Space Charge Limited Transport Research Articles

Two-dimensional model of space-limited current in the weakly collisional regime for an inhomogenous medium

This short report describes the phenomenon of space-charge-limited (SCL) current transport between ballistic and collision-dominated regimes for an inhomogeneous medium with a finite emission area. This intermediate regime can be considered as a weakly collisional regime where the collisional mean free path is comparable to the length of the medium. The SCL current density is calculated as a function of the degree of collision, inhomogeneity of the medium, and the geometrical properties of the emitting area. The inhomogeneity of the medium is characterized by a parameter of (0<α≤1), where α = 1 denotes a perfect homogenous medium. The calculated SCL current density is enhanced by finite emission area effects by a factor of 1+F×G, where F measures the mean position of the electrons in the medium, and G is a geometrical correction factor due to finite emission area. The enhancement is found to be higher in the collisional regime as compared to the ballistic regime. A higher inhomogeneity (with smaller α) also increases the enhancement. Smooth transition between the fully ballistic SCL transport (Child–Langmuir model) and the collision-dominated SCL transport (Mott–Gurney model) is demonstrated and verified, respectively, by the particle-in-cell code and the device simulator. This model is useful for the characterization of high-current SCL transport where the non-ideal conditions (such as inhomogenous medium and weakly collisional regime) cannot be described by the existing SCL models.

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.

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.

Diamond lateral FinFET with triode-like behavior

In this letter we report a diamond lateral FinFET fabricated using an ohmic regrowth technique. The use of ohmic regrowth separates the source/drain and gate fabrication, providing a viable means to improve ohmic contact resistance while protecting the top surface of the diamond channel from dry etch damage. Enabled by high channel quality, the diamond transistor behavior was shown to transit from a pentode-like to a triode-like characteristic when channel length decreased. For the first time, space charge limited transport in diamond FinFETs with a short channel length was demonstrated. We have analyzed the space charge limited transport from room temperature to 150 °C. This space charge limited transport, in combination with improved ohmic contacts, will enable diamond FinFETs for various high-power applications.

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.

High gradient pulsed quadrupoles for novel accelerators and space charge limited beam transport

Novel acceleration schemes like plasma wake-field based accelerators demand for high gradient focusing elements to match the Twiss parameters in the plasma to the transport lattice of the conventional accelerator beamlines, with typically much higher beta-functions. There are multiple candidates for achieving high gradient focusing fields, each one having certain drawbacks. Permanent magnets are limited in tunability, plasma lenses might degrade the transverse beam quality significantly and conventional magnets cannot reach very high gradients and often cannot be placed in direct proximity of the plasma accelerator because of their size. In this paper we present design considerations and simulations on compact, high gradient, pulsed quadrupoles, that could be used e.g. for final focusing of space charge dominated bunches into a LWFA (Laser Wake-Field Accelerator) at SINBAD or other facilities with similar demands. The target design gradient is 200 T/m at a physical aperture on the order of 10 mm.

Role of Near Substrate and Bulk Polymer Morphology on Out-of-Plane Space-Charge Limited Hole Mobility.

Charge transport is a central issue in all types of organic electronic devices. In organic films, charge transport is crucially limited by film microstructure and the nature of the substrate/organic interface interactions. In this report, we discuss the influence of active layer thickness on space-charge limited hole transport in pristine polymer and polymer/fullerene bulk heterojunction thin films (∼15-300 nm) in a diode structure. According to the results, the out-of-plane hole mobility in pristine polymers is sensitive to the degree of polymer chain aggregation. Blending the polymers with a fullerene molecule does not change the trend of hole mobility if the polymer tends to make an amorphous structure. However, employing an aggregating polymer in a bulk heterojunction blend gives rise to a marked difference in charge carrier transport behavior compared to the pristine polymer and this difference is sensitive to active layer thickness. In aggregating polymer films, the thickness-dependent interchain interaction was found to have direct impact on hole mobility. The thickness-dependent mobility trend was found to correspond well with the trend of fill factors of corresponding bulk heterojunction solar cells. This investigation has a vital implication for material design and the development of efficient organic electronic devices, including solar cells and light-emitting 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.

Intrinsic non-ohmic electronic transport properties of the transparent In–Zn–O compound nanobelts under ohmic contact and out of the space charge limited transport region

It is generally accepted that the nonlinear I–V characteristics for semiconductor nanostructures are mainly induced by the Schottky contacts or by the space charge limited transport mechanism. We perform I–V measurements on undoped and doped In–Zn–O compound nanobelts and confirm that their intrinsic non-ohmic transport behaviors are not caused by these mechanisms. A model based on the hopping assisted trap state electrons transport process is introduced to explain the nonlinear I–V characteristics and to extract their electrical parameters. An understanding of this trap-state influenced carrier transport can advance the progress of nanomaterials applications and enable us to distinguish their intrinsic transport behaviors from contact effects. The results also indicate that the material has good electrical properties and can be used as a potential substitute for In2O3.

Steady-state and high-frequency electron transport in GaN nanowires

The results of experimental and theoretical researches of steady-state and high-frequency electric characteristics of planar AlGaN/GaN heterostructure nanowires are presented. A strong depletion effect associated with electron trapping on the edge states is observed for narrow nanowires with widths ∼ 200 — 400 nm. The spatial distributions of the electron concentration and electrostatic potential are calculated. At small applied voltages (2 — 10 V) for nanowires of length equal to 600 μm we found non-linear current-voltage characteristics related to a space-charge limited transport regime. We found that for narrow wires transversal transport is essentially ballistic: this dramatically modifies the high-frequency properties of the electron gas, in particularly, leading to a resonant absorption of THz radiation.

Space-charge limited transport in large-area monolayer hexagonal boron nitride.

Hexagonal boron nitride (hBN) is a wide-gap material that has attracted significant attention as an ideal dielectric substrate for 2D crystal heterostructures. We report here the first observation of in-plane charge transport in large-area monolayer hBN, grown by chemical vapor deposition. The quadratic scaling of current with voltage at high bias corresponds to a space-charge limited conduction mechanism, with a room-temperature mobility reaching up to 0.01 cm(2)/(V s) at electric fields up to 100 kV/cm in the absence of dielectric breakdown. The observation of in-plane charge transport highlights the semiconducting nature of monolayer hBN, and identifies hBN as a wide-gap 2D crystal capable of supporting charge transport at high field. Future exploration of charge transport in hBN is motivated by the fundamental study of UV optoelectronics and the massive Dirac fermion spectrum of hBN.

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.

Conductance control in VO2 nanowires by surface doping with gold nanoparticles.

The material properties of semiconductor nanowires are greatly affected by electrical, optical, and chemical processes occurring at their surfaces because of the very large surface-to-volume ratio. Precise control over doping as well as the surface charge properties has been demonstrated in thin films and nanowires for fundamental physics and application-oriented research. However, surface doping behavior is expected to differ markedly from bulk doping in conventional semiconductor materials. Here, we show that placing gold nanoparticles, in controlled manner, on the surface of an insulating vanadium dioxide nanowire introduces local charge carriers in the nanowire, and one could, in principle, completely and continuously alter the material properties of the nanowire and obtain any intermediate level of conductivity. The current in the nanowire increased by nearly 3 times when gold nanoparticles of 10(11) cm(-2) order of density were controllably placed on the nanowire surface. A strong quadratic space-charge limited (SCL) transport behavior was also observed from the conductance curve suggesting the formation of two-dimensional (2D) electron-gas-like confined layer in the nanowire with adsorbed Au NPs. In addition to stimulating scientific interest, such unusual surface doping phenomena may lead to new applications of vanadium dioxide-based electronic, optical, and chemical sensing nanodevices.

Epitaxial growth of large area single-crystalline few-layer MoS2 with high space charge mobility of 192 cm2 V−1 s−1

We report on the vapor-solid growth of single crystalline few-layer MoS2 films on (0001)-oriented sapphire with excellent structural and electrical properties over centimeter length scale. High-resolution X-ray diffraction scans indicated that the films had good out-of-plane ordering and epitaxial registry. A carrier density of ∼2 × 1011 cm−2 and a room temperature mobility of 192 cm2/Vs were extracted from space-charge limited transport regime in the films. The electron mobility was found to exhibit in-plane anisotropy with a ratio of ∼1.8. Theoretical estimates of the temperature-dependent electron mobility including optical phonon, acoustic deformation potential, and remote ionized impurity scattering were found to satisfactorily match the measured data. The synthesis approach reported here demonstrates the feasibility of device quality few-layer MoS2 films with excellent uniformity and high quality.

Modeling of the Gate Leakage Current in AlGaN/GaN HFETs

A new physics-based model of the gate leakage current in AlGaN/GaN heterojunction field effect transistors (HFETs) is demonstrated. The model predicts accurately the gate-leakage current for a wide range of gate-drain voltage. The model is based on the formulation of tunneling and space charge limited (SCL) current flow. The gate leakage current is shown to flow through two paths: 1) the surface of the device establishes the primary path and the current transport mechanism is modeled using SCL transport in the presence of shallow traps and 2) the traps in the AlGaN layer assist in the tunneling of electrons from the gate to the 2-D electron gas, which flows to the drain electrode constituting the second path for the leakage current. The trap levels extracted from the model are consistent with the reports presented in the literature. The model appropriately explains the gate leakage current of the AlGaN/GaN HFETs for drain voltages up to 170 V and is verified by comparing the model results with the measured gate leakage data of industrial devices.

Temperature Dependence of Electron Transport in ZnO Nanowire Field Effect Transistors

Nanowires (NWs) have attracted considerable interests for electronic and optoelectronic device applications. However, the carrier transport mechanisms in these NW devices have not been well understood. Here we present the electron transport of ZnO NWs field effect transistors (FETs). Our results show that the electron transport of ZnO NW FETs is governed by the space charge limited model at temperatures below a trap temperature. Above the trap temperature, the electron transport is thermionic emission dominated. Based on the space charge limited model, an accurate method is developed for field effect mobility extraction. The extracted electron carrier mobility is strongly dependent on temperature with a peak value of 51 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs at 167 K. Under the space-charge limited transport, the field mobility is lower in comparison with the values extracted from the thermionic emission model. The electron mobility due to space charge scattering has a value of 483 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs at the trap temperature with temperature dependence of T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 ~ 5</sup> . The interface state density between the back gate dielectric and ZnO NWs is in the range 9.03×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> /cm-8.72×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> /cm at temperatures ranging from 77 to 227 K.

Charge transport in lightly reduced graphene oxide: A transport energy perspective

Significant variation in the charge transport behaviour in graphene oxide (GO) ranging from Schottky to Poole-Frenkel and to space charge limited transport exists. These have been extensively reported in the literature. However, the validity of such conventional charge transport models meant for delocalized carriers, to study charge transport through localised states in GO, a disordered semiconductor is open to question. In this work, we use the concept of transport energy (TE) to model charge transport in lightly reduced GO (RGO) and demonstrate that the TE calculations match well with temperature dependent experimental I-V data on RGO. We report on a temperature dependent TE ranging from a few 10 meV to 0.1 eV in slightly reduced GO. Last, we point out that, despite the success of several delocalised charge transport models in estimating barrier heights that resemble the TE level, they remain largely accidental and lack the insight in which the TE concept provides in understanding charge transport in RGO.

Colloidal chemically fabricated ZnO : Cu-based photodetector with extended UV-visible detection waveband

Polycrystalline ZnO : Cu-based film photodetectors with extended detection waveband (UV and visible light) were fabricated using facile colloidal chemistry and a post-annealing process. The obtained detectors are highly sensitive to visible light and can realize the response switch between UV and visible light. A native and extrinsic trap cooperatively controlled space charge limited (SCL) transport mechanism is proposed to understand this complex photoconduction behaviour.