Giant piezoresistance effect in silicon nanowires.

The piezoresistance effect of silicon has been widely used in mechanical sensors2–4, and is now being actively explored in order to improve the performance of silicon transistors. In fact, strain engineering is now considered to be one of the most promising strategies for developing high-performance sub10-nm silicon devices. Interesting electromechanical properties have been observed in carbon nanotubes8,9. In this paper we report that Si nanowires possess an unusually large piezoresistance effect compared with bulk. For example, the longitudinal piezoresistance coefficient along the k111l direction increases with decreasing diameter for p-type Si nanowires, reaching as high as 23,550 3 10 Pa – , in comparison with a bulk value of 294 3 10 Pa. Straininduced carrier mobility change and surface modifications have been shown to have clear influence on piezoresistance coefficients. This giant piezoresistance effect in Si nanowires may have significant implications in nanowire-based flexible electronics, as well as in nanoelectromechanical systems. Application of strain to a crystal results in a change in electrical conductivity due to the piezoresistance effect. To evaluate the piezoresistance effect (or electromechanical properties) in nanostructures, mechanical manipulations and electrical measurements must be performed simultaneously. Traditionally, this has been carried out by interfacing the nanostructures with lithographically defined electrodes. Such nanostructure–electrode interfaces, however, inevitably have stability and reliability issues when subject to mechanical forces. To avoid such interface problems, we have recently developed a chemical vapour deposition process to grow suspended nanowires for piezoresistance testing11. Silicon nanowires with k111l or k110l growth directions were grown in trenches on silicon-on-insulator (SOI) wafers to form bridge structures (Fig. 1; Supplementary Information, Fig. S1). Such bridges are themselves monolithically structured and fully functional devices, enabling direct probing of electromechanical properties. The joints between the nanowires and the trench sidewalls are mechanically robust. As shown in Fig. 1b, the nanowire grew backwards after impinging into the opposite wall, implying a self-welding mechanism. The deflection of these nanowires followed exactly the behaviour of double-clamped beams in our atomic force microscopy experiments, confirming the rigidity and equivalence of the two joints. The joints also make reliable electrical connections at the nanowire level. Cross-section transmission electron microscopy images show that there is no catalyst metal at either interface (Fig. 1c and e); hence, the wire–wall junctions are simply Si homojunctions with properties determined primarily by carrier distributions. Ohmic behaviour was readily established at the interfaces for the p-type nanowires studied here. The diameters and resistivities of nanowires can be readily controlled. In this study, the nanowires have diameters ranging from 50 to 350 nm and they were made p-type with resistivities of 0.003–10 V cm (see Methods). a