Dc Mobility Research Articles

Charge transport in polythiophene:fullerene:nanotube bulk heterojunction photovoltaic devices investigated by impedance spectroscopy

P3HT:PCBM bulk heterojunction devices incorporating SWNTs, which are predominantly metallic in character, have been analyzed using impedance spectroscopy to understand the effect of SWNTs on their charge carrier transport properties. SWNTs reduce the effective lifetime of injected charge carriers. Frequency dependence of capacitance and conductance of P3HT:PCBM devices show monotonic variations without any clear peak positions. Simulations of the complex admittance of the P3HT:PCBM devices under trap free space charge limited current within the framework of Scher–Montrol theory are used to qualitatively show that such characteristics are a signature of charge transport which is highly dispersive in nature. The position of peak τpeak in the imaginary part of impedance Im(Z), which is essentially same as the first transition frequency of Cole–Cole plot, has a direct relation with the effective dc mobility of charge carriers which varies with dispersion parameters. Using the dc mobility values and the voltage variation of peak frequency of Im(Z), the ratio of τdc to τpeak has been calculated. The magnitude of this ratio is indicative of the degree of dispersiveness in transport. It has been shown that, SWNTs at low concentrations tend to reduce the dispersiveness in charge transport.

Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy

THz spectroscopy has been applied to investigate the photo-induced and intrinsic conductivity in SnO2 nanowires using the Drude-Smith model. The refractive index of the nanowires was found to decrease from 2.4 to 2.1 with increasing THz frequency and the dc mobility of the non-excited nanowires was determined to be 72 ± 10 cm2/Vs. Measurements reveal that scattering times are carrier density dependent, while a strong suppression of long transport is evident. Intensity-dependent measurements provided an estimate of the Auger coefficient found to be γ = (7.2 ± 2.0) × 10−31 cm6/s.

Particle swarm approach for parameter optimization of quantum well nano structure

Analytical optimization techniques suffer from slow convergence in complex solution space. Heuristics-based swarm intelligence is an efficient alternative to analytical optimization techniques. In this paper, particle swarm optimization approach is utilized for better and efficient nano-device modeling. Mobility of two-dimensional hot electrons in modulation doped square quantum well of AlGaAs/GaAs which is determined using heated drifted Fermi–Dirac distribution function and relevant scattering mechanisms is taken as the fitness/objective function. The 2D carrier concentration, quantum well width and lattice temperature of the quantum well are taken as the input variables. The algorithm with three input variables is then utilized to get optimized values of input parameters to get desired ac and dc mobility values.

High Frequency Transport of Two-Dimensional Hot Electrons in CdS Quantum Well with Inserted Barrier Layer

High frequency transport of two-dimensional hot electrons in a CdS quantum well with an inserted barrier layer is studied in the framework of a heated drifted Fermi-Dirac distribution function for the carrier characterised by an electron temperature and a drifted crystal momentum. Carrier scattering by acoustic deformation potential, polar optic phonons and background ionized impurities are incorporated in the present calculations. The insertion of the thin barrier layer leads to a decrease in carrier scattering and to an increase in carrier mobility. The variation of ac and dc mobility with quantum well width and 2D electron concentration in the presence of field are also calculated. It was found that mobility increases with increase in both the channel width and the 2D carrier concentration. The 3 dB cut-off frequency is found to be enhanced reflecting better high frequency response upon introduction of thin barrier layer into the CdS quantum well.

Temperature dependent transient velocity and mobility studies in an organic field effect transistor

We have investigated the temperature-dependent behavior of a pentacene organic field effect transistor (OFET) by measuring dc current and voltage characteristics and the transient response of the OFET to a sharp voltage pulse applied at the source contact. Standard transistor equations were used to extract the dc field-effect mobility, μFE, and the turn on time of the OFET was used to extract a dynamic mobility, μdynamic. Temperature dependent measurements were used to extract both field-effect and dynamic activation energies, Ea,FE and Ea,dynamic, over a range of effective gate voltages in order to vary the charge carrier density in the channel. We found that at higher temperatures μdynamic>μFE, while at lower temperatures μdynamic<μFE. We attribute this varying relationship between dynamic and field effect mobilities to the fact that μFE is an average mobility of all the charge carriers in the channel, while μdynamic is a measure of the mobility of the fastest charge carriers in the channel. We also found Ea,dynamic to be greater than Ea,FE by approximately 7 meV at the same effective gate voltage. We attribute this to the fact that Ea,dynamic is extracted from transient measurements made during the process of channel formation, and therefore at lower charge carrier densities than the corresponding values of Ea,FE. We also note that Ea,dynamic is a measure of the activation energy of the most energetic charge carriers in the channel, while Ea,FE is a measure of the average activation energies of all charge carriers in the channel.

Electron transport in two dimensional electron gas formed at the heterojunction of Al xGa (1−x)N/GaN at microwave frequencies

Transport properties of the two dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN heterostructure ranging from 20 K to 300 K has been investigated theoretically considering the various scattering mechanisms like the acoustic, the piezo, the surface roughness, the alloy, the polar optical phonon and the dislocation scattering. The dc mobility is found to remain constant up to 150 K and then it decrease sharply with further increase in temperature. The ac mobility is also found to decrease with increase in the temperature. The real part of ac mobility, i.e. μ r decreases with the increase in the frequency very fast initially and then gradually attains a steady value. The imaginary part of the ac mobility μ im initially increases with the increase in the frequency and then decreases after reaching the maximum value. The value of the ac mobility reduces quite reasonably as the 2D carrier concentration increases at lower range of the frequency. At the carrier concentration of 5 × 10 17 m −2, the ac mobility remains constant through a wide range of frequencies. With the increase in the dislocation densities, the values of the ac mobility are found to decrease at the lower range of frequencies. The thermo electric power is positive at the 2D carrier concentration of 5 × 10 16 m −2, the value of which increases with the increase in the temperature and gradually attains a steady value. But the thermoelectric power at n 2D of 10 17 m −2 is found to be negative in the value. The value of ZT is found to increase with the temperature and attains the maximum value of 0.007 at 150 K and the value of ZT then decreases with increase in the temperature.

Low-temperature mobility of surface electrons and ripplon-phonon interaction in liquid helium

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction of surface and volume excitations of liquid helium, which rapidly decreases with temperature. Thus, the temperature dependence of the low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V s), where n_e is the surface electron density. The relation T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and \omega/2\pi < 30 Hz.

Hot-electron transport in graphene driven by intense terahertz fields

We investigate transport properties of graphene driven by intense terahertz (THz) fields, considering impurity, acoustic-phonon and optical phonon scatterings. The time evolutions of drift velocity and electron temperature are analyzed and the phase shift and distortion of drift velocity are demonstrated. We also examine the terahertz photoconductivity, finding that the suppression of linear dc mobility is greatly enhanced with decreasing frequency of the radiation.

A technique for determining the electric conductivity and mobility of charge carriers in lamellar semiconductor materials

A simple procedure for determining the components of the dc electric-conductivity tensor and mobility of charge carriers in lamellar semiconductor materials is proposed. The equations used to calculate the dc conductivity have been obtained on the basis of a solution to the relevant boundary-value electrodynamic problems and can be applied in studies of various types of lamellar and anisotropic semiconductor crystals. The described procedure has been tested experimentally.

Analysis of Bimodal Cavities for Microwave Hall Effect Measurements

In the field of non-contact methodology for the evaluation of carrier transport study, microwave Hall mobility technique using bimodal cavity is the most sensitive method available so far. Though there is no constraint for the use of different types or shapes of the bimodal cavities used, each cavity would have its own advantages and disadvantages. In this respect, it is very important to analyze the sensitivity of different cavities that could be used in this methodology. In this paper, different bimodal cavities [TE112 (X-band) cylindrical and TE111 (P-band) cylindrical, TE101 (X-band) cubical and a rectangular bimodal cavity (X-band resonating with TE103 and TE105 modes)] are used to evaluate the carrier mobility in various semiconductor samples. These cavities are then analyzed in terms of the sensitivity and the percentage of deviation from the DC mobility values.

Memory T cells Require Costimulation and Dendritic Cell Migration to Purge an Established Persistent Viral Infection (47.7)

Abstract Adoptive immunotherapy relies on transfer of exogenously derived pathogen-specific (memory) T cells and is a highly successful approach to purge an established persistent viral infection. For example, administration of adoptive immunotherapy to mice persistently infected with lymphocytic choriomeningitis virus (LCMV) (carrier mice) results in systemic eradication of the virus, including noncytopathic clearance of persistently infected CNS neurons. By visualizing GFP-tagged LCMV-specific memory CD8+ T cells on brain reconstructions, it was revealed that cytotoxic lymphocytes (CTL) distributed throughout the CNS within 8 days after immunotherapy. Flow cytometric and microscopic analyses revealed that conventional dendritic cells arrived in the CNS concurrent with the memory CTL and induced anti-viral cytokine production (e.g., TNF-alpha). When adoptive immunotherapy was performed in carrier mice depleted of DCs or with memory T cells deficient in TNF-alpha, the adoptively administered memory T cells failed to purge the persistent infection. We further demonstrated by employing CTLA-4-Fc blockade and CCR2 deficient recipients that memory CTL rely on costimulation and DC migration, respectively, for optimal secondary expansion and efficient viral clearance. These studies underscore the importance of DC mobility and memory CTL-DC interactions in the clearance of a persistent viral infection.

Terahertz transmission characteristics of high-mobility GaAs and InAs two-dimensional-electron-gas systems

Frequency-dependent complex conductivity of high-mobility GaAs and InAs two-dimensional-electron-gas (2DEG) systems is studied by terahertz time domain spectroscopy. Determining the momentum relaxation time from a Drude model, the authors find a lower value than that from dc measurements, particularly at high frequencies/low temperatures. These deviations are consistent with the ratio τt∕τq, where τq is the full scattering time. This suggests that small-angle scattering leads to weaker heating of 2DEGs at low temperatures than expected from dc mobility.

Mobility evaluation in transistors with charge-trapping gate dielectrics

Fast electron trapping in high-k gate dielectrics is shown to effectively increase the magnitude of the threshold voltage during the dc measurements of the drain current, which leads to underestimation of the intrinsic channel carrier mobility. An approach based on the pulse Id-Vg technique is proposed to estimate a correction factor to the dc mobility.

Validity of constant voltage stress based reliability assessment of high-/spl kappa/ devices

Charge trapping in high-/spl kappa/ gate dielectrics affects the result of electrical characterization significantly. DC mobility degradation and device threshold voltage instability and C-V and I-V hysteresis are a few examples. The charging effects in high-/spl kappa/ gate dielectric also affect the validity of conventional reliability test methodologies developed for SiO/sub 2/ devices. In this paper, we review high-/spl kappa/ materials specific phenomena that can affect the validity of constant-voltage-stress-based reliability test methods to address the direction of future reliability study on high-/spl kappa/ devices.

Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems

The many-body Monte Carlo method is used to evaluate the frequency dependent conductivity and the average mobility of a system of hopping charges, electronic or ionic on a one-dimensional chain or channel of finite length. Two cases are considered: the chain is connected to electrodes and in the other case the chain is confined giving zero dc conduction. The concentration of charge is varied using a gate electrode. At low temperatures and with the presence of an injection barrier, the mobility is an oscillatory function of density. This is due to the phenomenon of charge density pinning. Mobility changes occur due to the co-operative pinning and unpinning of the distribution. At high temperatures, we find that the electron-electron interaction reduces the mobility monotonically with density, but perhaps not as much as one might intuitively expect because the path summation favour the in-phase contributions to the mobility, i.e. the sequential paths in which the carriers have to wait for the one in front to exit and so on. The carrier interactions produce a frequency dependent mobility which is of the same order as the change in the dc mobility with density, i.e. it is a comparably weak effect. However, when combined with an injection barrier or intrinsic disorder, the interactions reduce the free volume and amplify disorder by making it non-local and this can explain the too early onset of frequency dependence in the conductivity of some high mobility quasi-one-dimensional organic materials.

Intervalenceband and plasmon optical absorption in heavily doped GaAs:C

By using direct numerical-solution techniques for the reflectance (R) and transmittance (T) equations of a multilayer structure, we have analyzed infrared R and T measurements on heavily doped p-type GaAs:C films grown by molecular beam epitaxy. The optical properties, for films with hole concentrations up to 1.4×1020 cm−3, were determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband) and intervalenceband contributions are in competition. Our results for the optical absorption coefficient resolve two separate peaks located (at high doping) at about 0.1 and 0.2 eV. By carrying out calculations of the intervalenceband (IVB) absorption processes for our dopings, we identify the peak near 0.2 eV with light-hole to heavy-hole IVB transitions, and we attribute the lower-energy peak to hole-plasmon excitations. Our experimental absorption spectra are very well described by a model combining the IVB contribution to the dielectric function with a plasmon contribution. The hole-plasmon parameters we obtain for highly doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled infrared mobility implied by the use of the usual effective-plasmon model) is in substantial agreement with the dc mobility.

Modulation of dendritic cell phenotype and mobility by tumor cells in vitro

To gain new insights into the functional interaction between DC and neoplastic cells, we have analyzed the effects of melanoma and colorectal cancer lines on the chemotaxis and the phenotype of monocyte-derived DC in vitro. Both types of tumor cells displayed effective chemoattractive capacity towards immature, but not mature DC. Furthermore, conditioned medium of discrete melanoma lines induced upregulation of CD80, CD86, MHC class I, and MHC class II molecules on immature DC. However, de novo expression of E-cadherin and strong upregulation of CD15 could also be detected in the absence of CD83 expression. Melanoma-conditioned DC exhibited an increased adhesion capacity to a melanoma cell line in vitro and did not migrate in response to SLC chemokine. Tumor-infiltrating CD15 + cells displaying DC morphology could also be detected by immunohistochemistry in the original tumor specimens from which discrete melanoma cell lines under investigation were derived. Colorectal cancer cell lines, although able to chemoattract immature DC, were apparently unable to modulate their phenotype. Altogether our results suggest that tumor cells can attract immature DC in vitro and, eventually, modulate their phenotype. As a result, DC mobility could be severely impaired.

Infrared studies of hole-plasmon excitations in heavily-doped p-type MBE-grown GaAs:C

Infrared reflectivity measurements (200--5000 ${\mathrm{cm}}^{\ensuremath{-}1})$ and transmittance measurements (500--5000 ${\mathrm{cm}}^{\ensuremath{-}1})$ have been carried out on heavily-doped GaAs:C films grown by molecular-beam epitaxy. With increasing carbon concentration, a broad reflectivity minimum develops in the 1000--3000 ${\mathrm{cm}}^{\ensuremath{-}1}$ region and the one-phonon band near 270 ${\mathrm{cm}}^{\ensuremath{-}1}$ rides on a progressively increasing high-reflectivity background. An effective-plasmon/one-phonon dielectric function with only two free parameters (plasma frequency ${\ensuremath{\omega}}_{p}$ and damping constant $\ensuremath{\gamma})$ gives a good description of the main features of both the reflectivity and transmittance spectra. The dependence of ${\ensuremath{\omega}}_{p}^{2}$ on hole concentration $p$ is linear; at $p=1.4\ifmmode\times\else\texttimes\fi{}{10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3},$ ${\ensuremath{\omega}}_{p}$ is 2150 ${\mathrm{cm}}^{\ensuremath{-}1}.$ At each doping, the damping constant $\ensuremath{\gamma}$ is large and corresponds to an infrared hole mobility that is about half the Hall mobility. Secondary-ion mass spectroscopy and localized-vibrational-mode measurements indicate that the Hall-derived $p$ is close to the carbon concentration and that the Hall factor is close to unity, so that the Hall mobility provides a good estimate of actual dc mobility. The observed dichotomy between the dc and infrared mobilities is real, not a statistical-averaging artifact. The explanation of the small infrared mobility resides in the influence of intervalence-band absorption on the effective-plasmon damping, which operationally determines that mobility. This is revealed by a comparison of the infrared absorption results to Braunstein's low-$p$ $p$-GaAs spectra and to a $\mathbf{k}\mathbf{\ensuremath{\cdot}}\mathbf{p}$ calculation extending Kane's theory to our high dopings. For $n$-GaAs, which lacks infrared interband absorption, the dc and infrared mobilities do not differ.

Comparative study of hole transport in poly(p-phenylene vinylene) derivatives

The dc mobility of holes in four poly(p-phenylene vinylene) (PPV) derivatives (three fully conjugated polymers with different side chains and one partially conjugated PPV) is examined as a function of temperature T and applied electrical field E. In all cases the mobility $\ensuremath{\mu}$ follows the empirical $\ensuremath{\mu}\ensuremath{\propto}\mathrm{exp}\ensuremath{\gamma}\sqrt{E}$ law. The specific temperature and electrical field dependence of the mobility is fitted within a (correlated) Gaussian disorder model. From the fits the energetic disorder, localization length, and average transport-site separation are determined. In the case of the fully conjugated polymers the different T and E dependencies of $\ensuremath{\mu}$ are completely determined by the energetic disorder. The relation between the obtained microscopic transport parameters and the specific chemical composition of the polymer material is discussed.

Frequency-dependent electrical response of holes in poly(p-phenylene vinylene)

Transport of holes in poly$(p$-phenylene vinylene) has been studied as a function of frequency, temperature, dc bias, and polymer thickness. For such a low mobility material we demonstrate that the specific frequency dependence of the response reflects the wide distribution in transit times of the injected carriers due to dispersive transport. The dc mobility is shown to correspond to the average of this dispersion and follows the empirical $\mathrm{ln}({\ensuremath{\mu}}_{\mathrm{dc}})\ensuremath{\propto}\sqrt{E}$ law.