© Naureen Ghafoor, 2008 ISBN: 978-91-7393-946-1 ISSN 0345-7524 Printed by LiU-Tryck, Linköping University, Sweden

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(2) © Naureen Ghafoor, 2008. ISBN: 978-91-7393-946-1 ISSN 0345-7524 Printed by LiU-Tryck, Linköping University, Sweden.

(3) All praises for Allah who is the entire source of knowledge and wisdom endowed to mankind and all respect for The Holy Prophet (PBUH) who is forever a torch of guidance.

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(5) Abstract This thesis treats the reective and structural properties of multilayer structures. Soft X-ray multilayer mirrors intended as near-normal incidence reective optics and polarizers in the water window (=2.4-4.4 nm) are the main focus. Such mirrors require multilayer periodicities between 1.2-2.2 nm, a large number 600 of multilayer periods (N), and atomically at interfaces. Bi-metallic multilayers were deposited by dual-target magnetron sputtering on Si(001) substrates at ambient temperatures. Material combinations were chosen to maximize the theoretical reectance based upon simulations. Hard and soft X-ray reectivities, hard Xray diraction, high resolution transmission electron microscopy, and electron diraction were used for structural characterization of the mirrors, whereas elastic recoil detection analysis and Rutherford back scattering spectroscopy were used for compositional analysis. Geometrical roughness and intermixing/interdiusion at the interfaces were investigated in connection with the impact of ion-surface interactions during growth of Cr/Ti, Cr/Sc, and Ni/V multilayers. This was achieved by comparing multilayers grown with or without highux low energy (Elrq ?30 eV) ion assistance. The use of modulated ion assistance resulted in a substantial improvement of interface atness and abruptness in each of the materials system. State-of-the art reectivities of 2.1 %(Cr/Ti), 20.7%(Cr/Sc), and 2.7%(Ni/V) were achieved. It is found that Cr/Sc multilayers exhibit the highest quality and as many as 600 periods can be grown, but accumulation of local atomic distortions (jaggedness) limits the reectivity at very large N. Moreover, it is shown that amorphous Cr/Sc multilayers exhibit smooth interfaces compared to nano-crystalline layers and that multilayers with period less than 2 nm are ‘self healing’, i.e., they re-gain smooth amorphous layers after morphological disturbances. Amorphization of Sc and Cr layers takes place below certain thicknesses. Ab-initio calculations indicate that the stabilization of the amorphous layer structure is due to a lowering of the total energy of the system by eliminating high energy incoherent interfaces between crystalline Sc and Cr. Light element incorporation in Cr/Sc multilayers was investigated through residual gas pressure variation. It is shown that multilayers retain their structural and optical properties within the high vacuum range of 2×1037 -to-2×1036 Torr. The incorporation of 34 at.% nitrogen at a higher residual gas pressure ( 2×1035 Torr) resulted in highly textured understoichiometric CrN{ /ScN| multilayers. As a result of nitrogen incorporation, interface widths as small as 0.29 nm, and near-normal incidence reectivity enhancement (at =3.11 nm) by 100 % (compared to pure Cr/Sc multilayers) was achieved. Light element incorporation was also found to be advantageous for the thermal stability of the multilayers. In-situ hard X-ray reectivity measurements performed during isothermal annealing in the temperature range 25-900 C has shown that understoichiometric CrN{ /ScN| are stable up to 350 C. As an alternative route to metallic multilayers, single crystal CrN/ScN superlattices, grown by reactive sputtering in N atmosphere onto MgO(001), were also investigated. The superlattice synthesis at 735 C, resulted in highly abrupt interfaces with minimal interface widths of 0.2 nm. As-deposited superlattices with only 61 periods showed an absolute soft X-ray reectance of 6.95% at =3.11 nm as well as very high thermal stability up to 850 C..

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(7) Populärvetenskaplig Sammanfattning Denna avhandling behandlar syntes, analys, och materialvetenskap rörande så kallade multilagerspeglar för mjuk röntgenstrålning. Speglarna är lämpade som optiska komponenter för instrument såsom röntgenmikroskop i våglängdsområdet 2,4 nm till 4,4 nm, även kallat vattenfönstret. Tack vare de senaste decenniernas stora teknologiska och vetenskapliga framsteg i att framställa mycket intensiva källor för mjuk röntgenstrålning, såsom tex synkrotronljuskällor, frielektronlasrar, och plasmagenererade källor, är det nu tänkbart att utnyttja denna strålning till nya tillämpningar som tidigare inte varit möjliga. Några exempel är; röntgenmikroskopi av biologiska preparat med upplösning ca 1/100 av det som är möjligt med synligt ljus, fotolitogra av strukturer endast några tiotals nm stora, samt för studier av dynamiska förlopp inom kemi och biologi med en tidsskala på femtosekundsnivå eller kortare. Även inom astronomin nns det ett stort intresse av att avbilda avlägsna naturliga röntgenkällor. För dessa ändamål krävs högupplösande optiska instrument med så få förluster som möjligt. Tyvärr uppvisar de esta material ett brytningsindex för mjukröntgen som är mycket nära 1 samt en kraftig stor absorption. Detta gör att det inte är möjligt att utforma de optiska elementen, dvs linser och speglar, på konventionellt sätt i sådana instrument. För att uppnå hög reektans för mjuk röntgenstrålning vid nära normalt infall utnyttjas därför möjligheten att skapa konstruktiv interferens mellan röntgenvågor som reekterats i ett stort antal mellanytor inne i en lagrad struktur av nanometertunna skikt, en så kallad interferensspegel, här kallad multilagerspegel. Det nns era stora utmaningar för att lyckas tillverka multilagerspeglar. Först och främst måste man hitta materialkombinationer som ger upphov till reektion i mellanytorna mellan materialen men som inte samtidigt absorberar all röntgenstrålning. Dessutom måste materialen gå att belägga på varandra i era hundra tunna lager, vart och ett endast ca 1 nanometer tjockt, med en ytojämnhet om endast några tiondels nanometer. Den absoluta tjockleks precision i varje lager som krävs för att uppnå perfekt konstruktiv interferens ligger på femtometernivå. Detta kräver stora kunskaper inom såväl materialvetenskap som beläggning av extremt tunna lmer och är huvudämnet för denna avhandling. I det här arbetet har fyra olika typer av multilagerbeläggningar undersökts: krom/titan (Cr/Ti), krom/skandium (Cr/Sc), nickel/vanadin (Ni/V) samt kromnitrid/skandiumnitrid (CrN / ScN). Materialvalen har baserats på teoretiska beräkningar som visat att dessa materialsystem genererar mycket god reektans i vattenfönstret. Varje kombination av metaller är optimal för en specik våglängd och de individuella lagertjocklekarna måste optimeras teoretiskt för varje enskilt fall. Beläggningarna av speglarna gjordes under lågt tryck, ca 1036 atmosfärer av argon och kväve, med så kallad magnetronsputtring där ödet av metallatomer noggrant kunde kontrolleras med hjälp av datorkontrollerade slutare. Under arbetets gång utvecklades en ny teknik, som med hjälp av ett stort öde av lågenergetiska joner till den växande lmen, resulterar i en tydlig förbättring av metallskiktens jämnhet. Genom att förna metoden, så att jonödet regleras under beläggningen av varje skikt, har även mellanytorna kunnat göras mer abrupta. I förlängningen ledde dessa forskningsresultat till att multilagerbeläggningar med överlägsen reektans kunde framställas. Till exempel uppmättes rekordreektansen 20% för röntgenstrålning av våglängden 3,11 nm för ett Cr/Sc multilager som tillverkats med sex hundra 0,605 nm tjocka Cr-lager varvade med lika många 0,985 nm tjocka Sc-lager. För Cr/Ti och Ni/V uppnåddes reektanserna 2,4% för 2,74 nm våglängd respektive 2,7% för 2,42 nm våglängd, vilken ännu är det högsta rapporterade värdet för så kortvågig röntgenstrålning. För Cr/Sc multilager har vi visat att lagren som beläggs har en oordnad, så kallad amorf, struktur av metallatomerna som har sitt ursprung i att multilagrets totala energi kan sänkas om mellanytor mellan kristallint Cr och kristallint Sc kan undvikas. Studier av eekterna av kväveupptag hos Cr/Sc multilagerspeglar under sputtringsprocessen har lett till ökad förståelse av materialsystemet. Till exempel har vi visat att kvävet.

(8) framförallt binder till de inre regionerna av Sc och inte så mycket till Cr-lagren eller i mellanytorna. Med kväve i strukturen har vi gjort speglar som tål höga temperaturer, vilket är av stor betydelse för tillämpningar baserade på högintensiva ljuskällor. Så kallade supergitter, dvs multilager tillverkade i form av en enda kristall med lagringen av material inom själva kristallen, visade sig vara mycket stabila upp till 850 ◦ C med bibehållen röntgenreflektans. Supergitter har dock den svagheten att de är svåra att belägga på substrat för optiska komponenter eftersom de kräver ett kristallint substrat och hög beläggningstemperatur. Ytterligare ett viktigt resultat är därför att polykristallina Cr/Sc multilagerspeglar med 34 atom-% kväve i strukturen, som tillverkats vid rumstemperatur på vanliga kiselsubstrat, visade sig stabila upp till 250 ◦ C och gav en förbättrad mjukröngenreflektans jämfört med rena Cr/Sc multilagerspeglar..

(9) Preface This doctorate thesis is a result of my PhD studies from 2004 to 2008 at the Thin Film Physics Division of the Department of Physics, Chemistry, and Biology (IFM) at Linköping University. Research on the development of soft X-ray multilayer mirrors was initiated about ten years ago at the department and I have been involved in this work since my Master’s diploma studies in 2002. The work has been presented at nine international conferences and is published in several scientic journals. At the end of this thesis seven of my articles, most related to the present work, are included. This research has been nancially supported through the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF)..

(10) Publications Included in this Thesis and My Contribution Paper I “Atomic scale interface engineering by modulated ion assisted deposition applied to soft X-ray multilayer optics”, F. Eriksson, N. Ghafoor, F. Schäfers, E. M. Gullikson, S. Aouadi, S. Rohde, L. Hultman, and J. Birch, submitted (2008) I performed TEM characterization and took active part in writing the manuscript. Paper II “Eects of ion-assisted growth on the layer denition in Cr/Sc multilayers”, N. Ghafoor, F. Eriksson, P. O. Å. Persson, L. Hultman, and J. Birch, Thin Solid Films 516, 982 (2008) I grew all the samples, performed TEM and XRD analyses, and wrote the article with the assistance of the co-authors. Paper III “Incorporation of nitrogen in Cr/Sc multilayers giving improved soft x-ray reectivity”, N. Ghafoor, F. Eriksson, E. M. Gullikson, L. Hultman, and J. Birch, Appl. Phys. Lett., 92, 091913 (2008) I did all the experiments except ERDA and SXR analyses, and wrote the paper with assistance from the co-authors. Paper IV “Eects of O and N impurities on the nanostructural evolution during growth of Cr/Sc multilayers”, N. Ghafoor, F. Eriksson, A. Mikhaylushkin, I. Abrikossov, E. M. Gullikson, M. Beckers, U. Kressig, L. Hultman and J. Birch, submitted (2008) I actively contributed in planning the experiments, designed and grew sophisticated stacked multilayers structures, performed extensive TEM work and HXR analyses, and I wrote the paper together with the co-authors. Paper V “Reectivity and structural evolution of Cr/Sc and nitrogen containing Cr/Sc multilayers during thermal annealing”, F. Eriksson, N. Ghafoor, L. Hultman, and J. Birch, submitted (2008) I participated in planning of the experiments, designed and grew the samples, performed all TEM characterization and took active part in analyses and writing of the nal manuscript. Paper VI “Single crystal CrN/ScN superlattice soft X-ray mirrors: Epitaxial growth, structure, and properties”, J. Birch, T. Joelsson, F. Eriksson, N. Ghafoor, and L. Hultman, Thin Solid Films, 514, 10 (2006) I took part in SXR measurements and participated in writing the nal manuscript. Paper VII “Interface engineered ultra-short period Cr/Ti multilayers as high reectance mirrors and polarizers for soft X-rays of =2.74 nm wavelength”, N. Ghafoor, P. O. Å. Persson, F. Eriksson, F. Schäfers, and J. Birch, Appl. Opt. 45, 137 (2006) I designed and grew the samples, performed all the characterization, made the analysis, and wrote nal manuscript with the help of the co-authors..

(11) Related Publications Paper VIII “Interface Engineering of Short-period Ni/V multilayer X-ray mirrors”, F. Eriksson, N. Ghafoor, F. Schäfers, E. M. Gullikson, and J. Birch, Thin Solid Films, 500, 84 (2006) Paper IX “Role of B4 C on structure, optical performance, and thermal stability of short period soft X-ray multilayers” N. Ghafoor, F. Eriksson, E. M. Gullikson, and J. Birch, in manuscript (2008) Paper X “Inuence of concurrent ion-bombardment during magnetron sputter deposition of Mo/Si multilayers”, J. Romero, N. Ghafoor, F. Eriksson, and J. Birch, in manuscript (2008). Conference Contributions During my PhD studies I have had the opportunity to attend several international conferences. Some of the conferences where I personally presented my research are listed here. • “Soft X-ray Cr/Sc multilayer mirrors: eects of B4 C, O and N impurities”, The 9wk international conference on Physics of X-ray Multilayer Structures (PXRMS), Montana, USA, Invited talk, O-2.1 (2008) • “Ion assisted growth of short period Ni/V multilayers”, The 15wk International Summer School on Vacuum Electron and Ion Technology (VEIT), Sozopol, Bulgaria, Contributed talk, OP-07 (2007) • “Cr/Sc Multilayer Mirrors: Inuence of impurities on amorphous-to-crystalline layer transformation and optical properties”, The 17wk International Vacuum Congress (IVC), Stockholm, Sweden, Contributed talk, TFS01-O4 (2007) • “Cr/Sc X-ray Multilayer Mirrors: Inuence of impurities on structure and performance of Cr/Sc multilayer mirrors”, Advances in X-ray/EUV Optics and Components II, Optics and Photonics (SPIE), San Diego, USA, Contributed talk, 6705-10 (2007) • “Interface engineered Cr/Sc multilayer mirrors for normal incidence reection of soft Xrays range at Sc (=3.1 nm) absorption edge”, International Summer School on Surfaces, Thin Films, Nanostructures and Applications (CIIT-ISESCOC), Lahore, Pakistan, Contributed talk, O-22 (2006) • “Roughness propagation in short-period Cr/Sc and Cr/Ti multilayers”, The 8wk international conference on Physics of X-ray Multilayer Structures (PXRMS), Sapporo, Japan, Contributed talk, S7-04 (2006) • “Nanostructural evolution of interface engineered Cr/Ti Multilayers”, The International Conference on Metallurgical Coatings and Thin Films (ICMCTF), San Diego, USA, Contributed talk, F3-03 (2005) • “Interface engineered Ti/Cr multilayer mirrors for normal incidence reection of soft Xray range at Ti-2p edge”, International Conference on Thin Films (ICTF-13/ACSIN-8), Stockholm, Sweden, Contributed talk, O-137 (2005) • “Ti/Cr multilayer mirrors for normal incidence in the soft x-ray range at the Ti-edge”, The 7wk international conference on Physics of X-ray Multilayer Structures (PXRMS), Sapporo, Japan, Poster presentation, P2-09 (2004).

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(13) Acknowledgements I never think of the future. It comes soon enough. Albert Einstein. So, the time has arrived for me, so to say, to wrap-up last six years of my work. Where my mind, in these long hours of writing nal manuscripts and thesis, is linking several dispersed concepts of multilayer science, it is also rewinding dierent stages of PhD. All the excitements when I had just seen nice reectivity peaks in Asterix, or wonderful layered structures in Galadriel, and occasional frustrations when B 4 C was so stubborn to start and ADAM had just refused to listen me; all feelings are now equally great memories. And above all this time is reminding me every single person involved in my work and/or has made my time joyful at IFM or outside work. Though, my vocabulary here is very limited, I would give a try to express my gratitude to, Jens Birch-my wonderful supervisor, your guidance, encouragement and endless patience during all these years has made me accomplish this thesis. Your fundamental knowledge in literally everything and philosophy of transferring as much knowledge as possible to everyone has always inspired me. Thanks Jens! for believing in me and giving me so much freedom in work. It has been great to have you as my supervisor. Thanks for, all the scientic discussions, helping me in every little thing, and for all the fun-travelling around the world. Lars Hultman-my group leader, for creating an inventive working environment in the group and for having an encouraging concern in my research. Although, I have had hard times to understand a high level microscopy language, it has been rewarding to have all ‘instant’ discussions with you. Fredrik Eriksson-my ancestor in the eld, for his signicant contribution in ‘getting started’ me with the sputtering, simulations, and more or less everything around. Thanks Fredrik, for great friendship we have had! it was really fun times at conferences, on summer trips and of course during the driving lessons. Per Persson-the microscopy guru, you simply, never have said no to me weather it concerns solving TEM issues or listening my general problem over the coee table. Co-Authors-I would like to acknowledge all the people who have helped me in collecting data and writing articles, presented in this thesis. I am also grateful to Carina Höglund and Andy Aquilla for giving me favours in measurements. Kalle Brolin, Thomas Lingefelt & Inger Eriksson, You all are masters in your elds. We cannot be successful without your help. Thanks a lot for keeping us working. Petter Larsson, I honestly think that you are too skilled to solve Naureen-troubles, and I truly appreciate all your help in the lab, especially to make ‘B4 C magnetron’ to work. I am thankful to all the past and present members of the Thin Film Physics, Plasm & Coating Physics, and Nanostructured Materials Divisions.for the cooperation and creating great working environment. Anders Elfving for a very nice friendship since my rst day at IFM.& Ming Zhao for always being there whenever I need help. Axel Flink, for a sincere friendship. Thanks for introducing me to the Swedish summer and Christmas, for all the fun trips and most of all for giving me company during the course of writing. Halldora, for providing me home away from home. Uzma Khalique, without your support I would not have been in Linköping. Birch’s family, I liked being with you all.... My Family, for all the support and encouragement especially from my Parents, you have always been a great source of strength. Naureen.

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(15) Contents Populärvetenskaplig Sammanfattning. 3. Preface. 5. Publication List. 6. Acknowledgements. 9. 1 Introduction. 13. 2 Soft X-ray Multilayer Mirrors 2.1 Multilayer X-ray Mirrors . . . . . . . . . . 2.2 Materials Selection for Soft X-ray Mirrors 2.3 Multilayer Design . . . . . . . . . . . . . . 2.4 Real Interfaces and Associated Roughness. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 15 15 18 21 21. 3 Multilayer Growth 3.1 Ion-surface Interactions . . . . . . . . . . . . . . . . . . . . 3.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Plasma Characteristics . . . . . . . . . . . . . . . . . 3.3 Interface Engineering by Ion-energy Modulation . . . . . . . 3.4 Multilayer Formation During Low-energy Ion Bombardment. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. 25 25 27 29 31 32. . . . . . . . . .. 37 37 37 40 40 43 43 44 44 46. . . . .. 46 47 47 48. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 4 Multilayer Characterization 4.1 X-ray Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Hard X-ray Reectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 X-Ray Diraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Soft X-ray Reectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 In-situ Hard X-ray Reectivity during Annealing . . . . . . . . . . . . . 4.2 Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Bright Field and Dark eld . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 High Resolution Transmission Electron Microscopy . . . . . . . . . . . . 4.2.3 Selected Area Electron Diraction . . . . . . . . . . . . . . . . . . . . . 4.2.4 Scanning Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Ion Beam Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Elastic Recoil Detection Analysis . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Rutherford Backscattering Spectroscopy . . . . . . . . . . . . . . . . . . 11.

(16) 12. CONTENTS. 5 Multilayer Roughness 51 5.1 Thermodynamical Roughening . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2 Accumulated Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5.3 Intermixing and Interdiffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6 Multilayer Impurities 59 6.1 Light Element Incorporation and Multilayer Mirror Properties . . . . . . . . . . 59 6.2 Related Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7 Summary and Contribution to the Field 7.1 Cr/Sc-based Multilayers . . . . . . . . . . . . . . . . . 7.1.1 Effects of Ion Assistance . . . . . . . . . . . . . 7.1.2 Cr/Sc Multilayers and Residual Gas Impurities 7.1.3 Ab initio Calculations . . . . . . . . . . . . . . 7.1.4 CrN/ScN Superlattices . . . . . . . . . . . . . . 7.1.5 Thermal stability . . . . . . . . . . . . . . . . . 7.2 Cr/Ti Multilayers . . . . . . . . . . . . . . . . . . . . . 7.3 Ni/V Multilayers . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. 65 65 65 65 66 68 68 69 69. 8 Additional Results and Future Outlook 8.1 Cr/Sc Multilayer Condenser Mirrors . . 8.2 B4 C Co-sputtering of Cr/Sc Multilayers 8.3 Mo/Si EUV Multilayers . . . . . . . . . 8.4 Low Temperature Growth . . . . . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. 71 71 71 73 73. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. Bibliography. 76. Papers. 83.

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(207) 14. CHAPTER 1. INTRODUCTION. multilayers have not been fully understood especially when multilayers are grown under highly non-equilibrium conditions with kinetic limitations. Another challenging area for the multilayer community is the methods of detailed multilayer characterization of buried interfaces. All these matters are, to some extent, addressed in this thesis and solutions to a number of so far unsolved research issues which are essential for mirror reflectivity and thermal stability are discussed This thesis is compiled into eight chapters. The next chapter deals with the general description of X-ray reflection from a multilayer and also the most commonly used terminologies in the field are introduced. Material selection and mirror design rules followed in this work are also described. Chapter 3 is a detailed outlook of metal multilayer growth related issues. Several characterization methods have been used during this work. Chapter 4 deals with the techniques and kind of information revealed from each of the analyzing technique. In Chapter 5 multilayer roughness evolution during growth is discussed with some examples of the present work. The work during the last two years was related to the study the effects of impurity incorporation into the multilayers and is shortly discussed in Chapter 6. Chapter 7 presents a summary of the papers constituting this thesis, and Chapter 8 is devoted to some related, yet unpublished, results and an outlook for future experiments..

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