Imec and Holst Centre
have developed an innovative sensor for measuring ultra-low concentrations of
NO2. Such sensors are important for applications that monitor environmental
pollution resulting from traffic, and in general, from all combustion motors.
The sensor’s active components are arrays of grown vertical InAs nanowires.
A typical sensor would contain 500 such nanowires, and will be sensitive to
NO2 concentrations of fewer than 100ppb at room temperature.
Artist impression of the NO2 gas sensor, showing the contacts and InAs nanowires.
The sensor’s nanowires are about 3µm in length and 50-100nm wide.
They are made from InAs, which is well-suited for gas sensing, because it has
an electron accumulation layer at the surface, making it sensitive to accumulated
charges. Gas molecules adsorb onto the nanowires, changing the current that
is flowing through the nanowires.
The semiconductor nanowires are contacted ohmically using an air bridge construction
(see picture). This construction has as advantage that it leaves the nanowire
surface free for gas adsorption. Because of the small bandgap of InAs, it’s
fairly easy to fabricate these ohmic contacts. The sensor can be reset, simply
by applying a stronger current.
The new sensor boasts several breakthroughs in nanowire technology. A key characteristic
is that the vertical nanowires are electrically contacted in the locations on
the substrate where they are grown. In other, comparable nanowire sensors, the
nanowires have to be placed on the substrate after being grown elsewhere. Another
major benefit of these sensing nanowires is that they function without heating,
making them much more power-efficient.
The new gas sensor has been developed in Holst Centre’s program for ultra-low-power
sensors. In a next step, the researchers will increase the sensitivity of the
sensor, as well as its detection selectivity. One goal is, for example, to make
a sensor that can distinguish between NO2 and NO. Also, new manufacturing techniques
are investigated, with the aim to use cost-effective silicon substrates for