Nanostructure Physics and Fabrication. Proceedings of the International Symposium, College Station, Texas, March 13*b115, 1989.

484 Pages·1989·8.063 MB·English

by  Mark A. Reed and Wiley P. Kirk (Eds.)

N A N O S T R U C T U RE
P H Y S I CS A ND
F A B R I C A T I ON
Proceedings of the International Symposium
College Station, Texas March 75-75, 7989
Edited by
Mark A. Reed
Central Research Laboratories
Texas Instruments Incorporated
Dallas, Texas
Wiley P. Kirk
Department of Physics
Texas A&M University
College Station, Texas
ACADEMIC PRESS, INC.
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Copyright © 1989 by Academic Press, Inc.
All rights reserved.
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United Kingdom Edition published by
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Library of Congress Cataloging-in-Publication Data
Nanostructure physics and fabrication : proceedings of the
international symposium, College Station, Texas, March 13-15, 1989 /
edited by Mark A. Reed, Wiley Ρ Kirk,
p. cm.
"Contains contributions presented at the First International
Symposium on Nanostructure Physics and Fabrication . . . [held] on the
campus of Texas A&M University from March 13 to 15, 1989" — Pref.
Includes bibliographies and index.
ISBN 0-12-585000-X (alk. paper)
1. Electronic structure—Congresses. 2. Quantum electronics-
-Congresses. 3. Superconducting quantum interference devices-
-Congresses I. Reed, Mark Α. II. Kirk, Wiley Ρ
III. International Symposium on Nanostructure Physics and
Fabrication (1st : 1989 : Texas A&M University)
QC176.8.E4N32 1990
530.4 'll-dc20 89-17516
CIP
Printed in the United States of America
89 90 91 92 9 8 7 6 5 4 3 2 1
Preface
This book contains contributions presented at the first In-
ternational Symposium on Nanostructure Physics and Fabrica-
tion. The symposium was attended by 160 researchers from
around the world, who met in College Station, Texas, on the
campus of Texas A&M University from March 13 to 15, 1989.
The purpose of the symposium was to bring together an inter-
disciplinary group of specialists in nanometer scale fabrication,
physics of mesoscopic systems, electronic transport, and mate-
rials scientists and technologists to discuss the current status of
nanometer scale electronic structures.
The symposium was organized into eleven sessions of 28
oral presentations that spanned three days, and was accompa-
nied by two lively late afternoon poster sessions in which approx-
imately forty technical posters w rere presented. These sessions
addressed topics on conceptual origins of nanostructures, lateral
periodicity and confinement, nanoconstruction and phase coher-
ent effects, quantum devices and transistors, equilibrium and
nonequilibrium response in nanoelectronic structures, quantum
ballistic transport, quantum dots, ballistic transport at quan-
tum point contacts, and quantum constriction and narrow wares.
In order to convey the many important ideas and the most
recent results of the symposium's contributors, we have orga-
nized this book into seven chapters. The first chapter represents
a condensed overview in which we attempt to unify and sum-
marize briefly the various individual contributions that follow
in the remaining six chapters. Because the field is very new, we
have also provided in the first chapter some introductory com-
ments and background material that has led to the formation
of the field. The book, therefore, provides the most up-to-date
view of a very active and growing subject that is gaining wide
interest.
The book's origin and contents are for the most part a re-
sult of the symposium's success, which in turn was a result of the
efforts and many fine suggestions put forward by the Organiz-
ing and Program Committee and the Advisory Committee. As
always, in the background of any successful conference is a co-
terie of students, research associates, and faculty colleagues who
see that things get done. Our coterie consisted of the following
individuals, whom we owe much gratitude: C. C. Andrews, K.
G. Balke, M. G. Blain, C. J. Brumlik, R. N. Burns, Z. Cai, F.
Cheng, J. F. Chepin, B. H. Chu, D. P. Dave, J. J. Garris, A.
Roy-Ghatak, W. B. Kinard, W. R. Klima, P. S. Kobiela, J. Lei,
F. Li, Y. X. Liu, J. R. McBride, E. C. Palm, M. A. Park, R.
Pathak, B. Radie, L. R. Richards, G. Spencer, T. Szafranski,
W. Szott, M. Tierney, R. Tsumura, A. L. Vandervort, C. Wang,
and W. K. Yue. We are indebted to Professor Glenn Agnolet for
his many helpful suggestions, to Professors Wayne M. Saslow,
Chia-Ren Hu, Joseph H. Ross, and Dr. John N. Randall for
their help in judging the posters, and especially to the Publi-
cations Secretary, Phil R. Reagan, for his invaluable assistance
in organizing the conference proceedings. Each of the referees,
who helped review the contributions, deserve much praise from
us. Our greatest thanks and appreciation, however, goes to the
Symposium Secretary, Mary J. Miller, who tackled innumerable
details with great proficiency. Finally, financial support from
our sponsors listed on a nearby page is gratefully acknowledged
and in particular we thank Dr. Larry R. Cooper for his special
interests in helping us secure the needed support.
Mark A. Reed Wiley P. Kirk
Dallas, Texas College Station, Texas
May 1989 May 1989
Organizing and Program Committee
Wiley P. Kirk,Chairman,Texas A&M University
Mark A. Reed, Co-Chair, Texas Instruments
Robert T. Bate, Texas Instruments
Alex L. de Lozanne, University of Texas, Austin
John N. Randall, Texas Instruments
Mark H. Weichold, Texas A&M University
Advisory Committee
Leroy Chang, IBM, T. J. Watson Research Center
Larry R. Cooper, Office of Naval Research
John D. Dow, University of Notre Dame
Robert C. Dynes, AT&T Bell Laboratories
David K. Ferry, Arizona State University
Karl Hess, University of Illinois
Klaus v. Klitzing, Max Planck Institut, Stuttgart
Michael Pepper, University of Cambridge
William J. Skocpol, Boston University
Horst L. Stornier, AT&T Bell Laboratories
Michael A. Stroscio, U. S. Army Research Office
Gerald L. Witt, Air Force Office of Scientific Research
Sponsors
Office of Naval Research
Air Force Office of Scientific Research
Defense Advanced Research Projects Agency
U.S. Army Research Office
American Vacuum Society, Texas Chapter
Texas Instruments, Inc.
Texas A&M University
Physics Department
Electrical Engineering Department
College of Science
Texas Engineering Experiment Station
Industrial Sponsors and Exhibitors
EKC Technology
Grant Associates
MKS
Group Five
Kurt J. Lesker
Balzers
Chapter 1
Overview and Background
"At any rate, it seems that the laws of physics present no barrier to reducing
the size of computers until bits are the size of atoms, and quantum behavior
holds dominant sway." Richard P. Feynman (from "Quantum Mechanical
Computers", Optics News, pp. 11-20, February 1985, published by the Optical
Society of America.)
OVERVIEW A ND B A C K G R O U ND
Mark Λ. Reed
Texas Instruments Incorporated
Dallas, Texas 75265
Wiley P. Kirk
Texas AfcM University
College Station, Texas 77843
1. I N T R O D U C T I ON
In 1957 J. R. Schrieffer [1] suggested that the narrow confinement poten-
tial of an inversion layer may lead to the observation of non-classical electron
transport behavior. This was unambiguously demonstrated in 1966 [2,3] by
low-temperature magnetotransport, and initiated an intense effort in the ex-
ploration of 2-dimensional electrons gas (2DEG) systems. The compelling
attraction of the field was and is the ability to observe macroscopic quan-
tum size effects and (to a certain extent) control system parameters that
determined these effects [4]. Until recently, only in areas of low-temperature
physics has there been any opportunity to study and gain experience with
macroscopic quantum mechanical effects. Unfortunately, the number of low-
temperature systems available for study has been few and mostly limited
to liquid and solid states of helium and a finite number of a few types of
superconductors. Moreover, these low-temperatures systems always incorpo-
rate rather complex many-body effects that make them exceedingly difficult
to control and understand on a first principle basis. Now, however, we are
rapidly approaching a level of solid-state structural fabrication in which the
energy and length scales are such that macroscopic quantum effects are man-
ifest. We have, for the first time, reached a stage where it is possible to
observe and perhaps even control large scale quantum effects in a variety of
materials and states of condensed matter.
NANOSTRUCTURE PHYSICS 3 Copyright © 1989 by Academic Press, Inc.
AND FABRICATION All rights of reproduction in any form reserved.
ISBN 0-12-585000-X
The advent of ultra-thin epitaxial film growth techniques ushered in the
era of reduced dimensionality physics, and it became clear that heterojunc-
tion interfaces with large energy band discontinuities were a reality. These
structures are inherently two-dimensional; thus investigations concentrated
on heterostructures, where quantum effects are due to confinement in the
epitaxial direction; i.e., quantum wells (more properly, "quantum planes")
which are 2-dimensional systems [5]. The ability to vary composition, band
offset, periodicity, and other variables results in nearly limitless possibilities in
creating structures for physical exploration as well as for electron and optical
device applications.
Within the last few years, advances in microfabrication technology have
allowed laboratories around the world to impose additional lateral dimen-
sions of quantum confinement on 2-dimensional systems with length scales
approaching those of epitaxial lengths in the growth direction. The achieve-
ment of quantum wires [6,7,8] and quantum dots [9] have demonstrated that
electronic systems with different dimensionality are now available to the ex-
perimentalist. The quantum limit of electronic transport has been demon-
strated [10,11], leading to a fundamentally distinct approach to the under-
standing of mesoscopic electronic systems.
Yet as late as 1986, "low dimensional structures" implicitly meant 2DEGs
or quantum wells. At the 1986 International Conference on the Physics of
Semiconductors in Stockholm, Sweden, there were less than 10 papers (out of
a total of 405 papers) on fabricated nanostructures. However, these few pa-
pers framed the beginnings of a new era in semiconductor transport. One of
the major motivational reasons for examining small electronic systems was the
observation of "universal conductance fluctuations" and weak localization ef-
fects in silicon [12] and GaAs [13] nanostructures. These works demonstrated
that coherent and random quantum interference, respectively, are observable
in few-electron systems, and perhaps could even be dominant in appropriate
structures. However, fabrication processes and tolerances were not sufficiently
well developed to create structures that would exhibit large quantum inter-
ference or quantum size effects; the observations were at most small, noisy
perturbations.
A conceptually more pleasing investigation at the time, at least to trans-
port physicists, was the observation of Aharonov-Bohm oscillations in small
metallic ring structures [14]. These observations appeared to be the first
clear observation of quantum interference of the electron wavefunction in a
nanofabricated structure. At the time, there appeared to be little connection
between this esoteric effect and conductance fluctuation work, except for their
embodiment in small electronic systems.
During this time, a renaissance was occurring in vertical electronic trans-
port through multilayer heterostructures. Large quantum tunneling effects
were achieved in resonant tunneling structures [15] nearly a decade after the
initial (rather disappointing) investigations by Chang et al. [16]. The im-
4
portance of this renaissance was not the specific realization of the tunneling
structure, but a realization that artificially-structured quantum states could
indeed show nonclassical electronic transport. Around the same time, the
achievement of ballistic transport (i.e., electronic transport in which carriers
(electrons) traverse without undergoing a scattering event) in vertical hot-
electron structures was reported [17,18].
The turning point in the understanding of nanometer scale electronic
transport was the development of reliable semiconductor fabrication tech-
niques on the nanometer scale, an example of which is the fabrication of
semiconductor quantum wires [7], which allowed the fabrication of electronic
systems in a totally new regime. The realization of ballistic structures had
been done in the epitaxial dimension, but not in the lateral dimensions due
to the small dimensional scales (tens of nanometers) involved. However, the
advancement of electron beam lithography and the incorporation of this di-
mensional scale onto high mobility 2DEGs allowed one, for the first time, to
create structures that were smaller than the relevant length scales (elastic,
inelastic, phase-breaking, etc). In this regime, quantum transport becomes
dominant and the wave nature of the electron becomes apparent. An elegant
example of the behavior of electrons on this length scale was the creation
of electron "wraveguides" [8], which are surprisingly akin to their microwave
counterparts that are nearly six orders of magnitude larger.
If structures are created where the interference of electron waves are a
dominant effect, how is the system measured? In such systems, the "contacts"
to the system are no longer ideal - they are now by definition part of the
entire (interacting and interfering) electron wave system. Thus, electrical
leads become intractably invasive [19]. This topic has spawned an active area
of current research, especially with regard to measurements in the quantum
limit.
An obvious area of interest is the limit of electrical conduction; i.e., what
is the relevant physics when the number of carriers (or current carrying chan-
nels) in the system are countably small. Though this limit was theoretically
explored by Landauer over 30 years ago, the realization of such systems has
only now been possible. The recent discovery of quantized conductance in bal-
listic "point contacts" [10,11] , which is predicted by the Landauer formalism,
has opened up a new era of nanostructure physics where one can fabricate
nanostructures whose behavior is dominated by quantum interference effects.
This new capability has enthused the experimentalist and theorist alike, an
excitement akin to the advent of quantum well technology, with limitless pos-
sibilities for physical exploration and device technology on the nanoscale. It
is within this context that the International Symposium on Nanostructure
Physics and Fabrication was held.
5