Venables J. Introduction to Surface and Thin Film Processes

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9 Postscript – where do we go from

here?

This short postscript wraps up the book, and provides pointers to further reading and

gathering information. The comments are personal impressions rather than cut and

dried issues. Please take them suitably lightly – and then get on with the rest of your

life which is, of course, all too short. Time is the only real enemy – someone must have

said that before.

9.1 Electromigration and other degradation effects in nanostructures

Degradation over time is an important part of materials science, and is inherent in the

metastabilty of all artiﬁcially tailored structures. It is a major reason why I will have to

replace the laptop computer, on which I have been composing this book, in the not-

too-distant future. There are many surface and thin-ﬁlm related degradation processes

involved, some of which have been discussed in the book, and some of which have been

omitted for reasons of space and time. Others are being researched and could use a

good review article. However, if I don’t ﬁnish this book now, I never will (another

example of degradation over time). I need to get on with other aspects of my life and

so do you.

Polycrystalline metal wires forming interconnects can fail via necking of ‘bamboo’

structures, for the same basic reason as the tungsten ﬁlaments discussed in section

6.2.1. Such a wire has slight changes in thickness along its length, being slightly thinner

where grain boundaries cross the wire, and slightly thicker either side (hence the name

bamboo). When a current is passed, the resistance heating is greater in the grain boun-

dary regions mostly because of the smaller cross section, and this leads to atom migra-

tion, principally by surface and grain boundary diﬀusion, towards the equilibrium

structure.

Once we have a thin wire, the surface energy is important, and we know from the dis-

cussion in section 1.2 that the equilibrium shape is close to a sphere, and is given by the

Wulﬀ plot. Thus a thin wire tries to become shorter and more rounded, eliminating

grain boundaries as it coarsens. This will happen more quickly for higher current den-

sities, and at higher operating temperatures, both of which are implicated in smaller

devices. Needless to say, eventually these eﬀects sever the wire in two: bad news for me,

and doubtless well calculated by the computer manufacturer who would love me to

297

purchase a new one, subject to avoiding recalls on existing models and law suits. Under

accelerated test conditions at high temperatures, this contraction, or expansion of a

wire under load, forms the zero creep method of measuring surface energy, as men-

tioned in section 1.2.1.

Metal interconnects are very important components in integrated circuits (ICs), and

have been mostly been made of aluminum, with copper being actively developed as a

replacement for use at higher current densities. As wires and FET gate electrodes, these

metals are laid down typically on the amorphous SiO

which is the insulating surface

on Si-based MOS devices; they will grow as polycrystalline layers by nucleation and

growth processes similar to those described in chapter 5. As contacts to silicon, they

can be either ohmic (to get current in or out) or form control elements as in the

Schottky barrier discussed in section 8.2.

Most metals are in fact very reactive to silicon itself. Metal–semiconductor interac-

tions is an enormous topic, which has been visited at various points in this book, but

not described systematically. This is an area where much work has been done, but it

feels to me as if the deﬁnitive review has not yet been written; in any case it probably

needs a whole book to itself. It is important to note here that substantial rearrange-

ments of both metal and semiconductor at the interface results from this reactivity, as

in the case of Ag/Ge or Si(111) shown in ﬁgure 1.20, and illustrated on the cover design.

In many cases barrier layers consisting of more tightly bound metals, alloys and/or sil-

icides, have been used to slow down interdiﬀusion across these interfaces and so

prolong device lifetimes (Lloyd 1999).

Experiments on many metals deposited on semiconductors have investigated these

interactions. As these are performed at elevated temperatures, some means of heating

the semiconductor substrate is needed. Often simple direct current has been used, and

as a result electromigration eﬀects have been uncovered, that is, a directional movement

of material due to a combination of electric potential and current. These eﬀects have

been reviewed by Yasunaga & Natori (1992), and have been found to occur in many

systems since; it is also a topic which probably needs an update in review form.

As we look towards semiconductor devices during the twenty-ﬁrst century, we can

see that the issues aired in chapter 8 and in this section are capturing the imagination

of relevant scientists and technologists (see, for example, Williams 1999). Although

they all agree about the current economic dominance of CMOS Al–Si–SiO

based IC

technology, there are increasing discussions of ‘the end of the roadmap’ or ‘what

happens below 0.1 mm [feature size]?’ or ‘what are the costs of the next generation of

Fabs?’. These concerns center around what became known as Moore’s law, named after

one of the founders of Intel, who noted the exponential decrease of IC feature size as

with time, corresponding roughly to a factor of two every three years over several

decades.

Much of this information is being gleaned by science journalists, who can distill

journal articles, and combine them with interviews of key industrial and academic

players, in language which may be accessible to a wider public. One such article, pub-

lished under the title Failure analysis in the nanometer world, quotes the head of failure

analysis at Texas Instruments, Lawrence Wagner, to the eﬀect that ﬁnding a fault in an

298 9 Postscript – where do we go from here?

IC today is like ‘looking for your lost keys from a helicopter hovering over Los

Angeles’; by the end of the next decade, it is going to be like ‘looking for your keys in

the state of California’ (Ouellette 1998). Thus we may well get the latest (and in this

case some of the best) information from the news and comments sections of profes-

sional and trade journals such as Physics Today, Physics World, The Industrial

Physicist, Materials Research Bulletin, or Vacuum Solutions, from special issues of

magazines such as New Scientist and Scientiﬁc American, or even from major news-

papers.

These articles tend not to be quoted in the archive literature; the thoughts expressed

are not necessarily original, but they are hot. An example is the New Scientist special

report of 7 November 1998: Inventing the future in Silicon Valley (Mullins et al. 1998).

This issue, for example, discusses copper interconnects, and refers to the journal Future

Fab International, which I have yet to locate. Who would have thought that such a title

existed without skipping though these general interest articles? Even in the age of com-

puterized information, such skipping is good exercise. However, in case you envisage a

future weighed down with new subscriptions born out of desperation to keep up with

the latest news, you can in fact relax. All these publications are (along with their adver-

tisers) trying to get your attention, and similar stories will appear in many places.

Another story on plans for copper interconnects and sub-0.2 mm technology in the

major ﬁrms appeared in the March 1999 issue of Semiconductor International; it will

not be the last such article. The advantages of copper, given its reactivity, are hotly dis-

puted (Lloyd 1999); this is another reason to leave the topic out of this book.

9.2 What do the various disciplines bring to the table?

Readers who have stayed with me thus far will have realized that by now I am skating

on thin ice (which is also good exercise until the ice cracks). I am coming to the end of

the book, and also to the limits of my own professional expertise. My own training is

in physics, and my professional interests lie mostly in the physics of materials, and

within that in the subjects aired here. So my opinions outside this area may well not be

professionally sound, and maybe I should keep them to myself in any case. But it is also

interesting to share ideas and see how others react.

Several professions have contributed to the topics discussed in this book. As judged

by the initial qualiﬁcations of the workers cited in the extensive reference list, this

includes mathematics and computer science, physics, chemistry, materials science and

engineering, and various branches of electrical, electronic, mechanical and chemical

engineering. This list has not to date included biology, biochemical and medical sci-

ences and biotechnology, but that is in the process of changing, and will change quickly

in future. One of the most understated success stories over the past century is the impact

that discoveries in physics have had on medical diagnosis and treatment; this achieve-

ment is simply vast, and should be more widely appreciated by the general public. One

doesn’t have to be clairvoyant to foresee a similar impact for micro-engineered surfaces

and thin ﬁlm devices, with chemical and/or biological selectivity and sensitivity,

9.2 What do the various disciplines bring to the table? 299

coupled to computer-based readout and analysis systems (Collins 1999). Does the

range of expertise needed to function in these areas mean that subject specialisms will

disappear in future? I don’t think so. But it does mean that subject specialists are almost

certain to have to work in teams, with a proper appreciation of the other person’s/

subject’s strengths and weaknesses.

The strength of the physicist’s approach is that it looks for overarching themes, such

as gravity, thermodynamics, quantum and statistical mechanics, and relativity, which

have immense generality and thus condition our ways of thinking. Experimental phys-

icists are adept at discovering and developing new instruments, at least in prototype

form, and these can completely change our perception of what is observable and inter-

esting. The most obvious example described here is the development of the various

types of microscope (TEM, FIM, STM, etc., described in chapter 3) and their appli-

cation to the study of kinetic processes described in chapter 5. Physicists are also good

(but not uniquely so) at making simple models of such processes, provided they remem-

ber the quote from Einstein used by Pettifor (1995) ‘as simple as possible, but not

simpler’. Other disciplines are fond of quoting a joke against physicists, asked to make

a model of a racehorse, who start oﬀ ‘imagine a spherical lump of muscle’ and go on

to talk about symmetry breaking, which leads to the production of a head, a tail and

four legs. There are many variants on this theme; such an approach can lead to a rep-

utation for arrogance which is counterproductive.

Chemists see themselves as centrally positioned in science, the guardians of the peri-

odic table, and the makers of new molecules par excellence. Historically this is indeed

the case, and it remains so, though perhaps less celebrated, to the present. In my expe-

rience, a training in chemistry results in the ability to assimilate huge numbers of facts,

often apparently unrelated, and then to try to make sense of them within whichever

model is to hand. Quantum chemistry is a great success on the theoretical side, even if

it must be annoying that sometimes the Nobel prizes for chemistry end up in the hands

of people trained as mathematicians and physicists, as happened in 1998, noted here

in chapters 6 and 7 and in the corresponding appendices J and K.

In parallel, experimental synthetic chemists try out large numbers of combinations

of reactions to produce new molecules, which includes new synthetic materials

and/or drugs, thus overlapping with materials scientists or biochemists, some of

which was described in section 8.4. Coupled with this is an ability to spot the main

chance: to decide an area is going nowhere and to move on to something more pro-

ductive, which is an ability that more introspective scientists sometimes lack. This is

of course not unique to chemists, but a chemists’ training seems to encourage it. Thus

it is perhaps not surprising that chemists, as much as engineers, are key to the pro-

duction of electronic devices by the various forms of CVD, described here brieﬂy in

section 2.5, which is the main technology behind most of the semiconductor devices

discussed in chapters 7 and 8. This technology is very largely empirically based, but

functions continuously on a massive scale, and remains a challenge to analytical

science and to process modeling. Chemists play similar roles in catalysis-based tech-

nologies in the petrochemical and related industry, indicated brieﬂy in sections 2.4

and 4.5.

300 9 Postscript – where do we go from here?

Surface and thin ﬁlm processes is an area where the boundaries between physics,

chemistry, materials science and engineering are particularly transparent, but even here

there are diﬀerent emphases in the choice of topic, and some mismatches in language

which can take time and eﬀort to sort out. The materials scientist and engineer also has

a very broad training, one which has emerged from the traditional disciplines of mining

and metallurgy. Many classes of crystalline and polymeric materials in bulk and thin

ﬁlm form are studied nowadays, of which this book has only discussed elemental

adsorbed gases, crystalline metals and semiconductors, and some ionic compounds in

any detail.

A training in materials science emphasizes materials properties and materials selec-

tion, and the role of microstructure in determining properties, involving the use of

microscopy in all its analytical forms. Such instrumentation is needed to visualize,

analyze and hence even to discuss the development of smaller devices which are now

being developed. However, too much emphasis on microscopy alone can sometimes

obscure the fact that much of our knowledge of the microscopic world has been

achieved by techniques which average over relatively large areas, coupled with a well-

founded model which interprets the data obtained at a microscopic level. This

approach is exempliﬁed by crystal and protein structure determination using X-rays,

and is illustrated here in section 3.2 by surface structure determination using electrons,

especially in conjunction with the corresponding microscopy.

The ﬁelds described in this book furnish good examples of serial versus parallel pro-

cessing. Point by point techniques, such as STM, SEM and STEM are ideally suited to

computer control and TV-like output, whereas the parallel processing diﬀraction-

microscopy techniques (LEED-LEEM, RHEED-REM or THEED-TEM) are in

general much faster, as emphasized in chapter 3. This serial-parallel choice is central

in engineering terms, in manufacturing and inspection, and is a major factor in the rise

of the ‘throw-away’ society. If ﬁnding individual faults in an IC will be like ‘looking for

your keys in California’, then serial processing, and individual repair, techniques are at

a great disadvantage compared to parallel processing of raw materials into new

products.

9.3 What has been left out: future sources of information

It is of course a pity that the ﬁeld of Surface and thin ﬁlm processes will not stay still

once I have ﬁnished this book. Such a thought condemns the writer to wanting to

update the manuscript continually, to hope for further editions in which all mistakes

will be corrected, and generally to wish that everything could be perfect in a manifestly

imperfect world. In the present case I have heaps of reprints, half-formed ideas and

vague thoughts on a wide range of topics from mechanical properties, indentation and

wear, the related ﬁeld of dislocations, quantum conduction in nano-wires, chemical

and bio-sensors, microelectromechanical systems (MEMS, yet another acronym), you

name it. Did I ever think I would get around to writing about these topics, or that I

could master them suﬃciently in a ﬁnite time or space? What a hope!

9.3 What has been left out: future sources of information 301

Sensors are an interesting case in point, since I mentioned them in the preface: it is

therefore reasonable that you would expect me to write about them in the body of the

text. These thin ﬁlm, and maybe surface-related devices, have spawned a huge litera-

ture. But the near-impossibility of getting inside it in a ﬁnite time ﬁnally got to me, after

reading a review of chemical sensors by the experts in the ﬁeld (Janata et al. 1994). This

review lists and says a few words about each of some 795 publications, notes the rapid

year on year growth of this number, and ends with a pretty justiﬁed complaint about

publishing practices in the ﬁeld, with a quote adapted from Marvin Minsky, one of the

founders of artiﬁcial intelligence and neural networks: “‘Look’, they say, ‘it did this’.

But they don’t consider it equally wonderful to say, ‘Look, it can’t do that.’”As a result,

there are lots of publications in this ﬁeld, but there are not quite so many working

devices.

Their more recent update on the same topic (Janata et al. 1998), gives the same treat-

ment to a further 929 publications, and reaches a similar though more positive conclu-

sion. Indeed, as I wrote this section, the June 1999 issue of the MRS Bulletin arrived

in the mail, containing a series of articles on Gas-Sensing Materials (Watson & Ihokura

1999); this re-emphasizes the importance of belonging to the appropriate professional

organization and scanning their journals. There is a lot of material in these reviews, but

I know it would be next to impossible to ﬁnd the full story simply by reading; I am a

long way oﬀ writing anything useful.

There have to be other, probably more attractive, visions of the future than adding

yet more words; I should quit while I’m ahead. The web oﬀers one such vision which

is deﬁnitely worth exploring; and it is already an amazing resource. Several of the ref-

erences cited in this book have been found from my oﬃce using the excellent

WebofScience™. At the same time one can, allowing for errors and omissions, ﬁnd out

the apparent impact of papers, and whether there are more recent reviews on similar

subjects. Just as putting my class notes up on the web originated this book, so the web

oﬀers a possible way of updating, or otherwise commenting on it, and of developing

related teaching activities and projects.

Most of the appendices which follow this chapter are of the type you would nor-

mally expect in such a book. Appendix D, on the other hand, is an introduction and

link to a series of web-based resources which will be updated, and can be used as a sup-

plement to the book in whatever way seems reasonable to the reader. Some of the other

appendices refer to web-based resources which can be accessed via Appendix D. In par-

ticular, I think it will be possible to use student projects, with their permission, to

develop both background material, and to explore further topics both with students

and colleagues, and present them in the form of web-pages for all to beneﬁt. In an ideal

world, this could be done collaboratively between co-workers in diﬀerent ﬁelds, insti-

tutions and countries. A start has been made here, and I am grateful to colleagues for

permission to include links to their web-based material; it will be interesting to see

‘where we go from here’.

Note added in proof: Dr Janata visited ASU in February 2000 to give a seminar, so we were able to discuss

these issues in person. The current publication rate in the sensor ﬁeld is about 1000 papers per year.

302 9 Postscript – where do we go from here?

Appendix A

Bibliography

The following references are mainly textbooks, which can be used for obtaining back-

ground material and/or extended coverage of the material in this book. The sections

where these sources are referred to are indicated in square brackets [*.* ].

Adamson, A.W. (1990) Physical Chemistry of Surfaces (John Wiley, 5th Edn) [1.1, 1.2].

Ashcroft, N.W. & N.D. Mermin (1976) Solid State Physics (Saunders College) [6.1, 6.3,

7.1].

Blakely, J.W. (1973) Introduction to the Properties of Crystal Surfaces (Pergamon) [1.1].

Briggs, D. & M.P. Seah (1990) Practical Surface Analysis, vols. I and II (John Wiley) [3.3].

Bruch, L.W., M. W. Cole & E. Zaremba (1997) Physical Adsorption: Forces and

Phenomena (Oxford University Press) [4.2, 4.4].

Buseck, P., J.M. Cowley & L. Eyring (Eds.) (1988) High Resolution Transmission

Electron Microscopy and Associated Techniques (Oxford University Press) [3.1].

Chen, C.J. (1993) Introduction to Scanning Tunneling Microscopy (Oxford University

Press) [3.1].

Clarke, L.J. (1985) Surface Crystallography: an Introduction to Low Energy Electron

Diﬀraction (John Wiley) [1.4, 3.2].

Craik, D. (1995) Magnetism: Principles and Applications (John Wiley) [6.3].

Davies, J.H. (1998) The Physics of Low-dimensional Semiconductors: an Introduction

(Cambridge University Press) [7.1, 8.2].

Desjonquères, M.C. & D. Spanjaard (1996) Concepts in Surface Physics (Springer) [1.1,

1.3, 4.5, 6.1, 7.1, Appendix K].

Dushman, S. & J. Laﬀerty (1992) Scientiﬁc Foundations of Vacuum Technique (John

Wiley); this updates the 1962 book with the same title by S. Dushman [2.1, 2.3,

Appendix G].

Feldman, L.C. & J.W. Mayer (1986) Fundamentals of Surface and Thin Film Analysis

(North-Holland) [3.1, 3.4].

Ferry, D.K. & S.M. Goodnick (1997) Transport in Nanostructures (Cambridge

University Press) [8.3].

Gibbs, J.W. (1928, 1948, 1957) Collected Works, vol. 1 (Yale Univerity Press, New

Haven); reproduced as (1961) The Scientiﬁc Papers, vol. 1 (Dover Reprint Series,

New York) [1.1].

Gil, B. (Ed.) (1998) Group III Nitride Semiconductor Compounds (Oxford University

Press) [7.3].

303

Glocker, D.A. & S.I. Shah (Eds) (1995) Handbook of Thin Film Process Technology

(Institute of Physics), parts A and B [2.3, 2.5].

Henrich, V.E. & P.A. Cox (1994, 1996) The Surface Science of Metal Oxides

(Cambridge University Press) [1.4, 4.5].

Hill, T.L. (1960) An Introduction to Statistical Thermodynamics (Addison-Wesley,

reprinted by Dover 1986), especially chapters 7–9, 15 and 16 [1.3, 4.2, Appendix E].

Hudson, J.B. (1992) Surface Science: an Introduction (Butterworth-Heinemann);

reprinted in 1998 and published by John Wiley [1.1, 1.2, 2.1, 4.5].

Jaros, M. (1989) Physics and Applications of Semiconductor Microstructures (Oxford

University Press) [8.2].

Jiles, D. (1991) Magnetism and Magnetic Materials (Chapman and Hall) [6.3].

Kelly, A. & G.W. Groves (1970) Crystallography and Crystal Defects (Longman) [1.3–4].

Kelly, M.J. (1995) Low-dimensional Semiconductors (Oxford University Press) [7.1,

8.2].

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Layers (The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis vol 8,

Elsevier) [5.1].

Kittel, C. (1976) Introduction to Solid State Physics (6th Edn, John Wiley) [6.3].

Liu, W.K. & M.B. Santos (Eds.) (1998) Thin Films: Heteroepitaxial Systems (World

Scientiﬁc) [5.1, 7.3].

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[1.4, 2.2–3, 3.2–3, 7.2, 8.1–2].

Masel, R.I. (1996) Principles of Adsorption and Reaction on Solid Surfaces (John Wiley)

[4.5].

Matthews, J.W. (Ed.) (1975) Epitaxial Growth, part A (Academic) [2.5]; part B

(Academic) [5.1].

Mönch, W. (1993) Semiconductor Surfaces and Interfaces (Springer) [7.2, 8.2].

Moore, J.H., C.C. Davis & M.A. Coplan (1989) Building Scientiﬁc Apparatus (2nd Edn,

Addison-Wesley) [2.3, 3.3].

O’Hanlon, J.F. (1989) A Users Guide to Vacuum Technology (John Wiley) [2.3,

Appendix G].

Pettifor, D.G. (1995) Bonding and Structure of Molecules and Solids (Oxford University

Press) [6.1, 7.1, 9.2, Appendix K].

Prutton, M. (1994) Introduction to Surface Physics (Oxford University Press) [1.4,

3.2–3, 3.5].

Rivière, J.C. (1990) Surface Analytical Techniques (Oxford University Press) [3.1, 3.3].

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Roth, A. (1990) Vacuum Technology (3rd Edn, North-Holland) [2.1–3, Appendix G].

Schroder, D.K. (1998) Semiconductor Material and Device Characterization (John

Wiley) [8.1].

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Smith, G.C. (1994) Surface Analysis by Electron Spectroscopy (Plenum) [3.3–4].

Stroscio, J. & E. Kaiser (Eds.) (1993) Scanning Tunneling Microscopy (Methods of

Experimental Physics, Academic) vol. 27 [3.1].

304 Appendix A

Sutton, A.P. (1994) Electronic Structure of Materials (Oxford University Press) [6.1,

7.1].

Sutton, A.P. & R.W. Balluﬃ (1995) Interfaces in Crystalline Materials (Oxford

University Press) [1.2, 6.1, 7.1, 8.2].

Sze, S.M. (1981) Physics of Semiconductor Devices (2nd Edn, John Wiley) [8.1].

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(Plenum NATO ASI) B360 [5.1].

Tsao, J.Y. (1993) Materials Fundamentals of Molecular Beam Epitaxy (Academic) [2.5,

7.3].

Walls, J.M. (Ed.) (1990) Methods of Surface Analysis (Cambridge University Press)

[3.1].

Wiesendanger, R. (1994) Scanning Probe Microscopy and Spectroscopy (Cambridge

University Press) [3.1, 7.2].

Woodruﬀ, D.P. & T.A. Delchar (1986, 1994) Modern Techniques of Surface Science

(Cambridge University Press) [3.2, 6.1].

Yu, P.Y. & M. Cardona (1996) Fundamentals of Semiconductors: Physics and Materials

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7.1–2].

Bibliography 305

Appendix B

List of acronyms

The following list gives the names of the acronyms used; the sections where they are

ﬁrst or most relevantly introduced are given in brackets.

0D, 1D, 2D, 3D: Zero, one, two or three (dimensions or dimensional) [throughout] as

in 2DEG: two dimensional electron gas [8.2.3]

AES: Auger electron spectroscopy [3.3, 3.4]

AFM: atomic force microscopy [3.1.3]

ALE: atomic layer epitaxy [1.4.4]

APCVD: atmospheric pressure CVD [2.5.5]

APW: augmented plane waves (band structure method) [6.1.2]

AR: angular resolved, as in ARUPS [3.3.1]

BCF: Burton, Cabrera & Frank (see reference list for this 1951 paper) [1.3.2, 5.5.1]

BEEM, BEES: ballistic energy emission microscopy, spectroscopy [8.1.3]

CBED, CBIM: convergent beam electron diﬀraction [3.1.4, 8.1.3], imaging [8.1.3]

CERN: Centre Européenne pour la Recherche Nucléaire (accelerator laboratory)

[2.1.3]

CFE: cold ﬁeld emisison [6.2.3]

C–I: commensurate–incommensurate (phase or transition) [4.4.2]

CAP, CIP, and CPP: Current at an Angle, In or Perpendicular to the Plane [8.3.3]

CMOS: complementary metal–oxide–semiconductor (device) [8.1, 9.1]

C–V: current–voltage, as in C–V proﬁling [8.2.3]

CVD: chemical vapor deposition [2.5.5]

DAS: dimer–adatom–stacking fault model [1.4.5, 7.2.3]

DFT: density functional theory [4.5.2, 6.1.2, Appendix J]

DLC: diamond-like carbon [6.2.2, 8.4.3]

DLEED: diﬀuse LEED [3.2.2]

DLTS: deep level transient spectroscopy [8.1.3]

DNA: deoxyribo-nucleic acid [8.4.4]

EAM: embedded atom model [4.5.2, 6.1.2]

EELS: electron energy loss spectroscopy [3.3.1]

EMT: eﬀective medium theory [4.5.2, 6.1.2]

ES: the Ehrlich–Schwoebel barrier [5.5.1]

ESCA: electron spectroscopy for chemical analysis [3.3.1]

306