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Single crystal CrN/ScN superlattice soft X-ray mirrors: epitaxial growth, structure, and properties
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
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2006 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 514, no 1-2, 10-19 p.Article in journal (Refereed) Published
Abstract [en]

Single crystal CrN/ScN superlattice films with modulation periods of 1.64 nm were grown on MgO(001) substrates. By utilizing a magnetically enhanced plasma in the vicinity of the substrate and a negative substrate bias, ion/metal nitride flux ratios of 45 and 144 were achieved during deposition of CrN and ScN, respectively. The effects of ion energies in the range [16–58 eV] and substrate temperatures in the range [535–853 °C] on the composition, interface width, crystal quality, and microstructure evolution were investigated using elastic recoil detection analysis, hard X-ray reflectivity, X-ray diffraction, and transmission electron microscopy (TEM). Minimal interface widths of 0.2 nm = 1/2 nitride unit cell were achieved at a growth temperature of 735 °C and ion energies of 24 and 28 eV for CrN and ScN, respectively. Under these conditions, also an optimum in the crystal quality was observed for near stoichiometric composition of CrN and ScN. TEM confirmed a cube-on-cube epitaxial relationship for the system with CrN(001)ScN(001)MgO(001) and CrN[100]ScN[100]MgO[100]. Also, the layers were coherently strained to each other with no misfit dislocations, threading dislocations, surface cusps, voids or gas bubbles present. Higher ion energies or lower deposition temperatures gave over-stoichiometric films with poor superlattice modulation while higher growth temperatures yielded a decreased crystal quality, due to loss of N. As-deposited superlattices with only 61 periods exhibited an absolute soft X-ray reflectance of 6.95% at an energy of 398.8 eV (Sc 2p-absorption edge) which is comparable to the performance of Cr/Sc. The compositional modulation and phase structure was stable during extended annealing at 850 °C, which is the highest thermal stability for an X-ray multilayer mirror. It is concluded that the ScN layers serve as effective diffusion barriers to hinder decomposition of the CrN layers and stabilize the pseudomorphic superlattice structure. Nanoindentation experiments showed that the hardness of the CrN/ScN superlattice films was 19 GPa.

Place, publisher, year, edition, pages
2006. Vol. 514, no 1-2, 10-19 p.
Keyword [en]
CrN, ScN, Superlattice, X-ray mirrors, Reactive magnetron sputtering
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-13065DOI: 10.1016/j.tsf.2006.02.011OAI: oai:DiVA.org:liu-13065DiVA: diva2:17761
Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2016-08-31
In thesis
1. Materials Science of Multilayer X-ray Mirrors
Open this publication in new window or tab >>Materials Science of Multilayer X-ray Mirrors
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis treats the reflective and structural properties of multilayer structures. Soft X-ray multilayer mirrors intended as near-normal incidence reflective optics and polarizers in the water window (λ=2.4-4.4 nm) are the main focus. Such mirrors require multilayer periodicities between 1.2-2.2 nm, a large number ~600of multilayer periods (N), and atomically flat interfaces. Bi-metallic multilayers were deposited by dual-target magnetron sputtering on Si(001)

Geometrical roughness and intermixing/interdiffusion 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 high-flux low energy (Eion<30 eV) ion assistance. The use of modulated ion assistance resulted in a substantial improvement of interface flatness and abruptness in each of theAb-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 2×10-7-to-2×10-6 Torr. The incorporation of 34 at.% nitrogen at a higher residual gas pressure ( ~2×10-5 Torr) resulted in highly textured understoichiometricx/ScNy multilayers. As a result of nitrogen incorporation, interface widths as small as 0.29 nm, and near-normal incidence reflectivity enhancement (at λ=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 reflectivity measurements performed during isothermal annealing in thex/ScNy 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 periodsλ=3.11 nm as well as very high thermal stability up to 850 °C.

Abstract [sv]

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, fotolitografi av

Det finns flera stora utmaningar för att lyckas tillverka multilagerspeglar. Först och främst måste man hitta materialkombinationer som ger upphov till reflektion i mellanytorna mellan materialen men som inte samtidigt absorberar all röntgenstrålning. Dessutom måste materialen gå att belägga på varandra i flera 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

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 reflektans i vattenfönstret. Varje kombination av metaller är optimal för en specifik våglängd och de individuella lagertjocklekarna måste optimeras teoretiskt för varje enskilt

För Cr/Sc multilager har vi visat att lagren som beläggs har en oordnad, så kallad amorf, struktur mellan metallatomerna som har sitt ursprung i att multilagrets totala energi kan sänkas om mellanytor mellan kristallint Cr och kristallint Sc kan undvikas.

Studier av effekterna av kväveupptag hos Cr/Sc multilagerspeglar under sputtringsprocessen har lett till ökad förståelse av materialsystemet. Till exempel har vi visat att kvävet 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 högintensiva ljuskällor. Så kallade supergitter, dvs multilager

Publisher
81 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1169
Keyword
Soft X-rays, Multilayer mirrors, Magnetron sputtering, Interface engineering
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-11343 (URN)978-91-7393-946-1 (ISBN)
Public defence
2008-04-18, Plank, Physics House, Linköping University, Linköping, 14:00 (English)
Opponent
Supervisors
Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2009-04-28
2. Nanostructural design of transition metal nitride thin films
Open this publication in new window or tab >>Nanostructural design of transition metal nitride thin films
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Early transition metal nitrides have found extensive use in a range of thin film applications. We are just beginning to understand their growth from vapor phase deposition and microstructure-property relationships. There are large fields to explore, not the least for the nanoand microstructural design in materials offered by a developed deposition process control and alloying by additional elements to grow, e.g., ternary nitrides or superlattices. The work in this thesis has been directed towards increasing the fondarnental understanding of the synthesis, characterization, and properties for some technologically relevant nitrides. Binary and ternary phases of transition metal nitrides, as well as artificial superlattice structures have been studied. In order to prepare materials of high purity, film deposition was performed by Ultra-High Vacuum Reactive Magnetron Sputtering.

In this thesis single-crystal Ti2A1N(0001) thin films were synthesized by epitaxial growth onto MgO(111) substrates at elevated temperature and using a 2Ti:A1 compound sputtering target. This is the first thin film deposition process reported for a nitride of the so called Ma+1AXn phase family of compounds - an inherently nanolaminated material that is characterized by metallic conductivity and ductility with retained ceramic strength, high-temperature stability, oxidation, and corrosion resistance. Th2A1N is found to have a room-temperature resistivity of 39 μΩcm, Young's modulus of 16-17 GPa, and hardness of 210 GPa. It is also found that nitrogen-depleted deposition conditions yield the growth of equilibriwn phases TiA1, Ti3A1 and Ti3A1N. For overstoichiometric deposition conditions with respect to Ti2A1N, a phase mixture with TiN was obtained. A super-structure in the TiN phase was also observed to form along the [111] direction at a repetition distance of 7.34 Å, possibly related to A1 segregation.

CrN/ScN superlattices were designed for use as soft x-ray mirrors and investigated with respect to thermal stability, hardness, and x-ray reflectivity properties. The combined performance of as-deposited superlattices films, with a compositional modulation period of 1.64 nm, both as a mirror and for thermal and mechanical stability was found to be far better than state of the art metallic Cr/Sc multilayers. In fact, the obtained reflectance of 6.95% at a wavelength of 3.1 nm is excellent, the structure is intact after annealing above 800 °C, and the hardness of 19 GPa makes the mirror effectively scratch-proof.

Stress measurements in the TiN/TaN system were performed in-situ. The obtained results showed that a technique based on curvature measurements by laser deflection on the sample during thin film deposition works when employed at the elevated temperatures typically used for sputtering of nitrides. Findings from the in-situ measurements show a correlation between the stress and the film microstructure and the phase composition of TaN layers. It is also shown how the individual layers in an TiN/TaN artificial superlattice affects the stress with a sub-nm resolution. The contribution from thermal stress is also detected and the fine increase in temperature due to exposure of energetic particles from the plasma can be calculated from that stress.

A multiphase region in nitrides was demonstrated to form in the NbxZr1-xN model system. The existence of such a multiphase or polytypic structure is predicted by first-principles density functional theory calculations that to occur in nitrides of compositional regions with valence electron concentrations that yield the same total energy for different crystal structures. Films with varying composition were grown and transmission electron microscopy studies revealed an increase in defect density for the x= 0.5 composition. Nanoindentation performed on such films showed an increase in hardness of - 20% compared to the binary nitrides. Analysis of the indents revealed that materials volumes had rotated away from the indenter, thus offering an alternative mechanism for plastic deformation compared to glide on preferred slip systems seen in the cubic binary nitrides. The materials design concept of such phase stability tuning for mechanical strengthening by a high density of phase interfaces is proposed to be expanded to other materials systems.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2005. 41 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 923
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-28395 (URN)13531 (Local ID)91-85297-33-X (ISBN)13531 (Archive number)13531 (OAI)
Public defence
2005-02-18, Planck, Fysikhuset, Campus Valla, Linköpings Universitet, Linköping, 14:00 (English)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-11-28Bibliographically approved

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Birch, JensJoelsson, TorbjörnEriksson, FredrikGhafoor, NaureenHultman, Lars

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