Quantum-Based Electronic Devices and Systems

The following sections are included:

• PREFACE

• CONTENTS

Recent advances in chemical self-assembly will soon make it possible to synthesize extremely powerful computing machinery from metallic clusters and organic molecules. These self-organized networks can function as Boolean logic circuits, associative memory, image processors, and combinatorial optimizers. Computational or signal processing activity is elicited from simple charge interactions between clusters which are resistively/capacitively linked by conjugated molecular wires or ribbons. The resulting circuits are massively parallel, fault-tolerant, ultrafast, ultradense and dissipate very little power.

We discuss novel nanoelectronic architecture paradigms based on cells composed of coupled quantum-dots. These ideas of a transistor-less approach represent a radical departure from conventional technology. We utilize a strategy which exploits the physical interactions between quantum-dots arranged in suitably designed cellular arrays. Boolean logic functions may be implemented in specific arrays of cells representing binary information, the so-called Quantum-Dot Cellular Automata (QCA). Cells may also be viewed as carrying analog information and we outline a network-theoretic description of such Quantum-Dot Nonlinear Networks (Q-CNN). In addition, we discuss possible realizations of these structures in a variety of semiconductor systems (including GaAs/AlGaAs, Si/SiGe, and Si/SiO₂), rings of metallic tunnel junctions, and candidates for molecular implementations.

In deep submicron silicon MOSFETs. GaAs-based HEMTs, and in new emerging heterostructure systems, such as AlGaN/GaN, electrons forming a two-dimensional (2D) conducting channel exhibit new interesting effects that might find important device applications. Some of these effects are related to the space dependence of the electron mass. Other effects are linked to a large sheet electron concentration, when electrons behave not as a 2D electron gas but rather as a 2D electron fluid. We consider plasma effects in this fluid and discuss plasma wave electronic devices that rely on these effects. We also discuss the properties of 2D electrons in silicon devices, where plasma effects might also play an important role in deep submicron MOSFETs.

Carbon nanotubes exhibit unusual electronic and mechanical properties which vary with subtle changes in microstructure, applied electromagnetic field and mechanical deformations, and introduction of topological defects. These novel properties offer unprecedent opportunities to study fundamental physics, fabricate advanced composition materials, and construct quantum devices at nanometer scales.

The interplay between geometrical confinement and materials considerations can efficiently reduce phonon-assisted transport, enabling scattering time and dissipation engineering in quantum devices. In resonant tunneling (RT) structures, quenching of phonon-assisted transmission leading to considerable reduction of the off-resonance valley-current is shown to occur in interband devices. In structures of low dimensionality such as quantum wires, electron-phonon scattering exhibits size effects and intersubband resonances which modulates the drift velocity and conductance of one-dimensional systems. Quantum dot nanostructures offer large flexibility for reduction and modulation of dissipative processes such as oscillatory hopping conductance induced by acoustic phonons in linear chains of quantum dots or negative differential resistance curve shaping in RT through quantum dot arrays.

Quantum mechanical devices utilize the wave nature of electrons for their operations whenever the electron mean-free-path exceeds the appropriate dimensions of the device structure. Some of the issues such as the tunneling time, the reduction of the dielectric constant and the drastic increase in the binding energy of dopants are discussed. Lacking an appropriate barrier for silicon, the majority of quantum devices are fabricated with compound semiconductors. In the past several years, certain schemes appeared, such as the resonant tunneling via nanoscale silicon particles imbedded in an oxide matrix, and the superlattice barrier for silicon consisting of several periods of Si/O. There appears some doubt about the tunneling nature of the former, and the possibility of dielectric breakdowns. This article aims to show that dielectric breakdowns can occur under fabrication conditions without using a controlled forming process. The latter results in epitaxially grown silicon beyond the superlattice barrier region, free of stacking fault defects, and thus is potentially important for silicon based quantum devices as well as serving as an SOI (silicon on insulator), without ion-implantation damage and oxygen inclusion. The replacement of SOI by the epitaxially grown Si/O superlattice barrier should promote the effort in high speed and low power MOSFET devices.

In the mid 1980s Averin and Likharev predicted that with the use of ultrasmall tunnel junctions a time correlation of electron flow through a junction could be observed, and permit the measurement of the effect of a net charge of less than one electron on the junction. Both effects were soon experimentally verified, and since that time there has been an explosion of work in the field of single electron devices. This chapter reviews the fundamental concepts behind the operation of such devices. It then describes some of the single electron effects studied in semiconductors. Superconducting devices are then contrasted to the semiconductor and the normal metal single electron devices. The details of some current applications are described, and a thumbnail sketch of current fabrication methods is given.

The consequences of quantum behavior of charge carriers in devices, while entertaining, is detrimental to the device performance. Chaos leads to unmanageable and unpredictable response in different devices to the same drive and bias inputs.

The theory of Bloch electron dynamics in spatially homogeneous electric fields of arbitrary strength and time dependence is presented. In the formalism, the electric field is described through the use of the vector potential, and the instantaneous eigenstates of the Hamiltonian are used as basis states to depict the Bloch dynamics and quantum properties. This approach leads to a natural indication of high- and low-field limits, and allows for the inclusion of general band-structure effects and multiband coupling in the quantum dynamics. A variety of dc electric field effects, such as Bloch oscillations, Zener tunneling, and localization, and ac electric field effects, such as interband absorption, and photon-assisted transport, will be discussed.

Schrödinger Equation (based) Monte Carlo (SEMC), a simulation method designed to bridge the gap from quantum to classical transport, is described. This method provides a non-perturbative, current conserving quantum mechanical treatment of carriers, phonons, and their coupling, yet the SEMC algorithm is analogous to and compatible with that of semiclassical Monte Carlo (SMC). Indeed, SEMC allows carriers to be followed through a sequence of stochastically sampled scattering events. Phase breaking and energy dissipation for charge carriers within this Schrödinger-equation-based method are modeled via the exchange of probability among oscillator degrees of freedom, mimic-ing the true process of carrier-phonon scattering. Carrier-phonon coupling potentials for SEMC are obtained by Monte Carlo sampling of (the spatial correlation functions of) the true carrier-phonon coupling potentials. Illustrative results demonstrate SEMC's ability to provide both physically and quantitatively accurate modeling of, in particular, long-range polar-optical phonon scattering, and to completely bridge the gap between phase-coherent and phase-incoherent transport.

As device dimensions in electronic and optoelectronic devices are reduced, the characteristics and interactions of dimensionally-confined longitudinal-optical (LO) and acoustic phonons deviate substantially from those of bulk semiconductors. Furthermore, as würtzite materials are applied increasingly in electronic and optoelectronic devices it becomes more important to understand the phonon modes in such systems. This account emphasizes the properties of bulk optical phonons in würtzite structures, the properties of LO-phonon modes and acoustic-phonon modes arising in polar-semiconductor quantum wells, superlattices, quantum wires and quantum dots, with a variety of cross sectional geometries and, lastly, the properties of optical phonons in würtzite materials as predicted by the dielectric continuum model. Emphasis is placed on the dielectric continuum and elastic continuum models of bulk, confined and interface phonons. This article emphasizes device applications of confined phonons in GaAs-based systems and provides a brief discussion of carrier-LO-phonon interactions in bulk würtzite structures. This account also includes discussions on the use of metal-semiconductor heterointerfaces to reduce scattering and on the role of phonons in Fröhlich, deformation and piezoelectric interactions in electronic and optoelectronic structures; specific device applications highlighted here include quantum cascade lasers, mesoscopic devices, thermoelectric devices and optically-pumped resonant intersubband lasers.