Space Charge Limited Current Regime Research Articles

Transition from the regime of thermionic emission to the space-charge limited current regime under strong Shottky effects

Transition from the regime of thermionic emission to the space-charge limited current regime under strong Shottky effects

Collisional space-charge-limited current with monoenergetic velocity: From Child–Langmuir to Mott–Gurney

All theories coupling electron emission theories ultimately approach the space-charge-limited current (SCLC) in vacuum, given by the Child–Langmuir (CL) law, for sufficiently high voltage, or the Mott–Gurney (MG) law for finite electron mobility and high (but not infinite) voltage. These analyses demonstrate the presence of an SCLC regime that cannot be described by either CL or MG. Here, we derive an exact solution for SCLC for general electron mobility and nonzero velocity. We recover the traditional CL with nonzero initial velocity at high voltage. For low mobility (or infinite collision frequency), we derive corrections to the MG law that depend on the ratio of initial velocity to the product of collision frequency and gap distance or initial velocity to drift velocity for low and high voltage, respectively. Increasing collisionality decreases the correction to SCLC for nonzero velocity, indicating that these corrections are less important for low-mobility materials (e.g., solids) than high-mobility materials (e.g., air or vacuum). For a given gap distance (collision frequency), increasing the collision frequency (gap distance) increases the voltage necessary to make the gap appear more like vacuum. These results provide a generalized SCLC for both collisionality and initial velocity when assessing the transitions between electron emission mechanisms.

Electrolyte‐Gated Vertical Transistor Charge Transport Enables Photo‐Switching

AbstractProposals for new architectures that shorten the length of the transistor channel without the need for high‐end techniques are the focus of very recent breakthrough research. Although vertical and electrolyte‐gate transistors are previously developed separately, recent advances have introduced electrolytes into vertical transistors, resulting in electrolyte‐gated vertical field‐effect transistors (EGVFETs), which feature lower power consumption and higher capacitance. Here, EGVFETs are employed to study the charge transport mechanism of spray‐pyrolyzed zinc oxide (ZnO) films to develop a new photosensitive switch concept. The EGVFET's diode cell revealed a current‐voltage relationship arising from space‐charge‐limited current (SCLC), whereas its capacitor cell provided the field‐effect role in charge accumulation in the device's source perforations. The findings elucidate how the field effect causes a continuous shift in SCLC regimes, impacting the switching dynamics of the transistor. It is found ultraviolet light may mimic the field effect, i.e., a pioneering demonstration of current switching as a function of irradiance in an EGVFET. The research provides valuable insights into the charge transport characterization of spray‐pyrolyzed ZnO‐based transistors, paving the way for future nano‐ and optoelectronic applications.

The evaluation of the Al/ZnO/Si/Al heterojunction diode current-conducting mechanisms

This work focuses on the electrical transport and conduction mechanisms — inside a sprayed — made zinc oxide-on-p-type silicon junction device. A fabricated Al/ZnO/Si/Al heterojunction is characterized at 300 and 380[Formula: see text]K in the dark. The aforementioned theories predict that the current density (J) is an exponential function of measured voltage (V) and work temperature (T). Exponential equations are linearized as a result of a current conduction mechanism (CCM) kind. Useful parameters like barrier height [Formula: see text] of trap are extracted from the slope B. Ohmic and space charge limited current (SCLC) regimes inside are studied at 300 and 380[Formula: see text]K recording a shift in exponent as a result of temperature. Fowler–Nordheim (FN) conduction mechanism inside Al/ZnO/Si/Al device is considered slope and [Formula: see text] values are determined for two cited work temperatures. Our device presents a thermionic emission (TE) and exhibits an effect of temperature on slope and intercept values of (ln[Formula: see text]/A*[Formula: see text]) function. The Poole–Frenkel (PF) emission in terms of 1/V data are presented for 300 and 380[Formula: see text]K. Besides, PF emission mechanism in terms of 1/[Formula: see text] is exhibited and parameters are influenced by temperature. Behavior of conductance as a result of temperature is detailed and evidenced.

Конкурирующие механизмы роста при формировании поликристаллической пленки MAPbI3

The results of studying thin polycrystalline perovskite layers of CH3NH3PbI3 (MAPbI3) are presented. The resulting MAPbI3 layers demonstrate a characteristic absorption spectrum, optical band gap, and photoresponse to irradiation in the visible region of the spectrum. Two crystallization mechanisms have been found in the MAPbI3 layer during heating, which ensure the formation of a film of crystallites with characteristic sizes of 100–200 nm and long dendritic structures with a length of more than 50 μm. A spacecharge-limited current regime has been registered, as well as hysteresis due to ion migration.

Visualization of Carrier Transport in Lateral Metal-Perovskite-Metal Structures and its Influence on Device Operation

The high performance of hybrid perovskite based devices is attributed to its excellent bulk-transport properties. However, carrier dynamics, especially at the metal-perovskite interface, and its influence on device operation are not widely understood. This work presents the dominant transport mechanisms in methylammonium lead iodide (MAPbI3) perovskite-based asymmetric metal-electrode lateral devices. The device operation is studied with inter-electrode lengths varying from 4 {\mu}m to 120 {\mu}m. Device characteristics indicate distinct ohmic and space-charge limited current (SCLC) regimes that are controlled by the inter-electrode length and applied bias. The electric-potential mapping using Kelvin-Probe microscopy across the device indicates minimal ion-screening effects and the presence of a transport barrier at the metal-MAPbI3 junction. Further, photocurrent imaging of the channel using near-field excitation-scanning microscopy reveals dominant recombination and charge-separation zones. These lateral devices exhibit photodetector characteristics with a responsivity of about 51 mA/W in self-powered mode and 5.2 A/W at 5 V bias, in short-channel devices (4 {\mu}m). The low device capacitance enables a fast light-switching response of ~12 ns.

Lasing in the space charge limited current regime

We introduce an analytical model for ideal organic laser diodes based on the argument that their intrinsic active layers necessitate operation in the bipolar space charge-limited current regime. Expressions for the threshold current and voltage agree well with drift-diffusion modeling of complete p-i-n devices and an analytical bound is established for laser operation in the presence of annihilation and excited-state absorption losses. These results establish a foundation for the development of organic laser diode technology.

Знакопеременная фотопроводимость в пленках PbSnTe : In в режиме тока, ограниченного пространственным зарядом

The dependence of the photoconductivity sign on the bias voltage, intensity and duration of illumination was studied in PbSnTe:In films in the space charge limited current regime. The role of traps with a complex energy spectrum, including the surface traps, in the observed effects is discussed

The Effect of Surface on Conductivity of PbSnTe:In/BaF2 Topological Crystalline Insulator in Space Charge Limited Current Regimes

The effect of surface chemical treatment on current-voltage (IV) characteristics of high-resistance MBE-grown PbSnTe:In films in space charge limited current (SCLC) regimes has been studied. At T=4.2 K depending on surface chemical treatment the current in films in SCLC regimes under no illumination could rise up to 104 times and even more. The surface chemical treatment also affected both the value and behavior of the photocurrent. The qualitative model for the observed phenomena has been discussed.

Charge carrier transport and electroluminescence in atomic layer deposited poly-GaN/c-Si heterojunction diodes

In this work, we study the charge carrier transport and electroluminescence (EL) in thin-film polycrystalline (poly-) GaN/c-Si heterojunction diodes realized using a plasma enhanced atomic layer deposition process. The fabricated poly-GaN/p-Si diode with a native oxide at the interface showed a rectifying behavior (Ion/Ioff ratio ∼ 103 at ±3 V) with current-voltage characteristics reaching an ideality factor n of ∼5.17. The areal (Ja) and peripheral (Jp) components of the current density were extracted, and their temperature dependencies were studied. The space charge limited current (SCLC) in the presence of traps is identified as the dominant carrier transport mechanism for Ja in forward bias. An effective trap density of 4.6 × 1017/cm3 at a trap energy level of 0.13 eV below the GaN conduction band minimum was estimated by analyzing Ja. Other basic electrical properties of the material such as the free carrier concentration, effective density of states in the conduction band, electron mobility, and dielectric relaxation time were also determined from the current-voltage analysis in the SCLC regime. Further, infrared EL corresponding to the Si bandgap was observed from the fabricated diodes. The observed EL intensity from the GaN/p-Si heterojunction diode is ∼3 orders of magnitude higher as compared to the conventional Si only counterpart. The enhanced infrared light emission is attributed to the improved injector efficiency of the GaN/Si diode because of the wide bandgap of the poly-GaN layer and the resulting band discontinuity at the GaN/Si interface.

Effects of vinylene and azomethine bridges on optical, theoretical electronic structure and electrical properties of new anthracene and carbazole based π-conjugated molecules

Abstract This study aims to synthesize and characterize a new semi-conducting 2,7-distyrylcarbazole-based molecule (M1) and its Schiff-base analog (M2). Those two molecules were synthesized via Wittig-Horner and diamine condensation reactions, respectively. The effects of replacing the vinylene ( CH CH ) linkage between carbazole donor unit and anthracene acceptor unit in molecule M1 by an azomethine ( CH N ) bridge in molecule M2 on physicochemical and electrical properties are reported in this work. Thermal characterization shows a semi-crystalline morphology in solid state for the two organic materials and a good thermal stability up to 370 °C in the case of M2. The observed non-fluorescence character of the Schiff-base molecule has been related to the photo-induced electron transfer (PET) process from the imine receptor moiety to the anthracene fluorophore unit. The effect of the addition of trifluoroacetic acid (TFA) on optical properties of the Schiff base molecule revealed a significant blue shift of the absorption spectrum and a turn-on of the fluorescence as well as visual changes. Computational analyses by Density Functional Theory (DFT) and time-dependent (TD-DFT) were then used to obtain conformation and the energy distributions of the molecular orbitals of the two compounds. The electrochemical behaviors were investigated by cyclic voltammetry and showed a reduction of the band gap of M2 as compared to M1 du to the electron-withdrawing effect the azomethine (–CH N–) bond. Finally, the charge-carrier mobility of the two compounds were extracted from single-layer diodes in the space-charge limited current regime (SCLC) and organic field effect transistors (OFETs) demonstrating lower hole mobilities in the case of M2 based thin film devices. The hole effective mobility of M2 (measured by SCLC) was one order of magnitude higher after exposition to trifluoricacetic acid vapors than in neutral films.

Axial p–n junction and space charge limited current in single GaN nanowire

The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p–n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p–n junction NWs grown by plasma-assisted molecular beam epitaxy. I–V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p–n junction NW. Thanks to an improved contact process, both the electric field induced by the p–n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p–n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116–125 nm. Following our previous work, the acceptor Na doping level was estimated to be 2–3 × 1017 at cm−3 assuming a donor level Nd of 2–3 × 1018 at cm−3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

Electric Current and Noise in Long GaN Nanowires in the Space-Charge Limited Transport Regime

We studied electric current and noise in planar GaN nanowires (NWs). The results obtained at low voltages provide us with the estimates of the depletion effects in the NWs. For larger voltages, we observe the space-charge limited current (SCLC) effect. The onset of the effect clearly correlates with the NW width. For narrow NWs, the mature SCLC regime was achieved. This effect has great impact on fluctuation characteristics of studied NWs. At low voltages, we found that the normalized noise level increases with decreasing NW width. In the SCLC regime, a further increase in the normalized noise intensity (up to 104 times) was observed, as well as a change in the shape of the spectra with a tendency towards slope −3/2. We suggest that the features of the electric current and noise found in the NWs are of a general character and will have an impact on the development of NW-based devices.

Strongly emissive plasma-facing material under space-charge limited regime: Application to emissive probes

A quasi-static theoretical 1D model is developed to describe the sheath structure of a strongly emissive plasma-facing material and is subsequently applied to emissive probes' experimental data—which are usually supposed to be an efficient tool to directly measure plasma potential fluctuations. The model is derived following the space-charge limited emission current model developed in Takamura et al., [Contrib. Plasma Phys. 44(1–3), 126–137 (2004)], adding the contribution of secondary emission due to back-diffusion of plasma electrons at the emitting surface. From this theory, current-voltage characteristics of emissive probes are derived. A theoretical relation between the floating potential of an emissive probe and plasma parameters is obtained and a criterion is derived to determine the threshold between the thermoemission limited current regime and space-charge limited current regime. In the space-charge limited regime, a first order expansion is then applied to the quasi-static relation to study the effect of plasma fluctuations on emissive probe measurements. Both the mean values and the fluctuations of the floating potential of an emissive probe predicted by the model, as well as the potential value at which the transition between emission current regimes occurs, are compared to three sets of experimental data obtained in two different plasma devices.

Mobility versus Alignment of a Semiconducting π-Extended Discotic Liquid-Crystalline Triindole.

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm2 V-1 s-1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound.

Towards a unified description of the charge transport mechanisms in conductive atomic force microscopy studies of semiconducting polymers.

In this work, conductive atomic force microscopy (C-AFM) is used to study the local electrical properties in thin films of self-organized fibrillate poly(3-hexylthiophene) (P3HT), as a reference polymer semiconductor. Depending on the geometrical confinement in the transport channel, the C-AFM current is shown to be governed either by the charge transport in the film or by the carrier injection at the tip-sample contact, leading to either bulk or local electrical characterization of the semiconducting polymer, respectively. Local I-V profiles allow discrimination of the different dominating electrical mechanisms, i.e., resistive in the transport regime and space charge limited current (SCLC) in the local regime. A modified Mott-Gurney law is analytically derived for the contact regime, taking into account the point-probe geometry of the contact and the radial injection of carriers. Within the SCLC regime, the probed depth is shown to remain below 12 nm with a lateral electrical resolution below 5 nm. This confirms that high resolution is reached in those C-AFM measurements, which therefore allows for the analysis of single organic semiconducting nanostructures. The carrier density and mobility in the volume probed under the tip under steady-state conditions are also determined in the SCLC regime.

Transport model for disordered organic nanocomposites

ABSTRACTElectric transport in disordered media is usually explained in terms of different transport regimes, such as SCLC (Space Charge Limited Current) or TCLC (Trap Charge Limited Current) regimes. These models lead to exponential dependencies of the current on voltage, e.g., quadratic for SCLC or higher order for TCLC, with transition regions between them where fitting is poor. Alternatively, a statistical distribution in space and energy of the disordered traps, e.g., Gaussian or exponential, allows explaining transport in disordered materials. In this work, we propose a modeling based on the density of states (DOS) function, fitted from normalized differential conductivity curves obtained from experimental current-voltage curves. In general a Gaussian function is used for low energies whereas one or more exponential functions are used for higher energies. The proposed model is used to reproduce experimental current-voltage curves of organic nanocomposites, with gold and silver nanoparticles within chitosan matrixes. A unique expression is obtained for a very accurate fitting the experimental current-voltage characteristics in the whole voltage range without transition regions.

Electric potential mapping by thickness variation: A new method for model-free mobility determination in organic semiconductor thin films

Charge transport, with charge carrier mobility as main parameter, is one of the fundamental properties of semiconductors. In disordered systems like most organic semiconductors, the effective mobility is a function of the electric field, the charge carrier density, and temperature. Transport is often investigated in a space-charge limited current (SCLC) regime in thin film single carrier devices, where an electric current is driven in the direction perpendicular to the surface. Direct evaluation of the current–voltage characteristics, however, is problematic, because parasitic contributions from injection or extraction barriers can falsify results.Here, we present a novel measurement and evaluation technique for key transport parameters. First, it allows for the direct determination of the potential profile in single carrier devices. It is obtained from a series of steady-state current–voltage measurements from devices with varying thickness (“electric potential mapping by thickness variation”, POEM). Second, the data can be evaluated to obtain the effective charge carrier mobility μ(F,n) as a function of the electric field F and the charge carrier density n. Single carrier transport is achieved by sandwiching the organic material under investigation between equally doped layers, i.e. p-i-p (resp. n-i-n) devices for hole (electron) transport investigations. The POEM concept is validated using drift-diffusion simulation data. It is furthermore experimentally applied to small molecular organic semiconductors, where the hole transport in a blend of zinc phthalocyanine (ZnPc) and C60 is characterized. In the measured range of F≈(1–5)×105V/cm and hole densities of approx. (1–5)×1016cm−3, the hole mobility is found to be in the range of (10−7–10−5)cm2/Vs, comprising a pronounced field activation with an activation constant of 0.01cm/V. A dependence of the mobility on the charge carrier density in the given range is not observed.The POEM approach does not require a given mobility function as input, i.e. it constitutes a model-free determination of the effective mobility μ(F,n). It is especially suitable for semiconductors which require complex mobility models, like hopping or trap-dominated transport in disordered systems, and relatively low mobilities, like e.g. neat or mixed organic semiconductors.

Antagonistic responses between magnetoconductance and magnetoelectroluminescence in polymer light-emitting diodes

Antagonistic responses between magnetoconductance (MC) and magnetoelectroluminescence (MEL) in the polymer light-emitting diodes with an interfacial layer between Al cathode and active layer are simultaneously measured. As the interfacial layer (tetraoctylammonium bromide) is used, the significant increase in the number of injected negative polarons and the blocking of positive polarons promote the triplets-(free polaron) reaction and provide a good explanation for the reason that electroluminescence (EL) efficiency is maximal in the trap free space charge limited current regime at high bias. By fitting of MC and MEL curves using Lorentzian and non-Lorentzian empirical equations, three magnetic field dependent mechanisms, which are the intersystem crossing between singlet/triplet polaron pairs, the triplets-(free polaron) reaction, and the triplets-(trapped polaron) reaction are elucidated. The distribution of the three components is tunable by varying the applied electric field, which primarily modulates the triplets-(free polaron) reaction rate. The results pave a new route toward understanding the mechanism of organic spintronics for developing of multifunctional devices.

Modeling the transition from ohmic to space charge limited current in organic semiconductors

This paper presents a model for charge transport in organic and polymeric diodes that provides a physical explanation of the transition from ohmic to space-charge limited current (SCLC) regimes. The proposed model is based on two established models: a unified model for the injection and transport of charge in organic diodes, including a proper boundary condition for the free charge density at the metal–organic interface; and a temperature and electric-field dependent mobility model. The model reproduces published experimental current–voltage characteristics (for different temperatures and device lengths). The modeling results highlight the importance of the boundary condition at the interface that is used to explain different trends and their transitions in the current–voltage characteristics: linear, quadratic and a higher than quadratic trend at high electric fields. The model uses a finite charge at the interface to eliminate any dependence of the parameters of the mobility model with the length of the organic layer. Importantly, this result differs from others that consider an infinite charge at the interface, and make use of a length-dependent mobility, or other more complex models that incorporate a dependence with the free charge density in order to avoid such a dependence with the device length.