Several mechanisms can cause atomic relocation as a result of ion irradiation. Experiments are reviewed that provide information on the atomic redistribution within a solid. Predictions of different models, when compared with these results, shed light on the dominant processes. Sucesses and problem areas are discussed and possible practical applications of the results are explored.

Large changes in the distributions of alloying components have been observed in a wide variety of alloys during ion bombardment. This metastable, radiation-induced segregation (RIS) results from the preferential transport of alloying components by defect fluxes. Since the number of radiationinduced defects can exceed the number of implanted ions by several orders of magnitude, RIS can be highly efficient in modifying near-surface alloy compositions. In this paper, the general features of RIS are described. Aspects which are pertinent to the modification of materials by ion beams are emphasized. Examples are given of surface modifications which utilize RIS effects.

Modifications of subsurface alloy composition by bombardment with ions of several keV energy were investigated theoretically, using a phenomenological model which includes the effects of various processes, such as preferential sputtering, displacement mixing, Gibbsian adsorption, radiation-enhanced diffusion, and radiationinduced segregation. The nonuniformity of damage rates, resulting from slow-down of the incoming ions, was also taken into account. The alloy composition evolution in time and space was calculated numerically for different temperatures, using concentrated Ni-Cu as a model alloy system. A good qualitative agreement between the present model calculations and recent experimental measurements was obtained.

Argon ions at 150 keV and fluences ranging from 5 × 1015 to 4.8 × 1017 ions/cm2 were used to bombard 60 nm chromium layers deposited on copper. The resulting concentration profiles were determined by AES and thin film microstructural analyses were performed using an STEM equipped with an energy dispersive X-ray spectrometer (EDS). The concentration profiles are in qualitative agreement with the theory of radiation induced mixing. At fluences up to 2.4 × 1017 Ar+/cm2, metastable solid solutions of increasing copper content in BCC chromium are formed. At the highest fluence (4.8 × 1017 Ar+/cm2 ), chromium rich precipitates form in the copper matrix.

Thin surface copper-nickel alloys were prepared by ion implantation at 90 keV. During the implantation of one pure element by the other the sputtered products were collected on catcher foils at different stages from the beginning of the implant through to the steady state configuration of the target surface. The collector foils and targets were analyzed to determine the behavior of the sputtering yields during implantation and for the change in surface composition at the selected fluence. The total sputtering yield for the target and the effective elemental sputtering yields for each component appear to be functions of the changing surface fractions, the self ion sputtering yield of the implanted species, and the elemental sputtering yield of the initial target species. A model relating these parameters is presented.

300 keV Kr ion irradiations with doses varying from 2× 1015 to 2× 1016 at/cm2 have been applied to initiate mixing of Cu-Au and Cu-W systems. As under normal thermodynamic conditions the Cu-Au system is miscible whereas the Cu-W system is not, the comparison of both systems provides a test for the current theories on ion-beam mixing. A pronounced difference in mixing phenomena is observed for both systems; in the Cu-Au system atomic displacements are one order of magnitude larger than those in the Cu-W system. In addition, a drastic temperature dependence of ion-beam mixing in the Cu-Au system has been found. The mixing is suppressed by lowering the substrate temperature during irradiation. These results show that radiation enhanced diffusion is the mechanism underlying the ion-beam mixing of Cu and Au. Results for the Cu-W system are consistent with a collisional mixing model.

After a relatively low dose Xe+ irradiation (∼ 2 × 1015 Xe/cm2), an initially discrete multilayered film of average composition Au7Si3 transforms to a uniform layer and forms a metastable phase. This is the well known ion mixing process. The same samples were subjected to higher dose irradiation (> 4 × 1015 Xe/cm2). It was found that the Si migrates to the sample surface and forms a nonuniform depth profile in BS spectra. At the same time, the metastable phase dissociates into Au and Si phases. As this process is opposite to that of ion mixing, we suggest to name this process ion demixing. These observations demonstrate that ion mixing and ion demixing can both occur in the same system, if it is properly chosen. Possible mechanism responsible for these processes are discussed.

The atomic mixing of evaporated Al/Sb films and of Al/Ag films on Al<110> crystal substrates by 400 keV Xe ion beams has been investigated. Concentration depth profiles were measured in situ by 1.5 MeV He scattering as a function of Xe fluence from 2 to 32×1015 Xe/cm2. The initial mixing rates are similar at 85 and 300 K; mixing proceeds by rapid motion of Al (≈15 Al/Xe) into and uniformly through the thickness of the Sb film and by a slow motion of Sb (≈0.5 Sb/Xe) into the Al<110> substrate. More rapid Sb mixing into Al occurs for polycrystalline Al. The rate for Al into Sb slows at concentrations approaching the stable AlSb phase. Appreciably higher rates of Sb mixing into Al (2.2 to 2.8 Sb/Xe) occur at 575 K. Mixing rates for the highly soluble system, Al/Ag, are compared to the nearly insoluble Al/Sb at 85 and 300 K. Appreciably higher rates are found for Ag than for Sb, suggesting the influence of chemical driving forces even at these low temperatures.

We have studied the atomic mixing of Sb layers in Al induced by irradiation with 110 keV Ar ions. Samples with 10 and 230 Å Sb layers sandwiched in Al were made by sequential vacuum deposition and analyzed by backscattering spectrometry. The mixing was characterized by finding the increase, σ2, in the variance of the Sb signal in the backscattering spectra. For 10 Å layers irradiated at 90 and 270 K, the profiles are Gaussian and σ2 increases linearly with dose of implanted Ar ions. The mixing is essentially independent of temperature up to ∼ 290 K, where it increases sharply. For 230 Å layers irradiated at 90 and 270 K, the profiles are approximately Gaussian but develop a flat top as the Al:Sb ratio approaches 1:1. At 90 K σ2 increases monotonically with dose up to 3.6 × 1016 Ar/cm2, but at 270 K it develops a plateau at a dose of ∼ 2.2 × 1016 Ar/cm2, which corresponds to Al:Sb = 1:1. No such plateau is observed at 90 K. Again, the mixing showed very little variation with temperature between 90 and 270 K, but rose abruptly above 300 K.

Techniques used to produce unique states of pure metals mixed into ceramic materials are presented. The samples were prepared by irradiating a 1-MeV Fe+ beam on A12O3 crystal surfaces over which a thin chromium or zirconium film had been evaporated. The limitations of using noble gas ion beams are noted. Micro Knoop hardness tests performed near the surfaces of the samples indicated a significant increase in the hardness of most samples prepared by ion beam mixing.

Epitaxial metastable (GaSb)1−Gex alloys with compositions across the pseudobinary phase diagram have been grown on (100) GaAs substrates by multitarget rf sputtering. An essential feature allowing the growth of these unique materials was low energy ion bombardment of the growing film during deposition to promote collisional mixing and enhance adatom diffusion. Annealing experiments indicated that the metastable films exhibit good high temperature stability. While the free energy difference between the single phase metastable and two phase equilibrium states is only ∼ 20 meV/atom, the activation barrier for the transformation is ∼ 3 eV. All films were ptype with room temperature carrier concentrations and mobilities between 1016 and 1019 cm−3 and 10 and 720 cm2/V∼sec, respectively, depending on film composition. Collisional mixing due to low energy ion bombardment of the growing film was also found to affect both the transformation rate kinetics as well as the reaction path during subsequent annealing of amorphous GaSb/Ge mixtures deposited on glass at 60°C. Finally, some initial results on the deposition of metastable fcc Cu/Cr alloys are presented.

The amorphous semiconducting phase (a-sc) of Si or Ge is so resistant to crystallization that rapid heating may bring it into a temperature regime in which it melts. Such melting might occur in one or the other of two ways, either homogeneously, by the reverse of the glass transition, to a viscous semi-conducting melt (ℓ-sc) or by transition, probably by nucleation and growth, to the molten metallic state (ℓm). Using the self-diffusion constant of the crystalline elements in conjunction with the Stokes-Einstein equation, upper limiting values of the glass transition (a-sc→ℓ-sc) temperatures of Si and Ge were calculated. These were of the order 0.6 to 0.65 Tcℓ for slow and 1.1 Tcℓ for ultra rapid heating, where Tcℓ is the equilibrium melting temperature of the crystal. Arguments are given that superheating to a temperature 1.15 to 1.25 Taℓ (a-sc↔ℓm in equilibrium at temperature T = Taℓ< Tcℓ) may be required for copious internal nucleation of im in a-sc. At lesser superheating the transition must be initiated at internal flaws (e.g. voids) or at the external surface of the a-sc film. Therefore the superheating at perceptible onset of the transition during rapid heating can vary widely from specimen to specimen, depending on the flaw concentration and how the external surface of a-sc was treated.

High resolution Rutherford backscattering and channeling techniques have been used to investigate the formation and stability of supersaturated solid solutions of As, Sb, In, Pb, Tℓ and Bi implants in (100) silicon. In all cases nearsubstitutional solid solubilities far exceeding maximum equilibrium solubility limits can be achieved by furnace annealing at temperatures ≤ 600°C. Details of the recrystallisation process indicate that the maximum impurity concentration which can be incorporated onto silicon lattice sites may be controlled by impurity size and associated strain effects at the amorphous-crystal boundary during epitaxial regrowth.

Transmission electron microscopy (plan-view as well as cross-section) and high-resolution Rutherford backscattering and channeling techniques have been combined to investigate residual defects and substitutional concentrations of In and Sb in <100> and <111> orientations of ion implanted silicon layers after solid-phase-epitaxial (SPE) growth at 475–600°C. The maximum concentrations of Sb and In in substitutional sites were found to be, respectively, 1.3 × 1021 and 5.0 × 1019 cm−3 , exceeding the respective retrograde maxima by factors of 18 and 60. These results provide direct evidence of solute trapping and metastable alloying under solid-phase growth conditions. Accumulation of solute at the crystalline-amorphous interface was observed only for indium above a certain interfacial concentration. At still higher interfacial concentrations, the planar interface was observed to become unstable and SPE growth was retarded.

Energy deposition and heat-flow determine the temperature distributions in pulsed-beam irradiated structures. In laser irradiated Si, transient conductance measurements indicated a liquid/solid interface velocity of 2.8 m/sec during crystallization in agreement with a heat flow model. With pulsed ionbeam annealing of metal-Si structures, melting starts at the interface; epitaxial NiSi2 layers have been formed. Ion-beammixing experiments on polycrystalline and epitaxial Au-Ag bilayers show that intermixing is more pronounced in the polycrystalline structures as is the case with thermal annealing. Superlattice structures are formed in the epitaxial structures.

Amorphous and three metastable crystalline phases have been formed in the Au-Si system by 300 keV Xe ion mixing using multilayered samples with average compositions of Au2Si8, AuSi, and Au7 Si3. Generally speaking, during the different stages of irradiation at liquid nitrogen temperature (LNT) or room temperature (R.T.), metastable crystalline phases are formed initially, and eventually an amorphous structure is obtained. Thermal decomposition of amorphous alloys yield different metastable crystalline phases. Some of the metastable crystalline phases can be formed directly from multilayered samples by steady-state thermal annealing. The formation mechanisms of metastable phases are discussed in terms of the processes involved in ion mixing and thermal annealing.

The influence of Xe+, As+ and Si+ irradiation on the Ni, Cr and Pt-silicide formed upon subsequent thermal annealings has been investigated. Helium MeV backscattering spectrometry and x-ray diffraction have been used to analyze the samples. The effect of silicon-metal interface contamination has been observed using an intentionally formed thin silicon oxide layer. The oxide layer was thick enough to prevent thermal silicide formation in unirradiated samples. In irradiated samples, the silicide forms thermally in spite of the presence of this oxide layer for Ni and Cr, but not for Pt. Similar results were obtained for Xe+, As+ and Si+ irradiations. We attribute this effect to ion mixing of the interfacial layer. The differences between results obtained with Ni, Cr and Pt are discussed. These results demonstrate that a low-dose irradiation can render the process of silicide formation upon thermal annealing tolerant to interfacial impurities. On the other hand, we also show that ion irradition can inhibit Pt and Ni silicide formation.

Single crystals of Al2O3 were implanted with chromium and zirconium to fluences of 1 × 1016 to 1 × 1017 ions cm−2. Rutherford backscattering-channeling studies showed the surface layers to be damaged but crystalline with the implanted ions randomly distributed. The microhardness and indentation fracture toughness were higher for the random solutions than for conventionally formed solid solutions. Changes in structure and properties caused by annealing in air at temperatures up to 1800°C were studied.

The formation of a continuous metallic layer in the doped region of LiF crystals implanted at low temperature, has been explained by the formation of bridges between next neighbouring alkali ions of the lattice around-each implanted ion. For a critical concentration of implanted ions it is possible to show using statistical arguments that conducting chains are formed by the union of these links, according to a percolation mechanism. We show that the assumption of a distribution of isolated implanted ions at low temperature is justified by the observation of their precipitation when the crystal is warmed up to room temperature. The transformation of the metallic optical absorption into the colloidal band associated with precipitates of implanted ions is correlated with the transition from a conducting state to an insulating state of the implanted layer. We show that this evolution towards an equilibrium situation may be reversed by a reirradiation, which is used as an external perturbation and that the conducting state associated with dispersed implanted ions is then once again obtained.

MgO single crystals implanted with alkali ions (Li+, Na+, K+ and Rb+),Fe+, In+ and Au+ ions have been studied after implantation with doses up to 1017 ions.cm−2 and after annealing at temperatures up to 1100°C.In order to characterize defects and precipitated phases, several techniques have been associated depending on the implanted ions : optical absorption, transmission electron microscopy, Rutherford backscattering spectrometry, conversion electron Mössbauer spectroscopy and X-ray diffraction at oblique incidence. Directly after implantation the intrinsic defects in the anionic sublattice (F, F+, F2-centers) and in the cationic sublattice (V−-centers) are observed. As to the implanted species, two precipitation processes are observed : (i) the implanted ions precipitate independently of the matrix elements. This is the case of alkali ions which form alkali metal precipitates. (ii) the implanted species precipitate with those of the matrix to form compounds. This is observed with iron which forms oxide precipitates and spinel ferrite. In the case of indium and gold, the precipitation process occurs with the cations of the matrix to form binary alloys : Mg3In and Au3Mg.