Observation of a rapid amorphization reaction
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S. Brennan Stanford Synchrotron Radiation Laboratory, Stanford, California 94309
A. P. Payne and B. M. Clemens Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 10 February 1992; accepted 6 April 1992)
We have observed a rapid amorphization reaction at ambient temperature in the Gd/Co system by employing grazing incidence x-ray scattering. We find that a 135 A crystalline Gd film is amorphized in less than 30 min by deposition of Co. We postulate that the rapidity of the reaction is due to surface diffusion of Co atoms after deposition to fast diffusion sites such as grain boundaries in the Gd film. Once the interfacial region has been amorphized these fast diffusion paths are sealed off from the surface, rapid diffusion of Co into the Gd crystalline layer is prevented, and the amorphization reaction stops.
One of the most important and interesting aspects of thin film growth is the interaction of two atomic species at the interface between layers. In some multilayer systems the interfaces between the constituents are completely coherent, resulting in long-range structural coherency throughout the sample. In other cases the interaction at the interface can be significant, and can (e.g., in glass forming systems) lead to the formation of disordered interfacial regions which very closely resemble amorphous alloys. Extensive disordered interfaces have been observed in several systems, including early transition metal/late transition metal pairs and rare earth/transition metal pairs, where multilayers with a bilayer period less than the interface width are observed to be compositionally modulated amorphous alloys.1"4 Because disordered interfaces are commonly observed in multilayers of glass forming systems, it is reasonable to suppose that the formation of disordered interfacial phases during deposition is related to the formation of amorphous alloys by reaction between crystalline phases. Disordered interfaces formed during deposition of multilayers can be quite thick. For instance, we and other investigators have observed that multilayers of Gd and Co are compositionally modulated amorphous alloys up to a bilayer period of 200 A; samples with larger bilayer periods show crystalline Gd and Co between the interfaces.5'6 There are three possible mechanisms by which this amorphous alloy could form. First, the Gd layer (for instance) could be inherently amorphous due to the kinetics of the deposition process. Second, a crystalline Gd layer could be amorphized by subsequent deposition of Co. Third, the formation of Gd crystals could be inhibited by diffusion of Co from an underlying Co layer. To elucidate the reaction mechanism that leads to the formation of these extended disordered regions during growth, we perform grazing incidence 1976 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 8, Aug 1992 Downloaded: 13 Mar 2015
x-ray scattering (GIXS) in situ during sputter deposition. We report here the first in situ observation of the formation of an
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