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Molecular Beam Epitaxy in the Lithium-Niobium-Oxygen System

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Molecular Beam Epitaxy in the Lithium-Niobium-Oxygen System MOLECULAR BEAM EPITAXY IN THE LITHIUM-NIOBIUM-OXYGEN SYSTEM by Mario Petrucci Department of Electrical and Electronic Engineering, University College London. A thesis submitted to University College, University of London for the degree of Doctor of Philosophy. October, 1989 1 ProQuest Number: 10610929 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10610929 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 To the memory of my father, Vincenzo Petrucci. 'We work not only to produce But to give value to time." Eugene Delacroix "Oh, Mr. Scientist: you take what you see And fashion a wondrous soliloquy; But greater magic is forged in what you write: Where night becomes day and day becomes night!" 2 ABSTRACT The N b -L i-0 materials system offers an extensive combination of properties useful to integrated optics. N b ^ and L iN b 03 already have applications in waveguiding devices, such as electro-optic modulators, Fresnel lenses, and SAW transducers. In several cases, however, it would be desirable to grow these oxides on lattice-matched substrates as epitaxial thin films of controlled composition, crystallinity, and thickness: this thesis describes work aimed at achieving this goal. For the first time, Molecular Beam Epitaxy (M BE) was employed in depositing polycrystalline and single-crystal niobium oxide layers on z-cut L iN b 0 3 and z-cut sapphire. Two new phases were grown epitaxially: a variant of niobium monoxide (termed "NbO-6C fee"), and a hexagonal phase (composition N b 0125, termed "hex-7t"). Thin-film niobium metal was also deposited on z-cut L iN b 03. The post-deposition oxidation of oxygen-deficient, MBE- grown niobium oxide was studied, and optical waveguiding achieved in the resulting pentoxide films. The problem of oxygen deficiency in the as-grown material was addressed, leading to the design, construction and investigation of a novel oxygen radical source. Structural, compositional, and optical data are presented for all layers. Process control was greatly enhanced, giving reproducible growth in this system for the first time. A 50keV RHEED unit was constructed, capable of probing insulating substrate and oxide-layer surfaces. Using this, original data was obtained for L iN b 03 substrates and their pre-deposition surface behaviour, the crystal inner potential (V 0) was deduced for L iN b 03, and improved electron diffraction data was obtained for grown layers. An understanding was achieved of the limitations and promise of the MBE technique as applied to the N b -L i-0 system. The kinetics of the growth process were examined: this included the first detailed study of oxygen sticking coefficients for growth with and without Li flux. The prospects of MBE were assessed for the deposition of Nb 205 /LiN b0 3 layers that can form the basis of device structures. 298 words. 3 ACKNOWLEDGEMENTS Financial support for the project was supplied by the DTI/SERC JOERS programme Integrated Optical Devices, and by EEC ESPRIT programme #866. SERC also provided funding under the aegis of a JOERS award Improved Lithium Niobate. GEC Hirst Research Centre supplied CASE support under the supervision of Dr. C. Dineen. My family, friends and UCL colleagues have supported me throughout this project. Henry Robinson was a first-rate sounding board (and provider of accurate home truths). With humour and argument, both he and Steve Reynolds made long hours memorable. Maria Jones, Fred Stride, David Darbyshire, Mark Abbott, Tony Overbury, Fay Morris, Kevin Lee, Andy Lynch and Adrian Bailey have all provided help, and have stood in as drinking partners at one time or another. Ralph Betts (now at the University of New South Wales, Australia) gave help with initial operation of the equipment and ideas for future work. Clive Hall helped with orientation and X-ray studies. Maurice Gillett, Alan Gorrod and co-workers at UCL workshop were assiduous allies during system alterations, and I. Rangue of the Physics Department at UCL provided assistance with vacuum hardware. Dr. H.J. Milledge, Dr. M. Mendelssohn and P.A. Woods (UCL, Crystallography Unit, Department of Geological Sciences) took Weissenberg .and inclined-beam X-ray photographs of grown films. Dr. Milledge’s expertise was vital to their interpretation; in fact, the assistance of this team has been central to the success of my project. The continued interest they have shown me, and their readiness to assist at the most difficult of times, can only be described as philanthropic. Much the same can be said of Dr. C. Dineen, W.G. Freeman, A.J. Vale and B.R. Brown of GEC Hirst Research Centre, who answered innumerable questions on XRF/XRD analysis and SOS, and also gave invaluable assistance with the X-ray analysis of niobium oxide films and powders. Thanks are also due to Dr. P.J. Dobson ofPhillips (Redhill), for guiding me safely through the labyrinthine corridors of RHEED analysis, and for putting right my unusual (but embarrassingly wrong) pronunciation of "Ewald". N. Hill, J. Eaves, R. Attwood and others of V.G. Hastings were involved in the evaporator modifications, and Dr. G. Lovis was more than helpful in his advice on the problems thus encountered. F. Tothill and co-workers of V.G. Scientific assisted with the RHEED hardware described in Chapter 3. Dr. C. Whitehouse, G. Williams, Dr. R. Hardiman and Dr. D. Gasson(RSRE, Malvern) were generous with RHEED equipment and consultation time. Dr. A.M. Glazer of The Clarendon Laboratory (Oxford) provided the crystal structure projections and access to the Inorganic Crystal Structure Data File at the SERC Chemical Data Bank System, Daresbury. Dr. R. King (National Physical Laboratory) kindly performed the ellipsometry measurements reported in Chapter 5. D. Hall and I. Sutherland of Loughborough Consultants Ltd. conducted the ex situ SIMS, AES and XPS studies. Professor E.H.C. Parker and R.F. Houghton (Warwick University) performed trial silicon MBE depositions; CVD silicon was donated by C.M. Shaw, D.B. Meakin and R.S. Kean of GEC Hirst Research Centre. No acknowledgement would be complete, however, without a mention of Professor C.W. Pitt, my supervisor. His encouragement, foresight and insight have been instrumental in converting this project from a feasibility to an actuality. Thank you, Chris. Finally, my (too often unexpressed) thanks go to Mary and Michelle, for listening. 4 TABLE OF CONTENTS PAGE (1) TITLE PAGE ....................................................................................................... 1 (2) ABSTRACT ....................................................................................................... 3 (3) ACKNOWLEDGEMENTS.......................................................................................... 4 (4) TABLE OF CONTENTS ............................................................................................ 5 (5) LIST OF TABLES ....................................................................................................... 10 (6) LIST OF FIGURES ....................................................................................................... 11 (7) LIST OF AUTHOR’S PUBLICATIONS .................................................................... 14 (8) CHAPTER 1: THE GROWTH OF NIOBIUM OXIDES AND LITHIUM-NIOBIUM OXIDES BY MOLECULAR BEAM EPITAXY: Project Concept & Background ...................15 1.1 ABSTRACT .......................................................................................................... 15 1.2 INTEGRATED OPTICS: BACKGROUND ............................................................... 15 1J USEFUL MATERIALS IN THE Nb-Li-0 SYSTEM ............................................... 16 1.3.1 Lithium Niobate, LiNb03 ............................................................................................ 16 1.3.2 Niobium Pentoxide, Nb20 5 .......................................................................................... 17 1.4 NIOBIUM OXIDE DEPOSITION TECHNIQUES .................................................... 20 1.4.1 Introduction 20 1.4.2 Growth Methods for Thin-Film Niobium Oxide: A Review .................................... 20 1.4.3 A Note on Lithium Niobate Deposition ....................................................................... 21 1.5 MOLECULAR BEAM EPITAXY ...............................................................................21 1.5.1 The MBE Process ..........................................................................................................21 1.5.2 MBE: Areas of A p p licatio n ......................................................................................... 22 1.5.3 MBE of Niobium Oxides: Potential Benefits ............................................................ 22 1.6 REASONS FOR NIOBIUM OXIDE DEPOSITION BY MBE; PROJECT AIMS .........................................................................................................24 1.6.1 Understanding of L i-N b -0 Growth by MBE; Improved M a te ria l ............................24 1.6.2
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