Single Electron Charging Energy Research Articles

Coulomb blockade from the shell of an InP-InAs core-shell nanowire with a triangular cross section

We report on growth of InP-InAs core-shell nanowires and demonstration of the formation of single quantum structures, which show the Coulomb blockade effect, over entire lengths of the nanowires. The core-shell nanowires are grown by a selective area growth technique via metal-organic vapor phase epitaxy. The as-grown core-shell nanowires are found to be of wurtzite crystals. The InP cores have a hexagonal cross section, while the InAs shells are grown preferentially on specific {11¯00} facets, leading to the formation of the core-shell nanowires with an overall triangular cross section. The grown core-shell nanowires are transferred onto a Si/SiO2 substrate and then contacted by several narrow metal electrodes. Low-temperature transport measurements show the Coulomb-blockade effect. We analyze the measured gate capacitance and single electron charging energy of the devices and demonstrate that a quantum structure which shows the Coulomb blockade effect of a many-electron quantum dot is formed over the full length of a single core-shell nanowire and consists of the entire InAs shell in the nanowire.

Equilibration of quantum Hall edge states by an Ohmic contact

Ohmic contacts are crucial elements of electron optics that have not received a clear theoretical description yet. We propose a model of an Ohmic contact as a piece of metal of the finite capacitance $C$ attached to a quantum Hall edge. It is shown that charged quantum Hall edge states may have weak coupling to neutral excitations in an Ohmic contact. Consequently, despite being a reservoir of neutral excitations, an Ohmic contact is not able to efficiently equilibrate edge states if its temperature is smaller than $\hbar\Omega_c$, where $\Omega_c$ is the inverse RC time of the contact. This energy scale for a floating contact may become as large as the single-electron charging energy $e^2/ C$.

Negative Differential Photoconductance in Gold Nanoparticle Arrays in the Coulomb Blockade Regime

We investigate the photoconductance of gold nanoparticle arrays in the Coulomb blockade regime. Two-dimensional, hexagonal crystals of nanoparticles are produced by self-assembly. The nanoparticles are weakly coupled to their neighbors by a tunneling conductance. At low temperatures, the single electron charging energy of the nanoparticles dominates the conductance properties of the array. The Coulomb blockade of the nanoparticles can be lifted by optical excitation with a laser beam. The optical excitation leads to a localized heating of the arrays, which in turn gives rise to a local change in conductance and a redistribution of the overall electrical potential in the arrays. We introduce a dual-beam optical excitation technique to probe the distribution of the electrical potential in the nanoparticle array. A negative differential photoconductance is the direct consequence of the redistribution of the electrical potential upon lifting of the Coulomb blockade. On the basis of our model, we calculate the optically induced current from the dark current-voltage characteristics of the nanoparticle array. The calculations closely reproduce the experimental observations.

Conduction Bottleneck in Silicon Nanochain Single Electron Transistors Operating at Room Temperature

Single electron transistors are fabricated on single Si nanochains, synthesised by thermal evaporation of SiO solid sources. The nanochains consist of one-dimensional arrays of ∼10 nm Si nanocrystals, separated by SiO2 regions. At 300 K, strong Coulomb staircases are seen in the drain–source current–voltage (Ids–Vds) characteristics, and single-electron oscillations are seen in the drain–source current–gate voltage (Ids–Vgs) characteristics. From 300–20 K, a large increase in the Coulomb blockade region is observed. The characteristics are explained using single-electron Monte Carlo simulation, where an inhomogeneous multiple tunnel junction represents a nanochain. Any reduction in capacitance at a nanocrystal well within the nanochain creates a conduction “bottleneck”, suppressing current at low voltage and improving the Coulomb staircase. The single-electron charging energy at such an island can be very high, ∼20kBT at 300 K.

Child–Langmuir law in the Coulomb blockade regime

The one-dimensional (1D) classical Child–Langmuir (CL) law has been extended to the Coulomb blockade (single to few electrons) regime, including the effect of single-electron charging. It is found that there is a threshold of voltage (Vth) equals to one-half of the single-electron charging energy for electron injection assuming zero barrier at the interface. For voltage in the range of 1<V/Vth<2, there is only one electron inside the gap, and the time-averaged single-electron injected current is equal or higher than the 1D CL current.

Probing the Orbital Levels of Engineered Fullerenic Molecules from a Nonvolatile Memory Cell

ABSTRACTThe Coulomb blockade behavior was observed for both C60-PCBM and C70-PCBM at room temperature utilizing a nonvolatile memory cell fabricated through a liquid-transfer process. Room-temperature and low-temperature (10K) electrical characterizations verified the blockade effect was originated from both molecular energy levels and single electron charging energy. Molecular orbital energy was extracted and shown good agreement with the literature [1].The successful integration and operation of this hybrid structure signified a strong potential for molecule-based electronic device design.

Study of Single Silicon Quantum Dots’ Band Gap and Single-Electron Charging Energies by Room Temperature Scanning Tunneling Microscopy

Scanning tunneling spectroscopy in the shell-filling regime was carried out at room temperature to investigate the size dependence of the band gap and single-electron charging energy of single Si quantum dots (QDs). The results are compared with model calculation. A 12-fold multiple staircase structure was observed for a QD of about 4.3 nm diameter, reflecting the degeneracy of the first energy level, as expected from theoretical calculations. The systematic broadening of the tunneling spectroscopy peaks with decreasing dot diameter is attributed to the reduced barrier height for smaller dot sizes and to the splitting of the first energy level.

Room temperature single electron charging in single silicon nanochains

Single-electron charging effects are observed at room temperature in single Si nanochains. The nanochains, grown by thermal evaporation of SiO solid sources, consist of a series of Si nanocrystals ∼10nm in diameter, separated by SiO2 regions. Multiple step Coulomb staircase current-voltage characteristics are observed at 300K in devices using single, selected, nanochains. The characteristics are investigated using a model where the nanochain forms a multiple tunnel junction. The single-electron charging energy for a nanocrystal within the multiple-tunnel junction is EC=e2∕2Ceff∼0.32eV, ∼12kBT at 300K.

First-principles study of a single-molecule magnetMn12monolayer on the Au(111) surface

The electronic structure of a monolayer of single-molecule magnets ${\mathrm{Mn}}_{12}$ on a Au(111) surface is studied using spin-polarized density-functional theory. The ${\mathrm{Mn}}_{12}$ molecules are oriented such that the magnetic easy axis is normal to the surface, and the terminating ligands in the ${\mathrm{Mn}}_{12}$ are replaced by thiol groups (-SH) where the H atoms are lost upon adsorption onto the surface. This sulfur-terminated ${\mathrm{Mn}}_{12}$ molecule has a total magnetic moment of $18{\ensuremath{\mu}}_{B}$ in the ground state, in contrast to $20{\ensuremath{\mu}}_{B}$ for the standard ${\mathrm{Mn}}_{12}$. The ${\mathrm{Mn}}_{12}$ molecular orbitals broaden due to the interaction of the molecule with the gold surface and the broadening is of the order of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is an order of magnitude less than the single-electron charging energy of the molecule so the molecule is weakly bonded to the surface. Only electrons with majority spin can be transferred from the surface to the sulfur-terminated ${\mathrm{Mn}}_{12}$ since the gold Fermi level is well above the majority lowest unoccupied molecular orbital (LUMO) but below the minority LUMO. The amount of the charge transfer is calculated to be 1.23 electrons from a one-dimensional charge density difference between the sulfur-terminated ${\mathrm{Mn}}_{12}$ on the gold surface and the isolated sulfur-terminated ${\mathrm{Mn}}_{12}$, dominated by the tail in the electronic distribution of the gold surface. A calculation of a level shift upon charging provides 0.28 electrons being transferred. The majority of the charge transfer occurs at the sulfur, carbon, and oxygen atoms close to the surface. The total magnetic moment also changes from $18{\ensuremath{\mu}}_{B}$ to $20{\ensuremath{\mu}}_{B}$, which is due to rearrangements of the magnetic moments on the sulfur atoms and Mn atoms upon adsorption onto the surface. The magnetic anisotropy barrier is computed including spin-orbit interaction self-consistently in density-functional theory. The barrier for the ${\mathrm{Mn}}_{12}$ on the gold surface decreases by $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in comparison to that for an isolated ${\mathrm{Mn}}_{12}$ molecule.

Single electron effects and Bloch oscillations in high-TC superconductive tunneling junctions

We investigated the effects of single electron charging energy in high-TC superconductors. Various phenomena originated from Coulomb blockade were observable in c-axis NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ superconducting tunnel junctions. The current-voltage characteristics show a Coulomb gap for Cooper pair tunneling when the charging energy exceeds the Josephson coupling energy. A crossover from Coulomb blockade of Cooper pair tunneling to a supercurrent is observed when the ratio of Josephson coupling energy to charging energy is increased. We found a regime in which aspects of the Coulomb blockade of tunneling coexist with features typical of Josephson tunneling. Under microwave irradiation of frequency f, the dynamic resistance dV∕dI as a function of the current I showed sharp singularities at I=2nef, n=±1,±2,…. This behavior appears to be very similar to the one predicted by the semiclassical theory of Bloch oscillations.

Coulomb blockade in a self-assembled GaN quantum dot

The authors report on transport phenomena in single self-assembled GaN quantum dots using metallic leads with a nanoscale gap. The nitride device works as a single electron transistor. Measurement of the stability diagram at 12K shows Coulomb blockade regions with a single electron charging energy of about 10meV.

Artificial Atom in Carbon Nanotubes and its Teraherz Response

Single quantum dots (QDs) have been fabricated in an individual single-wall carbon nanotube (SWCNT) simply by depositing source-drain metal on it. The SWCNT between the contacts behaved as a single QD. The single electron transport measurements have been carried out in a dilution refrigerator, which revealed unique artificial atom behavior. Those include clear observation of quantized energy states and their Zeeman splitting in magnetic fields, two or four electron shell structures, and quantum states of an interacting two-electron system. The above characteristics are well modeled by electrons confined in a one-dimensional hard wall potential. The unique features in the SWCNT artificial atom, as compared with semiconductor artificial atoms, are, first, that the single electron charging energy and the quantized level spacing are larger by more than an order, and, second that the simple shell structures are observable even with many electrons in the dot. To demonstrate the larger energy scales for the artificial atom, the teraherz (THz) photon assisted tunneling is presented.

Coulomb blockade anisotropic magnetoresistance and voltage controlled magnetic switching in a ferromagnetic GaMnAs single electron transistor

We observe low-field hysteretic magnetoresistance (MR) in a (Ga,Mn)As single electron transistor which can exceed three orders of magnitude. The sign and size of the MR signal is controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic MR is distinct from previously observed anisotropic MR effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single electron charging energy. Effective kinetic-exchange model calculations in (Ga,Mn)As show chemical potential anisotropies consistent with experiment and ab initio calculations in transition-metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin–orbit coupling. In addition we observe a significant gate voltage dependence of the magnetic switching field which we attribute to carrier density variations in the electric field exposed GaMnAs regions causing a substantial change in the magneto-crystalline anisotropy profile.

Coulomb Blockade Anisotropic Magnetoresistance Effect in a(Ga,Mn)AsSingle-Electron Transistor

We observe low-field hysteretic magnetoresistance in a (Ga,Mn)As single-electron transistor which can exceed 3 orders of magnitude. The sign and size of the magnetoresistance signal are controlled by the gate voltage. Experimental data are interpreted in terms of electrochemical shifts associated with magnetization rotations. This Coulomb blockade anisotropic magnetoresistance is distinct from previously observed anisotropic magnetoresistance effects as it occurs when the anisotropy in a band structure derived parameter is comparable to an independent scale, the single-electron charging energy. Effective kinetic-exchange model calculations in (Ga,Mn)As show chemical potential anisotropies consistent with experiment and ab initio calculations in transition metal systems suggest that this generic effect persists to high temperatures in metal ferromagnets with strong spin-orbit coupling.

Quantum-dot nanodevices with carbon nanotubes

We review our recent work on quantum-dot devices with carbon nanotubes. We conclude that the single-wall carbon nanotube quantum dot is an artificial atom with two- or four-electron shell structures. Zeeman splitting of single particle levels was observed, which is advantageous for the spin based quantum computing device (spin qubit) because the single spin is generated by putting one electron in the shell. Single-electron devices such as single-electron inverter and single-electron exclusive-OR gates have been fabricated, and their performance has been demonstrated at liquid-helium temperature. Despite the expected room-temperature operation from the single-electron charging energy, the operation temperature of our devices was limited to ∼10K because of the low height of the tunnel barrier.

Quantum effects in small-capacitance high temperature superconducting tunneling junctions

We investigated the effects of single electron charging energy in high temperature superconductors. Various phenomena originating from Coulomb blockade were observable in superconducting tunnel junctions. High quality tunneling junctions were fabricated from c-axis oriented NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ thin film multilayers by the pulsed laser deposition method. The current-voltage characteristics (CVCs) show a Coulomb gap for Cooper pair tunneling when the charging energy exceeds the Josephson coupling energy. We found a regime in which the CVCs exhibit sharply defined Coulomb steps due to single electron dynamics and nonlinear tunneling rates. From the obtained Coulomb staircase, the tunneling resistance shows a quantum effect: It is modulated by the tunneling current in the form h∕4e2RT∼[sin(πI∕I0)2∕(πI∕I0)]. We suggest an interpretation involving the quantum resistance h∕e2 and the competition between the charging, Josephson, and thermal energies of the system. Our results give a perspective on a solid-state quantum system with considerable interest for direct application in quantum computing.

Artificial atom solids based on metal nanocrystals: Formation and electrical properties

Metal nanocrystals can behave as “artificial atoms” due to their diameter-dependent single electron charging energy. Organically passivated nanocrystals with narrow size distributions can self-assemble into ordered arrays, offering the possibility of artificial atom solids with unique collective electronic properties, derived from both the size-dependent electronic properties of the individual nanocrystal cores and the inter-nanocrystal electronic coupling mechanisms. We review our recent progress on probing the electronic properties of artificial atom solids via variable temperature charge transport measurements on laterally contacted arrays of metal nanocrystals, together with development of combined synthesis and processing routes to manipulate these properties.

Spin-polarized Tunneling in Ultrasmall Vertical Ferromagnetic Tunnel Junctions

We have developed nanometer-scale vertical ferromagnetic tunnel junctions using a Si-based inorganic electron beam resist process, including barrier layer fabrication using metal evaporation in ozone atmosphere. The current–voltage (I–V) characteristics of Ni/NiO/Co multiple junctions with diameters of 20 nm have been measured in a magnetic field to investigate spin-polarized tunneling in the Coulomb blockade regime. The temperature dependence of the I–V curve indicates that Coulomb blockade phenomena occur at temperatures below 40 K, agreeing with the estimation of the single-electron charging energy from the device geometries. The magnetoresistance is strongly enhanced by magnetization reversal of Ni and Co, and the obtained MR ratio is greater than 100% in the Coulomb blockade regime at 15 K.

Coulomb blockade in a silicon/silicon–germanium two-dimensional electron gas quantum dot

We report the fabrication and electrical characterization of a single electron transistor in a modulation doped silicon/silicon–germanium heterostructure. The quantum dot is fabricated by electron beam lithography and subsequent reactive ion etching. The dot potential and electron density are modified by laterally defined side gates in the plane of the dot. Low temperature measurements show Coulomb blockade with a single electron charging energy of 3.2 meV.

Circuit with small-capacitance high-quality Nb Josephson junctions

We have developed a fabrication process for nanoscale tunnel junctions which includes focused-ion-beam etching from different directions. By applying the process to a Nb/(Al–)Al2O3/Nb trilayer, we have fabricated a Nb single-electron transistor (SET), and characterized the SET at low temperatures, T=0.04–40 K. The superconducting gap energy and the transition temperature of the Nb SET agree with the bulk values, which suggests high quality Nb junctions. The single-electron charging energy of the SET is estimated to be larger than 1 K.