We have fabricated metal constrictions having diameters as small as 3 nm. This is narrow enough that effects due to single defects within the constriction region are visible as changes in the resistance. We will discuss the voltage and temperature dependence of the individual defect motion that can lead to electromigration in these samples. We will also present observations of magnetic defects and the presence of time-independent conductance fluctuations in these ballistic devices.

Quantum dots represent a new class of electronic structures where the electronic configuration and degree of confinement or coupling can be tuned at will. These artificial mesoscopic structures mimic many atomic effects with a tremendous degree of flexibility. We have studied the transition from isolated to coupled quantum dots in quantum-dot and lateral-surface-superlattice structures. Our maguetocajacitance measurements probe three distinct regimes: a superlattice regime; a tight-bindin.g regime; and an isolated-quantum-dots regime. In the presence of an external magnetic field we find that the coupling of the magnetic flux with the periodic potential depends on the strength of the potential modulation. In strongly-coupled quantum wires and quantum dots, the commensurability between the superlattice period and the cyclotron orbit is reflected in a modification of the envelope of the magnetocapacitance oscillations. In weakly-coupled quantum dots, a new structure is observed. Aperiodic oscillations in the density of states are related to fractal-like behavior in the energy dispersion relation first predicted by Ilofstadter. In completely isolated quantum wires or quantum dots, the canonical magnetocapacitance oscillations are recovered with a Zeeman bifurcation of the states produced by the interplay of magnetic and spatial quantization. We will present our experimental work in light of theoretical models and discuss the fundamental physical phenomena that dominate the the.rmodynamic properties of electrons in quantum dots.

In the mesoscopic regime, where the characteristic length of a sample is comparable to the phase coherence length of electrons, quantum interference leading to a magnetoconductance periodic in the magnetic field coupled through a unit cell of a two-dimensional superlattice has been hypothesized for many years. We report here on the observation of such periodic effects in quasi-two-dimensional semiconductor structures with an additional two-dimensional periodic superlattice potential applied. In LSSLs prepared on MODFET material, magnetoconductance measurements made at 4.2K show Aharonov-Bohm type periodic oscillations with We periodicity in the flux coupled through each cell of the superlattice. In LSSLs on MESFET material, we find that the conductance is also periodic in the magnetic field, but with replicas of the negative magneto-resistance (signature of weak localization), with separations in magnetic field corresponding to integer changes in the flux per superlattice cell. In addition, the presence of a significant source-drain potential shifts the resonances in magnetic field. These effects are observed for relatively low magnetic fields, B<1 Tesla. Furthermore, conductance fluctuations are observed for samples whose dimensions are large compared to the inelastic mean free path.

We examine transport through several quantum electron waveguide geometries. Reflection and mode-mixing in transmission through bends in quantum waveguides are calculated and the impact on interconnections between devices discussed. We calculate the current-voltage relationship for a constriction in a quantum waveguide for applied voltages beyond linear-response regime. Strong resonance effects in waveguide cavities are found to persist even when cavity geometry is non-abrupt. We demonstrate the existence of current vortices in resonant cavities.

Ultra-submicron GaAs MESFETs have been fabricated with gate lengths ranging from 25 nm to 80 nm, using an electron-beam lithography process. The MESFETs were fabricated on vapor-phase grown wafers. The HEMT devices were fabricated on MBE grown wafers. Measurements of the transconductances of these devices, as a function of the effective gate length, exhibit transconductance degradation due to a diminishing aspect ratio. Velocity overshoot, saturation due to substrate current (MESFETs), real space transfer (HEMTs) and/or source dependent minimum acceleration lengths(both). In addition, the HEMTs with gate lengths less than 3Onm exhibit exponentially dependent current. This suggests that tunneling is the dominant current mechanism and the final limit to scaling of conventional FETs has been observed.

Modal expansions of the wave function and a mode-matching technique are used to calculate the transmission characteristics of semiconductor quantum wire structures assuming hard wall confmement in the transverse directions. Results for cascaded right-angle bends and periodic structures in a split-gate configuration are presented. A sharp transition to a plateau of zero conductance is observed for the double bend configuration. For periodic structures in the split-gate configuration, highly resonant behavior similar to that in tunneling resonant diodes is found. Calculated current-voltage characteristics for the case of two narrow constrictions are shown, exhibiting a region of negative differential resistance.

We present theoretical results for the conductance of a novel one-dimensional semiconductor superlattice (1DSL) structured in a ballistic constriction. We predict sharp switching between quantized conductance plateaus, as the Fermi level moves through the 1DSL miniband gaps, and strong resonant conductance oscillations. These effects are predicted to be observable even for systems with very few periods, and to be nearly independent of the geometry of the constriction openings.

It has recently been demonstrated[1,2] that zero-dimensional semiconductor structures ("quantum dots") can be fabricated with electrical contact to individual dots, and that the current voltage characteristics correspond to tunneling through the discrete density of states of a zero-dimensional system[3]. Because the density of states in such a quantum dot is a series of delta functions there is the potential for sharp transitions between tunneling and non-tunneling (on and off) states in devices fabricated from quantum dots. Such devices therefore could form the basis of a post-VLSI integrated circuit technology. These quantum dot devices are laterally-confined variations on the resonant tunneling diode (RTD). RTDs consist of a two dimensional quantum well surrounded by tunnel barriers. RTDs exhibit current peaks when electron energies in their contacts are aligned with quantum states in the well. As the quantum well states drop below the emitter conduction band edge, the current falls and there is negative differential resistance (NDR). Quantum dot diodes (QDDs) are RTDs which have lateral dimensions small enough to split the sub-bands in the quantum well into discrete energy states. This lateral confinement also creates 1-d sub-bands in the contact regions adjacent to the dot which become one dimensional quantum wires, leading to a more complex situation than exists in large area RTDs.

We have studied conductance fluctuations 5g due to quantum interference in mesoscopic modulation doped GaAs/AIGaAs heterostructures as a function of Fermi energy EF and as discrete time-dependent switching noise in various applied magnetic fields B at low temperatures. These systems have no, or negligibly small spin-orbit scattering. We observe a reduction of the variance by a net factor of four. This reduction results from a breaking of the spin degeneracy due to Zeeman splitting.

We study theoretically the effect of few elastic scatterers on electron transport in the ballistic regime. In laterally confined structures (quasi 1-d or 2-d), resonant transmission peaks occur when total electron energy equals any miniband energy. Unit transmission probability is approached when the scattering defect is small, and far from other scatterers, even if the scatterer is strong enough to decrease significantly the conductance away from resonance. Both numerical and analytical methods are used. Resonances occur for all shapes of confining potential.

Recent reports of very long electron mean free paths have raised the prospect of electronic devices operating on the basis of wave principles standard in physical optics. Such "optical" electron devices could be created within the 2DEG of a HEMT structure. We report here modeling of a proposed device which utilizes electron diffraction to perform several interesting functions, underscoring the potential multifunctionailty of quantum devices.

Quantum-mechanical model calculations of the conductance G of two parallel ballistic constrictions joining two two-dimensional electron gases show that G is nearly additive. This is due to the continuous distribution of incident modes and the collimation along the constrictions, and is almost independent of the width, length, Fermi energy and detailed shape of the constrictions. Due to coherence effects, the additivity breaks down, but only weakly so, when the constrictions are contiguous and very short and wide.

An attempt is made to study the Einstein relation for the diffuaivlty-mobility ratio of the electron in quantum well wires of Small-gap Semiconductors in the preaence of crossed electric and magnetic fields on the basis of a newly derived electron energy spectrum considering all types anisotropies in the band parameters. It is found taking n-CdGeAs as an example that, the same ratio increases with electron concentration and electric field in an oscillatory way. Besides, it decreases with thickness and the crystal field parameter Influence sinifigantly the ratio in the whole range of variables considered. We have also suggested an experimental method of determining the Einstein relation in degenerate materials having arbitrary dispersion law. The expression for quanturn well wires of parabolic semi-conductors are also obtained from our generalised expressions derived in the absence of cross field configuration.

Nanostructure fabrication for optoelectronic and quantum-effect devices can benefit from the greatly improved surface layer quality and low contamination offered by all vacuum processing. Finely focused ion beams can be used in a variety of ways for vacuum-compatable patterning on a nanometer size scale offering the potential of patterned devices with clean epitaxial interfaces and low dimensional confinement Several of these ion beam patterning techniques will be reviewed.

Nanometer-sized features as small as 400Ahave been fabricated in single-quantum-well GaAs/A1GaAs heterostructures for studies of quantum confinement effects in quantum dots. The features have been fabricated by dry-etching techniques using nanometer-sized etch masks by a novel surface deposition of colloidally-suspended spherical particles. SEM was used to examine the feature size.

We describe the use of Polymethylmethacrylate as both electron beam sensitive resist and ion etch mask for high-resolution pattern transfer. By using high-resolution electron beam lithography, chemically assisted ion beam etching, and in-situ metallization, we have fabricated ultra-narrow gates with lateral dimensions below 20 nm, spaced with < 50 nm pitch on high mobility 2D electron gas material. This technique, which is thought to provide extremely small lateral electron depletion lengths and well defined confienement potentials, allows us to produce new and more complicated structures for the study of quantum transport.

We describe a new electron beam lithography method for producing structures with lateral sizes smaller than the incident beam diameter. These patterns are transferred into GaAs/A1GaAs, InGaAs/GaAs and InGaAs/InP quantum well heterostructures using chemically assisted ion beam etching, thereby forming uniform arrays of pillars with lateral dimensions at or below 10 nm. To correlate the sizes of such structures with our exposure and development conditions, reflection electron microscopy observations are used.

It is suggested that 2D-electron-gas-systems, 500 nm wide and 100 ji m long, subjected to the conditions under which the quantum Hall-effect is observed, can behave like a 3osephson tunnel junction in that a phase-driven alternating current should exist between the two edge currents flowing parallel and anti-parallel to the longitudinal direction. Recent experimental results supporting this idea in low as well as in high frequency limits are discussed. The quenching of the Hall effect observed at low magnetic fields (around B=O) is shown to be the low frequency manifestation of this effect. Some newer experimental results at high values of B -- in the quantum-Hall regime -- show evidence of quantum interference arising out of the mixing of the edge currents by the high frequency phasedriven alternating currents.

The spin splittings E of the conduction subbands of zinc-blende semiconductors which possess inversion asymmetry of the microscopic crystal potentials are investigated theoretically for quantum wire structure. Due to inversion asymmetry, the 2 x 2 Hamiltonian in the spin basis has nonvanishing off-diagonal elements. We have solved the equivalent matrix elgenvalue problem obtained by expanding the eigenvectors in an N-term Fourier series chosen to satisfy the zero boundary conditions automatically. By increasing N step by step, the eigenenergies are shown to converge quickly to assigned accuracy. For a quantum wire with sizes L and L (I I [001]), and for free propagation wavevector k, we find that (1) when Ly is parallel to [0101, E increase linearly with k if L L2, but become negligible ifL =L; (2) when L is parallel to [1 10], \E as a function ofk can be Nshaped; and (3) when L and L are fixed and is rotated around LEshow 4 mm symmetry.

A new technique for high-resolution electron microscopy is described together with a number of applications involving semiconductor interfaces. Optically, this technique represents the realization of incoherent imaging at atomic resolution, with the advantages over conventional coherent imaging methods of improved resolution and an unambiguous, simply interpretable image. In addition, the image shows strong compositional sensitivity, thus providing a direct map of a material's structure and chemistry on the atomic scale.

Vibrational modes of the buried interfacial regions in strained layer SimGen SL's have been studied by Raman spectroscopy. The distinct but weak excitations depend on the strain distribution and are suggested to be related to localized modes of Si0.5Ge0.5 alloy layers at the interfaces. An extended annealing study is presented showing how these excitations become more pronounced as the interfaces are broadened. Long range order has been found by ThM in contrast to Raman scattering.

GaAs/A1GaAs quantum well (QW) widths were directly determined in-situ during MBE growth using reflection high energy electron diffraction (RHEED) intensity oscillations. Heavy-. and light-hole exciton transition energies in these QWs were measured using photoluminescence excitation (PLE) spectroscopy. We were able to obtain very good fits between calculated curves of these energies, using the standard quantum mechanical model for a QW, and the experimentally determined energies as a function of RHEEDdetermined QW widths. Thus, using this model and its fitting parameters, it is possible to derive QW widths accurately using only the data from PLE spectroscopy.

The silicon dioxide/silicon interface is critical in the electrical behavior of metal-oxidesemiconductor (MOS) field-effect transistors. As device dimensions shrink, roughness at this interface becomes increasingly important to electrical properties for two reasons. Roughness-induced local thickness variations lead to more significant fluctuating electric fields, and oxide growth temperatures must be reduced for thin oxides, leading to greater roughness. In this paper several experiments are described which provide information on the degree of roughness and its origin during the oxidation process. There are two types of experiments discussed: direct observation of atomic steps and structure during the very initial stages of room-temperature oxide growth on ultraclean Si(1 1 1) surfaces, and determination of roughness at conventional furnace-grown Si/SiO2 interfaces by a novel electron diffraction technique. The results of both these studies suggest that oxidation occurs primarily by the breaking of backbonds adjacent to the interface, and not by a terrace-ledge-kink mechanism. As a result, roughness is intrinsically created by the oxidation process, and can be removed only by a post-oxidation anneal. There is also evidence that the interface tension of Si/Si02 is sufficiently high, especially on Si(1 1 1), that it drives interface flattening during non-oxidizing thermal anneals.

In this work we study the growth of SiGe/Si superlattices and thick SiGe layers on ( 1 00), ( 1 1 1 ),and ( 1 1 0) Si surfaces at various temperatures by molecular-beam epitaxy (MBE) . We find that these three growth directions give rise to different growth morphologies and defect structures. The best growth is achieved on (100) surfaces, since growth on ( 1 1 1 ) and ( 1 1 0) surfaces are much more susceptible to twin formation. The growth direction, together with growth temperature, also dictates the onset of long-range ordering in SiGe layers. Our results indicate that ordering occurs only in thick, partially-relaxed SiGe layers grown on (100) surfaces at low temperatures but not in strained-layer superlattices grown under identical conditions. Thick SiGe layers or strained-layer superlattices grown on (1 1 1) or (1 10) surfaces at high or low temperatures do not exhibit ordering.

Interface structure can play a critical role in determining the properties of microelectronic devices. Because morphological features on the sub-nanometer scale are often involved, high resolution electron microscopy (HREM) is becoming increasingly used as a characterization tool. The application of HREM is illustrated in this article by reference to work on Si-Si02 and Ti-Si interfaces, which are both important in contemporary integratei circuits. q

Electron Beam Induced Current (EBIC) and Cathodoluminescence (CL) contrast of dislocations in GaAs show a strong temperature dependence. This is not only due to a variation of the recombination properties of the defects, which are a function of the defect energy level in the gap, but also due to the variation of diffusion length with temperature and their absolute value, which depends on doping type and doping concentration of the material. The experimental results are interpreted in the framework of model calculations for defect contrast and indicate a decreasing diffusion length and increasing recombination efficiency of dislocations with decreasing temperature in the range of 20 to 300 K.

To correlate microscopic with macroscopic properties and to investigate minority carrier recombination at the twins planes in dendritic web Si on a microscopic scale, cross-sectional EBIC has been developed. The twin planes are found to be transparent to minority carriers In as-grown material Irrespective of its quality. Upon cell processing, however, it is found that the twin planes in poor quality material become highly recombinative while recombination at regions close to the surface decreases. In good quality web material the twin planes remain benign after thermal cycling. DLTS measurements have also been performed and the results are consistent with EBIC. Crosssectional TEM results also correlate with the electrical measurements showing the appearance of a large density of defect clusters at the twin planes of the poor quality web upon thermal processing.

We have compared electrically and optically determined microscopic and macroscopic material parameters in Cu-doped, semi-insulating (SI) GaAs as a function of gettering. We find significant changes in the panchromatic scanning electron microscope room temperature cathodoluminescence (CL) images obtained before and after Cu contamination of undoped SI-GaAs. Electrical and optical measurements also indicate significant changes. These measurements include thermally stimulated current (TSC), Fourier infrared transform spectroscopy (FTIR), and photoconductive spectroscopy. After gettering by mechanical damage and subsequent heat treatment, the electrical characteristics revert to their pre-doping characteristics, indicating successful gettering.

ZnCdS semiconductor alloy films have been electrodeposited on conducting substrates. X-ray studies show polycrystalline nature exhibiting hexagonal structure for the solid solution. The lattice parameter variation obeys Vegard's law. SEM studies show decreasing grain size with increasing zinc content. Heat treated films show increased grain size thereby reducing grain boundary impurities. This lowers the resistivity of the deposited films.

We have observed low-frequency discrete resistance switching noise in a Si modulation-doped GaAs/ A!GaAs narrow MODFET. Resistance measurements were done at 1.5 and 4.2K as a function of applied magnetic field B to understand the origin of the resistance fluctuations. The noise magnitude diminishes rapidly as the field is increased and also decreases as the temperature T is raised. This can be understood in terms of strongly correlated spontaneous emptying and filling of ensembles or clusters of interacting localized electron states. The effect of increasing B or T is to weaken the interactions ultimately giving away to individual states fluctuating between two metastable configurations. Any relation of this noise to quantum interference effect is unclear.