X-ray diffraction on nanocrystalline thin films David Rafaja Institute of Physical Metallurgy, TU Bergakademie Freiberg (D) Michal Šíma PIVOT a.s. (CZ), PLATIT Advanced Coating Systems Ladislav Havela Department of Electronic Structures, Charles University Prague (CZ)

ISPMA 9, Prague2 Physical background A contribution to the explanation of the relationship between physical properties and real structure of matters Strong dependence of the magnetic behaviour of thin UN films on deposition conditions (microstructure) Strong dependence of the mechanical hardness of thin TiN films on deposition conditions (microstructure) Examples

ISPMA 9, Prague3 Magnetic susceptibility of UN thin films UN single crystals: paramagnetic below 53 K antiferromagnetic below 53 K Thin polycrystalline UN films: development of a ferromagnetic component below 100 K. Sample deposition: Reactive DC sputtering Target voltage: -800 V Ion current: 2.5 mA Plasma was maintained by injecting electrons with energy between -50 and -100 eV Substrate temperatures: -200°C, 20°C, 200°C, 300°C, 350°C, 400°C Deposition rates:  1 Å/s

ISPMA 9, Prague4 Hardness of Ti 1-x Al x N thin films A series of arc deposited Ti 1-x Al x N films with increasing aluminium contents TiAl N 2 + Ar Addition of Aluminium improves the hardness of the films, especially at high temperatures (up to 1000°C) Different colour and hardness of the coatings

ISPMA 9, Prague5 Microstructure of thin films Chemical and phase composition, chemical homogeneity Residual stress Stress-free lattice parameter Preferred orientation of crystallites (texture) Crystallite size and shape Microstrain Macroscopic and microscopic anisotropy of lattice deformation

ISPMA 9, Prague6 Experimental methods XRD  GAXRD with the parallel beam optics – phase composition and chemical homogeneity, residual stress, stress-free lattice parameters, crystallite size, microstrain, anisotropy of the lattice deformation   /  -scan on Eulerian cradle (pole figure) – texture  Symmetrical 2  /  -scan on Bragg-Brentano diffractometer – crystallite size and microstrain EPMA with WDX – chemical composition HRTEM – crystallite size and shape

ISPMA 9, Prague7 Phase composition (Uranium nitride) Phase composition 1.UN (Fm3m) mol.% 2.U 2 N 3 (Ia3) 10-20% mol.% UN (Fm3m) U: 4a (0, 0, 0) N: 4b (½, ½, ½) U 2 N 3 (Ia3) U: 8b (¼, ¼, ¼) U: 24d (-0.018, 0, ¼) N: 48e (0.38, 1/6, 0.398) Different lattice parameters Negligible differences in intensities 0 Atomic Percent Nitrogen T(°C) 400 UUN U2N3U2N3 UN 2 Schematic phase diagram of U-N

ISPMA 9, Prague8 Phase composition (Ti 1-x Al x N) Ti 4 Al 41 N 55 … AlN + Ti 1-x Al x N Ti 8 Al 38 N 54 … AlN + Ti 1-x Al x N Ti 19 Al 31 N 50 …Ti 1-x Al x N + AlN Ti 26 Al 24 N 50 … Ti 1-x Al x N + AlN Ti 37 Al 14 N 49 … Ti 1-x Al x N + AlN Ti 41 Al 7 N 52 … Ti 1-x Al x N + AlN (P6 3 mc) Ti 55 Al 2 N 43 … Ti 1-x Al x N (Fm3m) 001 WC 100 WC 101 WC 110 WC 002 WC 111 WC 200 WC 102 WC 100 AlN 002 AlN101 AlN 110 AlN 103 AlN 112 AlN 201 AlN 111 TiAlN 200 TiAlN 220 TiAlN 311 TiAlN222 TiAlN

ISPMA 9, Prague9 Phase composition (Ti 1-x Al x N) Diffraction line asymmetry, maximum in Ti 37 Al 14 N 49 Concentration gradient in Ti 1-x Al x N  TiAlN + AlN Ti 1-x Al x N (Fm3m) TiAlN + AlN Ti 55 Al 2 N 43 Ti 41 Al 7 N 52 Ti 37 Al 14 N 49 Ti 26 Al 24 N 50 Ti 19 Al 31 N AlN 220 TiAlN

ISPMA 9, Prague10 Residual stress and stress-free lattice parameters Elastic lattice deformation from X-ray diffraction: Bi-axial residual stress in thin films: The sin 2  -method for cubic thin films: sin 2  0 1 aa aa a || a0a0 2  n s  

ISPMA 9, Prague11 Residual stress and stress-free lattice parameters n  HKL   hkl      easy hard

ISPMA 9, Prague12 Preferred orientation of crystallites PVD Ti 1-x Al x N, texture {111} GAXRD at  = 3° Strong anisotropy of lattice deformation Simulation: fibre texture {111}

ISPMA 9, Prague13 Preferred orientation of crystallites PVD Ti 1-x Al x N, texture {100} GAXRD at  = 3° No anisotropy of lattice deformation Simulation: fibre texture {100}

ISPMA 9, Prague14 Preferred orientation of crystallites “111” “100” Ti 1-x Al x N PVD ~ 30° _ 111 _ 111 __ 111 _ 101 _ _ 101 _ ~ 30°

ISPMA 9, Prague15 Crystallite size and microstrain Williamson-Hall plot 1/D ~e~e Crystallite sizeMicrostrain Scherrer formula Line broadening only due to the crystallite size. Microstrain is neglected. Warren-Averbach or Krivoglaz methods Fourier analysis of diffraction profiles taken in symmetrical geometry Problems with low intensity of diffraction lines in thin films and with preferred orientation of crystallites.

ISPMA 9, Prague16 Microstructure of UN thin films Increasing substrate temperature Relaxation of the stress- free lattice parameter Relaxation of the residual stress Relaxation of the microstrain Weaker texture At high T s : Development of large crystallites Changes in the real structure of PVD UN thin films are predominantly caused by non-equilibrium deposition conditions

ISPMA 9, Prague17 Microstructure of Ti 1-x Al x N thin films Increasing Al-contents Decreasing stress-free lattice parameter (cell volume) Increasing residual stress Increasing microstrain Decreasing crystallite size Inclination of the texture direction (dominated by the geometry of the deposition process) Dominant phase fcc TiAlN hex AlN Crystallite size below 20 nm Minimum: ~ 3.3 nm Changes in the real structure of PVD UN thin films are due to the changes in the aluminium stoichiometry and due to the geometry of the deposition process

ISPMA 9, Prague18 Typical features observed in nanocrystalline fcc thin films  Fan-like distribution (scatter) of the “cubic” lattice parameters …is caused by mechanical interaction between neighbouring crystallites (compressive residual stress) …is related to the anisotropy of elastic constants and to the orientation of crystallites  Large compressive residual stress …is probably caused by atoms built in the host structure and by mechanical interaction between regions with different lattice parameters …is apparently increased by anisotropy of the lattice deformation top view

ISPMA 9, Prague19 Advanced information on microstructure of thin films XRD study  Lattice parameters + Texture  Structure model  Information on distribution of inter-atomic distances (local probe), but no lateral resolution n  HKL   hkl      Microstructure model and Texture model

ISPMA 9, Prague20 Typical features observed in nanocrystalline fcc thin films D < 0 PVD TiAlN films, GAXRD at  =3°  Negative crystallite size … anisotropic shape of crystallites … overestimated microstrain … coherent neighbouring crystallites  Large microstrain … anisotropic shape of crystallites … mutual coherence of neighbouring nano-crystals  Why nano-crystals develop in thin films ? …very high density of structure faults caused by the deposition process  nano-crystallites with large residual stress (local decomposition of TiAlN) …plastic deformation during the deposition because of large residual stress  nano-crystallites with large residual stress Needle-like crystallites Simulation using Height: 200 Å Width: 40 Å

ISPMA 9, Prague21 True crystallite size Symmetrical XRD HRTEM 35 – 50 Å Spatial modulation of interplanar spacing (chemical composition)  large residual stress (interaction between coherent domains)  large microstrain, “negative” crystallite size (large coherent domains with many structure faults)

ISPMA 9, Prague22 Relationship between deposition conditions, microstructure and physical properties Residual stress  change of the lattice parameter related to macroscopic directions, anisotropic variations of the inter-atomic distances Stress-free lattice parameter  change of the inter- atomic distances, indicates changes in stoichiometry Preferred orientation of crystallites  macroscopic anisotropy of physical properties, effect on the local lattice deformation Crystallite size  different effect of the grain boundaries Microstrain  local deformation of the crystal lattice, fluctuations in the inter-atomic distances

ISPMA 9, Prague23 Acknowledgements Grant Agency of the Czech Republic (Project number 106/03/0819) European Community (Program HPRI–CT-2001–00118) DFG (Priority Programme number 1062) Dr. T. Gouder, ITU Karlsruhe Dr. V. Klemm, Dr. D. Heger, Dipl.-Phys. G. Schreiber, Mrs. U. Franzke and Mrs. B. Jurkowska, TU BA Freiberg