###### Abstract
The latter phase is often called \"buffer layer\" or \"zero-layer\" graphene because it has the same honeycomb lattice structure as graphene. About 1/3 of the C atoms in this initial layer are covalently bound to the SiC surface and thus the buffer layer is devoid of the electronic properties of graphene [27]. Hydrogen intercalation may be employed to decouple the buffer layer from the substrate turning it into quasi-free-standing (QFS) MLG as the former covalent bonds are broken and the Si dangling bonds at a SiC surface are saturated with hydrogen [27; 28]. In UHV conditions the surface reconstructions up to the (\(6\sqrt{3}\times 6\sqrt{3}\))-R30\({}^{\circ}\) phase occur in the temperature range of 800-1200 \({}^{\circ}\)C [29]. Upon heating to a higher temperature, the buffer layer decouples from the SiC to form a graphene sheet and another buffer layer forms underneath. [PERSON] and [PERSON] [26] have shown that the temperature range within which the surface reconstructions occurs can be shifted up by as much as 200 \({}^{\circ}\)C in comparison to the case of an ultrahigh vacuum by increasing the Si background pressure to \(\sim\)8 \(\times\) 10\({}^{-7}\) Torr using disilane. Ar atmosphere efficiently enhances the Si pressure at the substrate surface since Ar atoms act as a diffusion barrier that limits the Si desorption from the surface. As a result, in Ar atmosphere graphene starts to form at higher temperatures as compared to growth in UHV. It has been shown that in an open Ar atmosphere with a pressure of \(\sim\)900 mbar graphene starts to form at temperatures above 1550 \({}^{\circ}\)C and the buffer layer forms between 1400 \({}^{\circ}\)C and 1550 \({}^{\circ}\)C [2; 4]. Forming the buffer layer at higher temperature has been theoretically suggested to be the key to grow high-quality graphene [30]. Experimentally it has also been shown that forming a smooth buffer layer at a temperature of \(T\simeq\) 1400 \({}^{\circ}\)C prevents giant step bunching and consequently it is possible to obtain a smooth surface covered with uniform MLG [17] even on wafers with a large miscut angle of 0.37\({}^{\circ}\)[4]. Introducing Ar at different temperatures during the graphitization process may provide an alternative pathway to influence the phase transition temperature between different surface reconstructions, and hence enable the growth of smooth MLG without the need of special pre-treatment. However, this approach has not been explored despite the intense investigation of buffer layer properties and optimization [4; 31; 32; 33; 34]. In this work, we report a comprehensive study of the effect of introducing Ar at different temperatures on the buffer layer formation and its properties in high-temperature sublimation for both \(n\)-type doped and high-purity semi-insulating (SI) 4H-SiC. The free charge carrier density and mobility parameters of the corresponding MLG and QFS-MLG are determined for different environmental conditions and discussed. A combined analysis of free charge carrier and structural properties provides insights into the graphitization processes in an enclosed environment and basis to design growth strategies depending on graphene targeted application.