Forward Dc Bias Research Articles

Study of A.C. conductivity and dielectric behaviour of hydrothermally synthesised molybdenum disulphide

This report presents an experimental study of electric and dielectric properties of hydrothermally synthesised molybdenum disulphide (MoS2) thin films using complex impedance spectroscopy (CIS). The measurements were taken in the bias voltage range 0 V–0.4 V and frequency 40 Hz–10 MHz. The CIS analysis depicts that both grain (bulk) as well as grain boundary effect has a significant role in the conduction mechanism of the compound. The frequency-dependent relaxation peak was observed in the impedance loss spectra with the upward rise in bias voltage. Also, we found that both real ( $${\varepsilon }{^{\prime}}$$ ) and imaginary part ( $${\varepsilon }^{{^{\prime}}{^{\prime}}}$$ ) of complex dielectric permittivity, both parts of electrical modulus (M′ and M′′) and AC conductivity of the MoS2 thin films are highly dependent on frequency. Existence of a single relaxation peak in electrical modulus plot further confirms the bulk effect in conduction. The varying ac conductivity plot with forward dc bias voltage endorses the occurrence of the hopping mechanism in the system, being well described by jump relaxation model (JRM). In short; this detailed study based on CIS shows the dielectric relaxation behaviour of MoS2 thin film and reflects the hopping induced conduction mechanism.

GMR Based Spin Valve like Magnetic Diode Exploiting Negative JMR in Heusler Alloy Thin Film/Semiconductor Heterostructures

Room temperature spin valve like magnetic diode behavior has been found in Co2MnSi/SiO2/n-Si heterostructure. The Co2MnSi (CMS) Heusler alloy film has been grown on n type Si (001) substrate with the help of Electron Beam Physical Vapour Deposition technique. We have investigated the transport properties of our heterostructure at various isothermal conditions in the temperature regime of 78-300 K. From the current (I) – voltage (V) characteristics of the junction, excellent rectifying tunnel diode like behavior have been seen near room temperature. The current (I) across the junction has been found to be increased with the application of magnetic B-field parallel to the CMS film clearly indicating negative Junction Magnetoresistance (JMR) of the heterostructure. It has been found to be negative 80% at 300 K at a magnetic field of 0.5 Tesla. When forward dc bias is applied to the heterostructure, the I-V characteristics are greatly influenced on turning on the field B = 0.5 T at 300 K, and the current reduces to the equal order of the magnitude of the reverse bias current. Hence our CMS/SiO2/n-Si heterostructure can perform in off (Ioff)/on (Ion) states with the use of zero/nonzero magnetic field like a spin valve at room temperature (300 K).

Bias Voltage-Dependent Impedance Spectroscopy Analysis of Hydrothermally Synthesized ZnS Nanoparticles

In this report, bias voltage-dependent dielectric and electron transport properties of ZnS nanoparticles were discussed. ZnS nanoparticles were synthesized by introducing a modified hydrothermal process. The powder XRD pattern indicates the phase purity, and field emission scanning electron microscope image demonstrates the morphology of the synthesized sample. The optical band gap energy (Eg = 4.2 eV) from UV measurement explores semiconductor behavior of the synthesized material. The electrical properties were performed at room temperature using complex impedance spectroscopy (CIS) technique as a function of frequency (40 Hz-10 MHz) under different forward dc bias voltages (0-1 V). The CIS analysis demonstrates the contribution of bulk resistance in conduction mechanism and its dependency on forward dc bias voltages. The imaginary part of the impedance versus frequency curve exhibits the existence of relaxation peak which shifts with increasing dc forward bias voltages. The dc bias voltage-dependent ac and dc conductivity of the synthesized ZnS was studied on thin film structure. A possible hopping mechanism for electrical transport processes in the system was investigated. Finally, it is worth to mention that this analysis of bias voltage-dependent dielectric and transport properties of as-synthesized ZnS showed excellent properties for emerging energy applications.

Spin valve-like magnetic tunnel diode exhibiting giant positive junction magnetoresistance at low temperature in Co2MnSi/SiO2/p-Si heterostructure

The rectifying magnetic tunnel diode has been fabricated by growing Co2MnSi (CMS) Heusler alloy film carefully on a properly cleaned p-Si (100) substrate with the help of electron beam physical vapor deposition technique and its structural, electrical and magnetic properties have been experimentally investigated in details. The electronic- and magneto-transport properties at various isothermal conditions have been studied in the temperature regime of 78–300 K. The current–voltage (I–V) characteristics of the junction show an excellent rectifying magnetic tunnel diode-like behavior throughout that temperature regime. The current (I) across the junction has been found to decrease with the application of a magnetic field parallel to the plane of the CMS film clearly indicating positive junction magnetoresistance (JMR) of the heterostructure. When forward dc bias is applied to the heterostructure, the I–V characteristics are highly influenced on turning on the field B = 0.5 T at 78 K, and the forward current reduces abruptly (99.2% current reduction at 3 V) which is nearly equal to the order of the magnitude of the current observed in the reverse bias. Hence, our Co2MnSi/SiO2/p-Si heterostructure can perform in off (Ioff)/on (Ion) states with the application of non-zero/zero magnetic field like a spin valve at low temperature (78 K).

Impedance Spectroscopy Study of Hydrothermally Synthesized Nano-semiconducting Bornite (Cu5FeS4)

Chalcogenide compound Bornite (Cu5FeS4) was prepared by hydrothermal synthesis method. Here, Citric acid (CA) was introduced in the hydrothermal reaction as a chelating agent to get the phase purity in the Bornite product. The room temperature Powder X-Ray Diffraction (PXRD) confirms the phase purity with cubic crystal structure. The Field Emission Scanning Electron Microscope (FESEM) image confirms the formation of flower shaped micro architecture consisting nano-flakes of the synthesized Bornite material and the optical band gap energy (=1.87 eV) was estimated from UV-VIS absorption spectra. Room temperature (303 K) Complex Impedance Spectroscopy (CIS) study for different forward dc bias voltages (0V-0.8V) on the thin film of synthesized material was performed. CIS analysis demonstrates the relaxation phenomenon and decrease in bulk resistance with increasing forward dc bias voltages reveals the involvement of bulk resistance in electrical conduction mechanism. Moreover the dependency of ac/dc conductivity on the forward dc bias voltage was analyzed on the basis of hopping mechanism for electrical transport processes.

Bias dependent conduction and relaxation mechanism study of Cu5FeS4 film and its significance in signal transport network

Here we have reported the electric and dielectric properties of a Bornite (Cu5FeS4) film using non-destructive complex impedance spectroscopy (CIS) in the frequency range 40 Hz–20 MHz and bias voltage range 0–0.8 V. A typical hydrothermal reaction was carried out to obtain the Cu5FeS4 material. Thereafter, we fabricated the film of the synthesized material and analysed its various properties as a function of frequency and dc bias voltages. The CIS analysis at room temperature (303 K) for different forward dc bias voltages shows that the bulk resistance plays a predominant role in conduction mechanism. The complex impedance (Nyquist) plots are well modelled with grain and grain boundary resistance by introducing proper ac equivalent circuit. The impedance loss spectra showed that the relaxation peak shifts towards the higher frequency with increasing bias voltages, which implies the possibility of a charge hopping between the localized charge states. Furthermore, we find that dielectric constant ( $${\varepsilon ^\prime }$$ ), dielectric loss ( $${\varepsilon ^{\prime \prime }}$$ ), electrical modulus ( $${{\text{M}}^*}$$ ) and ac/dc conductivity of the Cu5FeS4 film are strongly frequency dependent. The values of $${\varepsilon ^\prime }$$ and $${\varepsilon ^{\prime \prime }}$$ decreases whereas ac conductivity increases with increasing frequency. High frequency and low frequency dielectric constant of the material are found to be 3.48 and in the order of 104 respectively. Electrical modulus study is introduced for better explanation of conductivity relaxation phenomenon. Finally, we investigated the variation of ac/dc conductivity with forward dc bias voltages. This confirmed the presence of a hopping mechanism for electrical transport in our system, which can be best explained on the basis of jump relaxation model. Overall, this extensive study based on complex impedance spectroscopy demonstrates the dielectric relaxation behaviour of Cu5FeS4 film and also shed light on hopping induced conduction mechanism.

Al 0 .3 Ga 0.7 N PN diode with breakdown voltage >1600 V

Demonstration of Al0.3Ga0.7N PN diodes grown with breakdown voltages in excess of 1600 V is reported. The total epilayer thickness is 9.1 μm and was grown by metal-organic vapour-phase epitaxy on 1.3-mm-thick sapphire in order to achieve crack-free structures. A junction termination edge structure was employed to control the lateral electric fields. A current density of 3.5 kA/cm2 was achieved under DC forward bias and a reverse leakage current <3 nA was measured for voltages <1200 V. The differential on-resistance of 16 mΩ cm2 is limited by the lateral conductivity of the n-type contact layer required by the front-surface contact geometry of the device. An effective critical electric field of 5.9 MV/cm was determined from the epilayer properties and the reverse current–voltage characteristics. To our knowledge, this is the first aluminium gallium nitride (AlGaN)-based PN diode exhibiting a breakdown voltage in excess of 1 kV. It is noted that a Baliga figure of merit (V br 2/R spec,on) of 150 MW/cm2 found is the highest reported for an AlGaN PN diode and illustrates the potential of larger-bandgap AlGaN alloys for high-voltage devices.

Analysis of Electron Recombination in Dye Sensitized Solar Cells Based on the Forward Bias Dependence of Dark Current and Electroluminescence Characterization

The current-voltage and electroluminescence characteristics of dye-sensitized solar cells (DSSCs) are investigated under forward dc bias in a dark environment at room temperature. The results show that the presence of dyes can play an important role in improving device performance, and also produce defects at the TiO2/dye/electrolyte interfaces, which dominate the electron-hole recombination process and become a limiting factor in obtaining high-efficiency DSSCs. The goodness of fit between measured and calculated data supports these conclusions.

Far-infrared electroluminescence characteristics of an InGaP/InGaAs/Ge triple-junction solar cell under forward DC bias

The far-infrared electroluminescence characteristics of an InGaP/InGaAs/Ge solar cell are investigated under forward DC bias at room temperature in dark conditions. An electroluminescence viewgraph shows the clear device structures, and the electroluminescence intensity is shown to increases exponentially with bias voltage and linearly with bias current. The results can be interpreted using an equivalent circuit of a single ideal diode model for triple-junction solar cells. The good fit between the measured and calculated data proves the above conclusions. This work is of guiding significance for current solar cell testing and research.

Far-infrared electroluminescence characteristics of an Si-based photodiode under a forward DC bias current

At room temperature, the bias dependence of a far-infrared electroluminescence image of a photodiode is investigated in the dark condition. The results show that the electroluminescence image can be used to detect defects in the photodiode. Additionally, it is found that the electroluminescence intensity has a power law dependence on the dc bias current. The photodiode ideality factor could be obtained by a fitting a relationship between the electroluminescence intensity and the bias current. The device defect levels will be easily determined according to the infrared image and the extracted ideality factor value. This work is of guiding significance for current solar cell testing and research.

Fast pulsed electroluminescence from polymer light-emitting diodes

Transient electroluminescence (EL) from polymer light emitting diodes is investigated by measurements of the response to short voltage pulses. The carrier mobility is derived from the delay time between the electrical pulse and the onset of EL, μ≈3×10−4 cm2/V s. Bilayer devices with a polyethylene-dioxythiophene (PEDOT), hole injection layer are also studied. The delay time between the electrical pulse and the onset of EL is independent of the thickness of the injection layer, implying that the conducting PEDOT functions as a part of the electrode. When a dc forward bias is applied to the device, the delay time decreases, probably as a result of the shift of the emission zone towards the anode. The EL turn-on depends on the amplitude of the voltage pulse. The data are modeled by an equivalent circuit with a fixed capacitance connected in parallel with a nonlinear resistance. The solution of the differential equation depends on the exact form of the device’s I–V curve. Two analytical solutions are provided, and an analysis based on space-charge-limited current is presented. By applying a dc forward bias in advance to precharge the space-charge capacitance, the turn-on response time is reduced to 12 ns. The EL decay consists of two components with time constants of 15 ns and 1 μs. The decay does not depend on either the amplitude of the voltage pulse or the prebias.

Charge transport in a π-conjugated polymer: Generalized Langevin equation analysis

The complex dielectric constants of polymer light-emitting diodes using poly @2-methoxy-5-~2’-ethylhexyloxy!-1,4-phenylenevinylene# were measured, and the generalized Langevin equation was used to analyze the dielectric behavior in the frequency domain. We also proposed appropriate fitting functions for the voltage dependence of the fitting parameters. We confirmed that the generalized Langevin equation offers a very good approach to analyze and understand the transport properties of charge carriers in a p-conjugated polymer. A typical polymer light-emitting diode ~LED! consists of a thin layer of conjugated polymer sandwiched between two electrodes on top of a glass substrate. Under forward dc bias, electrons and holes are injected from the cathode and the anode, respectively, into the polymer. The injected charge carriers move through the polymer due to the applied external electric field over a certain distance until recombination takes place. Thus the operation of the polymer LED could be considered in three processes: injection, transport, and recombination of the charges. 1 To improve the performance of polymer LED’s, it is vital to understand which mechanism~s! control~s! the current density-voltage ( J-V) characteristics of a given device structure. 2 Transport at dc or at very low frequencies requires a kind of percolating network of transition, in which the weakest links ~intrachain connection! determine the magnitude of the conductivity. At highfrequencies, the charge carriers become localized in small regions of low-energy barriers. In this context, studying the conductivity in the frequency domain is essential due to its spectroscopic character. In practice also, for the pulse operation of the LED, we have to understand the current-voltage ( I-V) characteristics in the frequency domain, i.e., the relaxation process should be studied. Recently, ionic motions in amorphous LiNbO3 ,KNbO3, and PbTiO3 were described by using a modified generalized Langevin equation. 3 In this article, we analyzed the frequency-dependent dielectric constant of an indium tin oxide ~ITO!-poly@2-methoxy-5-~2’-ethyl-hexyloxy !-1,4phenylenevinylene #~ MEH-PPV!-aluminum ~Al! device using the generalized Langevin equation. Based on the results of this analysis, the conduction mechanism of the charge carrier is discussed. Polymer films were obtained by spin coating the filtered polymer solution onto the substrates with an ITO electrode. Then the polymer films on the ITO/glass substrates were subjected to a heat treatment in a vacuum oven for an hour. Aluminum was vapor deposited as the cathode at a working pressure below 4310 26 Torr, yielding an active size of 5-mm diameter. The I-V characteristics and the impedances of the samples were measured by a source-measurement unit ~Keythley model 236! and impedance gain/phase analyzer~SI1260, Solatron!, respectively. Figure 1 shows the I-V characteristics of the ITO/MEHPPV/Al device. At low voltages, up to around 1 V, the current is Ohmic with a slope equal to one, while at higher voltages, the current is bulk limited with slopes .3.3. Figs. 2~a! and 2~b! show the frequency-dependent real and imaginary part of the dielectric constants of the ITO/MEH-PPV/Al device, respectively, calculated from the impedance data. As shown in Fig. 2~a!, the real part of the capacitance does not change up to around 10 5 Hz and relaxes around 10 6 Hz. However, a strong frequency-dependent behavior in the imaginary part of the dielectric constants, i.e., low-frequency dispersion, is observed as seen in Fig. 2~b!. This indicates that the effect on the conduction of the remaining ions used during the polymerization process of MEH-PPV is negligible, and that only the hole is a major charge carrier. 4

Measurements of electric potential in a laser diode by Kelvin Probe Force Microscopy

The surface potential of the cleaved surface of a multilayer laser has been investigated by Kelvin Probe Force Microscopy (KPFM) in air and in non-contact mode. The sample is a laser structure emitting at 2.36 μm, grown by molecular beam epitaxy on GaSb(100) substrates, cleaved along the (110) surface. The structures comprise abrupt GaAlSbAs/GaSb heterojunctions. Using modified commercial equipment, we simultaneously obtain topography and Kelvin potential. Topography gives the localisation of the different layers of the structure, as the native oxide thickness is higher on Al-rich compounds than on GaSb. When applying forward or reverse DC bias to the device, shifts in surface potential are observed. They appeared to be directly correlated to the bulk voltage of the semiconductors. The measurements allow direct determination of the voltage-drops over the heterojunctions and in the active zone of the device, with these drops controlling the emitting efficiency of the laser.

The Double-surface Transistor

In a series of Letters to the Editor [1] appearing in a recent issue of this journal, there are described the physical construction and proposed theory of operation of a solid state semiconductor triode. This device, which is now called the type A transistor, comprises a block of high back-voltage germanium on one of the faces of which are two contacts, side by side with each other and close together. These contacts are called the emitter and collector, respectively. A large-area contact to the opposite face of the semiconductor block is called the base contact. The present communication describes another semiconductor triode, the double-surface transistor, in which the emitter and collector contacts bear on the two opposite faces of a thin wedge or slab of semiconductor. This slab is prepared from an ingot of high back-voltage germanium of N-type [2]. After being ground approximately to the desired shape, the slab is etched and provided with a suitable large-area base contact. For good gain characteristics it is advantageous that the thickness of the slab be no greater than about 0.01 cm at the place where the contacts bear upon it. These contacts should be within about the same distance of coming exactly opposite each other on the two faces of the slab. Tungsten, copper, and phosphor bronze have been used successfully as contact materials. This device, together with its electrical connections for use as a grounded-base amplifier, is illustrated schematically in Fig. 1. In operation a comparatively large dc reverse bias ( 50 to 100 V) is applied to the collector, while a comparatively small dc forward bias (a few tenths of a volt) is applied to the emitter. Because of positive feedback effects in the base contact and semiconductor body, the emitter bias voltage-to-base may in some cases be zero or even negative. The static characteristics of a double-surface transistor are presented in Fig. 2. Families of collector voltage versus collector current curves are given, with constant emitter current as parameter for the solid lines, and with constant emitter voltage as parameter for the dashed lines. Such a plot allows one to make judicious choice of dc operating point. It furnishes in addition complete information from which can be obtained, almost by inspection, the dynamic

Forward/Reverse dc bias or ac electroluminescence in poly(p-phenylene vinylene) prepared via sulfonium-salt and xanthate routes

In light-emitting diodes (LEDs) of standard configuration — Indium-Tin-Oxide (ITO)/polymer/metal, we observed light emission under ac or forward/reverse dc bias, using poly(p-phenylene vinylenes) prepared via the sulfonium salt (S-PPV) or xanthate (X-PPV) routes as active electroluminescent layers. S-PPV devices exhibited approximately symmetrical current-voltage characteristics under forward and reverse dc bias with the same onset voltage. The electroluminescence (EL) spectra either under a dc forward/reverse bias or ac were identical. X-PPV showed EL under reverse bias at higher voltage than in forward bias. Symmetrical current-voltage characteristics in SPPV devices is attributed to ionic impurities present in the polymer.

Alternating-current light-emitting devices based on conjugated polymers

Most polymer electroluminescent devices to date are represented as tunnel diodes and operate under direct-current (dc) driving field. Here we report the fabrication of symmetrically configured alternating-current (ac) light-emitting (SCALE) devices based on conjugated polymers. The new devices consist of an emissive polymer layer sandwiched between two redox polymer layers. This configuration enables the SCALE devices to work under both forward and reverse dc bias as well as in ac modes. The nearly ohmic electrode/redox polymer contacts improve the charge injection efficiency significantly and make the SCALE device operation insensitive to electrode work functions. Symmetric operation supports the key role of redox polymer/emissive polymer interface states.

Polymer Electroluminescence Using ac or Reverse dc Biasing

Poly(2,5-diheptoxy-p-phenylenevinylene-alt-2,5-dimethoxy-p-phenylenevinylene) (DHDMPPV) exhibits electroluminescence (EL) at room temperature when either a conventional dc forward bias, or a reverse bias, or an ac voltage is applied to the test device, which has a simple ITO (indium tin oxide)/polymer/Al configuration. DHDMPPV is a soluble derivative of poly(p-phenylenevinylene) (PPV) synthesized using a Wittig condensation polymerization. Its EL spectrum lies in the orange region with a maximum intensity at 582 nm and a shoulder at 620 nm. The EL spectra either under a dc forward or reverse bias or on the imposition of an ac voltage are qualitatively identical. The threshold biases are ∼5 V for dc forward bias, ∼6 V for dc reverse bias, and ∼5 V (10 V peak-to-peak) for ac operation, for a polymer emitter film thickness of ca. 80 nm. The EL emissions are bright and relatively stable at room temperature and can be easily seen under normal light conditions. Two other EL-active copolymers were also tested in similar configurations ; no significant dc reverse bias EL was observed. However, blends of DHDMPPV with another EL-active copolymer previously described show the same behavior as DHDMPPV itself ; for example, a 1 :1 weight ratio blend with a green-light-emitting multiblock copolymer shows only an enhanced orange emission from the narrow-band-gap DHDMPPV.

Symmetrically Configured AC Light-Emitting (Scale) Devices: Generalizations and Variations

ABSTRACTMost conjugated polymer-based light-emitting devices have been shown to be tunnel diodes which can only operate under forward DC driving field. Recently we have reported the fabrication of symmetrically configured AC light-emitting (SCALE) devices based on heterocyclic aromatic conjugated polymers. By adding an “insulating” layer (e.g. emeraldine base (EB) form of polyaniline) on both sides of the emitting layer, the SCALE devices emit light under both forward and reverse DC bias as well as AC driving voltage. The SCALE device structure ITO/J/emitterFl/M, has been shown to be quite general, and can be applied to a variety of electroluminescent polymers (emitter), insulating polymers (I) and electrode materials (M). Here we summarize and compare the performance of SCALE devices fabricated with different emitter, insulator, and electrode materials. The role of the insulating layer in the SCALE device operation is examined and a model that emphasizing the interface states is proposed to account for the device operation.

Spectral information coding by infrared photoreceptors

Spontaneous pulsing has been observed in circuits containing cryogenically cooled silicon p-i-n (p+-n-n+) diodes under dc forward bias. The intensity of infrared radiation incident on the diodes controls the pulse rate with no appreciable effect on the shape or size of the pulses. A strong similarity is noted between these properties and the nearly universal means of coding of visual information by animal photoreceptors and neural networks. It is proposed that exploitation of this remarkable analogy could lead to radically new approaches to acquisition and processing of infrared optical information. Infrared analogs of neural color coding and color vision are proposed based on analysis of p-i-n spectral response measurements.

Invited paper Generation and control of transform-limited optical pulses in subpico- and picosecond regions from semiconductor diode lasers

In this paper we summarize and discuss a series of experimental investigations, performed systematically in recent years in our laboratory, for the generation of transform-limited optical pulses with excellent coherence in the picosecond and subpicosecond ranges from semiconductor diode lasers in the near-infrared wavelength region. The two basic methods, mode locking and direct modulation, were developed and fully studied for this first achievement employing AlGaAs and InGaAsP laser diodes. Active and passive mode locking incorporating external cavities with AlGaAs diode lasers were successful in realizing transform-limited optical pulses with durations of 30 ps and 0-58 ps respectively. Moreover, a strong RF modulation scheme superimposed on the forward and backward DC bias current was demonstrated to be the most practical and reliable for generating single-mode ultra short optical pulses with excellent coherence from a distributed feedback (DFB) InGaAsP diode laser. This transform-limited behaviour was...