Solid state gas sensors for monitoring toxins in the environment, chemical exhaust or biological samples have received increasing attention over the recent years. A promising material for this application is graphene, having demonstrated single molecule detection capability and sensitivity towards a variety of gases [1]. In this contribution we present a transfer-free production method of nanocrystalline graphene field effect transistors (nGFETs), which can be used as very sensitive gas sensors.

By means of our PMMA-enhanced in situ catalytic chemical vapor deposition (CCVD) process [2] we fabricate few-layered nGFETs. By this method, hundreds of nGFETs are simultaneously fabricated on a single 2 inch oxidized, highly p-doped silicon wafer. After fabrication the individual metal catalyst sites are used as electrical contacts to the nanocrystalline graphene that bridges the gap along the insulating silicon dioxide surface (see figure 1a & b). Hereby, post-growth graphene-transfer and etching as well as cleaning steps are obsolete.

Material characterization has been done using a Horiba Labram HR800 Raman microscope with a 632nm laser, yielding spectra showing strong G (1590 cm^-1) and D (1350 cm^-1) signatures but only a weak 2D (2700 cm^-1) signal. Hence, a near edge X-ray absorption fine structure (NEXAFS) analysis at the carbon K-edge of the CCVD graphene was done and analyzed in respect to its C-C sp² bonding structure. For that purpose, reference spectra of graphene and graphene oxide, both supplied by Graphenea, have been recorded. The NEXAFS experiments were performed at the plane grating monochromator (PGM) [3] beamline of PTB at the BESSY II synchrotron employing radiometrically calibrated instrumentation. By means of a linear combination of reference NEXAFS spectra for graphene and graphene oxide, the composition of the CCVD grown graphene layers is derived.

By electrical characterization using an HP4156A precision semiconductor parameter analyzer we investigate the intrinsic electrical properties and gas sensing capabilities of our nGFETs using our self made vacuum probing station. The intrinsic properties have been tested under a vacuum pressure of 2*10^-5mbar revealing a negative shift of the charge neutrality point in the input characteristics of our nGFETs, thereby verifying atmospheric hole-doping to our as-fabricated devices. Afterwards, the nGFETs have been thermally annealed under vacuum to desorb remaining adsorbents in order to restore the intrinsic properties of the devices. Subsequently, the devices have been exposed down to a concentration of 200ppbv of ammonia. As a consequence a positive shift of the charge neutrality point is recorded, indicating an increase in electron doping. During this exposure the hysteresis effect of the nGFETs is enhanced and its origin will be discussed. We will also report on the dynamical behavior (e.g. response and recovery times) of our devices in comparison to a simple commercial MQ-5 gas sensor.

[1] F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov; “Detection of individual gas molecules adsorbed on graphene, Nat Mater, 6, 652 (2007).

[2] D. Noll, U. Schwalke; ”PMMA-enhancement of the lateral growth of transfer-free in situ CCVD grown graphene”, in 2016 13th International Multi-Conference on Systems, Signals & Devices (SSD), p. 458 (2016).

[3] F. Senf, U. Flechsig, F. Eggenstein, W. Gudat, R. Klein, H. Rabus, G. Ulm, J. Synchrotron Rad. (1998) 5, 780-782.

Acknowledgement

The authors would like to thank PD Dr. Emanuel Ionescu and Benjamin Juretzka from Technische Universität Darmstadt for the opportunity to record Raman spectra at the group of dispersive solids.