Irradiation of solids with energetic particles results in the reorganization of constituent target atoms, i.e., ion beam mixing (IM). At low temperatures, IM is characterized by prompt (10 −10 s) diffusion processes which are localized in the vicinity of the displacement cascade. Mixing at low temperatures can cause the system to depart far from the equilibrium state. At elevated temperatures, the diffusion of radiation-induced defects extends the mixing to longer times and greater distances. These delayed IM processes tend to return the system toward equilibrium. Recent experimental progress has led to a qualitative understanding of the fundamental aspects of IM in both temperature regimes. This has been achieved through systematic measurements of the influence of temperature, dose, dose-rate, cascade energy density and chemical interactions on IM. The results of these experiments will be reviewed and compared to IM models based on collisional, thermal spike and radiation-enhanced diffusion processes. The relation of IM to other fundamental radiation damage effects will also be discussed.

Fundamental aspects of ion beam mixing

Interface enabled defects reduction in helium ion irradiated Cu/V nanolayers

In this review we shall describe and discuss the basics of the ion mixing phenomenon. First, the process is examined by considering only ballistic and kinematic effects associated with collisions between projectile and target atoms. We shall examine how collision cascades and the evolution of dense energy spikes affect the mixing process. We then show how material properties, such as heat of mixing and cohesive energy, influence the mixing process. These properties lead to the physical basis for the occurrence of temperature-independent and temperature-dependent mixing regimes. Finally, we shall examine the phase formation possibilities in the ion-reacted layer, and how irradiation conditions and various material properties can influence the final structure that forms.

Ion beam mixing in metallic and semiconductor materials

A systematic study of ion-beam mixing of tracer impurities in thin metal films at low temperatures has been conducted. We have investigated the dependence of ion mixing on two matrix properties: atomic mass and cohesive energy. We have also studied the dependence of ion mixing on tracer impurity properties: its heat of mixing with the matrix and its thermal diffusivity in the matrix. The matrices investigated were thin films of C, Al, Ti, Fe, Ni, Cu, Mo, Ru, Ag, Hf, Ta, W, Pt, and Au. The tracer impurities, Al, Ti, Cr, Mn, Fe, Ni, Cu, Y, Nb, Mo, Ru, Ag, In, Sb, Hf, Ta, W, Pt, Au, and Bi, were deposited as ≤15 A layers near the midplanes of the specimens. All the tracer and matrix elements, except C, were deposited sequentially, without breaking vacuum. The samples were irradiated with 300–1000-keV Kr ions to doses 1015–1016 ions/cm2 at temperatures of 6 and/or 77 K. Most samples were analyzed at the irradiation temperature by He backscattering. A strong correlation between ion mixing and the matrix properties, atomic mass, and cohesive energy, was observed. A correlation between ion mixing and tracer impurity diffusion was also observed but not between ion mixing and the heat of mixing or relative mass of the impurity with the matrix. The results are interpreted within the framework of a thermal spike model of cascade diffusion.

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Low-Temperature Ion Beam Mixing in Metals

Ion-beam mixing by 500-keV xenon ions has been studied in targets consisting of 2000-A films of aluminium on a polycrystalline aluminium substrate, onto which has been evaporated a 500-A overlayer of copper. Both long- and short-range-mixing processes have been identified, by RBS analysis of the irradiated targets, as a deep copper tail in the aluminium and interfacial broadening, respectively. The long-range component varies linearly with xenon fluence, is temperature-independent in the interval 40–500 K, and is not influenced by the presence of an interfacial oxide layer between the copper and aluminium layers. The number of long-range-mixed atoms is in agreement with theoretical estimates of the recoil mixing. The short-range mixing, which is the dominating process, has a squareroot dependence on xenon fluence and is independent of temperature between 40 and 300 K, increasing rapidly at higher temperatures. The broadening attributed to the short-range mixing is explained by interstitial diffusion within the cascade. For small xenon fluences, interfacial oxide layers inhibited both short-range mixing and thermal diffusion. Higher xenon fiuences subdued the inhibition.

Han Westendorp

Papers

Ion beam mixing of CuAu and CuW systems