Epitaxial Ni-Al thin films on NaCl using a Ag buffer layer

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1 Epitaxial Ni-Al thin films on NaCl using a Ag buffer layer M. Yandouzi *, L. Toth, V. Vasudevan, M. Cannaerts, C. Van Haesendonck and D. Schryvers * * Electron Microscopy for Materials Science, University of Antwerp, Groenenborgerlaan 171, B Antwerp, Belgium Research Institute for Technical Physics and Materials Science of the H.A.S., Konkoly-Thege ut 29-33, H-1121 Budapest, Hungary Department of Materials Science & Engineering, University of Cincinnati, Cincinnati, OH , USA Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium [Received 21 February 2000 and accepted 10 July 2000] Abstract Epitaxial nanoscale [001] films of Ni x Al 100-x (x = 62.5) have been prepared by physical vapour deposition on to a thin film of Ag [001] on NaCl (001) faces with occasional hillocks. The Ag film contains numerous dislocations and stacking faults and has a root-mean-square surface roughness of 2 nm. The Ni-Al film is ordered in the B2 structure and reveals many dislocations as well as anti-phase boundaries between ordered domains. The formation of subgrains in the Ni-Al film results in severe height variations up to 30 nm across the surface. A cross-sectional model for the growth of both films is presented. 1. Introduction In order to study diffusive and displacive transformations in thin films by means of in-situ transmission electron microscopy (TEM) experiments, Ni x Al 100-x films with thicknesses in the nanoregime are produced by conventional vapour deposition. Direct deposition of the Ni-Al on rocksalt (NaCl) produced contiguous films consisting of small grains (5 20 nm lateral diameter) of B2 Ni-Al with only a limited amount of texture (Yandouzi et al 1997). This structure is known as the austenite phase, referring to the martensitic transformation occurring in bulk samples of this material when x is between 62 and 69 (Au and Wayman 1972, Smialek et al. 1973). The present paper describes the first results on the atomic and microstructure of a single crystal thin film of Ni 62.5 Al 37.5 deposited on an epitaxial buffer layer of Ag on the NaCl substrate schryvers@ruca.ua.ac.be Phylosophical Magazine Letters ISSN print/ ISSN online -C- Taylor & Francis Ltd

2 2. Experimental conditions The silver films were deposited by physical vapour deposition using thermal evaporation (PVD-TE) of Ag (99.99%) from a tungsten boat onto specially treated (001) NaCl (a NaCl = 0.564nm). The air cleaved NaCl crystal was treated with distilled water and chlorine gas prior to deposition as described by Safran et al. (Safran, et al. 1993). This yields the growth of an epitaxial single crystal Ag film at a relatively low substrate temperature of 120 C. This part of the sample preparation was carried out in a turbo pumped vacuum chamber (Balzers) with a pressure of 5x10-6 mbar during the deposition procedure. A film thickness of 15 nm was obtained by maintaining a constant deposition rate of 1.5 nm/s with a calibrated quartz crystal. After the Ag film was deposited, the substrate temperature was raised to 300 C and the Ni-Al alloy was deposited, again by PVD-TE from a single Ni 62.5 Al 37.5 alloy in a tungsten boat. A 50 nm thick film was deposited at a rate of 1.0 nm/s. The initial pressure (5x10-6 mbar) in the vacuum chamber increased to about 2x10-5 mbar during the deposition process. The Ni-Al/Ag/NaCl sample was then cooled to room temperature before venting the vacuum chamber with nitrogen gas. The films (Ni-Al + Ag) were floated off in water, followed by capturing on a Cu grid yielding plan view samples. The advantage of this procedure is that such samples are directly suitable for conventional as well as high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies without the need for further thinning which could severely influence the atomic and microstructure of these nanoscale films. The samples were investigated in a Philips CM20 and JEOL 4000EX microscope, both equipped with a LaB6 filament. The scanning probe measurements were performed in the non-contact atomic force microscopy (AFM) mode, with a commercial microscope (Park Scientific Instruments, M5 Autoprobe). The cantilever used is an ultrasharp silicon cantilever, with a spring constant of 3.2 N/m and a resonance frequency of 90 khz. Measurements on the freestanding Ag/Ni-Al films as well as on the Ag/Ni-Al on the NaCl substrate were performed, but no substantial differences were observed. 3. Results and discussion 3.1. Ag buffer film In order to understand the defects and microstructure of the Ni-Al film, the uncovered Ag buffer film was investigated first. Although most parts of the film form a continuous layer on the NaCl substrate, some regions appear to have captured fewer Ag atoms and reveal open holes or even a percolating film structure. A low magnification bright field (BF) TEM image of an area with holes in the Ag film is presented in figure 1a, including the SAED pattern valid for all three cases. The latter clearly shows that the entire Ag film is a face-centred cubic (fcc) single crystal with a [001] normal to the film surface and NaCl substrate. A lattice parameter of (0.409 ± 0.003) nm was measured, in perfect agreement with the bulk value. From the AFM image of a 2 x 2 µm 2 portion of a continuous Ag surface, shown in figure 1b, and the corresponding line scan in figure 1c, a root-mean-square surface roughness of 2 nm is obtained.

3 Figure 1. (a) BF TEM image of a typical morphology with holes of the [001] Ag film deposited onto (001) NaCl, including the corresponding SAED pattern. (b) AFM image of the surface of the continuous film. The line scan along the black line is shown in (c). From the relative orientation between the indexed SAED pattern and the edges of the holes in figure 1a, <100] and <110] traces are found to be most prominent for these edges. Although the holes could correspond with a growth stage intermediate between the percolating structure and the continuous film, the holes can also be induced by the existence of relatively large crystalline hillocks on the surface of the treated NaCl substrate. Occasionally, such hillocks are indeed observed in the TEM as leftovers of an incomplete NaCl dissolving process. From such observations, the epitaxial relation between NaCl and Ag could be identified as (001)[100] Ag // (001)[100] NaCl which confirms an earlier conclusion from the macroscopic manipulation of the samples. Moreover, separate AFM measurements on the mean area and density of the film holes and NaCl hillocks yield comparable values. In figure 2 some detail of the atomic and microstructure of the Ag film is presented. Observations under conventional two-beam bright field (BF) and weak-beam dark field (WBDF) conditions revealed the existence of numerous inclined dislocations in the Ag film. Contrast and g.b analyses utilising a number of low-index g-vectors contained in the [001] and [011] zones, coupled with stereographic trace analysis, established that most of the dislocations examined had b = a/2[0-11] lying on the (111) plane or b = a/2[101] lying on the (-111) plane. Most of these dislocations were of screw character, though a few with mixed/edge character were also observed which allowed determination of the above planes of the dislocations. During TEM observation, many of these unit dislocations were found to spontaneously dissociate into wide stacking faults on {111} planes bound by a/6<112> Shockley partial dislocations. Two such faults labelled A and B lying, respectively, on the (111) and (-111) planes, can be seen in figure 2(a). Contrast analysis also established that the stacking faults were intrinsic in nature, i.e. similar as observed in deformed bulk silver (Korner and Karnthaler 1981). Single layer and overlapping faults were observed, along with complex configurations in the form of L-shaped intersecting faults on two planes

4 and stacking fault tetrahedra. The lattice image in figure 2b shows a short inclined stacking fault ending on a/6<112> Shockley partial dislocations with line directions along <101], the latter being observed in projection as out-of-phase lines along the <100] directions. The measured shift of 1/5d 200 over these lines corresponds with the projection of the stacking fault displacement (Yandouzi, Vasudevan, Schryvers and Toth). Recombination of the Shockley partials into the unit dislocations and disappearance of the stacking faults was also a common occurrence. Dissociation was inhibited when the unit dislocation was faced with a pile-up of dislocations. Very similar observation to those described above were made by Phillips (Phillips 1960) in a detailed study by conventional TEM of evaporated Ag thin films. It is interesting to note that the stacking fault width (i.e. spacing between the Shockley partial dislocations) observed in the present study and by Phillips (Phillips 1960) is much greater than the few nm reported in deformed bulk silver (Korner and Karnthaler 1981). The latter is consistent with the equilibrium Shockley partial spacing of ~5 nm calculated using the reported values (Hirth and Lothe 1982) of the stacking fault energy (16 mj/m 2 ) and elastic constants (µ = 33.8 GPa, ν = 0.354) of bulk silver. Thus, it is believed that the dissociation of the unit dislocations into wide stacking faults observed in silver thin films is a dynamic effect caused by stresses generated from local bending and heating of the thin film by the electron beam, together with the facilitating effects of the thinness of the film and the presence of free surfaces, as was also concluded by Phillips (Phillips 1960). Figure 2. [001] TEM images of dislocations and stacking faults in monocrystalline Ag films obtained under (a) g=200 B2 WBDF and (b) HRTEM conditions Ni-Al film Figure 3 shows a conventional dark field (DF) image obtained by selecting a 100 ordering reflection of the Ni-Al B2 structure, as well as a [001] HRTEM image of the Ni-Al film on top of the Ag buffer layer with holes and an SAED pattern of a Ni-Al + Ag area. From the latter it is clear that the Ni-Al is a single crystal B2 film epitaxially grown on the Ag (001) surface. Basic <110] reflections of the Ni-Al B2

5 structure coincide with the <100] reflections of the Ag, while B2 <100] superlattice reflections are also visible. From these observations the epitaxy and orientation relation (001)[110] B2 // (001)[100] Ag can be inferred. The measured B2 lattice parameter is (0.28 ± 0.01) nm which indeed yields a perfect fit between the [110] B2 and [100] Ag directions, as is also clear from the overlapping lattices in the schematic of figure 3c. The spotted nature of the DF image indicates the existence of a subgranular structure of the epitaxial film. The Ni-Al material deposited inside the holes of the Ag film (grey regions in figure 3a) only reveals a limited texture comparable to that observed in earlier experiments when Ni-Al was directly deposited on (001) NaCl (Yandouzi et al.). The [001] HRTEM image in figure 3b reveals the expected square white dot pattern corresponding with the projected ordered configuration of the B2 material (Schryvers and Tanner 1990). Image simulations confirm that the 15 nm Ag film hardly affects the ordered character of the image of the overlapping structures (Yandouzi et al.). As the growth of the [001] Ni-Al structure can start by placing a Ni or an Al atom (implying the respective (001) B2 plane) on the Ag surface, anti-phase boundaries (APB) with a displacement vector R = 1/2[111] B2 can be expected, as seen from figure 3c. An example of such an interface is shown in the image of figure 3b, where the changing image contrast over the interface points towards an interface plane, which is not, observed edgeon. Figure 3. (a) 100 B2 DF TEM image with corresponding SAED pattern of [001] Ni-Al on (001) Ag. (b) [001] HRTEM image showing an APB in the ordered Ni-Al film. (c) Schematic of the epitaxy, including different nucleation configurations leading to the APB appearing in (b). The surface of the Ni-Al film was further investigated by AFM, from which the image in figure 4a was obtained. The measures along the line scan are presented in figure 4b. From this, the depth of the

6 holes is determined to be (45 ± 5) nm. Aside from these holes, the surface reveals numerous pillars protruding up to 30 nm above the rest of the surface. The lateral dimension of these pillars corresponds with the sub-grain size measured from the DF image in figure 3a. According to the growth diagram by Thornton (Thornton 1977), film growth at 300 C indeed induces preferential columnar growth. As confirmed by image simulations (Yandouzi et al.), the severe height differences of the Ni-Al film can explain the strong changes in HRTEM image contrast. Figure 4. (a) AFM image of a 1 x 1 µm 2 part of the Ni-Al surface on top of a Ag film containing holes. (b) Measures from the indicated line-scans 1 and 2 of (a). (c) Cross-sectional model for the NaCl/Ag/Ni-Al film growth. From the data presented above, the cross-sectional shape of the film can be inferred. When concentrating on a part of the (001) NaCl surface where hillocks of NaCl with simple crystallographic faces remain after treatment, the Ag buffer film will preferably grow in between the hillocks, e.g., starting at the corners between the hillocks and the surface. Since the AFM studies of the treated NaCl surface reveal the presence of hillocks with a height up to 100 nm (Yandouzi et al.), a plan view of the Ag film will indeed look like the image in figure 1a. From figure 3a, it is clear that, after deposition of the Ni-Al, the holes are filled with polycrystalline Ni-Al B2. The fact that AFM still reveals strong height differences between bottom and top of holes with shapes and sizes similar to the ones observed in the uncovered Ag film, indicates that this polycrystalline material is much thinner than the Ni-Al grown epitaxially on the flat Ag surface. Moreover, some sub-grains of the Ni-Al film strongly protrude above the average Ni-Al surface. From these combined experimental results the model illustrated in figure 4c is suggested. Acknowledgements Part of this work was made possible by a BOF grant of the University of Antwerp and by support from the Academy of Sciences in Belgium and Hungary. The financial support of a National Research

7 Grant in Hungary (OTKA T025042) is kindly acknowledged. The collaboration between the universities in Antwerp and Leuven has been supported in the framework of the Belgian Inter-University Attraction Poles (IUAP) program on Reduced Dimensionality Systems (PAI-IUAP No. 4/10). References Au, Y. K., and Wayman, C. M, Scripta Matall., 6, 1972, Hirth, J. P. and Lothe, J., 1982, Theory of dislocations, 2nd edition, 835. Korner, A. and Karnthaler, H. P. 1981, Phys. Stat. Sol. (a),68, 19. Phillips, V. A., 1960, Phil. Mag., 6, 571. Safran, G. Barna, P.B., Geszti, O. and Gunter, J. R., 1993, Thin Solid Films, 229, 37. Shryvers, D. and Tanner, L. E., 1990, Ultramicroscopy, 32, 241. Smialek, J. L. and Hehemann, R. F., 1973, Metall Trans., 4, Thornton, J. A., 1977, Ann. Rev. Mater. Sci., 7, 239. Yandouzi, M., Toth, L. and Schryvers, D., 1998, Nanostrctured Materials, 10, 99. Yandouzi, M., Vasudevan, V., Toth, L. and Schryvers, D., (to be published).