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Enhancement of the Figure of Merit of Silicon Germanium Thin Films for Thermoelectric Applications

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Enhancement of the Figure of Merit of Silicon Germanium Thin Films for Thermoelectric Applications Enhancement of the Figure of Merit of Silicon Germanium Thin Films for Thermoelectric Applications by Lo¨ısd'Abbadie Thesis submitted as a requirement for the Degree of Doctor of Philosophy School of Materials Science and Engineering Submitted: March 31st,2013 Supervisor: Prof. Sean Li PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES TheslsiD~ s""' S\Jmame 0< Fomily nome: crAbbadie Fnt name: lois Oth"r MJM/s: Pif!rre Stephane Abbtevi31ion fer degme as given in lhe University calendar:PhO School: MSE Faculty: SO..nce lotle: Enhancement of the Figure of Merit of Silicon Germanium Thin Films for Thermoelectric Applications A~ 350 words maximum: (PLEASE TYPE) Silicon Germanium thin films are !he most stable lbermoelcctric materials at high temperatures. Nonetheless, low efficiency and limited knowledge of such structures are still a challenge to research. Understanding !be various mechanisms taking place in !be maner and !heir relationships is !he next step to boost research and to enhance !be TE efficiencies. widening the range of applications. in this thesis, we develop a malbematical approach based on solid Sl3lC theory to calculate !he figure of merit from an electronic band structure. Added to DFr, this near ab initio melbod rapidly assesses virtual structures as possible TE materials. l.n this report, !be melbod is applied to bulk siliron germanium ~>ith good agreement with experimental results. Throughout tbe development of this method, we also conclude !bat TE semiconductors band-gaps are related to the range of temperature where !be material show higher val ues of zr. We also show that doping with donors and acceptors. which is a common enhancement, need optimi zation for high temperatures applications due to its contribution to the lbermal conductivity. Moreover !he carriers' mobility is a prevalent parameter but its calculation remains complex. So, we implement deformation potentiallbeory with DFr to calculate the electron-phonon interactions in silicon germanium alloys. We find no change of interactions wilb the alloy romposition. With enough computing power, these methods are applicable to low dimensions structures.ln addition to our lbeoretical study, we report a sputter deposition melhod of silicon germa:nium alloy thin ftlms wilb rontroUed composition and lhickoess, grown on a sputtered layer of silicon dioxide. XRD study shows the appearance of a crystal phase beginning at deposition temperatures of 650 C . Reflectivity and TEM provide a consistent measurement of deposition thickness in !he range of 20 to 100 om wilb average interfaces roughmess around 2 nm. Dec:lonl1lon _.,... to disposition of project tl>esisldlssertalion I heteb)f SratJ1 to lhe University of New Soo1h Wales or its agents the right to 0/ehlve and to make available my thesl$ or diosertatlon in whole or In part in lbe UniYe<say ibrarie$ in al fomlS of media, ,_0< ht>re after known. subject to the provisions of the Copyright Acl1968. 1 retain all pcopeey rights, SUCh as palent rights.. I also re!ain the righl to u.., in Mure worlts (sUCh as articles or bO<Jks) all or part of this thesis or dissertation. Ialso au1florise UniYe<say Miaofitns to <ISe lhe 3SO word abstract of rrry IronAbstracts tnterru>IJORal (this IS applicable to doctoral tl.e=Oiltt~ c ~· -·-·-6 '-- ~~ 1l; ~ 1\~~....&."'cl . \\ Iot;.j-z<>L~ . Tlw: ~ • : · ':"~lisa that there may be exoeptional circumstances requiring restridions on copy.ing or conditions on use. Requests for re:stJICtJC)n for a period of up to 2 years must be made .n wrrnng. Requests for a longer period of restnd•on may be cons1dered in exceptional c:ircumsta."K:eS and R!OUire the of lhe Dean of Gr.lduate Resealeh. FOR OFFICE USE ONLY Date of eornpleti0<1 of requirements fe< Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS Originality Statement 'I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or sub- stantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and con- ception or in style, presentation and linguistic expression is acknowledged.' Date Signature 31/03/2013 -i- Copyright Statement 'I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or 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 proprietary 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 have either used in substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Date Signature 31/03/2013 Authenticity Statement 'I certify that the Library deposit digital copy is a direct equivalent of the final of- ficially approved version of my thesis. No emendation of content has occured and if there are any minor variations in formatting, they are the result of the conversion to digital format.' Date Signature 31/03/2013 -ii- Acknowledgement First and foremost I offer my gratitude to my supervisor, Prof Sean Li, who has sup- ported me throughout my thesis with his knowledge whilst encouraging me during difficult parts of the work. His optimistic nature always sees the bright side of any situation, setting you to new interesting directions. Without his support, this thesis would not have been completed. One could not wish for a friendlier supervisor. I would like to thank my co-supervisor Dr Chucheng Yang for his meticulous ob- servation. I hope him well with his new research team. In the various laboratories, I have been aided by Dr Tan Thiam Teck (alias TT ) for his help and formation on most of the equipment, his insight in the management of my samples, his available knowledge and his patience. I also want to thank Dr Mohammad Hussein Naseef AL Assadi for his help and his corrections with DFT simulations. I also thank Dr Yu Wang from the solid state and elemental analysis unit and Dr Charlie Kong from the electron microscope unit for their help on the equipment and their advices on how to perfect measurements. I am grateful to Prof Charles Christopher Sorrell for his patience and his sometimes needed pushes. To Lana Strizhevsky who orientated me with patience through admin- istration procedures. To my friends from very diverse horizons whom with I shared some refreshing discussions during lunch breaks. Finally, I thank my parents, Dr G´erardd'Abbadie and Marie-Andr´eeXiste-d'Abbadie, for supporting me throughout all my studies and who always encouraged me to travel and see the world myself. I think of my two brothers and sister, Micha¨el,Aude and Luc whom I lived away from for too long. My uncle, Gilles Espitalier-No¨el, and aunty Marie-Paul Espitalier-No¨elfor their regards. I also thank my partner, Amandine Re- nard, for her patience and her support during all this time. -iii- Abstract Silicon Germanium thin films are the most stable thermoelectric materials at high temperatures. Nonetheless, low efficiency and limited knowledge of such structures are still a challenge to research. Understanding the various mechanisms taking place in the matter and their relationships is the next step to boost research and to enhance the TE efficiencies, widening the range of applications. In this thesis, we develop a mathematical approach based on solid state theory to calculate the figure of merit from an electronic band structure. Added to DFT, this near ab initio method rapidly assesses virtual structures as possible TE materials. In this report, the method is applied to bulk silicon germanium with good agreement with experimental results. Throughout the development of this method, we also conclude that TE semiconductors band-gaps are related to the range of temperature where the material show higher values of ZT . We also show that doping with donors and acceptors, which is a common enhancement, need optimization for high temperatures applications due to its contribution to the thermal conductivity. Moreover the carriers' mobility is a prevalent parameter but its calculation remains complex. So, we implement deformation potential theory with DFT to calculate the electron-phonon interactions in silicon germanium alloys. We find no change of interactions with the alloy composition. With enough computing power, these methods are applicable to low dimensions structures. In addition to our theoretical study, we report a sputter deposition method of silicon germanium alloy thin films with controlled composition and thickness, grown on a sputtered layer of silicon dioxide. XRD study shows the appearance of a crystal phase beginning at deposition temperatures of 650 ◦C . Reflectivity and TEM provide a consistent measurement of deposition thickness in the range of 20 to 100 nm with average interfaces roughness around 2 nm. -iv- CONTENTS Originality Statement . i Copyright Statement . ii Authenticity Statement . ii Acknowledgement . iii Abstract . iv List of Figures . viii List of Tables . xii 1 Introduction 1 2 State of the art thermoelectric materials 6 2.1 Development of thermoelectrics .
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