## 2 Experimental Details  
Buffer and MLG samples were prepared on the Si-face (0001) of on-axis SI and _n_-type doped 4H-SiC substrates (Cree, Inc., Durham, NC, USA) by high-temperature sublimation in Ar atmosphere [35] using the sublimation growth facilities at Linkoping University. The thickness and miscut angle of the SI and _n_-type doped wafers were 360 \(\upmu\)m and 0.09\({}^{\circ}\), and 340 \(\upmu\)m and 0.05\({}^{\circ}\), respectively. The substrates were chemical-mechanical polished (CMP) on the Si-face and optically polished on the C-face. Samples with different sizes of 10 mm \(\times\) 7 mm, 10 mm \(\times\) 10 mm or 15 mm \(\times\) 10 mm were fabricated. The substrates were first cleaned with acetone and ethanol, followed by the standard RCA1 and RCA2 cleaning procedures. Prior to transfer into the growth chamber, the substrates were treated with a hydrofluoric acid solution to remove the native oxide on the surface. A graphite crucible with a closed inner cavity has been designed with the Virtual Reactor software ([[http://www.str-soft.com/products/Virtual_Reactor/](http://www.str-soft.com/products/Virtual_Reactor/)]([http://www.str-soft.com/products/Virtual_Reactor/](http://www.str-soft.com/products/Virtual_Reactor/)) (accessed on 1 February 2021)) to provide uniform (within \(\sim\)0.5 \({}^{\circ}\)C) temperature distribution over 2-inch diameter wafer. The inner cavity design was optimized to minimize the lateral temperature variation resulting in a relatively complex shape. A sketch of the crucible is shown in Figure 1. A special graphite holder is used to position the SiC substrate in the crucible cavity. The crucible was placed into thermally-isolating porous graphite insulation and loaded into the growth chamber. The chamber is pumped down to vacuum level of \(\sim\)10\({}^{-6}\) mbar and the crucible was inductively heated. Initially, the temperature is ramped up in vacuum at a rate of \(\sim\)16 \({}^{\circ}\)C per min until the crucible temperature, measured with pyrometer on its surface, has reached 1300 \({}^{\circ}\)C. During this initial temperature ramp-up, Ar gas with pressure \(P_{\rm Ar}=850\) mbar was introduced into the chamber when the crucible temperature, \(T_{\rm Ar}\), was between 640 \({}^{\circ}\)C and 1300 \({}^{\circ}\)C. At the moment Ar was introduced the typical vacuum level was \(\sim\)5 \(\times\) 10\({}^{-5}\) mbar and it took about 5 min for the Ar pressure to reach \(P_{\rm Ar}=850\) mbar. During this time the temperature typically increased by about 100\({}^{\circ}\). Above 1300 \({}^{\circ}\)C, the temperature ramp-up continues at an increased rate of \(\sim\)70 \({}^{\circ}\)C per min until the targeted growth temperature, \(T_{\rm gr}\), is reached. The temperature is then kept constant for 0 min or 5 min, which we refer to as growth time, \(t_{\rm gr}\). During this final temperature ramp-up, \(P_{\rm Ar}\) slightly increased to \(P_{\rm Ar}=880\) mbar. Once the growth is finished, the inductive heating is switched off and the sample cools down passively at a rate of \(\sim\)65 \({}^{\circ}\)C per min. The MLG and buffer layer samples were grown at \(T_{\rm gr}=2000\) \({}^{\circ}\)C and \(T_{\rm gr}=1600\) \({}^{\circ}\)C, respectively. The growth conditions for all samples are listed in Table 1.  
Figure 1: A schematic of the crucible with the distribution of the temperature overplotted. Note that the SiC substrate is placed within a tightly closed inner cavity and it is completely surrounded by graphite. Micro-reflectance and micro-Raman scattering spectroscopy (\(\upmu\)-RS) maps were measured using the set-up described in Ref. [36]. A diode-pumped semiconductor laser with a wavelength of 532 nm (photon energy \(E_{\text{L}}\) = 2.33 eV) was used for the excitation. The full-width at half-maximum (FWHM) of the focused laser spot is \(\sim\)0.4 \(\upmu\)m using a 100\(\times\) objective. Typically, 30 \(\times\) 30 \(\upmu\)m\({}^{2}\) reflectance maps with step sizes of 0.3 \(\upmu\)m were measured at different locations of the sample.