Boron nitride substrates for high-quality graphene electronics

TL;DR

Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2).

Abstract

Graphene devices on standard SiO2 substrates are highly disordered, exhibiting characteristics far inferior to the expected intrinsic properties of graphene[1-12]. While suspending graphene above the substrate yields substantial improvement in device quality[13,14], this geometry imposes severe limitations on device architecture and functionality. Realization of suspended-like sample quality in a substrate supported geometry is essential to the future progress of graphene technology. In this Letter, we report the fabrication and characterization of high quality exfoliated mono- and bilayer graphene (MLG and BLG) devices on single crystal hexagonal boron nitride (h-BN) substrates, by a mechanical transfer process. Variable-temperature magnetotransport measurements demonstrate that graphene devices on h-BN exhibit enhanced mobility, reduced carrier inhomogeneity, and reduced intrinsic doping in comparison with SiO2-supported devices. The ability to assemble crystalline layered materials in a controlled way sets the stage for new advancements in graphene electronics and enables realization of more complex graphene heterostructres.

Figures (8)

• Figure 1: Optical images of graphene and$\mathrm{h}-\mathrm{BN}$ before (a and b, respectively) and after (c) transfer. Scale bar in each is $10 \mu \mathrm{~m}$. Inset shows electrical contacts. (d) Schematic illustration of the transfer process to fabricate graphene-on-BN devices (see text for details).
• Figure 2: (a)AFM image of monolayer graphene on BN with electrical leads. White dashed lines indicate the edge of the graphene flake. Scale bar is$2 \mu \mathrm{~m}$. (b) Histogram of the height distribution (surface roughness) measured by AFM for $\mathrm{SiO}_{2}$ (black triangles), h-BN (red circles) and graphene-on-BN (blue squares). Solid lines are Gaussian fits to the distribution. Inset: high resolution AFM image showing comparison of graphene and BN surfaces, corresponding to the dashed square in (a). Scale bar is $0.5 \mu \mathrm{~m}$
• Figure 3: Resistance versus applied gate voltage for (a) MLG and (b) BLG on h-BN. Inset in each panel shows the corresponding conductivity. For both devices, the temperature dependence of the conductivity minimum and high density resistivity are shown in (c) and (d), respectively. Solid and dashed lines in (d) are linear fits to the data. (e) Conductivity of a different MLG sample comparing the room-temperature transport characteristics measured as-transferred-to-h-BN (blue curve) and after annealing in$\mathrm{H}_{2} \mathrm{Ar}$ (black curve).
• Figure 4: (a)Longitudinal and Hall conductivity versus gate voltage at$B=14 \mathrm{~T}$ (solid line) and 8.5 T (dashed line) for MLG. (b) Longitudinal and Hall resistance versus gate voltage at $B=14 \mathrm{~T}$ for BLG. Inset shows a magnetic field sweep at fixed density. SdH oscillations begin at $\sim 0.4 \mathrm{~T}$ with LL symmetry breaking appearing at fields less than 6 T. $T \sim 2 \mathrm{~K}$ in both panels.
• Figure 5: FIG. S1: (a)Optical image of a representative h-BN flake exfoliated onto a $\mathrm{Si} / \mathrm{SiO}_{2}$ substrate. (b) AFM image of the region indicated in (a) by a dashed box. scale-bar is $0.5 \mu \mathrm{~m}$. The h-BN surface seen here measures $\sim 8 \mathrm{~nm}$ in height relative to the $\mathrm{SiO}_{2}$ backgraound. At this scale it is apparent the $\mathrm{h}-\mathrm{BN}$ surface is much smother than the underlying $\mathrm{SiO}_{2}$ substrate. (c) Height histogram of the h-BN surface measured for several different sample-thicknesses. A typical measurement from a $\mathrm{SiO}_{2}$ surface (solid black squares) and a HOPG wafer (open black circles) are shown for comparison. (d) h-BN surface roughness versus sample thickness measured from several different samples. Solid line is a guide-to-the-eye. Dashed line indicates resolution of our system, obtained by measuring the surface of HOPG under the same conditions.