Yield and Reliability of Nanocrystalline Graphene Field-Effect Gas Sensors
Authors: Noll Dennis , Schwalke Udo 
Published: February 2020

For the development of next-generation gas sensors with higher sensitivity, selectivity, responsivity and cost-effectiveness, carbon-based materials are a major research topic. Graphene, as a two-dimensional nanomaterial with a high surface-to-volume ratio and outstanding electronic properties, offers a good choice as a sensing layer [1]. However, for the successful application of reasonably priced graphene-based gas sensors a high fabrication yield and long-term stability are essential key requirements. Nevertheless, only few studies on the yield and reliability of graphene-based devices do exist [2].
Transfer-free nanocrystalline graphene field-effect transistors (ncGFETs), which are sensitive towards various toxic gases, including ammonia (NH3), nitrogen dioxide (NO2) and carbon monoxide (CO) can be fabricated by the use of our CMOS-compatible in situ catalytic chemical vapor deposition (CCVD) process. By this method we have fabricated hundreds of devices on oxidized two inch silicon wafers.
In this contribution we report results on process yield and reliability, obtained from backgate input characteristics of 524 devices fabricated on a single two inch wafer. Electrical characterization has been performed using a Keithley SCS 4200 semiconductor parameter analyzer. The characterized ncGFETs exhibit nominal device widths of 20 µm and lengths of 3 µm and have been parameterized at a drain-to-source bias of VDS = -300 mV. The measurements reveal that 81.68 % of the fabricated ncGFETs show typical ambipolar behavior as is known from graphene devices (fig. 1). A field-effect (FE), defined via current on/off ratio, of at least 2 is observed for all of the devices within a backgate electric field strength range of ±2.4 MV/cm. Beyond that, in these devices larger field-effects are observed with 59.81 % exceeding a field-effect of 10 to 15 (10 ≤ FE < 15), 24.3% between 15 and 20 (15 ≤ FE < 20) and 7.25% larger than 20 (FE ≥ 20) (fig. 2). Due to the differences in the strength of the field-effect, material variations of our nanocrystalline graphene are expected. Moreover, by means of a statistical evaluation of the shift of the charge neutrality point the different doping levels of our devices are investigated.
Furthermore, the ncGFETs show hysteresis effects that are enhanced by exposure to atmospheric environment and other gases, but are reduced in vacuum. Using the Pulsed Time-Domain Measurement (PTDM) technique [3] the surface charge trap density, which induces the hysteresis, is determined for various cases and its influence on the electrical and sensing characteristics of our device will be discussed. In addition, results on accelerated stress tests at high electrical fields and elevated temperatures will be presented as a first attempt to investigate the degradation and reliability of ncGFETs.
[1] F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat Mater, 6, 652 (2007).
[2] A. D. Smith, S. Wagner, S. Kataria, B. G. Malm, M. C. Lemme and M. Ostling, Ieee T Electron Dev, 64, 3919 (2017).
[3] R. S. Park, M. M. Shulaker, G. Hills, L. Suriyasena Liyanage, S. Lee, A. Tang, S. Mitra and H. S. Wong, ACS Nano, 10, 4599 (2016).

Figure 1