Showing papers on "Space charge published in 1993"

The influence of the strain‐induced electric field on the charge distribution in GaN‐AlN‐GaN structure

Abstract: We show that strongly pronounced piezoelectric properties play a key role in GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice‐mismatch‐induced strains generate polarization fields. We demonstrate that, in a GaN‐AlN‐GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain‐induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance‐voltage (C‐V) characteristics of the SIS stru...

Subpicosecond carrier transport in GaAs surface-space-charge fields.

Abstract: Above-band-gap pulsed optical excitation of electron-hole pairs within the surface-space-charge region of semiconductors alters the surface-space-charge field via free-carrier transport. We report on the direct observation of this ultrafast transient screening and the associated charge-carrier transport by applying reflective electro-optic sampling (REOS) with subpicosecond time resolution to (100)-oriented GaAs surfaces. The REOS measurements performed under different initial surface field conditions and various optical excitation densities are compared to numerical simulations of hot-carrier transport, including the calculation of the optical response. The simulations, which are based on a simple drift-diffusion model for optically excited electron-hole pairs, are in quantitative agreement with the experiment. The strength and sign of the static built-in field can be determined and the carrier drift velocities can be derived on a subpicosecond time scale.

Space Charge Segregation at Grain Boundaries in Titanium Dioxide: II, Model Experiments

Abstract: A quantitative study of space charge solute segregation at grain boundaries in TiO[sub 2] is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part 1. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are special in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor- and donor-doped composition, the Frenkel energy (assumed to be lower than the Schottky energy in TiO[sub 2]) can be separated into its individual terms. An average value for the titanium vacancy formation energy of g[sub v[sub Ti]] = 2.4 eV and an upper limit to the titanium interstitial formation energy of g[sub Ti[sub i]] = 2.6 eV are obtained.

Pulsed electro-acoustic method for measurement of space charge distribution in power cables under both DC and AC electric fields

Abstract: A modification of an earlier technique, to measure the distribution of space charge in the insulation of a coaxial cable, under both DC and AC electric fields, is described. The mathematical analysis is developed on the basis of acoustic waves propagating in media with cylindrical geometry. In order to measure the charge distribution in AC electric fields at any phase angle, a phase shift technique has been employed. The method of calibration of charge density is given, and the charge distribution in the insulation (2 mm in thickness) of XLPE cable has been measured under DC voltages (+or-40 kV, at room temperature) and AC voltage (17 kV peak, at 24 degrees C and 105 degrees C).

Space Charge Segregation at Grain Boundaries in Titanium Dioxide: I, Relationship between Lattice Defect Chemistry and Space Charge Potential

Abstract: The electrical potential difference between the interface and the bulk in TiO2 is obtained as a function of temperature, oxygen pressure, and acceptor or donor doping from a space charge model that explicitly includes the high-temperature lattice defect chemistry. Using defect equilibrium constants for TiO2 from previous literature studies, it is shown that for a space charge determined by ionic defect equilibration with the interface, the potential is negative in undoped and acceptor-doped TiO2 and positive at high donor concentrations. The isoelectric point lies in the donor-doped regime at high temperatures due to the contribution of defects from reduction, even for fairly oxidizing ambients.

Femtosecond thermionic emission from metals in the space-charge-limited regime

Abstract: We study femtosecond-laser-pulse-induced electron emission from W(100), Al(110), and Ag(111) in the subdamage regime (1–44 mJ/cm2 fluence) by simultaneously measuring the incident-light reflectivity, total electron yield, and electron-energy distribution curves of the emitted electrons. The total-yield results are compared with a space-charge-limited extension of the Richardson–Dushman equation for short-time-scale thermionic emission and with particle-in-a-cell computer simulations of femtosecond-pulsed-induced thermionic emission. Quantitative agreement between the experimental results and two calculated temperature-dependent yields is obtained and shows that the yield varies linearly with temperature beginning at a threshold electron temperature of ~0.25 eV The particle-in-a-cell simulations also reproduce the experimental electron-energy distribution curves. Taken together, the experimental results, the theoretical calculations, and the results of the simulations indicate that thermionic emission from nonequilibrium electron heating provides the dominant source of the emitted electrons. Furthermore, the results demonstrate that a quantitative theory of space-charge-limited femtosecond-pulse-induced electron emission is possible.

Electron paramagnetic resonance investigation of charge trapping centers in amorphous silicon nitride films

Abstract: We have explored the nature of the silicon dangling‐bond center in amorphous hydrogenated silicon nitride (a‐SiNx:H) thin films, and its relationship to the charge trapping centers using electron paramagnetic resonance (EPR) and capacitance‐voltage (C‐V) measurements. We have investigated the quantitative relationship between the concentration of silicon dangling bonds using EPR and the concentration of charge traps, measured by C‐V measurements, for both UV‐illuminated and unilluminated a‐SiNx:H thin films subjected to both electron and hole injection sequences. A theoretical framework for our results is also discussed. These results continue to support a model in which the Si dangling bond is a negative‐U defect in silicon nitride, and that a change in charge state of preexisting positively and negatively charged Si sites is responsible for the trapping phenomena observed in these thin film dielectrics.

Charges and forces on conductive particles in roll-type corona-electrostatic separators

Abstract: Mathematical modelling is a powerful tool in the optimization of electroseparation technologies and equipment. This paper proposes the use of a numerical method of field analysis to evaluate more accurately the charges and forces on millimeter-size conductive particles of virtually any shape in contact with an electrode. A program based on the boundary-element method was tested for this purpose, and provided the input data to an algorithm for the computation of particle trajectories. The results were in good agreement with the observations made on a laboratory electroseparator. The effect of space charge on the behavior of conductive particles was studied with a three-electrode system (a grounded grid, between a corona electrode and a metallic plate, energized by two high-voltage supplies of opposite polarities). The accuracy of numerical simulations of electroseparation processes is expected to increase when a mathematical model of particle motion in DC corona fields will be incorporated in the existing computer programs. >

Coulomb energy of traps in semiconductor space‐charge regions

Abstract: Large Coulomb barriers exceeding ΔE≊250 meV are estimated for capture and emission rates of trap centers in semiconductor space‐charge regions Depending on the charge state of the trap, the capture rate or both the capture and emission rates are activated or deactivated, respectively The Coulomb energy raises the equilibrium energy state of a trap center that is repulsively charged when occupied Quantitative agreement of the calculated Coulomb energy is obtained with trapping rates for single individual interface traps in metal‐oxide‐semiconductor (MOS) structures measured by random telegraph signals The Coulomb barrier is reduced in MOS capacitors by partial screening due to mobile charge carriers in the inversion channel The Coulomb energy can be externally controlled in MOS structures by the gate bias voltage

New approach to flashover in dielectrics based on a polarization energy relaxation mechanism

Abstract: The polaron concept is used to describe trapping of charge in a dielectric medium It is shown that trapping is associated with defects identified as being due to a local decrease of the electronic polarizability The polarization energy around a charge is estimated, using a self-consistent calculation of the local field In nonpolar dielectrics this energy is 5 eV per charge, in the limit of validity of the Clausius-Mosotti relation In polar dielectrics it can be much higher A flashover process is proposed as resulting from the destabilization of the space charge by a variety of perturbations (electrical, mechanical, thermal), followed by the mechanical relaxation of the lattice, initially polarized by the charge >

Excitation of the modified Simon–Hoh instability in an electron beam produced plasma

Abstract: An intermediate frequency (fci≤f≤fce) electrostatic instability has been observed in an electron beam produced, cylindrical plasma column. This instability has been identified as a new instability, the modified Simon–Hoh instability (MSHI), which has an instability mechanism similar to the Simon–Hoh instability (SHI). This instability can occur in a cylindrical collisionless plasma if a radial dc electric field exists and if this radial dc electric field and the radial density gradient are in the same direction. The origin of the dc electric field is found to be the difference between the ion and the electron radial density profiles. In such a plasma if the ions are essentially unmagnetized but the electrons are magnetized, a velocity difference in the θ direction can arise because of the finite ion Larmor radius effect. This leads to a space charge separation between the electron and ion density perturbations in the θ direction. The consequent perturbed azimuthal electric field Eθ1 and the enhancement of...

Observation of charge behavior in multiply low‐density polyethylene

Abstract: Effects of interfaces such as metal/polymer interfaces and polymer/polymer interfaces on the space‐charge distribution in multiply low‐density polyethylene have been investigated using a pulsed electroacoustic method. It has been found that the heterocharge was dominant in an artificial interface existing in a polyethylene sample. The time dependence of the heterocharge distribution in the interface on applied voltages and polarity was studied. From these charge distributions, the modified electrical field was calculated based on Poisson’s equation. The calculated result indicated that the actual field was stronger than the applied uniform field at the artificial interface. A new data display method for a three‐ or two‐dimensional plot is employed to display all measurement data on one plot in which the space charge becomes visible so that the results can be easily and conveniently understood.

Electric field calculations with the boundary element method

Abstract: The boundary element method is used to calculate the electric field profiles at needle tips commonly used for electrical treeing tests. Field distributions are also obtained for polyethylene containing a space charge, at the needle tip, and are compared with the values previously obtained by the finite difference method. >

High-current plasma lens

Abstract: The static and dynamic characteristics of a high-current plasma lens (PL) with a two-component quasi-neutral plasma medium formed by a wide-aperture, repetitively pulsed ion beam and secondary electron emission are investigated experimentally. Such a system provides magnetic isolation of electrons and ion beam focusing by means of electric fields due to uncompensated space charge. It is shown that careful selection of the magnetic field geometry, the number of lens electrodes, and the external potential distribution applied to them according to theoretical plasma optics principles allows the radial electric potential within the PL volume to be tailored to widely varying profiles. It is shown that there is a significant influence of the ion beam itself on the potential shape. Lens collective processes due to nonremovable radial gradients of the magnetic field in the ion beam focusing direction are investigated. It is shown that the conductivity mechanism and lens electrode leakage are connected with observed small-scale turbulence. A theoretical analysis of the linear growth phase of an instability is presented, and estimates are made of the nonlinear variable potential amplitude. >

Field‐effect conductance in amorphous silicon thin‐film transistors with a defect pool density of states

Abstract: A new computer program to analyze field‐effect conductance measurements has been developed. In this program a defect pool model, where the equilibrium density of state is determined by the Fermi level, has been incorporated. Transistors with finite band bending, due to fixed charge in the insulator, will therefore have a density of states that is spatially inhomogeneous. The inhomogeneous density of states means that the subthreshold slope of a device is not always controlled by the density of states near the interface, but can become dominated by the bulk density of states, contrary to simpler models. Both electron and hole branches are modeled simultaneously and self‐consistently with no assumptions made about the flatband voltage. Indeed, it is demonstrated that there is no flatband voltage in a transistor with an inhomogeneous density of state; however, a true flatband voltage can be achieved by a process of thermal bias annealing. Finite thickness effects and defect correlation energies are taken int...

A non-local kinetic model applied to microwave produced plasmas in cylindrical geometry

Abstract: In overdense HF or microwave induced plasmas a strong inhomogeneity of the maintaining electric field strength is usually found. From 'local' kinetic models, which usually employ the spatially homogeneous Boltzmann equation and assume that the electron distribution function is in equilibrium with the local electric field strength, an increase of the spectral emission towards the plasma boundary is predicted. In recent investigations, some contradictory experimental results have been found. Therefore a 'non-local' kinetic model is applied to this problem. In contrast to local models, in the non-local model the complete spatially inhomogeneous Boltzmann equation is solved so that the influence of the space charge potential and of the diffusive motion of electrons are taken into account. It is shown that the non-local model is capable of reproducing spatially resolved measurements of the spectral emission intensity much better than a local model.

High-voltage amorphous silicon thin-film transistors

Abstract: High-voltage MOSFETs have been fabricated in a large-area thin-film amorphous silicon technology. The transistors have an offset gate structure which allows them to operate at an excess of 400 V. Basic fabrication steps and structure are discussed. These transistors have a gate-controlled region in series with an offset region where the current is space charge limited. The I/sub D/-V/sub D/ characteristics exhibit a unique instability under drain voltage stress; there is a parallel shift in the I/sub D/ versus V/sub D/ characteristic to a higher V/sub D/. This instability arises from the creation of localized states in the a-Si during depletion. It is analyzed through experiment and two-dimensional simulation. Structural variations are described, including the application of a field plate, to stabilize transistor drive current. Reliability and process uniformity are discussed for arrays of transistors. An appropriate circuit simulation model is discussed. Operation of output drive circuits with high-voltage thin-film transistors (TFTs) is shown, and the application of these to an electrographic plotter is described. >

Charge trapping on different cuts of a single‐crystalline α‐SiO2

Abstract: A scanning electron microscope is employed for the investigation of charging on different cuts of an α‐SiO2. A method for the determination of trapped charges is proposed. Charging on different cuts is observed to decrease in the order of z cut, 30° cut, 45° cut, and 60° cut of the α‐SiO2. This phenomenon is related to permittivity, defect density, and stress of the samples. Details of the experiments and the method of charge determination are given.

Role of space charge in field emission cathodes

Abstract: Space charge plays an extremely important role in thermionic vacuum tubes. Normally it can be neglected in the operation of field emitter cathodes because the very high electric fields at the cathode negates the formation of space charge. However at very high current densities, it may be evident in field emission cathodes. This article explores the region where space charge affects the potential distribution in field emission cathodes of the type proposed for use in vacuum microelectronics applications. Rectangular, cylindrical, and spherical geometries are considered.

Emission from ferroelectric cathodes

Abstract: We have recently initiated an investigation of electron emission from ferroelectric cathodes. Our experimental apparatus consisted of an electron diode and a 250 kV, 12 Ω, 70 ns pulsed high voltage power source. A planar triode modulator driven by a synthesized waveform generator initiates the polarization inversion and allows inversion pulse tailoring. The pulsed high voltage power source is capable of delivering two high voltage pulses within 50 μs of each other and is capable of operating at a sustained repetition rate of 5 Hz. Our initial measurements indicate that emission current densities above the Child-Langmuir space charge limit, JCL, are possible. We explain this effect to be based on a non-zero initial energy of the emitted electrons. We also determined that this effect is strongly coupled to relative timing between the inversion pulse and application of the main anode-cathode pulse. We also have initiated brightness measurements of the emitted beam and estimate a preliminary lower bound to be on the order of 109 A/m2rad2. As in our previous measurements at this Laboratory, we performed the measurement using a pepper pot technique. Beamlet profiles are recorded with a fast phosphor and gated cameras. We describe our apparatus and preliminary measurements.

Observation of a charge limit for semiconductor photocathodes

Abstract: The Stanford Linear Accelerator Center is currently operating with a photocathode electron gun (PEG) to produce polarized electrons for its experimental program. Bunch intensities of up to 1011 electrons within 2 ns (8 A) are required from the electron gun. Operation of PEG has demonstrated a charge limit phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 1011 electrons (the expected space‐charge‐limited charge) when the photocathode quantum efficiency is low. Studies of this charge limit phenomenon observed with a GaAs photocathode are reported.

Time-resolved space charge-limited injection in a trap-free glassy polymer

Abstract: Time-resolved measurements of the dark current excited by application of a voltage step to the tetraphenylbenzidine containing condensation polymer PTPB fitted with ohmic contacts have been carried out as a function of field and temperature. At relatively high temperature, the experimental results are essentially in self-consistent conformity both with the predictions of trap-free space charge-limited (TFSCL) transport theory and with independent TOF measurements. Under trap-free transport conditions, the step field response of the dark current provides a unique means of following the temporal evolution of the drift mobility from the time scale defined by the average injected carrier transit time to a final steady state condition in which every state available to the transiting carriers has been statistically sampled. As the specimen temperature is lowered, increasing and systematic deviations from conventional TFSCLC theory are observed. The extent to which these deviations can be accounted for solely by modification of the theory to explicitly take into account the field dependence of the drift mobility or whether they are indicative of incomplete thermalization on the time scale of a transit time is analyzed.

Pyroelectric drive for light-induced charge transport in the photorefractive process

Abstract: We calculate the influence of thermal patterns and pyroelectric fields on photorefractive holographic recording in terms of a one center charge transport model, taking into account diffusion, photovoltaic current and space charge limitation. At pulse laser intensities the pyroelectric effect can enhance the holographic sensitivity and the saturation value of refractive index changes significantly.

Computer simulations of dense-branching patterns.

Abstract: We present a novel Monte Carlo simulation of electrochemical deposition that shows a smooth transition between diffusion limited aggregation and dense-branching morphology. The essential element of the simulation is to divide the growth field into two areas, a «space charge» region surrounding the aggregate in which particle motion is biased to branch tips, and a second region surrounding the first in which particle motion follows unbiased random walks. The interface between these areas is stable to perturbations of wavelength smaller than the size of the space charge region

High-field conduction in a-Se80Te20 and a-Se80Te10M10 (M=Ag, Cd or Sb)

Abstract: The present paper reports the measurement of DC conductivity at high fields in various binary and ternary glassy semiconducting alloys. The analysis of the data shows the existence of space-charge limited conduction in a-Se80Te20 and a-Se80Te10M10 (M=Ag, Cd or Sb). Using these measurements, the density of localized states near the Fermi level is also calculated for these alloys.

Field emission from structures with quantum wells

Abstract: Computer simulation studies of the properties of some novel types of field emitters have been carried out in the framework of the theory of quantum disc effects. The following cases have been analyzed in detail. (1) Emission from the field surface states (SS) exhibiting both discrete and continuous energy spectra. This case resembles the known effect of the work‐function deterioration ΔW∼(Ec−ESS), yet, here, prior to the emission into vacuum, the SS electrons must be excited the conductivity band via the Pool–Frenkel mechanism: ne≂C exp(aEs1/2/kT), or some other. (2) Emission from the δ‐doped structures via the internal field ionization of filled states, deposited near to surface in semiconductor or in super‐thin insulator layer. (3) Emission from the two‐dimensional (2D) quantum well (QW) surface space charge regions. In this case, the bottom of a subsurface c band is shifted by the quantization value E=Ec−Ei with Ei=(ℏ2/2m*)1/3 [3/2πeEs(i+3/4)]2/3∼Es2/3. Here Es=4π/2eNs is the strength of the surface el...

Space-charge effects in photovoltaic double barrier quantum well infrared detectors

Abstract: We show that the spatial distribution of the dopants strongly influences the transport asymmetry and the photovoltage observed in double barrier quantum well intersubband photodetectors. This influence can be quantitatively explained by the local space‐charge fields arising from an asymmetry of the doping profile with respect to the well centers. The resulting transport model correctly predicts both the observed transport asymmetry of the photocurrent and the opposite asymmetry of the dark current.