Silicon nanowires for advanced microelectronics: MBE gas-source growth and in situ devices

New nano-scale assembly technology, such as catalyzed or self-assembled growth of nanowires and quantum dots, may benefit integrated-circuit production by eliminating critical lithography steps. Many metals have been used to catalyze Si nanowire growth. Among them, Ti exhibits a diffusivity and solubility in silicon at least two orders of magnitude lower than that of Au, Fe, Zn, Ni, or Co at the same temperature. Therefore, Ti-catalyzed silicon nanowires are more compatible with integrated-circuit components and applications.; Using TiSi₂ islands as a catalyst, we have grown Si nanowires by molecular-beam epitaxy (MBE) with Si₂H₆ as a gas source. Approximately one monolayer of Ti was deposited on Si substrates and then annealed at high temperature to form TiSi₂ islands, which can nucleate Si nanowires in the subsequent growth, with diameters mainly between 20 nm and 40 nm.; Most TiSi₂ islands are identified as C49-TiSi₂ with the orientation: Si[110]//TiSi₂[100] (∼6% lattice mismatch) and Si(001)//TiSi₂(010). These islands do not nucleate Si nanowires, possibly due to a highly defective and strained interface with the Si substrate. Growth of Si nanowires is associated with nucleation by TiSi₂ islands with orientations other than the orientation mentioned above. In these wires, better lattice matching at the TiSi₂/Si interface is observed for major crystallographic planes. This is likely due to a lower energy barrier and/or a smaller driving force for Si atom attachment at the TiSi₂-Si interface resulting from the better lattice match. Additionally, abrupt changes in growth direction (kinking) frequently occur, typically accompanied by crystal twinning. Strain at the TiSi₂/Si interface is posited to be the main reason for the frequent twinning and kinking of nanowires.; The dopants As (donor) and B (acceptor) have been introduced during growth of Si nanowires. The in-situ p-n junctions produced by this technique display diode-like I-V behavior. A tungsten layer has also been deposited as a gate (electrically isolated) between the top contact of a wire and the substrate. The gated structures show modulation of the current through the wire, which demonstrates that surrounding-gate field-effect transistors made from Si nanowires are possible.