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Leonard J. Brillson

Surfaces and Interfaces ofElectronic Materials

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Leonard J. Brillson

Surfaces and Interfaces ofElectronic Materials

The Author

Prof. Leonard J. BrillsonOhio State UniversityElectrical & Computer Engineeringand PhysicsColumbus, [email protected]

All books published by Wiley-VCH arecarefully produced. Nevertheless, authors,editors, and publisher do not warrant theinformation contained in these books,including this book, to be free of errors.Readers are advised to keep in mind thatstatements, data, illustrations, proceduraldetails or other items may inadvertently beinaccurate.

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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim

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Printed in the Federal Republic of GermanyPrinted on acid-free paper

ISBN: 978-3-527-40915-0

V

Contents

Preface XVII

1 Introduction 11.1 Surface and Interfaces in Everyday Life 11.2 Surfaces and Interfaces in Electronics Technology 2

Problems 7References 8

2 Historical Background 92.1 Contact Electrification and the Development of Solid-State

Concepts 92.2 High-Purity Semiconductor Crystals 102.3 Development of the Transistor 102.4 The Surface Science Era 122.5 Advances in Crystal Growth Techniques 132.6 Future Electronics 15


3 Electrical Measurements 193.1 Schottky Barrier Overview 193.2 Ideal Schottky Barriers 203.3 Real Schottky Barriers 223.4 Schottky Barrier Height Measurements 253.4.1 Current–Voltage (J–V) Technique 253.4.2 Capacitance–Voltage (C–V) Technique 283.4.3 Internal Photoemission Spectroscopy (IPS) 293.5 Summary 33


Surfaces and Interfaces of Electronic Materials. Leonard J. BrillsonCopyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-40915-0

VI Contents

4 Interface States 374.1 Interface State Models 374.2 Simple Model Calculation of Electronic Surface States 394.3 Intrinsic Surface States 424.3.1 Experimental Approaches 424.3.2 Theoretical Approaches 434.3.3 Intrinsic Surface-State Models 444.3.4 Intrinsic Surface States of Silicon 454.3.5 Intrinsic Surface States of Compound Semiconductors 454.3.6 Dependence on Surface Reconstruction 484.3.7 Intrinsic Surface-State Summary 524.4 Extrinsic Surface States 524.4.1 Weakly Interacting Metal–Semiconductor Interfaces 524.4.2 Extrinsic Features 554.4.3 Schottky Barrier Formation and Thermodynamics 554.4.4 Extrinsic Surface-State Summary 624.5 Chapter Summary 62


5 Ultrahigh Vacuum Technology 675.1 Ultrahigh Vacuum Vessels 675.1.1 Ultrahigh Vacuum Pressures 675.1.2 Stainless Steel UHV Chambers 695.2 Pumps 705.3 Specimen Manipulators 765.4 Gauges 765.5 Deposition Sources 775.5.1 Metallization Sources 775.5.2 Crystal Growth Sources 785.6 Deposition Monitors 795.7 Summary 80

Problems 81References 81Further Reading 82

6 Surface and Interface Analysis 836.1 Surface and Interface Techniques 836.2 Excited Electron Spectroscopies 856.3 Principles of Surface Sensitivity 886.4 Surface Analytic and Processing Chambers 896.5 Summary 92

References 92Further Reading 92

Contents VII

7 Photoemission Spectroscopy 937.1 The Photoelectric Effect 937.2 The Optical Excitation Process 957.3 Photoionization Cross Section 957.4 Density of States 967.5 Experimental Spectrum 967.6 Experimental Energy Distribution Curves 977.7 Measured Photoionization Cross Sections 1007.8 Principles of X-ray Photoelectron Spectroscopy 1127.8.1 Chemical Species Identification 1127.8.2 Chemical Shifts in Binding 1147.8.3 Distinction between Near- and Subsurface Species 1147.8.4 Charging and Band Bending 1157.9 Excitation Sources 1197.10 Electron Energy Analyzers 1227.11 Summary 125


8 Photoemission with Soft X-rays 1298.1 Soft X-ray Spectroscopy Techniques 1298.2 Synchrotron Radiation Sources 1298.3 Soft X-Ray Photoemission Spectroscopy 1328.3.1 Basic Surface and Interface Techniques 1328.3.2 Advanced Surface and Interface Techniques 1378.3.2.1 Angular Resolved Photoemission Spectroscopy 1388.3.2.2 Polarization-Dependent Photoemission Spectroscopy 1398.3.2.3 Constant Final State Spectroscopy 1408.3.2.4 Constant Initial State Spectroscopy 1418.4 Related Soft X-ray Techniques 1418.5 Summary 143


9 Particle–Solid Scattering 1479.1 Overview 1479.2 Scattering Cross Section 1479.2.1 Impact Parameter 1479.2.2 Electron–Electron Collisions 1499.2.3 Electron Impact Cross Section 1509.3 Electron Beam Spectroscopies 1519.4 Auger Electron Spectroscopy 1539.4.1 Auger Transition Probability 1539.4.2 Auger versus X-ray Yields 1549.4.3 Auger Excitation Process 156

VIII Contents

9.4.4 Auger Electron Energies 1589.4.5 Quantitative Elemental Identification 1609.5 Auger Depth Profiling 1639.5 Summary 165


10 Electron Energy Loss Spectroscopy 16910.1 Overview 16910.2 Dielectric Response Theory 17110.3 Surface Phonon Scattering 17210.4 Bulk and Surface Plasmon Scattering 17410.5 Interface Electronic Transitions 17710.6 Atomic-Scale Electron Energy Loss Spectroscopy 18010.7 Summary 181

References 182

11 Rutherford Backscattering Spectrometry 18311.1 Overview 18311.2 Theory of Rutherford Backscattering 18411.3 Depth Profiling 18711.4 Channeling and Blocking 19011.5 Interface Studies 19211.6 Summary 195


12 Secondary Ion Mass Spectrometry 19712.1 Overview 19712.2 Principles 19712.3 SIMS Equipment 19912.4 Secondary Ion Yields 20312.5 Imaging 20612.6 Dynamic SIMS 20712.7 Organic and Biological Species 21112.8 Summary 211


13 Electron Diffraction 21313.1 Overview 21313.2 Principles of Low-Energy Electron Diffraction 21313.3 LEED Equipment 21513.4 LEED Kinematics 21613.5 Surface Reconstruction 217

Contents IX

13.6 Surface Lattices and Superstructures 21913.7 Silicon Reconstructions 22113.8 III–V Compound Semiconductor Reconstructions 22313.9 Reflection High-Energy Electron Diffraction 22713.8.1 RHEED Oscillations 23213.9 Summary 233


14 Scanning Tunneling Microscopy 23714.1 Overview 23714.2 Tunneling Theory 23914.3 Surface Structure 24414.4 Atomic Force Microscopy 24614.5 Ballistic Electron Emission Microscopy 24914.6 Atomic Positioning 25214.7 Summary 253


15 Optical Spectroscopies 25715.1 Overview 25715.2 Optical Absorption 25715.3 Modulation Techniques 26015.4 Multiple Surface Interaction Techniques 26215.5 Spectroscopic Ellipsometry 26315.6 Surface-Enhanced Raman Spectroscopy 26415.7 Surface Photoconductivity 26715.8 Surface Photovoltage Spectroscopy 26815.8.1 Theory of Surface Photovoltage Spectroscopy 26815.8.2 Surface Photovoltage Spectroscopy Equipment 27015.8.3 Surface Photovoltage Spectra and Photovoltage Transients 27115.8.4 Surface Photovoltage Spectroscopy of Metal-Induced Surface

States 27415.9 Summary 276


16 Cathodoluminescence Spectroscopy 27916.1 Overview 27916.2 Theory 28116.2.1 Scattering Cross Section 28116.2.2 Stopping Power 28216.2.3 Plasmon Energy Loss 283

X Contents

16.2.4 Electron Scattering Length 28516.2.5 Semiconductor Ionization Energies 28616.2.6 Universal Range-Energy Relation 28816.3 Monte Carlo Simulations 29116.4 Depth-Resolved Cathodoluminescence Spectroscopy 29316.4.1 Surface Electronic States 29516.4.2 Interface Electronic States 29516.4.3 Localized CLS in Three Dimensions 29716.4.3.1 Wafer-Scale Analysis of 2-DEG Layers 29816.4.3.2 Schottky Barriers 30016.4.3.3 Electronic Devices 30116.5 Summary 302


17 Electronic Materials’ Surfaces 30517.1 Overview 30517.2 Geometric Structure 30517.2.1 Surface Relaxation and Reconstruction 30517.2.2 Extended Geometric Structures 30617.2.2.1 Domains 30617.2.2.2 Steps 30617.2.2.3 Defects 31017.3 Chemical Structure 31117.3.1 Crystal Growth 31117.3.1.1 Bulk Crystal Growth 31117.3.1.2 Epitaxial Layer Crystal Growth 31417.3.2 Kinetics of Growth, Diffusion, and Evaporation 31717.4 Etching 31817.4.1 Etch Processes 31817.4.2 Wet Chemical Etching 31817.4.3 Orientation Effects on Wet Chemical Etching 31917.4.4 Impurities, Doping, and Light 32117.5 Electronic Implications 32317.6 Summary 323

Problems 324References 324Additional Reading 326

18 Adsorbates on Electronic Materials’ Surfaces 32718.1 Overview 32718.2 Geometric Structure 32718.2.1 Site Specificity 32818.2.2 Metal Adsorbates on Si 329

Contents XI

18.2.3 Metal Adsorbates on GaAs 32918.2.4 Epitaxical Overlayers 33118.2.4.1 Elemental Metal Overlayers 33118.2.4.2 Metal Silicides on Si 33118.2.4.3 Metal Epitaxy on Compound Semiconductors 33318.2.4.4 Epitaxical Metal–Semiconductor Applications 33518.3 Chemical Properties 33618.3.1 Metal Overlayers on Semiconductors 33618.3.1.1 Overlayer Growth Modes 33718.3.1.2 Thermodynamic Factors 33718.3.1.3 Nonequilibrium Energy Processing 33818.3.2 Macroscopic Interface Reaction Kinetics 33918.3.2.1 Silicide Phase Formation 33918.3.2.2 Thin Film versus Bulk Diffusion 34118.3.2.3 Mechanical and Morphological Effects 34318.3.2.4 Diffusion Barriers 34318.3.2.5 Atomic-Scale Metal-Si Reactions 34518.3.3 Compound Semiconductor Reactions 34518.4 Electronic Properties 34618.4.1 Physisorption 34618.4.2 Chemisorption 34718.4.3 Work Function Effects 35018.4.3.1 Charge Transfer 35018.4.3.2 Dipole Formation 35318.4.3.3 Ordered Adsorption 35418.4.3.4 Negative Electron Affinity 35518.4.3.5 Reconstruction Changes 35518.5 Summary 356

Problems 359References 360Additional Reading 363

19 Adsorbate–Semiconductor Sensors 36519.1 Adsorbate–Surface Charge Transfer 36519.1.1 Band-Bending Effects 36519.1.2 Surface Conductance 36619.1.3 Self-limiting Charge Transfer 36719.1.4 Transient Effects 36919.1.5 Orientational Dependence 36919.2 Sensors 37019.2.1 Sensor Operating Principles 37019.2.2 Oxide Gas Sensors 37119.2.3 Granular Gas Sensors 37419.2.4 Chemical and Biosensors 37519.2.4.1 Sensor Selectivity 376

XII Contents

19.2.4.2 Sensor Sensitivity 37719.2.5 Other Transducers 37719.2.6 Electronic Materials for Sensors 37919.3 Summary 379


20 Semiconductor Heterojunctions 38320.1 Overview 38320.2 Geometric Structure 38320.2.1 Epitaxial Growth 38320.2.2 Lattice Matching 38420.2.2.1 Lattice Match and Alloy Composition 38420.2.2.2 Lattice-Mismatched Interfaces 38620.2.2.3 Dislocations and Strain 38920.2.3 Two-Dimensional Electron Gas Heterojunctions 39320.2.4 Strained Layer Superlattices 39420.2.4.1 Superlattice Energy Bands 39420.2.4.2 Strain-Induced Polarization Fields 39620.3 Chemical Structure 39720.3.1 Interdiffusion 39720.3.1.1 IV–IV Interfaces 39720.3.1.2 III–V Compound Heterojunctions 39820.3.2 Chemical Reactions 39920.3.3 Template Structures 39920.3.3.1 Bridge Layers 39920.3.3.2 Monolayer Passivation 40020.3.3.3 Crystal Orientations 40020.3.3.4 Monolayer Surfactants 40120.3.3.5 Dipole Control Structures 40120.4 Electronic Structure 40220.4.1 Heterojunction Band Offsets 40220.4.2 Band Offset Characterization 40520.4.2.1 Macroscopic Electrical and Optical Methods 40520.4.2.2 Scanned Probe Techniques 40820.4.2.3 Photoemission Spectroscopy Techniques 41020.4.2.4 Band Offset Results 41220.4.3 Interface Dipoles 41220.4.3.1 Inorganic Semiconductors 41220.4.3.2 Organic Semiconductors 41820.4.4 Theories of Heterojunction Band Offsets 41920.4.4.1 Charge Neutrality Levels 42020.4.4.2 Local Bond Approaches 42120.4.4.3 Empirical Deep-Level Schemes 42220.4.5 Assessment of Theory Approaches 423

Contents XIII

20.4.6 Interface Contributions to Band Offsets 42320.4.6.1 Growth Sequence 42420.4.6.2 Crystallographic Orientation 42620.4.6.3 Surface Reconstruction: Band Bending versus Offsets 42720.4.6.4 Surface Reconstruction: Interface Bonding 42820.4.7 Theoretical Methods in Band Offset Engineering 42920.4.7.1 First-Principles Calculations 42920.4.7.2 Mathematical Approach 42920.4.7.3 Alternative Methods 43220.4.8 Application to Heterovalent Interfaces 43220.4.8.1 Polarity Dependence 43220.4.8.2 Interface Atomic Mixing 43320.4.8.3 Atomic Interlayers 43420.4.9 Practical Band Offset Engineering 43520.4.9.1 Spatially-Confined Nonstoichiometry 43620.4.9.2 Chemical Stability, Cross-Doping, and Interface States 43720.4.9.3 ‘‘Delta’’ Doping 43820.5 Summary 439


21 Metals on Semiconductors 44721.1 Overview 44721.2 Metal–Semiconductor Interface Dipoles 44821.3 Interface States 44921.3.1 Localized States 44921.3.2 Wavefunction Tailing (Metal-Induced Gap States) 45021.3.3 Charge Transfer, Electronegativity, and Defects 45321.3.4 Additional Intrinsic Pinning Mechanisms 45321.3.5 Extrinsic States 45321.3.5.1 Surface Imperfections and Contaminants 45521.3.5.2 Bulk States 45621.3.6 Interface-Specific Extrinsic States 45821.3.6.1 Interface Reaction and Diffusion 45821.3.6.2 Atomic Structural and Geometric Effects 46021.3.6.3 Chemisorption-Induced Effects 46121.3.6.4 Interface Chemical Phases 46221.3.6.5 Organic Semiconductor–Metal Dipoles 46321.4 Self-Consistent Electrostatic Calculations 46721.5 Fermi-Level Pinning Models 47121.6 Experimental Schottky Barriers 47121.6.1 Metals on Si and Ge 47221.6.1.1 Clean Surfaces 47221.6.1.2 Etched and Oxidized Surfaces 473

XIV Contents

21.6.1.3 N versus P-type Barriers 47421.6.1.4 Si, Ge Summary 47521.6.2 Metals on III–V Compound Semiconductors 47521.6.2.1 GaAs(110) Pinned Schottky Barriers 47621.6.3 InP(110) Unpinned Schottky Barriers 47921.6.3.1 Clean Surfaces 47921.6.3.2 Macroscopic Measurements 47921.6.4 GaN Schottky Barriers 48021.6.4.1 Other Binary III–V Semiconductors 48121.6.5 Ternary III–V Semiconductors 48221.6.6 Metals on II–VI Compound Semiconductors 48421.6.6.1 Sulfides, Selenides, and Tellurides 48521.6.6.2 ZnO: Dependence on Metals 48521.6.6.3 ZnO: Dependence on Native Point Defects 48521.6.6.4 ZnO: Dependence on Polarity 49021.6.7 Metals on IV–IV, IV–VI, and III–VI Compound

Semiconductors 49021.6.8 Compound Semiconductor Summary 49121.7 Interface Passivation and Control 49221.7.1 Macroscopic Methods of Contact Formation 49221.7.2 Processing Contacts 49321.7.2.1 Elemental Metals on GaAs 49321.7.2.2 Metal Multilayers on GaAs 50121.7.2.3 Useful Metallizations for III–V Compound Ohmic Contacts 50221.7.3 Atomic-Scale Control 50221.7.3.1 Reactive Metal Interlayers 50221.7.3.2 Less-Reactive Buffer Layers 50621.7.3.3 Semiconductor Interlayers 50621.7.4 Wet-Chemical Treatments 50821.7.4.1 Photochemical Washing 50821.7.4.2 Inorganic Sulfides 50921.7.4.3 Thermal Oxides and Hydrogen 50921.7.5 Semiconductor Crystal Growth 51021.7.5.1 Variations in Stoichiometry 51021.7.5.2 Misorientation/Vicinal Surfaces 51221.7.5.3 Epitaxical Growth of Binary Alloys on Compound

Semiconductors 51321.8 Summary 514


22 The Future of Interfaces 52322.1 Current Status 52322.2 Current Device Applications and Challenges 525

Contents XV

22.3 New Directions 52822.3.1 High-K Dielectrics 52922.3.2 Complex Oxides 53022.3.3 Spintronics 53222.3.4 Nanoscale Circuits 53422.3.5 Quantum-Scale Interfaces 53422.4 Synopsis 536

References 537

Appendices 539

Appendix 1: Glossary of Commonly Used Symbols 541

Appendix 2: Table of Acronyms 544

Appendix 3: Table of Physical Constants and Conversion Factors 548

Appendix 4: Semiconductor Properties 549

Appendix 5: Table of Preferred Work Functions 551

Appendix 6: Derivation of Fermi’s Golden Rule 552

Appendix 7: Derivation of Photoemission Cross Section fora Square Well 555

Index 557

XVII

Preface

This textbook is intended for students as well as professional scientists andengineers interested in the next generation of electronics and, in particular, in theopportunities and challenges introduced by surfaces and interfaces. As electronicstechnology improves with higher speed, higher sensitivity, higher power, andhigher functionality, surfaces and interfaces are becoming more important thanever. To achieve higher performance at the macroscopic level, one requires evenmore refined control of these junctions at the microscopic and, in fact, the atomicscale. With each advance, new techniques have been developed to measure andalter physical properties with increasing refinement. In turn, these studies haverevealed fundamental phenomena that have stimulated designs for new deviceapplications. This synergy between characterization, processing, and design spansseveral academic disciplines including physics, chemistry, materials science, andelectrical engineering.

Several excellent physics-based books are available that provide extensive mathe-matical analyses focused on specific effects that are also described here. However,the field of electronic surfaces and interfaces encompasses a wide range of chemicaland materials science phenomena that impact electronic properties. Rather thanfollow advanced treatments of specific effects, this book describes at an intermedi-ate level the full range of physical phenomena at surfaces and interfaces, the varietyof techniques available to measure them, and the physical issues to be addressedin order to advance electronics to the next level of performance. The author hopesto convey the excitement of this field and the intellectual challenges ahead. Healso wishes to thank many of his colleagues for paving the way for this bookwith their valuable discoveries and insights. Particular thanks are due to Prof. EliBurstein, who introduced him to the physics of metal–semiconductor interfaces,Dr Charles B. Duke, whose theoretical studies of semiconductor surface structureand tunneling provided a framework on which the experimental program was built,and Prof. Giorgio Margaritondo, who helped launch his soft X-ray photoemissionspectroscopy work on interfaces and introduced him to the international worldof synchrotron radiation science. Finally, his deepest gratitude goes to his wife,Janice, for her patience, understanding, love, and support during the year in whichthis book was written.


1

1Introduction

1.1Surface and Interfaces in Everyday Life

Surfaces and interfaces are all around us. Their properties are important inour daily lives and are basic to many of today’s advanced technologies. Thisis particularly true for the semiconductor materials that are used throughoutmodern electronics. The aim of this book is to present the physical principlesunderlying the electronic, chemical, and structural properties of semiconductorinterfaces and the techniques available to characterize them. Surfaces and interfacesare a cross-disciplinary field of science and engineering. As such, this bookemphasizes the principles common to physics, electrical engineering, materialsscience, and chemistry as well as the links between fundamental and practicalissues.

Surfaces and interfaces play a central role in numerous everyday phenomena.These include (i) triboelectricity, the transfer of charge between two materialsbrought into contact – such as the static electricity built up on a comb aftercombing one’s hair; (ii) corrosion, the oxidation of structural materials used in,for example, buildings, bridges, and aircraft; (iii) passivation, the prevention ofsuch chemical or biological processes using special protective layers; (iv) colloidchemistry, the wetting of surfaces and the dispersion of particles within fluids asemulsions or colloids, for example, paints and time-release capsule medicines;(v) tribology, the friction between sliding objects in contact and their interfacelubrication; (vi) cleaning and chemical etching, the removal of surface layers oradsorbed species; (vii) catalysis, the reduction in energetic barriers to speed up orimprove the yield of chemical reactions, for example, refining oil or burning coal;and (viii) optical interference, the rainbow of colors reflected off thin oil layers orthe internal reflection of light between stacks of materials only a few wavelengthsof light thick. On a much larger scale are (ix) electromagnetic interfaces betweenthe earth’s atmospheric layers that bounce short-wave radio signals around theworld and that alter the reflection or absorption of sunlight contributing to globalwarming.


2 1 Introduction

1.2Surfaces and Interfaces in Electronics Technology

Surfaces and interfaces are fundamental to microelectronics. One of the mostimportant microelectronic devices is the transistor, all functions of which dependon the boundaries between electronic materials. Figure 1.1 illustrates the threeaspects of this dependence. Here, current passes from a source metal to a drainmetal through a semiconductor, in this case, silicon (Si). A gate metal between thesource and the drain is used to apply voltages that attract or repel the charge carriersinvolved in the current flow. The result is control or ‘‘gating’’ of the current flow bythis third electrode. This basic device element is at the heart of the microelectronicsindustry.

The surfaces and interfaces are the key to the transistor’s operation, shown inFigure 1.1. Thus, the contact between the metal and Si is a metal silicide. Barrierscan form between metals and semiconductors that impede charge movementand introduce voltage drops across their interfaces. This barrier formation is acentral topic of this book. Microelectronics researchers found that promoting achemical reaction to form silicides, such as TiSi2 between Ti and Si, reducessuch transport barriers and the contact resistivity ρc at these metal–semiconductorinterfaces. This is illustrated, for example, in Figure 1.2a. Such interfacial sili-cide layers form low resistance, planar interfaces that can be integrated intothe manufacturing process. A challenge of this approach is to achieve very

Source DrainGate

Metal silicide

rC

Si

Figure 1.1 Source–gate–drain structure of a silicon transistor.

− − − − − − − − − − − − −Insulator

Dopant, impurity atoms

(a) (b) (c)

Metal

Reaction product

Semiconductor

Metal

Semiconductor

+ + + + + + + + + +

Semiconductor

Figure 1.2 Expanded view of a (a) interfacebetween metal and semiconductor with reacted layer,(b) gate–semiconductor interface with trapped charge ininsulator and at insulator–semiconductor junction, and(c) dopant or impurity atom diffusion into semiconductor.

1.2 Surfaces and Interfaces in Electronics Technology 3

thin, low ρc contacts without allowing reactions to extend far away from thejunction.

The second important interface appears at the gate–semiconductor junction,shown in Figure 1.2b. Here, the earliest transistor experiments [1] showed the pres-ence of fixed charges at this interface that prevented control of the source–draincurrent. This gate interface may involve a metal in direct contact with the semi-conductor or, more commonly, a stack of metal-on-insulator-on semiconductor toapply voltage bias without introducing additional current. Atomic sites within theinsulator and its semiconductor interface can immobilize charge and introducedipoles across the insulator–semiconductor interface. This localized charge pro-duces a voltage drop that offsets applied voltages at the gate metal, opposing thegate’s control of the source–drain current flow. Minimizing the formation of theselocalized charge sites has been one of the prime goals of the microelectronicsindustry since the invention of the transistor.

The third important microelectronic interface involves diffusion of atoms intoand out of the semiconductor. Atomic diffusion of atoms into the semiconductorthat donate or accept charge is used to control the concentration of free chargecarriers within specific regions of a device. Acceleration and implantation ofionized atoms is a common process to achieve such doped layers that extendinto semiconductor surfaces, here illustrated in Figure 1.2c. In addition, atomicdiffusion can occur between two materials in contact that are annealed at hightemperature. High-temperature annealing is often used to heal lattice damage afterimplantation or to promote reactions at particular device locations. However, suchannealing can introduce diffusion and unintentional doping at other regions ofthe device. Outdiffusion of semiconductor constituents is also possible, resultingin native point defects that can also be electrically active. Balancing these effectsrequires careful design of materials, surface and interface preparation, thermaltreatment, and device architectures.

Microelectronic circuits consist of many interfaces between semiconductors,oxides, and metals. Figure 1.3 illustrates how these interfaces form as siliconprogresses from its melt-grown crystal boule to a packaged chip. The Si bouleformed by pulling the crystal out of a molten bath is sectioned into wafers, whichare then oxidized, diffused, or implanted with dopants, and overcoated with variousmetal and organic layers. Photolithography is used to pattern and etch these wafersinto monolithic arrays of devices. The wafer is then diced into individual circuitsthat are then mounted, wire bonded, and packaged into chips.

Within each circuit element, there can be many layers of interconnected conduc-tors, insulators, and their interfaces. Figure 1.4 illustrates the different materialsand interfaces associated with a 0.18-μm transistor at the bottom of a multilayerAl–W–Si-oxide dielectric assembly [2]. Reaction, interdiffusion, and formation oflocalized states must all be carefully controlled at all of these interfaces during themany patterning, etching, and annealing steps involved in assembling the full struc-ture. Figure 1.4 also shows that materials and geometries change to compensatefor the otherwise increasing electrical resistance as interconnects between layersshrink into the nanoscale regime. This continuing evolution in microelectronics

4 1 Introduction

Ingot Ingot

Melt

Crystalgrowth

Wafer sliceand polish

Dice

Die attach Lead bond Encapsulateand test

Waferprobe

Wafer processing

Wafer

Oxidize, diffuseimplant,

evaporate, deposit

PatternEtch

Mask

Figure 1.3 Integrated circuit manufacturing process flow.

New interconnect materials and processes

Sematech0.18 μm today

Future0.1 μm

Aluminumconductors(six levels)

Copperconductors(eight levels)

Low-kdielectric

Copperplugs

Oxidedielectric

Tungstenplugs

The deviceE-Beam Mag Tilt Spot Det FWD 5 μm

Figure 1.4 Multilayer, multimaterial interconnect architec-tures at the nanoscale. Feature size of interconnects at rightis 45 nm [2].

underscores the importance of interfaces since the material volume associated withthese interfaces becomes a larger proportion of the entire structure as circuit sizesdecrease.

Many other conventional electronic devices rely on interfaces for their operation.Figure 1.5a illustrates the interface between a metal and semiconductor within asolar cell schematically as an energy E versus distance x band diagram. The Fermi


hn

e−

h+

Photons,microwaves,acoustic waves

EC

EF

EV(a) (b)

(c) (d)

Metal oxide

Gas molecule

hne−

Photoemissive cathode

Figure 1.5 Interfaces in conventional electronics:(a) solar cell, (b) gas sensor, (c) optoelectronic emitter, and(d) photoemissive cathode.

levels EF in the metal (solid line) and the semiconductor (dashed line) align ata constant energy, whereas the conduction band EC and the valence band EV inthe semiconductor bend near the interface. Incident photons of energy hν at thisinterface create electrons and holes that separate under the field set up by thesebent bands. This charge separation results in photoinduced current or voltagebetween the metal and the semiconductor.

Figure 1.5b shows what appears to be a transistor structure except that, unlikeFigure 1.1, there is no gate. Instead, molecules on this otherwise free surface adsorbon the surface, exchanging charge and inducing a field analogous to that of a gate.Figure 1.5c illustrates a circuit that generates photon, microwaves, or acousticwaves. The contacts that inject current or apply voltage to the generator layer arekey to its practical operation. Unless the resistance of such contacts is low, poweris lost at these contacts, reducing or totally blocking power conversion inside thesemiconductor. Figure 1.5d illustrates an interface involving just a semiconductorsurface that emits electrons when excited by incident photons. Chemical treatmentof selected semiconductors enables these surfaces to emit multiple electrons whenstruck by single photons. Such surfaces are useful as electron pulse generators orphotomultipliers.

Surfaces and interfaces have an even larger impact on electronics as devicesmove into the quantum regime. Figure 1.6 illustrates four such quantum electronicdevices schematically. Figure 1.6a illustrates the energy band diagram of a quantumwell, one of the basic components of optoelectronics. Here, the decrease in bandgapbetween EC and EV of one semiconductor sandwiched between layers of a largerbandgap semiconductor localizes both electrons and holes in the smaller gapmaterial. This joint localization enhances electron–hole pair recombination andlight emission. The quantum well is typically only a few atomic layers thick so

6 1 Introduction

hn

EC

EV

(a)

(c) (d)

(b)

ECEF

EV

e−

Quantum dots

Figure 1.6 Four quantum electronic devices. (a) Chargelocalization, recombination, and photon emission at a quan-tum well; (b) carrier confinement and transport at a semi-conductor inversion layer; (c) tunneling transport at anavalanche detector; and (d) carrier confinement in threedimensions at quantum dots.

that the allowed energies of electrons and holes inside the well are quantized atdiscrete energies. This promotes efficient carrier inversion and laser light emission.Imperfections at the interfaces of these quantum wells can reduce the quantizedlight emission by introducing competing channels for recombination that do notinvolve the quantized states in the well.

Figure 1.6b shows a schematic energy band diagram of a semiconductor withbands that bend down at the surface, allowing EF to rise above EC. The highconcentration of electrons is confined to within a few tens of nanometers or lessat the surface. This phenomenon is termed a two-dimensional electron gas (2DEG)layer. It forms a high carrier concentration, high mobility channel at the surfacethat is used for high-frequency, high-power devices, often termed high electronmobility transistors (HEMTs). Again, imperfections at the semiconductor interfacecan produce local electric fields that scatter charges, reduce mobility, and alter oreven remove the 2DEG region.

Figure 1.6c illustrates a structure consisting of alternating high- and low-bandgapsemiconductors characteristic of a cascade laser or of a very high frequencytransistor. In either case, charge must tunnel through the ultrathin (monolay-ers) ‘‘barrier’’ layers into quantized energy levels. Once again, the perfection ofthese interfaces is crucial for the charge to tunnel efficiently between layers.Finally, Figure 1.6d represents a real-space pair of quantum dots encapsulatedby other media. Such quantum dots with sizes of only a few nanometers alsohave quantized energy levels that yield efficient laser emission. Again, optical


emission is impacted if imperfections and recombination are present at theirinterfaces.

The materials that comprise all electronics consist of metals, semiconductors,and/or insulators. It is instructive to realize that these three materials differprimarily in terms of the energy separation between their filled and empty electronicstates. For metals, this bandgap is 0. For insulators, it can be quite large, typically5 eV or more. Semiconductors lie between these two regimes with intermediatebandgaps ranging from a few hundred meV or milli-electron volts to severalelectron volts.

These bandgaps and Fermi levels in the energy band diagrams of Figures 1.5and 1.6 point to another key aspect of interfaces – the alignment of energy levelsbetween the constituents. This chapter emphasizes that contacts and charge trans-port between metals, semiconductor, and insulators are essential to all electronicapplications. How their energy levels align is a fundamental issue that is still notwell understood. The question is, does this matter? The interface band structuredetermines how much energy difference exists between materials and thereby whatbarriers exist to charge movement between them. The question is, what affects thisphenomenon? There are three primary factors: (i) the constituents of the junctions,(ii) the conditions under which they form the interface, and (iii) any subsequentthermal or chemical processing. Therefore, to understand how surfaces and inter-faces impact electronics, it is important to know their properties at the microscopiclevel and how these factors shape these properties.

Surface and interface science continues to have enormous practical valuefor electronics. It has helped develop the semiconductor industry into thehigh-performance, high-value-added industry that it is today. Surface and interfacetools are essential for monitoring, controlling, and ultimately designing micro- andoptoelectronic clean room processes. They are also central to controlling propertiesof contacts. As such, they are integral to designing the next generation of electronicdevice materials and fabrication processes.

Problems

1. As electronics shrinks into the nanoscale regime, the actual numbers ofatoms become significant in determining the semiconductor’s physicalproperties. Consider a 0.1-μm Si field effect transistor with a 0.1 μm ×0.1 μm cross section and a doping concentration of 1017 cm−3. How manydopant atoms are there in the channel region? How many dopant atomsare there altogether?

2. Assume the top channel surface has 0.01 trapped electron per unit cell. Howmany surface charges are present? How much do they affect the channel’sbulk charge density?

3. What fraction of atoms is within one lattice constant of the channelsurfaces?

8 1 Introduction

4. Figure 1.4 illustrates the complexity of the transistor chip structure.Describe three interface effects that could occur in these structures thatcould degrade electrical properties during microfabrication.

5. Name three desirable solar cell properties that could be affected byinterface effects.

6. Name three interface effects that could degrade quantum well operation.

References

1. Bardeen, J. (1947) Phys. Rev., 71, 717.2. Lammers, D. (2007) Semiconductor

International, www.semiconductor.net/

article/CA6513618.html (accessed 18December 2007).

9

2Historical Background

2.1Contact Electrification and the Development of Solid-State Concepts

The history of electrical contacts extends far back in time past the Greco–Romanera. Contact electrification or, more precisely, triboelectricity was well known inancient times in the form of static electricity. Thus, the rubbing of cat’s fur withamber was an early example of charge transfer between solids. However, systematicobservation of this electrical phenomenon began much later when, in 1874, Braunused selenium to form a rectifier, which conducted electricity unequally withpositive versus negative applied voltage [1]. See Henisch’s Rectifying SemiconductorContacts [2] for a detailed review of early literature on semiconductor interfacephenomena.

Three decades later, Einstein explained the photoelectric effect at metal surfacesusing wave packets with discrete particle energies [3]. This established not only theparticle nature of light but also the concept of work function, one of the buildingblocks of energy band theory. Notably, his work on the photoelectric effect, ratherthan his many other achievements, was the basis for Einstein’s 1921 Nobel Prize.In 1931, Wilson presented a theoretical foundation for semiconductor phenomenawhich was based on the band theory of solids [4, 5]. Siemens et al. [6] and Schottkyet al. [7] subsequently showed that the semiconductor interior was not a factor incontact rectification. In 1939, Mott [8], Davidov [9, 10], and Schottky [11–13] allpublished theories of rectification. Each of their models involved a band-bendingregion near the interface. Mott proposed an insulating region between the metal andthe semiconductor that accounted for current–voltage measurements of copperoxide rectifiers. Davidov advanced the importance of thermionic work functiondifferences in forming the band-bending region. Schottky introduced the idea thatthe band-bending region could arise from stable space charge in the semiconductoralone rather than requiring the presence of a chemically distinct interfacial layer.Bethe’s theory of thermionic emission of carriers over the energy barrier [14],based in part on Richardson’s earlier work on thermionic cathodes [15], provided adescription of charge transport across the band-bending region that described therectification process for most experimental situations. These developments laid thegroundwork for describing semiconductor charge transfer and barrier formation.


10 2 Historical Background

See [2,16–20] for more extensive reviews of metal–semiconductor rectification,with particular emphasis on current transport and tunneling.

2.2High-Purity Semiconductor Crystals

A major roadblock to experimental studies was the need for semiconductors withhigh purity. Without sufficiently high crystal perfection and purity, the intrinsicsemiconductor properties were masked by the effects of impurities and latticeimperfections. By the 1940s, large high-purity semiconductor single crystals hadbecome available using crystal-pulling and zone-refining techniques [21–26]. Twosuch crystal-growth techniques – Czochralski and float zone – are illustrated inFigure 2.1.

2.3Development of the Transistor

By the mid-1940s, companies such as Bell Telephone had begun efforts aimedat producing amplifiers based on semiconductors rather than on vacuum tubes.A key event in this industrial research occurred when Bardeen, Shockley, andPearson discovered that electric charges immobilized on the semiconductor surfacewere interfering with the gate modulation of the charge and current inside thesemiconductor. Bardeen correctly interpreted this observation in terms of electronic

Silicon ingot

Polycrystallinemelt

Floating zoneprocess

Czochralskigrowth apparatus

Ingot

Wafers

Si O2 tube Si O2 liner

Moltenzone

Silicon

Heaterwinding

ImpuritiesSeed

Ingot

Crucible

Moltensilicon

Heatercoils

(a) (d)

(b) (c)

Figure 2.1 Pulling of an Si crystal from the melt (theCzochralski method): (a) silicon crystal being pulled with aseed crystal from molten silicon [25]. (b) Float-zone processwith impurity segregation away from the molten zone. (c)Cutout of Czochralski crucible interior, (d) Ingot sectioninginto wafers [26].

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