Electrostatic Potential Fluctuations Research Articles

Optical and electrical properties of Te doped AlGaAsSb/AlAsSb Bragg mirrors on InP

We present a comparative study carried out on the optical and electrical characteristics of undoped and Te doped AlGaAsSb/AlAsSb Bragg mirrors with 6.5 pairs of layers and bulk undoped and Te doped AlGaAsSb epilayers alloys lattice matched on InP, grown by molecular beam epitaxy, using SIMS, photoluminescence, reflectivity and IxV techniques. The temperature dependence of PL transitions observed in the Bragg mirrors are similar to that observed in bulk samples and associated with the donor and acceptor recombinations in alloys with electrostatic potential fluctuations described by quasi-donor-acceptor-pair (QDAP) models. We verified by SIMS the presence of a macro fluctuation in the Te profile concentrations in the growth direction of the doped Bragg mirror. The influence of doping, alloy and macrofluctuation of incorporated Te on the optical and electrical properties of doped Bragg mirror is analyzed. The AlGaAs/AlAsSb seems a very attractive option for VCSEL technology.

Electrostatic potential fluctuations in a Maxwellian plasma

The spatial correlation function of a Maxwellian plasma with perturbations arising in the electrostatic potential due to random ion density fluctuations is examined. The entropy is found from the one-particle distribution function using the Shannon formula and then, using the Einstein method, the probability distribution for the electrostatic potential fluctuation is determined. This straightforward procedure is demonstrated to be a powerful tool in studying plasma correlation functions when the system entropy can be computed.

Efficiency limitations of polycrystalline thin film solar cells: case of Cu(In,Ga)Se 2

Small-area Cu(In,Ga)Se 2 thin film solar cells have reached more than 19% efficiencies. Compared to other polycrystalline materials this efficiency value is remarkable. Nevertheless, the 19% for Cu(In,Ga)Se 2 range more than 6% (absolute) below the world's best single-crystalline Si cells and almost 14% below the upper theoretical limit of 33% for an ideal black body cell with infinitely large mobility and radiative recombination only. About 4% out of the 14% are of optical nature, additional 3% stem from the limited mobility/diffusion length and from band gap fluctuations with a standard deviation σ g≈50 meV due to spatial variations of composition and stoichiometry of the quaternary compound Cu(In,Ga)Se 2. Thus, about 26% efficiency would be possible if there were only these band gap fluctuations. Additional, voltage-dependent electrostatic potential fluctuations push down the efficiency further to 19%: The polycrystalline Cu(In,Ga)Se 2 which is unavoidably structurally inhomogeneous due to dislocations, grain boundaries, point defects, etc. is also electrostatically inhomogeneous because of charged defects. Electrostatic potential fluctuations at the valence and conduction band edge may be not only responsible for a high saturation current density but also for the ideality factor in the current/voltage curve. The band gap and electrostatic potential fluctuations make the effective band gap which controls the intrinsic carrier density smaller than the average optical gap. The (zero bias) electrostatic potential fluctuations are here derived from the ideality factors of the current/voltage curve. The ideality factor reflects the voltage-induced electrostatic homogenization of the sample. For the world's best Cu(In,Ga)Se 2 cells with an ideality factor of n id=1.5, we estimate zero bias electrostatic potential fluctuations with a standard deviation σ elec≈140 meV.

Turbulence velocimetry of density fluctuation imaging data

Analysis techniques to measure the time-resolved flow field of turbulence are developed and applied to images of density fluctuations obtained with the beam emission spectroscopy diagnostic system on the DIII-D tokamak. Velocimetry applications include measurement of turbulent particle flux, zonal flows, and the Reynolds stress. The flow field of turbulent eddies exhibits quasisteady poloidal flows as well as high-frequency radial and poloidal motion associated with electrostatic potential fluctuations and strongly nonlinear multifield interactions. The orthogonal dynamic programming technique, developed for fluid-based particle and amorphous shape (smoke) flow analysis, is investigated to measure such turbulence flows. Sensitivity and accuracy are assessed and sample results discussed.

Microphotoluminescence study of local temperature fluctuations in n-type (Cd,Mn)Te quantum well

Microphotoluminescence mapping measurements were performed in a magnetic field on a (Cd,Mn)Te quantum well, modulation n-doped with iodine at about 10 10 cm −2. Photoluminescence spectra contain neutral (X 0) and negatively charged (X −) exciton lines. The Zeeman effect shows a significant role of heating of the Mn system even under lowest excitation densities. The effective temperature of the magnetic system exhibits strong fluctuations anticorrelated with the total intensity of PL signal. An interpretation of these fluctuations in terms of the influence of non-radiative recombination centers is proposed (in two alternative versions). Maps of the local X 0/X − intensity ratio indicate a minor role of electrostatic potential fluctuations.

Localization of neutral and charged excitons in (Cd,Mn)Te quantum well: a microphotoluminescence study

Microphotoluminescence mapping measurements were performed on a (Cd,Mn)Te quantum well, iodine modulation-doped at about 10 1 0 cm - 2 . Intensity maps reveal dark lines with preferential directions [110] and [-110]. We attribute them to misfit dislocations propagating from substrate/buffer interface to the quantum well, as evidenced by a better spatial resolution observed at direct QW excitation compared to that at above-barrier energy. Photoluminescence spectra contain usual neutral (X) and negatively charged (X - ) exciton lines. Besides, a defect-related line (D) is observed at the low energy tail of the X - line. The presence of D line anticorrelates with that of X line, demonstrating an efficient trapping process of excitons at the defect centres. Lack of correlation between the X - and X intensities with direct excitation in the quantum well is interpreted as an evidence of electrostatic potential fluctuations.

A detailed theoretical and experimental noise study in n-on-p Hg 0.68Cd 0.32Te photodiodes

Both the method enabling numerical modeling of fluctuation phenomena in semiconductor devices as well as experimental results of noise current in Hg 0.68Cd 0.32Te n-on-p photodiodes are presented. While the concept of the applied numerical method was derived earlier [J. Appl. Phys. 90 (3) (2001) 1318], it has been enhanced in this paper by taking into account the influence of temperature fluctuations. The method is based on the assumption that the fluctuations of the generation–recombination processes, carrier mobility, heat generation and heat transfer result in the fluctuations of carrier and ionized impurity concentrations as well as fluctuations of carrier motion and temperature. These, in turn, may be expressed by the fluctuations of electrostatic potential, quasi-Fermi levels and temperature. The fluctuations of the electrostatic potential, quasi-Fermi level fluctuations and temperature fluctuations were calculated by solving the set of “transport equations for fluctuations” in which the fluctuations of generation–recombination processes, heat transfer and mobility act as random source terms. The method was applied to a theoretical analysis of the noise current in n-on-p Hg 0.68Cd 0.32 photodiodes. The noise spectrum in selected HgCdTe photodiodes is calculated and the contribution of different noise sources is determined. Theoretical results are compared with the experimental data. The planar homojunction devices were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching plasma induced conversion process [J. Electron. Mater. 29 (6) (2000) 841].

Nonlinear behaviour of resistive drift-Alfvén instabilities in a magnetized cylindrical plasma

Nonlinear behaviour of resistive drift-Alfvén mode is described by a three-field model composed of vorticity equation, Ohm's law and continuity equation. Resistive drift-Alfvén instabilities occur in the cylindrical plasma where both resistivity and plasma β value are finite. The resistive drift-Alfvén instabilities assuming single helicity generate poloidal sheared flow via the Reynolds stress by nonlinear mode couplings. The poloidal sheared flow stabilizes several unstable modes including the most unstable mode and leads to saturation of the nonlinear resistive drift-Alfvén modes. It is noted that some harmonics are destabilized by this flow. The level of magnetic fluctuations is much smaller than that due to density and electrostatic potential fluctuations. Particle flux due to the magnetic fluctuations is also extremely small compared to that due to electrostatic fluctuations. However, it is shown that the effect of magnetic fluctuations on the poloidal flow is significant. The magnetic fluctuations play a role of suppressing the poloidal flow drive by convective nonlinearity.

Experimental study of lower-hybrid drift turbulence in a reconnecting current sheet

The role of turbulence in the process of magnetic reconnection has been the subject of a great deal of study and debate in the theoretical literature. At issue in this debate is whether turbulence is essential for fast magnetic reconnection to occur in collisionless current sheets. Some theories claim it is necessary in order to provide anomalous resistivity, while others present a laminar fast reconnection mechanism based on the Hall term in the generalized Ohm’s law. In this work, a thorough study of electrostatic potential fluctuations in the current sheet of the magnetic reconnection experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)] was performed in order to ascertain the importance of turbulence in a laboratory reconnection experiment. Using amplified floating Langmuir probes, broadband fluctuations in the lower hybrid frequency range (fLH∼5–15 MHz) were measured which arise with the formation of the current sheet in MRX. The frequency spectrum, spatial amplitude profile, and spatial correlation characteristics of the measured turbulence were examined carefully, finding consistency with theories of the lower-hybrid drift instability (LHDI). The LHDI and its role in magnetic reconnection has been studied theoretically for decades, but this work represents the first detection and detailed study of the LHDI in a laboratory current sheet. The observation of the LHDI in MRX has provided the unique opportunity to uncover the role of this instability in collisionless reconnection. It was found that: (1) the LHDI fluctuations are confined to the low-beta edge of current sheets in MRX; (2) the LHDI amplitude does not correlate well in time or space with the reconnection electric field, which is directly related to the rate of reconnection; and (3) significant LHDI amplitude persists in high-collisionality current sheets where the reconnection rate is classical. These findings suggest that the measured LHDI fluctuations do not play an essential role in determining the reconnection rate in MRX.

An effect of thermal disorder on the electronic structure of MgO

We have studied an effect of thermal disorder on the electronic structure of MgO using a combination of classical molecular dynamic simulations at different temperatures with static quantum-chemical calculations of snapshot atomic configurations. The results demonstrate that thermal fluctuations of the on-site electrostatic potential are determined by fluctuations of positions of neighbouring ions. the former are responsible for the tails in the electron density of states (DOS). The electron states from the DOS tails extended into the band gap are localised on several ions. Those corresponding to the valence (conduction) band tails have reduced (increased) effective charges. We demonstrate that the electron-phonon coupling causing charge fluctuations is stronger for acoustic mode(s).

Two-field transport models for magnetized plasmas

Two-field fluid models for low-frequency density and electrostatic potential fluctuations in a purely toroidal magnetized low-β plasma are discussed. In particular, we present some simple models suitable for investigating plasma transport close to marginal stability. For gradient-driven fluctuations, linear normal mode analysis of the interchange and resistive drift wave instabilities are reviewed, with attention to the relative phase and amplitude of density and electrostatic potential fluctuations. This reveals many characteristic features of drift-wave fluctuations that are also observed in laboratory experiments and during ionospheric irregularities. Finally, these results are used to investigate the nonlinearly conserved energy functionals, discussing the effect of magnetic field curvature and particle collisions on energy, enstrophy, and cross-correlation between density and vorticity fluctuations. Implications for turbulent fluctuations and nonlinear transport in laboratory and ionospheric plasmas are discussed.

Convergence of Molecular and Macroscopic Continuum Descriptions of Ion Hydration

Using computer simulations we study the boundary between explicit molecular and macroscopic continuum dielectric descriptions of ion hydration. The continuum model reasonably describes electrostatic potential fluctuation contributions to the hydration free energy, but fails to capture first-order potential contributions in the uncharged state. Once this contribution is accounted for, cationic and anionic free energies become indistinguishable with increasing size, in agreement with continuum model predictions. The ion radius at which the hydration free energies converge scales as the square root of the absolute charge (υ = 0.54 ± 0.04) and compares to the length scale ion−water dipole interactions equal the thermal energy.

Trion and exciton dephasing measurements in modulation-doped quantum wells: A probe for trion and carrier localization

To investigate the properties of a two-dimensional carrier gas at intermediate densities, we perform picosecond transient four-wave mixing experiments on trions (charged excitons) and neutral excitons in modulation p-doped ${\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mg}}_{x}{\mathrm{Zn}}_{y}\mathrm{Te}$ quantum wells. The determination of trion and exciton dephasing rates reveals a localization of both trions and holes in potential fluctuations induced by the ionized remote acceptors. We demonstrate that trions can be efficiently used as a charged optical probe sensitive to electrostatic potential fluctuations which are imperceptible for neutral excitons.

Effect of solvent fluctuation on the electronic transitions of formaldehyde in aqueous solution

The absorption spectra of nπ* and ππ* transitions in formaldehyde aqueous solution were studied by the reference interaction site model self-consistent-field (RISM-SCF) method. The electrostatic potential fluctuations acting on the solute sites originating from the solvent fluctuations were obtained by calculating the derivative of the solute-solvent radial distribution function analytically, and these were utilized to estimate the spectral bandwidths. The contribution from the solute vibrations was also examined. As a result, a blue shift of 1998 with bandwidth of 2987 cm−1 was obtained for the nπ* transition. The ππ* transition, on the other hand, showed a redshift of 1598 with the bandwidth of 5474 cm−1. The solvent fluctuation effect contributes to the bandwidths by 617 and 137 cm−1 for the nπ* and ππ* transition, respectively. We further analyzed the simulated absorption band shapes using effective charges on the atoms and the charge response kernel calculated for each state.

Application of wavelet spectral analysis to plasma fluctuation measurements using beam emission spectroscopy

Beam emission spectroscopy (BES) measures localized density fluctuations in fusion plasma turbulence. A wavelet transform method of spectral analysis is applied to BES data to extract fluctuations without the long time-averaging characteristic of Fourier transform spectral analysis methods. This technique is used to observe an abrupt (∼100 μs) L–H transition on DIII–D. Wavelet-based cross correlation analysis can be used to obtain fluctuations of poloidal rotation velocity, and suggests opportunity for measurement of electrostatic potential fluctuation measurements.

Transient photoconductivity study of localized-state distributions in metallophthalocyanines

The origin of an exponential distribution of localized states in metallophthalocyanines (MPcs) has been studied on cobalt phthalocyanine (CoPc) and iron phthalocyanine (FePc) dispersed in polymer binders in terms of transient photoconductivity measurements. It is found that the localized-state distributions, calculated from transient photoconductivity data, are characterized by an exponential distribution on which a Gaussian distribution is superimposed. It is also found that the characteristic temperature of the exponential distribution in CoPc and FePc is dependent on their dielectric constants. It is shown that this experimental finding is in agreement with the theoretical prediction deduced from electrostatic potential fluctuations caused by MPc+ and O2− in the MPcs. From the similarity found in the localized-state distributions of other MPcs, it is suggested that such electrostatic potential fluctuation is a common origin of an exponentia1 distribution of localized states in MPcs.

An energy estimate for a perturbed Hasegawa-Mima equation

It is commonly believed that drift waves and drift-wave turbulence play a major role in the understanding of anomalous transport at the plasma edge of a tokamak fusion reactor. A one-field equation describing the electrostatic potential fluctuations in this regime is the so-called Hasegawa-Mima equation. If this equation is driven by some instability and damped by some hyperviscous term, the energy grows exponentially in time which is not consistent with the approximations made in the derivation of the equation. Numerical simulations of a perturbed Hasegawa-Mima equation which includes in addition a so-called nonlinearity showed that the energy saturates at a finite level. In this paper this numerical observation is proven analytically.

Turbulent heat and particle flux response to electric field shear

Turbulent heat and particle fluxes are shown to have different responses to E×B shear flow in edge plasmas. Recent work has shown that E×B shear can affect the cross phase between electrostatic potential and pressure fluctuations as well as fluctuation levels, thus affecting the heat and particle fluxes. Resistive pressure gradient driven turbulence (RPGDT) is considered including a collisional electron temperature equation. While the particle flux is sensitive to moderate amounts of shear in the radial electric field, the heat flux is insensitive to moderate shear. In the collisional regime, the difference in the sensitivity of the heat and particle fluxes to E×B shear is due to the large parallel heat conductivity. The large parallel heat conductivity dominates the temperature response to electrostatic perturbations. In lower collisionality regimes, the difference in the dissipation rates between density and temperature is negligible, but other effects can still produce different responses in the heat and particle fluxes.

The influence of magnetic fluctuations on collisional drift-wave turbulence

A two-dimensional isothermal collisional drift-wave turbulence model including magnetic fluctuations is studied numerically. The model has as limits the electrostatic collisional drift-wave and two-dimensional magnetohydrodynamic systems. The electromagnetic and electrostatic regimes for thermal gradient-driven (drift-wave) turbulence are decided by the parameter β̂=(4πnT/B2)(L2s/L2n), where Ls and Ln are the parallel and background profile scale lengths, respectively. Significant electromagnetic effects were found only for β̂∼10 for most parameters, and were most pronounced in the strongly adiabatic regime for drift waves. The principal effect of the magnetic fluctuations is magnetic induction in the parallel force balance for electrons, which is linear. This diminishes the adiabaticity of the system by reducing the immediacy of the dissipative coupling between the density and electrostatic potential fluctuations. The transport was still found to be dominantly electrostatic even for β̂=10, although its level decreased with β̂ due to reduced coherency in the coupling between E×B velocity and density fluctuations.

Transport reduction via shear flow modification of the cross phase

As a model example of the effect of shear flow on the cross phase between electrostatic potential and pressure fluctuations, a nonlinear theory of resistive pressure gradient driven turbulence (RPGDT) in a shear flow is presented. This work builds on numerical studies of RPGDT, which have shown that both flow shear and curvature can affect the cross phase as well as the fluctuation levels. In this work, we show that the effect of shear flow on transport can be expressed through the temporal response of pressure to potential. It is shown heuristically that even in the case where the fluctuation levels are not modified, the flow shear still acts to reduce the phase angle between potential and pressure fluctuations, thereby suppressing transport. The scaling of the cross phase with flow shear and flow curvature is presented.