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Nanostructural design of transition metal nitride thin films
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-28395Local ID: 13531ISBN: 91-85297-33-X (print)OAI: oai:DiVA.org:liu-28395DiVA: diva2:249201
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
List of papers
1. Single-crystal Ti2AlN thin films
Open this publication in new window or tab >>Single-crystal Ti2AlN thin films
2005 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 11, 111913- p.Article in journal (Refereed) Published
Abstract [en]

We have produced pure thin-film single-crystal Ti2AlN(0001), a member of the Mn+1AXn class of materials. The method used was UHV dc reactive magnetron sputtering from a 2Ti:Al compound target in a mixed Ar–N2 discharge onto (111) oriented MgO substrates. X-ray diffraction and transmission electron microscopy were used to establish the hexagonal crystal structure with c and a lattice parameters of 13.6 and 3.07 Å, respectively. The hardness H, and elastic modulus E, as determined by nanoindentation measurements, were found to be 16.1±1 GPa and 270±20 GPa, respectively. A room-temperature resistivity for the films of 39 μΩ cm was obtained.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-45490 (URN)10.1063/1.1882752 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2016-08-31
2. Deposition of single-crystal Ti2AlN thin films by reactive magnetron sputtering from a 2Ti:Al compound target
Open this publication in new window or tab >>Deposition of single-crystal Ti2AlN thin films by reactive magnetron sputtering from a 2Ti:Al compound target
2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 102, no 7, 074918- p.Article in journal (Refereed) Published
Abstract [en]

Single-crystal Ti2AlN (0001) thin films were grown on (111) oriented MgO substrates kept at 830 °C by ultrahigh vacuum dc reactive magnetron sputtering from a compound 2Ti:Al target in a mixed Ar/N2 discharge. The effects of variations in the nitrogen partial pressure on the phase composition of the films were studied. Results from transmission electron microscopy, x-ray diffraction, and elastic recoil detection analysis show a narrow region for growth of Ti2AlN MAX phase with respect to the nitrogen content in the discharge. Perovskite Ti3AlN and intermetallic Ti3Al and TiAl phases dominate at nitrogen depletion. For overstoichiometric deposition conditions with respect to Ti2AlN, a phase mixture with NaCl-structured TiN is obtained. Epitaxial growth is observed with a layer-by-layer mode on the 0001 basal planes for all phases. A superstructure in the TiN phase is also observed along [111] with the repetition distance of 7.34 Å, most likely related to Al segregation. Nanoindentation shows that the film hardness increases from 11 to 27 GPa with increasing nitrogen content and corresponding phase transformations from Ti–Al intermetallics to Ti3AlN, Ti2AlN, and TiN.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-39389 (URN)10.1063/1.2785958 (DOI)48153 (Local ID)48153 (Archive number)48153 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2016-08-31
3. Single crystal CrN/ScN superlattice soft X-ray mirrors: epitaxial growth, structure, and properties
Open this publication in new window or tab >>Single crystal CrN/ScN superlattice soft X-ray mirrors: epitaxial growth, structure, and properties
Show others...
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.

Keyword
CrN, ScN, Superlattice, X-ray mirrors, Reactive magnetron sputtering
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-13065 (URN)10.1016/j.tsf.2006.02.011 (DOI)
Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2016-08-31
4. Residual stress formation in multilayered TiN/TaNx coatings during reactive magnetron sputter deposition
Open this publication in new window or tab >>Residual stress formation in multilayered TiN/TaNx coatings during reactive magnetron sputter deposition
Show others...
2000 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 18, no 6, 2884-2889 p.Article in journal (Refereed) Published
Abstract [en]

Multilayered physical vapor deposited TiN/TaNx coatings were deposited on cemented carbide substrates using a dual target magnetron sputtering system. The coatings were investigated with respect to the influence of nitrogen partial pressure during deposition on the residual stress developed in the coatings. Furthermore, the fracture strength of the material, i.e., the magnitude of the tensile stress that the coating can support without cracking, was evaluated. It was found that, by increasing the nitrogen partial pressure, it is possible to change the stress from compressive to tensile. The highest tensile stress was about 3.6 GPa. Despite this high stress, the coating displayed no cracking. This implies that it is possible to grow TiN/TaNx multilayered coatings with high tensile fracture strength using dual magnetron sputtering and a high deposition temperature (about 680 °C). 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-47552 (URN)10.1116/1.1308594 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2016-08-31
5. Stress evolution in TiN and TaN layers and multilayers prepared by reactive magnetron sputtering and studied with in-situ laser reflection curvature technique
Open this publication in new window or tab >>Stress evolution in TiN and TaN layers and multilayers prepared by reactive magnetron sputtering and studied with in-situ laser reflection curvature technique
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Time-resolved in-situ curvature measurements have been used to measure the stress evolution during UHV-based reactive magnetron sputter deposition of TiN and TaN layers and multilayers at 750° C onto Si wafers. It was fmmd that the stress in the initial TiN layers is compressive and reaches a maximum value of -1.2 GPa at 10 nm, there after the stress evolves to tensile during steady state columnar growth. It is suggested that the initial compressive stress is caused by the affinity of reducing any SiO2 to TixSiyOz and the heat transfer from the discharge to the substrate, corresponding to a temperature raise of ~35 °C on the substrate. The stress evolution in TaN layers gwwn on TiN template layers depends strongly on the nitrogen partial pressure in the discharge, from 1 to 3.5 mTorr. This can be correlated with the condition that TaN undergoes phase transformations as the nitrogen partial pressure increase. At low nitrogen content the dominating phase is the hexagonal γ-Ta2N phase and the stress is as high as -1.0 GPa. At higher nitrogen content the layer is essentially stress free with a mixture of cubic δ-TaN and hexagonal ε-TaNphases. This nitrogen dependence combined with the high time sensitivity of the measuring technique provides means to control the stress or phase composition in a given film during growth. It is also shown that the stress evolution in the individual layers in a TiN/faN superlattice with a period as small as 5 nm can be resolved.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85686 (URN)
Available from: 2012-11-28 Created: 2012-11-28 Last updated: 2016-08-31
6. Phase stability tuning in the NbxZr1−xN thin-film system for large stacking fault density and enhanced mechanical strength
Open this publication in new window or tab >>Phase stability tuning in the NbxZr1−xN thin-film system for large stacking fault density and enhanced mechanical strength
2005 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 13, 131922- p.Article in journal (Refereed) Published
Abstract [en]

The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1−xN (0 ⩽ x ⩽ 1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x = 0.5 composition. The nanoindentation hardness of the films varied between 21 GPa for the binary nitrides, and 26 GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred 〈10〉{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x = 0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-28403 (URN)10.1063/1.1884743 (DOI)13539 (Local ID)13539 (Archive number)13539 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2016-08-31

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