A new chemical sensor based on reverse-biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias-dependent molecular detection sensitivity and low operating power. The device takes advantage of graphene’s atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO 2 and 3 times higher for NH 3 in ambient conditions, while consuming ∼ 500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confi rmed using capacitance-voltage measurements.
1 . Introduction
Graphene has drawn a huge research interest in the past several years due to its extraordinary material properties [1] including remarkably high charge carrier mobility of 200 000 cm 2 V −1 s −1 in suspended form, [2] very high surface to volume ratio due to its essentially two-dimensional (2D) nature, [3] and very low Johnson noise in the limit of no charge carriers. [3–5] These properties make graphene very attractive for molecular sensing applications, since the adsorbed molecules can readily affect its conductivity through charge transfer. [3] Demonstration of its ultra-high sensitivity, down Tunable Reverse-Biased Graphene/Silicon Heterojunction Schottky Diode Sensor Amol Singh ,* Md. Ahsan Uddin , Tangali Sudarshan , and Goutam Koley to a single gas molecule, [3] has generated widespread interest in its potential application for molecular detection based on changes in conductance, [3,6,7] surface work function, [6] frequency of the surface acoustic waves, [8] as well as low frequency noise spectra. [7,9]
4 . Experimental Section
Graphene samples used in this work were synthesized through chemical vapor deposition (CVD) on Cu (catalyst) foils (Alfa Aesar, 99.999% purity) in a quartz tube furnace. The synthesis was performed at 1035 °C and 10 Torr pressure with CH 4 as the precursor (nominal fl ow rate of 40 sccm) along with H 2 (nominal fl ow rate 50 sccm), following earlier reports. [35,36]
