Fabrication and quantum properties of nanostructured silicon

Physical properties of nanostructured silicon are reviewed in comparison to those of single-crystalline silicon (c-Si) and amorphous silicon (a-Si). Particular emphases are placed on low-dimensional silicon chains (e.g. polysilane) and nanocrystalline silicon (nc-Si) particles (e.g. porous silicon (PS)). These materials can be obtained by nanofabrication technology based on chemical, electrochemical, or dry-processing techniques. Due to induced strong quantum confinement effects, the optical and related properties of nanostructured silicon become free from direct- and indirect-transition regime. Then various functions appear as a novel semiconducting system. Polysilane consisting of a linear silicon backbone chain and capped organo-substituents acts as a self-assembled quantum wire. Nanocrystalline porous silicon composed of highly packed isolated or interconnected silicon nanocrystallites with an average diameter of 2–3 nm efficiently luminesces visible light. Highly efficient electroluminescent nc-Si diodes have been fabricated based on silicon substrates. In addition, nc-Si devices operate as a negative-resistance diode with an electroluminescent behavior, a light-emissive nonvolatile memory, a ballistic electron emitter, and an ultrasound generator. Scientific significance and technological potential of silicon nanostructuring are discussed from viewpoints of exploring advanced materials and developing functional integration.