James H. Luscombe

Current issues in nanoelectric modelling

The author presents a review of numerical methods for modelling the electronic properties of quantum nanostructure devices. The appropriate boundary conditions for solving the Poisson and Schrodinger equations in modeling the self-consistent screening potential and electron states are emphasised. Besides providing a framework for understanding the physics of nanoscale structures, realistic computer modeling constitutes a valuable tool for designing quantum devices that the author argues enables the development of a nanoelectronic technology along with the associated advances in fabrication technologies. Nanoelectronic devices make use of the quantized energy levels of confined electrons to control the flow of charge. The potential energy environment that gives rise to such levels is, however, a strongly sensitive function of the geometry and layer properties of the device structure. The relevant device variables must therefore be rather precisely specified, and the most cost-efficient means of developing realistic designs is to use modeling tools that are based on fundamental physical laws.

Models for nanoelectronic devices

Quantum nanoelectronic devices are a new class of devices, still under development, which operate by fully utilizing an electrons quantum-mechanical behavior at very small length scales. These devices are highly promising technologically, offering the prospect of continued miniaturization beyond the limiting length scales set by conventional devices. A review is given of theoretical models for nanoelectronic devices. Whereas modeling conventional devices is relatively well developed, incorporating quantum mechanics into device models is not. The authors argue that a spectrum of modeling tools of varying degrees of sophistication is required to meet the needs of the various stages of quantum device development. They review the applicability of: firstly, finite-temperature Thomas-Fermi theory to such quantum devices as resonant tunneling diodes, resonant tunneling transistors and quantum dot nanostructures; and secondly, the Wigner distribution function to resonant tunneling diodes.

Coupled-quantum-well field-effect resonant tunneling transistor for multi-valued logic/memory applications

A vertical field-effect resonant tunneling transistor is demonstrated consisting of a triple-barrier, double-well resonant tunneling diode (3bRTD) that can be depleted by the action of side gates. The 3bRTD features a double peak current-voltage characteristic in which the second valley current is less than the first valley current. Combination of the resonant tunneling transistor and a constant current load is shown to yield both binary and ternary logic and memory functions. >

Resonant Tunneling Transistors

Resonant tunneling transistors can perform more logic per transistor than conventional transistors. By exploiting their unique characteristic, circuit functional density and speed can be increased without changing the lithographic design rule. In addition, resonant tunneling transistors, in which the control electrode directly modulates the carrier transport, scale to smaller dimensions than conventional transistors. In this paper, the measured negative differential resistance and transconductance characteristics of several resonant tunneling transistors are described.

Electric field coupling to quantum dot diodes

We have fabricated two different structures in the GaAs/AlGaAs heterojunction system to quantify the transfer characteristics of the quantum dot subjected to external, local electric fields. The first structure is a single quantum dot diode with an annular field electrode placed adjacent to the double‐barrier structure by a self‐aligned fabrication process. A second structure consists of a pair of independently contacted quantum dot diodes separated by several hundred angstroms. The fabrication processes and transport properties for both of these structures are described. We have also attempted for the first time to form quantum dots in InGaAs/AlAs system lattice matched to InP and have observed that strong conductance fluctuations not related to lateral size quantization can occur. These fluctuations arise from the formation of rotation‐induced finite superlattices.

Lateral confinement in quantum nanostructures: Self‐consistent screening potentials

Self‐consistent lateral confining potentials and carrier density functions are computed for quantum nanostructures utilizing a finite‐temperature Thomas–Fermi approximation for the conduction electrons and the assumption of a uniform background of donor charges. The formation of the confining potential is the result of a nonlinear, electrostatic screening process which is determined by the Fermi level pinning properties of the lateral surfaces, the doping level, and the lateral dimensions. We find that the ability to populate nanostructures with carriers depends sensitively upon the details of the system.

Anharmonic oscillator model of a quantum dot nanostructure

The energy level spacings observed recently in resonant tunneling through an axially symmetric double‐barrier quantum dot structure are reproduced by an isotropic two‐dimensional anharmonic oscillator model. We propose that the extent of the depletion region in such a device effectively controls the possible angular momentum states of those electrons contributing to the current‐voltage curve.

Self-consistent screening potentials in quantum nanostructures: role of confined states☆

Abstract We obtain the finite-temperature density function and confining potential for electrons in a laterally-confined cylindrical quantum wire from a self-consistent solution of the coupled Poisson-Schrodinger equations. Except in special regimes, the results are remarkably similar to those we obtained previously with a Thomas-Fermi approximation.

Fabrication of lateral resonant tunneling devices

Lateral resonant tunneling transistors have been fabricated in the InAlAs/InGaAs material system lattice matched to InP. Lateral tunnel barriers are formed in the plane of a two‐dimensional electron gas confined at a modulation‐doped heterointerface by depletion regions induced by top‐contact metal gates. The device is structurally similar to a dual‐gate modulation‐doped field effect transistor with nanoscale gates. The metal gates are written by e‐beam lithography. Device results include multiple negative differential resistance peaks for temperatures as high as 20 K. Using the substrate as a backgate, multiple regions of negative transconductance are also observed.

Resonant tunneling quantum-dot diodes: physics, limitations, and technological prospects

The authors discuss the electronic structure and properties of the present generation of resonant-tunneling quantum-dot structures. Quantum dots are zero-dimensional semiconductor nanostructures, i.e., structures in which an electron is quantum-mechanically confined in all three spatial dimensions. Quantum dots appear to represent a viable structure to allow the continued downscaling of critical circuit geometries beyond the currently perceived limits for conventional VLSI devices. As they are currently fabricated, however, quantum-dot diodes have impediments which prevent the full realization of their potential. The authors assess these limitations and discuss measures for their solution. >