Luminescence and the Light Emitting Diode. The Basics and Technology of LEDS and the Luminescence Properties of the Materials

245 Pages·1978·4.2 MB·English

by  E. W. Williams, R. Hall and B.R. Pamplin (Auth.)

Other Titles of Interest
Other Titles in the International Series in
The Science of the Solid State (Editor B. R. Pamplin)
Vol. 1. GREENAWAY & HARBEKE: Optical Properties and Band
Structures of Semiconductors
Vol.2. RAY: II-VI Compounds
Vol. 3. NAG: Theory of Electrical Transport in Semiconductors
Vol.4. JARZEBSKI: Oxide Semiconductors
Vol. 5. SHARMA and PUROHIT: Semiconductor Heterojunctions
Vol. 6. PAMPLIN (editor): Crystal Growth*
Vol. 7. SHAY and WERNICK: Ternary Chalcopyrite Semiconductors:
Growth, Electronic Properties and Applications
Vol. 8. BASSANI and PASTORI PARRAVICINI: Electronic States and
Optical Transitions in Solids
Vol. 9. SUCHET: Electrical Conduction in Solid Materials (Physico-
chemical Bases and Possible Applications)
Vol.10. TANNER: X-Ray Diffraction Topography
Vol. 11. ROY: Tunnelling and Negative Resistance Phenomena in
Semiconductors
An Important new review journal**
Progress in Crystal Growth and Characterization
Editor-in-Chief B. R. PAMPLIN
*Now available in flexicover
**Free specimen copy available on request
LUMINESCENCE AND THE
LIGHT EMITTING DIODE
The Basics and Technology of LEDS and the Luminescence
Properties of the Materials
by
E.W. WILLIAMS
I.C.I., Corporate Laboratory, Runcorn, Cheshire
and
R. HALL
Thorn Lighting Limited, Leicester
PERGAMON PRESS
OXFORD NEW YORK TORONTO SYDNEY PARIS FRANKFURT
U.K. Pergamon Press Ltd., Headington Hill Hall,
Oxford OX3 OBW, England
U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park,
Elmsford, New York 10523, U.S.A.
CANADA Pergamon of Canada Ltd., 75 The East Mall,
Toronto, Ontario, Canada
AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street,
Rushcutters Bay, N.S.W. 2011, Australia
FRANCE Pergamon Press SARL, 24 rue des Ecoles,
75240 Paris, Cedex 05, France
FEDERAL REPUBLIC Pergamon Press GmbH, 6242 Kronberg-Taunus,
OF GERMANY Pferdstrasse 1, Federal Republic of Germany
Copyright © 1978 E. W. Williams and R. Hall
All Rights Reserved. No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means: electronic, electrostatic,
magnetic tape, mechanical, photocopying, recording or otherwise, without
permission in writing from the copyright holders
First edition 1978
Library of Congress Cataloging in Publication Data
Williams, E. W.
Luminescence and the Light Emitting Diode
Includes bibliographical references.
1. Light emitting diodes. 2. Luminescence.
I. Hall, R., joint author. II. Title.
TK7871.89.L53W54 1977 621.3815'22 77-4427
ISBN 0-08-020442-2 (Hardcover)
ISBN 0-08-020441-4 (Flexicover)
In order to make this volume available as economically and as rapidly as possible the
authors' typescripts have been reproduced in their original forms. This method
unfortunately has its typographical limitations but it is hoped that they in no way
distract the reader.
Printed in Great Britain by Cox & Wyman Ltd, Fakenham
EDITORS' PREFACE
The light-emitting diode (LED) is now an electronic component in everyday
use in pocket calculators and other alphanumeric displays and indicator lamps
on stereo equipment and computers. This book provides a timely discussion of
the basic physics and solid state science behind these devices. Both the
authors have seen them develop from an idea to fulfilment but in different
ways. Ted Williams has worked for many years, and on both sides of the
Atlantic, on absorption and emission of light from semiconductors. Bob Hall,
on the other hand, is a scientist who specialised in the production of light
emitting solid state devices. They form an excellent matched pair for the
production of this book which shines light on optical processes in semi-
conductors and the useful devices which can be made.
When I left Cambridge in 1957 and joined a leading British electronics company,
my section leader explained the working of a solar cell. He handed me a
silicon slice 2 centimetres square which was the latest solar cell available
and said: "This process must be reversible - try and produce the inverse
effect and generate light from a pn junction." Three months later, after
searching the literature for the causes of my failure, I reported that it
might be possible when dislocation free silicon becomes available. It was not
until the development of the III-V compounds in the early sixties that the
path was clear for the first real light emitting diode.
Now as a component of more and more displays, the visible LED clearly has an
established future. The exciting promise of optical or infra red communi-
cations systems using LED or laser sources and photodiode detectors is going
to play a part in the next phase of the electronic revolution. It is foreseen
that the growth of communications will continue to expand at a rapid rate,
with optical fibres soon bringing the video telephone, television, information
and access to computers into the home.
This book provides a very readable account of the basic physics and technology
of LEDs and pn junction lasers and of the materials behind them.
Brian Randall Pamplin,
Scientific Advisers & Co.,
15 Park Lane, Bath, England.
ix
AUTHORS' PREFACE
The advent of the pocket calculator and the digital watch has ensured the
place of the LED display in electronic-device history. It was therefore felt
timely to bring out a student monograph on the LED. Although this book was
written as a course book for third-year graduate and post-graduate students it
is hoped that it will also be helpful to a much wider range of readers who
would like to know more about the LED and the material from which it is made.
The book begins with an introduction to the crystal structure and growth, and
the optical and electrical properties of LED materials. Following this
introduction in the first three chapters, the detailed fabrication of the LED
is given in Chapter 4 and this should prove useful both for student projects
and for non-graduate engineers with an interest in the LED. After this is
Chapter 5 the luminescence of the material and the diode light emission is
considered from a simplified theoretical point of view. The sixth chapter is
a very brief review of solid-state lasers made from LED materials.
The last four chapters in the book should prove useful for both student
projects and laboratory experiments. Chapter 7 describes the equipment used
to measure luminescence, cathodoluminescence and the diode electro-optic
characteristics. "Luminescence in LED materials" is the subject of Chapter 8
and this concentrates on the three most important LED materials at the present
time: GaP, GaAsi- P and GaAs. Other LED materials such as SiC and the newer
x x
ternary semiconducting compounds are surveyed in Chapter 9. Also included in
this chapter is detailed information on commercially available LEDs. The book
closes with a brief chapter on "Applications" which gives a few ideas for
student projects and should also give the electronic amateur a taste for some
of the novel uses that this versatile device can be put to.
Finally, the authors would like to acknowledge the guidance of
Dr. B.R. Pamplin and the many helpful discussions with their colleagues at
Thorn Lighting and RSRE. The typing of Chris Steven and copying and drawing
office facilities at ICI Corporate and Mond Research Laboratories was very
much appreciated. This book would never have been written without the
patience of our wives Margaret and Diane during the many hours we were absent
and so the book is dedicated to them.
This book is published with the permission of Thorn Lighting and ICI Corporate
Laboratory.
XI
1 INTRODUCTION
The light-emitting diode (LED) display became a success when the pocket-
calculator boom began in the early 1970s. It is still the choice of the
majority of calculator, digital watch, and electronic instrument makers
because it is such a reliable display in comparison to the other types that
are presently available.
Some of the other reasons why the LED is so popular are:
1. Long life
2. Compatible with integrated circuits
3. Small size and weight
4. Ruggedness
5. Multi-colour displays and tailored wavelength of light emission are
possible
6. Good temperature stability
7. Fast switching times
8. Cold light - no heating
9. Low noise optical switches are possible when the LED is combined with a
silicon photodetector
10. Low drive voltage makes solar-cell-powered displays an attractive
prospect.
Behind the success story of the LED lie many hard years of research into the
preparation and properties of semiconductors. One of the key factors in this
research has been the use of luminescence to study the semiconducting
material from which the LED is made at every stage of the process. With the
assistance of luminescence, semiconducting compounds and alloys have been
characterized so that the impurities and defects and their densities can be
identified. As a direct result of this the materials preparation techniques
were improved to such an extent that an efficient LED could be made with
light emission in either the infra-red or visible.
Photoluminescence measurements also provided very strong evidence that the
emission in these efficient LED devices occurred on the p-side of the junction.
1
2
Luminescence and the LED
Figure 1.1. clearly shows this for a diffused GaAs diode (Ref. 1) at 77 K.
The photoluminescence of the p-region of the diode almost exactly matches the
diode emission spectrum whereas the photoluminescence from the n-side is well
displaced to higher energies.
H h H h SLIT WIDTH
2 IOO
80l·-
PHOTO -
60
LUMINESCENCE
n-SIDE
GaAs
I 40 ' np+ DIODE
EMISSION
1-42 1-4 6 1-48 ISO
PHOTON ENERGY (eV)
Fig. 1.1. A comparison of photoluminescence at 77 K from
the n and the p+ side of a np+ diode with the electro-
luminescence from the LED provides strong evidence that
the light emission occurs on the p-side of the forward-
biased diode. (After Carr and Biard (Ref. 1)).
This meant that to get the maximum amount of light out and hence optimise the
power efficiency or the ratio of light out to electrical power in, the LED had
to be mounted with the p-side uppermost.
The LED materials that are judged to be the most useful fall into three
classes:
(a) III-V binary compounds like GaAs and GaP.
(b) III-V ternary alloys like GaAsP formed by alloying GaAs with GaP.
(c) Ternary compounds that have similar properties to the binary compounds.
For example, Cu In Se2«
This book will be mainly restricted to discussing the properties of materials
and devices that fall into these three categories. There are others which
fall outside these guidelines but their properties are much less understood
and they will only be referred to briefly.
All of the three types of semiconductor are derived from the "family tree" of
elements that is shown in Fig. 1.2. The parents in the "family tree" are the
Introduction 3
Group I H m W Y "SI YD.
Fig. 1.2. The family tree of elements from which semi-
conductors are derived.
group IV elements of the periodic table. Silicon is now by far the most
important of these. In solid silicon only the four outer electrons bound to
the nucleus take part in the bonding. These electrons are referred to as the
valence electrons.
Goodman (Ref. 2) showed that it was possible to use this valence rule of four
valence electrons per atom to predict possible new semiconductors by the
process of cross-substitution. The binary and ternary compounds of the three
types discussed above, can all be derived in this way. Gallium arsenide is a
cross-substitutional derivative of germanium obtained by substituting gallium
and arsenic for two germanium atoms. Similarly ternaries can be derived from
binaries and so on. Figure 1.3. illustrates the process of deriving the stable
ternary compound CuGaTe2- Unfortunately not all ternary compounds derived in
this way are stable. This technique has been more recently modified by
Pamplin (Ref. 3) to include vacant lattice sites in the "four electron per
site rule". Large numbers of new ternary and quaternary adamentine compound
semiconductors can now be predicted with more certainty.
Before we get carried away in our speculative search for new semiconductors
let us return to a brief look at the history of III-V materials. According
to Hilsum and Rose-Innes (Ref. 4), the paper by Thiel and Koelsch in 1910
(Ref. 5) which reports the preparation of InP was the first to report on a
III-V compound. It then took 42 years before Welker (Ref. 6) stressed the
special semiconducting properties of this group of compounds in 1952. LEDs
were first reported by Wolff et al. (Ref. 7) and Braunstein (Ref. 8) in 1955.
However, it was not until the laser properties of these materials was
discovered in 1962 (Refs. 9-12) that interest in the LED really began. It
still took almost 10 more years of research before the LED became a commercial
success.
4 Luminescence and the LED
Now red LEDs can be seen everywhere. In pocket calculators, watches,
instruments of all types, displays and indicator lamps in a wide variety of
applications. Green, amber and yellow lamps have been improved recently and
may soon be as common as the red ones. However, blue ones have so far proved
very inefficient and a lot more research will have to be done before we can
have all the primary colours and the possibility of a full colour display.
CROSS - SUBSTITUTION
TOTAL VALENCE
NO ELECTRONS
VALENCE PER
ELECTRONS ATOM
16 4 4 Si W Si
16 2Cd 2 Te IT-YI CdTc
I 6 4 Cu Ga Te I - m-YI CuGaTe,
2 2
Fig. 1.3. The process of cross-substitution for
deriving binary and ternary semiconducting compounds.
Introduction 5
REFERENCES
1. W. N. Carr and J. R. Biard, J. Appl. Phys. 35, 277 7(1964).
2. C. H. L. Goodman, J. Phys. Chem. Solids, 6_, 30 5(1958) .
3. B. R. Pamplin, J. Phys. Chem. Solids, 25, 675 (1964).
B. R. Pamplin, J. de Physique, C3.53 (1975).
4. C. Hilsum and A. C. Rose-Innes, Semiconducting III-V Compounds,
Pergamon Press, p.l (1961).
5. A. Thiel and H. Koelsch, Z. anorg. Chem. 65-66, 28 8(1910).
6. H. Welker, Z. Naturforsch. 11, 744 (1952).
7. G. A. Wolff, R. A. Herbert and J. D. Broder, Phys. Rev. 100, 114 4(1955).
8. R. Braunstein, Phys. Rev. 99, 1892 (1955).
9. D. N. Nasledov, A. A. Rogachev, S. M. Ryvkin and B. V. Tsarenkov,
Soviet Phys. - Solid State, _4, 782 (1962) translated from Fiz. Tverd.
Tela, 4_, 1062 (1962) .
10. T. M. Quist, R. J. Keyes, W. E. Krag, B. Lax, A. L. McWhorter,
R. H. Rediker and H. J. Zeiger, Appl. Phys. Letters, 1_, 9 1(1962).
11. M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill and G. Lasher,
Appl. Phys. Letters, _1, 62 (1962).
12. R. N. Hall, G. E. Fenner, J. D. Kingsley, R. J. Soltys and
R. 0. Carlson, Phys. Rev. Letters, 9_, 366 (1962) .