Space Charge Limited Current Transport Research Articles

SnO-SnO<sub>2</sub> Nanocomposites Based pn Diode: <i>In Situ</i> Synthesis, Characterization and Fabrication of Device for pn Diode Applicability

We report on an approach for the in-situ synthesis (chemical method based) of SnO-SnO2 nanocomposites followed by characterisation (including morphological, chemical, structural and optical) and investigation of the electrical properties of the nanocomposites with reference to the as-synthesized SnO2 nanoparticles. Compared to spherical SnO2 particles, the SnO phase is found existing in the form of sheet like morphology. It has been found that through controlling of the Sn:OH precursor ratio is effective for the achievement of SnO phase. Compared to the pristine SnO2 nanoparticles, the current-voltage (I-V) characteristics of the nanocomposites show the p-n junction characteristics. The observation of rectification ratio 2.05 indicates the excellent rectifying property of the nanocomposites due to the presence of p-type SnO phase. Further, exploration of the I-V characteristics has revealed the dominance of space-charge limited current transport mechanism for the nanocomposites sample. The lattice defects are discovered to be the cause of the transport mechanism in the nanocomposites sample.

Analytical model of space charge limited current for a cylindrical porous trap-limited dielectric

In this study, analytical models for space charge limited current (SCLC) transport in a porous (or disordered) trap-limited dielectric cylindrical configuration are developed. The method used in this paper is first verified by reproducing the well-known analytical results for planar cases developed decades ago based on the traditional approach. By considering the porous solid as a fractional object characterized by a parameter α≤1, we formulate its fractional capacitance and determine the SCLC transport by using the transit time approach. At α=1, it will recover the well-known Mott–Gurney law and Mark–Helfrich law for trap-free and trap-limited cases, respectively. For cylindrical geometry, our findings show an analytical form that is not available from the traditional methods. We anticipate that the proposed analytical model will serve as a useful tool for characterizing the current–voltage measurements in SCLC transport in dielectric breakdown and organic electronics, where spatial porosity of the materials is inevitable. The introduced fractional parameter α extracted from such characterization can facilitate the quantitative determination of the relationship between spatial porosity and charge mobility.

Ballistic-like space-charge-limited currents in halide perovskites at room temperature

The emergence of halide perovskites in photovoltaics has diversified the research on this material family and extended their application toward several fields in the optoelectronics, such as photo- and ionizing-radiation-detectors. One of the most basic characterization protocols consists of measuring the dark current–voltage (J−V) curve of symmetrically contacted samples for identifying the different regimes of the space-charge-limited current (SCLC). Customarily, J∝Vn indicates the Mott–Gurney law when n≈2 or the Child–Langmuir ballistic regime of SCLC when n=3/2. The latter has been found in perovskite samples. Herein, we start by discussing the interpretation of J∝V3/2 in relation to the masking effect of the dual electronic–ionic conductivity in halide perovskites. However, we do not discard the actual occurrence of SCLC transport with ballistic-like trends. Therefore, we introduce the models of quasi-ballistic velocity-dependent dissipation (QvD) and the ballistic-like voltage-dependent mobility (BVM) regimes of SCLC. The QvD model is shown to better describe electronic kinetics, whereas the BVM model results are suitable for describing both electronic and ionic kinetics in halide perovskites as a particular case of the Poole–Frenkel ionized-trap-assisted transport. The proposed formulations can be used as the characterization of effective mobilities, charge carrier concentrations and times-of-flight from J–V curves, and resistance from impedance spectroscopy spectra.

Using junction temperature to adjust the simulation of temperature effects on the current–voltage characteristics of two-layer organic light emitting diodes

In this study, the space charge limited current transport model was applied to simulate the current density–voltage characteristics of two-layer NPB/Alq3 organic light emitting diodes (OLEDs). Several parameters for simulations were moderately adjusted, and the best set of parameters were obtained. Upon increasing the applied voltage, the junction temperature of the OLED increased, further influencing the transport of the carriers. When the junction temperature was used as the reference temperature in analysis of the relationship between voltage and temperature, the simulated current density–voltage characteristics were found to be very consistent with the experimental data.

Highly symmetric and delayed excitonic emission response and space charge-limited current transport in β-irradiated WSe2 and WS2 nanoflakes

We report on the highly symmetric excitonic emission, manifested Raman active modes, and electrical properties of WSe2 and WS2 nanoflakes subjected to energetic β-particles. Raman spectra have revealed numerous mixed modes and a declining trend of the E12g-to-A1g intensity ratio with radiation dose (0–4.8 kGy). The nanoscale WSe2 exhibits a remarkable luminescence peak, located at ~ 675 nm and characterized by a combinatorial effect of both 2s and 2p excitons, whereas the nanoflakes of WS2 offered a testimony to charged excitons (trions) in the range ~ 624–630 nm. Moreover, a moderate (from ~ 2.29 to ~ 2.88 ns) and a nearly sevenfold enhancement in the slow life-time decay parameters have been observed for the irradiated WSe2 and WS2 nanoflakes, respectively. Furthermore, the rectifying nature of the I–V trends suggests formation of metal–semiconductor nanojunctions with transport characteristics described by parameters such as ideality factor (η) and barrier height (φb), both of which gave a declining trend with increasing β-dose. As substantiated by transport characteristics and first principle calculations, a semi-metallic behavior of WSe2 was also realized due to the introduction of excessive chalcogen defects at the highest radiation dose. The results would strengthen our insights prior to their deployment in next-generation nanodevices integrating nano-electronics and nano-photonics at large.

Space charge-limited current transport in thin films of alkyl-functionalized silicon nanocrystals

We describe the fabrication and electrical characterization of all-silicon electrode devices to study the electronic properties of thin films of silicon nanocrystals (SiNCs). Planar, highly doped Si electrodes with contact separation of 200 nm were fabricated from silicon-on-insulator substrates, by combination of electron beam lithography and reactive ion etching. The gaps between the electrodes of height 110 nm were filled with thin-films of hexyl functionalized SiNCs (diameter 3 nm) from colloidal dispersions, via a pressure-transducing PDMS (polydimethylsiloxane) membrane. This novel approach allowed the formation of homogeneous SiNC films with precise control of their thickness in the range of 15–90 nm, practically without any voids or cracks. The measured conductance of the highly resistive SiNC films at high bias voltages up to 60 V scaled approximately linearly with gap width (5–50 μm) and gap filling height, with little device-to-device variance. We attribute the observed, pronounced hysteretic current–voltage (I–V) characteristics to space-charge-limited current transport, which—after about twenty cycles—eventually blocks the current almost completely. We propose our all-silicon device scheme and gap filling methodology as a platform to investigate charge transport in novel hybrid materials at the nanoscale, in particular in the high resistivity regime.

Effects of interfacial layer on the electrical properties of n-ZnO/p-Si heterojunction diodes between 260 and 340 K

Electrical characteristics and transport properties of n-ZnO/p-Si heterojunction diodes were investigated by current–voltage (I–V) measurements between 260 and 340 K. The ZnO layer was deposited onto p-Si substrate via RF sputtering and the films were annealed at 673 K and 973 K, separately. X-ray diffraction analyses indicate that the both the films are highly oriented along (002) plane. Fourier-transform infrared spectroscopy measurements demonstrate that an interfacial layer (IL) is formed after 973 K annealing. The rectifying behavior of the diodes annealed at 673 K is better than the diodes annealed at 973 K due to the well interface and oxide quality. Saturation current (Io), barrier height (ϕb) and ideality factor (n) markedly vary with ambient temperature. The Io of the diodes annealed at 973 K is higher than the diodes annealed at 673 K which may be attributed to different activation energies of the defects. The ϕb values of the diodes annealed at 973 K significantly decrease, while the n values increase as compared to the diodes annealed at 673 K at the same ambient temperature. Forward bias I–V curves suggest that the current conduction can be approximated to space charge limited current transport between 340 and 300 K, while defect assisted tunneling becomes more dominant to carrier conduction between 300 K and 260 K for the diodes annealed at 673 K. On the other hand, the tunneling transport dominates the current conduction of the diodes annealed at 973 K at all ambient temperatures. The obtained results have depicted that presence of the parasitic IL significantly degrades the electrical performance of diodes.

Synthesis and study of morphological, optical and electrical properties of new organic semi conducting polymers containing isosorbide pendant group

Tow isosorbide containing poly(phenylene vinylene-arylene vinylene)s have been synthesized via Wittig polycondensation: PPVIs-Carb (arylene: carbazole) and PPVIs-Naph (arylene: naphtalene). The macromolecular structures were characterized by NMR and FTIR spectroscopies. The polymers are soluble in common organic solvents and show good film-forming abilities thanks to the presence of the polar isosorbide side groups. These organic materials showed an amorphous behavior. A higher polarity of the surface was obtained for PPVIs-Naph. The both polymer films are photoluminescent and exhibit an optical gap of 2.3eV. The carbazole-based polymer emits in yellow-green while a green-yellow emission was obtained in the naphtalene conatainig polymer with higher fluorescence quantum efficiency. The HOMO/LUMO energy levels were determined by cyclic voltammetry and showed higher ionization potential and electronic affinity for PPVIs-Carb. The study of the ITO/polymer thin film/Al devices showed a space-charge limited current transport mechanism with charge carrier mobility in order of 10−8cm2V−1s−1.

Operation mechanism of high performance organic permeable base transistors with an insulated and perforated base electrode

The organic permeable base transistor is a vertical transistor architecture that enables high performance while maintaining a simple low-resolution fabrication. It has been argued that the charge transport through the nano-sized openings of the central base electrode limits the performance. Here, we demonstrate by using 3D drift-diffusion simulations that this is not the case in the relevant operation range. At low current densities, the applied base potential controls the number of charges that can pass through an opening and the opening is the current limiting factor. However, at higher current densities, charges accumulate within the openings and in front of the base insulation, allowing for an efficient lateral transport of charges towards the next opening. The on-state in the current-voltage characteristics reaches the maximum possible current given by space charge limited current transport through the intrinsic semiconductor layers. Thus, even a small effective area of the openings can drive huge current densities, and further device optimization has to focus on reducing the intrinsic layer thickness to a minimum.

Fractional-dimensional Child-Langmuir law for a rough cathode

This work presents a self-consistent model of space charge limited current transport in a gap combined of free-space and fractional-dimensional space (Fα), where α is the fractional dimension in the range 0 < α ≤ 1. In this approach, a closed-form fractional-dimensional generalization of Child-Langmuir (CL) law is derived in classical regime which is then used to model the effect of cathode surface roughness in a vacuum diode by replacing the rough cathode with a smooth cathode placed in a layer of effective fractional-dimensional space. Smooth transition of CL law from the fractional-dimensional to integer-dimensional space is also demonstrated. The model has been validated by comparing results with an experiment.

Quantum theory of space charge limited current in solids

We present a quantum model of space charge limited current transport inside trap-free solids with planar geometry in the mean field approximation. We use a simple transformation which allows us to find the exact analytical solution for the steady state current case. We use our approach to find a Mott-Gurney like behavior and the mobility for single charge carriers in the quantum regime in solids.

Analysis of current transport properties in nonpolar a-plane ZnO-based Schottky diodes

Using current-voltage (I–V) measurements, we investigated the temperature-dependent transport properties in Ag/nonpolar a-plane ZnO Schottky diodes. The bias-dependent ideality factors were altered by the different temperatures and showed a hump at lower temperatures. The series resistance of the diode depended on the temperatures, which was related to the number of free carriers contributing to the series resistance. For high forward bias, the slope m obtained from the lnI–lnV curves decreased with increasing temperature, assuring the space-charge-limited-current (SCLC) model controlled by an exponential distribution of traps. The reverse-biased current transport was associated with the Schottky effect, with a thermally-assisted tunneling for lower voltages and the Poole-Frenkel effect for higher voltages. The density of localized states (Nt) was obtained by applying the theory of SCLC transport, which yielded a Nt value of 8.32 × 1011 eV−1cm−3.

Analytical re-derivation of space charge limited current in solids using capacitor model

In this paper, we have used a capacitor model to reproduce the known analytical formulas of space charge limited current transport inside both trap-free and trap-filled solids in planar geometry. It is found that the approach is simple when compared to the traditional method as the latter involves solving second order differential equation. Exact analytical results can also be obtained for cylindrical diode with an outer radius much larger than the inner radius.

Space‐charge‐limited current transport in nitride HFETs

AbstractHigh‐voltage microwave AlGaN/GaN HFETs operating under high‐power conditions suffer from degraded RF performance and linearity due to a nonlinear resistance effect in the gate‐source region. During RF operation, the nonlinear resistance is due to the onset a of space‐charge‐limited current (SCLC) transport mechanism within the device channel. Under high current injection conditions, SCLC transport can set in and, consequently, the source resistance becomes a function of the injected charge causing a rapid increase and limiting device performance. The threshold for SCL current is dependent on the donor‐like states on the AlGaN surface which are responsible for supplying electrons to the channel. To alleviate the effect of nonlinear source resistance we show on an un‐gated HFET channel model that the critical current density of space‐charge effects and thus the onset of nonlinear source resistance can be shifted above the normal operating current density of the device by modification of the charge on the AlGaN surface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Trap-limited mobility in space-charge limited current in organic layers

Space-charge limited current transport in organic devices, relevant to the operation of a range of organic optoelectronic devices, is analyzed in the frequency domain. The classical multiple trapping picture with one transport state and one trap level is used as the basis for the descriptions. By varying the energetic and kinetic properties of the traps, we show that the admittance and the capacitance spectra are considerably modified depending on the interplay between the trap-limited mobility and the trap kinetics. We point out that capacitance steps at low-frequency, usually found in experiments, are observed only for slow traps.

Electrochemical deposition and characterization of phosphorous doped p-CdTe thin films

An electrodeposition process for formation of low resistivity phosphorous doped p-type CdTe films is described. The deposition is carried out in organic electrolyte which enables ionic doping by phosphorous in situ with the growth of CdTe films. Te to P atomic ratios in the film, their electrolytic concentrations and activation energy analysis establishes that p-conversion is due to acceptor states at E v + 0.05 eV formed by P atoms selectively occupying Te sites in CdTe. The lower resistivity limit of 5–10 Ω cm of p-CdTe films is set by generation of interstitial P related defects if the P concentration exceeds a threshold value of 6 × 10 −4M. Analysis of space charge limited current transport in these films establishes hole concentrations of 8 × 10 16 cm −3 and associated low defect densities around 9 × 10 14 cm −3 obtained through good stoichiometric control inherent to this technique. Codeposition of P modifies nucleation and growth, resulting in p-CdTe films having hexagonal structure and oriented crystallites of over 0.8 μm grain size. Due to these properties p-CdTe films are well suited for application to solar cells.