## 3 Results and Discussion
### Buffer Layer Formation
Finally, the sample with \(T_{\rm Ar}\) = 1300 \({}^{\circ}\)C is severely charging and consists mostly of SiC substrate with the buffer layer just beginning to form, as revealed by \(\upmu\)-LEED (Figure 3d). In this case, the reflectance map (Figure 3h) appears quasi-uniform as the buffer layer nuclei are significantly smaller in comparison with the laser spot size. Based on the Raman scattering spectroscopy, reflectance mapping as well as \(\upmu\)-LEED results, we can conclude that with the increasing temperature at which Ar is introduced, the formation of the buffer layer is suppressed and shifted to a higher temperature. The same trend is also consistently observed when the buffer layers are formed on SI 4H-SiC substrates. Our investigations further indicate that the SiC substrate areas not covered by the buffer layer are oxidized. There are three possible scenarios: (i) oxidation occurs after the buffer layer formation due to ambient exposure when the samples are removed from the reactor; (ii) oxidation occurs after the buffer layer formation during cooling down and (iii) oxidation occurs during the annealing process. Scenario (ii) and (iii) necessitate residual oxygen in the growth system. Oxidation of buffer and MLG samples as a result of residual oxygen has been previously observed for both conventional and high-temperature sublimation growth [44,45]. It has been suggested that since the graphitization process does not take place in ultra-high vacuum (oxygen-free) conditions, oxygen may be present as a result of oxygen-containing adsorbates on graphite parts and/or inner walls of the reactor. Different growth strategies to obtain high-quality MLG and/or buffer layer (e.g., for QFS-MLG applications) should be employed depending on whether scenario (i), (ii) or (iii) transpires. Figure 3: (**a–d**) Microphe low-energy electron diffraction (\(\upmu\)-LEED) patterns taken at electron energy of 50 eV **(a,b)** and 40 eV **(c,d)** and (**e–h**) 30 \(\upmu\)m \(\times\) 30 \(\upmu\)m normalized reflectance maps of buffer layer samples with \(T_{\rm Ar}\) = 800 \({}^{\circ}\)C (**a,e**),\(T_{\rm Ar}\) = 900 \({}^{\circ}\)C **(b,f**), \(T_{\rm Ar}\) = 1150 \({}^{\circ}\)C (**c,g**) and \(T_{\rm Ar}\) = 1300 \({}^{\circ}\)C **(d,h)**. \(T_{\rm Ar}\) are indicated in the up right corner of the respective images. The difference in reflectance between bare substrate and fully covered with buffer layer is \(\sim\)0.01. In order to elucidate which of the above scenarios takes place, we will discuss in the following the structural evolution of SiC during the sublimation process in Ar atmosphere. Both SiC restructuring and surface reconstruction are expected to be affected by the presence of Ar, which influences the gas pressure at the crystal-vapor interface and the mean free path length. Ar atmosphere effectively enhances the Si pressure since it leads to a reduced Si evaporation rate. The stability of steps on the SiC surface at a given temperature is also affected by Si pressure since the surface Si is in equilibrium with the gas phase Si as well as the bulk SiC. At higher Si pressures higher temperatures are needed to initiate Si decomposition from the terrace [30] and decomposition proceeds rather from the step resulting in smoother surface morphology as compared to ultrahigh vacuum [1; 2]. Ar atmosphere also influences the mass transport of various species. Another consequence of the enhanced Si pressure in Ar is that Si depletion close to the SiC is slowed down and a higher temperature is needed to trigger and complete the buffer layer formation (consequently graphene formation). Indeed, it has been demonstrated that the phase transformation temperatures associated with different surface reconstructions on the Si-face SiC can be shifted by several hundred degrees Celsius by balancing the rate of Si evaporation with an external flux of Si [26].