Revealing Phase Separation Mechanisms in Alloy Thin Films

The Impact of Electron Microscopy

  • Revealing Phase Separation Mechanisms in Alloy Thin Films - The Impact of Electron MicroscopyRevealing Phase Separation Mechanisms in Alloy Thin Films - The Impact of Electron Microscopy
  • Revealing Phase Separation Mechanisms in Alloy Thin Films - The Impact of Electron Microscopy
  • Fig. 1: Dark field TEM image and selected area diffraction pattern of Cu-Ag alloy film of eutectic composition.
  • Fig. 2: High resolution electron microscopic image of Cu-Ag film (a), its Fourier transform diffraction pattern showing pairs  of Cu and Ag reflections (b) and schematic drawing with indexing of the diffraction pattern (c).
  • Fig. 3: Twisting rows (arrows) of 3-5 nm nanocrystals in (Ti0.41Al0.57)Y0.02N film.
  • Fig. 4: High resolution image (a) of a (Ti0.41Al0.57)Y0.02N columnar crystal showing wavy lattice fringes corresponding to structure in Fig. 3. Selected area diffraction pattern of an area about 1 μm in diameter (b) of the same film and the Fourier transform diffraction pattern (c) of the area, shown in (a).

Effective and reproducible tailoring of different properties of thin films and coatings is possible, if the growth processes and mechanisms are known on atomic level. The structure zone diagram and growth mechanisms for one-component thin films have been worked out on the basis of electron microscopic evidences and are summarized in [1]. These mechanisms include atomic arrangements only on the growth surface, usually recalled as kinetic segregation resulting in nucleation and growth of phases. The model also successfully treats the effect of contaminants in the processes.

A similar and comprehensive knowledge would be necessary for the films composed from two- or more components. As direct observation of atomic movements is difficult we often have to trace these processes by indirect methods and, sometimes aided by simulations, deducing the atomic processes from all possible structural details of the as grown films. These structural details include the formed phases, their morphology (size and shape of grains), orientation relations (texture), surface properties as well as very local analytical information on the distribution of components and impurities. Transmission electron microscopy is one of the most direct methods for providing the above nanometres- or even atomic scale structural information. Combining various TEM methods makes possible to reveal the phase separation mechanisms in nanostructured two-component films and to understand better the self-organization mechanisms behind them.

Experimental

Two binary systems have been chosen to show the feasibility of TEM in revealing the phase separation processes in them. Thin films of practically non-mixing components were co-deposited, all films in a broad composition range. Cu and Ag were thermally evaporated from two tungsten sources and deposited at room temperature [2]. (Ti1-xAlx)1-yYyN thin films were grown by reactive unbalanced magnetron sputtering from compound target in N2+Ar gas mixture at 550oC [3]. Structural characterization was carried out by conventional TEM using a Philips CM-20 microscope at 200 keV and high resolution measurements were carried out using a JEOL 3010 microscope at 300 keV.

For chemical analysis of the films a Noran Energy Dispersive Spectrometer attached to the CM20 microscope and a Gatan Tridiem Electron Energy Loss Spectrometer (EELS) on the JEOL 3010 microscope were applied.

Phase Separation in Eutectic Ag-Cu Film

Cu and Ag have the same type of unit cell with about 10% misfit between the lattice parameters. They are non-mixing in the whole composition range at room temperature. Co-depositing them, the phase separation processes can be well investigated. For studying of the initial growth stage of the films the best choice is using high resolution microscopy. After nucleation the films grow forming islands, different in size. It can be shown that their phase state, prior to coalescence, depends on the size. No phase separation could be established in the smallest particles of 1-2 nm in size. Among single(alloy)-phase particles of 2-5 nm, full or partial separation of the components in particles of similar sizes can be observed. A Monte Carlo simulation was also used to understand better the possible phase separation processes in the Cu-Ag system. Correlating the HREM measurements and the results of the simulations we concluded, that the Cu-Ag alloy particles can start phase separation by spinodal decomposition and can proceed to full separation and nucleation [4].

In thicker, continuous Cu-Ag films establishing the phases and morphology can provide several pieces of information and by the help of the structure zone diagram it is possible to conclude the film forming mechanisms. Conventional TEM gives the possibility to record diffraction patterns supplying information about the phase state and texture of the films, while dark field images help to establish the grain morphology and grain size. This can be demonstrated on the close to eutectic composition Cu-Ag films. Dark field images revealed that the film is built up from narrow, V shaped columns (fig. 1) corresponding to zone T in the structure zone diagram of single-phase films [1]. The film displays also strong <111> texture, which is observable from the splitting of diffraction rings (fig. 1). The texture evolution can be attributed to the competing crystal growth (characteristic for zone T). The shape and orientation of the grains provides the evidence that the growth occurred in single-phase alloy, i.e. during film growth Cu-Ag solid solution columns formed. However, evaluating the diffraction pattern shows the presence of a two-phase alloy (fig. 1) where the crystallites of the phases are epitaxially related to each other, though the columns do not show strong dark filed contrast using the textured 111 Cu and Ag reflections.

Investigation at higher magnification reveals that the separation of the two phases occurs by redistribution of components on an atomic level within the same lattice. The columns of the films are composed of 2-3 nm epitaxial domains of Cu and Ag rich solid solutions with misfit dislocations between them (fig. 2). The epitaxial nano-domains could only form by spinodal decomposition from the originally single-phase Cu-Ag solid solution; this being the phase separation process in which the new phases preserve the orientation of the host matrix. The spinodal process is taking place in the buried volume of the growing crystallites below the growth front. And, as a result, each column in the as grown film can be described as a nano-composite of epitaxial crystallite-domains [2].

Phase Separation in the (Ti1-xAlx)1-yYyN Film

The tracing for phase separation mechanisms can be looked over in a more complicated system, evidenced by structural features detected in the as grown structure.

TiAlN films are developed for high temperature oxidation resistant hard coatings. TiN and AlN are both close packed cubic and hexagonal structures, accordingly. They are non-mixing at the growth temperature in the whole composition range. Structural characteristics typical for one phase growth (V-shaped columnar morphology and texture) have been observed also in (Ti0.41Al0.57)Y0.02N thin films. The formation mechanism of the columnar grains must also include the growth of supersaturated solid solution of single-phase hcp (Ti0.41Al0.57)Y0.02N host phase. Furthermore, a special structure has been revealed in the dark field image of columns, made by a TiN reflection and consisting from twisting chains of nanocrystals (fig. 3). High resolution images have shown that the chains are composed of 3~5 nm in size fcc (TiN) and hcp (AlN) crystallites alternating in the chains and exhibiting the epitaxial relation {111}TiN//{0001}AlN and <110>TiN//<11-20>AlN with each other. Their formation occurs by phase separation through spinodal decomposition. Beside the epitaxial relation between the separating phases the size of their crystallites, their coherent interfaces and the uniform size distribution all are supporting the spinodal character of the phase separation process. In addition the character of the diffraction patterns preserves the same two phase information down to the size of the investigated area about 10 nm in diameter. The peculiarity of the process is that the separating phases are not pseudomorph. An fcc (TiN) phase is forming by spinodal decomposition in the hcp (Ti0.41Al0.57)Y0.02N matrix. The process can take place without nucleation due to the similarity of the hcp and fcc lattices. The formation of the TiN phase takes place on the 0001 hcp planes where, in addition to Ti accumulation, the change of the stacking sequence of the hcp lattice occurs.

Summary

Utilizing the possibilities of different imaging, diffraction and analytical techniques available in transmission electron microscopy is very useful for obtaining complete structural information on the nanostructure of even complicated systems consisting of several components. This can substantially contribute to the knowledge, needed for describing and identifying the atomic growth and phase separation processes in multicomponent thin film systems.

Some binary systems with large miscibility gap, independently of the similarities in their individual structure, tend to grow in the form of single-phase structures. However, the forming phase depends on the dissimilarity of the expected equilibrium phases. The Cu-Ag and (Ti0.41Al0.57)Y0.02N films grow in the form of a supersaturated solid solution phase, fcc and hcp, accordingly.

The observed growth morphologies for the eutectic Cu-Ag and for the (Ti0.41Al0.57)Y0.02N films correspond to columnar morphology and the corresponding texture is developing by competitive crystal growth providing the proof on single-phase competing crystal growth in zone T of the structure zone diagram as revealed by TEM.

High resolution electron microscopy revealed that initial phase separation by spinodal decomposition occurred in the bulk of the crystal structure. It is taking place still as a growth event and is responsible for the formation of the epitaxial two phase nano-composite within each column.

Acknowledgement

The Hungarian National Science Foundation (OTKA, K-81808 project) is acknowledged for financial support. F. Misják also acknowledges the financial support from the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.

References
[1] Petrov I. et al.: J. Vac. Sci. Technol. A 21, 117-127 (2003)
[2] Misják F. et al.: Thin Solid Films 516, 3931-3934 (2010)
[3] Székely L. et al.: submitted to Surface and Coatings Technology
[4] Misják f. et al.: In: Proceedings of 15th European Microscopy Congress, Manchester, GB, 0178 (2012)

Authors
Dr. György Radnóczi

Dr. Fanni Misják
Dr. Péter B. Barna
Research Centre for Natural Sciences
Hungarian Academy of Sciences
Budapest, Hungary

Dr. Domokos Biró 
Faculty of Technical and Human Sciences
Sapientia-Hungarian University of Transylvania
Târgu-Mures, Romania

Contact

Hungarian Academy of Science

1525 Budapest
Hungary
Phone: +36 1 39 22 22 2

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