Gas sensing device using as-grown vertical InAs nanowires

Holst Centre has developed a nanoscale gas sensing device as part of its ultra-low-power sensors program. Based on gold-free grown vertical InAs nanowire arrays, the system is sensitive to NO2 concentrations of fewer than 100ppb at room temperature. The semiconductor nanowires are contacted ohmically using an air bridge construction, leaving the nanowire surface free for gas adsorption.

The achievement - first reported in a paper published in Nano Letters in May - boasts several key landmarks in nanowire technology. Key amongst these is that the vertical nanowires are electrically contacted in the locations on the substrate where they are grown. Alternative solutions place the nanowires on the substrate after growing them elsewhere. Another major benefit of the sensing nanowires is that they work without heating, making them highly power efficient.

InAs makes good sense
The InAs nanowires are about 3?m in length and 50-100nm wide, and are contacted in an exposed air-bridge formation. A typical sensor contains 500 nanowires. InAs is a good material for gas sensing because it exhibits an electron accumulation layer at the surface, making it sensitive to accumulated charges, and allows relatively easy fabrication of ohmic contacts (due to its small band gap). Analogous to the gate of a transistor, gas molecules adsorbing onto the nanowires modify the current flow through the nanowires. The sensor is reset, simply by applying a stronger current.

The ability to detect NO2 is important for applications for monitoring environmental pollution resulting from combustion or automotive emission. One of the next development steps is to increase detection selectivity as well as sensitivity, for example for the distinction between NO2 and NO. Manufacturing techniques are also being investigated, aiming to make a silicon substrate a viable alternative for high yield solutions.