Table of contents

Transport coefficients are derived which govern the current response of phase coherent conductors in the presence of slowly oscillating external perturbations. We derive the low-frequency admittance if oscillating voltages are applied to the contacts of the sample and discuss the response to an oscillating magnetic flux. Whereas the dc-conductance is determined only by the equilibrium electrostatic potential, the adiabatic transport coefficients discussed here depend on the electrostatic potential that is established in the presence of transport. The symmetry of the transport coefficients under flux reversal is discussed. Novel experiments to find Aharonov-Bohm oscillations in capacitance coeffcients are suggested.

The quantized value of conductance in a quantum wire is discussed near the Mott transition. The way in which the quantized value collapses is also predicted as a function of temperature and length of the wire by taking account of both impurity scattering and mutual Coulomb interactions, i.e. the case of dirty Tomonaga-Luttinger liquid.

Effects of the electron-electron interaction on the transport in quasi one-dimensional systems are studied theoretically. The theory is applied to multi-subband quantum wires, and the temperature dependence of the resistance is predicted.

On the basis of the composite electron picture, we analyze bilayer quantum Hall systems at the Landau level filling factor ν=1/m with m an odd integer. A composite electron is an electron bound to statistics flux. It is shown that various Josephson phenomena occur, but not for the Meissner effect. A physical picture of the commensurate-incommensurate phase transition in the magnetic order is described by analogy with the superconductor Josephson junction. Plasmon excitations are analyzed in each phase. The activation energy anomaly which was recently discovered experimentally is accounted for by the change of the plasmon excitations associated with this phase transition.

A method of full quantum mechanical calculation of energy bands of antidot lattices is developed based on a scattering (S) matrix which describes scattering at a junction of quantum wires. A comparison of calculated energy bands with results of diagonalization of the Hamiltonian in a lattice model shows that an S matrix including all traveling and a few evanescent modes gives a sufficiently accurate result.

We study how the dc transport through a semiconductor quantum dot varies when time-dependent external fields are applied. We find (i) the shift of the Coulomb blockade region due to the interband excitation, (ii) the resonant peak structures of the Coulomb oscillation due to the intraband excitation, and (iii) the electron pumping effect in the quantum dot modulated by the microwave field with frequency lower than the level spacing of the quantum dot.

We demonstrate correlated trapping events in a GaAs/Al_xGa_1-xAs one-dimensional (1D) channel. Due to the Coulomb potential of electrons trapped near the 1D channel, continuous trapping-detrapping of these electron causes fluctuations in the bottle-neck potential, and thus random telegraph signals (RTSs) in the conductance of the channel. RTS appears when the Fermi level crosses the single-electron energy levels of traps. Our results show that when two trap states are at the Fermi level, two RTSs do not occur at the same time, but do interact with each other. This implies that there is electrostatic interaction between the two trapping events.

We have obtained clear experimental evidence for the oscillatory magnetoresistance effect in a two-dimensional electron gas under the influence of a spatially modulated magnetic field, which has been theoretically predicted. The magnetoresistance curve contains an oscillatory component due to the magnetic modulation as well as one due to the potential modulation, and their relative amplitudes can be changed by adjusting the gate bias.

Device-length dependence of the breakdown of the integer quantum Hall effects is studied by using devices with narrow constricted regions of different lengths L at T=1.5 – 4.2 K and Landau-level filling factors of ν= 2, 4 and 6. Samples are fabricated from GaAs/AlGaAs heterostructures. Sharp increase in the diagonal resistance R_x at a critical current disappears, and is replaced by a continuous and gradual increase in short constricted regions of L < 30 µm. The observed L-dependence supports an avalanche-type carrier multiplication mechanism triggered by electron-superheating.

We write a coherent state Feynman path integral for tunneling between Landau levels by developing a continuum representation for the Landau level index and also by showing that a gradient expansion of a sufficiently smooth potential can be written in terms of creation and annihilation operators for the cyclotron motion. We restrict our consideration to only the lowest two Landau levels, n=0 and n=1. We employ a steepest descent method for determining the coupled equations of motion for the path of the guiding center drift coordinate R(τ) and the single Landau index continuous coordinate θ(τ) in a smooth but otherwise arbitrary 2D potential.

The density of states and the conductivity tensor in antidot arrays in magnetic fields are calculated numerically by the self-consistent Born approximation (SCBA). The peak positions of the density of states agree well with the quantization condition for several short-periodic orbits. The behavior of calculated magnetoresistivity agrees with that of experimental data. However, the behavior of the conductivity tensor is very complicated, and it is not explained simply by the periodic orbit expression for the conductivity tensor.

Local current distribution in the presence of nonequilibrium distribution between different edge states in a quantum Hall effect regime is theoretically studied by explicitly taking into account extra charge added into edge states. The extra charges produce a Hall electric field that induces Hall current I _H. Total current is the sum of I _H and the chemical-potential edge current I _CE carried by the extra charges themselves. The calculation shows that I _CE ≪I _H. When the local current distribution is represented by a "response current density j _R," it generally spreads out into the interior region of a conductor, while its amplitude increases as the sample boundaries are approached. When the local current distribution is represented by a "Fermi-surface current density j _F," it is strictly localized at the edge states.

In order to investigate anisotropic transport in various types of lattices, two-dimensional electron gas in the hexagonal and rectangular antidot lattices are measured. Magnetoresistance is isotropic in hexagonal lattice, but anisotropic in the rectangular lattice with circular antidots. The influence of antidot shapes upon anisotropic transport is also investigated in the rectangular lattice. It is found that the anisotropic transport is sensitive to the arrangement and the shape of antidot potential and anomalous peaks are observed in rectangular lattice with ellipsoidal antidots. The origin of the anomalous peaks is discussed. In both lattices quenching of the Hall effect is observed and the positions of commensurate peaks in Hall resistance shift to higher field than that in magnetoresistance. These effects cannot be explained by the pinned orbit model.

The magnetic electron focusing effect (MEFE) is investigated using a device with two extra probes connected by a gate-controlled byway channel. The focusing peak height is inversely proportional to the byway channel resistance. The results show that the focusing peaks are caused by the current through the byway. It is found that, at most, about 9% of the total current flows in the byway when the focusing effect occurs. Moreover, a countercurrent flows in the byway when the focusing effect does not occur.

The frequency dependence of the capacitance between two-dimensional electron systems (2DESs) and the gate has been studied at the quantum Hall (QH) plateaus. The capacitance minima at the QH plateaus are highly dependent on the area of edge channels at high frequencies. The residual contribution of bulk channel to the capacitance has been estimated, and the widths of edge channels have been evaluated. The diagonal conductivity σ_xx of less than 10^-11 S has been obtained accurately by capacitance measurements at the QH plateaus. The relationship between σ_xx and capacitance has been discussed.

To explore commensurate oscillations in antidot lattice, the dependence on the antidot array number (N) perpendicular to the current flow direction is studied. With decreasing N, the peaks of the oscillations become small. Even at N=1, the peaks do not vanish. The relationship between the oscillations and the current flow direction is investigated in rectangular antidot lattices which are rotated at five different angles (θ) with respect to the current flow direction. At θ=0°, the shorter side of the cell is perpendicular to the current flow direction. The main peak magnetic field of the oscillations is determined by the period of the shorter side of the cell. The peak height decreases with increasing θ and vanishes at θ=90°.

Over the past few years, there have been many reports of the breakdown of universality in conductance fluctuations in mesoscopic systems when a high magnetic field is applied, particularly in the case of quantum wires. Normally, conductance fluctuations are described by a single scaling parameter–the coherence length l_φ. The increase in the magnetic coherence length B _c is generally attributed to a decrease in l_φ which is not observed in the amplitude of the fluctuations. Worse, in high mobility material, the fluctuations seem to increase in amplitude, which is also inconsistent with a decrease in l_φ. Here, we argue that (1) the wire behavior is governed by surface scattering, (2) there is a formation of edge states in the high magnetic field, and (3) a breakdown of diffusive transport and transition to quasi-ballistic transport in the edge states in high-mobility material. The transition to high field behavior occurs when the cyclotron orbit at the Fermi surface becomes smaller than the width, so that proper treatment of the amplitude of the fluctuations and of the magnetic correlation length remains in keeping with theoretically expected behavior without loss of universality.

We study magneto-resistance fluctuations in GaAs/AlGaAs, ballistic quantum dots. At low temperatures, and at sufficiently low magnetic fields, the fluctuations obscure any average features in the magneto-resistance. As the magnetic field is increased, such that the cyclotron orbit size becomes much smaller than the dot dimensions, however, a strong decay in their high frequency content is observed. We associate this behaviour with the formation of well defined edge states in the dot, and in order to account for our observations apply a simple model, which considers the flux enclosed by skipping orbits localised at the dot walls.

We have studied the field dependence of the correlation field, B _c, and the amplitude, δg, of the conductance fluctuations in the low-temperature magnetoresistance of split-gate wires. B _c is found to increase as the field increases and δg is also found to show a similar dependence. Two kinds of scattering processes were found to affect phase breaking of electron interference in the quantum wire. These appear at low and high magnetic fields, and are considered to arise from width-dependent and width-independent scattering processes, respectively. In addition, the inelastic scattering length has been obtained from the phase breaking rate, analyzed via the slope of the field dependence of B _c.

We examine the geometry dependence of the conductance fluctuations in a quantum wire, using the recursive Green's function technique, by changing the width of a wire with fixed length. In the experimental situation, the quantum wire is `connected' to `wide' and `long' disordered contact regions which are often ignored in calculations. This more complicated quantum wire geometry lends itself to a numerical approach but would be very difficult to tackle from the viewpoint of the diagrammatic perturbation theory. We can include these disordered contact regions easily in our calculations, and our numerical results suggest that the presence of these contacts tends to reduce the fluctuations. This is a consequence of entering the transport `localization regime', where the sample length is of the order of the localization length, for the longer structure with the disordered contacts.

We have observed an anomalous magnetoresistance (MR) in a mesoscopic aluminum wire near the superconducting transition temperature. The anomaly exists in a region of low magnetic field less than 12.5 G and within the narrow temperature region where the Ginzburg-Landau coherence length exceeds half the distance between the voltage probes. It consists of a "mesa" or "peak"-type MR centered around zero field (negative MR) with zero resistance outside the field region; in other words, the sample undergoes a normal-superconducting-normal transition as the magnetic field increases. Satellite peaks around the mesa MR were also observed for a very narrow range of temperature. We speculate that the sample size and dimensionality play an important role in these phenomena, but the origin is unclear.

We have measured the magnetoresistances of mesoscopic wires fabricated in AlGaAs/GaAs two-dimensional electron gas. The nonlocal magnetoresistances show oscillatory behaviors in the transition regions between successive quantum Hall states. We find that the nonlocal oscillations are well explained by the mixing effect of edge states, whose longitudinal wave vectors can be obtained by using the linear confining potential approximation near the edge. Our analysis implies that the confining field strength should in effect decrease as the magnetic field increases.

Using the level-spacing distribution and the total probability function of the numbers of levels in a given energy interval we analyze the crossover of the level statistics between the delocalized and the localized regimes. By numerically calculating the electron spectra of systems of up to 32³ lattice sites described by the Anderson Hamiltonian it is shown that the distribution P(s) of neighboring spacings is scale-independent at the metal–insulator transition. For large spacings it has a Poisson-like asymptotic form P(s)∝exp (- As/Δ), where A ≈1.9. At the critical point we obtain a linear relationship between the variance of the number of levels <[δn(ε)]² > and their average number <n(ε)> within the interval ε. The constant of proportionality is less than unity due to the repulsion of the levels. Both P(s) and <[δn(ε)]² > are determined by the probability density Q_n(ε) of having exactly n levels in the energy interval ε. The distribution Q_n(ε) at the critical point is found to be size-independent and to obey a Gaussian law near its maximum, where n∼<n >.

We analyze the noise characteristics associated with the fluctuation of the number of electrons in two terminal elements. We find that the local current noise on leads around a point contact varies spatially. The local current noise resulting from short wavelength fluctuations differs from the standard one resulting from long wavelength fluctuations. The excess shot noise near the point contact decreases towards the leads, and the characteristic length is the mean free path of electrons ℓ. At distances longer than ℓ from the contact, the local noise approaches the pure thermal noise given by 4k _BTG _Q with the quantized conductance G _Q. We also find that the tunneling current of resonant tunneling devices (RTDs) is strongly influenced by the existence of the immobile charges captured by the localized traps. By treating the electron capture and emission processes kinematically, we simulate random telegraph noise (RTN).

If a small region of electron gas is separated from its leads by tunnel junctions, the quantization of charge and energy gives rise to sharp peaks in the conductance as a function of electron density, one for each electron added to the isolated region. Because the charge and energy are quantized, we call this an artificial atom. The peaks in conductance occur when the Fermi energy in the leads is degenerate with one of the quantized energy levels of the artificial atom, thus providing a measure of the chemical potential µ_N of the artificial atom with a fixed number N of electrons. Detailed comparison of µ_N at high magnetic field with theory shows that exchange profoundly affects the ground state of electrons confined in this way. In particular, it stabilizes the singlet state to an unexpectedly high field, above which there is a gradual transition to the spin polarized state.

We study the operation of quantum cellular automata (QCA) devices in the presence of stray charge. The operation of linear arrays of QCA cells, called binary wires, relies on Coulombic interaction between the cells, which is affected by the presence of such stray charge. The position of the charge determines whether or not the devices function properly, and it is possible to determine the "forbidden" region near the array in which the presence of stray charge causes device failure. We calculate this forbidden region by directly diagonalizing the Hamiltonian for the system including the stray charge. We find that the QCA binary wire is unaffected by stray charge at a distance greater than the intercellular repeat distance of the wire.

Aligned InGaAs quantum dots as small as 20 nm were naturally formed at multi-atomic step edges by metalorganic chemical vapor deposition growth. In addition, it was found that anisotropic structure of InGaAs along the step edges toward the [110]A direction appears with the increase of growth time of InGaAs. This phenomenon may be useful in the formation of quantum wires.

We have demonstrated a method of forming silicon and cobalt silicide nanoparticles (<10 nm) in CaF₂ grown on a Si(111) substrate using codeposition of Si, Co and CaF₂. It is shown that the size and density of silicon and silicide particles can be controlled by the substrate temperature and the evaporation rate of CaF₂ and Si. At a growth temperature of around 200°C and a flux ratio of Co:Si:CaF₂ of about 1:2:2, an average diameter of 7 nm was obtained.

We report a novel GaAs/InGaAs/GaAs quantum dot structure formed in a tetrahedral-shaped recess (TSR) pattered on a (111)B GaAs substrate with anisotropic chemical etching. The pseudomorphic heterostructure shows two clear photoluminescence (PL) peaks, which are attributed to an In anisotropic incorporation on (111)B compared to (111)A. Cathodoluminescence at a lower energy peak with InGaAs of 2.5 nm shows bright image at the bottom of TSRs, which indicates the local minimum of potential energy is at the bottom of the TSR.

We have fabricated AlGaAs/GaAs quantum dot structures using selective area metalorganic vapor phase epitaxy (MOVPE). First, GaAs pyramidal structures with fourfold symmetric {011} facet sidewalls are formed on SiN_x-masked (001) GaAs with square openings. Once the pyramidal structures were completely formed, no growth occurs on the top or sidewalls of the pyramids. Furthermore, the shape and width of the top area observed using a scanning electron microscope (SEM) and an atomic force microscope (AFM) is shown to be highly uniform. This indicates that self-limited growth occurs. Next, using these uniform pyramids, GaAs quantum dots are overgrown on top of the pyramids under different growth conditions. Sharp photoluminescence (PL) spectra are observed from uniform quantum dots.

GaAs/AlAs quantum dot structures have been fabricated by atomic hydrogen-assisted molecular beam epitaxy (MBE). The substrates used were (111)B GaAs, on which tetrahedral-shaped pits surrounded by three equivalent triangular-shaped {111}A side facets have been produced by an anisotropic wet chemical etching. The deposition of GaAs on (111)A facets were decreased as a consequence of growth selectivity under atomic hydrogen irradiation. A high selectivity in the growth between on (111)B and (111)A has been uniquely utilized for the fabrication of quantum dot structures.

Optical techniques play a significant role in studies of nano-structures. The electronic structures of quantum dots, for example, vary with the geometric sizes in an ensemble, resulting in broadened spectral lines. Recently, different forms of local spectroscopic techniques have been applied to investigate such inhomogeneously broadened emission lines. In this paper we report on three methods for local spectroscopy: cathodo-luminescence, luminescence induced by a scanning tunnel microscope and microphotoluminescence. Each of these techniques is shown to have the capacity to investigate single quantum dots, with linewidths in the range 40–1000 µeV. Besides demonstrating the possibility of imaging and spectroscopically studing individual dots, we also demonstrate the possibility of investigating single impurity atoms, in imaging as well as in emission spectroscopy modes.

A novel conductive transparent (CT) tip has been developed that effectively collects tunneling-electron luminescence with a large solid angle of 2 sr in the direction of the most intensive photoemission. Using this CT tip, the spectrum of tunneling-electron luminescence from cleaved Zn-doped GaAs was obtained at 10 K.

Coherent multiatomic steps with extremely straight edges are naturally formed on vicinal (001) GaAs surfaces during metalorganic vapor phase epitaxial growth. GaAs quantum well wires (QWWs) are formed on these self-organized multiatomic steps. In our previous study, a thin AlGaAs layer was grown on GaAs with multiatomic steps as a lower barrier of QWWs. However, the height and spacing of the steps slightly fluctuate on AlGaAs layer surfaces. Therefore, in this experiment, AlAs layer instead of AlGaAs layer was used as a lower barrier layer to improve the uniformity of the height and spacing of the steps. Atomic force microscopy observations and photoluminescence (PL) measurements at 20 K revealed that the underlying coherent GaAs multiatomic steps were well traced by the AlAs barrier layer rather than the AlGaAs barrier layer. Furthermore, we measured the polarization anisotropy of the PL spectra from the QWWs with AlAs. These results suggest that uniform QWWs are successfully formed using multiatomic steps on vicinal (001) GaAs surfaces.

We demonstrated the lateral-size control of GaAs/AlAs trench-buried quantum wires (QWRs) in the region below 20 nm by using GaAs/AlAs superlattice layers (SLs). Scanning electron microscopy images and photoluminescence properties of the trench-buried QWRs revealed that the trench width can be controlled by varying the number of SLs and reduced to about 13 nm by growing 7 pairs of SLs. The GaAs wires in the trenches have a tendency to grow so as to maintain a constant cross-sectional area, which leads to reduction of the energetic broadening of the quantum sub-levels caused by pattern size fluctuation.

We have performed photoluminescence experiments on modulated barrier In_0.13Ga_0.87As/GaAs quantum wires in normal magnetic fields up to B = 10.5 T. By using high excitation powers we observed emission from several lateral subbands. In comparison to low excitation spectra the emission features are shifted to lower energies due to band gap renormalization. Calculated luminescence spectra are in excellent agreement with the experimental spectra and have been used to study many-particle effects in these wires. We have found clear indications for an enhancement of the renormalization in quantum wires as expected due to the increase of the Coulomb interaction.

We report the formation of GaAs quantum wires using giant step structure formed during molecular beam epitaxial growth of AlGaAs on vicinal (110) GaAs surfaces. Atomic force microscope observation indicates that the steps extend to over several µm and are coherently aligned. The growth of an AlGaAs/GaAs quantum well (QWL) on the giant step structure forms quantum wires (QWRs) along the step edges. Carrier confinement into the QWRs is caused by the increase of well width (well-width modulation) and the decrease of Al composition in the AlGaAs barriers (barrier-compositional modulation), which are confirmed by transmission electron microscope observation. Redshift and strong polarization parallel to the wire direction in the photoluminescence spectra support carrier confinement into the GaAs QWRs.

Epitaxial overgrowth of GaAs over nanometer-sized tungsten patterns is studied experimentally. The relationships between the quality of the overgrowth and geometrical parameters, such as crystallographic orientation, metal width, metal thickness, and grating period, are investigated. The overgrown structures are characterized using a scanning electron microscope at top surfaces and at cleaved edges, and by I-V measurements of Schottky diodes formed between the metal grid and the overgrown GaAs. It is established that it is possible to obtain a high-quality overgrowth layer with a completely planarized growth front within 200 nm of growth.

In order to evaluate the hot electron coherent length from the resonance characteristics of resonant tunneling diodes (RTDs), the dependence of the resonant level width on various broadening mechanisms is investigated. From the dependence of the resonant level width on the well width, it is shown that the individual contributions to resonant level broadening (phase relaxation and well width variation) can be analyzed separately. The resonant level widths were measured experimentally and compared with theoretical results, and the coherent length was evaluated to range from 80 to 120 nm at 4.2 K in GaInAs at hot electron energies of 50 to 100 meV.

We report on electron-beam patterned hydrocarbon residues on silicon dioxide as chemical initiators in high temperature HF vapor etching for the production of nanolithographic masks (∼20 nm feature size). These are of utility in reactive ion etching based pattern transfer to the underlying substrate and, potentially, as dielectric components in nanoscale devices. Metallic cobalt, deposited through the oxide mask, has been used to generate 12–20 nm wide lines of cobalt silicide.

Staircase current-voltage (I-V) characteristics, observed at 77 K in narrow 2DEG channels irradiated by a single line scan of a focused ion beam (FIB), is reported in detail. These staircases are interpreted as evidence of single electron tunneling through a naturally occurring specific Coulomb island in the random potential fluctuations created by FIB damage. Clear comparison is made between the I-V's taken from wide channels and those from narrow channels. Based on orthodox calculations of the I-V characteristics, it is shown that highly asymmetric tunnel junctions are needed to explain our data. This is consistent with the random nature of the potential landscape in the FIB damaged region.

Based on a proposed growth model of Si selective epitaxy on Al₂O₃, the selective epitaxy of Si on both Al₂O₃ and SiO₂/Si substrates was investigated. It is clarified that the incubation time for the Si growth on Al₂O₃ depends on electron dose density and growth conditions. The incubation time increased with increasing electron dose density from 10¹⁵ to 10¹⁸ electrons/cm². Thickness of the selectively grown layer is determined by taking the product of the growth rate and the incubation time difference. Selective Si deposition on SiO₂/Si substrates was studied using an electron-beam irradiation method, because SiO₂ is also the oxide material as Al₂O₃, and the same phenomena could be expected from our proposed mechanism. It was confirmed that the selective growth of Si was possible on the SiO₂/Si substrate modified by electron beam irradiation. However, Si deposition on SiO₂ occurred on the irradiated area, in contrast to the case of Si on Al₂O₃.

Precise magnetoresistance measurements on microstructures of photolithographically patterned PbSe epilayers have been performed in the magnetic field range up to 17 T. Unusual, reproducible magnetoconductance fluctuations have been observed. The fluctuation amplitude is about one order of magnitude greater than that predicted for universal conductance fluctuations. Moreover, the fluctuation frequency spectra correspond to the Aharonov-Bohm effect on electron trajectories up to 50 times smaller than the electron mean free path. This strongly points to the ballistic, not diffusive, origin of the observed phenomenon.

An ultrafine fabrication technique for hot electron interference/diffraction devices was developed. The alignment of two nanostructures by e-beam direct writing before and after crystal growth was reported for the first time. The aligned structure consists of 70 nm pitch grating GaInAs/InP buried structure and 70 nm pitch stripe electrode of Cr/Au.

In our quest to demonstrate electron directional coupling, the coherent tunneling of electrons between two electron waveguides, we have investigated split-gate dual electron waveguide devices. With the structure biased in a "leaky" electron waveguide configuration we have carried out extensive observations of one-dimensional (1D) to two-dimensional (2D) tunneling between a waveguide and a neighboring two-dimensional electron gas. These tunneling spectroscopy experiments have provided the first glimpse of the one-dimensional density of states of a 1D electronic system. We have also carried out experimental observations of 1D to 1D tunneling between two electron waveguides. Demonstrating electron directional coupling will still require the development of a new generation of ultrashallow heterostructures with sharp confining potential barriers.

Eigenenergies and eigenfunctions in coupled electron waveguides with Schottky gates are numerically calculated by solving the Schrödinger equation and the Poisson equation self-consistently in order to estimate the strength of coupling between two electron waveguides. It is concluded that very low middle-gate voltage is required for strong coupling. In order to realize strong coupling, a modified structure which has a middle gate with a small gap is proposed and fabricated using electron beam lithography and the lift-off technique.

We have theoretically investigated the coherent transport in quantum-mechanically coupled quantum wire structures by using the tight-binding Green's function method. Extension of the method to a four-terminal geometry is found to be essential in predicting transmission characteristics of quantum-wire electron wave directional coupling structures. It is found that the interwire transmission probability can be tuned from 0.02 to 0.95 by applying external gate electric fields. Thus, we have confirmed the basic operation of field-effect switching action. Furthermore, we have investigated the effect of a potential gradient along the channel on the transmission coefficients and found that the potential gradient significantly improves the interwire transmission probabilities.

To investigate the possibility of estimating hot electron phase coherence at high temperature, the current-voltage (I-V) characteristic of a triple-barrier resonant tunneling diode (TBRTD) is analyzed taking phase breaking into account. When the central barrier width is larger than 7 nm in GaInAs/InP TBRTD, the width of the peak in the I-V curve is determined predominantly by the phase breaking. Therefore the peak width provides the information of the phase coherent length of the hot electrons. It is shown that the phase coherent length can be estimated up to 100 K using TBRTD.

A new type of quantum field effect directional coupler with finite coupling length is studied with the equivalent network approach. The coupler analyzed is composed of GaAs/Ga_1-xAl_xAs and has finite external potential barriers and longitudinal discontinuities. Although this structure is analogous to that in an optical counterpart, there is no radiation loss. Numerical results show that the effects of the longitudinal interference appear as a short-periodic behavior of the transmission probability and the effects of the beat of two normal modes appear as a long-periodic behavior of the transmission probability.

The confinement potential in etched quantum wire structures is calculated self-consistently as a function of the etch profile in the shallow as well as the deeply etched regime. The etching effect is taken into account by introducing surface charges at the etched semiconductor interfaces. In single GaAs/AlGaAs heterostructures a remarkable decrease of the confinement in the growth direction is found in the deeply etched regime depending on the wire width. The effect is explained by side depletion of the wire and the ionized donor layer. In GaAs/AlGaAs quantum wells, side depletion results in an asymmetric well potential shifting the electronic wave function towards the top interface of the well.

Quasi-one-dimensional quantum electron and hole states are suggested in modulation-doped n-Al_xGa_1-xAs/u-GaAs/u-Al_xGa_1-xAs corrugated double heterojunctions. The interfaces between n-Al_xGa_1-xAs/u-GaAs and u-GaAs/u-Al_xGa_1-xAs are saw-tooth corrugated by bends with period 850 Å and bending angle 90°. The thickness of the sandwiched GaAs layer is 150 Å. Quantum-mechanical simulations are performed for electrons and holes. The electrons are found to be densely packed in quasi-one-dimensional states located in the convex corner of the GaAs layer near the doped layer of n-Al_xGa_1-xAs. The hole states are also quasi-one-dimensional and are located in the convex corner of the GaAs layer near the undoped layer of u-Al_xGa_1-xAs. Consequently, spatial separation of ground level electron and hole quantum states can be achieved.

We propose quantum interferometric spectroscopy (QIS), a novel technique for characterizing heterostructures. Theoretical consideration of tunneling currents of a double-barrier (DB) structure utilizing QIS revealed that individual structural fluctuations of DB structure which cause current differences can be precisely specified. We applied QIS to analyze local current-voltage spectra of DB structures measured by a scanning force microscope using a current-voltage spectra method. We demonstrated that QIS can detect a one-monolayer fluctuation in thickness and fluctuation of less than 0.01 in In mole fraction in the InGaAs well layer on a nanometer-scale.

We have exactly treated the problem of the multichannel electrostatic Aharonov-Bohm effect in a ring structure with a voltage-controlling gate. We have found that, at zero temperature, the conductance oscillation patterns are complex because of contributions from individual channels. However, as the temperature is raised, the oscillations originated from the lower channels quickly vanish, and the overall conductance is almost completely governed by the topmost channel only, thereby making the situation very similar to the single-channel case in characteristics.

The dynamic and nonlinear quantum transport in mesoscopic devices, especially, in a quantum wire and a quantum well laser are studied based on the Wigner function model. In the simulation of a quantum wire, the contacts to the quantum wire are modeled carefully. It is found that in the nonlinear transport regime, the space charge significantly affects the current-voltage characteristics of the quantum wire. Further, the dynamic responses of the quantum wire are studied when the bias voltage is switched abruptly. In the simulation of a quantum well laser, the bipolar quantum transport is discussed by solving the three Liouville equations for electron, heavy-hole and light-hole simultaneously. The bottleneck phenomenon of carrier injection into the multi-quantum wells is discussed.

An extreme example of surface segregation is found in Sn-doped GaAs grown by molecular beam epitaxy (MBE). Abrupt changes in the doping profile are not possible, instead the dopant concentration decreases exponentially towards the wafer surface from the point at which doping was terminated. In this work it is shown that segregation can be suppressed by implanting the Sn from a very-low-energy (50 to 300 eV) ion beam during growth. The effect of ion implantation energy is studied using secondary ion mass spectroscopy (SIMS) to measure the depth profile of the implanted Sn. It is found that the level of incorporation can be increased by up to a factor of eight using a 300 eV ion energy.

We report the observation of multiple negative differential resistance (NDR) in a metal (CoSi₂)/insulator (CaF₂) resonant tunneling hot electron transistor structure. Multiple NDR observed here can be attributed to the modulation of the transmission probability of hot electron waves due to quantum interference in the conduction band of the insulator (CaF₂) collector barrier layer between two metal (CoSi₂) layers. By reducing the influence of the Schottky diode at the CoSi₂/Si interface, relatively clear and low-voltage NDR is observed. It is found, by a simulation including parasitic elements, that the collector resistance and leakage current greatly influence the current voltage characteristics.

We have fabricated single-electron-tunneling transistors using silicon which is a useful material for device applications. The device is composed of thin polycrystalline silicon film patterned by electron-beam lithography and its thermally grown oxidized film. We have observed, in this device, periodic conductance oscillations as a function of gate voltage and nonlinear resistances as a function of drain voltage at 4.2 K. These experimental results are in agreement with the theory of Coulomb blockade. We conclude that the observed behavior results from the charging energy of single-electron tunneling.

The dynamics of the current and the electron number in quantum-dot turnstile devices, in which the heights of the tunneling barriers are modulated by external rf signals, is studied theoretically within the adiabatic approximation. The displacement current is more accurately quantized than the tunneling currents flowing through both the left and the right barriers, since the leakage current is compensated. We find numerically that an electron tunnels through the barrier before its height reaches the minimum value. The tunneling phases of rf signals, at which an electron can actually tunnel, change as the amplitude of rf signals and the barrier height and width are varied. We also discuss the conditions for the current quantization. The dc I-V curve which we obtain shows a plateau at each quantized current nef with width e²/C+ΔE and sharp steps between neighboring plateaus. Whenever the dc current is quantized, the change of the number of electrons in the quantum dot during one half-period of rf signal is an integer.

The effect of potential fluctuations on a semiconductor quantum dot is exhibited. The height of a series of single peaks in the Coulomb oscillation (CLO) becomes irregular in the presence of potential fluctuations. As the source-drain voltage becomes larger than the quantized level spacing, a single peak broadens and turns into a multiple peak. The asymmetry of the barrier heights and the potential fluctuations in the dot are crucial in this regime. The influence of potential fluctuations on wave functions plays an important role in the transport phenomena in submicrometer devices near pinch-off.

Photonic band gaps are calculated for various structures of semiconductor 2-dimensional (2D) photonic crystals for effective control of spontaneous emission. When the structure consists of a semiconductor and air, only circular holes periodically formed on the semiconductor substrate show the absolute 2D photonic gaps. However, it was seen that circular rods whose surface is covered with a dielectric film show a wide TM absolute gap. The latter is a promising structure due to the ease of fabrication and the small nonradiative recombination expected by the passivated semiconductor surface.

A formula describing the intensity of spontaneous emission outside a planar microcavity is derived quantum mechanically. It reflects the quantum mechanical interference of the atom and the emitted photon via multiple reflections in the cavity. The expression derived for the power spectrum reproduces the results of the classical theory by Huang et al.. [Appl. Phys. Lett. 61 (1992) 2961]. The quantum mechanical expression for the electric field amplitude for a single emission event is given together with its evolution equation.

Strong photon localization in mesoscopic-scaled optical waveguide structures has been analyzed through two numerical approaches. The conventional electromagnetic wave analysis and the forced vibrational analysis methods are applied to 2-dimensional mesoscopic-scaled optical waveguide structures. The results indicate that both analysis methods give similar results for the density of states, the photonic band gap and the strong photon localized states according to the introduction of aperiodic structures of the refractive indices in the waveguides. The calculated mode patterns of the midgap states in the photonic band gap indicate that the features of strongly localized states depend on the aperiodicity, and light is localized strongly within a region of several micrometers in the random aperiodic waveguide. We also discuss fabrication techniques for realistic quasi-random aperiodic structures and the smearing out effect by the cladding layer which prevents the strong localization of light.

We consider photon creation through the dynamical Casimir effect in a semiconductor material. Unlike previous studies, we evaluate the number of created photons using a microscopic model in which polarization degree of freedom is included in the theory as a microscopic variable, under realistic situations in which material parameters vary by a finite magnitude within a finite time, in a material which exhibits strong dispersions in the dielectric constant. Our results differ strikingly from previous results.

The effect of inelastic scattering on electron reflection in multiquantum barriers has been examined for the first time by using the damped resonant tunneling model. The electron reflectivity exhibits marked deterioration to values below unity at discrete energies in the virtual barrier. The largest dip in reflectivity is about 15% for an intraband relaxation time of 0.16 ps. It is also shown that this deterioration can be reduced by utilizing a strain-compensated superlattice in the multiquantum barriers.

The energy relaxation phenomena of photoexcited hot carriers in a quasi-one-dimensional (quasi-1D) structure are investigated using the photoluminescence intensity correlation method. It was found for the first time that the anti-correlation dip appears on application of the external electric field, if the kinetic energy of electrons is nearly equal to the GaAs LO phonon energy. Results are discussed in terms of the imbalance between the cooling effect due to the electron-phonon interaction and the heating effect due to the external field. It is suggested that the energy relaxation time and/or the electron mobility is enhanced in quasi-1D structure.

We present experimental and theoretical evidence for Wannier-Stark (WS) oscillations in the DC electric current through reverse-biased highly doped p-i-n GaAs diodes. The intrinsic region in the diode contained seven (GaAs)₅/(AlAs)₂ multi-quantum wells. Carrying out these transport experiments at low temperatures, we found periodic WS oscillations in the second derivative of the Zener current, which is in qualitative agreement with theoretical predictions based on a realistic multiband and multichannel scattering theory. These findings resolve long-standing controversies about the existence of Wannier-Stark levels in the Zener tunneling current.

An interface matrix which incorporates mixing between Γ and X conduction-band valleys is used for calculation of optical intensities in GaAs/AlAs short-period superlattices. A characteristic oscillation of the intensity as a function of the monolayer number for superlattices with lower X states is destroyed easily in the presence of interface fluctuations.

Self-sustained current oscillations have been found in doped GaAs-AlAs superlattices and are investigated experimentally and theoretically. The electric-field distribution in doped superlattices becomes unstable if the carrier density is not sufficiently large to form stable domains. The instability results in self-oscillations of the current, which are due to an oscillatory motion of the domain boundary in the superlattice. A discrete drift model taking into account several regions of negative differential velocity and a large electron density clearly establishes the origin of these oscillations.

Resonant tunneling transmission is studied theoretically for symmetrical rectangular quadruple-barrier structures with deep triple wells. Analytical expressions for the transmission coefficient and the resonance condition are derived for the first time. Two desired resonance levels may be obtained almost independently by selecting appropriate values of well width, which may be favorable for resonant tunneling device fabrication. The transmission coefficient versus electron energy is investigated, and it is shown analytically that the transmission spectrum is Lorentzian in shape near energies of resonance. Wave functions of an electron at the resonance level are examined, and the confining phenomenon is studied.

Using a multiband effective-mass theory, we have calculated the transverse-electric (TE) and transverse-magnetic (TM) gains of a strained quantum-well laser subject to an external electric field. Our results show that when the well is under a moderate tensile strain, the field can switch the polarization of the room-temperature stimulated emission from the TE to the TM mode.

Photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) study is performed on unpassivated and passivated AlGaAs/GaAs near-surface quantum wells (QWs) in order to clarify the mechanism of the recently found large PL intensity increase which was achieved by a novel interface control technique utilizing a SiO₂/Si₃N₄/Si structure including an ultrathin silicon interface control layer (Si ICL). It is shown that the novel Si ICL technique produces a coherent interface structure free of oxides and nitrides, and this removes surface states without introducing additional confined electronic states that interfere with the fundamental e1-hh1 transition of the near-surface QW. The present technique seems to be applicable to passivation of various kinds of compound semiconductor quantum structures.

Magnetic-field dependence of luminescence energy due to recombination of a two-dimensional electron and a photogenerated free hole in one-side-doped n-type GaAs/AlGaAs single quantum wells is calculated. The luminescence energy shows an anomalous shift when the Fermi energy is in an energy gap between adjacent Landau levels. The direction of this energy shift depends on the well width. This is due to well-width dependence of screening strength of two-dimensional electron gas for holes.

We observed temperature dependence of resistance in two-dimensional random and regular small tunnel junction arrays, in the normal and superconducting states. Precursor behaviors of the charge Kosterlitz-Thouless (K-T) transitions were observed for the first time. We also observed the vortex K-T transition in random arrays. The results are consistent with theoretical phase diagrams.

We have confirmed the effects of interference on the superconducting critical current of a gate-fitted superconductor-normal metal-superconductor junction in the clean limit. As the normal metal, the junction uses a two-dimensional electron gas (2DEG), which is well confined in an InAs-inserted-channel InAlAs/InGaAs heterostructure. The superconducting critical current is measured as a function of the gate voltage and shows oscillations as a function of the 2DEG carrier concentration. This oscillation is explained by the Fabry-Pérot interference of the quasi-particles that undergo two normal reflections between two Andreev reflections.

Bound states of quasiparticles in a normal metal placed between two different pair potentials are investigated. The energy levels of the bound states become discrete to satisfy a quantum condition, and are strongly affected by the macroscopic phases of pair potentials in the superconductors. The most interesting feature is formation of bound states on the Fermi level. Our model intuitively and systematically explains qualitative features of the bound states in various situations such as vortex cores and surfaces.

We have studied flux line confinement by regular arrays of submicron holes (triangular and square antidot lattices) in superconducting films. Critical current density j _c and pinning force f _p are very strongly enhanced due to pinning of single or multi-quanta vortices by the antidots. Direct measurements of j _c and f _p for several well-defined antidot diameters D, proves that pinning centers with size considerably larger than the temperature-dependent coherence length ξ(T) are much more efficient than those with D\congξ(T).

We have investigated the charge transport through superconducting single-electron transistors. With a sample which has large charging energy E _c> Δ, where Δ is a superconducting energy gap of the electrodes, we studied the positions of the so-called Josephson-quasiparticle (JQP) peaks. We demonstrate that the JQP cycles coexist with the normal quasiparticle current even at the bias voltages above the Coulomb blockade threshold. Bias voltage conditions for observing these peaks are also classified for various parameter regions.

Basic properties of the quantum dot structure, which is a normal region created inside superconducting matter on a subnanometer scale, are predicted theoretically. The bound states change dramatically as the number of the magnetic flux quanta changes by one. We have clarified the origin of the "nearly midgap states" by intuitive consideration based on the unified picture of interference of the quasiparticle proposed by Kashiwaya.

We have observed the breakdown of charge-vortex duality in small Josephson junction arrays with finite widths. The duality is broken because of the difference in the sign of their image potential at the edges of the arrays. We show that the critical tunneling resistance for superconductor-insulator transition of the Josephson junction arrays depends on the width.

We investigate how coherent Cooper-pair tunneling depends on the competition between Josephson coupling energy E _J and charging energy E _C. Measurements were performed on a circuit consisting of two dc-SQUIDs in series, with a gate capacitively coupled to the central island. As the E _J/E _C ratio decreased, coherent Cooper pairs tended to tunnel incoherently. Measured data show good agreement with theoretical calculations in strong and weak coupling limits. This experiment implies the manifestation of Heisenberg's uncertainty principle in a superconductor.

Very small lead particles have been deposited with random distribution on top of high mobility GaAs/AlGaAs two-dimensional electron gas (2DEG) samples, and the effects on the transport properties of the 2DEG have been studied. Particles more than 5 µm in diameter cause magnetic disorder in the 2DEG in their superconducting state, giving rise to prominent structures in the magnetoresistance. In contrast, spherical particles of 80 to 300 nm diameter do not cause magnetic effects. Electrostatic effects of the nm-sized particles on the transport properties of wide 2DEG structures are very weak, whereas the magnetoresistance properties of a narrow channel were altered significantly in the presence of only a few lead particles smaller than 300 nm. The influence of particles of diameter <300 nm on wide 2DEG samples could be strongly enhanced by melting the particles on the surface.