Room Temperature Current-voltage Characteristics Research Articles

N-polar GaN p-n junction diodes with low ideality factors

High-quality N-polar GaN p-n diodes are realized on single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy. The room-temperature current–voltage characteristics reveal a high on/off current ratio of >1011 at ±4 V and an ideality factor of 1.6. As the temperature increases to 200 °C, the apparent ideality factor gradually approaches 2. At such high temperatures, Shockley–Read–Hall recombination times of 0.32–0.46 ns are estimated. The measured electroluminescence spectrum is dominated by a strong near-band edge emission, while deep level and acceptor-related luminescence is greatly suppressed.

Realization of highly rectifying Schottky barrier diodes and pn heterojunctions on κ-Ga2O3 by overcoming the conductivity anisotropy

Novel devices based on orthorhombic κ-Ga2O3 could enable solar blind infrared detection or high-electron mobility transistors with large two-dimensional electron gas densities. Here, we report on the current transport parallel to the growth direction of κ-Ga2O3 layers grown by pulsed-laser deposition on highly conductive Al-doped ZnO back contact layers. Besides ohmic Ti/Al/Au contact layer stacks, vertical Pt/PtOx/κ-Ga2O3 and Pd/PdOx/κ-Ga2O3 Schottky barrier diodes and NiO/κ-Ga2O3 and ZnCo2O4/κ-Ga2O3 pn-heterodiodes are investigated by current–voltage measurements. While a lateral current transport is severely suppressed to less than 10−9Acm−2 due to rotational domains, we record a significant current flow through the ohmic contacts in the vertical direction of >0.1Acm−2. The Schottky barrier diodes and the pn-heterojunctions exhibit rectification ratios of up to seven orders of magnitude. Room temperature current–voltage characteristics of diode ensembles as well as temperature-dependent measurements for selected Pt-based diodes reveal a mean barrier height of ϕBm≈2.1eV and ideality factors down to η≈1.3.

DSSC using Wrightia tinctoria (Roxb.) R Br.: A trail

This paper focuses on the structure of Dye Sensitized Solar Cell (DSSC) engineered using Titanium dioxide (TiO2) nano-particles sensitized by extract from leaves of Wrightia tinctoria (Roxb.) R. Br. TiO2 thin films are screen printed over flexible Cu foil which serves as the back contact for the device structure. Fluorine doped Tin Oxide (FTO) grown over glass substrate using chemical spray pyrolysis technique is used as the front electrode with KOH as the electrolyte. The optical characterization of the dye is conducted using optical absorption and emission spectroscopic studies. A DSSC of the configuration (Cu|TiO2|Dye(danthapala)|KOH(Electrolyte)|FTO) was engineered. The electrical parameters of the engineered Dye Sensitized Solar Cells were characterized. The two-diode model was used to extract the series and shunt resistance, saturation current and diode ideality factor for the device from the room temperature current–voltage characteristics.

Spin-flip scattering in magnetic tunnel junctions: From current-voltage characteristics to spin-transfer torque

We examine room temperature current-voltage (IV) characteristics of $\mathrm{CoFeB}\phantom{\rule{0.16em}{0ex}}|\phantom{\rule{0.16em}{0ex}}\mathrm{MgO}\phantom{\rule{0.16em}{0ex}}|\phantom{\rule{0.16em}{0ex}}\mathrm{CoFeB}$ type of perpendicularly magnetized tunnel junctions developed for memory applications. From their nonlinear bias voltage dependence, a conductance ``cross-scaling'' is seen that is consistent with the involvement of inelastic spin-flip scattering in electrodes. A phenomenological model is constructed that connects the parameters of spin-flip scatter-related inelastic events with both magnetoresistance and spin-transfer torque. The model provides measurable, electrode-specific properties such as interface exchange stiffness as it affects spin-torque performance.

Demonstration of highly repeatable room temperature negative differential resistance in large area AlN/GaN double-barrier resonant tunneling diodes

Here, we present a systematical investigation of AlN/GaN double-barrier resonant tunneling diodes (RTDs) grown by plasma-assisted molecular beam epitaxy on metal-organic chemical vapor deposition GaN-on-sapphire templates. The processed devices featured an active region composed of 2.5 nm GaN quantum well sandwiched by two 1.5 nm AlN barriers and RTD mesa diameter ranging from 1 to 20 μm. Room temperature current–voltage characteristics exhibited a repeatable negative differential resistance (NDR) free of degradation and hysteresis after 1000 times subsequently up-to-down voltage sweeps across different sizes. High peak-to-valley current ratios of 1.93 and 1.58 were obtained at room temperature for 1 and 12 μm diameter devices, respectively, along with peak current densities of 48 and 36 kA/cm2 corresponding to peak voltages of 4.65 and 5.9 V. The peak current density decreased quickly initially and then was less susceptible to this averaging effect with increasing the device diameter. Temperature-dependent measurements revealed that the valley current density displayed a positive relationship to the temperature, and an abruptly increasement was observed for the devices with a diameter of 20 μm when the temperature rose over 230 K. We attributed this abnormal phenomenon to the increased contribution from acoustic and longitudinal optical (LO) phonon scattering, especially for the LO phonon scattering. The area dependence of electrical performance suggested that the leakage pathway through dislocations played a vital role for charge transport and there existed a threshold of dislocation density for NDR characteristics. These results promote further study for future implementation of III-nitride-based RTD oscillators into high-frequency and high-power terahertz radiation.

Compositional tuning of negative differential resistance in a bulk silver iodide memristor

Silver iodide a negative differential resistance (NDR) memristor

Room-Temperature Negative Differential Resistance in Amorphous Carbon: The Role of Electron Trapping Defects at Device Interfaces

Hysteresis and NDR have been observed in the room-temperature current–voltage characteristics of lateral devices featuring planar graphitic electrodes separated by a nitrogen-doped amorphous carbon (a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> ) layer. The devices were fabricated using energetic physical vapor deposition and electron-beam lithography. Devices formed on a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> layers deposited with N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> pressure of 0.10 mTorr exhibited NDR. This NDR occurred at a well-defined voltage/electric field but only after the voltage of the opposite polarity was applied. Based on this behavior, a model is proposed in which the NDR occurs by field-induced trapping/detrapping at one of the graphite/a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> interfaces within the device. The results of this investigation highlight the importance of interfaces and defects in determining the characteristics of a-C memristive devices.

N-polar GaN/AlN resonant tunneling diodes

N-polar GaN/AlN resonant tunneling diodes are realized on a single-crystal N-polar GaN bulk substrate by plasma-assisted molecular beam epitaxy growth. The room-temperature current–voltage characteristics reveal a negative differential conductance (NDC) region with a peak tunneling current of 6.8 ± 0.8 kA/cm2 at a forward bias of ∼8 V. Under reverse bias, the polarization-induced threshold voltage is measured at ∼−4 V. These resonant and threshold voltages are well explained with the polarization field, which is opposite to that of the metal-polar counterpart, confirming the N-polarity of the resonant tunneling diodes (RTDs). When the device is biased in the NDC-region, electronic oscillations are generated in the external circuit, attesting to the robustness of the resonant tunneling phenomenon. In contrast to metal-polar RTDs, N-polar structures have the emitter on the top of the resonant tunneling cavity. As a consequence, this device architecture opens up the possibility of seamlessly interfacing—via resonant tunneling injection—a wide range of exotic materials with III-nitride semiconductors, providing a route towards unexplored device physics.

Fabrication of interdigitated electrode (IDE) based ZnO sensors for room temperature ammonia detection

Interdigitated electrode (IDE) sensor has been successfully fabricated, characterised and successfully used for ammonia sensing. Silver (Ag) is used as IDE which is fabricated by using DC magnetron sputtering while ZnO sensing layer with three different dopants (Cd, Mg and Y) have been deposited over the Ag sputtered IDE by RF magnetron sputtering. The deposited sensing layer has been characterised by X-ray diffraction (XRD), Photoluminescence (PL) and Hall effect which reveal that typical ZnO characteristics. The room temperature current-voltage characteristics of the fabricated device show the ohmic behaviour for sensors SD1 (10 mol% Cd doped ZnO) and SD2 (10 mol% Mg doped ZnO) while Schottky behaviour for the sensor SD3 (2 mol% Y doped ZnO). The fabricated IDE sensors were exposed to the various concentration of ammonia, and the enhanced ammonia response has been observed compared to thin film sensors. The sensors SD1 and SD2 show higher ammonia response than SD3. However, fast response and speed recovery time were realised for device SD3 compared to SD1 and SD2. Among the three sensors, SD1 shows the maximum ammonia response. The enhanced ammonia sensing of fabricated sensors explained by band bending mechanism in detail.

Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm2

We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast process that goes beyond the limitations of current III-Nitride high electron mobility transistors.

Hysteresis-free DC conduction in zinc oxide films with a conducting polymer counter electrode

A conducting polymer, namely poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been employed as an alternative metal counter electrode to study steady state charge conduction through a spin-coated 5.2 µm thick ZnO film in a sandwich structure on a fluorine doped tin oxide coated glass substrate. The room temperature current–voltage characteristics exhibit rectifying behaviour without hysteresis as the bias voltage is cycled between ± 10V for varying voltage sweep rate from 100 to 1000 mV/s. Thermionic emission is believed to be the dominant conduction mechanism at forward bias voltage V ≤ 2.5 V whereas the space charge limited current conduction becomes effective in a higher voltage region V > 2.5 V. The barrier height (φb), ideality factor (n) and series resistance (Rs) are found to be strongly temperature dependent parameters showing the increase of {phi _b} and simultaneous decreases of n and Rs within the range of 218–298 K. The behaviour of the temperature dependent charge carrier mobility in the higher voltage region has also been discussed. The use of PEDOT:PSS is quite promising as an alternative to metal electrodes in semiconductor devices.

Effect of compositional interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors grown on SiC

We have investigated the effect of strain-compensating interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors (DBRs). Samples with 10.5 mirror pairs were grown through plasma-assisted molecular beam epitaxy on SiC. Room-temperature current–voltage characteristics were measured vertically in mesas through 8 of the 10.5 pairs. The sample with no interlayers yields a mean specific series resistance of 0.044 Ω cm2 at low current densities, while three samples with 5/5-Å-thick, 2/2-nm-thick, and graded interlayers have resistivities between 0.16 and 0.34 Ω cm2. Thus, interlayers impair vertical current transport, and they must be designed carefully when developing conductive DBRs.

Investigation of high-temperature charge transport mechanism in Al–Gd2O3–Al-based metal–insulator–metal (MIM) structure

In this paper, the charge conduction mechanism at high temperature in Al–Gd2O3 (MIM) structure has been investigated by performing temperature-dependent current–voltage measurements in the temperature range 280–390 K. MIM structure is realized by electron beam evaporation system where thin films of Gd2O3 (40, 60 and 80 nm) and Al metal on both sides of dielectric film were deposited on glass substrate. The possibility of different transport mechanisms has been testified by plotting various graphs. The nonlinear behavior of LnV versus LnI and V 1/2 versus LnV/I graphs ruled out the possibility of space-charge-limited conduction (SCLC) and Poole–Frenkel mechanism in Al–Gd2O3–Al MIM structure. The straight lines LnI–V 1/2 graphs at various temperatures confirmed that Schottky emission is the dominant transport mechanism in Al–Gd2O3–Al structure. The calculated values of field barrier lowering coefficient at different measurement temperatures were in good agreement with the theoretical prediction confirming conduction is via Schottky emission. The field-dependent Ln(I/T 2) versus 1000/T plots were obeyed a linear relationship according to Schottky emission theory. Furthermore, the dielectric thickness dependence room-temperature current–voltage characteristics of Al–Gd2O3–Al MIM structure were showed strong dependence of current on dielectric film thickness according to Schottky emission theory of conduction current.

Irreversibility in room temperature current–voltage characteristics of NiFe2O4 nanoparticles: A signature of electrical memory effect

Room temperature I–V characteristics study, both in presence and absence of magnetic field (1800Oe), has been performed on NiFe2O4 nanoparticles, having different particle size (Ф~14, 21 and 31nm). Our experiments on these nanoparticles provide evidences for: (1) electrical irreversibility or hysteretic behaviour; (2) positive magnetoresistance and (3) magnetic field dependent electrical irreversibility or hysteresis in the sample. “Hysteretic” nature of I–V curve reveals the existence of electrical memory effect in the sample. Significantly, such hysteresis has been found to be tuned by magnetic field. In order to explain the observed electrical irreversibility, we have proposed a phenomenological model on the light of induced polarization in the sample. Both the positive magnetoresistance and the observed magnetic field dependence of electrical irreversibility have been explained through magnetostriction phenomenon. Interestingly, such effects are found to get reduced with increasing particle size. For NiFe2O4 nanoparticles having Ф=31nm, we did not observe any irreversibility effect. This feature has been attributed to the enhanced grain surface effect that in turn gives rise to the residual polarization and hence electrical memory effect in NiFe2O4 nanoparticles, having small nanoscopic particle size.

Optical Properties and Junction Characteristics of 6-(5-Bromothiohen-2-yl)-2,3-Dihydro-1-Methyl-3-Oxo-2-Phenyl-1H-Pyrazolo[4,3-b]Pyridine-5-Carbonitrile Films

In this study, 6-(5-bromothiohen-2-yl)-2,3-dihydro-1-methyl-3-oxo-2-phenyl-1H-pyrazolo[4,3-b]pyridine-5-carbonitrile (BDPC) powder was synthesized. BDPC powder showed a polycrystalline structure, whereas the thermally evaporated films had an amorphous structure. The optical parameters such as absorption coefficient and refractive index were calculated in the spectral range 200–500 nm. Spectral distribution analysis of the absorption coefficient revealed that the films had an indirect band transitions with energy gaps of 2.57 eV and 3.5 eV. According to the single oscillator model, the oscillation energy, dispersion energy, and dielectric constant were estimated. The room-temperature current–voltage characteristics of the fabricated Au/BDPC/p-Si/Al heterojunction showed diode-like behavior. The ideality factor, the barrier height and series resistance were determined based on thermionic emission theory and Norde’s function. At reverse bias, the current was interpreted in terms of the Schottky and pool-Frenkle effects in low and high voltages, respectively. The built-in voltage, carrier concentration and barrier height were obtained using capacitance–voltage characteristics.

ZrxCo0.8−xNi0.2−xFe2O4-graphene nanocomposite for enhanced structural, dielectric and visible light photocatalytic applications

Zirconium doped nickel cobalt ferrite (ZrxCo0.8−xNi0.2−xFe2O4) nanoparticles and ZrxCo0.8−xNi0.2−xFe2O4-graphene nanocomposites were synthesized by a cheap and facile co-precipitation method. Annealing was done at 750°C for 6.5h. Spinel cubic structure of prepared nanoparticles was confirmed by X-ray powder diffraction (XRD) technique. Crystalline size of nanoparticles was observed in the range of 18–27nm. Graphene was synthesized by Hummer's method. Formation of rGO was confirmed by UV-visible spectroscopy (UV-vis) and XRD. ZrxCo0.8−xNi0.2−xFe2O4-graphene nanocomposites were prepared by ultra-sonication route. Grain size of nanoparticles and dispersion of nanoparticles between rGO layers was determined by Scanning electron microscopy (SEM). In application studies of nanoparticles and their nanocomposites, photocatalytic efficiency of nanoparticles under visible light irradiation was observed by degradation of methylene blue. Charge transfer resistance was measured by electrochemical impedance spectroscopy (EIS) and the variation in dc electrical resistivity was analyzed by room temperature current voltage characteristics (I-V). Dielectric constant was also evaluated in frequency range from 1MHz to 3GHz. All these investigations confirmed the possible utilization of these materials for a variety of applications such as visible light photocatalysis, high frequency devices fabrication etc.

Resistance switching memory characteristics of CaF2/Si/CaF2 resonant-tunneling quantum-well heterostructures sandwiched by nanocrystalline Si secondary barrier layers

A novel resistance switching memory using CaF2/Si/CaF2 resonant-tunneling quantum well heterostructures sandwiched by nanocrystalline Si (nc-Si) as secondary barrier layers has been proposed and the room temperature current–voltage characteristics of the basic resistance switching memory operation have been demonstrated. A resistance switching voltage of 1.0 V, a peak current density of approximately 42 kA/cm2, and an ON/OFF ratio of 2.8 were observed. In particular, more than 28000 write-read-erase cyclic memory operations have been demonstrated by applying pulsed input voltage sequences, which suggests better endurance than the device using a CaF2/CdF2/CaF2 heterostructure.

Electrical transport in transverse direction through silicon carbon alloy multilayers containing regular size silicon quantum dots

Electrical transport in the transverse direction has been studied through a series of hydrogenated silicon carbon alloy multilayers (SiC-MLs) deposited by plasma enhanced chemical vapor deposition method. Each SiC-ML consists of 30 cycles of the alternating layers of a nearly amorphous silicon carbide (a-SiC:H) and a microcrystalline silicon carbide (μc-SiC:H) that contains high density of silicon quantum dots (Si-QDs). A detailed investigation by cross sectional TEM reveals preferential growth of densely packed Si-QDs of regular sizes ∼4.8nm in diameter in a vertically aligned columnar structure within the SiC-ML. More than six orders of magnitude increase in transverse current through the SiC-ML structure were observed for decrease in the a-SiC:H layer thickness from 13nm to 2nm. The electrical transport mechanism was established to be a combination of grain boundary or band tail hopping and Frenkel–Poole (F-P) type conduction depending on the temperature and externally applied voltage ranges. Evaluation of trap concentration within the multilayer structures from the fitted room temperature current voltage characteristics by F-P function shows reduction up-to two orders of magnitude indicating an improvement in the short range order in the a-SiC:H matrix for decrease in the thickness of a-SiC:H layer.

Performance characteristics of CdTe drift ring detector

CdTe and CdZnTe material is an excellent candidate for the fabrication of high energy X-ray spectroscopic detectors due to their good quantum efficiency and room temperature operation. The main material limitation is associated with the poor charge transport properties of holes. The motivation of this work is to investigate the performance characteristics of a detector fabricated with a drift ring geometry that is insensitive to the transport of holes. The performance of a prototype Ohmic CdTe drift ring detector fabricated by Acrorad with 3 drift rings is reported; measurements include room temperature current voltage characteristics (IV) and spectroscopic performance. The data shows that the energy resolution of the detector is limited by leakage current which is a combination of bulk and surface leakage currents. The energy resolution was studied as a function of incident X-ray position with an X-ray microbeam at the Diamond Light Source. Different ring biasing schemes were investigated and the results show that by increasing the lateral field (i.e. the bias gradient across the rings) the active area, evaluated by the detected count rate, increased significantly.

Ultraviolet Photodetection Properties of ZnO/Si Heterojunction Diodes Fabricated by ALD Technique Without Using a Buffer Layer

The fabrication and characterization of a Si/ZnO thin film heterojunction ultraviolet photodiode has been presented in this paper. ZnO thin film of ~100 nm thick was deposited on <100> Silicon (Si) wafer by atomic layer deposition (ALD) technique. The Photoluminescence spectroscopy confirms that as-deposited ZnO thin film has excellent visible-blind UV response with almost no defects in the visible region. The room temperature current-voltage characteristics of the n-ZnO thin film/p-Si photodiodes are measured under an UV illumination of 650 <W at 365 nm in the applied voltage range of ±2V. The current-voltage characteristics demonstrate an excellent UV photoresponse of the device in its reverse bias operation with a contrast ratio of ~ 1115 and responsivity of ~0.075 A/W at 2 V reverse bias voltage.