Insights into few-layer epitaxial graphene growth on $4H\text{-SiC}\left(000\overline{1}\right)$ substrates from STM studies

Epitaxial carbon was grown by heating $\left(000\overline{1}\right)$ silicon carbide (SiC) to high temperatures $\left(1450–1600°\text{C}\right)$ in vacuum. A continuous graphene surface layer was formed at temperatures above $1475°\text{C}$. X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were extensively used to characterize the quality of the few-layer graphene (FLG) surface. The XPS studies were useful in confirming the graphitic composition and measuring the thickness of the FLG samples. STM studies revealed a wide variety of nanometer-scale features that include sharp carbon-rich ridges, moiré superlattices, one-dimensional line defects, and grain boundaries. By imaging these features with atomic-scale resolution, considerable insight into the growth mechanisms of FLG on the carbon face of SiC is obtained.

Figure 1

(Color online) The $\text{C}1s$ XPS spectra, collected at $\theta =0°$, (a) from a reference HOPG substrate and (b) from a FLG sample grown at $1500°\text{C}$ on SiC. The similarity of the two XPS spectra indicates the presence of graphitic carbon on SiC. Both spectra were obtained at a photoemission angle of $0°$. A closer examination of the region between 288 and 295 eV from both samples provides evidence for shake-up satellites (insets).

Figure 2

(Color online) STM images of graphene grown at $1475°\text{C}$ shows the two growth morphologies. In (a), an $800\times 800{\text{nm}}^2$ region, the rough graphene region is on the left while the smooth graphene region is on the right. In (b), a $50\times 50{\text{nm}}^2$ image showing the detailed morphology of the rough graphene. In (c), a $2\times 2{\text{nm}}^2$ atomic-resolution image of the rough region in (b) reveals a hexagonal lattice. The lattice parameter $a=0.245\text{nm}$ of the hexagonal lattice indicates the presence of graphene. The original STM scan was processed with wavelet analysis (Ref. 49). Scan parameters are $I_{\text{set}}=5.0\text{nA}$ and $V_{\text{bias}}=72\text{mV}$ for (a) and $I_{\text{set}}=3.0\text{nA}$ and $V_{\text{bias}}=100\text{mV}$ for (b) and (c).

Figure 3

(Color online) STM images of a region from the graphene grown at $1500°\text{C}$. In (a), a $500\times 500{\text{nm}}^2$ image showing the presence of a grain boundary in the upper half of the image. In (b), a $150\times 150{\text{nm}}^2$ image showing the parallel 1D features within the grain boundary. Scan parameters are $I_{\text{set}}=2.0\text{nA}$ and $V_{\text{bias}}=100\text{mV}$ for (a) and (b).

Figure 4

(Color online) (a) An STM image of a $200\times 200{\text{nm}}^2$ region shows a 1D superlattice. (b) A line profile along the line $ABCDE$, which follows the profile of the 1D superlattice. Scan parameters are $I_{\text{set}}=2.0\text{nA}$ and $V_{\text{bias}}=300\text{mV}$.

Figure 5

(Color online) (a) An STM image of a $5\times 5\mu {\text{m}}^2$ region shows fine ridges, 5–10 nm high, crossing the sample. (c) A $1.5\times 1.5\mu {\text{m}}^2$ region located in the center of (a) reveals a superlattice, the boundaries of which are indicated by the dashed lines. This superlattice is bounded by the ridge on the left-hand side and is discussed further in Fig. 7. A profile (b) across (c) shows a 6-nm-high and 40-nm-wide ridge. Steps in the underlying SiC substrate are indicated by vertical black bars (1.1 nm tall). Scan parameters are $I_{\text{set}}=5.7\text{nA}$ and $V_{\text{bias}}=72\text{mV}$ for (a) and $I_{\text{set}}=1.0\text{nA}$ and $V_{\text{bias}}=300\text{mV}$ for (c).

Figure 6

(Color online) (a) An STM image of a $20\times 20{\text{nm}}^2$ region shows a moiré superlattice. (b) The 2D-FFT of the moiré superlattice shows the hexagonal superlattice $\left(\mathcal{D}=4.44\pm 0.31\text{nm}\right)$. Image (b) is a 2D-FFT of a $60\times 60{\text{nm}}^2$ region; the larger image is necessary for a high-resolution FFT. (c) A $6\times 6{\text{nm}}^2$ zoom of (a) shows the hexagonal lattice of the top graphene layer. (d) From the 2D-FFT of (a), the atomic lattice is found to be $0.23\pm 0.09\text{nm}$, close to the accepted value of 0.246 nm. The scale bars in (b) and (d) are the length of the $k$ vector where $1/k$ is the lattice periodicity. Scan parameters are $I_{\text{set}}=5.7\text{nA}$ and $V_{\text{bias}}=72\text{mV}$.

Figure 7

(Color online) (a) An STM image of a $600\times 600{\text{nm}}^2$ region shows a moiré superlattice continuing across two step edges (both 1.1 nm high) in the SiC substrate. The dashed box in (a) indicates the $150\times 150{\text{nm}}^2$ region in (b), which was scanned at a higher resolution. The superlattice is clearly visible in (c), a $50\times 50{\text{nm}}^2$ magnified region of the center of (b). A profile (d) drawn across the step edge in (b) illustrates that no apparent row of atoms is missing at the step edge. To enhance the apparent periodicity, the height of the step edge was subtracted from this profile. Scan parameters are $I_{\text{set}}=1.0\text{nA}$ and $V_{\text{bias}}=300\text{mV}$ for (a) and $I_{\text{set}}=1.5\text{nA}$ and $V_{\text{bias}}=50\text{mV}$ for (b) and (c).

Figure 8

(Color online) (a) An STM image $\left(1000\times 700{\text{nm}}^2\right)$ shows the extent of the moiré region with a periodicity of $\mathcal{D}=12.7\pm 2.1\text{nm}$, as indicated by the solid line. The exceptionally jagged edge of the moiré region is illustrated by the $350\times 250{\text{nm}}^2$ inset. Scan parameters are $I_{\text{set}}=1.0\text{nA}$ and $V_{\text{bias}}=300\text{mV}$ for (a) and $I_{\text{set}}=2.0\text{nA}$ and $V_{\text{bias}}=100\text{mV}$ for (b).

Figure 9

(Color online) (a) An STM image, $500\times 500{\text{nm}}^2$, shows a 1-nm-high line defect that runs parallel for $\sim 200\text{nm}$ of its length to a 0.65 nm step edge. Carbon ripples emanate from the line defect and are found to cross over the step edge. In (b), an STM image of a $300\times 300{\text{nm}}^2$ zoom of the rippled region of (a) reveals that the superlattice is coexistent with the ripples. In (c), the hexagonal periodicity of the superlattice is confirmed by a 2D-FFT of (b). The superlattice is a superposition of two moiré patterns, with periodicities of $\mathcal{D}=9.3\pm 1.2\text{nm}$ (bright inner spots) and $\mathcal{D}=6.4\pm 0.7\text{nm}$ (dim outer spots). In (d), a profile of the ripples along the straight blue line in (b). Scan parameters are $I_{\text{set}}=1.0\text{nA}$ and $V_{\text{bias}}=500\text{mV}$.