Space Charge Build-up Research Articles

Role of space charge in scanned probe oxidation

The growth rate and electrical character of nanostructures produced by scanned probe oxidation are investigated by integrating an in situ electrical force characterization technique, scanning Maxwell-stress microscopy, into the fabrication process. Simultaneous topographical, capacitance, and surface potential data are obtained for oxide features patterned on n- and p-type silicon and titanium thin-film substrates. The electric field established by an applied voltage pulse between the probe tip and substrate depends upon reactant and product ion concentrations associated with the water meniscus at the tip-substrate junction and within the growing oxide film. Space-charge effects are consistent with the rapid decline of high initial growth rates, account for observed doping and voltage-pulse dependencies, and provide a basis for understanding local density variations within oxide features. An obvious method for avoiding the buildup of space charge is to employ voltage modulation and other dynamic pulse-shaping techniques during the oxidation pulse. Voltage modulation leads to a significant enhancement of the growth rate and to improvements in the aspect ratio compared with static voltage pulses.

Space charge buildup in quantum-well infrared photodetectors leading to low-power nonlinear photoresponse

Space charge buildup in quantum-well infrared photodetectors (QWIPs) is shown to be the main reason for the low-power nonlinear photoresponse of the QWIPs. We present a model that takes into account the space charge distribution across the quantum wells and derive a simple expression for the intensity dependent responsivity of the QWIP. A comparison of our model with a set of intensity dependent responsivity measurements performed on a 50-period QWIP shows a very good agreement with our model as long as the QWIP is far from saturation.

TRANSPORT ACROSS BOUNDARY LAYERS IN IONIC CRYSTALS PART II: STATIONARY CHEMICAL DIFFUSION

Defect concentration and charge density profiles are derived for a mixed conductor MO 1+δ with two carriers under a small chemical potential gradient. The description starts from the formulation of the flux equations for the diffusing species of relevance (e.g. O 2− and 2e − in an oxide), takes account of the Poisson equation and is restricted to dilute systems and linear regime. Several examples are discussed in detail: (i) it is shown analytically that chemical diffusion involving carriers with different diffusion coefficients leads to a kinetic build-up of subsurface space charges; (ii) the influence of sluggish surface reactions on the steady state flux and the concentrations profiles is studied on both a “black box” and a mechanistic level; (iii) the impact of different prototype grain boundaries (amorphous blocking core, depletion of carriers in adjacent space charge layers) on chemical diffusion is treated in detail numerically.

Resonant tunneling in asymmetrical double-barrier structures under an applied electric field

Resonant tunneling is studied theoretically for the asymmetrical rectangular double-barrier structures with deep well when a dc bias field is applied. Analytical expressions for the transmission coefficient and the resonance condition are derived by assuming that the effect of the space charge buildup in the well is neglected. It is found that the resonant tunneling with unity transmission (unity resonance) occurs when both the phase difference condition for resonance (PDCR) and the maximum condition for peak value hold simultaneously, while occurs the under-unity resonance that the peak value of transmission coefficient at resonance is below unity when only PDCR holds. The values of transmission peak and the resonance energy are investigated by two methods, Airy function analysis and multistep rectangular approximation. Wave-functions of an electron at the resonance level are calculated and the confining phenomenon is confirmed. Moreover, the effect of the right-hand side barrier height on the transmission coefficient characteristics is investigated.

Photoelectrochemistry ofn-silicon semiconductor in fluoride media

The effect of adding [Ru(bpy)3]2+ complexes during photoelectrochemical etching ofn-silicon on the photocurrent generation is studied. Remarkable enhancement of photocurrent is induced due to the presence of the dye redox system. Redox stabilization of silicon via electron transfer process based on remarkable interfacial interaction between reducing species and the photoelectrode is efficiently achieved. Transient photoelectrochemical measurements during anodic dissolution ofn-silicon in the presence of the complex redox system resulted in inhibition of photocurrent oscillations which was observed at high potentials in the absence of dye redox system. Practically, all photogenerated holes reaching the silicon electrode surface will be efficiently reacted with the coordination redox system resident at the electrode surface. A mechanism of current oscillations, based on periodic buildup and decay (hole/electron recombination) of space charge within the superficial oxide layer is proposed.

Power deposited on a dielectric by multipactor

We use a simple transmission line model to evaluate the RF power deposited on a dielectric window by a multipactor discharge. The calculation employs Monte Carlo simulation, using realistic secondary electron yield curves as input, and taking into account the distributions in the emission velocities and emission angles of the secondary electrons. Beam loading on the external RF, as well as the evolution of the DC electric field due to dielectric charging, are also accounted for. It is found that the buildup of the multipactor space charge, rather than beam loading, causes saturation. Over a wide range of operating conditions and materials, it is found quite generally that the multipactor delivers on the order of 1 percent, or less, of the RF power to the dielectric. A simple estimate is given in support of this ratio, using the susceptibility diagram that was constructed from kinematic considerations. Comparison with experimental results is given.

Excitation efficiency and field non-uniformity in ZnS-based thin-film electroluminescent devices grown by atomic layer epitaxy

Transient measurements of the excitation efficiency were performed in ZnS-based a.c. driven electroluminescent devices grown by atomic layer epitaxy. The results evidence the build-up of a bulk space-charge in the semiconductor layer from the first pulse on, even at low levels of the transferred charge (about ). At these levels, the semiconductor - insulator interfaces contribute to charge carrier injection and provide primary electrons for multiplication in the bulk. Models of space-charge generation by field emission from the bulk only are not compatible with experimental data. The multiplication rates seem rather high ; however, filamentary conduction may lead to an overestimation of .

Impedance spectroscopy of SrBi2Ta2O9 and SrBi2Nb2O9 ceramics correlation with fatigue behavior

In this research, a fatigue model for ferroelectric materials is proposed. The reasons for the electrical fatigue resistance of SrBi2Ta2O9 (SBT), SrBi2Nb2O9 (SBN), and PbZr1−xTixTixO3 (PZT) are discussed in terms of the bulk ionic conductivities of the compounds. To obtain the bulk ionic conductivity of SBT and SBN, we have used impedance spectroscopy which provides an effective method that allows us to separate the individual contributions of bulk, grain boundaries, and electrode-ferroelectric interfaces from the total capacitor impedance. The bulk ionic conductivities of SBT and SBN (∼10−7 S/cm) are much higher than those of the perovskite ferroelectrics, e.g., PZT (∼10−11–10−10 S/cm). The high ionic conductivities led us to conclude that the good fatigue resistance of SrBi2Ta2O9 and SrBi2Nb2O9 is due to easy recovery of defects. Specifically, oxygen vacancies entrapped within the capacitors are easily released, resulting in limited space charge buildup and domain wall pinning during the polarization reversal process. However, the oxygen vacancies in PZT are trapped at trap sites to become space charges, resulting in capacitor fatigue.

Effect of growth phase on intrinsic Stark effect in CdS–ZnSe superlattices

The operation of the intrinsic Stark effect at visible wavelengths is reported, for both cubic and wurtzite type-II CdS–ZnSe strained layer superlattices grown on CdS buffer layers on (111)A GaAs. An observed increase of the effective piezoelectric coefficient from the measured cubic bulk value is attributed to a nonlinear effect, as previously reported for CdTe quantum wells [R. André, J. Cibert, Le Si Dang, J. Zeman, and M. Zigone, Phys. Rev. B 53, 6951 (1996)]. Exciton peak shifts of 70 meV per decade change of excitation intensity are similar in magnitude to those found in CdS–CdSe superlattices of similar period. These shifts are attributed to screening of the internal piezoelectric fields by photoexcited carriers. Exciton peak shifts that are smaller by an order of magnitude appear in CdS–ZnSe samples, grown on (100) ZnSe buffers and therefore predicted to exhibit no piezoelectric effects of any order. These small shifts, which we attribute to space charge buildup, are a characteristic feature of type-II superlattices.

Electronic and defect processes in oxides. The polaron in action

The basic processes involving space charge are charge localization (including trapping) and charge transport. These processes may be associated with energy localization. At the molecular scale, energy localization is an initiator of electrical breakdown and aging phenomena. At the mesoscopic scale, energy localization generates the damage observed macroscopically and is associated with discharge. Processes on one length scale determine behavior on another scale, Most applications concentrate on macroscopic space charge, yet processes on the atomic scale determine the build-up and consequences of space charge. In non-metals, one key concept is that of self-trapping (formation of small polarons). This survey discusses the phenomenon of self-trapping and its effects on charge transport, charge trapping, and the measurement of localized charges. Most examples will relate to oxides, including bulk transition metal oxides, oxide interfaces, thin layers of oxide on silicon, and laser- and radiation-induced charges in alumina. The paper also links some of the ideas to the outstanding problems in the generation of space charge, including contact charging, tribocharging, and impact charging.

Temporal behavior of bidimensional photorefractive bright spatial solitons.

The time behavior of bright spatial solitons in biased photorefractive media is investigated within the framework of a bidimensional band transport model. Biasing the photorefractive media requires an externally applied electric field or the presence of a photovoltaic effect. These two basically different phenomena are shown to be equivalent and additive. The mechanism of space-charge field buildup is analytically expressed, leading to a time-dependent wave propagation equation in generic photorefractive media. The temporal behavior of the soliton solutions to this equation is investigated. It shows the evolution of the soliton beam from the time the external electric field is applied to the final steady-state soliton. On the way, the so-called quasisteady soliton is retrieved, along with its properties. Furthermore, the photovoltaic soliton is described by the wave propagation equation: its behavior is the same as that of the steady-state soliton, the transient states included. Finally, low-power photorefractive bright spatial solitons are generated in a ${\mathrm{Bi}}_{12}$Ti${\mathrm{O}}_{20}$ crystal with a He-Ne laser and their temporal behavior is investigated, thus providing an experimental validation of our theoretical considerations.

Space charge formation and relaxation in ion-bombarded poly(imide) Kapton films

The space charge formation and relaxation in poly(imide) Kapton thin films, deep-implanted with heavy ions in the MeV amu −1 range, have been studied by on-line measurements, with the pressure wave propagation method. The measured space charge depth profiles correspond to the damage depth profiles due to the nuclear collisions. The space charge buildup in the bulk thus seems to be linked to some end-of-range defects acting as trapping sites. Ionization-induced defects seem to play a negligible role in the formation of the bulk space charge.

Application of Piezoelectric Paints to Damage Detection in Structural Materials

Piezoelectric paints were prepared using lead zirconate titanate (PZT) ceramic powder as a pigment and epoxy resin as a binder. The obtained paints were coated on aluminum plates and were cured at room temperature, thus forming the paint films having thicknesses of 25 to 300 /tm and a PZT volume fraction of 53%. These films were then poled at room temperature, and were evaluated with regard to the sensitivity as an acoustic emission sensor in the frequency range of 0-1.2 MHz. The paint film sensitivity obtained at a given poling field depends on the film thickness, even after being normalized by dividing by the film thickness. The current-voltage characteristic of the paint film reveals that the film-thickness dependence of the normalized sensitivity is interpreted in terms of the space-charge buildup at the PZT/epoxy interface during the poling process of the paint film.

Effects of annealing on relaxation behavior and charge trapping in film-processed poly(phenylene sulfide)

The relaxation behavior of poly(phenylene sulfide) (PPS) RytonTM film has been studied as a function of annealing temperatures, Ta, ranging from 30°C to 140°C. Previously, this type of semicrystalline PPS film was shown to possess a very large fraction of constrained, or rigid, amorphous chains. Here we investigate relaxation of amorphous chains using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermally stimulated depolarization current (TSDC). DSC studies suggest that annealing causes the as-received PPS film to relax some of its rigid amorphous fraction and increase its crystallinity, for Ta > Tg. DMA results show a corresponding increase in the temperature location of the dissipation peak and a decrease in its amplitude when Ta increases above 100°C. Analysis of the TSDC p-peak due to injected space charges trapped at the crystal/amorphous interphase provides additional information about amorphous phase relaxation. This peak does not exist in amorphous film, or as-received film, or as-received films annealed at lower Ta. The p-peak does exist in cold-crystallized films and as-received films annealed at higher Ta. We suggest that crystallinity is a necessary, but not sufficient, condition for observation of a p-peak. In addition to crystallinity, the sufficient conditions for observation of a p-peak are existence of (1) a sharp and distinct crystal/amorphous interphase, to provide charge trapping, and, (2) a large fraction of liquid-like amorphous phase, to provide a pathway for charge transport. These conditions are not met in PPS semicrystalline films with very imperfect crystals and large amounts of rigid amorphous phase. In such films, the rigid amorphous phase has, like the crystal phase, a restricted molecular mobility that causes it also to restrict the mobility of space charge. The implications are that film PPS processed with a large amount of rigid amorphous phase chains will have superiour barrier properties to the build-up of interfacial space charge. © 1996 John Wiley & Sons, Inc.

Photoinhibition of the quantum confined Stark effect in piezoelectric multiple quantum wells.

We show that photoinduced space-charge buildup in piezoelectric [111]-oriented multiple quantum wells (MQW's), with average electric fields opposing the field in the barriers, inhibits the shifting of optical transitions by externally applied electric fields. This effect is due to the screening of the average electric field as photogenerated electrons and holes drift by this average field towards opposite locations in the MQW region. The resulting dipole flattens the envelope potential and hence precludes the change of energy levels with variations of external field. This behavior has been observed in different device configurations employing ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/GaAs MQW embedded in a p-i-n diode by low-temperature photoluminescence (PL) and photocapacitance spectroscopies under different bias conditions. In addition to these ``self-locked'' transitions, we also observe other peaks in the PL spectra that result from the charge accumulation effect and that are qualitatively explained using Hartree calculations. \textcopyright{} 1996 The American Physical Society.

Degenerate parametric amplification in photorefractive media: theoretical analysis

We present for the first time, to our knowledge, a theoretical analysis of the nonlinear dynamic space-charge field buildup when a photorefractive medium is exposed simultaneously to two running gratings. It is shown that the two gratings are nonlinearly coupled, hence, for an appropriate choice of parameters, the two gratings interact, causing one grating to be amplified at the expense of the other. This process is referred to as degenerate parametric amplification. In principle an infinite amplification is possible because, even in cases for which no signal grating (i.e., the one to be amplified) is externally seeded into the crystal, a finite signal-grating amplitude is obtained after relaxation of the amplification process.

Evidence of strong correlation between space-charge buildup and breakdown in cable insulation

Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a nondestructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8/spl times/ the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

Space charge buildup in tight-binding superlattices induced by electron sequential tunneling

High-field domain formation in semiconductor structures with tight-binding GaAs/AlGaAs superlattices has been studied using capacitance-voltage (C-V) measurements. Accumulation of the electrons in the quantum wells during the extension of the high-field domain through the superlattice as a result of increasing bias is detected as a sharp increase in the capacitance. The comparison between the C-V and current-voltage (I-V) characteristics is discussed in terms of the extension of the high-field domain through the entire superlattice. Measurements performed at 77 K show clear oscillations in the I-V and the C-V characteristics, from which the space charge accumulation in the superlattice can be calculated.

Theory of electron multipactor in crossed fields

A general theory of multipactor in orthogonal electric and magnetic fields is given. The model consists of two parallel plates of known secondary emission properties, across which a time varying voltage is applied, and between which a constant magnetic field is applied. Expressions are derived for the resonant phases at which the RF-driven cascades occur; these reduce to previously derived expressions in the limit of the vanishing magnetic field. In addition, this work obtains the conditions governing the stability of the motion about those phases, as well as a dynamic constraint from imposing the restriction that each impact on a plate is the first impact that is allowed by the equations of motion. Chaotic effects from the random ejection velocities of the secondaries are addressed for the first time. It is proven that the phase focusing effect from the radio frequency (RF) interaction will overcome the dispersive effect from the random emission, provided that the mean square emission velocity is sufficiently small; in that case stability of the multipactor is determined by the stability of an ‘‘ideal’’ multipactor with zero emission velocity. That reduces the multipactor parameter space to only two control parameters: the normalized RF electric and DC magnetic field strengths. The multipactor prone region is mapped over the entire parameter space by applying both dynamic and secondary yield criteria; previous studies have been limited to specific cases. Finally a simplified model is adopted to address collective effects and multipactor saturation from the space charge buildup. In addition to the mutual electron repulsion, we find that the previously neglected interaction of the electron sheath with the induced image charge on the plates is important to saturation.

Current pulses in dc glow discharges in electronegative gas mixtures

Regular oscillations in the current are predicted in low-pressure, planar discharges under certain conditions in electronegative gas mixtures in which the attachment rate coefficient is large at low values of E/P, the ratio of the electric-field strength to the gas pressure. The frequency of the oscillations is about 10 kHz, and depending on the conditions of pressure, gap spacing, and applied voltage, the current wave form varies from a near-sinusoidal shape to regularly repeating and well-separated spikes with a peak current density on the order of or less than 1 mA/cm2. The instability which gives rise to these oscillations is due to attachment, and the oscillations result from alternate phases of space charge buildup and decay. Thus, the current oscillations predicted here in planar discharges are analogous to Trichel pulses, periodic current spikes which are observed in negative point-plane corona discharges in electronegative gases.