Ion Mixing Processes
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ION MIXING PROCESSES MARC-A. NICOLET, T. C. BANWELL, AND B. M. PAINE California Institute of Technology, Pasadena, CA
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ABSTRACT We consider ion mixing in the low temperature regime, Mixing of most thin where it is insensitive to temperature. markers is Gaussian, independent of irradiation flux, and However, the mixing in some varies linearly with fluence. media varies widely between markers of similar mass and In appears to correlate with thermal diffusion constants. bilayer systems, the profile of long-range mixing is exponential, and the number of mixed atoms scales linearly with fluence. This can be modeled successfully with simple Short range mixing scales with the collisional theory. but again shows strong square root of the fluence, We correlations with known bulk chemical properties. forces and low energy conclude that chemical driving transport mechanisms such as interstitial migration play major roles in ion mixing, even at low temperatures.
INTRODUCTION Experiments with bilayer, multilayer and marker samples all demonstrate that the sample temperature is a primary factor in the ion mixing process. regimes exist whose The available evidence suggests that two distinct position in the temperature scale can be indicated by a critical temperature T . Above T , ion mixing produces results that depend on the temperature; 9 The general below T , t e outcome is insensitive to temperature. interpretation of this fact is that above T , the perturbation produced in is principally determined by the thermal motion of the sample In this regime, the details of the primary (radiation-induced) defects. obliterated by -the subsequent collisional process are largely The outcome becomes sensitive thermally-induced rearrangements of atoms. not only to temperature, but also to the specific chemical and structural properties of the irradiated material. Below T c , extensive migration of defects is absent, so the observed atomic distributions reflect the primary collisional processes of the ion-solid interaction more closely than above T . We consider here primarily this low temperature (i.e. T < T ) regime and review some of the data that are available from marker and bilayer experiments. MARKER EXPERIMENTS In a marker experiment, a very thin layer of an atomic species is imbedded in an otherwise uniform host material of a different species. The Backscatter~ng spectra are taken before and after ion irradiation. increase, Q , in the energy varian e of th• marker signal due to ion mixing is obtained from the difference Q . - Q . of the variances after ayd before irradiation. The variance in repth, y , is then given by (Q/N[ I) , where N is the atom density of the host medium and [(] is the stopping cross section factor for scattering of He ions from the marker in that host medium. The principal facts established so far, from such marker experiments are: (i) Marker spreading is observed only when the ion beam penetrates to the marker. (ii) The spreading is Gaussian. Notable exceptions are Pd and Pt in Si Mat. Res. Soc. Symp
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