## 3 Results and Discussion  
### Buffer Layer Formation  
Figure 2 shows \(\mu\)-Raman spectra of buffer layers on \(n\)-type 4H-SiC, for which the Ar gas was introduced at \(T_{\rm Ar}\) = 800 \({}^{\circ}\)C, 900 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C and 1300 \({}^{\circ}\)C, respectively (BL1-BL4, Table 1). The Raman spectra reveal features in the range of 1200-1700 cm\({}^{-1}\), typical for the buffer layer [31; 33; 42]. The band around 1330 cm\({}^{-1}\) appears to be on par in terms of intensity with the band around 1580 cm\({}^{-1}\) for all samples. It has been argued that the buffer layer Raman spectrum is not composed of discrete peaks but rather reflects the vibrational density of states [42]. The integrated intensity ratio of the D-band around 1330 cm\({}^{-1}\) (D\({}_{\rm BL}\)) and the G-band 1580 cm\({}^{-1}\) (G\({}_{\rm BL}\)) can be used to evaluate the content of \(sp^{3}\) hybridization [31] or discuss correlations associated with buffer structure in general [33]. We will come back to this question when comparing buffer layers grown on \(n\)-type and SI 4H-SiC. However, what is important to the present discussion is the observation that the intensities of the two bands scale down with increasing \(T_{\rm Ar}\) (see Figure 2). The analysis of the Raman scattering maps shows that the areas with lower reflectivity are associated with lower intensity of the D\({}_{\rm BL}\) and G\({}_{\rm BL}\) bands, which we attribute to lower buffer layer coverage. Furthermore, we estimate that the difference of the reflectance between regions that are barely covered with buffer and those with full coverage is \(\sim\)1%. Hence, reflectance mapping can also be employed to obtain information on the buffer layer uniformity on a large-scale. The \(\mu\)-LEED patterns and the respective 30 \(\mu\)m \(\times\) 30 \(\mu\)m reflectance maps of the buffer layer samples from Figure 2 are shown in Figure 3. The \(\mu\)-LEED pattern of the sample with \(T_{\rm Ar}\) = 800 \({}^{\circ}\)C (Figure 3a) displays well resolved (\(6\sqrt{3}\times 6\sqrt{3}\))-R30\({}^{\circ}\) surface reconstruction [11]. The uniform buffer layer coverage, for this sample, is corroborated by LEEM I(V) (not shown) and the reflectance map (Figure 3e), which reveals uniform intensity distribution. A clear buffer layer can also be inferred from the \(\mu\)-LEED pattern of the buffer layer with \(T_{\rm Ar}\) = 900 \({}^{\circ}\)C (Figure 3b), however, some charging on the surface is observed. The latter could be associated with oxidized SiC areas not covered by the buffer  
Figure 2: Normalized average \(\mu\)-Raman scattering spectra obtained over 3 \(\mu\)m \(\times\) 3 \(\mu\)m maps for the buffer layer samples with different \(T_{\rm Ar}\), indicated in the inset. layer. For \(T_{\rm Ar}\) = 1150 \({}^{\circ}\)C even stronger charging is observed in \(\upmu\)-LEED and patches of oxidized Si are identified by XPEEM (Figure 4). A mixture of the buffer layer and oxidized Si is inferred for this sample. Further confirmation of the suppressed buffer layer formation in the case of \(T_{\rm Ar}\) = 900 \({}^{\circ}\)C and \(T_{\rm Ar}\) = 1150 \({}^{\circ}\)C comes from the respective reflectance maps (Figure 3f,d), which show nonuniform intensity distribution with dark and bright areas. The size of the dark areas with suppressed buffer layer formation increases with increasing \(T_{\rm Ar}\) up to 1150 \({}^{\circ}\)C. This sample also shows the highest RMS of 0.7 nm as compared to 0.35 nm and 0.5 nm for the buffer layers with \(T_{\rm Ar}\) = 800 \({}^{\circ}\)C and \(T_{\rm Ar}\) = 900 \({}^{\circ}\)C, respectively. Note the resemblance between the XPEEM image (Figure 4) and the reflectance map (Figure 3g). We have previously reported a decrease in the relative reflectance of MLG with respect to the SiC substrate due to the presence of the oxide layer at the interface [43].