Space Charge Limited Current Region Research Articles

Modulating multiple leakage current mechanisms in the [LaNiO3/Ba0.67Sr0.33TiO3]3 multilayer heterostructure thin films via dielectrics/electrode interface modifications

Ferroelectric (FE) multilayer heterostructure films have attracted significant attention due to their superior dielectric performance, which shows increasing opportunities in the application of energy storage capacitors or high-capacitance density systems. However, the leakage current density in FE multilayer heterostructure thin films is closely linked to the thickness of each single FE layer and the number of hetero-structure interfaces. In Pt/[LaNiO3/Ba0.67Sr0.33TiO3]3 (Pt/[LNO/BST]3) multilayer thin films, the dominant leakage current mechanism is Poole–Frenkel (PF) emission at room temperature. The space charge limited current (SCLC) and the co-dominated leakage current mechanism from the PF and Schottky (SC) emissions were observed under higher operating temperatures. After inserting an ultrathin SrTiO3 (STO) layer between the Pt/[LNO/BST]3 multilayer thin films and the top Au electrode, the SCLC region was replaced by an Ohmic region, and the SC emission was not detected in the temperature range of 288–368 K. Moreover, the calculated zero-field energy barriers ϕPF of PF emission are higher than the prepared multilayer thin films without the STO layer. Consequently, the leakage current density of Pt/[LNO/BST]3 thin films showed a significant decrease.

Analysis of the improved thermal stability of Al-doped ZnO-adopted organic solar cells

The stable performance of organic solar cells (OSCs) at high temperatures is a critical issue for their commercialization. Although a few studies have reported the improved stability of OSCs that adopted Al-doped ZnO (AZO) as an electron transport layer (ETL), systematic research that validates the origins of this improvement has been lacking. In this work, we investigated the underlying mechanism for the improved stability of an OSC with AZO. By adopting AZO, the power conversion efficiency reached 8.65% and retained 70% of its initial value at 85 °C for 6 h, which was an increase in stability of 46% compared to that of an OSC with pristine ZnO. We found that the improved stability of the OSC with AZO originated from the decrease in oxygen defects and the space-charge limited current region through trap-related analyses and the Mott-Gurney law. Therefore, the results supported the enhanced thermal stability of OSCs that incorporated AZO as an ETL.

CdS/Si nanofilm heterojunctions based on amorphous silicon films: Fabrication, structures, and electrical properties* *Project supported by the Natural Science Foundation of Henan Province, China (Grant No. 202300410304) and the Key Research Project for Science and Technology of the Education Department of Henan Province, China (Grant No. 21A140021).

Shortening the distance between the depletion region and the electrodes to reduce the trapped probability of carriers is a useful approach for improving the performance of heterojunction. The CdS/Si nanofilm heterojunctions are fabricated by using the radio frequency magnetron sputtering method to deposit the amorphous silicon nanofilms and CdS nanofilms on the ITO glass in turn. The relation of current density to applied voltage (I–V) shows the obvious rectification effect. From the analysis of the double logarithm I–V curve it follows that below ∼ 2.73 V the electron behaviors obey the Ohmic mechanism and above ∼ 2.73 V the electron behaviors conform to the space charge limited current (SCLC) mechanism. In the SCLC region part of the traps between the Fermi level and conduction band are occupied, and with the increase of voltage most of the traps are occupied. It is believed that CdS/Si nanofilm heterojunction is a potential candidate in the field of nano electronic and optoelectronic devices by optimizing its fabricating procedure.

Investigation of negative magneto-conductance properties of cobalt phthalocyanine thin films

The opto-electrical and magnetic field effects on cobalt phthalocyanine (CoPc) thin film single layer device were explored. The root mean square roughness of thin film was 5.8 nm. The mobility was estimated for space charge limited current region in the current–voltage characteristics. Magnetic field effect on injection current was studied in terms of organic magneto-conductance. The negative magneto-conductance (MC) was observed in CoPc thin film diode at different bias voltages. The negative values of MC were found to be 10.05, 8.29, 6.84, 4.98 and 3.13% for applied bias voltages 1, 2, 3, 4 and 5 V, respectively. The CoPc thin film absorbed the UV–visible wavelength range which were represented by two bands of CoPc molecule, B band and Q band. MC study of device under dark and light reveals the formation of trap assisted bipolarons and their quenching by the formation of photogenerated excitons. The device was checked for the resistive and capacitive behaviour by impedance spectroscopy. The contact resistance is approximately constant ~ 66 Ω at all applied bias voltages. The bulk and interface resistance vary from 1.8 to 0.6 kΩ and 50.06 to 1.43 kΩ, respectively. Similarly, the bulk and interface capacitance vary from 55.81 to 10.56 nF and 50.47 to 558.09 nF, respectively for applied bias voltages.

Air-stable, high current density, solution-processable, amorphous organic rectifying diodes (ORDs) for low-cost fabrication of flexible passive low frequency RFID tags

We report on long-term air-stable organic rectifying diodes (ORD) on flexible substrates based on a solution deposited amorphous organic semiconductor (OSC) material, consisting of a co-polymer of arylamine and a fused aromatic species, reaching charge-carrier mobilities of μ=0.05cm2/Vs (space charge limited current region) and current densities of up to 100A/cm2 at 10V levels with rectification ratios of 104 operating in the 10kHz range. The ORDs exhibit a high degree of air-stability without any passivation with extremely reliable reproducibility. ORDs were fabricated on polyethylene naphthalate foils in a vertical sandwich structure with gold and aluminium as the injecting and blocking electrodes respectively via evaporation with the OSC spin-coated in between (Type1). In order to improve device performance of the ORDs, poly(ethylenedioxythiophene):poly(styrenesulfonate) was μ-dispensed as a hole-injection layer (Type2). The results for Type1-diodes show a really narrow spread of the diode characteristics, whereas for Type2 diodes the spread is slightly more but still acceptable. These ORDs prove themselves better than conventional pentacene diodes both in terms of reliability/repeatability of the diode performance and air-stability without an encapsulation layer, and goes towards the enabling of a viable and reliable low-cost fabrication method of radio frequency identification-tags using organic semiconductors.

Investigation of temperature-dependent electrical properties of p-VOPc/n-si heterojunction under dark conditions

An organic/inorganic heterojunction p-VOPc/n-Si was fabricated and its electrical properties were investigated. Temperature-dependent dark current–voltage (I–V) characteristics of the heterojunction exhibited rectification behaviour with a rectification ratio of 405 at ±1 V and room temperature. The current–voltage characteristics of the cell showed ohmic conduction at low voltages followed by a space charge-limited current (SCLC) conduction dominated by an exponential trap distribution at higher voltages. At room temperature, the series and shunt resistances were found to be approximately 1.4 and 100 kΩ, respectively. Diode ideality factor n was found to be 3.2 at room temperature and dropped to 1.9 at 363 K. Room temperature mobility of vanadyl phthalocyanine (VOPc) was extracted from the I–V characteristics in the SCLC region and was found approximately 15.5 × 10−3 cm2 V−1 s−1. The effective barrier height, ФB, was estimated as 0.77 eV. The effect of temperature, on various heterojunction parameters was recorded under dark conditions and at temperatures ranging from 300 to 363 K.

Current transport mechanisms for heterojunctions of a-Se on various crystalline wafers (n-Si, p-Si and n-GaAs)

Heterojunction diodes fabricated by thermal evaporation of p-type amorphous selenium (a-Se) on various crystalline wafers (n-Si, p-Si and n-GaAs) are analyzed by measuring their current–voltage ( J– V) characteristics. The measured J– V characteristics for the investigated devices of configuration Au/a-Se/c-wafer/Al, exhibit a rectifying behavior and the bulk effect of the a-Se layer. For low forward voltage, the conduction mechanism is dominated by recombination of the carriers in the amorphous side of the space charge region. At higher voltage, the J– V characteristics could be divided into two regions: an ohmic region and a space charge limited current region. The values of the activation energy obtained from the ohmic region are in agreement with those obtained from dc conductivity measurements in the same range of temperature. The reverse bias activation energy values at different temperatures are in agreement with those obtained from the temperature dependence of the forward saturation current, supporting the proposed recombination mechanism of conduction.

Current Transport Mechanisms for MSM-Photodetectors on ZnO:N Thin Films

AbstractMetal-semiconductor-metal photodetectors (MSM-PDs) on ZnO:N thin films deposited by radiofrequency (RF) sputtering and with post N+ ion implantation processing were fabricated using a ZnO/Si structure. A 10 times reduction in dark current was observed compared to the devices on an as-deposited ZnO thin film without ion implantation. These MSM-PDs gave performances of a photo-to-dark current ratio of 2030 and responsivity (R) = 2.7 A/W; the pulse response was a 12.3 ns rise time and 15.1 ns fall time using a femto-second pulse. Temperature-dependent current -voltage (I-V-T) characteristics of the MSM-PDs were observed and the space charge limited current (SCLC) theory was applied to determine the current transport mechanisms. In the SCLC region, J∼Vm gave m to determine the current transport mechanism and the value of m changes with temperatures and applied voltages. Current transport is governed by the ZnO structure rather than the electrodes.

Thermally stimulated current and space charge limited current mechanism in film of the gold/zinc oxide/gold type

The thermally stimulated current (TSC) and space charge limited current (SCLC) properties of n-type spray pyrolyzed ZnO thin film were investigated using TSC and current–voltage measurements. TSC measurements of the n-type film revealed the presence of one peak. The results of the characteristic parameters of the peak are T max = 179.6 K , peak current = 9.305 × 10 - 6 A . The values of the capture cross-section of the trap S t and attempt to escape frequency ν of a trapped electron for the sample were found to be 6.99 × 10 - 30 cm 2 and 2.783 × 10 - 4 s - 1 , respectively. The activation energy of the trap was found to be 0.00318 ± 1.43 × 10 - 4 eV . The observed trap level allows TSC. The current–voltage characteristic at 10 K shows ohmic and space charge limited current regions (SCLC).

Carrier Transport Mechanisms in Organic Light-Emitting Devices with a N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine:tris-(8-hydroxyquinoline) Aluminum Mixed Layer Acting as an Emitting Layer

The current density-voltage (J-V) measurements on organic light-emitting devices (OLEDs) with and without a N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB):tris-(8-hydroxyquinoline) aluminum (Alq3) mixed layer were performed to investigate the carrier transport mechanism. The slopes of the double logarithmic J-V curves for the OLEDs with and without the mixed layer in the space-charge-limited current (SCLC) region were 6.5 and 2, respectively. The dramatic increase in the slope of the double logarithmic J-V curve for the OLEDs with the mixed layer was attributed to the decrease in the mobility difference of the charge carriers at the NPB/Alq3 heterointerface resulting from the minimization of the energy barriers, and the carrier transport behaviors in the SCLC region were significantly affected by the unintentional virtual electrode, serving as a charge carrier reservoir, that formed at the heterointerface.

Charge carrier transport in thermally oxidized metal/PS/p-Si and metal/PS/n-Si structures

Metal/PS/c-Si structures with porous silicon (PS) layers of 55–75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400–960 °C. We studied the effect of oxidation on the charge carrier transport in these structures. It was demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current–voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness deff of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential deff-dependence of the conductance in the ohmic region observed at low bias is explained by carrier tunnelling through potential barriers formed by SiOX isthmuses between silicon crystallites. The results can be used for fabrication of electronic devices based on oxidized PS and similar materials.

Tunneling Conductivity in Thermally Oxidized Porous Silicon

AbstractMetal/PS/c-Si structures with porous silicon (PS) layers of 55-75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400-960°C. The effect of oxidation on the photoluminescence and the transport of charge carriers in these structures were studied. We demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current – voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential dependence of the ohmic conductance on the thickness of SiOX tissue is explained by tunneling of carriers through potential barriers formed by the tissue surrounding silicon crystallites.

Electric Conduction of Electroluminescent Metal Chelate Thin Films

Electric conduction in vacuum-deposited films of electroluminescent metal-chelates, tris-(8-hydroxyquinoline) aluminum (Alq3) and zinc bis-(2-(o-hydroxypheyl)-benzoxazolate)) (ZnPBO), were studied by current (I)-voltage (V) measurement. The I-V characteristics demonstrated that the films exhibited trap-controlled conduction. For the Alq3 film, conductivity σ was estimated to be 3.0 × 10−15 Scm−1 from the characteristic in ohmic current region. From the I-V characteristic in space charge limited current region, further, carrier mobility μ of the Alq3 film was estimated to be 5.0 × 10−7 cm2V−1s−1; this value is in good agreement with electron mobility evaluated by the time-of-fight technique. For ZnPBO, the values of σ and μ were estimated to be 2.0 × 10−14 Scm−1 and 7.0 × 10−6 cm2V−1s−1, respectively.

Laser diodes in photon number squeezed state

A laser diode with an intrinsic layer as the space charge limited current region is expected to emit a low noise (less than the shot noise level) light. However, when one applies the intrinsic layer to the laser diode, severe difficulty is faced. Because the intrinsic layer has a very high resistivity, the applied voltage to operate the laser diode is too large and causes catastrophic damage to the laser diode. Here we propose novel laser diodes which emit a low noise light. The first is an AlGaAs laser diode having an undoped layer between the active layer and the cladding layer which acts as the space charge limited current region. Fano factor, F/sub m/, of this laser diode is 28% smaller than the shot noise level (standard quantum limit, F/sub m/=1) at 21 mA (output power, P/sub 0/=20 mW). The second one is an InGaAsP laser diode having two tunnel barrier layers whose bandgap energy is larger than that of the cladding layer. The region between the barriers acts as the space charge limited current region, Fano factor, F/sub m/ of this laser diode is 47% smaller than the shot noise level at 21 mA (P/sub 0/=10 mW). On the other hand, an AlGaAs laser diode with the two tunnel barrier layers has Fano factor, F/sub m/ which is 43% smaller than the shot noise level at 21 mA (P/sub 0/=20 mW). The calculated amplitude noise spectral densities of the latter two laser diodes are in good agreement with the calculated values from Langevin method. However, the calculated amplitude noise spectral density of the former laser diode does not agree with the calculated value from Langevin method. This disagreement is also discussed.

Carrier Transport Properties of Sputter-Deposited CdS/CdTe Heterojunction

CdS, CdTe and CdS/CdTe heterojunction films were deposited on glass substrates by RF sputter deposition using undoped CdS and CdTe targets, respectively. The deposited CdS and CdTe films have high resistivities greater than 106 Ω·cm and 109 Ω·cm, and preferential orientations at (002) and (111) planes, respectively. CdS/CdTe heterojunction film shows good rectification properties. The carrier transport properties of the CdS/CdTe heterojunction were investigated by means of current-voltage measurements at different temperatures. The forward current is classified into the tunneling-recombination current, recombination current, space charge limited current (SCLC) and trap filling of SCLC regions by considering applied voltage. The reverse current is dominated by the generation recombination current in the depletion region of the CdTe layer.

Conduction properties of thin N +/I/N + a-Si:H structures

The conduction properties of n +/i/n + sandwich structures have been studied experimentally as a function of intrinsic layer thickness. Room temperature J-V characteristics and ohmic conduction activation energies have been measured on series of samples with the same intrinsic layer deposition conditions and various thicknesses. The data may be accounted for assuming spatially homogeneous intrinsic silicon physical properties and taking into account the effects of band bendings in the vicinity of the n +/i interfaces. The approach of scaling that takes place in the space charge limited current region of the J-V characteristics, in the limit of large thicknesses, is discussed.

Physical models for amorphous-silicon thin-film transistors and their implementation in a circuit simulation program

A semianalytic theory to describe both the current-voltage and capacitance-voltage characteristics of amorphous silicon thin-film transistors on the basis of their physics of operation is presented. In this model, the drain current is directly related to the electron concentration at the source side of the channel. This enables one to describe the various regimes of operation of these devices (i.e. subthreshold or above threshold) using only one equation. The output conductance of these devices in saturation is also considered, and it is shown that the finite output impedance is a consequence of the drain voltage modulating the effective channel length by creating a space-charge limited current region of variable length near the drain. The results of this model are in good agreement both with experimental data and the results of comprehensive two-dimensional simulations. These device models have been successfully incorporated into a SPICE circuit simulation program. >