Two-dimensional Charge Research Articles

A virtual phase CCD detector for synchrotron radiation applications

A two-dimensional charge coupled device (CCD) detector, based on the Texas Instruments ‘‘virtual phase’’ CCD, has been developed for synchrotron radiation applications. Simultaneous near-edge and multilayer scattering experiments have been carried out with the detector on an energy-dispersive synchrotron beamline. The detector was used in an optical mode where the CCD element is coupled to a phosphor screen by a pair of focusing and demagnifying lenses. We report on the performance of the detector in this mode.

Effect of an applied magnetic field on interface excitations in finite layered structures. III.

The interface excitations associated with double inversion layers in a finite semiconductor-insulator-semiconductor structure are investigated. The layered structure is assumed to be subjected to a magnetostatic field (${\mathrm{B}}_{0}$) applied parallel to the direction of propagation and to the interfaces (Faraday configuration). The exact dispersion relation which governs the propagation characteristics of two-dimensional charge carriers under the influence of an applied magnetic field has been derived on the basis of local theory. The model the author has developed is sufficiently realistic in that it does not ignore the three-dimensional effects in the bulk semiconductor layers. The general dispersion relation which is found to be size dependent and a function of the dielectric properties of the constituent layers has been solved numerically, without any restrictions. The exact numerical results demonstrate the dependence of the coupled modes of excitations on the insulator thickness and the size of the structure. The dispersion characteristics have been discussed in light of recent work in other configurations.

Measurement and modeling of the sidewall threshold voltage of mesa-isolated SOI MOSFETs

Five-terminal silicon-on-insulator (SOI) MOSFETs have been characterized to determine the threshold voltage at the front, back, and sidewall as a function of the body bias. The threshold voltage shift with the body bias at the front and back interfaces can be explained by the standard bulk body effect equation. However, the threshold voltage shift at the sidewall is smaller than predicted by this equation and saturates at large body biases. This anomalous behavior is explained by two-dimensional charge sharing between the sidewall and the front and back interfaces. An analytical model that accounts for this charge sharing by a simple trapezoidal approximation of the depletion regions and correctly predicts the sidewall threshold voltage shift and its saturation is discussed. The model makes it possible to measure the sidewall threshold even when it is larger than the front threshold voltage. >

Theoretical investigation of collective excitations in HgTe/CdTe superlattices. I. Intrasubband excitation.

In this paper, we study in detail the electron collective excitations of HgTe/CdTe superlattices with a simple model superlattice, taking into account the overlap of wave functions at the interfaces for interface states. The linear-response theory has been used with success to calculate the density response of the superlattice to an external perturbation, including many-body effects and electron\char21{}heavy-hole coupling in a reasonable way for intrasubband modes. Some attractive features are found compared with those in type-I and -II superlattices, such as, localized modulated three-dimensional optical and acoustical plasmons, and so on. Moreover, the coupling of interface states with heavy-hole-like subbands is discussed. Also, a comparison of the collective excitation spectra between the quasi-two-dimensional and purely two-dimensional charge interfaces, along with a discussion on their relevance to experiment, is presented.

Two-dimensional proton conduction at a membrane surface: Influence of molecular packing and hydrogen bonding

Evidence for two-dimensional protonic charge transport along a monolayer/water interface is presented, supporting the concept of localised charge transport along membrane surface. Charge transport proceeds only at and above a critical molecular packing density, and probably occurs via a network of hydrogen-bonded water molecules and lipid headgroups.

Extended and local plasmons in a lateral superlattice

A lateral superlattice system has been induced in a metal-oxide-silicon system with a periodically modulated oxide thickness. The originally two-dimensional (2D) charge density was N S1 in the region t 1 and N S2 for the rest t 2 = a−t 1 of the period a (a ≈ 700nm). For samples with t 1⩾3t 2 we find that at small wavevectors q = 2π a , where the plasmon oscillations extend over several superlattice periods, the plasmon frequency is governed by the averaged charge density of the system. For large q ⩾ 4 · ( 2π a ) the plasmon frequency increases significantly stronger than expected from the √q dependence of a 2D system. This indicates that the plasmons become the local mode of a part of the superlattice period, i.e., of the region t 1.

Tunneling from quasi two-dimensional space charge layers

We calculate the lifetime of quasi-2D electrons, which are confined at a heterostructure interface but have the possibility of tunneling through the barrier into the continuum. Exact results are obtained from the width of the local density-of-states in the channel. A highly accurate quasi-3D approximation is derived, which treats the bound electrons as free incoming waves. The method is then applied to a GaAs-GaAlAs heterostructure, discussing resonance phenomena and the influence of nonparabolicity.

Density and capacitance profiles and edge effects in a two-dimensional charge layer on a dielectric surface

The density and capacitance profiles and edge effects in a two-dimensional (2D) layer of electrons held on a liquid helium surface between two horizontal plates of a parallel-plate capacitor are studied by solving Laplace's equation on a computer. An effective length for experimental cells is defined to take into account nonuniform charge density and capacitance near the edges of the cells. The profiles and edge effects are studied as a function of charge density on the helium surface, helium depth inside the cell, repelling voltages on guard electrodes around the capacitor plates, and the frequency of excitation. The results should be useful in designing cells for experiments and better analyzing the results of measurements.

Performance of high‐resolution monitors for digital chest imaging

High-resolution cathode-ray tubes (CRT's) are currently the most viable soft-copy display for digital radiography. We present here methods for measuring large-area contrast ratio and detail contrast ratio. A two-dimensional charge coupled device (ccd) array signal-averaged with a video frame buffer permits linear microradiometric measure of individual beam lines. Results from three different 1000-line monitors demonstrate the shift variance of resolution. The detail contrast ratio (or modulation depth) was found to vary from 100% to less than 10% across the face of one CRT. Dynamic focus in both the horizontal and vertical deflection circuitry proved effective in reducing this shift variance. Comparisons of three phosphors demonstrate the utility of long persistence phosphors (P164) for static display in producing brighter images with less flicker. Recommendations for CRT design and selection for high-resolution digital radiography are included.

Two-dimensional charge-control model for MODFET's

A dc model for MODFET's accounting for two-dimensional effects is proposed. In this model, charge control is realized by solving the two-dimensional Poisson equation in the depleted AlGaAs region. The transport picture used for the two-dimensional electron gas (2-DEG) in the AlGaAs/GaAs heterojunction relies on the quasi-Fermi level together with a field-dependent mobility and therefore includes 2-DEG diffusion effects. Our approach reduces the analysis to a single integral equation. I-V curves, which provide a good fitting to the reported experimental data, are obtained using a smooth velocity-field curve. The channel voltage, 2-DEG concentration, parallel electric-field, and drift velocity along the channel are given in this study and provide a clear picture of current saturation. The model is consistent with the approximate two-region saturation picture but provides a smoother transition. We observe a large diffusion current component along the channel in addition to the drift current. However, the total saturation current obtained has nearly the same value as found from the two-region model. This new model with two-dimensional charge control provides much insight into the current saturation mechanism of the MODFET.

Effects of finite charge-layer thickness in Raman scattering by plasmons in semiconductor superlattices

A response-function theory of light scattering by plasmons in the bulk of semiconductor superlattices is employed to study effects due to the finite thickness of the charge layers. It is shown that in the non-retarded limit of this theory there may be two low-frequency plasmon peaks in the light scattering spectrum, compared with only one bulk plasmon peak in the two-dimensional charge sheets model of semiconductor superlattices. The spectral lineshapes and intensities corresponding to the plasmon excitations are calculated. This theory is applied in particular to GaAs/ Al x Ga 1- x As superlattices, allowing comparison with experimental data.

Two-Dimensional Green's Function of a Semi-Infinite Anisotropic Dielectric in the Wavenumber Domain

A closed-form expression for the two-dimensional Green's function of a semi-infinite anisotropic dielectric in the wavenumber space is presented, and then the validity and definiteness of the ob- tained expression for arbitrary values of the wavenumber vector is proven. The derived Green's function is the Fourier transform of the potential response to a point-charge source located on the surface of a constantly stressed semi-infinite anisotropic dielectric. Therefore it is the most significant part in calculating the two-dimensional charge density and field distribution of the surface acoustic wave interdigital transducers in the case of the electrostatic approximation. The inte- grals associated with the inverse Fourier transform of the derived Green's function are discussed.

Optical control of two-dimensional electron density in a single asymmetric quantum well

We report a novel charge-transfer effect which takes place in a single asymmetric quantum well in a GaAsAlGaAs heterostructure grown by molecular beam epitaxy. Under illumination, the two-dimensional charge density in the quantum well decreases, and we observe the effect of this decrease on the recombination energy of the resulting photoluminescence. Additionally, we have observed a decrease in the conductance of the 2D electrons. With high intensity illumination, both of these changes saturate. We fit both the shift and its saturation by a model which includes the effect of 2D charge density on band-bending and the exchange and correlation energy, as well as the return of electrons to the quantum well by tunneling.

The Design and Manufacture of Wedge and Strip Anodes

We describe the Wedge and Strip Anode (WSA), a two-dimensional position sensitive charge read-out system. Two new optimised WSA's have been developed, giving improved position resolution over previous WSA's for a given charge gain. We show that due to the effective absence of partition noise, WSA's outperform resistive disk anodes at all charge levels. We present measurements of the performance of these optimsed read-outs. Finally, we have designed and built a software controlled WSA manufacturing facility, which can produce any pattern of up to 400 × 500 mm2 with an absolute accuracy of ± 2.5μm.

Magnetization studies of the field induced transitions in (TMTSF)2ClO4

The metallic salt (TMTSF)2ClO4 undergoes a series of magnetic field induced transitions at low temperatures above a threshold field Hk. We present magnetization studies of this system which enablish the isotropic and anisotropic behavior of the two-dimensional charge system and which demonstrate the orbital nature of the transitions. Finally we discuss the theoretical progress in understanding this rich physical system, including some unusual possibilities to explain some of the observed behavior.

Pattern evolution in incommensurate modulated systems

A simplified derivation is given for the equations of motion of discommensurations and dislocations in incommensurate modulated systems. Computer simulation studies are presented for a simplified version of these equations where we reproduce some of the processes experimentally observed in evolution of two-dimensional charge density wave patterns.

Radiative recombination associated with a two-dimensional space charge layer in silicon

We present photoluminescence measurements in Si-MOS structures in presence of a two-dimensional space charge layer. A unique feature is observed: the electron-hole droplet and exciton recombinations are quenched, and a new radiative recombination channel appears, in presence of an electron space charge layer at the (1 1 1) and the (1 1 0) surfaces and of a hole space charge layer in the (1 0 0) surface. The dependence of the energy of this new band on the two-dimensional carrier concentrations is given and a simple model to explain the observed features is proposed.

Observation of bulk and edge magnetoplasmons in a two-dimensional electron fluid.

Two-dimensional magnetoplasmons are studied in a classical gas of electrons bound to a liquid-helium surface. A new and unpredicted resonant mode has been observed. The density and magnetic field dependences of this new mode differ strikingly from normal magnetoplasmons. These new resonant modes are interpreted as edge magnetoplasmons of the two-dimensional charge sheet.

Critical behavior of two-dimensional spin models and charge asymmetry in the Coulomb gas

Many two-dimensional spin models can be transformed into Coulomb-gas systems in which charges interact via logarithmic potentials. For some models, such as the eight-vertex model and the Ashkin-Teller model, the Coulomb-gas representation has added significantly to the insight in the phase transitions. For other models, notably theXY model and the clock models, the equivalence has been instrumental for almost our entire understanding of the critical behavior. Recently it was shown that theq-state Potts model and then-vector model are equivalent to a Coulomb gas with an asymmetry between positive and negative charges. Fieldlike operators in these spin models transform noninteger charges and magnetic monopoles. With the aid of exactly solved models the Coulombgas representation allows analytic calculation of some critical indices.

Influence of short-channel effects on dopant profiles obtained from the DC MOSFET profile method

Distortions in the dopant density profile obtained from dc MOSFET measurements due to short-channel effects are not properly predicted by present two-dimensional charge sharing, or charge conservation, models. The comparison of dopant profile data with predictions based on charge conservation models is a powerful technique for evaluating the accuracy of these models.