Materials modification by electronic excitation

537 Pages·2001·3.942 MB·English

by  Itoh N., Stoneham A.M.

MATERIALS
MODIFICATION BY
ELECTRONIC
EXCITATION
N. ITOH
A. M. STONEHAM
CAMBRIDGE UNIVERSITY PRESS
Materials modiWcation by electronic excitation
Electronic excitation is a means to change materials properties.This book analyses the impor-
tant features ofthe changes induced by electronic excitation,identiWes what is critical,and pro-
vides a basis from which materials modiWcation can be developed successfully. Electronic
excitation by lasers or electron beams can change the properties of materials. In the last few
years,there has been a mix of basic science,of new laser and electron beam tools,and of new
needs from microelectronics,photonics and nanotechnology.This book extends and synthesises
the science,addressing ideas like energy localisation and charge localisation,with detailed com-
parisons of experiment and theory.It also identiWes the ways this understanding links to tech-
nological needs, like selective removal of material, controlled changes, altering the balance
between process steps, and possibilities of quantum control. This book will be of particular
interest to research workers in physics,chemistry,electronic engineering,and materials science.
NoriakiItohhas been Emeritus Professor ofPhysics at Nagoya University since 1995.Until
recently he was Professor of Physics at the Faculty of Information Science,Osaka Institute of
Technology.He spent much of his career at Osaka and Nagoya Universities.After completing
his PhD,he was Research Associate,Lecturer and Associate Professor of Nuclear Engineering
at the Faculty of Engineering, Nagoya University. At Nagoya University, he became in turn
Associate Professor and Professor of Nuclear Engineering at the Faculty of Engineering,
Professor of Physics in the Faculty of Science and finally Emeritus Professor. He served as
department heads occasionally. He has been Emeritus Professor of Nagoya University since
1995.He was a Council Member of the Japanese Society of Applied Physics.He was Editor of
Radiation EVects and Defects in Solids,and a Member of the Advisory Boards of Journal of
Physics Condensed Matter, Nuclear Instruments and Methods B and Review of Solid State
Science.His main interests in materials science and in experimental physics are in the ways in
which materials are modiWed by irradiation with photons,electrons and ions.His publications
include books on radiation eVects on solids as well as about 300 papers. As an experimental
physicist he has been stimulated by theorists whose main interest is modelling of atomic
processes.
Marshall Stoneham is Massey Professor of Physics and Director of the Centre for
Materials Research at University College London.In 1989,he was elected a Fellow ofthe Royal
Society. He is a Fellow of the Institute of Materials, of the Institute of Physics, and of the
American Physical Society.He spent much ofhis career at Harwell,becoming in turn Leader of
the Solid State and Quantum Physics Group, Head of Materials Physics and Metallurgy
Division,Director of Research for AEA Industrial Technology,and Wnally AEATechnology’s
Chief Scientist.He is a Director of Oxford Authentication Ltd and,as Vice President of the
Institute of Physics,chairs the Board of Directors of Institute of Physics Publishing.He is a
Senior Fellow ofCorning.His wide interests in materials science and in basic and applied physics
include the ways in which materials modelling can be used to advantage.His publications include
books on defects in solids and on the reliability of non-destructive inspection,as well as nearly
400 papers.As a theorist,he particularly enjoys the stimulus of working alongside good exper-
imenters.
This page intentionally left blank
MATERIALS MODIFICATION BY
ELECTRONIC EXCITATION
N. ITOH
Nagoya University
and
A. M. STONEHAM
University College London
PUBLISHED BY CAMBRIDGE UNIVERSITY PRESS (VIRTUAL PUBLISHING)
FOR AND ON BEHALF OF THE PRESS SYNDICATE OF THE UNIVERSITY OF
CAMBRIDGE
The Pitt Building, Trumpington Street, Cambridge CB2 IRP
40 West 20th Street, New York, NY 10011-4211, USA
477 Williamstown Road, Port Melbourne, VIC 3207, Australia
http://www.cambridge.org
' N. Itoh and A. M. Stoneham 2001
This edition ' N. Itoh and A. M. Stoneham 2003
First published in printed format 2000
A catalogue record for the original printed book is available
from the British Library and from the Library of Congress
Original ISBN 0 521 55498 5 hardback
ISBN 0 511 01736 7 virtual (netLibrary Edition)
Contents
Preface pagexii
1 Concepts:Excitation,polarons and electronic structure 1
1.1 Basic ideas about the localisation of charge and energy 1
1.1.1 The polaron concept 3
1.1.2 Excitation of metals and insulators:What is special about
insulators? 6
1.2 Methods of excitation 7
1.2.1 Excitation by electromagnetic radiation 7
1.2.2 Excitation by electrons 15
1.2.3 Other forms of particle excitation 19
1.2.4 Other forms of excitation 23
1.3 Structure at the atomic scale 25
1.3.1 Structural issues:Where do crystalline and amorphous
materials diVer? 25
1.3.2 The varied forms of ‘amorphous’ 27
1.3.3 Mesostructure 28
1.4 Basic issues of electronic structure 31
1.4.1 Band structures:General features for crystalline and
amorphous solids 31
1.4.2 Approaches to electronic structure 34
1.4.3 Special cases 35
1.4.4 Localising charge 38
1.5 Excitation and excited states 45
1.5.1 Optical excitation 46
1.5.2 Excitation by ionising radiation 48
1.5.3 Excitation at higher energies 49
1.5.4 Excitation at higher intensities 52
1.6 Excitation of defects and recovery after excitation 52
v
vi Contents
2 Energy deposition and redistribution in solids 57
2.1 Interactions of charged particles with solids 58
2.2 Theory of the interaction of charged particles with solids 67
2.3 Issues:Beyond the standard models 71
2.4 Challenges:Non-equilibrium situations 74
2.5 Thermal diVusion:Processes near thermal equilibrium 75
2.5.1 The phenomenology of diVusion rates:The Arrhenius
and Meyer–Neldel (compensation) expressions 76
2.5.2 Special cases of diVusion 77
2.6 Transport and capture processes 79
2.6.1 Geminate recombination 79
2.6.2 Rate theory and defect aggregates 81
3 Electron–lattice coupling and its consequences 85
3.1 Basics of electron–lattice coupling 85
3.2 The conWguration coordinate diagram 88
3.2.1 The basic conWguration coordinate model 89
3.2.2 Choices of conWguration coordinate 90
3.2.3 Simple cases:The F centre 91
3.2.4 Optical transitions 93
3.2.5 Charge transfer transitions 95
3.3 Relaxation energies and defect stability 96
3.3.1 Stability and instability 97
3.3.2 Examples of charge state stability 98
3.3.3 Stability of self-trapped polarons:Strategies 99
3.3.4 Stability of small polarons:Static approaches 101
3.3.5 Stability of small polarons:Microscopic calculation of the
relaxation energy 102
3.3.6 Small-polaron formation energy:Energy cycles 105
3.3.7 SpeciWc properties of the self-trapped exciton (STX) state 106
3.4 Mobilities and charge transport in non-metals 108
3.4.1 Experimental data on mobilities 108
3.4.2 Small polarons and large polarons:Ideas about motion 108
3.4.3 Self-trapped excitons versus self-trapped holes:Exciton
bandwidths 110
3.4.4 Classical diVusion of ions and other over-the-barrier
processes 110
3.4.5 DiVusion of self-trapped carriers 113
3.5 Non-radiative transitions I:Cooling transitions 114
3.5.1 Cooling of atomic motion 114
Contents vii
3.5.2 Transitions from one energy surface to another 120
3.5.3 Cooling of electronic excitation:Free carrier states 122
3.5.4 Cooling of electronic excitation:Capture and cooling of
bound carrier states 125
3.6 Non-radiative transitions II:Absolute rates 127
3.6.1 Kinetics and dynamics 127
3.6.2 Multiphonon non-radiative transitions 128
3.7 Non-radiative transitions III:Localisation processes and their
rates 132
3.7.1 Routes to the self-trapped state 132
3.7.2 Quantum molecular dynamics approaches 135
3.7.3 Solvation of an electron in water 135
3.7.4 Frozen Gaussian methods 136
4 Self-trapping 138
4.1 Self-trapped carriers in halides 138
4.1.1 Self-trapped electrons 140
4.1.2 Self-trapped holes 141
4.1.3 Relaxation processes of self-trapped holes 145
4.1.4 Extrinsic and perturbed self-trapped holes 149
4.2 Self-trapped carriers in oxides 150
4.3 Self-trapped excitons in halides 152
4.3.1 AgCl 154
4.3.2 Alkali halides with the NaCl structure 155
4.3.3 Other halides 166
4.4 Self-trapped excitons in oxides 171
4.4.1 Self-trapped excitons in oxides with closed-shell cations 171
4.4.2 Self-trapped excitons of oxides with open-shell cations 180
4.5 Self-trapped excitons in crystalline semiconductors 181
Summary 185
5 Local lattice modiWcation by electronic excitation of halides 187
5.1 Excitonic mechanisms for defect formation 188
5.1.1 Adiabatic potential energy surfaces and relaxation
channels 188
5.1.2 Experimental evidence for three channels for defect pair
formation in alkali halides 193
5.1.3 Branching between the relaxation channels from exciton to
defect pair 195
5.1.4 Thermal conversion from self-trapped exciton to defect
pair 201
viii Contents
5.1.5 Other materials in which the excitonic mechanism is
eVective 205
5.2 Defect formation by other mechanisms 207
5.2.1 Defect formation from interacting excitons 207
5.2.2 Defect generation by two-hole localisation 208
5.2.3 The photographic process in silver halides 209
5.2.4 Photochromic and photosensitive glasses 214
5.2.5 Creation of defect pairs in the cation sublattice 215
5.3 Defects created by ionising radiation 216
5.3.1 Defect pairs created at low temperatures 217
5.3.2 Stabilisation of interstitials 219
Summary 223
6 Local lattice modiWcation by electronic excitation of crystalline
insulating oxides 224
6.1 Basic phenomena 224
6.1.1 Oxides and halides:Basics and similarities 224
6.1.2 Self-trapping in oxides 225
6.1.3 Charge transfer and colour 226
6.1.4 Non-linear processes and negativeU 228
6.1.5 Amorphisation 229
6.2 EVects induced under electron beam excitation 230
6.2.1 Damage and degradation 230
6.2.2 Amorphisation by electron beams 232
6.2.3 Transient defects 233
6.3 Electrical breakdown and related phenomena 234
6.3.1 Metal–insulator transitions in oxide Wlms 235
6.3.2 Electrical breakdown in simple ceramic oxides,like MgO
and alumina 237
6.3.3 Breakdown in the oxide on silicon 238
6.3.4 Radiation-induced electrical degradation 242
Summary 244
7 Local lattice modiWcation of semiconductors by electronic excitation 245
7.1 General comparisons:Switching between states and motion 245
7.2 Enhanced diVusion 247
7.2.1 Characteristics of enhanced diVusion 247
7.2.2 Routes to enhanced diVusion 250
7.2.3 Understanding enhanced diVusion 252
7.2.4 Types of enhanced diVusion 253
7.3 Local heating models (‘hot-spot’or ‘phonon-kick’mechanisms) 253
7.3.1 The model of Weeks,Tully,and Kimerling 254
Contents ix
7.3.2 The model of Masri and Stoneham 254
7.3.3 The model of Sumi 255
7.3.4 Other general issues 256
7.4 Local excitation models,including the Bourgoin–Corbett
mechanism 259
7.4.1 Case I:Energy extrema at the same site 260
7.4.2 Case II:Energy surfaces with extrema at diVerent sites 261
7.4.3 The Bourgoin–Corbett model 263
7.4.4 Analogous systems:Metastability and reorientation 264
7.5 How can the mechanisms be distinguished from each other? 266
7.5.1 Consistency arguments 267
7.5.2 Reasonableness arguments 267
7.5.3 Are charge state changes possible and significant? 268
7.6 Issues in enhanced diVusion:Further discussion of mechanisms 269
7.6.1 Competing processes:Isotope eVects in electrical isolation 269
7.6.2 Dislocation growth and motion 270
7.6.3 Enhanced oxidation 273
Summary 274
8 Local lattice modiWcation of amorphous materials by electronic
excitation 275
8.1 Electrons,holes,and excitons in amorphous solids 280
8.1.1 The optical absorption edge 280
8.1.2 Motion of electrons and holes 282
8.2 Optical absorption and luminescence 284
8.2.1 Amorphous silicas:a-SiO 287
2
8.2.2 Chalcogenides 292
8.2.3 Diamond-like carbon (a-C:H;DLC) and amorphous
silicon (a-Si:H) 294
8.3 Defect formation 300
8.3.1 Amorphous silicas:a-SiO 301
2
8.3.2 Chalcogenides 309
8.3.3 Amorphous silicon:a-Si:H 311
8.4 Photo-induced structural changes:Photodarkening 313
8.5 Ion-beam-induced structural changes 319
8.5.1 Ion-induced crystallisation and amorphisation of silicate
glasses 319
Appendix:Basic defects in glasses 321
Summary 324
9 Atomic emission and surface modiWcation 325
9.1 Energy absorption near surfaces 325