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Undoped Algaas/Gaas Quantum Dots with Thermally Robust Quantum Properties

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Undoped Algaas/Gaas Quantum Dots with Thermally Robust Quantum Properties THE UNIVERSITY OF NEW SOUTH WALES SCHOOL OF PHYSICS Undoped AlGaAs/GaAs Quantum Dots with Thermally Robust Quantum Properties Andrew Ming See A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy September 2011 1 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: SEE First name: MING Other name/s: ANDREW Abbreviation for degree as given in the University calendar: School: SCHOOL OF PHYSICS Faculty: FACULTY OF SCIENCE Title: Undoped AlGaAs/GaAs Quantum Dots with Thermally Robust Quantum Properties Abstract 350 words maximum: (PLEASE TYPE) In a modulation-doped AlGaAs/GaAs heterostructure, electrons in the two-dimensional electron gas (2DEG) are provided by ionization of Si dopants in the AlGaAs layer. To reduce the effect of Coulomb scattering between ionized dopants and electrons in the 2DEG, an undoped AlGaAs spacer is grown between the doped AlGaAs and the undoped GaAs layers, which reduces large-angle scattering, thus increasing the mobility and electron mean free path. In a traditional semiclassical picture of an open quantum dot, if the electron mean free path exceeds the dot width, transport becomes ballistic and the corresponding magneto-conductance fluctuations (MCF) are considered as a Fourier sum of periods arising from all possible Aharonov-Bohm loops that intercept the quantum point contacts and are formed by scattering from the dot walls alone. As a result, these devices, known as semiconductor billiards; were seen as ideal for studies of dynamical chaos in the quantum mechanical limit. However, modulation-doped devices are not without their problems. For example, it has recently been demonstrated that small-angle disorder scattering causes unpredictable changes in the device’s electronic properties each time it is cooled for use. This finding forces a careful reconsideration of our notions of ballistic transport in these devices. Another problem associated with modulation-doped devices is the temporal instability due to rapid switching of the dopants between ionized and de-ionized states, hindering the development of ultrasensitive quantum devices. This thesis reports the development of undoped quantum dots, where the ionized dopants are removed and the 2DEG is populated electrostatically by applying a positive bias to a degenerately doped cap. Our “induced” devices produce MCF that are reproducible with high fidelity after thermal cycling to 300 K. By performing a comparative analysis between nominally identical undoped and modulation-doped billiards, we conclude that small-angle scattering dominates transport in dots. Our work has important implications for studies of quantum chaos and ballistic transport. Additionally, measurements of our small undoped quantum dot operating in the Coulomb blockade regime showed features of excited state transport and spin- dependent transport blockade. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS COPYRIGHTSTATEMENT ‘IherebygranttheUniversityofNewSouthWalesoritsagentstherighttoarchiveand tomakeavailablemythesisordissertationinwholeorpartintheUniversitylibrariesin allformsofmedia,noworhereafterknown,subjecttotheprovisionsoftheCopyright Act1968.Iretainallproprietaryrights,suchaspatentrights. Ialsoretaintherighttouseinfutureworks(suchasarticlesorbooks)allorpartofthis thesisordissertation.IalsoauthorizeUniversityMicrofilmstousethe350wordabstract ofmythesisinDissertationAbstractInternational(thisisapplicabletodoctoraltheses only). IhaveeitherusednosubstantialportionsofcopyrightmaterialinmythesisorIhave obtainedpermissiontousecopyrightmaterial;wherepermissionhasnotbeengrantedI have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed……………………………………………........................... Date……………………………………………........................... AUTHENTICITYSTATEMENT ‘IcertifythattheLibrarydepositdigitalcopyisadirectequivalentofthefinalofficially approvedversionofmythesis.Noemendationofcontenthasoccurredandifthereare any minor variations in formatting, they are the result of the conversion to digital format.’ Signed……………………………………………........................... Date……………………………………………........................... 2 ORIGINALITYSTATEMENT ‘Iherebydeclarethatthissubmissionismyownworkandtothebestofmyknowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any otherdegreeordiplomaatUNSWoranyothereducationalinstitution,exceptwhere dueacknowledgementismadeinthethesis.Anycontributionmadetotheresearchby others,withwhomIhaveworkedatUNSWorelsewhere,isexplicitlyacknowledgedin thethesis.Ialsodeclarethattheintellectualcontentofthisthesisistheproductofmy ownwork,excepttotheextentthatassistancefromothersintheproject'sdesignand conceptionorinstyle,presentationandlinguisticexpressionisacknowledged.’ Signed…………………………………………….............. Date…………………………………………….............. 3 Acknowledgements Firstandforemost,IwouldliketothankmysupervisorsA/Prof.AdamMicolich andProf.AlexHamiltonfortheirunconditionalsupportandinspirationsthroughoutthe course of my PhD candidature. Adam and Alex have taught me a lot about semiconductor physics, research methods, experimental techniques, science communication skills as well as life in general. I am very grateful to have them as supervisors,mentorsandfriends. Secondly,IwanttosaythankstomycollegesattheQuantumElectronicDevices (QED)groupformakingthisjourneysoenjoyable:toDr.OlehKlochanforshowingme howtorunthefridgeandsetupthemeasurementsproperly;toDr.JackCochranefor histechnicalsupport;toDr.WarrickClarkeforgettingmestartedinthecleanroom;to Daisy Wang for simulating the transport scattering times, and to Dr. Ted Martin, Dr. LasseTaskinen,Dr.LapHangHo,Dr.AdamBurke,Dr.ZacharyKeane,Sarah Macleod, Jason Chen, Sebastian Fricke, Patrick Scriven, LaReine Yeoh, Ashwin Srinivasan, Rifat Ullah,SpencerRussettfor,DavidWaddingtonandRoyLiforhelpfuldiscussions. I would also like to acknowledge the Australian National Fabrication Facility (ANFF) for providing us with the cleanroom and measurement facilities. I want to especiallythankBobStarrettandDavidBarberfortheirassistanceincryogenics;Dr.Eric Gauja,Dr.FayHudson,Dr.JoannaSzymanska,Dr.AndreasFuhrer,Dr.AndreaMorello andFrankWrightforusefuladviceindevicefabrication.IwanttoalsothankProf.Saskia Fischer,Prof.MarkEriksson,Prof.AndrewSachrajda,Prof.KlausEnsslinandProf.Sven Roggeforinsightfuldiscussionsthatledtosomeoftheresultspresentedinthisthesis. Additionally, I want to thank everyone in the School of Physics whom I have great pleasure working with: to the Head of School Prof. Richard Newbury for his support in writing this thesis; to David Jonas and Kristien Clayton for resolving our computerͲrelated issues; to Prof. Michael Gal, Prof. Michael Ashley, Sue Hagon, Ranji Balalla,AlbertMcMaster,JoselitoConducto,StephenLo,PatriciaFurst,DaveRyanand mygoodfriendPatrickMcMillian,fortheiradministrativesupport. 4 DuringAugustͲSeptember2010,IhadtheopportunitytocollaboratewithProf. RichardTaylorandhisgroupattheUniversityofOregonduringmyfourweeksofvisit.I wanttothankRichard,Dr.BillyScannell,IanPilgrim,RickMontgomeryandPeterMorse forawonderfulexperience(well,apartfromeatingthatBaconMapleAle!). DuringthecourseofmyPhDcandidature,Ihadconcurrentlyparticipatedinthe Research Commercialization Training Scheme (CTS): I would like to thank Dr. Wallace BridgeformakingthisgraduatecourseavailableforPhDstudentsacrossanumberof universitiesinAustralia;Ihadawonderfultimelearningaboutthecommercialsideof research,aswellascompetinginthefinalsofthePeterFarrellCup2008withDr.Kai WeiChuandSidneyHsiong. Lastbutnotleast,Iowemymostsinceregratitudetomyparents–Leonand Amy See, who have worked so hard in Hong Kong in order to support my studies in AustraliasinceYear9.Withouttheirunconditionalloveandsupport,Iwouldneverhave made it this far. I also want to thank my grandparents for always looking after me. Finally, I want to thank my beloved wife Selina for her patience and understanding duringthetoughtimesinthisjourney. 5 ListofPublications Journalpublications x “ImpactofSmallͲAngleScatteringonBallisticTransportinQuantumDots”–A.M. See,I.Pilgrim,B.C.Scannell,R.Montgomery,O.Klochan,A.M.Burke,M. Aagesen,P.E.Lindelof,I.Farrer,D.A.Ritchie,R.P.Taylor,A.R.HamiltonandA.
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