Smart electronic materials  fundamentals and applications

Thu vi$n - 0H Quy Nhori C a m b r id g e SMART ELECTRONIC MATERIALS Smart materials respond rapidly to external stimuli to alter their physical properties They are used in devices that are driving advances in modem information technol­ ogy and have applications in electronics, optoelectronics, sensors, memories and other areas This book fully explains the physical properties of these materials, including semiconductors, dielectrics, ferroelectrics, and ferromagnetics Fundamental con­ cepts are consistently connected to their real-world applications It covers structural issues, electronic properties, transport properties, polarization-related properties, and magnetic properties o f a wide range of smart materials The book contains carefully chosen worked examples to convey important con­ cepts and has many end-of-chapter problems It is written for first year graduate students in electrical engineering, material sciences, or applied physics programs It is also an invaluable book for engineers working in industry or research laboratories A solution manual and a set of useful viewgraphs are also available for instructors by visiting http://www.cambridge.org/ 0521850274 S I N G H obtained his Ph.D in Solid State Physics from the University of Chicago He is currently a professor in the Applied Physics Program and in the Department of Electronic and Computer Science at the University of Michigan, Ann Arbor He has held visiting positions at the University o f California in Santa Barbara He has authored over 250 technical articles He has also authored eight textbooks in the area of applied physics and technology His area of expertise is novel materials for applications in intelligent devices jasprit SMART ELECTRONIC MATERIALS Fundamentals and Applications JASPRIT SINGH University of Michigan TRƯỜNG ĐẠI HỌC QUV NHƠN THƯ VIỆN &sữ/ A ' 'W S3 C A M B R ID G E UNIVERSITY PRESS CAMBRIDGE UNIVERSITY PRESS University Printing House, Cambridge CB2 8BS, United Kingdom Cambridge University Press is part of the University of Cambridge it furthers the University's mission by disseminating knowledge in the pursuit of education, learning and research at the highest international levels of excellence www.cambridge.org information on this title: www.cambridge.org/9780521850278 © Cambridge University Press 2005 This publication is in copyright Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press First published 2005 A catalogue record fo r this publication is available from the British Library ISBN 978-0-521-85027-8 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites Is, or will remain, accurate or appropriate CONTENTS PR EFACE page xi INTRO DUCTIO N *'» S mart materials: an introduction XIII Input - output decision ability xiv xiv 2.1 Device based on conductivity changes 2.2 Device based on changes in optical response XV B iological systems: nature's smart materials xix R ole of this book xxii STRUCTURAL PROPERTIES 1.1 Introduction 1.2 C rystaline materials 1.2.1 Basic lattice types 1.2.2 Some important crystal structures 1.2.3 Notation to denote planes and points in a lattice: Miller indices 1.2.4 Artificial structures: superlattices and quantum wells 1.2.5 Surfaces: ideal versus real 1.2.6 Interfaces 12 16 17 19 1.3 D efects in crystals 20 1.4 H eterostructures 23 1.5 N on-crystalline materials 24 25 26 27 31 1.5.1 1.5.2 1.5.3 1.5.4 1.6 Polycrystalline materials Amorphous and glassy materials Liquid crystals Organic materials S ummary 31 Contents 1.7 Problems 1.8 Further reading 33 37 QUANTUM MECHANICS AND ELECTRONIC LEVELS 2.1 Introduction 2.2 N eed for quantum description 2.2.1 Some experiments that ushered in the quantum age 2.3 S chrodinger equation and physical observables 2.3.1 Wave amplitude 2.3.2 Waves, wavepackets, and uncertainty 39 40 40 48 52 54 2.4.1 Electronic levels in a hydrogen atom 2.4.2 Particle in a quantum well 2.4.3 Harmonic oscillator problem 57 58 62 67 2.5 From atoms to molecules: coupled wells 69 2.6 Electrons in crystalline solids 2.6.1 Electrons in a uniform potential 2.6.2 Particle in a periodic potential: Bloch theorem 2.6.3 Kronig-Penney model for bandstructure 77 80 85 87 2.7 S ummary 93 2.8 Problems 93 2.9 Further reading 99 2.4 Particles in an attractive potential: bound states ELECTRONIC LEVELS IN SOLIDS 100 3.1 Introduction 100 3.2 Occupation of states: distribution function 100 3.3 Metals, insulators, and superconductors 3.3.1 Holes in semiconductors 3.3.2 Bands in organic and molecular semiconductors 3.3.3 Normal and superconducting states 104 104 107 108 B andstructure of some important semiconductors 110 3.4.1 Direct and indirect semiconductors: effective mass 111 3.4 vil Contents 3.5.1 Electrons in metals 3.5.2 Mobile carriers in pure semiconductors 116 117 120 3.6 D oping of semiconductors 126 3.7 T ailoring electronic properties 131 131 132 3.5 M obile carriers 3.7.1 Electronic properties of alloys 3.7.2 Electronic properties of quantum wells 3.8.1 Disordered materials: extended and localized states 136 138 3.9 S ummary 141 3.10 Problems 141 3.11 Further reading 146 3.8 L ocalized states in solids CHARGE TRANSPORT IN MATERIALS 148 4.1 Introduction 148 4.2 A n overview of electronic states 149 4.3 Transport and scattering 151 154 4.3.1 Scattering of electrons 4.4 M acroscopic transport properties 4.4.1 Velocity-electric field relations in semiconductors 4.5 C arrier transport by diffusion 4.5.1 Transport by drift and diffusion: Einstein's relation 4.6 Important devices based on conductivity changes 4.6.1 Field effect transistor 4.6.2 Bipolar junction devices 4.7 T ransport in non-crystalline materials 4.7.1 Electron and hole transport in disordered systems 4.7.2 Ionic conduction 162 162 173 175 178 179 184 186 187 191 4.8.1 Thin film transistor 4.8.2 Gas sensors 193 193 195 4.9 S ummary 195 4.10 Problems 199 4.11 F urther reading 200 4.8 Important non-crystalline electronic devices Contents LIGHT ABSORPTION AND EMISSION 202 5.1 Introduction 202 5.2 Important material systems 204 Optical processes in semiconductors 5.3.1 Optical absorption and emission 5.3.2 Chargei injection, quasi-Fermi levels, and recombination 5.3.3 Optical absorption, loss, and gain 2^7 210 219 225 5.4 Optical processes in quantum wells 226 5.5 Important semiconductor optoelectronic devices 231 231 238 243 5.3 5.5.1 Light detectors and solar cells 5.5.2 Light emitting diode 5.5.3 Laser diode 5.6 5.6.1 Excitonic state 251 252 5.7 Summary 255 5.8 Problems 255 5.9 Further reading 262 Organic semiconductors: optical processes & devices DIELECTRIC RESPONSE: POLARIZATION EFFECTS 264 6.1 Introduction 264 6.2 Polarization in materials: dielectric response 6.2.1 Dielectric response: some definitions 265 265 6.3 Ferroelectric dielectric response 273 6.4 T ailoring polarization: piezoelectric effect 275 6.5 Tailoring polarization: pyroelectric effect 285 6.6 D evice applications of polar materials 6.6.1 Ferroelectric memory 6.6.2 Strain sensor and accelerometer 287 287 288 6.6.3 Ultrasound generation 6.6.4 Infrared detection using pyroelectric devices 289 289 IX Contents 6.7 S ummary 291 6.8 Problems 291 6.9 F urther readin g 295 OPTICAL MODULATION AND SWITCHING 296 7.1 Introduction 296 7.2 L ight propagation in materials 297 7.3 M odulation of optical properties 302 303 309 7.3.1 Electro-optic effect 7.3.2 Electro-absorption modulation 7.4.1 Electro-optic modulators 7.4.2 Interferroelectric modulators 312 316 318 7.5 S ummary 323 7.6 Problems 325 7.7 F urther reading 325 7.4 Optical modulation devices MAGNETIC EFFECTS IN SOLIDS 326 8.1 Introduction 326 8.2 M agnetic materials 326 8.3 Electromagnetic field magnetic materials 327 8.4 P hysical basis for magnetic properties 331 8.5 Coherent transport: quantum interference 335 335 338 8.5.1 Aharonov Bohm effect 8.5.2 Quantum interference in superconducting materials 8.6 D iamagnetic and paramagnetic effects 8.6.1 Diamagnetic effect 8.6.2 Paramagnetic effect 8.6.3 Paramagnetism in the conduction electrons in metals 340 340 341 345 Contents X 8.7 Ferromagnetic effects 8.7.1 Exchange interaction and ferromagnetism 8.7.2 Antiferromagnetic ordering 8.8 A pplications in magnetic devices 8.8.1 Quantum interference devices w > 8.8.2 8.8.3 8.8.4 8.8.5 8.9 Summary 8.10 Problems 8.11 Further reading 352 352 354 355 357 359 359 359 362 IMPORTANT PROPERTIES OF SEMICONDUCTORS 363 P-N DIODE: A SUMMARY 368 B.1 B.2 tn tjn ^ Application example: cooling by demagnetization Magneto-optic modulators Application example: magnetic recording Giant magnetic resistance (GMR) devices 346 346 348 Introduction P-N junction B.2.1 P-N Junction under bias 368 372 FERMI GOLDEN RULE 380 LATTICE VIBRATIONS AND PHONONS 386 DEFECT SCATTERING AND MOBILITY 383 E.1 E.2 E.3 E.4 A lloy scattering Screened Coulombic scattering Ionized impurity limited mobility Alloy scattering limited mobility INDEX u 404 E 397 S c r e e n e d C o u lo m b ic s c a tte r in g r(a rb itra ry units) > F igu re E.2: Comparison of screened and unscreened Coulomb potentials of a unit positive charge as seen by an electron The screening length is A- W hen the free carrier density is high so that the carriers are degenerate, A2 = (E.16) 2c E f where E f is the Fermi energy As noted, the effect of screening is to reduce the range of the potential from a /r variation to a ex p (—\ r ) / r variation This is an extrem ely im portant effect and is shown schem atically in Fig E.2 We now calculate the m atrix element for the screened Coulombic potential U (r) = - -v 47re r (E.17) where Z e is the charge of the impurity We choose the initial normalized state to be Ik) = ex p (ik • r ) / W and the final state to be |k') = exp(ik' • v ) / y / V , where V is the volume of the crystal The m atrix element is then = fc Z t f e- I (k - k l r - - r 2d r sin O'dd'd' i rc V J r Carrying out the (f> integration which gives a factor of n , we have rpr ^kk' kk fOO / pi = ~T~TF^ I = Z e * gT Ai reV |k' - k | + A2 AirtV J0 r dr I d ( c o s V Are ~ i | k ~ k|rCOS* / 398 D e fe c t s c a tte r in g a n d m o b ility F igu re E.3: As a consequence of the elastic scattering, there is a simple relation between the magnitude of the scattered wavevector and the scattering angle We note that |k ;| = |k| since the scattering is elastic Then, as can be seen from Fig E.3, | k - k ' | = 2A sin (0/2) (E 18) where is the polar scattering angle Z e ^kk' - V c k sin 2(0 /2 ) 4- A2 T he scattering rate is unow by u g j v M 5given » v u h \ V e ) “ r r