Control of epitaxial growth of Fe-based nanocrystals on Si substrates using well-controlled nanometer-sized interface

摘要

We have developed an epitaxial growth technique for Fe-based nanocrystals (NCs) on Si substrates with high selectivity of their crystal structure. Ge NCs with controlled shape and strain were initially epitaxially grown on Si substrates covered with an ultrathin SiO_2 film. Using these well-controlled Ge NCs as nucleation sites, Fe-based NCs could be formed with crystal-structure selectivity. In Fe deposition on the Ge NCs at room temperature, bcc-Fe NCs were formed, where epitaxial growth was influenced by the Ge NC shapes related to surface coverage. For Fe deposition at 250-300 ℃, Fe-Ge alloying occurred without intermixing with Si. The epitaxially grown crystal structures were determined by the strain state of the Ge NCs: Fe_(1.7)Ge NCs with a B8_2 structure for spherical strain-relaxed Ge NCs with a lattice constant close to that of bulk Ge, and ε-FeGe NCs with a B20 structure for flattened strained Ge NCs with a lattice constant close to that of bulk Si. All the NCs had sharp interfaces, where interfacial alloying in the Fe-Si-Ge system was well controlled. This growth technique can be used as a general technique enabling epitaxial growth of well-controlled transition metal-based films and nanostructures.