Z. Erdélyi

Interdiffusion in amorphous Si/Ge multilayers by Auger depth profiling technique

It has been shown by the Auger depth profiling technique that the concentration profile at the initially sharp Si/Ge interface in amorphous Si/Ge multilayers shifted but remained still sharp after a heat treatment at 680 K for 100 h. At the same time the fast diffusion of Si resulted in the formation of an almost homogeneous Ge(Si) amorphous solid solution, while there was practically no diffusion of Ge into the Si layer. This is direct evidence on the strong concentration dependence of the interdiffusion coefficient in amorphous Si/Ge system, and it is in accordance with the previous indirect result obtained from the measurements of the decay of the small angle Bragg peaks, as well as with finite difference simulations.

Investigation of the interplay of nickel dissolution and copper segregation in Ni/Cu(111) system

Abstract The effect of segregation on the dissolution of a thin Ni layer (3–14 equivalent-monolayers (eq-ML)) into a semi-infinite Cu substrate has been investigated at different temperatures (600–730 K) by Auger electron spectroscopy. Computer simulations have also been carried out to understand the details of the dissolution and a sophisticated method is proposed to analyse the experimental kinetics. The simulations indicated a step-wise character of dissolution and an interesting interference between segregation and dissolution. Because of the strong concentration dependence (there is a fast diffusion in Cu and there is practically no diffusion into Ni), the interface remains sharp and shifts proportionally with time until it does not reach the next-to-the-last Ni layer. Then the dissolution continues by the saturation of Cu in the topmost layer, according to the segregation isotherm. After the complete coverage of the surface by Cu, the final homogenisation takes place by the total dissolution of the next-to-the-last layer. According to these results, it was possible to separate the dissolution from the segregation and to determine the average speed of the interface shift, which is proportional to the intrinsic diffusion coefficient.

On the range of validity of the continuum approach for nonlinear diffusional mixing of multilayers

Abstract Using finite difference calculations in continuum equations and using deterministic kinetic equations for calculations in a discrete lattice, the problem of the asymmetry, related to the strong concentration dependence of the diffusion coefficient in the diffusional mixing of binary multilayers, was investigated assuming that the stress effects are negligible and the binary system is ideal. Assuming exponential concentration dependence, it was obtained that the nonlinearity is manifested not only in the change of the range of the validity of the continuum approach (the validity limit was shifted to higher modulation length by about a factor of 10 in the range investigated), but also in peculiar concentration distribution and its time evolution. A fast homogenization took place in both models on the side where the diffusion is faster, and here the distribution remained practically flat and only the amplitude of the composition modulation decreased with time. The shift of a sharp boundary between t...

Thermal stability of amorphous and crystalline multilayers produced by magnetron sputtering

Abstract For the demonstration of the high diversity of different applications of the magnetron sputtering technique in the fabrication of multilayers the production, characterisation and investigations on the thermal stability of epitaxial metallic Mo\V as well as amorphous Si\Ge semiconductor multilayers are presented. It is demonstrated, that combination of low and high angle X-ray diffraction as well as TEM is a very essential tool for the characterisation of the modulated structures and it is also very useful in the investigation of the thermal stability. The experimental results are also compared with numerical calculations made by finite element method for the diffusional homogenisation, controlled by bulk diffusion. It was shown, that in the case of the amorphous Si\Ge system the intermixing is possible in amorphous state, and diffusional stresses do not cause a significant curvature on the thermal decay curves of the first order peak of the low angle Bragg reflection (ln(I\I0) vs t), but they play an important role in the strong porosity formation in the silicon side. Furthermore it was illustrated, that the initial curvature on the ln(I\I0) vs t curve is due to the strong concentration dependence of the intrinsic diffusion coefficients. In epitaxial Mo\V multilayers the thermal behaviour is different. There is a fast transformation of the layered structure into a heterogeneous partly reacted structure, which consists of regions of a reacted Mo(V) alloy and the remaining Mo\V layers. This can not be interpreted by bulk diffusion, but rather by the formation of grain-boundaries inside the Mo layer and probably by a grain-boundary motion induced alloying.

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...

Dissolution and off-stoichiometric formation of compound layers in solid state reactions

In many technological processes, intermediate phases are produced by solid state reactions (SSRs) of pure materials initially layered onto each other. It is generally accepted that during heat treatments of such structures, stoichiometric compound films form and then grow, or if such a layer already exists then it growths further. We show that in many realistic cases the compound layer forms and starts to grow highly off stoichiometrically. Moreover, an initially existing stoichiometric compound layer may dissolve then off stoichiometrically reform. Our findings are of primary importance for nanotechnologies where early stages of SSR are utilized; e.g., metallization for silicon-based devices in micro-/nanoelectronics.

Determination of grain-boundary diffusion of Ag in nanocrystalline Cu by the Hwang–Balluffi method

From the Ag and Cu Auger signals measured on the surface of nanocrystalline Cu/Ag thin bilayers, the temperature dependence of the parameter ω′ for Ag grain-boundary diffusion in Cu (ω′=δkbDb/δsks, where δ and δs are the width of the grain boundary and the segregated layer and kb and ks are the grain-boundary and surface segregation factors, respectively) have been determined by the Hwang–Balluffi method in the C-kinetics regime over the temperature range of 393–428 K. These values have been compared with triple products, P=δkbDb, determined in the temperature range 584–804 K by Bernardini et al. [Phil. Mag. A 73, 237 (1996)] using radio tracer technique in the B-kinetics regime and the temperature dependence of the surface segregation factor has been extracted. The surface segregation energy (34±16 kJ/mol) agrees well with other data published in the literature.

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.

Transition from anomalous kinetics toward Fickian diffusion for Si dissolution into amorphous Ge

Over the last years, several experimental and theoretical studies of diffusion kinetics on the nanoscale have shown that the time evolution (x∝tkc) differs from the classical Fickian law (kc=0.5). However, all work was based on crystalline samples or models, so far. In this letter, we report on the diffusion kinetics of a thin amorphous Si layer into amorphous Ge to account for the rising importance of amorphous materials in nanodevices. Employing surface sensitive techniques, the initial kc was found at 0.7±0.1. Moreover, after some monolayers of Si dissolved into the Ge, kc changes to the generally expected classical Fickian law with kc=0.5.

Interface sharpening in miscible Ni/Cu multilayers studied by atom probe tomography

Interfaces of Ni/Cu multilayers were studied by atom probe tomography. To this aim, specimens with sharp or artificially smeared interfaces were prepared and investigated before and after annealing at 773 K. Owing to three-dimensional subnanometer resolution of the atom probe, local chemical analysis of layer interfaces becomes possible without interferences of grain boundaries or geometric roughness. In contrast to the classical expectation for a miscible system, but in agreement with more recent theoretical considerations, diffusion reduces the chemical width of the interfaces by up to 50%.