Multistep Tunneling Research Articles

Growth and electrical characterization of two-dimensional layered MoS2/SiC heterojunctions

The growth and electrical characterization of the heterojunction formed between two-dimensional (2D) layered p-molybdenum disulfide (MoS2) and nitrogen-doped 4H silicon carbide (SiC) are reported. The integration of 2D semiconductors with the conventional three-dimensional (3D) substrates could enable semiconductor heterostructures with unprecedented properties. In this work, direct growth of p-type MoS2 films on SiC was demonstrated using chemical vapor deposition, and the MoS2 films were found to be high quality based on x-ray diffraction and Raman spectra. The resulting heterojunction was found to display rectification and current-voltage characteristics consistent with a diode for which forward conduction in the low-bias region is dominated by multi-step recombination tunneling. Capacitance-voltage measurements were used to determine the built-in voltage for the p-MoS2/n-SiC heterojunction diode, and we propose an energy band line up for the heterostructure based on these observations. The demonstration of heterogeneous material integration between MoS2 and SiC enables a promising new class of 2D/3D heterostructures.

Tunneling anisotropic magnetoresistance inC60-based organic spintronic systems

${\mathrm{C}}_{60}$ fullerenes are interesting molecular semiconductors for spintronics since they exhibit weak spin-orbit and hyperfine interactions, which is a prerequisite for long spin lifetimes. We report spin-polarized transport in spin-valve-like structures containing ultrathin (10 nm) ${\mathrm{C}}_{60}$ layers, ferromagnetic (FM) epitaxial face-centered-cubic (fcc) Co (111) contacts, ${\mathrm{AlO}}_{x}$ tunnel barriers, and nonmagnetic Al counter electrodes. Even though genuine spin-valve behavior cannot occur for only one FM contact, we find significant tunneling anisotropic magnetoresistance (TAMR) upon rotating the in-plane magnetization, originating from spin-orbit interaction (SOI) induced anisotropy of the fcc (111) Co bands. The uniaxial magnetocrystalline anisotropy of the Co electrodes results in a predominantly twofold symmetric in-plane TAMR effect. We investigated the TAMR effect in the direct tunneling regime (2 nm ${\mathrm{C}}_{60}$), at the transition point to two-step tunneling (4 nm ${\mathrm{C}}_{60}$), and in the multistep regime (8 nm ${\mathrm{C}}_{60}$). A sizable TAMR of 4.5% is found at 5 K under application of a 500-mT in-plane magnetic field for ${\mathrm{C}}_{60}$ layers of 2 nm, which is strongly suppressed at 8 nm thickness, indicating that TAMR may strongly contribute to the ``spin-valve'' signal for direct tunneling, but not for multistep tunneling. The TAMR effect is proposed to be due to a combination of SOI induced modulation of the tunneling DOS upon rotating the in-plane magnetization of the fcc epitaxial Co thin film, resonant tunneling processes involving interfacial states, and different Bychkov-Rashba SOI at the different interfaces.

Oxygen‐deficiency‐induced suppression of JT distortion and stabilization of charge ordering in La0.2Sr0.8MnO3−δ

Structural phase‐transition studies employing low‐temperature transmission electron microscopy and low‐temperature X‐ray diffraction have been carried out on nearly stoichiometric (δ = 0.01) and on off‐stoichiometric (δ = 0.12) versions of La0.2Sr0.8MnO3−δ manganite. The nearly stoichiometric La0.2Sr0.8MnO3−δ undergoes a cubic to Jahn–Teller distorted C‐type antiferromagnetic tetragonal phase transition at 260 K. For off‐stoichiometric La0.2Sr0.8MnO3−δ the Jahn–Teller distorted tetragonal phase transition is totally suppressed and the basic perovskite lattice remains cubic but shows a charge‐ordered phase. Stabilization of charge‐ordered phase in the absence of cooperative Jahn–Teller distortion has been attributed to Coulomb repulsion and Hunds coupling energy. The off‐stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic, and calorimetric measurements. A smooth crossover from variable‐range hopping transport to a power‐law dependence of resistivity (ρ) on temperature (T), i.e., ρ = CT−α has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.

Tunnel magnetoresistance in organic spin valves in the regime of multistep tunneling

A model of a spin valve in which electron transport between the magnetized electrodes is due to multistep tunneling is analyzed. Motivated by recent experiments on organic spin valves, we assume that spin memory loss in the course of transport is due to random hyperfine fields acting on electron while it waits for the next tunneling step. Amazingly, we identify the three-step configurations of sites, for which the tunnel magnetoresistance (TMR) is negative in the absence of interfacial effects, suggesting that the resistance for antiparallel magnetizations of the electrodes is smaller than for parallel magnetizations. We analyze the phase volume of these configurations with respect to magnitudes and relative orientations of the on-site hyperfine fields. The effect of sign reversal of TMR is exclusively due to interference of the spin-flip amplitudes on each site; it does not emerge within commonly accepted probabilistic description of spin transport. Another feature specific to multistep inelastic tunneling is bouncing of electron between nearest neighbors while awaiting a ``hard'' hop. We demonstrate that this bouncing, being absolutely insignificant for conduction of current, can strongly affect the spin memory loss. This effect is also of interference origin.

Optical and electrical characteristics of ZnO/Si heterojunction

Self-assembled ZnO nanorods have been synthesized on a seeded Si substrate by a simple chemical bath deposition method at a temperature of 80°C. Room-temperature photoluminescence analysis revealed material of high optical quality with a low density of defects that can be reduced by post growth annealing. Current–voltage measurements on these devices showed excellent rectification. Junction characteristics were also studied using capacitance–voltage measurements and showed that the junction characteristics are mainly determined by the properties of the p-Si substrate. Based on the energy band diagram and possible interface states at the junction, it was suggested that the current transport in the device is predominantly determined by hopping of charge carriers between localized states through a multi-step tunneling process.

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors based on metalorganic vapor-phase epitaxy-grown epilayers

Charge transport properties of p-CdTe/n-CdTe/n+-Si diode-type nuclear radiation detectors, fabricated by growing p-and n-type CdTe epilayers on (211) n+-Si substrates using metalorganic vapor-phase epitaxy (MOVPE), were studied by analyzing current-voltage characteristics measured at various temperatures. The diode fabricated shows good rectification properties, however, both forward and reverse biased currents deviate from their ideal behavior. The forward current exhibits typical feature of multi-step tunneling at lower biases; however, becomes space charge limited type when the bias is increased. On the other hand, the reverse current exhibits thermally activated tunneling-type current. It was found that trapping centers at the p-CdTe/n-CdTe junction, which were formed due to the growth induced defects, determine the currents of this diode, and hence limit the performance of the nuclear radiation detectors developed.

Electrical and photovoltaic properties of SnSe/Si heterojunction

Thin film of SnSe is deposited on n-Si single crystal to fabricate a p-SnSe/n-Si heterojunction photovoltaic cell. Electrical and photoelectrical properties have been studied by the current density–voltage (J–V) and capacitance–voltage (C–V) measurements at different temperatures. The fabricated cell exhibited rectifying characteristics with a rectification ratio of 131 at ±1V. At low voltages (V<0.55V), the dark forward current density is controlled by the multi-step tunneling mechanism. While at a relatively high voltage (V>0.55V), a space charge-limited-conduction mechanism is observed with trap concentration of 2.3×1021cm−3. The C–V measurements showed that the junction is of abrupt nature with built-in voltage of 0.62V which decreases with temperature by a gradient of 2.83×10−3V/K. The cell also exhibited strong photovoltaic characteristics with an open-circuit voltage of 425mV, a short-circuit current density of 17.23mAcm−2 and a power conversion efficiency of 6.44%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50mWcm−2.

Improvement of optical transmittance and electrical properties for the Si quantum dot-embedded ZnO thin film

A Si quantum dot (QD)-embedded ZnO thin film is successfully fabricated on a p-type Si substrate using a ZnO/Si multilayer structure. Its optical transmittance is largely improved when increasing the annealing temperature, owing to the phase transformation from amorphous to nanocrystalline Si QDs embedded in the ZnO matrix. The sample annealed at 700°C exhibits not only high optical transmittance in the long-wavelength range but also better electrical properties including low resistivity, small turn-on voltage, and high rectification ratio. By using ZnO as the QDs’ matrix, the carrier transport is dominated by the multistep tunneling mechanism, the same as in a n-ZnO/p-Si heterojunction diode, which clearly differs from that using the traditional matrix materials. Hence, the carriers transport mainly in the ZnO matrix, not through the Si QDs. The unusual transport mechanism using ZnO as matrix promises the great potential for optoelectronic devices integrating Si QDs.

Current Transport Mechanism of n-Type Nanocrystalline FeSi&lt;sub&gt;2&lt;/sub&gt;/Intrinsic Si/p-Type Si Heterojunctions Fabricated by Facing-Targets Direct-Current Sputtering

n-Type nanocrystalline FeSi2/intrinsic Si/p-type Si heterojunctions were successfully fabricated by FTDCS and their forward current-voltage characteristics at low temperatures were analyzed on the basis of thermionic emission theory. The analysis of J-V characteristics exhibits an increase in the ideality factor and a decrease in the barrier height at low temperatures. The values of ideality factor were estimated to be 2.26 at 300 K and 9.29 at 77 K. The temperature dependent ideality factortogether with the constant value of parameter A indicated that a trap assisted multi-step tunneling process is the dominant carrier transport mechanism in this heterojunction. At high voltages, the current transport mechanism is dominated by SCLC process.

Carrier transport mechanisms of n-ZnO/ZnMgO/p-GaN heterojunctions revealed by temperature-dependent current–voltage characteristics

The heterojunction diodes with n-ZnO/ZnMgO/p-GaN structure have been deposited on the commercially available p-GaN/sapphire substrates by a simple process of ultrasonic spray pyrolysis. To demonstrate the effect of ZnMgO layer as electron blocking layer, the carrier transport mechanisms of both n-ZnO/p-GaN and n-ZnO/ZnMgO/p-GaN heterojunctions were systemically investigated by temperature-dependent current–voltage measurements. The results revealed that the multistep tunneling is the dominant carrier transport mechanism in n-ZnO/ZnMgO/p-GaN heterojunctions under forward current, accurately controlled ZnMgO layer thickness can effectively block electron injection from ZnO to GaN and promote hole injection from GaN to ZnO. Therefore, the design and application of ZnO based heterostructure devices will benefit significantly from these achievements.

Shot Noise in Epitaxial Double-Barrier Magnetic Tunnel Junctions

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the magnetic configuration of the junction—the P-state with the magnetic moments of all three ferromagnetic layers aligned parallel, the AP1 state with the central electrode magnetized opposite to its neighbors, and two different AP2 states with the magnetic moment of the upper or bottom electrode aligned opposite to the other two. We also show that the asymmetry between both MgO barriers is another important factor which affects the shot noise. Some voltages present an enhancement in both conductance and shot noise, which indicates that resonant tunneling through quantum well states formed in the middle magnetic layer is taking place. When the junctions are heated to 60 K, the resonance tunneling anomalies in the shot noise smear out, but they survive in the differential conductance. On the other hand, at low voltages (below 200 mV) and low temperatures (below 4 K) the shot noise tends to decrease, probably due to multi-step tunneling via localized defect states in the tunnel MgO barrier. The theoretical model of sequential tunneling proposed for this system, which takes into account spin relaxation, successfully describes the experimental observations in the bias range between 200 mV and 500 mV, where the influence of tunneling through barrier defects and resonant states inside the central electrode is negligible.

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO2/p-Si heterojunctions are fabricated by the deposition of a TiO2 film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO2/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.

Scaling description of non-Ohmic transport in manganites

The current–voltage characteristics of several manganite systems like La0.87MnO3–(Mn2O3)0.13, La0.60Y0.07Ca0.33MnO3 and Nd0.55Sm0.3375Sr0.1125MnO3 show non-Ohmic behaviour at different temperatures. The onset voltage V0(T) at which conductance starts deviating from its zero-bias value Σ0(T) scales as V0(T)∼Σ0(T)xT with xT being the onset exponent. A detailed scaling analysis with these manganite systems reveal that the onset exponents xT are different in the paramagnetic insulating and ferromagnetic metallic phases. Besides, the magnitudes of xT are different in different manganite systems. Non-Ohmic conduction and the onset exponent are described with the help of inelastic multi-step tunneling model and scaling concepts.

The Multistep Tunneling Analogue of Conductivity Mismatch in Organic Spin Valves

AbstractCarbon‐based, molecular semiconductors offer several attractive attributes for spintronics, such as exceptionally weak spin‐orbit coupling and compatibility with bottom‐up nanofabrication. In spite of the promising properties of organic spin valves, however, the physical mechanisms governing spin‐polarized conduction remain poorly understood. An experimental study of C60‐based spin valves is presented and their behavior is modeled with spin‐polarized tunneling via multiple intermediate states with a Gaussian energy distribution. It is shown that, analogous to conductivity mismatch in the diffusive regime, the magnetoresistance decreases with the number of intermediate tunnel steps, regardless of the value of the spin lifetime. This mechanism has been largely overlooked in previous studies of organic spin valves. In addition, using measurements of the temperature and bias dependence of the magnetoresistance, inhomogeneous magnetostatic fields resulting from interfacial roughness are identified as a source for spin relaxation and dephasing. These findings constitute a comprehensive understanding of the processes underlying spin‐polarized transport in these structures and shed new light on previous studies of organic spin valves.

Transport-mechanism analysis of the reverse leakage current in GaInN light-emitting diodes

The reverse leakage current of a GaInN light-emitting diode (LED) is analyzed by temperature dependent current–voltage measurements. At low temperature, the leakage current is attributed to variable-range-hopping conduction. At high temperature, the leakage current is explained by a thermally assisted multi-step tunneling model. The thermal activation energies (95–162 meV), extracted from the Arrhenius plot in the high-temperature range, indicate a thermally activated tunneling process. Additional room temperature capacitance–voltage measurements are performed to obtain information on the depletion width and doping concentration of the LED.

Experimental evidence of direct contact formation for the current transport in silver thick film metallized silicon emitters

Great advances have been achieved in the development of silver pastes. The use of smaller silver particles, higher silver content, and, thus, less glass frit allow modern silver pastes to contact high resistive emitters without the necessity of a selective emitter or subsequent plating. To identify the microscopic key reasons behind the improvement of silver paste, it is essential to understand the current transport mechanism from the silicon emitter into the bulk of the silver finger. Two current transport theories predominate: i) The current flows through the Ag crystallites grown into the Si emitter, which are separated by a thin glass layer or possibly in direct contact with the silver finger. ii) The current is transported by means of multistep tunneling into the silver finger across nano-Ag colloids in the glass layer, which are formed at optimal firing conditions; the formation of Ag crystallites into the Si surface is synonymous with over-firing. In this study, we contact Si solar cell emitters with different silver pastes on textured and flat silicon surfaces. A sequential selective silver-glass etching process is employed to expose and isolate the different contact components for current transport. The surface configurations after the etching sequences are observed with scanning electron microscopy. Liquid conductive silver is then applied to each sample and the contact resistivity is measured to determine the dominant microscopic conduction path system. We observe glass-free emitter areas at the tops of the pyramidal-textured Si that lead to the formation of direct contacts between the Ag crystallites grown into the Si emitter and the bulk of the silver finger. We present experimental evidence that the major current flow into the silver finger is through these direct contacts.

Charge transport in ultrathin iron-phthalocyanine thin films under high electric fields

The temperature dependent current–voltage (J–V) characteristics of 20 nm thick iron-phthalocyanine films are investigated in the temperature range of 300–30 K, and in the bias range of ±200 V. In the temperature range of 300–100 K, the charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J∼V) to exponentially distributed shallow trap mediated space–charge-limited conduction (J∼Vα, α≥2) to space–charge-limited conduction with field enhanced mobility (lnμ∼E1/2). However, at temperatures less than 100 K, the charge transport is electrode-limited, and undergoes a bias dependent transition from Schottky (lnJ∼V1/2) to multistep tunneling. From shallow trap mediated space–charge-limited conduction data the estimated room temperature mobility was found to be ∼1.9 cm2 V −1 s−1. The high mobility of films is attributed to better structural organization due to the face-on stacking of molecules, which is supported by x-ray diffraction and UV–visible spectroscopy data.

Nonlinearity exponents in lightly doped conducting polymers

The I-V characteristics of four conducting polymer systems such as doped polypyrrole, poly(3,4-ethylenedioxythiophene), polydiacetylene, and polyaniline in as many physical forms have been investigated at different temperatures, quenched disorder, and magnetic fields. Transport data clearly show the existence of a single electric-field scale in each system. Based upon this observation, a phenomenological scaling analysis is performed, leading to the extraction of a numerical value for a nonlinearity exponent called ${x}_{M}$ which serves to characterize a set of I-V curves. The conductivity starts deviating from an Ohmic value ${\ensuremath{\sigma}}_{0}$ above an onset electric field ${F}_{o}$ which scales according to ${F}_{o}\ensuremath{\sim}{\ensuremath{\sigma}}_{0}^{{x}_{M}}$. The electric-field-dependent data are shown to be described by the multistep tunneling model of Glazman-Matveev [JETP 67, 1276 (1988)] in a near-perfect manner over nine orders of magnitude in conductivity and five orders of magnitude in electric field. Furthermore, ${x}_{M}$ is found to possess both positive and negative values lying between $\ensuremath{-}1/2$ and $3/4$. There is no theory at present for this exponent. Some issues concerning applicability of the Glatzman-Matveev model are discussed.

Enhanced electroluminescence from the fluorine-plasma implanted Ni/Au-AlGaN/GaN Schottky diode

The effect of fluorine-plasma (F-plasma) implantation on the current-voltage (I-V) and electroluminescence (EL) characteristics of Ni/Au-AlGaN/GaN Schottky diodes have been investigated. The observed EL spectrum is dominated by the GaN near band edge emissions. The threshold current of the EL emission for F-plasma implanted diodes is significantly lower than that for the previously reported diodes without the F-plasma implantation. This reduction of threshold current results from the presence of negatively charged F-centers in AlGaN layer which leads to upward band bending of AlGaN layer and enhanced hole injection by multi-step tunneling process through AlGaN layer. The magnitude of the upward banding in AlGaN layer is estimated to be 0.36 eV by analyzing the forward-biased I-V characteristics.

The forward bias current density–voltage–temperature (J–V–T) characteristics of Al–SiO2–pSi (MIS) Schottky diodes

Metal–insulator–semiconductor Schottky diodes were fabricated to investigate the tunnel effect and the dominant carrier transport mechanism by using current density–voltage (J–V) and capacitance–voltage (C–V) measurements in the temperature range of 295–370 K. The slope of the ln J–V curves was almost constant value over the nearly four decades of current and the forward bias current density J is found to be proportional to Jo (T) exp(AV). The values of Nss estimated from J–V and C–V measurements decreased with increasing temperature. The temperature dependence of the barrier heights obtained from forward bias J–V was found to be entirely different than that from the reverse bias C–V characteristics. All these behaviours confirmed that the prepared samples have a tunnel effect and the current transport mechanism in the temperature range of 295–370 K was predominated by a trap-assisted multi-step tunnelling, although the Si wafer has low doping concentration and the measurements were made at moderate temperature.