Molecular beam allotaxy: a new approach to epitaxial heterostructures

The present status of a new epitaxial growth method, named molecular beam allotaxy (MBA), is reviewed. The method allows one to grow single-crystalline heterostructures in a new way; that is, the desired layer forms during annealing of a precipitate layer. The precipitates are embedded within a single-crystalline matrix, grown by molecular beam epitaxy. The key point is that the epitaxial growth of the matrix persists from the substrate through the spacings between the inclusions to the overlayer. The precipitates coarsen and coalesce into a uniform layer during a subsequent high-temperature treatment, the second processing step. We discuss the present theoretical model of the layer formation during annealing which is based on inhomogeneous Ostwald ripening and particle coalescence. We place particular emphasis on the growth of precipitates in single-crystalline silicon, since MBA is based on precipitate growth and these nanoparticles exhibit interesting phenomena, such as self-ordering. We discuss the influence of the growth parameters on the layer quality and provide optimized growth and annealing parameters. MBA has been used so far mainly for the growth of epitaxial silicide heterostructures. The metallic disilicide CoSi2 on Si(100) serves as a model system because of its interesting material properties and its potential for microelectronic and optoelectronic applications. The structural and electrical properties of the obtained epitaxial Si/CoSi2/Si(100) heterostructures are reviewed. We also discuss the growth of a ternary silicide, Co1-gammaPd gamma Si-2, showing that MBA can be applied also for ternary compounds. We see a great potential in the semiconducting silicides beta-FeSi2 and RU2Si3, which can be grown by MBA. As a further test of the MBA method we investigated the growth of a buried SiOx layer in Si(100). The use of epitaxial silicides as compliant substrates and as building blocks for new microelectronic and optoelectronic devices is briefly reviewed.