bombardment. In recoil mixing, atoms are relocated by single collision events. Recoil mixing is predominately seen at large angles as a result of soft collisions, with the number of atoms undergoing recoil implantation varying linearly with ion dose. Recoil implantation, however, is not the dominant process in ion beam mixing. Most relocated atoms are part of a collision cascade in which recoiled atoms initiate a series of lower energy lattice displacements, which is referred to as cascade mixing. Ion beam mixing can be further enhanced by heat spike effects Ion mixing (IM) is essentially similar in result to interdiffusion, hence most models of ion mixing involve an effective diffusion coefficient that is used to characterize thickness of the reacted layer as a function of ion beam implantation over a period of time. The diffusion model does not take into account the miscibility of substrate and layer, so for immiscible or low-miscibility systems it will overestimate the degree of mixing, while for highly miscible systems the model will underestimate the degree of mixing. Thermodynamic effects are also not considered in this basic interdiffusion equation, but can be modeled by equations that consider the enthalpies of mixing and the molar fractions of the target species, and one can thereby develop a thermodynamic effective diffusion coefficient reflecting temperature effects (which become pronounced at high temperatures). Advantages and disadvantages Advantages of ion beam mixing as a means of synthesis over traditional modes of implantation include the process' ability to produce materials with high solute concentrations using lower amounts of irradiation, and better control of band gap variation and diffusion between layers. The cost of IM is also less prohibitive than that of other modes of film preparation on substrates, such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Disadvantages include the inability to completely direct and control lattice displacements initiated in the process, which can result in an undesirable degree of disorder in ion mixed samples, rendering them unsuitable for applications in which precise lattice orderings are paramount. Ion beams cannot be perfectly directed, nor the collision cascade controlled, once IM effects propagate, which can result in leaking, electron diffraction, radiation enhanced diffusion (RED), chemical migration and mismatch. Additionally, all ion mixed samples must be annealed. See also List of coating techniques References External links Ion Beam Mixing at Answers.com Chemical processes Semiconductor device fabrication