Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation

We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of $4H$-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 °C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer.

Figure 1

Au-intercalated quasi-free-standing monolayer graphene. (a) A schematic of Au atoms on top of the Bu-L and intercalation of Au atoms after thermal drive, lifting the Bu-L from SiC (0001) to produce QFS monolayer graphene. (b) Schematics of Au atoms (yellow) on half of the Bu-L substrate before thermal drive, and diffusion of Au (light yellow, Au diffused) after thermal drive. The blue circles are Pd contacts. The bottom figures are optical micrographs of the areas in the red boxes, showing the diffusion of Au over millimeter lengths. The black circles are Pd contacts and the scale bar is 100 µm. (c), (d) Raman spectroscopy around the Raman $D$ and $G$ peaks (c) and 2D peaks. The spectra given for the Bu-L (black), Au-deposited Bu-L (red), Au-diffused area (green), and Au-deposited area after intercalation (blue) show the emergence of graphene $D, G$, and 2D peaks after decoupling the Bu-L. (e), (f) STM topography images ($z\mathrm{range}=13\mathrm{nm}$) after 4-Å Au intercalation, where the Au-deposited area shows Au clusters (e) and the Au-diffused area (f) is cluster free with clear terraces on the surface.

Figure 2

Synchrotron studies of Au-intercalated quasi-free-standing monolayer graphene. (a) LEEM image (field of $\mathrm{view}=25\mu \mathrm{m}$, electron energy 3.2 eV) at the boundary of Au-deposited and Au- diffused areas after Au intercalation ($t_{\mathrm{Au}}=4\AA$). The black patches in the image were originally monolayer graphene domains (by-product of buffer growth) turned into bilayer after intercalation. (b), (c) Micro-LEED patterns ($\mathrm{SV}=45$ eV, sampling area 1.5 μm) recorded from the marked areas shown in (a) with corresponding colors and shapes. Au-deposited (b) and diffused (c) regions that show only diffraction from monolayer graphene and no traces of $6\sqrt{3}\times 6\sqrt{3}R{30}^{\circ }$ structure (which is the fingerprint of the buffer layer). (d) LEEM IV curves from both Au-deposited and Au-diffused areas recorded from the marked areas shown in (a) with corresponding colors and shapes. A dip in the IV LEEM at 5.5 eV is a result of intercalation and consequent formation of free-standing graphene. (e) Micro-XPS spectra ($h\nu =150\mathrm{eV}$, sampling area 5 μm) of Au $4f$ at different areas show the presence of Au on Au-deposited (black) and diffused (blue) areas and a gradual disappearance of Au signal as the distance from the boundary increases; violet (0.5 mm), pink (2 mm) away, respectively. (f) micro-XPS spectra ($h\nu =350\mathrm{eV}$, sampling area 5 μm) of C $1s$ on Au-deposited (black) and diffused (blue) areas.

Figure 3

Electron transport studies on Au-intercalated quasi-free-standing monolayer graphene (a), (b). Schematics of top-gate structures with $h-\mathrm{BN}/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ gate dielectrics: (a) top contact (D1) and (b) edge contact (D2) configurations. (c) An optical micrograph of D1. The dashed areas indicate the sample covered by $h-\mathrm{BN}$. Measurement configuration: Current bias (1-2), $V_{\mathrm{XX}}$ (3-4), $V_{\mathrm{XY}}$ (5-3), gate voltage $V_{\mathrm{g}}$ (6-2). The scale bar is 5 µm. (d) Top-gate voltage dependence of the two devices D1 and D2 at $T=2\mathrm{K}$ [$I=100\mathrm{nA}$ (D1) and 1 µA (D2)] showing Dirac points in both devices. The red lines are the fitted curves to the equation, $R_{\mathrm{XX}}=\frac{1}{e{\mu }_C}\left(\frac{L}{W}\right)\frac{1}{\sqrt{n_g^2+n_0^2}}$. (e) Gate-voltage dependence of Hall carrier density of D1 (black, $I=100\mathrm{nA}$) and D2 (blue, $I=1\mu \mathrm{A}$) at $T=2\mathrm{K}$, showing transition from electrons to holes across the DP.

Figure 4

Two-charge-carrier analysis. (a) Asymmetric parts of $R_{\mathrm{XY}}\left(B\right)$ of D2 at $T=2\mathrm{K}$ at different top-gate voltages. Red lines are fits to our experimental data using a two-band model, with ${\rho }_{\mathrm{XY}}\left(B\right)=-\left(\frac{1}{e}\right)B\frac{(p{{\mu }_h}^2-n{{\mu }_e}^2)+{{\mu }_h}^2{{\mu }_e}^2(p-n)B^2}{{(p{\mu }_h+n{\mu }_e)}^2+{{\mu }_h}^2{{\mu }_e}^2{(p-n)}^2{B}^2}$, in which $n$ ($p$) and ${\mu }_e$ (${\mu }_h$) are the electron (hole) densities and mobilities, respectively. (b) Gate-voltage dependence of $n, p$, and Hall electron (hole) density $n_{\mathrm{H}}\left(p_{\mathrm{H}}\right)$ obtained from the single-carrier model. (c) Gate-voltage dependence of ${\mu }_e, {\mu }_h$, and Hall mobilities ${\mu }_{\mathrm{H}}$ obtained from the single-carrier model. (d) ${\mu }_e/{\mu }_h$ and n/p at different gate voltages.

Figure 5

Positive magnetoconductivity. Symmetrized longitudinal magnetoconductivity $\mathrm{\Delta }{\sigma }_{\mathrm{XX}}={\sigma }_{\mathrm{XX}}\left(B\right)–{\sigma }_{\mathrm{XX}}\left(0\right)$ of D1 at $T=2\mathrm{K}$ (current bias $I=100\mathrm{nA}$) at different gate voltages shows positive magnetoconductance. (See Fig. S7 for details of symmetrization [21].)