Carrier Jumps Research Articles

Nanoscopy of Single Charge Carrier Jumps in a Conjugated Polymer Nanoparticle

We present super-resolution (∼2 nm spatial resolution per frame, 1 ms time resolution) tracking of single charge carriers in nanoparticles of the conjugated polymer PFBT. The motion of the charge carrier is determined from fluctuations in the centroid position of the particle fluorescence spot arising from fluorescence quenching by the polaron. A single polaron is observed to hop between a few sites, consistent with dispersive charge transport in a disordered energy landscape. In some shorter segments of the trajectory, there is repeated hopping between two sites, whereas for longer segments and full trajectories, random walk-like behavior consistent with multiple sites is observed. The hopping times range from a few milliseconds to seconds, following a power law distribution, while the hopping distances range from 2 to 5 nm, roughly following an exponential distribution. From the polaron hopping time distribution, we estimate a barrier height of 430–570 meV, with a nearest-neighbor distribution ranging f...

A compact model of organic thin-film transistors for display device

Organic transistor has been widely studied for next generation flexible semiconductor process. In this paper, we proposed the organic thin-film transistors (OTFT) model for the display device at the moderately high temperature (above 273K). The model based on hopping theory of Gaussian disordered density-of-states (DOS) considers the dependences relationship of the carrier mobility with temperature and carrier concentrations. Charge transport is controlled by carrier jumps from trap states around the Fermi level to tail trap states at large carrier concentrations. The sub-threshold model is modified using an exponential function to make the model cover all regimes of OTFT operation. A pentacene-based thin film transistor (TFT) model has been designed using Verilog-A language. The model has been verified by device simulations and measurements.

Conductive layers in diamond formed by hydrogen ion implantation and annealing

High conductivity is extremely difficult to obtain in diamond due to its wide band gap and low solubility of dopands. The goal of the investigation was to form a conductor inside HPHT synthetic diamond plates with initial high sheet resistivity ρs (∼1012Ω/sq) for 400μm thickness. We used metastable character of diamond structures relative to the graphitization of defective layers formed by 50keV hydrogen molecular ions at high fluence Φ=(1−13)×1016cm−2 ion implantation. High temperature (HT) (500–1600°C) and vacuum or high pressure (VP/HP) (3×10−3/4×109Pa) thermal annealing were chosen to provide the annealing regimes where the graphitic carbon is the most stable phase. Sheet resistance, dropped down up to nine orders of magnitude (ρs∼103Ω/sq), as well as Raman spectroscopy, and AFM measurements were used to determine electrical, optical and geometrical properties of multilayered heterostructures formed in the set of experiments. Temperature dependences of the conductivity show, that after highest fluencies and annealing temperatures the conductivity is quasimetallic and electronic system is above metal–insulator transition (MIT). At lower fluences and/or annealing temperatures the system is under MIT with the transport of charge carriers being well described by variable range hopping (VRH) mechanism with variable decay length of wave function for localized states. Two or three order of magnitude differences in the conductivity in VP and HP annealed samples are attributed with the higher dimensions of graphite nanocrystals in the case of vacuum annealing. This suggestion coincides with Raman spectra and optimum hopping length for carrier jumps in VRH model for conductivity in the buried layers.

Microscopic study of carrier transport in the organic semiconductor zinc‐phthalocyanine

AbstractNominally undoped zinc‐phthalocyanine (ZnPc) was investigated using Muon Spin Rotation (μSR) to probe microscopic carrier transport properties. The study focused on the relaxation of the positive muon's polarisation produced by spin‐flip scattering with charge carriers. An energy of 71(8) meV was found for the temperature activation of carrier jumps, a value that does not match the activation energies known in ZnPc from electrical measurements, and that was attributed to a fast transport component in this material. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of backward carrier jumps on variable-range hopping in disordered materials

A variable-range hopping model is described that accounts for the effect of back-and-forth carrier jumps between occasionally close hopping sites isolated from the rest of the hopping network. It is shown that accounting for this effect for constant density of states (DOS) does not change the Mott ${T}^{\ensuremath{-}1/4}$ law but yields a modified coefficient that can be considered as a percolation threshold for dc conductivity in a positionally and energetically disordered hopping system. In a system with an exponential DOS distribution, the backward jumps significantly reduce dc conductivity but only slightly affect its functional dependence upon temperature.

Crystallization-water-stimulated dynamic conductivity

In water-molecule-doped barium peroxide and barium oxide, a step increase in the dynamic conductivity to ∼10−3 Ω−1 cm−1 was found. The increase is observed when water molecules are present in two nonequivalent states in the lattice, with concentrations of the molecules n t of ≥2.2×1021 cm−3. At n>n t , the conductivity does not depend on the number of molecules in the lattice but is temperature-dependent, obeying the law σ(T = C 1exp(−E 1/kT) + C 2exp(−E 2/kT. The run of the σ(n, T) curve is explained by trapping electrons that result from H2O dissociation and by two sorts of carrier jumps between localized and delocalized states.

Some remarks on ac conduction in disordered solids

Alternating current conduction in disordered solids is discussed, from a general point of view. As regards experiment, it is argued that the observed power-law behavior of the frequency-dependent conductivity, σ(ω), is probably not fundamental, that the Ngai relation between dc and ac conductivity activation energies follows from independent experimental facts, that the shape of the modulus peak has no fundamental significance, and that there are interesting mechanical analogies to the observed ac electrical behavior. As regards hopping models for ac conduction, it is shown that three commonly used arguments against the existence of a distribution of activation energies are all incorrect. Also, it is shown that σ( ω) ≠ σ(0) only if there are correlations in the directions of different charge carrier jumps; in particular this result implies σ( ω) = σ(0) for all frequencies in the continuous time random walk (CTRW) model. In the final section a number of open problems are listed, and suggestions are made for future work.

Dielectric properties of slightly hydrated collagen: time-water content superposition analysis.

AbstractThe frequency dependences of the dielectric constant, ε′, and the loss factor, ε″, in collagen were measured at several water contents from 0.1 to 0.3 g/g collagen over a frequency range of 30 Hz to 100 kHz and at a temperature of 20°C. Remarkable dispersion was observed at the lower frequencies for higher water contents. According to accumulated results on the thermodynamic and structural investigations, the dispersion has some analogy to the surface conduction proposed by B. V. Hamon [(1953) Aust. J. Phys. 6, 304–315]. An empirical relation bewteen ε″ and frequency, f, ε″ ∝︁ fn, where 0 < n < 1, suggests that the dielectric and conductive properties of hydrated collagen are related to carrier jumps between neighboring sites. For the polarization mechanism of this dispersion, we supposed a model of the transfer of protons between absorbed water molecules, and found that the time–water content superposition procedure is applicable to slightly hydrated collagen. The results derived from the superposition procedure show that the water content, ϕ, is related to the conductivity, σ, or the dielectric loss factor by the following equations: σ (ϕ, f) = const. × ϕnm−1f1−n and ε″ (ϕ, f) = const. ϕnmf−n, respectively, where m is a constant independent of ϕ and f. These results agree with that derived by another treatment of the same data. The role of water molecules in the conduction and polarization in slightly hydrated collagen is considered to be not far from that assumed in the model.

Time of flight of carriers in simple aromatic hydro-carbon layers I: Theory

It has been found that the drift velocity of carriers in polycrystalline layers of simple aromatic hydrocarbons increases proportionally to the electric field and saturates at a field of 2×10 6 V m −>1. In addition it has been inferred that the predominant conduction mechanism in polycrystalline simple aromatic hydro-carbon layers is a hopping process between localized states within the “impurity” band at the Fermi level. A simple model to explain the phenomenon was put forward earlier. In this paper the problem is discussed more exactly, using the assumption that so-called “difficult” jumps of carriers play a significant role in the transport of carriers in polycrystalline simple aromatic hydrocarbon layers.

A physical process for 1/f noise in thin metallic films

The concept of power spectra for nonstationary stochastic processes is examined and a definition of ``expected power spectrum'' is employed. This allows a derivation of the spectrum for a simple process where the current arises in a conductor consisting of a collection of sites by virtue of carrier jumps. If the noise may be described by a series of noise generators associated with each volume element of the conductor we are led to a spectrum essentially in accord with the experimental observations of noise in thin films subject to a certain condition. It is shown that this condition can be satisfied if a stochastic model is chosen in which the conduction takes place by a tunnelling mechanism between small islands of conductivity.