Amorphization of Thin Multilayer Films by Ion Mixing and Solid State Reaction
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AMORPHIZATION REACTION
OF THIN MULTILAYER
FILMS BY ION MIXING AND SOLID STATE
M. VAN ROSSUM*, U. SHRETER, W. L. JOHNSON, AND M-A. NICOLET California Institute of Technology, Pasadena, CA. 91125 *Present Address: Instituut Voor Kern- en Stralingsfysika, Leuven University, Belgium
ABSTRACT We have compared the formation of amorphous alloys from Ni-Hf multilayer films by ion mixing and solid state diffusion. We find that ion mixing and solid state reaction produce significant differences in the composition range of the amorphous phase inside the mixed samples. Moreover, the thermochemical parameters which are of primary importance for the solid state reaction also influence the behavior of the Ni-Hf system under ion mixing.
INTRODUCTION The formation of amorphous phases by ion mixing (IM) of thin multilayered films is now well documented [1]. To improve the understanding of this process, it would be desirable to compare amorphization by IM with other methods. A direct comparison with traditional quenching techniques is not straightforward, since the latter produce the amorphous phase from a mixed liquid or gas phase, whereas IM acts directly on the elemental constituents in the solid state. However, a novel approach has recently been developed that allows the formation of amorphous binary alloys by thermal interdiffusion of solid multilayer films E2]. Direct comparative experiments with IM can therefore be carried out on the same samples, provided that the chosen system fulfills the requirements for amorphization under both procedures. The conditions for solid state reaction (SSR) have been stated as follows [2]: (i) the binary system should exhibit a large negative heat of mixing in the solid phase in order to provide a strong chemical driving force for the reaction, (ii) one of the elements has to be an "anomalously fast diffuser" in the other, so that a temperature range can be found in which the amorphous phase formation is kinetically favored over the nucleation of crystalline compounds. On the other side, the requirements for amorphization by IM have been summarized in the "structural difference rule" [11, which states that an amorphous phase is most likely to be formed if the lattice structures of the crystalline multilayer constituents are different. The Ni-Hf system was selected for our investigation on the basis of these criteria: it fulfills the structural difference rule, the elements exhibit a large negative heat of mixing [3], and Ni is known as an anomalously fast diffuser in Hf [4). Moreover, the large mass difference between the two elements facilitates the characterization of the mixing process by backscattering spectrometry.
EXPERIMENTAL PROCEDURE Ni-Hf multilayer films were prepared from metallic targets by electron-gun evaporation on thin pyrex or oxidized Si substrates. The thickness of the individual layers was kept below 150 A in order to maximize the mixing efficiency. The total thickness of samples intended for IM was typically 600 A, which matches the penetration depth (mean projected
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