Graphene Nanoribbon Field-Effect Transistors on Wafer-Scale Epitaxial Graphene on SiC substrates

We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ~10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ~0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of GNRs, the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature, and interband tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors, and remain potential candidates for electronic switching devices. 2 Implementation of 2-dimensional (2D) graphene for digital logic devices has proven challenging because of the material’s zero band gap [1]. Various alternate digital logic device structures have been proposed that take advantage of interlayer tunneling, graphene-3D semiconductor heterostructure, and properties that exploit the light-like energy dispersion of carriers in 2D graphene [2-6]. From the point of view of realizing conventional field-effect transistors, well-controlled graphene nanoribbons (GNRs) mimic the excellent electrostatic properties of carbon nanotubes (CNTs) and offer hope for graphene-based digital logic devices [7, 8]. The ultrathin body can enable scaling down to 10 nm or below while still keeping shortchannel degradation effects at bay. GNRs suffer from edge-roughness scattering effects compared to CNTs, but GNRs provide better large-area scalability, planar fabrication opportunity, and heat dissipation capacity than CNTs [9]. The availability of broken bonds at the edges provides a window of opportunity for chemical doping [10], which remains difficult in CNTs due to saturated sp 2 chemical bonds. A number of “beyond-CMOS” devices, such as the GNR tunneling field-effect transistor (TFET) [11] can be realized if controlled GNRs can be fabricated on large-area substrates. Thus, progress in the fabrication and characterization of wafer-scale GNRs stands to potentially enable a host of applications in the future. The creation of controlled band gaps by quantum confinement of carriers in GNRs remains a significant challenge [12~21]. To date, graphene nanoribbon field effect transistors (GNRFETs) down to 10 ~ 20 nm channel width have been fabricated from exfoliated graphene [13, 14] and chemical vapor deposition (CVD) grown graphene [15, 16] using conventional topdown lithography and etching methods. Bottom-up techniques such as chemically derived 3 GNRFETs down to sub-5 nm width have been fabricated, and show substantial band gaps with ION/IOFF ~10 6 at room temperature [17]. GNRFETs have also been fabricated by unzipping CNTs [18-20]. More recently, GNRs down to 5 nm has been directly grown on SiC substrates using ion implantation followed by laser annealing [21]. But the bottom-up techniques are not yet site-controlled and reproducible, and are currently incompatible with conventional lithographic processes for circuit implementations. Epitaxial graphene (EG) grown on single-crystal, semi-insulating SiC wafers satisfy many of the above criteria [22, 23]. Furthermore, devices based on EG require fewer processing steps and are more immune to contamination compared to CVD-grown large-area graphene due to the absence of a transfer process. GNRFETs can mimic properties of CNTFETs and remove needs of alignment and random mixtures of metallic and semiconducting channels. The major challenge in realizing GNRs is in achieving ~5 nm widths with smooth edges. In this pursuit, GNRFETs stand to benefit from recent process developments in Silicon FinFET technology, in which arrays of ~5 nm wide Si fins have been demonstrated with robust structural integrity [24]. Process variation challenges of such narrow fins have been addressed for next-generation CMOS technology [25]. Despite the importance of EG, substantial energy gaps have not yet been demonstrated in GNRFETs made in EG on SiC [26]. Furthermore, there are no studies that correlate experimentally measured transport properties and theoretical models for EG-GNRs. In this work, we report the fabrication of top-gated ~10 nm wide GNRFETs by lithography on large area EG on SiC substrates. We observe for the first time, the opening of a substantial energy gap inversely proportional to the GNRFET width of EG-GNRs. By relating the measured transport with theoretical modeling, we find that the transport properties of narrow epi-GNRs are similar 4 to well-behaved narrow-bandgap semiconductors, contrary to carrier localization effects reported extensively in wider GNRs fabricated on exfoliated graphene [27-31]. The reasons for these observations will be discussed. Fig. 1. (a) ~ (d) Optical microscope image of epitaxial graphene nano-ribbon (GNR) FETs on wafer size SiC substrate. (e) Scanning electron microscope (SEM) image of GNR having 10 nm widths with source and drain metal. (f) SEM image of HSQ array ribbon patterns, consisting of 13 nm line with and 17 nm space, showing no deformation and collapse. The HSQ patterns play a role as a mask to etch graphene during O2 Plasma. Finally, GNR remains after removing the HSQ mask. The starting material in this work was epitaxial-graphene grown on a 4 inch diameter Siface 6H-SiC substrate. The epitaxial growth conditions are described in earlier reports [22] and this epitaxial graphene on SiC is expected to have lower residual charge than transferred graphene (2~5 x10 11 cm -2 ) on SiO2 due to the absence of transfer process [32, 33]. Figure 1 shows the final device images including single GNR and arrays of GNRs. Hydrogen