Surface Charge Sensitivity of Silicon Nanowires:  Size Dependence

Abstract

Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.

Contactless monitoring of the diameter-dependent conductivity of GaAs nanowires

“…Furthermore, the conductivity of undoped or lowdoped GaAs and other semiconductor NWs-that is expected to undergo substantial changes with decreasing width and in the presence of surface charges on the side walls [16]-can barely be characterized by conventional transport measurements. Chemical modifications of the surface are another poorly controlled factor that influences the transport properties, since these differ for NWs made of different compound semiconductors [6,17].…”

On the origin of enhanced sensitivity in nanoscale FET-based biosensors

“…The use of such nanostructures is justified by the belief that nanoscale biosensors are more sensitive, with sensitivity defined as the relative change in drain current or a shift in threshold voltage in response to a change in bound biomolecule density. A few experiments specifically studied the effect of shrinking nanowire radii on sensitivity, albeit with varying structures, analytes, and sensing circumstances, and found that shrinking a sensor's dimensions indeed improves its sensitivity (4)(5)(6). The enhanced sensitivity has been loosely attributed to the increase in the sensor's surface area-to-volume ratio, which is a direct result of shrinking its dimensions.…”

Contact detection for nanomanipulation in a scanning electron microscope

“…nanotubes and nanowires) are of low cost and can be integrated into micro-structures to form nanodevices. Existing wafer-scale nanotube/nanowire integration methods, such as directed assembly [15], contact printing [16], and dielectrophoresis [17] are inexpensive; however, they are incapable of precisely controlling nanostructure parameters, such as the number nanostructures and nanostructure size, which influence device performance [18][19][20][21][22][23][24][25]. This results in a large device-to-device variability as their performance is mainly dependent on these nanostructure parameters.…”

“…These nanostructure attributes have been shown in nanowire electrical transport studies [18,19], gas-phase chemical sensing [20,21], aqueous sensing of pH and ionic species [22,23], and nanoribbon FETs [24,25] to influence device performance. Therefore these nanowire/nanotube integration methods will result in devices with higher device-to-device variability.…”