Molecular Beam Epitaxy: From Research to Mass Production

720 Pages·2012·69.097 MB·English

by  Mohamed Henini

Molecular Beam Epitaxy
From research to mass production
Edited by
Mohamed Henini
Professor of Applied Physics
School of Physics & Astronomy
Nottingham Nanotechnology and Nanoscience Centre
University of Nottingham
Nottingham NG7 2RD, UK
AMSTERDAM(cid:1)BOSTON(cid:1)HEIDELBERG(cid:1)LONDON(cid:1)NEWYORK(cid:1)OXFORD
PARIS(cid:1)SANDIEGO(cid:1)SANFRANCISCO(cid:1)SINGAPORE(cid:1)SYDNEY(cid:1)TOKYO
Elsevier
225WymanStreet,Waltham,MA02451,USA
LinacreHouse,JordanHill,OxfordOX28DP,UK
Copyright(cid:1)2013ElsevierInc.Allrightsreserved
Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmittedinanyformorbyanymeanselectronic,
mechanical,photocopying,recordingorotherwisewithoutthepriorwrittenpermissionofthepublisher
PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRightsDepartmentinOxford,UK:phone(+44)(0)
1865843830;fax[phone];email:[e-mail]tyourrequestonlineby
visitingtheElsevierwebsiteathttp://elsevier.com/locate/permissions,andselectingObtainingpermissiontouseElseviermaterial
Notice
No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability,
negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material
herein.
BritishLibraryCataloguinginPublicationData
AcataloguerecordforthisbookisavailablefromtheBritishLibrary
LibraryofCongressCataloging-in-PublicationData
Molecularbeamepitaxy:fromresearchtomassproduction/editedbyMohamedHenini.
pagescm
Summary:“Thismulti-contributorhandbookdiscussesMolecularBeamEpitaxy(MBE),anepitaxialdepositiontechniquewhich
involveslayingdownlayersofmaterialswithatomicthicknessesontosubstrates.ItsummarizesMBEresearchandapplicationin
epitaxialgrowthwithclosediscussionanda‘howto’onprocessingmolecularoratomicbeamsthatoccuronasurfaceofaheated
crystallinesubstrateinavacuum.MBEhasexpandedinimportanceoverthepastthirtyyears(intermsofuniqueauthors,papersand
conferences)fromapureresearchdomainintocommercialapplications(prototypedevicestructuresandmoreattheadvancedresearch
stage).MBEisimportantbecauseitenablesnewdevicephenomenaandfacilitatestheproductionofmultiplelayeredstructureswith
extremelyfinedimensionalandcompositionalcontrol.Thetechniquescanbedeployedwhereverprecisethin-filmdeviceswith
enhancedanduniquepropertiesforcomputing,opticsorphotonicsarerequired.ThisbookcoverstheadvancesmadebyMBEbothin
researchandmassproductionofelectronicandoptoelectronicdevices.Itincludesnewsemiconductormaterials,newdevicestructures
whicharecommerciallyavailable,andmanymorewhichareattheadvancedresearchstage”eProvidedbypublisher.
ISBN978-0-12-387839-7(hardback)
1. Molecularbeamepitaxy.2. OptoelectronicdevicesdMaterials.3. SemiconductorsdMaterials. I.Henini,Mohamed,editorof
compilation.
QC611.6.M64M6452012
621.381ddc23
2012032168
ISBN:978-0-12-387839-7
ForinformationonallElsevierpublications
visitourwebsiteatstore.elsevier.com
PrintedandboundintheUSA.
1213141516 10987654321
Preface
Semiconductor devices have developed at an astounding The topic of MBE remains a lively one in the tech-
pace following the invention of the transistor by Bardeen nical and commercial world, and it is the purpose of this
andBrattainin1947.Thediscoveryofthetransistorhadan book to consider developments and changes since its
extraordinary impact on the electronic industry. In the inception. The book demonstrates the versatility of the
1960s, development of molecular beam epitaxy (MBE) MBE technique and its main achievements both in
deposition facilitated the fabrication of heterojunction fundamental research and manufacturing, and provides
transistors and novel active devices with “tailor-made” a wide range of current and hot topics from contributors
characteristics, and provided access to new device fromcompaniesandresearchcenters.Becauseofthenew
phenomena.Thesuccessesofthisgrowthtechniquemaybe developments in MBE and its use for mass production of
measured by the number of papers published, conferences electronic, optoelectronic and photonic based-devices,
that have taken place over the past 30 years, and the attri- this book will without doubt be welcomed addition in
butions of several Nobel prizes that have been directly the market. From semiconductor researchers to semi-
linked to results obtained from MBE grown samples. In conductor manufacturers, there will be something for
addition,thistechniqueisnowcommerciallyusedtomass everyone in this book.
produce devices for electronic, optoelectronics and As everyone knows editing a book is hard, butI found
photonics applications. Many more exciting and novel editingthisMBEbookevenharder.Nobookcanbewritten
devices are at the advanced research stage. Some of the withoutmanyhoursofhardworkbymanypeople.Iwould
great advantages of MBE are its versatility and ability to like first to express my special thanks, appreciation and
incorporate awide range of sources that provide the flexi- gratitude to the authors/co-authors for their considerable
bility of heteroepitaxial integration of semiconductor efforts. I enjoyed working with the people from Elsevier,
materials with different properties. and I thank them for their valuable help and assistance at
The demands on a mass-production MBE system are different stagesofthe editorial process.
stricter than a research machine because of the high Finally, I would like to thank my wife and two daugh-
specifications on parameters such as uniformity in layer ters for their patience and understanding for many mood
thickness, doping and composition profiles, and low swings accompanying my efforts as editor of this book
surface defect density. Other important parameters for duringthe past year anda half.
commercialandeconomicalpurposesincludelargewafer
area, high throughput, long uptime and versatility. All MohamedHenini
these major factors are responsible for the continued SchoolofPhysicsandAstronomy,
success of MBE as an epitaxial growth technique for NottinghamNanotechnologyandNanoscienceCentre,
electronic and optoelectronic devices. The MBE field is UniversityofNottingham,Nottingham,UK
still dynamic despite that it is over 30 years old. This is
demonstrated by a number of novel physics-based device
concepts that use the idea of band gap engineering.
vii
Contributors
S. Andrieu, Institut Jean Lamour (UMR CNRS 7198), Adam Duzik, Department of Materials Science and
Universite´ de Lorraine, Vandoeuvre les Nancy cedex, Engineering, University of Michigan, Ann Arbor, MI,
France USA
Donat J. As, University of Paderborn, Warburger Strasse, James N. Eckstein, Department of Physics, University of
Paderborn, Germany Illinois, Urbana, IL, USA
V. Avrutin, Department of Electrical and Computer Roman Engel-Herbert, Department of Materials Science
Engineering, Virginia Commonwealth University, and Engineering, The Pennsylvania State University
Richmond, VA, USA
Secondo Franchi, CNR-IMEM Institute, Parco delle
Zahida Batool, Advanced Technology Institute and Scienze, Parma, Italy
Department of Physics, University of Surrey,
Alex Freundlich, Center for Advanced Materials,
Guildford, UK
University of Houston, Houston, USA
Abdelhak Bensaoula Ph.D., Depts. of Physics and
Rafael Fritz, Fachbereich Physik, Philipps-Universita¨t
Electrical and Computer Engineering, University of
Marburg, Marburg, Germany; Materials Science
Houston, Houston, TX, USA
Center, Philipps-Universita¨t Marburg, Marburg,
Oliver Bierwagen, Paul-Drude-Institut fu¨r Germany
Festko¨rperelektronik, Hausvogteiplatz, Berlin, Germany
Chaturvedi Gogineni, School of Electrical, Computer,
UniversityofCalifornia,SantaBarbara,California,USA
and Energy Engineering & Center for Photonics
Victorz Blinov, Rzhanov Institute of Semiconductor Innovation,ArizonaStateUniversity,Tempe,AZ,USA
Physics, Siberian Branch of the Russian Academy of
MirceaGuina,OptoelectronicsResearchCentre,Tampere
Science, Novosibirsk, Russia
University of Technology, Tampere, Finland
Chris Boney Ph.D., Dept. of Physics, University of
DrewHanser,VeecoInstruments,Inc.,St.Paul,MNUSA
Houston, Houston, TX, USA
M. Heiblum, Department of Condensed Matter Physics,
Sangam Chatterjee, Fachbereich Physik, Philipps-
Braun Center for Submicron Research, Weizmann
Universita¨t Marburg, Marburg, Germany; Materials
Institute of Science, Rehovot, Israel
Science Center, Philipps-Universita¨t Marburg,
Isaac Herna´ndez-Caldero´n, Physics Department,
Marburg, Germany
Cinvestav, Ave. IPN 2508, 07360 Mexico City, DF,
Alexej Chernikov, Fachbereich Physik, Philipps-
Mexico
Universita¨t Marburg, Marburg, Germany
Konstanze Hild, Advanced Technology Institute and
J.Y. Chi, National Chung Hsing University, Graduate
Department of Physics, University of Surrey,
InstituteofOptoelectronicEngineering,Taichung,Taiwan
Guildford, UK
Dr. Alan Colli, Nokia Research Center, Broers Building,
Yoshiji Horikoshi, School of Science and Engineering,
21 JJ Thomson Avenue, CB3 0FA Cambridge, United
Kagami Memorial Laboratory for Materials Science
Kingdom
and Technology, Waseda University, Japan
BruceDavidson,InstituteforthePhysicsofMatter,CNR,
Thomas J.C. Hosea, Advanced Technology Institute and
Trieste, Italy
Department of Physics, University of Surrey,
Molly Doran, Veeco Instruments, Inc., St. Paul, MN Guildford, UK; Department of Physics and Ibnu Sina
USA Institute, Faculty of Science, Universiti Teknologi
Malaysia, UTM, Skudai, Johor, Malaysia
K. Dumesnil, Institut Jean Lamour (UMR CNRS 7198),
Universite´ de Lorraine, Vandoeuvre les Nancy cedex, Alex Ignatiev, Center for Advanced Materials, University
France of Houston, Houston, USA
ix
x Contributors
Sebastian Imhof, Institut fu¨r Physik, Technische Engineering, University of British Columbia,
Universita¨t Chemnitz, Chemnitz, Germany Vancouver,Canada
J. Kossut, Institute of Physics, Polish Academy of F. Matsukura, WPI-Advanced Institute for Materials
Sciences, al. Lotniko´w, Warsaw, Poland Research, Tohoku University, 2-1-1 Katahira, Aoba-ku,
Sendai,Japan;CenterforSpintonicsIntegratedSystems,
S.V.Ivanov,IoffePhysical-TechnicalInstituteofRAS,26
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,
Politekhnicheskaya street, St. Petersburg, Russia
Japan; Laboratory for Nanoelectronics and Spintronics,
Dr. Roland Ja¨ger, Royal Philips Electronics N.V.
ResearchInstituteofElectricalCommunication,Tohoku
Zenan Jiang, Department of Physics, Simon Fraser University,2-1-1Katahira,Aoba-ku,Sendai,Japan
University, Burnaby, Canada
Joanna Mirecki Millunchick, Department of Materials
ShirongJin,AdvancedTechnologyInstituteandDepartment Science and Engineering, University of Michigan,
of Physics, University of Surrey, Guildford, UK Ann Arbor, MI, USA
Shane R. Johnson, School of Electrical, Computer, and Patricia M. Mooney, Department of Physics, Simon
Energy Engineering & Center for Photonics Fraser University, Burnaby, Canada
Innovation,ArizonaStateUniversity,Tempe,AZ,USA
E. Moreau, Institute of Electronics, Microelectronics and
A.V. Katkov, National Tsing-Hua University, ESS Nanotechnology (IEMN, UMR 8520 CNRS) Av.
Department, Hsinchu, Taiwan Poincare´ Villeneuve d’Ascq France
q. K1opotowski, Institute of Physics, Polish Academy of H. Morkoc¸, Department of Electrical and Computer
Sciences, al. Lotniko´w, Warsaw, Poland Engineering, Virginia Commonwealth University,
Richmond, VA, USA
Martin Koch, Fachbereich Physik, Philipps-Universita¨t
Marburg, Marburg, Germany AlexanderNikiforov,RzhanovInstituteofSemiconductor
Physics, Siberian Branch of the Russian Academy of
Stephan W. Koch, Fachbereich Physik, Philipps-
Science, Novosibirsk, Russia
Universita¨t Marburg, Marburg, Germany
Jiro Nishinaga, Waseda Institute for Advanced Study,
NobuyukiKoguchi,L-NESSandDipartimentodiScienza
Waseda University, Okubo, Shinjuku, Tokyo, Japan
deiMateriali,Universita´ degliStudidiMilanoBicocca,
PRESTO, JST, Honcho Kawaguchi, Saitama, Japan
Milano, Italy
Gang Niu, Lyon Institute of Nanotechnologies (INL),
Kolja Kolata, Fachbereich Physik, Philipps-Universita¨t
Ecole Centrale de Lyon, CNRS UMR 5270, France
Marburg, Marburg, Germany Materials; Science Center,
Philipps-Universita¨tMarburg,Marburg,Germany Mark O’Steen, Veeco Instruments, Inc., St. Paul, MN
USA
Naohiro Kuze, Asahi Kasei Microdevices Corporation,
R&D Center, Samejima, Fuji, Shizuoka, Japan KunishigeOe,KyotoInstituteofTechnology,Department
of Electronics Matsugasaki, Sakyo, Kyoto, 606-8585.
Ryan B. Lewis, Department of Physics and Astronomy,
Japan
University of British Columbia, Vancouver, Canada;
Department of Electrical and Computer Engineering, Seongshik Oh, Department of Physics, Rutgers
University of Victoria, Victoria, Canada University, Piscataway, NJ, USA
Klaus Lischka, University of Paderborn, Warburger H. Ohno, WPI-AdvancedInstituteforMaterialsResearch,
Strasse, Paderborn, Germany Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,
Japan;CenterforSpintonicsIntegratedSystems,Tohoku
XianfengLu,SchoolofElectrical,Computer,andEnergy
University, 2-1-1 Katahira, Aoba-ku, Sendai, Japan;
Engineering & Center for Photonics Innovation,
Laboratory for Nanoelectronics and Spintronics,
Arizona State University, Tempe, AZ, USA;
ResearchInstituteofElectricalCommunication,Tohoku
Department of Physics and Astronomy, University of
University,2-1-1Katahira,Aoba-ku,Sendai,Japan
British Columbia, Vancouver, Canada
U¨. O¨zgu¨r, Department of Electrical and Computer
Dr. Faustino Martelli, Istituto per la Microelettronica e i
Engineering, Virginia Commonwealth University,
Microsistemi, Consiglio Nazionale delle Ricerche, Via
Richmond, VA, USA
del fosso del cavaliere 100, 00133 Roma, Italy
Oleg Pchelyakov, Rzhanov Institute of Semiconductor
Mostafa Masnadi-Shirazi, Department of Electrical and
Physics, Siberian Branch of the Russian Academy of
ComputerEngineering,UniversityofVictoria,Victoria,
Science, Novosibirsk, Russia
Canada; Department of Electrical and Computer
Contributors xi
Dmitry Pridachin, Rzhanov Institute of Semiconductor Angela Thra¨nhardt, Institut fu¨r Physik, Technische
Physics, Siberian Branch of the Russian Academy of Universita¨t Chemnitz, Chemnitz, Germany
Science, Novosibirsk, Russia
Thomas Tiedje, Department of Electrical and Computer
EricReadinger,VeecoInstruments,Inc.,St.Paul,MNUSA Engineering, University of Victoria, Victoria, Canada
Nathaniel A. Riordan, School of Electrical, Computer, Min-Ying Tsai, University of California, Santa Barbara,
and Energy Engineering & Center for Photonics California, USA
Innovation,ArizonaStateUniversity,Tempe,AZ,USA
V. Umansky, Department of Condensed Matter Physics,
Oleg Rubel, Department of Physics, Lakehead University Braun Center for Submicron Research, Weizmann
& Thunder Bay Regional Research Institute, Thunder Institute of Science, Rehovot, Israel
Bay, Ontario, Canada
D.Vignaud,InstituteofElectronics,Microelectronicsand
Dr. Silvia Rubini, Istituto Officina dei Materiali CNR, Nanotechnology (IEMN, UMR 8520 CNRS) Av.
Laboratorio TASC, Area Science Park-Basovizza, Poincare´ Villeneuve d’Ascq France
S.S. 14 Km. 163,5, 34149 Trieste, Italy
Bertrand Vilquin, Lyon Institute of Nanotechnologies
Guillaume Saint-Girons, Lyon Institute of (INL), Ecole Centrale de Lyon, CNRS UMR 5270,
Nanotechnologies (INL), Ecole Centrale de Lyon, France
CNRS UMR 5270, France
Kerstin Volz, Fachbereich Physik, Philipps-Universita¨t
Stefano Sanguinetti, L-NESS and Dipartimento di Marburg, Marburg, Germany; Materials Science
Scienza dei Materiali, Universita´ degli Studi di Center, Philipps-Universita¨t Marburg, Marburg,
Milano Bicocca, Milano, Italy Germany
Achim Scho¨ll, Experimentelle Physik 7 and Ro¨ntgen Shu Min Wang, Photonics Laboratory, Department of
Research Center for Complex Material Systems Microtechnology and Nanoscience, Chalmers
(RCCM), Universita¨t Wu¨rzburg, Am Hubland, University of Technology, Go¨teborg, Sweden
Wu¨rzburg, Germany; Gemeinschaftslabor fu¨r
Guang Wang, Center of Material Science, Institute of
Nanoanalytik, Karlsruhe Institute of Technology
Technical Physics, College of Sciences, National
(KIT), Karlsruhe, Germany
University of Defense Technology, Changsha, P. R.
Frank Schreiber, Institut fu¨r Angewandte Physik, China
Universita¨t Tu¨bingen, Auf der Morgenstelle,
Maitri Warusawithana, Department of Physics, Florida
Tu¨bingen, Germany
State University, Tallahassee, FL, USA
I.V.Sedova,IoffePhysical-TechnicalInstituteofRAS,26
Z.R. Wasilewski, National Research Council of Canada,
Politekhnicheskaya street, St. Petersburg, Russia
Ottawa, Ontario, Canada; Present Address:
Ichiro Shibasaki, Toyohashi University of Technology, Department of Electrical and Computer Engineering,
Hibarigaoka, Tempaku-cho, Toyohashi, Japan Waterloo Institute of Nanotechnology, University of
Waterloo, Waterloo, Ontario, Canada
Leonid Sokolov, Rzhanov Institute of Semiconductor
Physics, Siberian Branch of the Russian Academy of Mark E. White, University of California, Santa Barbara,
Science, Novosibirsk, Russia California, USA
S.V. Sorokin, Ioffe Physical-Technical Institute of RAS, P. Wojnar, Institute of Physics, Polish Academy of
26 Politekhnicheskaya street, St. Petersburg, Russia Sciences, al. Lotniko´w, Warsaw, Poland
James S. Speck, University of California, Santa Barbara, Qi-Kun Xue, State Key Laboratory of Low Dimensional
California, USA Quantum Physics, Department of Physics, Tsinghua
University, Beijing, P. R. China
Gunther Springholz, Institut fu¨r Halbleiter- und
Festko¨rperphysik, Johannes Kepler Universita¨t Linz, Masahiro Yoshimoto, Kyoto Institute of Technology,
Linz, Austria Department of Electronics Matsugasaki, Sakyo,
Kyoto, 606-8585. Japan
Stephen J. Sweeney, Advanced Technology Institute and
Department of Physics, University of Surrey, Xiaofang Zhai, Hefei National Laboratory for Physical
Guildford, UK Sciences at the Microscale, USTC, Hefei, China
John C. Thomas, Department of Materials Science and MaoZheng,DepartmentofPhysics,UniversityofIllinois,
Engineering, University of Michigan, Ann Arbor, MI, Urbana, IL, USA
USA
Chapter 1
Molecular beam epitaxy: fundamentals,
historical background and future
prospects
Secondo Franchi
CNR-IMEMInstitute,ParcodelleScienze,Parma,Italy
Chapter Outline
1.1 Introduction 1 1.5.2.1 MonteCarlosimulationsoftheMBE
1.2 BasicsofMBE 2 2Dgrowthprocess 17
1.3 ThetechnologyofMBE 4 1.5.2.2 EvidencebyRHEEDofthe2Dlayer-by-
1.3.1 MBEmachines 4 layergrowthmechanism 19
1.3.2 MBEgrowthchambers 4 1.5.2.3 Otherevidencesofthe2Dlayer-by-layer
1.3.3 Sourcesofmolecularbeams 5 two-dimensionalgrowthmechanism 21
1.3.3.1 Thermal-evaporationcells 6 1.5.2.4 Consequencesofthelayer-by-layer
1.3.3.2 Valvedcrackercells 7 growthmechanism 23
1.3.4 VariantsoftheMBEprocess 7 1.5.2.5 Effectsofinterfaceroughness
1.3.4.1 GassourceMBE,metalorganicMBE ondeviceproperties 24
andchemicalbeamepitaxy 8 1.5.2.6 Layer-by-layergrowthmechanismin
1.3.4.2 NitrideMBEandreactiveMBE 8 lattice-matchedorlowlylattice-
1.3.4.3 Group-IVMBE 9 mismatchedstructures 25
1.3.4.4 Migrationenhancedepitaxy(MEE) 1.5.3 Three-dimensionalgrowthmechanism 25
andatomiclayerMBE(ALMBE) 10 1.5.3.1 Three-dimensionalgrowthofself-
1.3.4.5 Dropletepitaxy 10 assemblednanoislands 25
1.3.5 Measurementsofmolecularbeamfluxes 10 1.5.3.2 Strainrelaxationasthedriving
1.3.6 Controlofcompositionanddopingprofilesalong forcefor3Dgrowth 25
growthdirection 11 1.5.3.3 Metamorphicstructuresandquantumdot
1.4 DiagnostictechniquesavailableinMBEsystems 12 strainengineering 29
1.4.1 RHEEDandsurfacereconstructions 13 1.5.4 MBE-assistedgrowthofone-dimensionalstructures 31
1.4.2 STMandsurfacereconstructions 14 1.6 Historicalbackground 33
1.4.3 Othertechniques(AES,SIMS,XPSandUPS) 15 1.7 Futureprospects 35
1.5 ThephysicsofMBE 16 1.8 Conclusions 36
1.5.1 Group-Vandgroup-IIIspeciesonIIIeVsurfaces 16 References 38
1.5.2 2Dlayer-by-layergrowthmechanism 17
1.1 INTRODUCTION
vapour-phaseepitaxy(VPE),metalorganicVPE(MOVPE)
In the last four decades, it has been largely proved that and molecular beam epitaxy (MBE), along with its metal-
epitaxial technologies for material growth have unique organicvariant (MOMBE).
advantages over simpler counterparts, in spite of their In this chapter it will be shown that MBE and variants
generally higher technological costs. Interesting epitaxial are particularly well suited for the growth of advanced
technologies developed since the end of the 1960s are epitaxialstructures;indeed,MBEallowsforthegrowthof
MolecularBeamEpitaxy.http://dx.doi.org/10.1016/B978-0-12-387839-7.00001-4
Copyright(cid:1)2013ElsevierInc.Allrightsreserved. 1
2 Molecular Beam Epitaxy
(i) materials with reduced concentrations of thermody- (SIMS), X-ray photoemission spectroscopy (XPS) and
namical defects, due to the relatively low growth temper- ultraviolet photoemission spectroscopy (UPS) are briefly
atures;(ii)structureswherecompositionordopingprofiles reviewed in Section 1.4. Section 1.5.1 deals with surface
in the growth direction can be modulated in abrupt or phenomena of group-III and group-V species on IIIeVs
continuous ways, a feature that has opened the way to (a) substrates, which determine the kinetic mechanisms of
the preparation of new epitaxial structures, (b) the fabri- MBE growth of IIIeVs and which have been supposedly
cation of innovative devices, (c) researches that were extendedalsotoothersystems;inSection1.5the2Dlayer-
awardedthe2000Nobelprizeinphysics(e.g.,Alferov[1]; by-layer growth mechanism is discussed, which results in
NobellectureandKroemer[2];Nobellecture);and,finally, thegrowthofinterfacesbetweenlattice-matched(orlowly
(iii) quantum structures, where engineered composition mismatched) materials smooth on the atomic scale; in
and doping profiles confine carriers in two- or three- particular, proofs of the occurrence of such a mechanism
dimensional regions with sizes smaller or comparable to are considered, which were obtained both experimentally
the de Broglie wavelength of carriers (Esaki [3]; von and by Monte Carlo simulation of the growth process.
Klitzing [4];Tsui [5] and Sto¨rmer [6];Nobel lectures). Section 1.5.3, instead, deals with the 3D growth of self-
In this chapter it will be emphasised that MBE growth assembled nanoislands, which takes place when the mate-
takes place according to two mechanisms that allow the rials in structures are lattice-mismatched by more than
fabrication of structures where carriers may undergo two- 2e3%;itwillbeshownthatthismechanismisthenatural
(2D) or three- (3D) dimensional quantum confinement; candidate for the preparation of quantum dot structures,
indeed,mostofquantumstructures(suchasquantumwells, characterisedbythe3Dconfinementofcarriers;inSection
superlattices,selectivelydopedheterostructures,structures 1.5.3itisshownhowstraincanbeconsideredasadegreeof
for resonant tunnelling, as well as quantum dots) and freedom to engineer the band structure of metamorphic
related deviceshave been demonstrated by MBE. structures.InSection1.5.4ashortconsiderationisgivento
ManyclassesofmaterialshavebeenpreparedbyMBE: the MBE-assisted growth of nanowires based on the
semiconductors, such as IIIeVs, IIeVIs, IVeVIs and vapoureliquidesolid mechanism. A re´sume´ of the histor-
IVeIVs, and also oxides, magnetic materials and metals; ical background of MBE is given in Section 1.6, while in
however, most of the work on the development of tech- Section 1.7someprospects ofMBE are presented; finally,
nology and physics of MBE reviewed in this chapter has inSection 1.8 afewconclusions are drawn.
been doneon IIIeVs.
Inthischapter,aftertheshortintroduction(Section1.1),
1.2 BASICS OF MBE
the basic elements of MBE technology are considered in
Section 1.2. Section 1.3 deals with different aspects of Molecular beam epitaxy (MBE) is an epitaxial process by
MBE such as (i) layouts of MBE machines used for which growth of materials takes place under UHV condi-
researchpurposes(Section1.3.1),(ii)growthchambersfor tionsonaheatedcrystallinesubstratebytheinteractionof
research (Section 1.3.2), and (iii) sources of molecular adsorbed species supplied by atomic or molecular beams
beams (Section 1.3.3); in Section 1.3.4 a description is [7]. The layers or deposits have: (i) the same crystalline
given of the numerous variants of MBE, developed either structure of the substrate or a structure with a similar
forgrowingdifferentmaterialsorfortakingadvantagesof symmetryand(ii)alatticeparameterdifferingfromthatof
particularfeaturesofMBEgrowthmechanisms.Then,two thesubstratebynomorethanw10%.Thebeamsgenerally
points are considered, which are particularly relevant for have thermal energy and are produced by evaporation or
the growth of structures with engineered composition and sublimation of suitable materials contained in ultra-pure
doping profiles: (i) the measurement of molecular beam crucibles.
fluxes (Section 1.3.5) and (ii) the accurate control of such In the archetypal case of IIIeV semiconductors grown
profiles(Section1.3.6).Oneofthemostinterestingfeatures on IIIeV substrates, such as InGaAsP on GaAs, adsorbed
of MBE is the availability of diagnostic techniques in group-III atoms and group-V molecules migrate on the
growthorinanalysischambers,allconnectedunderultra- heated substrate surface until theyinteract in proximity of
high vacuum (UHV). The use of these techniques, that in suitable vacant lattice sites, where they are incorporated
a few instances are available in-situ (in the growth into the solid phase (Section 1.5.1); in InGaAsP solid
chamber)andinrealtime(duringthegrowth),accountsfor solutions, In and Ga, as well as As and P, are randomly
the deep understanding of growth mechanisms, which, in distributed in the group-III and group-V sublattices,
turn, explains why MBE processes can be controlled very respectively.AtomicbeamsofelementssuchasSiandBe
accurately and with high yields. Techniques such as providen-type andp-typedoping inIIIeVcompounds.
reflection high-energy electron diffraction (RHEED), Asitwillbediscussedinmoredetailbelow,beamsmay
scanning tunnelling microscopy (STM), Auger electron alsobeproducedbyinjectioninthegrowthenvironmentof
spectroscopy (AES), secondary ion mass spectrometry gaseous species, such as AsH and PH (gas source MBE
3 3
Chapter | 1 Molecularbeamepitaxy: fundamentals,historicalbackgroundandfutureprospects 3
(GSMBE)) or of volatile metalorganic compounds carried constant,theAvogadro’soneandtheabsolutetemperature,
byhydrogen flows (metalorganicMBE, (MOMBE)). respectively [14]. In the case of incorporation of C, an
Cellsproducingionisedbeamswithnon-thermalenergy impurity deriving from CO e a typical MBE background
have been proposed: they may generate Zn-ion beams to specieseM ¼28ands w1(cid:3)10(cid:4)3.SinceT¼300K
CO C
dope IIIeVs [8] or As- and Sb-ion beams for Si ([9] [11], and s ¼ 1 (Section 1.5.1), it follows that, for GaAs
Ga
respectively)(Section 1.3.4.3); the ions are accelerated by growth rates of 1 ML/s (w1 mm/h), in order to have C
electricfields(afewhundredseV)towardsgrowinglayers concentrations of 1(cid:3)10(cid:4)9, it is required that the CO
in order to improve the sticking of dopants. Another backpressureisp ¼1.6(cid:3)10(cid:4)12Torr,avaluewhichfully
CO
example of beams with non-thermal energy is that of justifiesthe requirementofUHVconditions.
supersonic beams (a few tens eV) which are used to A very interesting consequence of the UHV growth
enhancesurfacemigrationofadsorbedspeciesand,then,to environmentisthatMBEgrowthtakesplaceinamolecular
improvethe morphology of deposits [12]. regime, as opposed to a viscous one, typical of vapour-
Interestingexamplesofepitaxialrelationshipsbetween phase technologies such asMOVPE, which is anepitaxial
layers and substrates are those between (i) IIIeV (100) technologies competing with MBE [15]. The molecular
zinc-blende layers and substrates (such as GaAs/GaAs, regime [14] is characterised by mean free paths between
AlGaAs/GaAs, InGaAs/GaAs, InGaAsP/InP and low N- collisions of atoms and molecules in the beams larger or
contentInGaAsN/GaAs);(ii)(0001)wurtziteInGaNlayers comparabletocriticallengthsofthegrowthsystem,suchas
on (111) Si, a-SiC and c-sapphire substrates; and (iii) distancesbetweencellsandsubstrates(<0.2m).Underthis
(0001) wurtzite CdS on (111) InP, with the (11e20) CdS regime, atoms and molecules basically do not interact
prismatic planes parallel tothe (1e10) InPones [13]. duringtheirpathsand,then,mechanicalbeamshutterscan
As mentioned above, important requirements of MBE beusedtoswitchonandoffthebeamsdirectedtowardsthe
growth are (i) the use of substrates heated at temperatures substrate;insuchaway,thecompositionofthenourishing
that depend on materials to be grown (Section 1.6.2) and phase can be abruptly changed in times given by the
(ii)UHVconditionsinthegrowthenvironment.Asfor(i), actuation times of shutters, that is, in the order of 0.1 s.
therelativelyhighsubstratetemperatures(500e600(cid:1)Cfor SincetheMBEgrowthrategenerallyis,atmost,intheML/s
GaAs) activate the efficient migration of adsorbed species range, the thickness of interfaces between layers with
onthegrowthsurface,whichisnecessaryfortheformation different composition or doping can be in the order of or
of an ordered lattice; this topic and the existence of smaller than tenths of an ML (1 ML ¼ 0.28 nm for (100)
optimum growth-temperature windows will be discussed GaAs).
more thoroughly while presenting the kinetic models of In order to confirm that MBE growth generally takes
MBE growth (Section 1.6.2). On the other hand, UHV place under a molecular regime, it is useful to recall that
conditionsarerequiredfor(i)minimisingtheincorporation accordingtothekinetictheoryofgasesthemeanfreepathl
of unintentional impurities from growth environment and, betweencollisionsofatomsormoleculesatapressurepis
then,obtaininghigh-puritymaterialsand(ii)optimisingthe givenby
surface morphology, which could be affected by surface
k T
contamination byspecificimpurities such asC[7]. l ¼ B
21=2ppD2
As for impurity incorporation, let us consider the case
of GaAs, which in many cases of basic and technological whereDisthediameterofatomsormoleculesinthebeam
interest should have a content of unintentional dopants of [14]. Therefore, for typical beams and MBE or MOMBE
1014 cme3 or lower. The concentration of an impurity i operatingpressuresof10(cid:4)6e10(cid:4)4Torr,itfollowsthatl¼
incorporated into the solid during growth is given by the 5e0.05m, respectively(Sections 1.2 and1.3.4.1).
ratioc¼(s N)/(s N ),whereN isthenumberofatoms It is interesting to compare these values with those of
i i Ga Ga k
k(k¼i,Ga)perunitsurfaceimpingingonthesurfaceand MOVPEthatischaracterisedbyoperatingpressuresinthe
s is the incorporation coefficient of the species k; s , in 10e760-Torrrange;consequently,lisshorter than50 mm
k k
turn, is given by the ratio between numbers of atoms k and, then, the regime is viscous. Therefore, any surface
incorporated and that of atoms impinging on the surface. immersedinthegasflowissurroundedbyaboundarylayer
WhileaccordingtothekineticmechanismsofMBEgrowth (BL or stagnant layer) due to the finiteviscosity of gases;
of GaAs (Section 1.5.1) N is related to the growth rate, BLs are defined as regions close to any surface where the
Ga
fromthe kinetictheoryof gases it follows that tangential component of gas velocity rapidly varies from
(cid:1) N (cid:3)1=2 vanishingly small values at the surface to values deter-
N ¼ p A mined by the main gasflow[15].
i i 2pM k T
i B The existence of BLs has significant effects on the
where p,M, k , N and Tare the partialpressure and the properties of epitaxial layers. In particular, since mass
i i B A
molecular weight of the species i, the Boltzmann’s transportinBLstakesplacebydiffusionandsincedifferent
4 Molecular Beam Epitaxy
chemicalspeciesmayhavesignificantlydifferentdiffusion
coefficients, after any abrupt change in the vapour-phase
composition,differentspeciesreachthegrowingsurfaceat
different times; therefore, layers of uncontrolled composi-
tion and doping are grown, until the new steady state is
reached. In other words, BLs actually act as temporary
sinks or reservoirs of the nourishing species which have
been switched on or off, respectively. Since BLs exist not
onlyaroundthesubstrate,butalsoclosetothereactorwalls
and in the gas piping, times as long as minutes may be
required to reach steady-state compositions of vapour
FIGURE1.1 SchematicviewofamodularmultichamberMBEmachine.
phase; therefore, the existence of BLs may result in
broadening of interfaces between materials with different
compositionand/ordoping;thebroadeningcanextendover
process(Section1.4).Suchanavailabilityisstrictlyrelated
hundreds of MLs or more, unless special precautions are
to the relatively small chemical aggressiveness of MBE
takeninthedesignandtheoperation ofepitaxial reactors,
growth environment.
such as low-pressure operation [15].
The use of multiple growth chambers is of interest for
Instead,itisworthrecallingthattheUHVconditionsof
both: (i) increasing the yield of a single machine that can
MBE growth environment (i) result in the absence of BLs
simultaneously carry out different growth runs and (ii)
and (ii) allow the use of mechanical shutters; both these
preparing heterostructures consisting of layers with
featuresare instrumental inachieving interfaces abrupton
significantlydifferentcompositions,whichcanbegrownin
the atomic scale, when no hindrance is set by kinetic
different chambers to avoid any cross-contamination. A
growth mechanisms (Section 1.5.1). The sharpness of well-known example is that of structures made of IIIeV
interfacesisaprerequisiteforthegrowthofnanostructures and IIeVI layers, since group-II and -VI species act as
where effects of quantum confinement may show up. unintentional dopants in IIIeVs, as do III- and V-group
atoms in IIeVIs.
Introduction chambers are provided with heaters to
1.3 THE TECHNOLOGY OF MBE
thoroughlyoutgaswaferholdersbeforetheirinsertioninto
1.3.1 MBE machines growth chambers. In case of IIIeV substrates, the outgas-
sing process is carried out at temperatures of hundreds of
MBE machines may have different layouts, depending on (cid:1)
Cfor tens ofminutes.
(i)whethertheyareusedforresearchorproduction,(ii)the
materialstobegrownand(iii)thespecificvariantofMBE
technology thatisactuallyimplemented.
1.3.2 MBE growth chambers
InthissectionwewillbrieflydealwithMBEmachines
forresearch,whicharebettersuitedforadiscussiononthe The basic elements of an MBE growth chamber are
fundamentalsofMBEtechnology.Moredetailsaregivenin (Figure 1.2) (i) a stainless steel vessel, that in case of
reviewpapers and books[7,14,16e21]. research machines generally has a diameter smaller than
Modern machines generally have a modular design, 20 inches, but that is much larger in case of production
where each module is optimised for a definite process. systems,(ii)cellstoproducemolecularbeams,whichwill
Figure 1.1 shows a typical MBE machine for research on be described in the next sections, (iii) shutters to switch
the growth of archetypal IIIeVs; it consists of (i) two molecularbeamsonandoff,(iv)asubstrateholderheatable
(cid:1)
growth chambers; (ii) an analysis chamber fitted with uptotemperaturesofseveralhundredsof Cand(v)aload-
different analytical techniques (Section 1.4); and (iii) lock system which allows for the introduction of wafers
a preparation chamber for in-situ processes such as etch- into the growth chamber and for their extraction without
ings,metallisationsorinsulatordepositions.Thewafersare breakingUHVinthegrowthenvironment.Anelectrongun
introduced in and extracted from the system through indi- is generally provided to study electron diffraction
vidually pumped introduction and extraction chambers, (RHEED) from growing surfaces in real time (Section
respectively, so that growth, analysis and preparations 1.4.1)
chambers, as well as transfer modules, are always under As discussed above, to limit the incorporation of
UHV. The transfers are carried out by means of suitable contaminants which may affect material quality and
translators. The availability of diagnostic techniques in morphology,thechambersareevacuatedtobasepressuresof
both growth and analysis chambers accounts for both the a few 10e11 Torr. For solid source MBE (SSMBE), this is
accurate control and the deep understanding of the MBE generallyachievedusingacombinationofion,titaniumand