Electron ratchet effect in semiconductor devices and artificial materials with broken centrosymmetry

Studies on nonlinear electron transport in nanometer-sized semiconductor devices with broken centrosymmetry are reviewed. In these devices, an applied alternating (rocking) electric field induces a net flow of electrons in the direction perpendicular to that of the applied field. Such an electron ratchet effect has been observed in a number of differently designed devices, fabricated from two types of semiconductor material systems. The functionality is interpreted with an extended Büttiker–Landauer formula. We show that the devices operate at both cryogenic and room temperatures and at frequencies up to at least 50 GHz. Based on a similar microscopic mechanism, we have also constructed, to the best of our knowledge, the first artificial electronic nanomaterial that operates at room temperature. The promising possibilities for practical applications, such as rectification, microwave detection, second-harmonic generation, etc., are also discussed. Received: 16 January 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Escape time model for the current self-oscillation frequencies. in weakly coupled semiconductor superlattices

A semiclassical model was developed to predict the frequencies of current self-oscillations in weakly coupled semiconductor superlattices (SLs). The calculated frequency is derived from the classical round trip time in one well and the tunneling probability through the barrier, using the well and barrier width, effective masses and band offsets as well as the resulting energies of the sub- and minibands as input parameters. For all SLs, the measured frequency dependence on the sample parameters can be well described by our model. For weakly (strongly) coupled SLs, the calculated frequencies are somewhat above (below) the observed ones. The changeover from one behavior to the other occurs for SLs with miniband widths of a few meV. Received: 2 August 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Flux-dependent anti-crossing of resonances in parallel non-coupled double quantum dots

We present novel resonant phenomena through parallel non-coupled double quantum dots (QDs) embedded in each arm of an Aharonov-Bohm (AB) ring with magnetic flux passing through its center. The electron transmission through this AB ring with each QD formed by two short-range potential barriers is calculated using a scattering matrix at each junction and a transfer matrix in each arm. We show that as the magnetic flux modulates, a distortion of the grid-like square transmission occurs and an anti-crossing of the resonances appears. Hence, the modulation of magnetic flux in this system can have an equivalent effect to the control of inter-dot coupling between the two QDs.     

Single-electron quantum dots in silicon MOS structures

We present experimental results for two types of quantum dots, which are embedded within a silicon metal-oxide-semiconductor structure. Evidence is found for single-electron charging at low temperature, and for an asymmetric shape of the dot. First results of simulations of these dots are presented. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Charging ratchets: Coulomb blockade and rectification

Discrete ratchets describe directed motion of a ‘reaction coordinate’ through a cycle of states in response to some varying external parameter. Such systems, in the simple, history-independent case, are described by a Markov process which in turn leads to a master equation with a transition matrix. Thus the ratchet property is reduced to a characteristic of the parameter-dependent symmetry of matrices. In the standard model of tunneling through a set of quantum dots in the Coulomb-blockade regime, a master equation is also used to describe the evolution through states in ‘dot-occupancy space’, leading to transport of electrons from a source to a drain. The symmetry of the transition matrix in this case is also a function of external parameters, notably the applied gate voltages and source–drain voltage, as well as depending on the configuration of dots and their tunnel couplings. We show that rectification and other ratchet behavior is a common feature of tunneling transport in the Coulomb-blockade regime. We also show that specific arrangements of dots and their tunnel couplings can be designed to enhance the ratchet effect. Finally, we show that the strong rectification of Coulomb-blockaded systems results from the reduction in phase space accessible to the system as it traverses the states in the reaction cycle. Received: 16 October 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Quantum corrections to the conductance of a square quantum dot with soft confinement

We study the conductance of a square quantum dot, modeling the potential with a self-consistent Thomas-Fermi approximation. The resulting potential is characterized by level statistics indicative of mixed chaotic and regular electron dynamics within the dot in spite of the regular geometry of the gates defining the dot. We calculate numerically, for the case of a quantum dot with soft confinement, the weak localization (WL) correction. We demonstrate that this confining potential may generate either Lorentzian or linear lineshapes depending on the number of modes in the leads. Finally, we present experimental WL data for a lithographically square dot and compare the results with numerical calculations. We analyze the experimental results and numerical simulations in terms of semiclassical and random matrix theory (RMT) predictions and discuss their limitations as far as real experimental structures are concerned. Our results indicate that direct application of the above predictions to distinguish between chaotic and regular dynamics in a particular cavity can not always lead to reliable conclusions as the shape and magnitude of the WL correction can be strongly sensitive to the geometry-specific, non-universal features of the system. Received 13 May 1998     

Error diagrams and temporal correlations in a fracture model with characteristic and power-law distributed avalanches

The field-induced carrier redistribution between the subbands of a semiconductor superlattice is treated using the density matrix approach. The unit cell of the superlattice consists of one quantum well with three occupied subbands. Carrier scattering on polar-optical phonons is described within the microscopic bulk phonon model. At the tunneling resonance, an intrinsic population inversion is observed. The temperature dependence of the population inversion is determined. Received 25 October 2002 / Received in final form 27 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: kl@pdi-berlin.de     

Carrier recombination and high-barrier Schottky diodes on silicon

Small-area high-barrier Schottky diodes have a very high dynamic resistance. Consequently, special care is needed when measuring the current-voltage characteristic of such diodes. The reported observation of carrier recombination in the depletion layer of high-barrier IrSi/Si Schottky diodes at room temperature is shown to be due to instrumental loading of the diodes. Careful measurements show that carrier recombination is observed only below 200 K and is dependent on the dimension of the diode.     

Multishell conduction in multiwalled carbon nanotubes

The full electronic complexity of multiwalled carbon nanotubes may be explored by sequentially removing individual carbon shells. This technique is employed to directly measure the number of shells contributing to conduction at room temperature, as well as the contribution of each shell to the overall conductance. By exploring the gate dependence of the conductance, the random alternation between semiconducting and metallic shells can also be observed. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

High Field Electrical Conduction in Pre-Formed A1-ZnS-AI Thin Films in Metal-Insulator-Metal Devices

The high field electrical conduction mechanism for the widely used ZnS thin films in the microelectronic industry is investigated. Experimental data on the dc conduction as a function of the applied bias for the A1-ZnS-A1 devices is carefully compared with the theoretical equations given by Schottky and Poole-Freukel. The resu/ts yield the value of the coefficient of the barrier lowering compatible with the Schottky theory rather than the Poole-Frenkel theory, which are also in agreement with the results reported earlier by Maekawa [Phys. Rev. Lett. 24 （1970） 1175]     

Persistent current and transmission probability in the Aharonov-Bohm ring with an embedded quantum dot

Persistent current and transmission probability in the Aharonov-Bohm (AB) ring with an embedded quantum dot (QD) are studied using the technique of the scattering matrix. For the first time, we find that the persistent current can arise in the absence of magnetic flux in the ring with an embedded QD. The persistent current and the transmission probability are sensitive to the lead-ring coupling and the short-range potential barrier. It is shown that increasing the lead-ring coupling or the short-range potential barrier causes the suppression of the persistent current and the increasing resonance width of the transmission probability. The effect of the potential barrier on the number of the transmission peaks is also investigated. The dependence of the persistent current and the transmission probability on the magnetic flux exhibits a periodic property with period of the flux quantum.     

Quantum waveguide theory of a fractal structure

The electronic transport properties of fractal quantum waveguide networks in the presence of a magnetic field are studied. A Generalized Eigen-function Method (GEM) is used to calculate the transmission and reflection coefficients of the studied systems unto the fourth generation Sierpinski fractal network with node number N=123$N=123$. The relationship among the transmission coefficient T, magnetic flux Φ and wave vector k is investigated in detail. The numerical results are shown by the three-dimensional plots and contour maps. Some resonant-transmission features and the symmetry of the transmission coefficient T to flux Φ are observed and discussed, and compared with the results of the tight-binding model.     

Charge transfer via interfaces, especially of nanoscale materials

Received: 27 March 1998     

Crystalline Si thin-film solar cells: a review

The present review summarizes the results of research efforts in the field of crystalline silicon thin-film solar cells on foreign substrates. The large number of competing approaches can be broadly classified according to the grain size of the crystalline Si films and the doping of the crystalline absorber. Currently, solar cells based on microcrystalline Si films on glass with an intrinsic or moderately doped absorber film achieve efficiencies around 10%, whereas thin-film cells fabricated from large-grained polycrystalline Si on high-temperature-resistant substrates have efficiencies in the range of 15%. The paper discusses the limitations of various approaches and describes recent developments in the area of thin, monocrystalline Si films that may open the way towards 20% efficient thin-film Si solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Correlations of conductance peaks and transmission phases in deformed quantum dots

We investigate the Coulomb blockade resonances and the phase of the transmission amplitude of a deformed ballistic quantum dot weakly coupled to leads. We show that preferred single-particle levels exist which stay close to the Fermi energy for a wide range of values of the gate voltage. These states give rise to sequences of Coulomb blockade resonances with correlated peak heights and transmission phases. The correlation of the peak heights becomes stronger with increasing temperature. The phase of the transmission amplitude shows lapses by between the resonances. Implications for recent experiments on ballistic quantum dots are discussed. Received 17 July 1998