements in the temperature range of 10–300K, and these measurements are realized even for Al/p-Si/Al diode. The porosity properties of porous Si layer are investigated by a cross-sectional SEM view and a PL measurement. The good rectification for a Schottky like diode which was formed by Al metallization of porous silicon (PS) on p+-Si substrate was observed and it continues even in 6K temperature, despite Al/p-Si/Al diode freezing out at 20K. Moreover, it is reported that the Al/PS/p+-Si/Al heterojunction device reveals the excellent junction parameters with an ideality factor very close to unity, a high barrier height which is 0.63eV and a small value of series resistance that is 30Ω, at 300K. However, the high serial resistance values for this heterojunction device compared to that of the crystalline Si based on the Al/p-Si/Al diode are related to PS. At high temperatures, it is noted that the current transport mechanism is through the thermoionic-field emission mechanism. However, at low temperatures, the unreliable variation of the Richardson plot is seen, and no distinct Gaussian distribution is seen in the relation of Φb vs. q/2kT; and this behaviour is attributed to the highly doped silicon. Furthermore, in this study, the current transport mechanism is discussed by using the Fowler Nordheim tunnelling theory in PS, and it is proposed that tunnelling via the migration/diffusion is the main mechanism of charge transport in PS.

Nanoscale Electrical Investigation of Layer‐by‐Layer Grown Molecular Wires

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.

Structural analysis, magnetic and electrical properties of samarium substituted lithium–nickel mixed ferrites

A series of Sm-doped Li–Ni ferrites with formula of (Li0.5Fe0.5)0.4Ni0.6SmyFe2−yO4, where 0.0≤y≤0.1 were prepared by double sintering ceramic technique. The structure was characterized by X-ray diffraction, which has confirmed the formation of single-phase spinel structure. The samarium concentration dependence of lattice parameters obeys Vegard's law. The octahedral site radii increased with Sm content while the tetrahedral site radii decreased. Deviation from the ideal crystal structure (Δ) is found to decrease with Sm substitution, and the hopping length on the octahedral site is found to increase with Sm content. Hall measurement confirmed p-type conductivity behavior for Sm-doped ferrite and the main charge transport mechanism is hopping of halls between Ni2+ and Ni3+. Sintering at 1300°C resulted in low resistivity ferrite, which was found to increase with Sm content. Resistivity is governed by both charge carrier mobility and carrier concentration. It decreases with frequency, and this behavior with frequency is discussed according to Koop's theorem. The dielectric constant is found to decrease more rapidly at low frequencies than at higher frequencies while the dielectric constant increases with Sm content. The decrease in ε″ with frequency agrees with Deby's type relaxation process. Maximum in ε″ is observed when the hopping frequency is equal to the external electric field frequency. The variation in tanδ with frequency shows a similar nature to that of ε″ with frequency. The magnetization under applied magnetic field for the samples exhibits a clear hysteretic behavior. The scanning electron microscope (SEM) studies showed that the domain walls may tend to be trapped (pinned) by non-magnetic inclusions, precipitates and voids. The saturation magnetization (MS) increases with the sintering temperature, while the coercivity (HCi) is found to decrease.

Photoconductivity of oxidized nanostructured PbTe(In) films

Photoconductivity of as-grown and oxidized nanocrystalline PbTe(In) films has been studied in the dc and ac modes at temperatures 4.2–300 K. The electric transport in the films is defined by two mechanisms: conductivity through barriers at grain boundaries and transport along inversion channels at the grain surface. Modification of the transport mechanisms induced by oxidation is considered. Relatively weak oxidation results in an increase in the contribution of grain barriers to conductivity followed by an enhancement of the photoconductivity amplitude. Instead, this contribution drops in the case of deep oxidation resulting in a photoresponse reduction. It is shown that the main mechanism of charge transport in deeply oxidized films at low temperatures is hopping along inversion channels at the grain surface. It is demonstrated that the photoconductive response of nanocrystalline materials may be optimized by variation of the oxidation level, measurement frequency and temperature.

Analytical current–voltage relationships for electron tunneling across rough interfaces

Conductive rough surfaces separated by an insulating ultrathin film comprise the core of various engineering and scientific systems. Electron tunneling through the potential barrier imposed by the insulating film is the main mechanism of charge transport across these interfaces. The strong dependence of the tunnel current on the characteristics of the interface provides an in situ means of studying the electromechanical behavior of hindered interfaces in terms of the current–voltage response. Analytical relationships for the current density versus applied voltage that include the real tunneling area are used to characterize the electrical and mechanical properties of the interfaces of rough conductive surfaces coated by an insulating thin film. It is shown that the contact load, real contact area, surface topography, mechanical properties, and insulating film characteristics, such as thickness, dielectric constant, and potential barrier, can be determined from simple electrical measurements and the use of the present analytical approach.

Charge transport in Fe-p-InP diode structures

The mechanism of charge transport in Fe-p-InP diode structures and its dependences on illumination and magnetic field were investigated. It is shown that a double injection in a drift approximation into a high-resistivity π-layer is the main mechanism of charge transport. Phenomena of the suppression of a forward current with light (negative photoresponse) and a sharp increase in the differential resistance in a magnetic field are observed and discussed.

Chaos in resonant-tunneling superlattices.

Spatio-temporal chaos is predicted to occur in n-doped semiconductor superlattices with sequential resonant tunneling as their main charge transport mechanism. Under dc voltage bias, undamped time-dependent oscillations of the current (due to the motion and recycling of electric field domain walls) have been observed in recent experiments. Chaos is the result of forcing this natural oscillation by means of an appropriate external microwave signal.

Dynamics of parametric conical scattering of orthogonally polarized waves in BaTiO3

The temporal evolution of nonlinear scattering which is induced by simultaneous action of an ordinary and an extraordinary wave impignig on a photorefractive crystal in the plane perpendicular to the c-axis is investigated. Diffusion is presumed to be the main charge transport mechanism. The extended analysis takes into account scattered light with monotonically increasing intensity as well as transient scattering. The comparison of theoretical results with measurements of a BaTiO3 crystal yields good agreement.

Diffusion of viologens in Nafion film studied by a combined electrochemical-spectrophotometric method

The diffusion of three viologens with differing overall charges has been studied in Nafion-modified electrodes. A new technique based on spectroscopic observation of the monomer/dimer equilibria of the reduced viologens yields apparent diffusion coefficients (D app ) of the radicals in Nafion. D app is found to be dependent on the concentration of the viologen species. It is proposed that the hydrophilic channels in Nafion play a significant role in determining D app and that physical diffusion and not electron hopping appears to be the main charge-transport mechanism

1/f noise in varistors

An experimental investigation was made of low-frequency fluctuations of the open-circuit voltage of mass-produced Varistors made of silicon carbide. It was concluded that the main charge transport mechanism in varistors may be the Frenkel ionization of impurities.