Facile fabrication of ordered Si _1-xGe _x nanostructures via hybrid process of selective epitaxial growth (SEG) and self-assembled nanotemplates

In this study, a facile fabrication method of the ordered Si _1-xGe _x nanodots (NDs) and nanowires (NWs) was successfully developed via hybrid process of selective epitaxial growth (SEG) of Si _1-xGe _x and self-assembled nanotemplates, i.e., anodic aluminum oxide (AAO) and diblock copolymer (DBC) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA). Si _{1- x}Ge _x films were selectively grown on the Si windows against the oxide area at a growth temperature of 550 °C by repeating the unit cycle consisting of two consecutive steps; Si _{1- x}Ge _x deposition step using Si ₂H ₆ and GeH ₄ and Cl ₂ exposure step for removing the nuclei or deposits formed on the oxide area during the preceding Si _{1- x}Ge _x deposition step. The chemical composition of the Si _1-xGe _x films was readily controlled by adjusting the flow rate of GeH ₄ from 20 sccm to 45 sccm while that of Si ₂H ₆ gas was fixed at 10 sccm, giving rise to the variation of Ge composition in Si _1-xGe _x from 22.2% to 34.0%. In order to fabricate well-ordered Si _{1- x}Ge _x nanostructures, Si windows with hexagonal arrangement were fabricated using AAO and PS-b-PMMA. AAO was prepared through multi-step anodization of the Al films of Al/Si(0 0 1) substrate under suitable anodizing conditions. Subsequently, ordered Si windows were fabricated by removing the barrier layer at the bottom of the AAO membrane by reactive ion etching (RIE). In case of PS-b-PMMA, SiO ₂ templates with ordered Si windows were fabricated through replication of nano-cylindrical pattern of PS-b-PMMA to the 20-nm thick SiO ₂ layers of SiO ₂/Si. By utilizing the ordered Si windows obtained from both AAO and PS-b-PMMA, Si _{1- x}Ge _x was selectively grown on Si windows against the oxide area, viz., aluminum oxide in AAO and SiO ₂ templates. Hexagonally ordered NDs and freestanding NWs were facilely fabricated on the Si substrates after removing the AAO and SiO ₂ templates.