  We demonstrate humidity sensing using a change of electrical resistance of a
single- layer chemical vapor deposited (CVD) graphene that is placed on top of
a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor
towards the most common constituents in air, its signal response was
characterized individually for water vapor (H2O), nitrogen (N2), oxygen (O2),
and argon (Ar). In order to assess the humidity sensing effect for a range from
1% relative humidity (RH) to 96% RH, devices were characterized both in a
vacuum chamber and in a humidity chamber at atmospheric pressure. The measured
response and recovery times of the graphene humidity sensors are on the order
of several hundred milliseconds. Density functional theory simulations are
employed to further investigate the sensitivity of the graphene devices towards
water vapor. Results from the interaction between the electrostatic dipole
moment of the water and the impurity bands in the SiO2 substrate, which in turn
leads to electrostatic doping of the graphene layer. The proposed graphene
sensor provides rapid response direct electrical read out and is compatible
with back end of the line (BEOL) integration on top of CMOS-based integrated
circuits.
