Atomic transport in Xe-irradiated Ni/SiO 2 bilayers
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M-A. Nicolet California Institute of Technology, Pasadena, California 91125 (Received 2 June 1988; accepted 3 August 1988) Parametric experiments are described that characterize the process of atomic relocation in N i / SiO2 bilayers induced by Xe irradiation. The parameters varied are the irradiation temperature ( - 196to +500°C)andtheXeirradiationdose(0.01-15Xl0 1 5 cm- 2 ).Backscattering spectrometry of the irradiated samples after removal of the unreacted Ni film is the main analytical tool. A phenomenological model is given that describes the results quantitatively. It is deduced that secondary recoil implantation followed by subsequent redistribution within the cascade's lifetime produces the dominant transport mechanism responsible for incorporating Ni into the SiO2.
I. INTRODUCTION There have been many investigations of the physical changes in multicomponent targets produced by ion irradiation. l~* Induced modifications include short- and long-range redistribution of atoms within the target, and the formation of altered, or even new metastable, phases.1"5 The high local effective temperature (about 104 °C) and rapid quench rate ( > 1014 °C/s) associated with the ion impact event suggests that this could be used to fabricate useful new materials. A number of basic mechanisms are known to contribute to atomic transport in ion mixing. 13 Recoil implantation from single high-energy collisions is an anisotropic process with a characteristic linear fluence dependence.4 A very large number of target atoms experience short-range displacements induced by high generation collisions. The motion within a cascade is predominately isotropic, although the anisotropic flux of secondary recoils can be significant. Electronic interactions introduce chemical effects after the average particle energy in the cascade has diminished to about 1 eV. For binary metal systems, there is a strong correlation between the extent of ion-irradiation-induced mixing and thermochemical properties. Much of the mixing in metal-metal systems apparently occurs in this thermalized regime.2 Chemical interactions can bias the random walk motion within this regime and produce a Darken effect. Mixing is confined to the individual cascades at low temperatures. Johnson etal. have proposed for these systems that metallurgical phases are formed after the modified concentration profile is established.2'5 Long-lived defects generated by the cascade can affect other transport mechanisms. These defects enhance atomic mobility and promote compound formation in metal-Si systems irradiated at elevated temperatures.6"8 An initial sample configuration often investigated is the A/B bilayer formed by two adjacent distinct A and 1072
J. Mater. Res. 3 (6), Nov/Dec 1988
B films. In both metal-metal and metal-Si systems, lowtemperature ion irradiation generally produces a continuous composition variation resembling a diffusion profile. The typical deviation is due to homogeneous compound formation in the intermixed region at elevated temperatures. '~3 Ternary metal-S
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