G.L. Katona

Linear growth kinetics of Nanometric silicides in Co/amorphous-Si and Co/CoSi/amorphous-Si thin films

Evolution of the reaction zone on the nanoscale has been studied in bi- and multilayered Co/a-Si as well as in trilayered Co/a-CoSi/a-Si and Co/CoSi/a-Si thin film diffusion couples. The kinetics of the phase boundary movement during solid state reaction has been followed with special interest of the initial stage of the diffusion, i.e. effects happening on the nanoscale (short time, short distance). The interfacial reactions have been investigated in situ by synchrotron radiation. The formed phases were also characterized by transmission electron microscopy and resistance measurements. The effect of phase nucleation and shift of phase boundaries have been separated in order to determine the “pure” growth kinetics of the crystalline CoSi and Co2Si product phases at the very early stages. Deviations have been found from the traditional diffusion controlled parabolic phase growth. Computer simulations based on a kinetic mean field model illustrated that the diffusion asymmetry (large difference in diffusion...

Nanoscale investigations of shift of individual interfaces in temperature induced processes of Ni–Si system by secondary neutral mass spectrometry

We describe a method for measurement of nanoscale shift of interfaces in layered systems by a combination of secondary neutral mass spectrometry and profilometer. We demonstrate it by the example of the investigation of interface shifts during the solid state reaction in Ni/amorphous-Si system. The kinetics of the shrinkage of the initial nanocrystalline Ni film and the amorphous Si layer as well as the average growth kinetics of the product phases were determined at 503 K. The results show that nanoscale resolution can be reached and the method is promising for following solid state reactions in different thin film systems.

Kinetic pathways of diffusion and solid-state reactions in nanostructured thin films

Mass transport and solid-state reactions in nanocrystalline thin films are reviewed. It is illustrated that diffusion along different grain boundaries (GBs) can have important effects on the overall intermixing process between two pure films. These processes can be well characterized by a bimodal GB network, with different (fast and slow) diffusivities. First the atoms migrate along fast GBs and accumulate at the film surface. These accumulated atoms form a secondary diffusion source for back diffusion along slow boundaries. Thus the different GBs of the thin films can be gradually filled up with the diffusing atoms and composition depth profiles reflect the result of these processes. Similar processes can be observed in binary systems with intermetallic layers: instead of nucleation and growth of the reaction layer at the initial interface, the reaction takes place in the GBs and the amount of the product phase grows by the motion of its interfaces perpendicular to the GBs. Thus, the entire layer of the pure parent films can be consumed by this GB diffusion-induced solid-state reaction (GBDIREAC), and a fully homogeneous product layer can be obtained.

Formation of CuxAu1−x phases by cold homogenization of Au/Cu nanocrystalline thin films

Summary It is shown, by using depth profiling with a secondary neutral mass spectrometer and structure investigations by XRD and TEM, that at low temperatures, at which the bulk diffusion is frozen, a complete homogenization can take place in the Cu/Au thin film system, which leads to formation of intermetallic phases. Different compounds can be formed depending on the initial thickness ratio. The process starts with grain boundary interdiffusion, which is followed by a formation of reaction layers at the grain boundaries that leads to the motion of the newly formed interfaces perpendicular to the grain boundary plane. Finally, the homogenization finishes when all the pure components have been consumed. The process is asymmetric: It is faster in the Au layer. In Au(25nm)/Cu(50nm) samples the final state is the ordered AuCu3 phase. Decrease of the film thicknesses, as expected, results in the acceleration of the process. It is also illustrated that changing the thickness ratio either a mixture of Cu-rich AuCu and AuCu3 phases (in Au(25nm)/Cu(25nm) sample), or a mixture of disordered Cu- as well as Au-rich solid solutions (in Au(25nm)/Cu(12nm) sample) can be produced. By using a simple model the interface velocity in both the Cu and Au layers were estimated from the linear increase of the average composition and its value is about two orders of magnitude larger in Au (ca. 10−11 m/s) than in Cu (ca. 10−13 m/s).

Influence of the substrate choice on the L10 phase formation of post-annealed Pt/Fe and Pt/Ag/Fe thin films

Pt/Fe and Pt/Ag/Fe layered films were deposited by DC magnetron sputtering on MgO(001), SrTiO3(001), and Al2O3(0001) single crystalline substrates at room temperature. The films were post-annealed between 623 K and 1173 K for 30 s in flowing N2 atmosphere. The onset of the L10-FePt phase formation in films deposited on MgO(001) and SrTiO3(001) substrates was observed after annealing between 773 and 873 K, while chemical L10 ordering sets in for Pt/Fe bilayers on Al2O3(0001) at lower temperatures accompanied by strong (001)-texture. It is concluded that elastic stress, arising from the difference in thermal expansion coefficients between film and substrate, promotes ordering and texture formation.

Influence of the annealing atmosphere on the structural properties of FePt thin films

FePt thin films with a thickness of 30 nm were deposited by dc magnetron sputtering at room temperature onto SiO2(100 nm)/Si(100) substrates. These films were post-annealed in a temperature range of 500 °C to 900 °C for 30 s in three different atmospheres—N2, Ar, and forming gas (Ar+H2 (3 vol. %)). Irrespective of the annealing atmosphere, the chemically ordered L10 FePt phase has formed after annealing at 500 °C. Higher annealing temperatures in N2 or Ar atmosphere resulted in a strong increase in grain size and surface roughness but also in the appearance of a pronounced (001) texture in the FePt films. However, these films show the presence of iron oxide. In contrast, annealing in forming gas atmosphere suppressed the oxidation process and resulted in a reduced grain size and lower surface roughness. However, no (001)—but a strong (111)—texture was obtained after annealing at 700 °C, which might be related to the reduced unit cell tetragonality and incorporation of hydrogen to the FePt lattice. Thus, t...

Low-temperature formation of the FePt phase in the presence of an intermediate Au layer in Pt /Au /Fe thin films

Pt /Fe and Pt /Au /Fe layered films were deposited at room temperature by dc magnetron sputtering on Al2O3(0 0 0 1) single crystalline substrates and heat treated in vacuum at 330 °C with different durations (up to 62 h). It is shown by secondary neutral mass spectrometry depth profiling and x-ray diffraction that the introduction of an additional Au layer between Pt /Fe layers leads to enhanced intermixing and formation of the partially chemically ordered L10 FePt phase. The underlying diffusion processes can be explained by the grain boundary diffusion induced reaction layer formation mechanism. During the solid state reaction between Pt and Fe, the Au layer moves towards the substrate interface replacing the Fe layer. This was explained by the much faster diffusion of Fe, as compared to Pt, along the grain boundaries in Au. Enhancement of the process and formation of the ordered FePt phase in the presence of the Au intermediate layer were interpreted by the effect of stress accumulation during the grain boundary reactions: the disordered FePt phase formed initially at different Au and Pt grain boundaries can experience appropriate compressive stress along the {1 0 0} directions, which can initiate the formation of the chemically ordered L10 FePt phase.

Investigation of solid-state reaction in Ag/Sn nanostructured thin films at room temperature

Diffusion intermixing processes in nanostructured Ag/Sn thin-film system at room temperature were investigated by means of secondary neutral mass Spectrometry depth profiling technique. As it was confirmed by X-ray diffraction too, the reaction started already in the as-deposited sample. Since the bulk diffusion was frozen at room temperature, the Ag3Sn phase was formed along the grain boundaries (GBs), gradually consuming the interior of grains, and was grown perpendicular to the GBs. At the same time, formation and growth of a small compact reaction layer near the interface were observed and the shift of the bordering parallel interfaces was controlled by GB diffusion. From the kinetics of the diffusion process in the above two mechanisms, both the interface velocity in the diffusion-induced grain boundary motion regime as well as the coefficient of parabolic growth in the planar growth regime were determined.

Anomalous Kinetics and Regimes of Growth of Intermetallic Phases during Solid State Reactions in Nanosystems

Two interesting features of formation and growth of intermetallic phases in nanoscale solid state reactions will be discussed:Linear-parabolic “normal” growth: it will be summarized that at the very early stages of the growth of an already existing new phase (i.e. when nucleation problems can be neglected) the linear kinetics can be observed due to the so-called diffusion asymmetry. Indeed, it was shown that if the ratio of the diffusion coefficients differ by orders of magnitude in the parent materials (and so also in the new phase), during the growth of a phase bordered by parallel interfaces from the parent phases (normal growth geometry), the shift of the individual interfaces can be linear at the beginning and a transition to the parabolic regime can take place even after a shift of several tens of nanometres. In addition, an AB compound in contact with the pure A and B phases can be dissolved if the diffusion in B is much faster than in either A and AB. This means that the thickness of this phase should decrease, or even can be fully dissolved, at the beginning and only after some time—when the composition in B will be high enough allowing the re-nucleation of this AB phase—will the AB phase grow further.The common problem of two stages of solid state reactions will be revisited: usually the growth can be divided into two stages: a) the formation (nucleation) and lateral growth of the new phases and b) the “normal” growth of the already continuous phase. It was concluded in different previous reviews that in stage b) in the majority of cases the parabolic growth was observed in accordance with the above i) point: the linear-parabolic transition length was typically below 1 μm, which was the lower limit of detection in many previous investigations. On the other hand recently the application of the linear-parabolic growth law for the analysis of experimental data obtained in nanoscale reactions became very popular, not making a clear distinction between a) and b) stages. It will be emphasized here that care should be taken in all cases when the experimental methods applied provide information only about the increase of the amount of the reaction product and there is no information where and how the new phase (s) grow. We have illustrated in a series of low temperature experiments - where the bulk diffusion processes are frozen - that even in this case a full homogeneous phase can be formed by cold homogenization called Grain Boundary Diffusion Induced Solid State Reaction (GBDIREAC). In this case first the reaction starts by grain-boundary (GB) diffusion and nucleation of the new phase at GBs or their triple junctions, then the growth of the new phase happens by the shift of the new interfaces perpendicular to the original GB. This is a process similar to the diffusion induced grain-boundary motion (DIGM) or diffusion induced recrystallization (DIR) phenomena and in this case the interface shift, at least in the first stage of the reaction until the parent phases have been consumed, can be considered constant. This means that the amount of the phase increases linearly with time, giving a plausible explanation for the linear kinetics frequently observed in stage a).

Determination of the compositions of the DIGM zone in nanocrystalline Ag/Au and Ag/Pd thin films by secondary neutral mass spectrometry

Summary Alloying by grain boundary diffusion-induced grain boundary migration is investigated by secondary neutral mass spectrometry depth profiling in Ag/Au and Ag/Pd nanocrystalline thin film systems. It is shown that the compositions in zones left behind the moving boundaries can be determined by this technique if the process takes place at low temperatures where solely the grain boundary transport is the contributing mechanism and the gain size is less than the half of the grain boundary migration distance. The results in Ag/Au system are in good accordance with the predictions given by the step mechanism of grain boundary migration, i.e., the saturation compositions are higher in the slower component (i.e., in Au or Pd). It is shown that the homogenization process stops after reaching the saturation values and further intermixing can take place only if fresh samples with initial compositions, according to the saturation values, are produced and heat treated at the same temperature. The reversal of the film sequence resulted in the reversal of the inequality of the compositions in the alloyed zones, which is in contrast to the above theoretical model, and explained by possible effects of the stress gradients developed by the diffusion processes itself.