Figure compares the D
2-TPD spectra for Si (Sample A), (√3 × √3)R30°-reconstructed 3C-SiC(111)/Si(111) (Sample B), (6√3 × 6√3)R30°-reconstructed 3C-SiC(111)/Si(111) (Sample C), and (1 × 1) graphene/3C-SiC(111)/Si(111) (Sample D) surfaces [
7]. The spectrum for Si(111) (Sample A) has two peaks at ~660 and ~780 K, which are ascribable to the deuterium desorption from D
2-Si and D-Si, respectively [
9]. The dominance of the latter peak indicates that the surface of the Si(111) substrate is largely terminated with monodeuteride. In the spectrum of the (√3 × √3)R30°-reconstructed 3C-SiC(111)/Si(111) surface (Sample B), a new predominant peak appears at ~960 K. This peak is attributable to the deuterium desorption from D-Si backbonded with carbon atoms (D-Si(C)) [
10]. Concomitant with the appearance of the predominant peak of D-Si backbonded with carbon atoms, the peaks of D
2-Si (660 K) and D-Si (780 K) backbonded with silicon atoms decrease. The absence of peaks in the region ranging from 1,100 to 1,300 K indicates that no carbon atoms are present in the topmost layer. This is in good agreement with the Raman spectrum in Fig. , showing no graphene-related peaks. The surface of the (√3 × √3)R30°-reconstructed 3C-SiC(111)/Si(111) is thus proven to be Si-terminated.
On the (6√3 × 6√3)R30°-reconstructed 3C-SiC(111)/Si(111) (Sample C), the TPD spectrum consists mainly of D
x-Si(C). Here,
x = 1 stands for the peak at 960 K and
x = 2 for the peak at 850 K [
10]. The peaks due to D
x-Si (500–800 K) almost disappear. It has been demonstrated in a previous report that the adsorption of hydrogen atoms onto the (6√3 × 6√3)R30°-reconstructed SiC surface breaks the bondings between the reconstructed layer and the SiC interface, so that the surface carbon atoms in the layer are converted into graphene layer, and the SiC interface is terminated with hydrogen [
11,
12]. The main peak at 960 K is thus attributable to the deuterium desorption from D-terminated SiC interface that results from the breakage of the bonds between the (6√3 × 6√3)R30°-reconstructed layer and the underneath SiC surface. Small features appear in the temperature range of 1,100–1,400 K. They are due to desorption of deuterium atoms bonded on sp
2-bonded carbon atoms by D-exposure [
13,
14]. The appearance of these peaks is in agreement with the presence of the (6√3 × 6√3)R30°-reconstructed layer because the reconstructed layer mainly consists of sp
2-bonded atoms, and part of the carbon atoms are covalently bonded with the SiC surface [
15]. This D
2-TPD result is in good agreement with the Raman spectrum of the reconstructed layer. The appearance of the broad G band (~1,600 cm
−1) indicates the formation of the cluster of sp
2-bonded carbon atoms, i.e., nanographene. The appearance of the D band (~1,350 cm
−1) and the absence of the G′ band (~2,700 cm
−1), however, indicate a low degree of the crystallinity of the sp
2-bonded network of the carbon atoms. C1s core-level spectrum of the (6√3 × 6√3)R30°-reconstructed layer (Fig. ) actually supports the idea. In the core-level spectrum, the surface components S1 (~285.2 eV) and S2 (~284.0 eV) appear as well as the component due to the bulk of the SiC thin film (~283.0 eV). S1 and S2 are due to the carbon atoms connected with the SiC surface and the sp
2-bonded carbon atoms, respectively, in the (6√3 × 6√3)R30°-reconstructed layer [
15]. The intensity ratio of S1 and S2 peak is 2, as that of the spectrum of (6√3 × 6√3)R30°-reconstructed layer on 6H-SiC(0001). Thus, the 3C-SiC(111)/Si(111) surface is proven to become C rich and (6√3 × 6√3)R30° reconstructed by annealing the Si-terminated (√3 × √3)R30°-reconstructed surface [
15]. This change in the surface chemistry of 3C-SiC(111)/Si(111) is quite similar to that of 6H-SiC(0001) [
3,
4,
15].
The TPD spectrum of the graphene overlayer on 3C-SiC(111)/Si(111) (Sample D) drastically changes, as demonstrated in Fig. . Two distinct peaks around 1,100 K and 1,300 K are due to desorption of deuterium bonded with sp
2-bonded carbon atoms in the graphene overlayer, while the peaks due to deuterium desorption from D-Si of the SiC thin film and Si substrate (<1,000 K) are not observed. The Raman spectrum of the graphene overlayer (Fig. ) displays the G′ band, in addition to the G and the D bands. The presence of the G′ band indicates the well-ordered graphene overlayer because the appearance of G′ band is the consequence of the high degree of the crystallinity of the graphene layer [
16]. Further, the G′ band consists of multiple components, as can be seen from the line shape of the G′ band. This indicates that the graphene layers are Bernal stacked [
16]. This is supported by the C1s core-level spectrum (Fig. ). In the spectrum, the peak due to sp
2-bonded carbon atoms (~284.3 eV) is dominant, and the component due to the (6√3 × 6√3)R30°-reconstructed layer (~285.2 eV) is still observed. It can be thus concluded that the graphene overlayer grows on the (6√3 × 6√3)R30°-reconstructed layer as the epitaxy of graphene on 6H-SiC(0001). This can explain the disappearance of the peaks (D
x-Si(C)) that is related with the (6√3 × 6√3)R30°-reconstructed layer in the TPD spectrum of sample D, because graphene overlayer blocks adsorption or desorption of deuterium onto the surface.
1 and Maki Suemitsu1,2
