Tunneling Of Charge Carriers Research Articles

Improved power conversion efficiency in n-MoS2/AlN/p-Si (SIS) heterojunction based solar cells

In the present work, MoS2/Si (n-p) and MoS2/AlN/Si (Semiconductor-Insulator-Semiconductor (SIS)) heterojunctions based solar cells have been fabricated by depositing ultrathin layers of MoS2 and AlN on p-type Si substrate by DC magnetron sputtering technique. The SIS heterojunction exhibits higher current and large threshold voltage as compared to conventional p-n junction in Current-Voltage (I-V) characteristics. It suggests that the tunnelling of charge carriers can be a better transport mechanism for solar cell applications. With the AlN layer insertion, the power conversion efficiency of the device increases from 2.92% to 3.53%. The insertion of ultrathin insulating AlN layer in between the n-MoS2/p-Si heterojunction lowers the static charge transfer and makes more effective tuning of the Fermi level. The obtained results demonstrate a novel route for the fabrication of efficient photovoltaic devices for next-generation energy harvesting applications.

Direct Probe of Room-Temperature Quantum-Tunneling Processes in Type-II Heterostructures Using Terahertz Emission Spectroscopy

Charge-carrier-transport phenomena on nanoscopic length and ultrashort timescales are of great interest for a multitude of ultrafast dynamic processes occurring in chemical reactions as well as modern devices such as solar cells or transistors. However, the investigation of such ultrashort current pulses is very challenging and mostly based on indirect methods such as spectroscopic changes induced by the charge transport. Here we monitor the short current pulse generated by the dissipative tunneling of charge carriers through a barrier between two adjacent quantum wells via its radiated legacy in the terahertz frequency range. By examining quantum structures with intermediate barriers of different thickness, we demonstrate that the spectrum of the emitted radiation is a direct measurement of the charge-transfer dynamics associated with the tunneling process. Our findings indicate that these incoherent tunnel currents do not start instantaneously but build up on a timescale of approximately 170 fs.

Interlayer Band‐to‐Band Tunneling and Negative Differential Resistance in van der Waals BP/InSe Field‐Effect Transistors

AbstractAtomically thin layers of van der Waals (vdW) crystals offer an ideal material platform to realize tunnel field‐effect transistors (TFETs) that exploit the tunneling of charge carriers across the forbidden gap of a vdW heterojunction. This type of device requires a precise energy band alignment of the different layers of the junction to optimize the tunnel current. Among 2D vdW materials, black phosphorus (BP) and indium selenide (InSe) have a Brillouin zone‐centered conduction and valence bands, and a type II band offset, both ideally suited for band‐to‐band tunneling. TFETs based on BP/InSe heterojunctions with diverse electrical transport characteristics are demonstrated: forward rectifying, Zener tunneling, and backward rectifying characteristics are realized in BP/InSe junctions with different thickness of the BP layer or by electrostatic gating of the junction. Electrostatic gating yields a large on/off current ratio of up to 108 and negative differential resistance at low applied voltages (V ≈ 0.2 V). These findings illustrate versatile functionalities of TFETs based on BP and InSe, offering opportunities for applications of these 2D materials beyond the device architectures reported in the current literature.

On high-field conduction and I–V measurements in quaternary Se–Te–In–Pb nano-chalcogenide thin films

In this paper, multi-component Se79-xTe15In6Pbx (x = 0, 1, 2, 4, 6, 8 ,and 10 at.wt.%) nano-chalcogenide alloys were prepared by using melt quenching technique and their electrical properties and sub-micron structural features have been reported and discussed in detail. Sharp characteristic peaks in x-ray diffraction (XRD) spectra signify nano-range of each alloy under investigation and validated by field emission scanning electron microscopy (FESEM). The thin films of investigated nano-chalcogenide alloys were deposited on glass substrates by thermal evaporation technique. In-depth analysis of DC conductivity for studied nano-chalcogenide alloys was carried out in the temperature range 303–333 K and in the voltage range 0–150 V, to realize the conduction mechanism. I–V characteristics illustrate ohmic behavior in the voltage range 0–50 V while at high voltage non-ohmic behavior is detected. Moreover, pre-exponential factor (σo) along with activation energy (ΔE) deduced from the variation of DC conductivity with temperature, which reveals that thermally assisted tunneling of charge carriers among the localized states occurs in band tails responsible for conduction in the analyzed systems. The detailed analysis indicates that SCLC theory is not appropriate to describe the conduction process, as ln(I/V) against V plots is almost straight, although slope of these plots does not reduce linearly with temperature. The linear relation among ln(I) against V1/2 demonstrates that the conduction mechanism is either Poole–Frenkel type or Schottky emission. The obtained outcomes demonstrate that Poole–Frenkel conduction mechanism is most appropriate to describe the conduction. The deduced values of activation energy are less than 0.98 eV, which once again validated the presence of Poole–Frenkel type conduction in the examined composition. The deduced results are further corroborated by photoluminescence studies.

Enhancement of the DC performance of a PNPN hetero-dielectric BOX tunnel field-effect transistor for low-power applications

A comparative analysis is performed on a PNPN hetero-dielectric buried oxide (BOX) tunnel field-effect transistor (FET) with the conventional versus various modified structures, including the effects of using a high-k dielectric (HfO2) below the gate electrode and/or a low-bandgap material (InAs) as a source pocket. The use of InAs as a low-bandgap material greatly improves the tunneling of charge carriers from the source to channel region in the tunnel FET. Meanwhile, technology computer-aided design (TCAD) simulations reveal that inclusion of the high-k gate oxide with InAs as the source pocket in the PNPN structure results in a huge improvement in the electrostatic characteristics of the proposed structure in terms of both the ON-state current and subthreshold swing (SS). The proposed structure achieves a SS of 4.64 mV/dec and an ON-state current density of $$1.5\times 10^{-3}$$ A/$$\mu $$m.

Quantum-tunneling controlled thermoelectricity in polymers

Abstract Thermoelectric energy conversion within a homogeneous conductor is governed by the charge carrier mobility and the density of electronic states, intrinsic trades-off limit efficiency. Heterogeneous conduction created by the spatial variation of disorder is considered undesirable because it is conceptually difficult and not easily reproduced. However, could it be the basis for unique, exploitable thermoelectric effects? Here we show that the heterogeneous conduction typically found in conducting polymer films can result in thermoelectricity governed by only quantum mechanical tunneling of charge carriers through nanoscale Coulomb barriers at order-disorder boundaries. We conclude this to be the basis of a new category of thermoelectric system centered on heterogeneous conductors or conducting networks. We show that thermoelectricity of this origin can be engineered without doping or intrinsic trades-off. Our findings point to a wide range of alternatives to the doped inorganic semiconductor paradigm.

Imaging work and dissipation in the quantum Hall state in graphene.

Topology is a powerful recent concept asserting that quantum states could be globally protected against local perturbations1,2. Dissipationless topologically protected states are therefore of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often susceptible to various dissipative mechanisms, which are difficult to study directly because of their microscopic origins. Here we use scanning nanothermometry3 to visualize and investigate the microscopic mechanisms that undermine dissipationless transport in the quantum Hall state in graphene. Simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries as a result of edge reconstruction. Instead of local Joule heating, however, the dissipation mechanism comprises two distinct and spatially separated processes. The work-generating process that we image directly, which involves elastic tunnelling of charge carriers between the quantum channels, determines the transport properties but does not generate local heat. By contrast, the heat and entropy generation process-which we visualize independently-occurs nonlocally upon resonant inelastic scattering from single atomic defects at graphene edges, and does not affect transport. Our findings provide an insight into the mechanisms that conceal the true topological protection, and suggest routes towards engineering more robust quantum states for device applications.

Electric field and oxygen concentration-dependent transport properties of nano-graphene oxide

Electrical transport properties of the nano-graphene oxide were investigated by measuring current-voltage characteristics in the wide temperature range of 15 K∼450 K. The n-GO is composed of nanometer-sized intact graphene-like sp2 domains embedded in the sp3 matrix which acts as a charge transport barrier between the highly conductive sp2 domains. The oxygen in the n-GO has the concentration of 4.43 at% in the form of oxygen functional groups. Below the conduction band, four discontinuous localized states with the activation energies of 1.92 meV, 3.27 meV, 5.54 meV, and 6.58 meV were observed. These activation energies decrease with decreasing oxygen concentration and increasing external electric field in the n-GO material. Moreover, we found that the direct tunneling of charge carrier through the sp3 barrier was a dominant transport mechanism for the n-GO material. Also, unlike the activation energy of charge carrier, the transport barrier was independent of both the concentration of the oxygen functional groups and external electric field. The transport barrier was mainly determined by insulation property of the sp3 structure.

Structural dependent, dielectric and conduction analysis of CdSe based quantum dots

Nowadays, the effect of temperature on nanocrystal’s dielectric has become a routine study while their structural effects on dielectric properties, lacks the attention of researchers. The correlation of different shell arrangement on dielectric enhancement in CdSe based quantum dots, is addressed in this letter. Detrimental impact of surface defects on dielectric properties of quantum dots is investigated and shown that surface defects of bare core increases dielectric loss, dissipation factor and decreases conductivity of the quantum dots. Dielectric properties of quantum dots is evaluated after deposition of different shells and multishells. It is observed that loss factor is declined in core/shell(multishell) quantum dots due to passivation of surface defects and consequently ease in charge carrier tunneling. Higher conductivity of the core/shell and multishell structures has also been demonstrated here. Structural and optical investigation of the quantum dots after shell deposition is also considered.

Fowler-Nordheim tunneling characteristics of graphene/hBN/metal heterojunctions

The heterostructures of low-dimensional materials are considered promising candidates for future electronic devices. In this work, we manifest the interlayer hexagonal boron nitride (hBN) thickness dependent tunneling characteristics of graphene/hBN/metal heterojunction back-gated field effect transistors (FETs). Direct tunneling of charge carriers through the ultrathin hBN layer is noticed at low bias voltages. Besides, Fowler-Nordheim tunneling takes place at high bias voltages revealing an increase in the barrier height with an interlayer film thickness. In addition, the back-gate dependent tunneling properties of graphene/hBN/metal heterojunction back-gated FETs are investigated. Furthermore, the Fermi level of graphene is tuned via back-gate voltages that results in the modification of the tunneling current, threshold voltage, and barrier height of the device. This could be an effective approach to modify the tunneling features of vertical heterostructures for their prospective role in next generation high performance electronic devices.

Electrical conduction mechanism in films of Se80−xTe20Bix (0 ≤ x ≤ 12) glassy alloys

This paper reports the study of DC electrical conductivity of films of Se80−xTe20Bix (0 ≤ x ≤ 12) glasses prepared using physical vapor deposition method. The films were structurally characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM results indicate the formation of nanorods within the films. The electrical conductivity of the samples was studied using Keithley electrometer in the temperature range 303–373 K. The results show that conduction in these samples takes place via thermally assisted tunnelling and variable range hopping of charge carriers corresponding to higher and lower temperature ranges, respectively. Further, it was found that the conductivity increases with increase in Bi concentration in Se–Te system. This has been explained on the basis of chemically ordered network model. It was also found that nanorod formation improves the electrical conductivity of Se–Te–Bi system compared to bulk Se–Te–Bi system.

Investigation of electrical and thermal property of poly(vinyl alcohol)–calcium titanate nanocomposites

The poly(vinyl alcohol):CaTiO3 nanocomposites were prepared by solution casting method using triethanolamine as a dispersive solvent in different weight percentages. The prepared nanocomposites have been characterized by FTIR and XRD for structural analysis. FTIR spectra show the important peaks of CaTiO3 and PVA which confirms the formation of nanocomposites. XRD pattern shows that the CaTiO3 has orthorhombic structure and its crystal structure does not distorted even after dispersion in PVA matrix. The TGA graph confirms that the addition of CaTiO3 (0.3 wt%) enhance the thermal stability of the nanocomposites. Further, the temperature dependent conductivity shows the increase in conductivity due to the tunnelling of charge carriers. Dielectric constant and dielectric loss of the 0.3 wt% of nanocomposites is low compare to other nanocomposites and hence its conductivity is high. It is found that the damping loss is 0.002% as indicated in quality factor and tangent loss is almost constant after 104 Hz which indicates that these nanocomposites can be used as low k-dielectric materials in electronic application.

On Anomalously High Photosensitivity of CdS1 –xSex Granular Films

An anomalously high photosensitivity of $${\text{Cd}}{{{\text{S}}}_{{{\text{1--}}x}}}{\text{S}}{{{\text{e}}}_{x}}$$ granular films observed in the experiment is explained. It is assumed that the charge transfer mechanism based on surface states existing at the granule boundaries can be responsible for an anomalously high photocurrent-to-dark current ratio recently found in the study of the photoconductivity of $${\text{Cd}}{{{\text{S}}}_{{{\text{1--}}x}}}{\text{S}}{{{\text{e}}}_{x}}$$ granular films. It has been shown that the current of charge carriers in surface states flowing through the gaps between the granules at a certain film topology can exceed by many orders of magnitude the current associated with tunneling of charge carriers between the granules.

Controlling ambipolar current of dopingless tunnel field-effect transistor

Ambipolarity in tunnel field-effect transistor (TFET) is a subject of grave concern in the current scenario of the semiconductor industry as this property of device limits its usability in CMOS circuit applications. In this concern, this paper presents a new approach to suppress the ambipolar behavior of dopingless TFET (DL TFET) by controlling the lateral band-to-band tunneling at the drain/channel interface. To execute this, a metal strip is embedded inside the oxide region between gate and drain terminals to modulate the energy bands for preventing the tunneling of charge carriers. The energy bands are widened at the drain-channel junction using this technique and correspondingly ambipolarity is reduced by a factor of $$10^{11}$$ . The variation in the energy bands is examined under different negative gate bias for the proposed device and conventional device which has shown that the energy barrier for the proposed device remains wider for different voltages under the influence of metal embedded. Alignment of the metal strip is varied to obtain the desired performance of the device. In this regard, optimization of work function and length of metal strip along with its position is demonstrated considering its impact on the ambipolar current and ON-state current, which gave 4.0 eV work function and 20 nm as the optimized work function and length of the metal strip, respectively.

Fabrication of Strain Gauges via Contact Printing: A Simple Route to Healthcare Sensors Based on Cross-Linked Gold Nanoparticles

In this study, we developed a novel and efficient process for the fabrication of resistive strain gauges for healthcare-related applications. First, 1,9-nonanedithiol cross-linked gold nanoparticle (GNP) films were prepared via layer-by-layer (LbL) spin-coating and subsequently transferred onto flexible polyimide foil by contact printing. Four-point bending tests revealed linear response characteristics with gauge factors of ∼14 for 4 nm GNPs and ∼26 for 7 nm GNPs. This dependency of strain sensitivity is attributed to the perturbation of charge carrier tunneling between neighboring GNPs, which becomes more efficient with increasing particle size. Fatigue tests revealed that the strain-resistance performance remained nearly the same after 10.000 strain/relaxation cycles. We demonstrate that these sensors are well suited to monitor muscle movements. Furthermore, we fabricated all-printed strain sensors by directly transferring cross-linked GNP films onto soft PDMS sheets equipped with interdigitated electrodes. Due to the low elastic modulus of poly(dimethylsiloxane) (PDMS), these sensors are easily deformed and, therefore, they respond sensitively to faint forces. When taped onto the skin above the radial artery, they enable the well-resolved and robust recording of pulse waves with diagnostically relevant details.

Carrier dynamics in a tunneling injection quantum dot semiconductor optical amplifier

The process of tunneling injection is known to improve the dynamical characteristics of quantum well and quantum dot lasers; in the latter, it also improves the temperature performance. The advantage of the tunneling injection process stems from the fact that it avoids hot carrier injection, which is a key performance-limiting factor in all semiconductor lasers. The tunneling injection process is not fully understood microscopically and therefore it is difficult to optimize those laser structures. We present here a numerical study of the broadband carrier dynamics in a tunneling injection quantum dot gain medium in the form of an optical amplifier operating at $1.55\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Charge carrier tunneling occurs in a hybrid state that joins the quantum dot first excited state and the confined quantum well--injection well states. The hybrid state, which is placed energetically roughly one longitudinal optic phonon above the ground state and has a spectral extent of about $5\phantom{\rule{3.33333pt}{0ex}}\text{meV}$, dominates the carrier injection to the ground state. We calculate the dynamical response of the inversion across the entire gain spectrum following a short pulse perturbation at various wavelengths and for two bias currents. At a high bias of $200\phantom{\rule{3.33333pt}{0ex}}\text{mA}$, the entire spectrum exhibits gain; at $30\phantom{\rule{3.33333pt}{0ex}}\text{mA}$, the system exhibits a mixed gain-absorption spectrum. The carrier dynamics in the injection well is calculated simultaneously. We discuss the role of the pulse excitation wavelengths relative to the gain spectrum peak and demonstrate that the injection well responds to all perturbation wavelengths, even those which are far from the region where the tunneling injection process dominates.

Electrical properties of Se80-xTe20Sbx (0 ≤ x ≤ 12) nanorods

The present study reports the electrical properties of Se80−xTe20Sbx (0 ≤ x ≤ 12) nanorods as a function of temperature and composition. The amorphous bulk powder of Se80−xTe20Sbx (0 ≤ x ≤ 12) glasses were prepared using melt quenching method and the nanostructured thin films were prepared using physical vapor condensation method. These thin films were characterized using XRD, SEM and TEM. TEM images indicate the formation of nanorods within the thin film samples. I–V measurements were carried out in temperature range 303–373 K and electrical conductivity of nanostructured Se80−xTe20Sbx (0 ≤ x ≤ 12) thin films was determined. Two conduction mechanisms were observed in the Se80−xTe20Sbx (0 ≤ x ≤ 12) nanostructured system. In higher temperature range (343–373 K), conduction takes place through thermally assisted tunnelling of charge carriers while in the temperature range 303–343 K, variable range hopping of charge carriers contributes to conduction. VRH was also tested for lower temperature (263–303 K) range for SeTeSb3 sample and was found to be the conduction mechanism at lower temperatures also.

Characterization of the Al/Ge/In2Se3/Ga2S3/Al hybrid tunneling barriers designed for Gigahertz/Terahertz applications

In the current work, we report the design and performance of a tunneling amorphous thin film hybrid device made of Ge/In2Se3/Ga2S3 sandwiched between two aluminum thin films. Each of the stacked semiconducting layers are of 200 nm thicknesses. The hybrid device which is composed of a p-n junction between two Schottky shoulders is designed to have two valence band offsets of 0.59 and 0.84 eV at the Ge/In2Se3 and at In2Se3/Ga2S3 interfaces, respectively. The offsets which caused two quantum confinements forces the device to exhibit field effect assisted thermionic and thermionic transport mechanisms under the reverse and forward biasing conditions, respectively. When an alternating current signal is imposed between the terminals of the device, the device conducted by the quantum mechanical tunneling of charge carriers and by the correlated barrier hopping above and below 0.180 GHz, respectively. The hybrid structure are also observed to exhibit series and parallel resonance at the switching frequency between the two conduction mechanisms. It also exhibited negative differential capacitance effect in the frequency domain of 0.18–1.80 GHz. For the Al/Ge/In2Se3/Ga2S3/Al hybrid structure the microwave cutoff frequency reached 100 GHz. The latter value is promising as it indicates the ability of using the device as fast switches and microwave/Tera wave traps.

Breakdown of Zero-Energy Quantum Hall State in Graphene in the Light of Current Fluctuations and Shot Noise

We have investigated the cross-over from Zener tunneling of single charge carriers to avalanche type of bunched electron transport in a suspended graphene Corbino disk in the zeroth Landau level. At low bias, we find a tunneling current that follows the gyrotropic Zener tunneling behavior. At larger bias, we find avalanche type of transport that sets in at a smaller current the larger the magnetic field is. The low-frequency noise indicates strong bunching of the electrons in the avalanches. On the basis of the measured low-frequency switching noise power, we deduce the characteristic switching rates of the avalanche sequence. The simultaneous microwave shot noise measurement also reveals intrinsic correlations within the avalanche pulses and indicate decrease of correlations with increasing bias.

Facile Synthesis of Water-Soluble Graphene Quantum Dots/Graphene for Efficient Photodetector

ABSTRACTGraphene quantum dots (GQDs) are zero-dimensional material with characteristics derived from functionalized graphene precursors are graphene sheets a few nanometers in the lateral dimension having a several-layer thickness. Combining the structure of graphene with the quantum confinement and edge effects, GQDs possess unique chemical behavior and physical properties. Intense research activity in GQDs is attributed to their novel phenomena of charge transport and light absorption and photoluminescence excitation. The optical transitions are known to be available up to 6 eV in GQDs, applicable for ultraviolet photonics and optoelectronics devices, biomedical imaging capabilities and technologies. We present facile hydrothermal and solvothermal methods for synthesizing homogenous dispersed and uniform sized GQDs with a strong greenish and violet blue emission peaks at ∼10-14% yield. This approach enabled a large-scale production of aqueous GQD dispersions without the need for chemical stabilizers. The structure and emission mechanism of the GQDs have been studied by combining extensive characterization techniques and rigorous control experiments. We further demonstrate the distinctive advantages of such GQDs as high-performance photodetectors (PDs). Here we also report high-efficient photocurrent (PC) behaviors consisting of multilayer GQDs sandwiched between monolayer graphene sheets. It is conceivable that the observed unique PD characteristics proved to be dominated by tunneling of charge carriers which occurs through the multiple energy states within the bandgap of GQDs, based on bias-dependent variation of the band profiles. This results in novel dark current and PC behaviors. The external quantum efficiency (η) is predicted to be 47% at applied potential 2 V. These findings highlight rich photophysics and comparable performance of graphene/graphene oxide hybrids opening up potential applications as optoelectronic devices.