Space Charge Redistribution Research Articles

Enhancing polarization retention in BiFeO3 domain wall nanodevices through controlled space charge redistribution

Ferroelectric thin films with binary switchable polarization offer promising applications in domain wall (DW) memory. However, achieving reliable polarization retention in size-reduced devices remains challenging. In this study, we present a planar ferroelectric BiFeO3 DW nanodevice demonstrating stable polarization retention through electrical cycling. By precisely controlling the device size, including the width and gap of planar Pt electrodes, we observed a back-switching process in switched polarization within 20 min, as characterized by ex-situ piezoresponse force microscopy (PFM). To overcome this issue, long-time alternating voltage pulse cycling effectively suppressed the back-switching behavior, leading to symmetrized polarization coercive voltages of the device. PFM characterization further confirmed the persistence of the artificially created DW between the electrode pair. This improved polarization retention was attributed to the migration of mobile charged defects, such as oxygen vacancies, at the domain boundary. The presence of these defects generated a local built-in field, which screened the net polarization induced by the charged DW. Upon erasing the DW, the local built-in field gradually dissipated during opposite voltage poling, resulting in restored poor polarization retention and asymmetric coercive voltage in the nanodevices. This work presents a promising approach to utilize defect redistribution for stabilizing the switched polarization in the development of high-density ferroelectric DW memory.

Space Charge Redistribution in Epoxy Mold Compounds of High-Voltage ICs at Dry and Wet Conditions: Theory and Experiment

Space charge profile plays an important role in defining the charge transport and breakdown field of an insulator. Non-destructive experimental techniques (e.g., PEA measurement) have recently been used to track in-situ the time-evolution of space charge profiles within complex insulators, such as epoxy mold compounds. Unfortunately, the published results are often inconsistent (both positive and negative charge build-up have been reported), and the discrepancy remains unexplained. In this paper, we (i) derive a physics-based compact analytical model of homocharge injection, (ii) explain the time-and voltage-dependent redistribution of homo- and heterocharge, (iii) investigate experimentally and explain theoretically the importance of moisture ingress regarding space charge redistribution, and finally, (iv) summarize strategies to suppress space-charge related instability in modern ICs encapsulated by epoxy mold compounds.

Importance of charge self-consistency in first-principles description of strongly correlated systems

First-principles approaches have been successful in solving many-body Hamiltonians for real materials to an extent when correlations are weak or moderate. As the electronic correlations become stronger often embedding methods based on first-principles approaches are used to better treat the correlations by solving a suitably chosen many-body Hamiltonian with a higher level theory. The success of such embedding theories, often referred to as second-principles, is commonly measured by the quality of self-energy Σ which is either a function of energy or momentum or both. However, Σ should, in principle, also modify the electronic eigenfunctions and thus change the real space charge distribution. While such practices are not prevalent, some works that use embedding techniques do take into account these effects. In such cases, choice of partitioning, of the parameters defining the correlated Hamiltonian, of double-counting corrections, and the adequacy of low-level Hamiltonian hosting the correlated subspace hinder a systematic and unambiguous understanding of such effects. Further, for a large variety of correlated systems, strong correlations are largely confined to the charge sector. Then an adequate nonlocal low-order theory is important, and the high-order local correlations embedding contributes become redundant. Here we study the impact of charge self-consistency within two example cases, TiSe2 and CrBr3, and show how real space charge re-distribution due to correlation effects taken into account within a first-principles Green’s function-based many-body perturbative approach is key in driving qualitative changes to the final electronic structure of these materials.

Structural instability of transition metals upon ultrafast laser irradiation

Precision in laser material structuring is critically defined by the energy flow during the irradiation process, particularly in ultrafast regimes. Alternative to thermal evolutions, nonequilibrium electronic excitation can exercise a direct influence on the atomic bonding delivering a potentially rapid destructuring process. In this context, the atomic disordering of transition metals (Cr, Ni, and Ti) induced by nonequilibrium electronic excitation typical for ultrafast laser processing is studied with an emphasis on the role of the $d$-band filling and crystalline structure. The density functional theory is used to obtain, from first principles, structural stability criteria and nonthermal disordering pathways on timescales shorter than picosecond electron-phonon dynamics. We show that the hot electrons distort charge distribution in the cold crystalline arrangement and increase the entropy of the system, thus driving the mechanical expansion. The calculated nonequilibrium free-energy potentials indicate that a solid destabilization is possible when electron temperature reaches a universal value of around 2 eV for all considered metals. Under a uniaxial lattice relaxation expected in the laser ablation of surface layers, the charge redistribution and loss of lattice stability is shown to be oriented along specific crystal directions. Moreover, we show that the interatomic potential destabilization is energetically more favorable for Ni than for Cr and Ti. Lattice dynamics is affected by space-charge redistribution induced by Fermi smearing associated with anisotropic expansion due to electron pressure gradients. This ultrafast destructuring mechanism is shown to be a general feature of partially filled $d$-band transition metals.

Numerical Simulation to Evaluate the Effects of Upward Lightning Discharges on Thunderstorm Electrical Parameters

A theoretical discussion of the discharge effects of upward lightning simulated with a fine-resolution 2D thunderstorm model is performed in this paper, and the results reveal that the estimates of the total induced charge on the upward lightning discharge channels range from 0.67 to 118.8 C, and the average value is 19.0 C, while the ratio of the induced charge on the leader channels to the total opposite-polarity charge in the discharge region ranges from 5.9% to 47.3%, with an average value of 14.7%. Moreover, the average value of the space electrostatic energy consumed by upward lightning is 1.06×109 J. The above values are lower than those related to intracloud lightning discharges. The density of the deposited opposite-polarity charge is comparable in magnitude to that of the preexisting charge in the discharge area, and the deposition of these opposite-polarity charges rapidly destroys the original space potential well in the discharge area and greatly reduces the space electric field strength. In addition, these opposite-polarity charges are redistributed with the development of thunderstorms. The space charge redistribution caused by lightning discharges partly accounts for the complexity of the charge structures in a thunderstorm, and the complexity gradually decreases with the charge neutralization process.

Effect of the carbon component on the electrical and optical properties of nanocellulose-based composites

The results of X-ray studies of the structure of components of composite materials based on milled microcrystalline cellulose are presented. The 3D model of the atomic arrangement in the short-range order of amorphous carbon can be described by a mechanical mixture of two types of clusters in the ratio of 1 : 2. One type of clusters is formed by two planar graphene single layers shifted relative to each other and containing vacancies, and the other type is presented by six graphene grids. The cellulose matrix with silicon nanoparticles has a low photoluminescence-signal degradation rate. The introduction of fullerenes into nanomaterial as a third nanofraction, as well as the action of ozone, leads to anomalous luminescence kinetics under UV (ultraviolet) photoexcitation, which can be associated with competing processes of hydrogen and oxygen adsorption on the surface of silicon nanoparticles. A change in the ionic conductivity of the porous cellulose matrix upon exposure to ozone can be used to develop effective ozone detectors. Such a filler as amorphous-crystalline carbon causes not only ionic but also electronic conductivity in the sample; however, the processes of space-charge redistribution remain dependent only on the ion-current component. An increase in the total current passing through the pressed sample eliminates the need for a further increase in the signal in the design of ozone sensors.

Finite element analysis of the Poisson–Boltzmann equation coupled with chemical equilibriums: redistribution and transport of protons in nanophase separated polymeric acid–base proton exchange membranes

The finite element analysis is applied to the study of the redistribution and transport of protons in model nanophase separated polymeric acid–base composite membranes by the Poisson–Boltzmann equation coupled with the acid and base dissociation equilibriums for the first time. Space charge redistribution in terms of proton and hydroxide redistributions is observed at the interfaces of acidic and basic domains. The space charge redistribution causes internal electrostatic potential, and thus, promotes the macroscopic transport of protons in the acid–base composite membranes.

Space charge redistribution in bi-layered resistance switching materials

In modeling resistance switching of metal oxides, migration of ion species is assumed to explain the local modulation of energy barriers or microscopic changes of stoichiometry. This article addresses an additional aspect of ions migration, that is, space charge redistribution in metal oxides. Modeling charge migration and its impact on current-voltage (I-V) characteristics were evaluated based on a bi-layered switching material composed of a ‘transport layer’ and a ‘tunnel layer’. A dipolar charge profile was assumed to describe a low resistance state, and it was found that the I-V slope of the negative differential resistance regime increases with the initially assumed dipolar space charge density. A similar behavior was demonstrated in experimental I-V characteristics, which were successfully reproduced by the developed model. Space charge redistribution in metal oxides provides additional insight towards obtaining a more complete understanding of resistance switching phenomena.

Investigation of analog memristive switching of iron oxide nanoparticle assembly between Pt electrodes

The analog memristive switching of iron oxide (γ-Fe2O3) nanoparticle assembly was investigated. The γ-Fe2O3 nanoparticles were chemically synthesized with ∼10 nm in diameter and assembled to be a continuous layer as a switching element in Pt/nanoparticles/Pt structure. It exhibited the analog switching that the resistance decreased sequentially as repeating −V sweeps and pulses while increased as applying +V. The capacitance-voltage curves presenting hysteresis with flatband voltage shift and distortion of their shapes with respect to the applied voltage supported the redistribution of space charges in nanoparticle assembly that might induce resistive switching. The polarity-dependent analog resistance change proportional to pulse voltage, time, and number of pulses was analogy to potentiation and depression of adaptive synaptic motion.

Space charge induced electroluminescence spectra shift in organic light-emitting diodes

The electroluminescence spectra of organic light-emitting diodes can be tuned by controlling the space charge density in the emission layer, and both red and blue shifts in the electroluminescence spectra are realized. The origin of the space charge induced spectra shift is ascribed to the space charge redistribution as response to the dipole moment variation of the organic emitters at excitation and ground states. The space charge density in the emission layer can be controlled by simply designing the device structure and layer configuration, which may have potential for optimizing the color coordinates of organic light-emitting diodes.

Symmetric Ferroelectric Switching in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

We report the observation of asymmetric switching dual peaks in ferroelectric copolymer films. These dual peaks occurs when the poling electric field is just below the coercive field and can be removed by continuous application of high enough switching voltage. Our experimental observations can be explained by the injection and the redistribution of space charges in ferroelectric films.

Accelerated and decelerated polarization reversal in thin vinylidene fluoride/trifluoroethylene copolymer films

Ferroelectric switching characteristics during continuous polarization reversal were investigated in thin vinylidene fluoride/trifluoroethylene copolymer films by using a train of bipolar on-off electric field pulses. It was found that polarization reversal is accelerated as the pulse width Δt is decreased whereas it is decelerated as Δt is increased. For a given Δt, the switching time t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> depends on the pulse height E in accordance with an exponential law t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ∞e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">/E</sup> in that E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> increases from 0.5 GV/m to 1.2 GV/m as Δt is increased from 10 μs to 1000 s whereas t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ∞ remains constant at 8 ns. These results are interpreted in terms of the dependence of t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> on previous poling history that produces diverse polarization states containing various defects and disorder that affect the nucleation probability and growth velocity in the following switching process. Particular acceleration at short /Delta/t was attributed to unswitched polarization that aids in the generation of critical nuclei of reversed domains. Marked deceleration at long /Delta/t was attributed to structural stabilization observed as slow dielectric relaxation during the off period. The effects of ferroelectric heating and space charge redistribution are discussed in relation to acceleration and deceleration, respectively.

Space-charge limited conduction in doped polypyrrole devices

Doping dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were carried out on polypyrrole devices in metal-polymer-metal sandwich structure. Temperature dependent I-V measurements infer that space-charge limited conduction (SCLC) with exponential trap distribution is appropriate for the moderately doped samples, whereas trap-free SCLC is observed in lightly doped samples. Trap densities and energies are estimated, the effective mobility is calculated using the Poole–Frenkel model, and the mobility exhibits thermally activated behavior. Frequency dependent capacitance-voltage characteristics show a peak near zero bias voltage, which implies that these devices are symmetric with a negligible barrier height at the metal-polymer interface. Low frequency capacitance measurements have revealed a negative capacitance at higher voltages due to the processes associated with the injection and redistribution of space-charges.

The Strong Dependence of Polarization Fatigue on Poling-Voltage Conditions in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

Recently, polarization fatigue in ferroelectric polymers has attracted much attention due to their potentials in nonvolatile memory devices, although the mechanism behind is still an open question. In this letter, we report the strong dependence of polarization fatigue on poling voltages, which indicates that the application of a poling electric field slightly below the coercive field can greatly degrade the fatigue endurance. This phenomenon may be directly related to the fatigue mechanism, and its origin is discussed based on the understanding of the injection and redistribution of space charges in ferroelectric films.

Charge retention behavior of preferentially oriented and textured Bi3.25La0.75Ti3O12 thin films by electrostatic force microscopy

The authors report charge retention in preferentially (117) oriented and textured c-axis oriented ferroelectric Bi3.25La0.75Ti3O12 thin films by electrostatic force microscopy. Surface charges of the films were observed as a function of time in a selected area which consists of a single-poled region and a reverse-poled region. The highly (117) oriented film shows the extended exponential decay with characteristic scaling exponents, n=1.5–1.6. The preferentially c-axis oriented film shows a remarkable retained behavior regardless of the poling. Decay and retention mechanisms of the regions are explained by space-charge redistribution and trapping of defects in the films.

Dynamics of space and polarization charges of ferroelectric thin films measured by atomic force microscopy

Retention behavior and local hysteresis characteristics in Pb(Zr 0.52Ti 0.48)O 3 (PZT) thin films on Pt electrodes have been investigated by electrostatic force microscopy (EFM). A sol–gel method is used to synthesize PZT thin films and drying conditions are carefully explored over a wide range of temperature. Decay and retention mechanisms of single-poled and reverse-poled regions of the ferroelectric thin films are explained by space charge redistribution. Trapping behavior of space charges is dependent on the nature of interface between ferroelectric thin films and bottom electrodes. Local measurement of polarization–electric field curves by EFM shows inhomogeneous space charge entrapment.

Polarization behaviors of (Bi3.15Nd0.85)Ti3O12 thin films deposited by radio-frequency magnetron sputtering

Ferroelectric (Bi3.15Nd0.85)Ti3O12 (BNdT) thin films of random orientation have been successfully deposited on Pt∕Ti∕SiO2∕Si(001) by radio-frequency magnetron sputtering, followed by rapid thermal annealing at 700 °C. They exhibit a remanent polarization 2Pr of 23.2μC∕cm2 and a coercive field EC of 112kV∕cm at an applied voltage of 10 V. These BNdT films also show a switchable polarization (ΔP=Psw−Pns) of 12.9μC∕cm2 at 5 V, together with an almost fatigue-free behavior up to 1.4×1010 switching cycles at both 100 and 150 °C. They demonstrate desirable retention and imprint behaviors at 100 °C, while a further increase in temperature up to 150 °C led to an acceleration in both retention loss and imprint behavior, which can be accounted for by the thermally assisted redistribution of defects and space charges. Studies of the ac dependence of relative permittivity suggest the occurrence of domain-wall pinning, which is a commonly observed phenomenon in oxide ferroelectric thin films.

Transformation kinetics of electric field domains in weakly coupled GaAs/AlGaAs superlattices in a transverse electric field

Transformation kinetics of the resonant-tunneling domain structure in a superlattice in a rapidly varying electric field is investigated using real-time studies of current response. It is shown that the kinetics is mainly determined by a lag in redistribution of the space charge that forms the domain boundary. A nonmonotonic oscillatory dependence of the transient-process duration on the amplitude of a voltage pulse is observed, as well as the effect of the weak dependence of the transformation time on the displacement of the domain boundary, which indicates that the transformation processes is discrete. Possibilities for controlling the switching processes in a multistable system of current states in weakly coupled semiconductor superlattices are discussed.

Electric-Field-Induced Second Harmonic Generation from Vacuum-Evaporated Metallophthalocyanine Film/Metal Electrode Interface

The optical second harmonic generation (SHG) was used to study the redistribution of space charges, which was caused by photoillumination, at the nanosized interface between metal electrode and phthalocyanine (Pc) film. The redistribution of space charges was observed using the SHG spectra, where the space-charge-induced electric field (SCIEF) broke the centrosymmetry of the Pc molecule and gave rise to another effective second order nonlinear polarization aside from the one induced by the second order electric quadrupole. Furthermore, an external bias was applied to the sandwiched metal-copper phthalocyanine (CuPc)-meal sample to induce the second order nonlinear polarization. The quadratic relationship between SHG intensity and the external applied bias was observed. On the basis of the four-layer model, the electric-field-induced SHG (EFISHG) process was analyzed. The theoretical results showed good agreement with the experimental data.

Influence of the recharging process on the dark current noise in quantum-well infrared photodetectors

We show that the dark curent noise spectrum of an In0.30Ga0.70As/GaAs quantum-well infrared photodetector (QWIP) is characterized by two plateau-like frequency regions. At high frequencies, the observed noise current is due to the generation–recombination noise of carriers, which have been emitted thermionically from the quantum wells into the continuum. In the low-frequency regime, an additional contribution to the noise current is caused by the redistribution of space charges, that occurs on a time scale similar to the dielectric relaxation time.