Low-temperature Current-voltage Characteristics Research Articles

Magnetic two-dimensional field effect transistor

We study experimentally low-temperature current-voltage characteristics of n-type GaAs/GaAlAs modulation doped quantum wells under the influence of an external magnetic field. In particular, we use samples additionally doped in the well with Be acceptors. As showed previously, negatively charged acceptor ions can localize conduction electrons by a joint effect of a quantum well and an external magnetic field. It is found that, in the acceptor-doped samples, the Hall electric field resulting from the presence of magnetic field plays the role of the gate voltage. At sufficiently high magnetic fields, the drain current has a constant value independent of the drain voltage. It is argued that the above phenomenon is due to the electron localization with the resulting decrease of conducting electron density in the crossed-field configuration. We propose to exploit the observed unusual behaviour as a device called “magnetic two-dimensional electron gas field effect transistor” operating at low temperatures.

Stability, sub-gap current, 1/f-noise, and elemental depth profiling of annealed Al:Mn-AlOX-Al normal metal-insulator-superconducting tunnel junctions

In this paper we report a study of the effect of vacuum annealing at 400°C on the properties of normal metal-insulator-superconductor (NIS) tunnel junctions, with manganese doped aluminium (Al:Mn) as the normal metal, aluminum as the superconductor and amorphous aluminum oxide as the tunneling barrier (Al:Mn-AlOx-Al). The annealing treatment improves the stability of the junctions, increases their tunneling resistance and does not have a negative impact on the low-temperature current-voltage characteristics. The measured 1/f resistance noise of the junctions also changes after annealing, in the best case decreasing by over an order of magnitude. All these observations show that annealing is a viable route to improve NIS junction devices after the sample has been fabricated.

Negative differential resistance in n-type noncompensated silicon at low temperature

We present the results on low temperature current-voltage characteristics of noncompensated Si doped by Sb. In the temperature range 1.9–2.25 K and at electrical fields smaller than 1 V/cm, the negative differential resistance (NDR) was observed. The external magnetic field enhances the region of the NDR. We attribute this effect to the delocalization of the D− states in the upper Hubbard band due to the accumulation of the charge injected by current.

Tailoring phase slip events through magnetic doping in superconductor-ferromagnet composite films.

The interplay between superconductivity (SC) and ferromagnetism (FM) when embedded together has attracted unprecedented research interest due to very rare coexistence of these two phenomena. The focus has been mainly put into the proximity induced effects like, coexistence of magnetism and superconductivity, higher critical current, triplet superconductivity etc. However, very little attention has been paid experimentally to the role of magnetic constituent on triggering phase slip processes in the composite films (CFs). We demonstrate that less than 1 at.% of magnetic contribution in the CFs can initiate phase slip events efficiently. Due to advanced state-of-the-art fabrication techniques, phase slip based studies have been concentrated mainly on superconducting nanostructures. Here, we employ wide mesoscopic NbGd based CFs to study the phase slip processes. Low temperature current-voltage characteristics (IVCs) of CFs show stair-like features originated through phase slip events and are absent in pure SC films. Depending on the bias current and temperature, distinct regions, dominated by Abrikosov type vortex-antivortex (v-av) pairs and phase slip events, are observed. The results presented here open a new way to study the phase slip mechanism, its interaction with v-av pairs in two dimensions and hence can be useful for future photonic and metrological applications.

Low-temperature (77–300 K) current-voltage characteristics of 4H-SiC p +-p-n + diodes: Effect of impurity breakdown in the p-type base

The effect of impurity breakdown on the low-temperature (77–300 K) current-voltage (I–V) characteristics of 4H-SiC diodes with a p-type base has been studied. Experimental samples were fabricated from CVD-grown (chemical vapor deposition) commercial p+-p-n+ 4H-SiC structures. A high electric field in the p-type base was created by applying a forward bias to the diodes. It was found that, at temperatures of 136, 89, and 81 K, the commonly observed “diode” portion of the I–V characteristics is followed by a portion in which the current grows more rapidly due to the impact ionization of frozen-out Al acceptor atoms in the ground (unexcited) state. At temperatures of 81 and 77 K, this portion is followed by one with a negative differential resistance due to the regenerative dynistor-like switching of the diode, caused by impact ionization of aluminum atoms in the excited state.

Resistive Width of Superconducting Transition in Titanium Nitride Thin Film

The transition to the superconducting state of the ultrathin (5 nm thick) titanium nitride film was studied. It is found that the nonmonotonic temperature dependence of the resistance, R(T), is a consequence of competition between the contributions to conductivity from quantum corrections stemming from the electron-electron interaction in the diffusion and Cooper channels. It is shown that the appreciable decrease in the resistance at T > Tc (Tc is the superconducting transition temperature) results from the superconducting fluctuations. We present the results of the analysis of low-temperature (T < Tc ) current-voltage characteristics V(I). It is found that they follow the power-law behavior V∝ I α(T) , with α(T) increasing rapidly with the decreasing temperature. The Berezinskii-Kosterlitz-Thouless transition temperature, TBKT, and width of the superconducting transition ∆T = Tc − TБКТ were determined

Self-heating and nonlinear current-voltage characteristics in bilayer graphene

We demonstrate by experiments and numerical simulations that the low-temperature current-voltage characteristics in diffusive bilayer graphene (BLG) exhibit a strong superlinearity at finite bias voltages. The superlinearity is weakly dependent on doping and on the length of the graphene sample. This effect can be understood as a result of Joule heating. It is stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG is more sensitive to temperature due to the higher density of electronic states at the Dirac point.

Bistable low temperature (77 K) impurity breakdown in p-type 4H-SiC

Low-temperature (77 K) forward current-voltage characteristics of 4H-SiC p +-p-n +-n (substrate) mesa epitaxial diode structures have been measured. The characteristics are S-shaped, which is accounted for by the bistable nature of the impact ionization of frozen-out acceptor atoms of aluminum.

Quantum-mechanical modeling of current-voltage characteristics of Ti-silicided Schottky diodes

Based on the interfacial-layer and quantum-mechanical (QM) carrier transport approach, a theoretical model is proposed to predict the anomalous behavior of low-temperature current-voltage (I-V) characteristics of Ti-silicided Schottky diodes. Physical parameters such as barrier height, ideality factor, series resistance, and effective Richardson constant of silicided Schottky diodes are extracted from the forward experimental I-V characteristics. Simulations of both the forward and reverse I-V characteristics have also been performed using extracted parameters. Results are compared with the models, such as, thermionic-emission-diffusion and thermionic-emission with barrier lowering reported in the literature. It is shown that for Ti-silicided Schottky diodes, the use of QM transport model provides a better agreement with the experimental data.

Coulomb-blockade transport in quasi-one-dimensional polymer nanofibers

We report the low temperature current-voltage (I-V) characteristics studies in quasi-one-dimensional conducting polymer nanofibers. We find a threshold voltage ${V}_{t}$ below which little current flows at temperatures below $30--40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For $V>{V}_{t}$ current scales as ${(V∕{V}_{t}\ensuremath{-}1)}^{\ensuremath{\zeta}}$ where $\ensuremath{\zeta}\ensuremath{\sim}1.8--2.1$ at high biases. Differential conductance oscillations are observed whose magnitude increases as temperature decreases below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We attributed the observed low temperature I-V behavior to Coulomb-blockade effects with a crossover to Luttinger liquidlike behavior at high temperature. We demonstrate that at low temperatures such a doped conjugated polymer fiber can be considered as an array of small conducting regions separated by nanoscale barriers, where the Coulomb-blockade tunneling is the dominant transport mechanism.

Injection currents in narrow-gap (Pb1−x SnxTe):In insulators

The low-temperature current-voltage characteristics of narrow-gap (Pb1−xSnxTe):In have been studied experimentally and calculated for a wide range of electric fields. It is shown that the obtained data are satisfactorily described in terms of a space-charge-limited current model in the presence of traps. The concentration and energy depth of the traps have been estimated.

Low temperature electronic transport and electron transfer through organic macromolecules

It is shown that at low temperatures and moderate electron dephasing the electron transmission function reveals a structure containing information about donor/acceptor sites effectively participating in the electron transfer processes and primary pathways of electrons tunneling through molecular bridges in macromolecules. This important information can be obtained as a result of analysis of experimental low temperature current-voltage characteristics for chosen molecules.

Investigation of trap-assisted tunneling current in InAs/(GaIn)Sb superlattice long-wavelength photodiodes

Trap centers with an energy level positioned 1/3 of the band gap below the effective conduction band edge are observed in the electroluminescence spectra of InAs/(GaIn)Sb superlattice photodiodes with a cutoff wavelength of 11 μm. The trap centers are recognized by simulating the low-temperature current–voltage characteristics of the diodes. Excellent quantitative agreement on both, the I–V characteristic and the differential resistance between the experimental data and the theoretical prediction is achieved. The quantitative simulation of the I–V characteristics shows, that the 77 K performance of InAs/(GaIn)Sb photodiodes is dominated by generation-recombination processes even at long wavelengths. Above 50 K, tunneling currents are not of importance.

Low-temperature current–voltage characteristics of MIS Cu/n-GaAs and inhomogeneous Cu/n-GaAs Schottky diodes

The current–voltage (I–V) characteristics of metal–insulating layer–semiconductor Cu/n-GaAs and inhomogeneous Cu/n-GaAs Schottky barrier diodes were determined in the temperature range 80– 300 K . The evaluation of the experimental I–V data reveals a nonlinear increase of the zero-bias barrier height ( qΦ 0) for the inhomogeneous Cu/n-GaAs Schottky barrier diodes and a linear increase of the zero-bias barrier height ( qΦ 0) for Cu/n-GaAs Schottky barrier diodes with an interfacial layer. The ideality factor n decreases with increasing temperature for all diodes. Furthermore, the changes in Φ 0 and n become quite significant below 150 K and the plot of ln( I 0/ T 2) versus 1/ T exhibits a non-linearity below 180 K for the inhomogeneous barrier diodes. Such behavior is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the interface. The value of the Richardson constant was found to be 5.033 A/ cm 2 K 2 , which is close to the theoretical value of 8.16 A/ cm 2 K 2 used for the determination of the zero-bias barrier height.

GaAs-based modulation-doped quantum-well infrared photodetectors for single- and two-color detection in 3-5 μm

Double-barrier quantum-well infrared photodetectors are promising for operation in the midinfrared region. In this paper, we present a series of novel molecular beam epitaxy (MBE)-grown devices based on modulation-doped (MD) AlGaAs-AlAs-GaAs structures that exhibit a remarkable responsivity at zero bias (0.05 A/W) at 4.6 /spl mu/m. Since the photovoltaic properties are strongly dependent on the symmetry of the potential profile, we have systematically varied the position of the dopant in the barriers for a series of single-color detectors. Low-temperature photocurrent spectra and current-voltage (I-V) characteristics (in the dark and under illumination) show that the location of the dopant is a relevant design parameter, due to its role in the photovoltaic behavior (i.e., the presence or absence of zero bias signal). The performance of the MD devices is compared with that of a detector with doping in the center of the well and otherwise the same structure. In particular, the responsivity and detectivity seem to be higher for the MD detectors than for well-doped samples, especially when the dopant is located in the barrier closest to the substrate. Therefore, we have chosen that MD dopant profile when designing and growing, to our knowledge, the first 3-5 /spl mu/m two-color detector, with simultaneous detection at 3.8 and 4.4 /spl mu/m.

Conduction mechanism of Si single-electron transistor having a one-dimensional regular array of multiple tunnel junctions

Uniformly doped Si single-electron transistors consisting of a one-dimensional regular array of multiple tunnel junctions (MTJs) and islands have been fabricated. The Coulomb blockade effect is found to play an important role in carrier conduction in the MTJ system at low temperatures (6 K). The conduction mechanism can be interpreted well by considering soliton. The soliton extends less than three islands in our MTJs, and the energy of a single soliton is found to be 0.024 eV from an analysis of low-temperature current–voltage characteristics. For high-temperature operation, it is effective to reduce the parasitic capacitance of each island, which leads to an increase in soliton length.

Electron transport measurements of Schottky barrier inhomogeneities

We report nonmonotonicities in the low-temperature current versus gate voltage characteristics of PtSi/Si Schottky Barrier metal–oxide–semiconductor field-effect transistors. Direct tunneling through the Schottky barrier is shown to limit the current and be superimposed with resonant peaks and oscillations. These structures are attributed to resonant tunneling through impurities located close to the interface and nonuniformities of the heterojunction. We thus demonstrate barrier height variations in electron transport through a relatively large metal/semiconductor contact area. The inhomogeneities result in different average Schottky barrier heights between devices, and cause height variations as a function of carrier concentration within a metal/semiconductor interface.

Mesoscopic NbSe3 wires

We have fabricated wire structures with (sub)micron sizes in the charge-density wave conductor Nbse 3- Electrical transport measurements include complete mode-locking on Shapiro steps and show that the patterning has not affected the CDW material. Our mesoscopic wires show strong fluctuation and hysteresis effects in the low-temperature current-voltage characteristics, as well as a strong reduction of the phase-slip voltage. This reduction can not be explained with existing models. We suggest that single phase-slip events are responsible for a substantial reduction of the CDW strain in micron-sized systems.

Development of an energy barrier at the metal-chain–metallic-carbon-nanotube nanocontact

We perform ab initio pseudopotential calculations for a nanocontact between a metallic carbon nanotube and a copper chain. We find that the on-top position is the most stable geometry of the copper chain on the nanotube and a local energy gap of $\ensuremath{\lesssim}0.1 \mathrm{eV}$ opens as the mirror symmetry of the nanotube is broken by the presence of copper. A weak ionic bonding is formed between the tube and the copper chain and the charge density in the conducting channel around the Fermi level is reduced at the contact region. Therefore, the electronic transport across the contact occurs essentially through the tunneling process. Appearance of such a locally semiconducting property in the intrinsically metallic carbon nanotube may explain the unusual low-temperature current-voltage characteristics exhibiting a huge contact resistance or a quantum dot behavior.

C-axis tunneling between adjacent superconducting CuO2 bi- or trilayers

We have investigated the subgap tunneling characteristics in single crystals of Bi2Sr2CaCu2O8+δ and thin films of Tl2Ba2Ca2Cu3O10+δ with intrinsic Josephson junctions in the resistive state. The low temperature current voltage characteristics (IVC) exhibits a high subgap conductivity. The results are compared to a calculated IVC for tunnel junctions between two identical d-wave superconductors. The calculated quasiparticle branch agrees reasonable with the measured IVC. However, the presence of a small s-wave component can not be excluded. Besides the global IVC we studied fine structures which were observed in the IVC of both materials. These structures can be explained by a resonant coupling of infrared active c-axis phonons to the Josephson ac currents between adjacent pairs of superconducting layers.