Coulomb Blockade Energy Research Articles

Coulomb blockade: Toward charge control of self-assembled GaN quantum dots at room temperature

We present capacitance–voltage [C(V)] measurements of self-assembled wurtzite-GaN quantum dots (QDs). The QDs are embedded in a charge-tunable diode structure and were grown by molecular beam epitaxy in the Stranski–Krastanov growth method. The internal electric fields present in GaN and its alloys together with its wide bandgap make this material system an ideal candidate for high-temperature quantum applications. Charges and the internal electric fields influence the energy spacing in the QDs. We correlate photoluminescence measurements with C(V) measurements and show single-electron charging of the QDs and a Coulomb blockade energy of around 60 meV at room temperature. This finding demonstrates the possibility of quantum applications at room temperature.

Influence of electric polarization on Coulomb blockade in a super-paraelectric clusters assembly

We study electron transport in an assembly of epitaxial Cr2O3 nanoparticles embedded in a MgO tunnel barrier: an unusual variation in the Coulomb blockade charging energy is observed as a function of the size of the clusters. In striking contrast to the expected increase in charging energy when decreasing the cluster size, an almost constant behavior is observed. We argue here that the spontaneous superparaelectric moment carried by the cluster core is the origin of this unusual behavior since it drives the dielectric constant in this cluster assembly. This phenomenon could be exploited to fabricate devices with single valued Coulomb blockade energy despite a statistical dispersion in the cluster size.

An analytical approach to model capacitance and resistance of capped carbon nanotube single electron transistor

The single electron transistor (SET) as a nanoscale transistor operates according to the electron tunneling via two tunnel junctions. Since selecting a suitable island material plays a key role in electron transfer through the tunnel junctions, in this research capped Carbon NanoTube (CNT) is utilized for the SET island which produces the quantum capacitance (CQ). Its low value decreases the total capacitance (CT). Subsequently the coulomb blockade (CB) energy and the critical temperature are reduced. Moreover the resistance of the capped CNT as a two dimensional material is very low thus its effect on the total resistance can be neglected. The result of an investigation on the capped CNT SET tunnel junction shows that the tunneling time of electron into or out of island decreases therefore the operation speed of capped CNT SET increases. Furthermore both the resistance and the quantum capacitance are modeled and analyzed. Comparison studies of proposed models indicate that the capped CNT length and applied bias voltage play effective roles on the resistance. Additionally the capped CNT SET is simulated and the capped CNT SET stability diagrams are analyzed. The simulation result shows that the lower capped CNT length exhibits a smaller coulomb diamond regions. The lower CB range leads to faster capped CNT SET and a better reliability can be achieved.

Single-electron transistor with an island formed by several dopant phosphorus atoms

We present the results of an experimental study of electron transport through individual phosphorus dopants implanted into a silicon crystal. We developed an original technique for single-electron transistor fabrication from silicon-on-insulator material with an island formed by single phosphorus atoms. The proposed method is based on well-known CMOS compatible technological processes that are standard in semiconductor electronics and may be used in most research groups. The large Coulomb blockade energy value of the investigated single-electron transistor (∼20 meV) allows one to observe single-electron effects in a wide temperature range up to 77 K. We measured and analyzed stability diagrams of fabricated experimental structures. We demonstrated a single-electron transistor with controllable electron transport through two to three phosphorus dopants only.

Tunneling mechanism and contact mechanics of colloidal nanoparticle assemblies

Nanoparticle assemblies with thiol-terminated alkyl chains are studied by conducting atomic force microscopy (c-AFM) regarding their use as strain gauges for touch-sensitive panels. Current–force spectroscopy is used as a characterization tool complementary to the macroscopic setup since it allows a bias to be applied to a limited number of junctions, overcoming the Coulomb blockade energy and focusing on the contact electromechanics and the transport mechanism across the ligand. First, transition voltage spectroscopy is applied with varying force to target the underlying tunneling mechanism by observing whether the transition between the ohmic and exponential current–voltage behavior is force-dependent. Secondly, current–force spectroscopy in the ohmic range below the transition voltage is performed. The current–force behavior of the AFM probe in contact with a nanoparticle multilayer is associated with the spread of force and current within the nanoparticle lattice and at the level of adjacent particles by detailed contact mechanics treatment. The result is twofold: concerning the architecture of sensors, this work is a sample case of contact electromechanics at scales ranging from the device scale down to the individual ligand molecule. Regarding transport across the molecule, the vacuum tunneling mechanism is favored over the conduction by coherent molecular states, which is a decision-making aid for the choice of ligand in applications.

A sensitive charge scanning probe based on silicon single electron transistor**Project supported by the Instrument Developing Project of the Chinese Academy of Sciences (No. YZ201152), the National Natural Science Foundation of China (No. 11403084), the Fundamental Research Funds for Central Universities (Nos. JUSRP51510, JUDCF12032), and the Graduate Student Innovation Program for Universities of Jiangsu Province (No. CXLX12_0724).

Single electron transistors (SETs) are known to be extremely sensitive electrometers owing to their high charge sensitivity. In this work, we report the design, fabrication, and characterization of a silicon-on-insulator-based SET scanning probe. The fabricated SET is located about 10 μm away from the probe tip. The SET with a quantum dot of about 70 nm in diameter exhibits an obvious Coulomb blockade effect measured at 4.1 K. The Coulomb blockade energy is about 18 meV, and the charge sensitivity is in the order of 10−5−10−3 e/Hz1/2. This SET scanning probe can be used to map charge distribution and sense dynamic charge fluctuation in nanodevices or circuits under test, realizing high sensitivity and high spatial resolution charge detection.

Calculating electronic tunnel currents in networks of disordered irregularly shaped nanoparticles by mapping networks to arrays of parallel nonlinear resistors

We have shown both theoretically and experimentally that tunnel currents in networks of disordered irregularly shaped nanoparticles (NPs) can be calculated by considering the networks as arrays of parallel nonlinear resistors. Each resistor is described by a one-dimensional or a two-dimensional array of equal size nanoparticles that the tunnel junction gaps between nanoparticles in each resistor is assumed to be equal. The number of tunnel junctions between two contact electrodes and the tunnel junction gaps between nanoparticles are found to be functions of Coulomb blockade energies. In addition, the tunnel barriers between nanoparticles were considered to be tilted at high voltages. Furthermore, the role of thermal expansion coefficient of the tunnel junction gaps on the tunnel current is taken into account. The model calculations fit very well to the experimental data of a network of disordered gold nanoparticles, a forest of multi-wall carbon nanotubes, and a network of few-layer graphene nanoplates over a wide temperature range (5-300 K) at low and high DC bias voltages (0.001 mV–50 V). Our investigations indicate, although electron cotunneling in networks of disordered irregularly shaped NPs may occur, non-Arrhenius behavior at low temperatures cannot be described by the cotunneling model due to size distribution in the networks and irregular shape of nanoparticles. Non-Arrhenius behavior of the samples at zero bias voltage limit was attributed to the disorder in the samples. Unlike the electron cotunneling model, we found that the crossover from Arrhenius to non-Arrhenius behavior occurs at two temperatures, one at a high temperature and the other at a low temperature.

Transport behaviors and mechanisms in cuspidal blockade region for silicon single-hole transistor

The Si single-hole transistor displays the anomalously-extended cuspidal blockade region, which is elongated toward the 45°-tilted direction normal to gate vs. drain bias voltage regions in the Coulomb blockade diagram. This is attributed to the formation of an ultra small Si quantum dot (QD) into the gate-all-around (GAA) stack. Namely, the large one-electron-addition energy (= 447 meV) from the 2-nm-size Si QD enables the clear Coulomb-blockade events at room temperature, and the large voltage gain from the GAA stack allows the cuspidal extension of the blockade region through the renormalization of Coulomb-blockade energies at the adjacent bias points near the initial Coulomb-blockade state.

Investigation of the electrical properties of metallic nanoclusters

Palladium nanoclusters were prepared by magnetron sputtering and inert gas condensation technique and deposited between two gold electrodes. We have investigated the current-voltage characteristics at temperatures between 45 and 300 K. The conduction-voltage profile allows calculating the number of potential barriers, n electrons have to cross while conducting through a percolation path. n is found to increase with decreasing temperature. The observed temperature dependence of the conductance is Arrhenius-like above 140 K, with activation energy much smaller than the calculated Coulomb blockade energy. It suggests that some of the nanoclusters are fused together. At temperatures below 140 K the conductance-temperature data was fitted with Efros-Shklovskii model for conduction where variable range hopping is dominating the electron transport mechanism. The presence of variable range hopping may interpret the increase of n with decreasing temperature.

Spintronic mechanics of a magnetic nanoshuttle

We investigate theoretically the prospects for using a magnetic nanoelectromechanical single-electron tunneling (NEM-SET) device as an electronic spin filter. We find that strong magnetic exchange forces on the net spin of the mobile central dot of the NEM-SET structure lead to spin-dependent mechanical displacements ("spin polarons"), which give rise to vastly different tunnelling probabilities for electrons of different spin. The resulting spin polarization of the current can be controlled by bias and gate voltages and be very close to 100% at voltages and temperatures below a characteristic correlation energy set by the sum of the polaronic and Coulomb blockade energies.

Electron transport in discontinuous gold films and the effect of Coulomb blockade and percolation

Understanding the electron transport in disordered assemblies of weakly coupled nano-sized metal clusters is important for many applications. Here, we investigate experimentally and theoretically the electron transport properties of metal cluster assemblies in the form of discontinuous gold films. Discontinuous films of different average island size are produced by sputter deposition, and the resistance and the non-linear current-voltage (I-V) characteristics of the films are measured as a function of temperature. To interpret the experimental electron transport data, a conduction percolation model is employed where broad probability distributions for both the tunnel junction gaps and the Coulomb blockade energies are used. Excellent agreement between experimental data and model calculations is found. In particular, the non-Arrhenius resistive behavior, the I-V power-law behavior, and the I-V characteristics at large bias voltage are all shown to be due to a conduction percolation mechanism governing disordered networks of nano-sized metal islands connected by tunnel junctions.

Nonmechanical Conductance Switching in a Molecular Tunnel Junction.

We present a molecular junction composed of a donor (polyacetylene strands) and an acceptor (malononitrile) connected together via a benzene ring and coupled weakly to source and drain electrodes on each side, for which a gate electrode induces intramolecular charge transfer, switching reversibly the character of conductance. Using a new brand of density functional theory, for which orbital energies are similar to the quasiparticle energies, we show that the junction displays a single, gate-tunable differential conductance channel in a wide energy range. The gate field must align parallel to the displacement vector between donors and acceptor to affect their potential difference; for strong enough fields, spontaneous intramolecular electron transfer occurs. This event radically affects conductance, reversing the charge of carriers, enabling a spin-polarized current channel. We discuss the physical principles controlling the operation of the junction and find interplay of quantum interference, charging, Coulomb blockade, and electron-hole binding energy effects. We expect that this switching behavior is a generic property for similar donor-acceptor systems of sufficient stability.

Effect of Coulomb blockade, gold resistance, and thermal expansion on the electrical resistance of ultrathin gold films

Controlling the electrical resistance of granular thin films is of great importance for many applications, yet a full understanding of electron transport in such films remains a major challenge. We have studied experimentally and by model calculations the temperature dependence of the electrical resistance of ultrathin gold films at temperatures between 2 K and 300 K. Using sputter deposition, the film morphology was varied from a discontinuous film of weakly coupled meandering islands to a continuous film of strongly coupled coalesced islands. In the weak-coupling regime, we compare the regular island array model, the cotunneling model, and the conduction percolation model with our experimental data. We show that the tunnel barriers and the Coulomb blockade energies are important at low temperatures and that the thermal expansion of the substrate and the island resistance affect the resistance at high temperatures. At low temperatures our experimental data show evidence for a transition from electron cotunneling to sequential tunneling but the data can also be interpreted in terms of conduction percolation. The resistivity and temperature coefficient of resistance of the meandering gold islands are found to resemble those of gold nanowires. We derive a simple expression for the temperature at which the resistance changes from non-metal-like behavior into metal-like behavior. In the case of strong island coupling, the total resistance is solely determined by the Ohmic island resistance.

Quantum-critical region of the disorder-driven superconductor–insulator transition

We investigate low temperature transport properties of thin TiN superconducting films, differing by the degree of disorder. At zero magnetic field we find an extremely sharp separation between the superconducting- and insulating phases, indicating a direct superconductor–insulator transition without an intermediate metallic phase. We show that in the critical region of the transition a peculiar highly inhomogeneous insulating state with superconducting correlations forms. The insulating films exhibit thermally activated conductivity and huge positive magnetoresistance at low magnetic fields. A sharp depinning transition at some voltage V T is observed in the I– V curves at very low temperatures. We propose a percolation type of depinning with the threshold voltage determined by the Coulomb blockade energy for the Cooper pairs between neighboring self-induced superconducting islands, with V T being the total voltage along the first conduction path. The observed hysteretic behavior of the threshold and steps on the d I/d V vs. V curves support this percolation picture of the depinning transition.

Electron- and hole-related electrical activity of InAs/GaAs quantum dots

The electron- and hole-related electrical activity of the InAs/GaAs quantum dot system has been demonstrated with a use of the high-resolution Laplace and conventional DLTS methods combined with below GaAs bandgap illumination. Without the illumination, the DLTS signal refers to the emission process of electrons bound by the dot confining potential in the conduction band. In this experimental mode, single- and double-electron states of the dot could be observed. The resolution of the method allows both charge states of the dot to be observed in the same Laplace DLTS spectrum and thus the relative occupancy factor for both charge states can be determined. This gives the value of the Coulomb blockade energy (∼11 meV). When prior to the DLTS signal detection a short below-bandgap illumination resonant with the dot excitonic energy is applied, a non-equilibrium hole occupancy in the dot is created. This allows the same dots to be studied in order to establish the hole-referred electrical dot activity governed by the confining potential in the valence band.

Tunnel magnetoresistance and Coulomb blockade in a planar assembly of cobalt nanoclusters embedded in TiO2

Magnetic and magnetotransport properties of Co nanoclusters embedded in a TiO2 matrix grown by pulsed laser deposition have been studied. The results show a tunneling magnetoresistance associated with spin-polarized multiple quantum tunneling through the Coulomb energy barrier (Coulomb blockade effect). A shift between the fields at the magnetoresistance maxima and the coercive fields in the Coulomb blockade regime is observed. This shift decreases with increasing temperature and disappears when the thermal energy becomes stronger than the Coulomb blockade energy.

First-Principles Theory of Quantum Well Resonance in Double Barrier Magnetic Tunnel Junctions

Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.

The excitation spectrum of quantum antidots

Source-drain bias measurements of transport across quantum antidots reveal a ladder of excited states analogous to the excitation spectrum of quantum dots. The antidot excitation spectrum provides an unambiguous method of determining the Coulomb-blockade charging energy and the energy difference between antidot single-particle states. The energy-level spacings and the presence of strong Kondo resonances in this regime cannot be explained within a non-interacting model.

Role of quantum capacitance in coupled low-dimensional electron systems

We have investigated the charging behavior of a layer of self-assembled InAs quantum dots placed in close vicinity to a two-dimensional electron gas (2DEG). As the gate bias is changed, the number of electrons in each system is altered simultaneously. Based on the quantum capacitance of the involved layers we develop a general model to determine the charging state of coupled low-dimensional systems from capacitance-voltage (CV) spectroscopy. The model is then applied to the special case of a layer of self-assembled quantum dots coupled to a 2DEG. As a complementary method, we have employed Hall voltage measurements. We find that the measurement of the two-dimensional carrier density through lateral transport provides a direct insight into the vertical charging process of the quantum dot system. Six individual charging peaks related to the occupation of the $s$ and $p$ shells of the dots can be resolved. In agreement with results from CV spectroscopy, Coulomb blockade and quantization energies can be extracted. Moreover, the Hall measurement offers a higher peak-to-valley ratio and a better estimate for the number of simultaneously charged dots than the capacitance data.

Coulomb gap, Coulomb blockade, and dynamic activation energy in frustrated single-electron arrays

We have used modern supercomputer facilities to carry out extensive numerical simulations of statistical properties of 1D and 2D arrays of single-electron islands with random background charges, in the limit of small island self-capacitance. In particular, the spectrum of single-electron addition energies shows a clear Coulomb gap that, in 2D arrays, obeys the Efros-Shklovskii theory modified for the specific electron-electron interaction law. The Coulomb blockade threshold voltage statistics for 1D arrays is very broad, with r.m.s. width $\delta V_t$ growing as $<V_t > \propto N^{1/2}$ with the array size $N$. On the contrary, in square 2D arrays of large size the distribution around $<V_t> \propto N$ becomes relatively narrow $(\delta V_t/< V_t> \propto 1/N)$, and the dc $I$-$V$ curves are virtually universal. At low voltages, the slope $G_0(T)$ of $I$-$V$ curves obeys the Arrhenius law. The corresponding activation energy $U_0$ grows only slowly with $N$ and is considerably lower than the formally calculated "lowest pass" energy $E_{max}$ of the potential profile, thus indicating the profile "softness".