###### Abstract  
In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H-SiC. We show that by introducing Ar at higher temperatures, \(T_{\rm Ar}\), one can shift the formation of the buffer layer to higher temperatures for both \(n\)-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher \(T_{\rm Ar}\) is proposed and discussed. Increased \(T_{\rm Ar}\) is also shown to reduce the \(sp^{3}\) hybridization content and defect densities in the buffer layer on \(n\)-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which \(T_{\rm Ar}\) plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different \(T_{\rm Ar}\) and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 \({}^{\circ}\)C, and which is found to be independent of \(T_{\rm Ar}\). Higher \(T_{\rm Ar}\) leads to higher free charge carrier mobility parameters in both intrinsically \(n\)-type and ambient \(p\)-type doped monolayer graphene. \(T_{\rm Ar}\) is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application. eptaxial graphene on SiC; buffer layer; quasi-free-standing graphene; monolayer graphene; high-temperature sublimation; terahertz optical Hall effect; free charge carrier properties +
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Accepted: 13 February 2021  
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large-size MLG domains can be obtained [1; 2; 15]. However, small inclusions of bi-layer graphene (BLG) are typically present, most often formed on the step edges (due to the small miscut of nominally on-axis wafers) or in association with surface defects [15; 16]. Hydrogen pre-treatment has widely been used to provide step-like surface morphology with atomically flat terraces and typical step height of 0.75 nm. Consequently, BLG always forms on the step edges of hydrogen etched SiC and giant step bunching is observed in the graphitization process [2; 17]. The two layers in the BLG are AB-stacked, hence possessing a parabolic band structure in contrast to the linearly dispersing bands (Dirac cones) at the K points of the first Brillouin zone of MLG. As a result, BLG inclusions may degrade significantly the transport properties of graphene on the Si-face of SiC and limit its applications [18; 19]. Several approaches dispensing with H etching have been explored to eliminate giant step bunching. For example, we have shown that high-temperature sublimation (\(T>\) 1800 \({}^{\circ}\)C) in Ar atmosphere in closed graphite crucible delivers wafer-scale MLG with negligible BLG inclusions and without hydrogen pre-treatment [1; 15; 20; 21; 22; 23; 24]. Other open-reactor strategies involve pre-conditioning of the SiC wafer by annealing in Ar and/or use of polymer layer, which enables smooth and uniform BLG-free MLG [4; 17; 25]. Formation of MLG on the Si-face SiC is preceded by consecutive surface reconstructions as the wafer is heated up [26]. The surface undergoes reconstruction from the Si-enriched (\(3\times 3\)) phase to the C-enriched (\(6\sqrt{3}\times 6\sqrt{3}\))-R30\({}^{\circ}\) phase.