Tunneling Current Measurements Research Articles

Current-in-plane tunneling measurement of oxygen-functionalized few-layer boron nitride lateral barriers

A surface tunneling current measurement was used to study the electronic performance of hexagonal boron nitride (hBN) and surface-functionalized hBN in a variety of configurations and conditions.

A millikelvin scanning tunneling microscope in ultra-high vacuum with adiabatic demagnetization refrigeration.

We present the design and performance of an ultra-high vacuum scanning tunneling microscope (STM) that uses adiabatic demagnetization of electron magnetic moments for controlling its operating temperature ranging between 30 mK and 1K with an accuracy of up to 7 μK rms. At the same time, high magnetic fields of up to 8T can be applied perpendicular to the sample surface. The time available for STM experiments at 50 mK is longer than 20 h, at 100 mK about 40 h. The single-shot adiabatic demagnetization refrigerator can be regenerated automatically within 7 h while keeping the STM temperature below 5K. The whole setup is located in a vibrationally isolated, electromagnetically shielded laboratory with no mechanical pumping lines penetrating its isolation walls. The 1K pot of the adiabatic demagnetization refrigeration cryostat can be operated silently for more than 20 days in a single-shot mode using a custom-built high-capacity cryopump. A high degree of vibrational decoupling together with the use of a specially designed minimalistic STM head provides outstanding mechanical stability, demonstrated by the tunneling current noise, STM imaging, and scanning tunneling spectroscopy measurements, all performed on an atomically clean Al(100) surface.

Identification of DNA bases using nanopores created in finite-size nanoribbons from graphene, phosphorene, and silicene

Graphene’s success for nanopore deoxyribonucleic acid (DNA) sequencing has shown that it is possible to explore other potential single-atom and few-atom thick layers of elemental 2D materials beyond graphene (e.g., phosphorene and silicene) and also that these materials can exhibit fascinating and technologically useful properties for DNA base detection that are superior to those of graphene. Using density functional theory (DFT), we study the interaction of DNA bases with nanopores created in finite-size nanoribbons from graphene, phosphorene, and silicene. Due to the small size of DNA bases, the bases interact with only a small section of the nanoribbon; hence, using a finite-size model is appropriate for capturing the interaction of bases and 2D membrane materials. Furthermore, by using a finite-size model, our system is approximated as a molecular system, which does not require a periodic DFT calculation. We observe that binding energies of DNA bases using nanopores from phosphorene and silicene are similar and generally smaller compared to those from graphene. This shows that minimal sticking of DNA bases to the pore is expected for phosphorene and silicene devices. Furthermore, nanopores from phosphorene and silicene show a characteristic change in the density of states for each base. The bandgaps of phosphorene and silicene are significantly altered due to interaction with DNA bases compared to those of graphene. Our findings show that phosphorene and silicene are promising alternatives to graphene for DNA base detection using advanced detection principles such as transverse tunneling current measurement.

Consideration on SiO2/4H-SiC Band Alignment Modulation by NO Annealing

NO post-oxidation annealing (NO-POA), introducing nitrogen at the SiO2/4H-SiC interface, is an excellent method not only for the reduction of the interface trap density, but also for the improvement of the MOSFET performance. However, a significant negative shift of the threshold voltage of MOSFET is frequently observed after NO-POA, which can be a critical issue for the industrial application. Therefore, it is important to understand the SiO2/4H-SiC interface after nitridation from various perspectives. Here, we experimentally demonstrated that the shift of the flat-band voltage is directly related to the change of the SiO2/4H-SiC band alignment. We estimated the flat-band voltage without a fixed charge effect and observed the negative shift of flat-band voltage on (0001) & (11-20) 4H-SiC. Moreover, we observed the shift of the band alignment of various samples by F-N tunneling current measurement and XPS analyses. There was a relationship between the conduction band offset and NO-POA duration for both crystal orientations and we suggested a model with dipole layer formation as an origin. On (0001) & (11-20) 4H-SiC, due to the increase of the conduction band offset, flat-band voltage showed a negative shift but the leakage current level also decreased, which can be an advantage for the reliability property. These findings show new opportunities to modulate the band alignment between SiC and SiO2 just by substituting a foreign element for the atoms at the interface.

Scanning Tunneling Thermometry

Temperature imaging of nanoscale systems is a fundamental problem which has myriad potential technological applications. For example, nanoscopic cold spots can be used for spot cooling electronic components while hot spots could be used for precise activation of chemical or biological reactions. More fundamentally, imaging the temperature fields in quantum coherent conductors can provide a wealth of information on heat flow and dissipation at the smallest scales. However, despite significant technological advances, the spatial resolution of temperature imaging remains in the few nanometers range. Here we propose a method to map electronic temperature variations in operating nanoscale conductors by relying solely upon electrical tunneling current measurements. The scanning tunneling thermometer, owing to its operation in the tunneling regime, would be capable of mapping sub-angstrom temperature variations, thereby enhancing the resolution of scanning thermometry by some two orders of magnitude.

Thickness-Insensitive Properties of α-MoO3 Nanosheets by Weak Interlayer Coupling.

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO3 nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO3 is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO3 in MoO3 showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO3 of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO3 nanosheets for nanodevice applications.

A tunneling current measurement scheme to observe Majorana-zero-mode-induced crossed Andreev reflection

We propose a scheme to observe the Majorana-zero-mode-induced crossed Andreev reflection by measuring tunneling current directly. In this scheme a metallic ring structure is utilized to separate electron and hole signals. Since tunneling electrons and holes have different propagating wave vectors, the conditions for them to be constructively coherent in the ring differ. We find that when the magnetic flux threading the ring varies, it is possible to observe adjacent positive and negative tunneling current peaks of about equal amplitudes.

Evaluation of the Kinetic Property of Single-Molecule Junctions by Tunneling Current Measurements.

We investigated the formation and breaking of single-molecule junctions of two kinds of dithiol molecules by time-resolved tunneling current measurements in a metal nanogap. The resulting current trajectory was statistically analyzed to determine the single-molecule conductance and, more importantly, to reveal the kinetic property of the single-molecular junction. These results suggested that combining a measurement of the single-molecule conductance and statistical analysis is a promising method to uncover the kinetic properties of the single-molecule junction.

Detecting sub-nanometer transverse vibrations on a piezo crystal oscillator surface, using time series tunneling current measurements

Vibration characteristics of a piezo crystal oscillator surface are studied using time series measurements of tunneling current. Using this technique, the fluctuations in the tunneling current between a scanning tunneling microscopy tip and the surface of a piezo crystal oscillator are studied, which reveal sub-nanometer vibrations with a sensitivity of 10−2 A°Hz . As the excitation frequency applied to the crystal is varied, the vibrations on the oscillator surface exhibit a resonant response. Furthermore, we detected unconventional sub-nanometer perpendicular vibration modes excited on the crystal surface. These vibrations are in a direction transverse to the surface of the crystal oscillator, whose conventional vibration mode is in a horizontal plane parallel to the surface. We also find near resonance higher harmonics of the perpendicular mode. Thus, the piezo crystal oscillator together with the time series tunneling current measurements offer a convenient simultaneous drive and detection system with a wide operating frequency range.

Detecting Single-Nucleotides by Tunneling Current Measurements at Sub-MHz Temporal Resolution.

Label-free detection of single-nucleotides was performed by fast tunneling current measurements in a polar solvent at 1 MHz sampling rate using SiO2-protected Au nanoprobes. Short current spikes were observed, suggestive of trapping/detrapping of individual nucleotides between the nanoelectrodes. The fall and rise features of the electrical signatures indicated signal retardation by capacitance effects with a time constant of about 10 microseconds. The high temporal resolution revealed current fluctuations, reflecting the molecular conformation degrees of freedom in the electrode gap. The method presented in this work may enable direct characterizations of dynamic changes in single-molecule conformations in an electrode gap in liquid.

Tunable UV-visible absorption of SnS2 layered quantum dots produced by liquid phase exfoliation.

4H-SnS2 layered crystals synthesized by a hydrothermal method were used to obtain via liquid phase exfoliation quantum dots (QDs), consisting of a single layer (SLQDs) or multiple layers (MLQDs). Systematic downshift of the peaks in the Raman spectra of crystals with a decrease in size was observed. The bandgap of layered QDs, estimated by UV-visible absorption spectroscopy and the tunneling current measurements using graphene probes, increases from 2.25 eV to 3.50 eV with decreasing size. 2-4 nm SLQDs, which are transparent in the visible region, show selective absorption and photosensitivity at wavelengths in the ultraviolet region of the spectrum while larger MLQDs (5-90 nm) exhibit a broad band absorption in the visible spectral region and the photoresponse under white light. The results show that the layered quantum dots obtained by liquid phase exfoliation exhibit well-controlled and regulated bandgap absorption in a wide tunable wavelength range. These novel layered quantum dots prepared using an inexpensive method of exfoliation and deposition from solution onto various substrates at room temperature can be used to create highly efficient visible-blind ultraviolet photodetectors and multiple bandgap solar cells.

MIMO $$H_{\infty }$$ H ∞ Integral Control Design for 2D Tunneling Current Measurement System: STM Application

Ultrahigh measurement precision of the tunneling current (which is a quantum mechanical phenomenon and of the order of nano-amperes) is one of the key requirements in many of its applications like in STM (Scanning Tunneling Microscope). STM is used to generate the image of any conducting sample surface with atomic scale resolution, and the image quality highly depends on the precision of the tunneling current. Multi-axis piezoelectric actuator is attached with the tip of STM to move it precisely over the sample surface in vertical as well as in horizontal directions. The desired precision of the tunneling current is adversely affected not only because of the sample surface variations and the measurement noise, but also because of the cross-couplings of multi-axis piezoelectric actuator. In this paper, a dynamic modeling of MIMO (multi-input-multi-output) plant having vertical tunneling current measurement system with horizontal scanning system in the presence of cross-coupling phenomenon is proposed. Then, a robust MIMO \(H_{\infty }\) integral controller is analyzed in order to achieve the desired measurement precision of the tunneling current. Results are compared with SISO (single-input-single-output) \(H_{\infty }\) integral controller and the conventionally used classical PI (proportional-integral) controller. The simulation results achieved with proposed MIMO \(H_{\infty }\) integral controller show improved performances than those obtained with SISO \(H_{\infty }\) integral controller and classical PI controller.

Effect of Tunnel Junctions and Coulomb Blockade on Semiconducting Property of Networks of Single-Wall Carbon Nanotubes

The optical and electrical characterization techniques were employed to demonstrate both experimentally and theoretically (parallel nonlinear resistor model) that the existence of tunnel junctions between semiconducting single-wall carbon nanotubes implies that the Coulomb blockade effect dominates their semiconducting property. Multiwall carbon nanotubes were added to a network of single-wall carbon nanotubes, and Raman spectroscopy and UV–vis–NIR spectroscopy enabled us to observe the semiconducting property of individual single-wall carbon nanotubes in a network with a high concentration of multiwall carbon nanotubes. However, the field effect measurement at 300 K on the network of single-wall carbon nanotubes and the electrical tunnel current measurements over a wide temperature range (4–300 K) at different bias voltages (0.001–10 V) on all of the networks revealed that the networks can be treated as networks of disordered metallic nanoparticles and the semiconducting property of single-wall carbon nanotubes is significantly diminished. Furthermore, we demonstrate that the electron tunneling in networks of carbon nanotubes cannot be fully described by models such as Efros–Shklovskii, Mott variable range hopping, electron cotunneling, or fluctuation-induced tunneling because of the diameter and length distributions in the networks. The effect of tunnel junctions on the performance of such networks for sensing applications and field effect transistors is discussed.

Electrical characterization of organic monolayers/silicon hybrid structures

Hybrid structures consisting of an n-Si substrate covered with a cationic polyethylenimine layer and an anionic dye layer were fabricated. Their electrical properties were characterized using tunnel current measurements by a tungsten probe. The obtained I-V curves were analyzed using a modified diode equation. We found out that adsorption of the cationic polyelectrolyte decreases the barrier height of n-Si/organic monolayer/W structure, while adsorption of the low molecular weight anionic dye increases it. The results demonstrate that along with thermionic mechanism other ones are present which prevail at low voltage. In particular, electron tunneling dominates for a single monolayer on the silicon substrate, while, apparently, the Pool-Frenkel mechanism is typical for two monolayers.

Tunneling current noise in the fractional quantum Hall effect: When the effective charge is not what it appears to be

Fractional quantum Hall quasiparticles are famous for having fractional electric charge. Recent experiments report that the quasiparticle's effective electric charge determined through tunneling current noise measurements can depend on the system parameters such as temperature or bias voltage. Several works proposed to understand this as a signature for edge theory properties changing with energy scale. I consider two of such experiments and show that in one of them the apparent dependence of the electric charge on a system parameter is likely to be an artefact of experimental data analysis. Conversely, in the second experiment the dependence cannot be explained in such a way.

Improving sequencing by tunneling with multiplexing and cross-correlations

Sequencing by tunneling is a next-generation approach to read single-base information using electronic tunneling transverse to the single-stranded DNA (ssDNA) backbone while the latter is translocated through a narrow channel. The original idea considered a single pair of electrodes to read out the current and distinguish the bases [1, 2]. Here, we propose an improvement to the original sequencing by tunneling method, in which $N$ pairs of electrodes are built in series along a synthetic nanochannel. While the ssDNA is forced through the channel using a longitudinal field it passes by each pair of electrodes for long enough time to gather a minimum of $m$ tunneling current measurements, where $m$ is determined by the level of sequencing error desired. Each current time series for each nucleobase is then cross-correlated together, from which the DNA bases can be distinguished. We show using random sampling of data from classical molecular dynamics, that indeed the sequencing error is significantly reduced as the number of pairs of electrodes, $N$, increases. Compared to the sequencing ability of a single pair of electrodes, cross-correlating $N$ pairs of electrodes is exponentially better due to the approximate log-normal nature of the tunneling current probability distributions. We have also used the Fenton-Wilkinson approximation to analytically describe the mean and variance of the cross-correlations that are used to distinguish the DNA bases. The method we suggest is particularly useful when the measurement bandwidth is limited, allowing a smaller electrode gap residence time while still promising to consistently identify the DNA bases correctly.

Fabrications of insulator-protected nanometer-sized electrode gaps

We developed SiO2-coated mechanically controllable break junctions for accurate tunneling current measurements in an ionic solution. By breaking the junction, we created dielectric-protected Au nanoprobes with nanometer separation. We demonstrated that the insulator protection was capable to suppress the ionic contribution to the charge transport through the electrode gap, thereby enabled reliable characterizations of liquid-mediated exponential decay of the tunneling conductance in an electrolyte solution. From this, we found distinct roles of charge points such as molecular dipoles and ion species on the tunneling decay constant, which was attributed to local structures of molecules and ions in the confined space between the sensing electrodes. The device described here would provide improved biomolecular sensing capability of tunneling current sensors.

Experimental setup for non-destructive measurement of tunneling currents in semiconductor devices

A new experimental setup used to perform non-destructive measurement of electrical quantities on semiconductor devices is described in this paper. The particular case of tunneling current measurement in n-type semiconductor–oxide–semiconductor (SOS) capacitors, whose dielectrics play a crucial role in non-volatile memories, has been investigated. When the gates of such devices are polarized with a sufficient bias voltage while the other terminals are grounded, tunnel conduction of electrons through the thin oxide layer is allowed. Typical tunneling current measurements obtained with this advanced setup are presented and compared to the results yielded by older standard experimental protocols. An application to the experimental observation of the temperature dependence of the tunneling current is proposed. Conclusions about the benefits of this kind of electrical measurements are then drawn.

Robust digital control approach for high performance tunneling current measurement system

This paper is devoted to the digital control system design for high performance measurement of tunneling current. A common approach for such applications is to use a conventional Proportional Integral (PI) control. In this paper, a robust digital design method is instead considered, based on combined pole placement with sensitivity function shaping, and allowing for better performance tuning in terms of precision, robustness and disturbance rejection. The resulting control scheme looks like some enhanced PID controller, and is validated over an experimental setup, developed in GIPSA-lab (Grenoble Image Parole Signal Automatique) research center. The corresponding simulation and experimental results show improved performances with respect to those obtained with the more conventional PI control technique.

Diffusion of CO2 on the Rutile TiO2(110) Surface

The diffusion of CO2 molecules on a reduced rutile TiO2(110)-(1×1) surface has been investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM feature associated with a CO2 molecule at an oxygen vacancy (VO) becomes increasingly streaky with increasing temperature, indicating thermally activated CO2 diffusion from the VO site. From temperature-dependent tunneling current measurements, the barrier for diffusion of CO2 from the VO site is estimated to be 3.31 ± 0.23 kcal/mol. The corresponding value from the DFT calculations is 3.80 kcal/mol. In addition, the DFT calculations give a barrier for diffusion of CO2 along Ti rows of only 1.33 kcal/mol.