Morphology and Magneto-Transport in Exfoliated Graphene on Ultrathin Crystalline β-Si₃N₄(0001)/Si(111)

In the last decade, graphene and graphene-derived systems have attracted wide attention in view of possible applications in electronics and sensing that are envisioned in light of graphene’s unique electrical, mechanical, thermal, and optical properties combined with its intrinsic 2D nature. Experimentally accessible transport properties of graphene on standard SiO₂ substrates are however limited by roughness, charge scattering, and impurities. Alternative dielectric substrates with small lattice mismatch with graphene and possibly with a high-k dielectric constant are therefore desirable for improving graphene-based device performance. Hexagonal boron nitride (h-BN) is appealing for this purpose owing to its relatively smooth surface virtually free of charge traps. Indeed, graphene devices on exfoliated h-BN substrates have higher mobilities than those fabricated on SiO₂. However, their technological relevance is limited by the small size of the exfoliated h-BN flakes. Among other high-k dielectric materials, Si₃N₄ is regarded as one of the best candidates for this application. An ultrathin layer (less than 1 nm) of wafer-scale crystalline β-Si₃N₄(0001) can be readily grown on Si(111) substrates and is known to passivate the Si (111) surface.

We used a polymer-assisted method for the transfer of exfoliated graphene onto crystalline β-Si₃N₄, an approach that is reliable and compatible with surface-science techniques, known to be highly demanding in terms of cleanness and atomic order. After an initial preparation and characterization of the nitride surface at the atomic level under ultra-high vacuum (UHV) conditions, a graphene flake was transferred on top of it. Micro-Raman spectroscopy was employed to verify graphene quality and electron-beam lithography followed by metal evaporation was used to fabricate Ohmic contacts on the graphene layer in a Hall bar configuration. The graphene surface was subsequently characterized by scanning tunneling microscopy (STM) under UHV conditions, indicating that even after complete-device fabrication the structural properties of the graphene are preserved. Back-gate modulation on the graphene channel and weak localization were investigated by magneto-transport measurements at 4.2 K.

Figure 1(a) shows a 10 × 10 nm² image of the graphene, demonstrating its high quality. In panel (b), we show a magnification of the graphene surface with atomic resolution (4 × 4 nm²). The background was subtracted and a profile traced (along the blue line in panel (b)): see panel (d). The image shows a lattice period of 0.256 nm, in agreement with literature. The image shows a triangular lattice instead of the typical honeycomb lattice structure characteristic of noninteracting graphene. We attribute this behavior to the effect of a slight curvature, as shown by the larger size image of panel (a). The local electronic properties of the graphene flake were investigated by scanning tunneling spectroscopy (STS) measurements. The STS spectrum reported in Figure 1(c) shows a gap-like feature at around 0 V (i.e., at the Fermi level) and a dip at +0.21 V, indicated as V_D. This feature is attributed to the energy position of the Dirac point, indicating p-type doping in the graphene.

Figure 1: (a) 10 × 10 nm² STM image of the graphene flake after device fabrication. (b) A magnification (4 × 4 nm²) of the highlighted square area in panel (a) shows the atomically resolved structure of the graphene flake. (c) STS spectrum measured on the same area as panel (a). (d) A line profile of panel (b) is shown. It shows that ten periods correspond to 2.56 nm.

Current–voltage curves measured at 4.2 K in two-probe configuration (at back gate voltage V_BG = 0 V) are linear indicating the presence of good Ohmic contacts between graphene and the metal electrodes. Figure 2(a) shows the gate-voltage dependence of the graphene resistance R₂₄, measured at 4.2 K. Starting from a back-gate voltage of about 0 V the resistance strongly increases with increasing back-gate voltage, reaching a local maximum at about 3.5 V, which suggests that the charge neutrality point is located there. This indicates that the graphene is p-doped, consistently with the STM data. The R₂₄ versus V_BG data was measured up to a back-gate voltage of 20 V, a range in which no leakage current from the gate to the graphene could be detected. Carrier concentration and mobility were obtained through Hall effect measurements. We performed a series of magneto-transport measurements at different back-gate voltages at 4.2 K. An example is shown in Figure 2(b) for V_BG = −2.65 V. Note also the peak at B = 0 T that we attribute to weak localization. The sharp magnetoresistance peak indicates a high mobility and a long phase coherence length of the sample. The negative sign of the slope in Figure 2(b) indicates p-type behavior of the graphene channel, in agreement with the data shown in Figure 2(a).

Figure 2: (a) Channel resistance R as a function of back gate voltage V_BG. (b) Resistance R as a function of magnetic field B at back gate voltage V_BG = −2.65 V.

Publications:

1. Sedighe Salimian, Shaohua Xiang, Stefano Colonna, Fabio Ronci, Marco Fosca, Francesco Rossella, Fabio Beltram, Roberto Flammini, and Stefan Heun: Morphology and Magneto-Transport in Exfoliated Graphene on Ultrathin Crystalline β-Si₃N₄(0001)/Si(111), arXiv:2004.09170 [cond-mat.mes-hall].
2. Sedighe Salimian, Shaohua Xiang, Stefano Colonna, Fabio Ronci, Marco Fosca, Francesco Rossella, Fabio Beltram, Roberto Flammini, and Stefan Heun: Morphology and Magneto-Transport in Exfoliated Graphene on Ultrathin Crystalline β-Si₃N₄(0001)/Si(111), Adv. Mater. Interfaces 7 (2020) 1902175.

Presented at:

1. Shaohua Xiang, Stefano Colonna, Fabio Ronci, Stefan Heun, Roberto Flammini: Silicon nitride as graphene substrate in device design, Materials.it 2016, Aci Castello, Catania, Italy, 12 – 16 December 2016 (poster). [Abstract] [Poster]
2. Sedighe Salimian, Shaohua Xiang, Stefano Colonna, Fabio Ronci, Marco Fosca, Francesco Rossella, Fabio Beltram, Roberto Flammini, and Stefan Heun: Quantum transport in exfoliated graphene on ultrathin crystalline β-Si3N4, E-MRS Spring Meeting 2019, Nice, France, 27 – 31 May 2019 (oral). [Abstract] [Talk]
3. Stefano Colonna: Material characterization at NanoMicroFab, NanoInnovation 2019, Rome, Italy, 11 – 14 June 2019 (oral). [Abstract] [Talk]
4. Sedighe Salimian: Quantum transport in dual-channel InAs/InP/GaAsSb core-shell nanoscale devices and Graphene/ultrathin-Si3N4 heterostructure device, NANO Colloquia 2020, Pisa, Italy, 21 May 2020 (oral). [Abstract] [Talk]