(PDF) Critical View on Buffer Layer Formation and Monolayer Graphene Properties in High-Temperature Sublimation

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Article Critical view on buffer layer formation and monolayer graphene properties in high-temperature sublimation V. Stanishev1 , N. Armakavicius1,2 , C. Bouhafs1 , C. Coletti3 , P. Kühne1,2 , I.G. Ivanov4 , A.A. Zakharov5 , R. Yakimova4 and V. Darakcheiva1,2 1 Terahertz Materials Analysis Center, Department of Physics, Chemistry and Biology, IFM, Linköping University, 581 83 Linköping, Sweden 2 Center for III-Nitride technology C3NiT - Janzén, Department of Physics, Chemistry and Biology, IFM, Linköping University, 581 83 Linköping, Sweden 3 Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza S. Silvestro, 12, 56127 Pisa PI, Italy 4 Department of Physics, Chemistry and Biology, IFM, Linköping University, 581 83 Linköping, Sweden 5 MaxLab, Lund University, S-22100 Lund, Sweden * Correspondence:

Version December 31, 2020 submitted to Appl. Sci. 1 Abstract: In this work we have critically reviewed the processes in high-temperature sublimation 2 growth of graphene in Ar atmosphere using enclosed graphite crucible. Special focus is put on buffer 3 layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding 4 monolayer garphene on 4H-SiC. We show that by introducing Ar at lower temperatures, TAr , one can 5 shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating 6 substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr 7 is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content 8 and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating 9 substrates results in ordered buffer layer with significantly improved structural properties, for which 10 TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene 11 and quasi-freestanding monolayer graphene with different TAr and different environmental treatment 12 conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors 13 on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 14 2000◦ C, and which is found to be independent of TAr . Higher TAr leads to higher free charge carrier 15 mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr 16 is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer 17 graphene. These findings are discussed in view of interface and buffer layer properties in order to 18 construct a comprehensive picture of high-temperature sublimation growth and provide guidance 19 for growth parameters optimization depending on the targeted graphene application. 20 Keywords: Epitaxial graphene on SiC; buffer layer; quasi-free standing graphene; monolayer 21 graphene; high-temperature sublimation; terahertz Optical Hall effect; free charge carrier properties 22 1. Introduction 23 Epitaxial graphene on SiC substrates[1–4] holds promise for myriad of future electronic and 24 sensing applications.[5–9] In particular, on the Si-face of SiC, the number of graphene layers can 25 be well controlled and uniform monolayer graphene (MLG) can be obtained. Epitaxial graphene 26 grown in ultra-high vacuum (UHV) on Si-face SiC consists of small domains with a typical size of Submitted to Appl. Sci., pages 1 – 16 www.mdpi.com/journal/applsci © 2021 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 2 of 16 27 200–500 nm.[10–15] In such instances the surface roughens during the graphitization even when 28 growth starts from atomically-flat surface. If the graphitization is performed in argon (Ar) atmosphere, 29 smoother surface and large-size MLG domains can be obtained.[1,2,15] However, small inclusions 30 of bi-layer graphene (BLG) are typically present, most often formed on the step edges (due to the 31 small miscut of nominally on-axis wafers) or in association with surface defects.[15,16] Hydrogen 32 pre-treatment has widely been used to provide atomically flat terraces and which results in typical 33 step height of 0.75 nm. Consequently, BLG is always forms on the step edges of hydrogen etched SiC 34 and giant step bunching is observed in the graphitization process.[2,17] The two layers in the BLG are 35 AB-stacked, hence possessing a parabolic band structure in contrast to the linearly dispersing bands 36 (Dirac cones) at the graphene K point of MLG. As a result, BLG inclusions may degrade significantly 37 the transport properties of graphene on the Si-face of SiC and limit its applications. 38 Several approaches dispensing with H etching have been explored to eliminate giant step 39 bunching. For example, we have shown that high-temperature sublimation (T > 1800◦ ) in Ar 40 atmosphere in enclosed graphite crucible delivers wafer-scale MLG with negligible BLG inclusions 41 and without hydrogen pre-treatment.[1,15,18? ? –20] Other open-reactor strategies involve 42 pre-conditioning of the SiC wafer by annealing in Ar and/or use of polymer layer, which enables 43 smooth and uniform BLG-free MLG.[4,17,21] 44 Formation of MLG on the Si-face SiC is preceded by consecutive surface reconstructions as the 45 wafer is heated up.[22] √ √ surface undergoes reconstruction from the Si-enriched (3 × 3) phase to The 46 the C-enriched (6 3 × 6 3)-R30◦ phase. The latter phase is often called "buffer layer" or "zero-layer" 47 graphene because it has the same honeycomb lattice structure as graphene. About 1/3 of the C atoms 48 in this initial layer are covalently bound to the SiC surface and thus the buffer layer is devoid of the 49 electronic properties of graphene.[23] Hydrogen intercalation may be employed to decouple the buffer 50 layer from the substrate turning it into quasi-free-standing MLG as the formerly covalent bonds are 51 broken and all Si atoms of the SiC are saturated with hydrogen.[23,24] √ √ 52 In UHV conditions the surface reconstructions up to the (6 3 × 6 3)-R30◦ phase occur in the 53 temperature range of 800 − 1200◦ C. Upon heating to higher temperature the buffer layer decouples 54 from the SiC to form a graphene sheet and another buffer layer forms underneath. Tropm and 55 Hannon[22] have shown that the temperature range within which the surface reconstructions occur 56 can be shifted up by as much as 200◦ C in comparison to the case of ultrahigh vacuum by increasing the 57 Si background pressure to ∼ 8 × 10−7 Torr using disilane. Ar atmosphere efficiently enhances the Si 58 pressure on the substrate surface since Ar atoms act as a diffusion barrier that limits the Si desorption 59 from the surface. As a result, in Ar atmosphere graphene starts to form at higher temperature as 60 compared to growth in UHV. It has been shown that in an open Ar atmosphere with a pressure of 61 ∼ 900 mbar graphene starts to form at temperatures above 1550◦ C and the buffer layer forms between 62 1400◦ C and 1550◦ C[2,4]. 63 Forming buffer layer at higher temperature has been theoretically suggested to be the key to grow 64 high quality graphene [25]. Experimentally it has also been shown that forming a smooth buffer layer 65 at temperature of T ' 1400◦ prevents giant step bunching and consequently is it possible to obtain a 66 smooth surface covered with uniform MLG[17] even on wafers with large miscut angle of 0.37◦ [4]. 67 Introducing Ar at different temperatures during the graphitization process may provide an alternative 68 pathway to influence the phase transition temperature between different surface reconstructions, 69 and hence enable growth of smooth MLG without the need of special pretreatment. However, this 70 approach has not been explored despite the intense investigation of buffer layer properties and 71 optimization.[4,26–28] 72 In this work, we report a comprehensive study on the effect of introducing Ar at different 73 temperatures on the buffer layer formation and properties in high-temperature sublimation for 74 both n-type and semi-insulating 4H-SiC. The free charge carrier density and mobility parameters of 75 the corresponding MLG and quasi-free standing MLG are determined for different environmental 76 treatment conditions and discussed. A combined analysis of free charge carrier and structural Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 3 of 16 Figure 1. A schematic of the crucible with the distribution of the temperature overplotted. 77 properties provides insights into the graphitization processes in enclosed environment and basis 78 to design growth strategies depending on graphene targeted application. 79 2. Experimental details 80 Buffer and monolayer graphene samples were prepared by high-temperature sublimation of 81 Si-face (0001) nominally on-axis 4H-SiC wafers in Ar atmosphere[29] using the sublimation growth 82 facilities at Linköping University. Semi-insulating (SI) and n-type doped 4H-SiC from Cree, Inc. were 83 used. The wafers were chemical mechanical polished (CMP) on the Si-face and optically polished on the 84 C-face. Samples with different sizes, depending on the analysis of 10 mm ×7 mm, 10 mm ×10 mm or 85 15 mm ×10 mm were fabricated. The substrates were first cleaned with acetone and ethanol, followed 86 by the standard RCA1 and RCA2 cleaning procedures. Prior to the transfer in the growth chamber the 87 substrates were treated with hydrofluoric acid solution to remove the native oxide on the surface. 88 A closed graphite crucible has been designed with the Virtual Reactor software1 to provide 89 uniform (within ∼ 0.5◦ C) temperature distribution over 2-inch diameter wafer. A sketch of the 90 crucible is shown in Fig. 1. The inner cavity design was optimized to minimize the lateral temperature 91 variation resulting in a relatively complex shape. A special graphite holder for the SiC wafer is 92 positioned in the middle of the crucible. The crucible was placed into thermally-isolating porous 93 graphite foam and loaded into the furnace. The chamber is pumped down to vacuum level ∼ 94 10−6 mbar. Initially the temperature is ramped up in vacuum at a rate of ∼ 16◦ C per min until the 95 crucible temperature, measured with pyrometer on its surface, has reached 1300◦ C. During this initial 96 temperature ramp-up, Ar with pressure PAr = 850 mbar was quickly introduced into the chamber 97 when the crucible temperature was between 640◦ C and 1300◦ C. At the moment Ar is introduced the 98 typical vacuum level is ∼ 5 × 10−5 mbar. Above 1300◦ C, the temperature ramp-up continues at an 99 increased rate of ∼ 70◦ C per min until the growth temperature of Tgr = 2000◦ C is reached. Growth 100 times vary between 0 s and 5 min. At the growth temperature PAr slightly increases to PAr = 880 mbar. 101 Once the growth is finished, the RF heading is switched off and the sample cools down passively at a 102 rate of ∼ 65◦ C per min. 103 Micro-reflectance and micro-Raman scattering spectroscopy (µ-RS) maps were measured using 104 the set-up described in Ref. [30]. A diode-pumped semiconductor laser with a wavelength of 532 nm 105 (photon energy EL =2.33 eV) was used for the excitation. The full-width at half-maximum (FWHM) 106 of the focused laser spot is ∼ 0.4 µm using a 100× objective. Typically, 30×30 µm2 reflectance maps 107 with a step sizes of 0.3 µm were measured on different locations of the sample. Raman maps were also 1 http://www.str-soft.com/products/Virtual_Reactor/ Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 4 of 16 0.8 TAr= 800° C TAr= 900° C TAr=1150° C 0.6 TAr=1300° C Normalized intensity 0.4 0.2 0.0 1000 1200 1400 1600 1800 Raman shift [cm−1] Figure 2. Normalized average µ-Raman scattering spectra obtained over 3 µm×3 µm maps for the buffer layer samples with different TAr , indicated in the inset. 108 measured and a Raman spectrum of a bare 4H-SiC substrate was subtracted to obtain clean Raman 109 spectra of graphene and buffer layers. 110 The surface morphology of the graphene and buffer layers was characterized by tapping mode 111 atomic force microscopy (AFM) (Veeco Dimension 3100). Micro low-energy electron diffraction 112 (µ-LEED), low energy electron microscopy (LEEM), X-ray photoelectron emission microscopy (XPEEM) 113 and X-ray photoelectron microspectroscopy (micro-XPS) were used to investigate the structural 114 properties and chemical composition of the buffer layer samples. The experiments were performed 115 using the ELMITEC-LEEM III instrument at the I311 beamline of the MAX-Lab synchrotron radiation 116 facility in Lund, Sweden. 117 Contactless Terahertz (THz) cavity-enhanced (CE) optical Hall effect (OHE) measurements were 118 performed for the determination of graphene free charge carrier properties using the custom-built 119 ellipsometry instrumentation at the THz Matreials Aanalysis Center [31]. The OHE describes the 120 magnetic field induced optical birefringence generated by free charge carriers under the influence of the 121 Lorentz force and can be measured by Mueller matrix ellipsometry [32]. The CE measurements were 122 performed at room temperature by placing the sample on either of the two sides of a permanent magnet 123 with field strength of B = 0.548 T and an external cavity of ∼ 100 µm [33]. In − situ environmental 124 control gas cell was employed to measure in different gases and relative humidity (RH) parameters 125 [31,34]. Data collected at magnetic fields B = +0.548 T and B = −0.548 T and their differences 126 were simultaneously analyzed using a stratified optical model with parameterized model dielectric 127 tensor (MDF) following the methodology described in Ref. [32]. The model consists of a perfect 128 mirror (magnet), air gap, 4H-SiC substrate and a graphene layer. The dielectric function of 4H-SiC in 129 THz range was first determined by analyzing measurements of a bare substrate. The substrate MDF 130 parameters were then kept fixed during the analysis of the graphene samples. The MDF of graphene 131 is described by Drude contribution in the presence of magnetic field [31,32]. The free charge carrier 132 mobility µ and sheet density Ns of graphene were determined by non-linear least-squares fit of the ∗ 133 q Müller matrix elements to the measured ones. The effective mass m was parametrized as calculated 134 m∗ = (h2 Ns )/(4πv2F ) following Ref. [35], where vF = 1.02 × 106 m s−1 is the Fermi velocity and Ns 135 is the carrier sheet density. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 5 of 16 Figure 3. (a)-(d) µ-LEED patterns taken at electron energy of 50 eV (a-b) and 40 eV (c-d) and (e)-(h) 30 µm×30 µm reflectivity maps of buffer layer samples with TAr = 800◦ C (a,e),TAr = 900◦ C (b,f), TAr = 1150◦ C (c,g) and TAr = 1300◦ C (d,h). TAr are indicated in the up right corner of the respective images. 136 3. Results and discussion 137 3.1. Buffer layer formation 138 Figure 2 shows µ-Raman spectra of buffer layers on n-type SiC, for which the Ar was introduced 139 at TAr = 800◦ C, 900◦ C, 1150◦ C and 1300◦ C, respectively. To prevent the formation of MLG the wafers 140 were heated only to Tgr = 1600◦ C (zero growth time). The Raman spectra reveal features in the 141 1200-1700 cm−1 range, typical for the buffer layer [26,28,36]. The band around 1330 cm−1 appears to 142 be on par in terms of intensity with the band around 1580 cm−1 for all samples. It has been argued that 143 the buffer layer Raman spectrum is not composed of discrete peaks but rather reflects the vibrational 144 density of states.[36] The integrated intensity ratio of the two bands around 1330 cm−1 (DB ) and 145 1585 cm−1 (GB ) can be used to evaluate the content of sp3 hybridization [26] or discuss correlations 146 associated with buffer structure in general [28]. We will comeback to this question when comparing 147 buffer layers grown on n-type and SI SiC. However, what is important to the present discussion is the 148 observation that the intensities of the two bands scale down with increasing TAr (See Fig.2). As all 149 spectra are normalized to the SiC substrate, this decrease in intensity could be associated with reduced 150 buffer layer coverage. A direct correlation between reflectivity and Raman scattering mapping shows 151 that lower intensity areas in the reflictivity maps are associate with lower buffer layer coverage. Hence, 152 reflectivity mapping can also be employed to obtain information on the buffer layer uniformity on a 153 large-scale. 154 The µ-LEED patterns and the respective 30µm by 30µm reflectivity maps of the buffer layer 155 samples from Fig.2√ are shown √ in Fig.3. The µ-LEED pattern of the sample with TAr = 800◦ C displays 156 well resolved (6 3 × 6 3) surface reconstruction (Fig.3 (a)). The uniform buffer layer coverage is 157 corroborated by LEEM I(V) (not shown) and the reflectivity map (Fig.3 (e)), which reveals uniform 158 intensity distribution. A clear buffer layer can also be inferred from the µ-LEED pattern of the buffer 159 layer with TAr = 900◦ C (Fig.3 (b)), however, some charging on the surface is observed. The latter could 160 be associated with oxidized SiC areas not covered by buffer layer. For TAr = 1150◦ C even stronger 161 charging is observed in µ-LEED and patches of oxidized Si are identified by XPEEM (Fig.4). A mixture 162 of buffer layer and oxidized Si is inferred for this sample. Further confirmation of the suppressed buffer 163 layer formation in the case of TAr = 900◦ C and TAr = 1150◦ C comes from the respective reflectivity maps Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 6 of 16 Figure 4. Si 2p oxide XPEEM image taken at photon energy of 133 eV and electron energy of 26 eV with 40-µm field-of-view for the buffer layer sample with TAr = 1150◦ C. The bright areas correspond to higher content of SiOx but even the dark areas of the image have some oxide component. 164 (Fig.3 (f,d)), which show nonuniform intensity distribution with dark and bright areas. The size of the 165 dark areas with suppressed buffer layer formation increases with increasing TAr up to 1150◦ C. This 166 sample also shows the highest RMS of 0.7 nm as compared to 0.35 nm and 0.5 nm for the buffer layers 167 with TAr = 800◦ C and TAr = 900◦ C, respectively. Note the resemblance between the XPEEM image 168 (Fig.4) and the reflectivity map (Fig.3 (d)). It is well known that native SiOx may act as anti-reflective 169 coating for a particular thickness and we have previously reported a decrease of the relative reflectivity 170 of MLG with respect to the SiC substrate due to presence of oxide layer at the interface [37]. Finally, 171 the sample with TAr = 1300◦ C is severely charging and consists mostly of SiC substrate with the buffer 172 layer just beginning to form, as revealed by µ-LEED (Fig.3 (d)). In this case, the reflectivity map (Fig.3 173 (h)) appears uniform as the buffer layer nucleii are significantly smaller in comparison with the laser 174 spot diameter. 175 Based on the Raman scattering spectroscopy, reflectivity mapping as well as µ-LEED results we 176 can conclude that with increasing the temperature, at which Ar is introduced, the formation of the 177 buffer layer is suppressed. The same trend is also consistently observed when the buffer layers are 178 formed on SI SiC substrates. Our investigations further indicate that the areas of the SiC substrate areas 179 not covered by buffer layer are oxidized. There are three possible scenarios: i) oxidation occurs after 180 the buffer layer formation due to ambient exposure when the samples are removed from the reactor; ii) 181 oxidation occurs after the buffer layer formation during cooling down and iii) oxidation occurs during 182 the annealing process. Scenario ii) and iii) necessitate residual oxygen in the growth system. Oxidation 183 of buffer and monolayer graphene samples as a result of residual oxygen has been previously observed 184 for both conventional and high-temperature sublimation growth [38,39]. It has been suggested that 185 since the graphitization process does not take place in ultra-high vacuum (oxygen-free) conditions, 186 oxygen may be present as a result of oxygen-containing adsorbates on graphite parts and/or inner 187 walls of the reactor. Different growth strategies to obtain high quality MLG and/or buffer layer (e.g., 188 for quasi-freestanding MLG applications) should be employed depending on whether scenario i), ii) or 189 iii) transpires. 190 In order to elucidate which of the above scenarios takes place we will discuss in the following the 191 structural evolution of SiC during the sublimation process in Ar atmosphere. Both SiC restructuring 192 and surface reconstruction are expected to be affected by the presence of Ar which influences the 193 effective gas pressure at the crystal-vapor interface and the mean free path length. Ar atmosphere 194 effectively enhances the effective Si pressure since it leads to reduced Si evaporation rate. The stability 195 of a step at a given temperature is also affected by Si pressure since the surface Si is in equillibrium 196 with the gas phase Si as well as the bulk SiC. At higher Si pressures higher temperatures are needed 197 to initiate Si decomposition from the terrace[25] and decomposition proceeds rather from the step 198 resulting in smother surface morphology as compared to ultrahigh vacuum.[2,40] Ar atmosphere also 199 influences the mass transport of various species and thus affects the etching rate, which may depend Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 7 of 16 200 on the susceptor material, confinement scheme, reactor design etc.[39,41] Another consequence of 201 the enhanced effective Si pressure in Ar is that Si depletion close to the SiC is slowed down and 202 a higher temperature is needed to trigger and complete the buffer layer formation (consequently 203 graphene formation) in Ar atmosphere. Indeed, it has been demonstrated that the phase transformation 204 temperatures associated with different surface reconstructions on the Si-face SiC can be shifted by 205 several hundred degrees Celsius by balancing the rate of Si evaporation with an external flux of Si 206 [22]. Therefore, with decreasing TAr in our experiments the effective Si pressure is expected to be 207 increased and thus the buffer layer formation is expected to be shifted to higher temperatures. In 208 other words, for TAr = 1300 ◦ C when the reconstruction occurs in vacuum one would expect a better 209 developed buffer layer compared to TAr = 800 ◦ C when the reconstruction occurs in Ar. Surprisingly, 210 we find the opposite trend from the Raman scattering spectroscopy, reflectivity mapping and µ-LEED 211 results. These findings are not compatible with scenarios i) and ii) in which oxidation of uncovered 212 areas occurs after buffer layer formation. A potential explanation for the observed suppression of 213 buffer layer formation at higher TAr is provided by scenario iii) in which the observed oxidation occurs 214 during the annealing process. 215 It has been shown that intermediate SiOx on the Si-face of SiC is stable up to a temperature of 216 1200◦ C and it is difficult to be fully eliminated even at 1400◦ C. [42] Thus, if oxidation occurs during 217 annealing and Ar is introduced at temperatures higher than 1200◦ C the oxide layer will prevent the buffer layer formation. As the oxide layer starts to gradually be removed above ◦ C Ar 218 √ 1200-1400 √ 219 effectively enhances the Si gas pressure and suppress the phase transformation to (6 3 × 6 3) surface 220 reconstruction. As a results after heating up to 1600 ◦ C the sample with TAr = 1300◦ C shows only initial 221 stage of buffer layer and is mostly uncovered SiC (Fig.3(d)). At TAr lower than 1200◦ C, Ar reduces the 222 mean free path of oxygen suppressing oxide formation and allowing complete (partial) buffer layer 223 formation for TAr = 800 ◦ C (900◦ C - 1150◦ C.) We note that no charging or any indication of oxidation 224 is observed in the buffer layer sample with TAr = 800 ◦ C, which may be understood in view of the 225 reduced mean free path at lower temperatures. 226 Scenario iii) has several important implications for the growth strategies to obtain high-quality 227 graphene by high-temperature sublimation. As the buffer layer becomes the first graphene layer upon 228 annealing, forming the buffer layer and consequently graphene at higher temperatures should be 229 favorable in terms of surface roughness and uniform restructuring as they affect positively free charge 230 carrier mobility. At the same time, one can argue that if the buffer layer forms at lower temperatures 231 it can be conditioned during the annealing process until the temperature of graphene formation is 232 reached, reducing density of defects such as vacancies or/and sp3 -defects. Another interesting question 233 is to compare the properties of quasi-freestanding MLG of buffer layer obtained at different TAr and 234 understand which mechanism has a decisive role. To address these questions we have investigated 235 the free charge carrier properties of MLG and quasi-free standing (QFS) MLG samples for which the 236 Ar was introduced at different TAr . The MLG and QFS-MLG were grown on SI substrates in order 237 to reliably measure the free charge carrier properties. Interestingly, a difference between the Raman 238 scattering spectra grown at the same conditions on n-type and SI SiC is observed. 239 3.2. Comparison between buffer layers grown on n-type and SI 4H-SiC 240 A comparison of the Raman spectra of buffer layers on n-type and SI SiC obtained at TAr = 800◦ C 241 is presented in Fig.5 (a). The Raman spectrum of the buffer layer grown on n-type substrate displays 242 DB (around 1335 cm−1 ) and GB (around 1585 cm−1 ) bands with similar intensities. The latter is slightly 243 asymmetric due to a band at around 1530 cm−1 (see also Fig.2). Such Raman spectrum is typical 244 for carbon-rich graphitic clusters bonded to SiC [23] and can be associated with a large degree of 245 disorder [43]. On the other hand, the buffer layer grown at the same conditions but on SI substrates 246 exhibits blue shift of the DB and the GB bands, and the band at around 1530 cm−1 becomes more 247 pronounced. These are typical vibrational characteristics of well connected buffer layer domains [4]. 248 Further information about disorder and the content of sp3 hybridization can be obtained from the Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 8 of 16 1.2 buffer n−type 4H−SiC (a) MLG n−type 4H−SiC (b) 1.0 buffer SI 4H−SiC MLG SI 4H−SiC relative intensity 0.8 0.6 0.4 0.2 0.0 1000 1200 1400 1600 1000 1200 1400 1600 Raman shift [cm−1] Raman shift [cm−1] Figure 5. A comparison between the average µ-Raman scattering spectra for buffer layer samples with TAr = 800◦ C: (a) on n-type and SI 4H-SiC, and (b) the buffer layer features in fully-formed MLG graphene at TGr = 2000◦ C for 0 s on n-type and SI 4H-SiC. 249 histograms of the GB band position (Fig.6 (a) and (c)) and the ratios of the DB and GB bands areas, 250 ADB /AGB , (Fig.6 (b) and (d)). The GB band energy changes from 1583 cm−1 to 1606 cm−1 and the 251 ADB /AGB changes from 2.0 to 1.3 comparing the buffer layers grown on n-type and SI substrates, 252 respectively. A similar trend is also found for the case of TAr = 1300◦ C (Fig.6 (e) and (g)). According 253 to the amorhization trajectory presented for nano-crystalline graphite in Ref.[44], these changes can 254 be associated with significant reduction of the sp3 hybridization content for the case of the SI SiC. 255 The ADB /AGB is further related to the degree of disorder introduced by the presence of sp3 defects, 256 which is proportional to the average distance between the defects [43]. Accordingly, the density of 257 defects in the buffer layer grown on the SI substrate is 46% lower and the crystallite size is 35% larger. 258 Again, very similar trend is found for the buffer layer with TAr = 1300◦ C (Fig.6 (f) and (h)). The 259 observed differences between the two types of substrates could be understood considering the fact 260 that electron concentration generally enhances thermal conductivity. Hence, temperature variations 261 should occur slower for the SI substrates during the heating up bringing the graphitization process 262 closer to thermodynamic equillibrium and allowing the formation of well connected buffer layer with 263 lower density of defects. It is interesting to note that the vibrational features of the buffer layer formed 264 underneath monolayer graphene, grown at TMLG = 2000◦ C for 0 s, (Fig.5 (b)) become even finer and 265 bear closer resemblance with the buffer vibrational density of states [36]. Note that the spectral features 266 are identical for the buffer layers on conductive and SI substrates. This further highlights the important 267 roles of the carbon-rich environment and the high temperature for the formation of high quality buffer 268 layer. 269 Comparing the buffer layers grown on n-type substrates and different TAr , a moderate blue-shift 270 of the G-like band position for TAr = 1300◦ C to 1593 cm−1 with respect to the sample with TAr = 800◦ C 271 (1583 cm−1 ) can be seen (Fig.5 (a) and (e)). This can be explained by a reduced sp3 hybridization 272 content as expected due to the higher temperature at which the reconstruction occurs. At the same 273 time, the ADB /AGB increases from 2.0 to 2.7 (Fig.5 (b) and (f)), which could be related to a reduced 274 crystallite size with 30%. This finding is in accordance with out µ-LEED results showing that the 275 buffer layer with TAr = 1300◦ C has just begun to form. We now turn our attention to the buffer layers 276 grown with different TAr on SI 4H-SiC substrates. The same trend of suppressed reconstruction with 277 increasing TAr is found. In fact, for the case of TAr = 1300◦ C heating up to 1600◦ C (0 s) did not result 278 into a buffer layer formation and heating up to 1800◦ C (0s) was needed for a clear buffer layer Raman 279 spectrum to be obtained. Interestingly, the buffer layers TAr = 800◦ C and TAr = 1300◦ C exhibit very 280 similar GB positions (Fig.5 (c) and (g)) and ADB /AGB ratios (Fig.5 (d) and (h)), indicating similar sp3 281 hybridization contents and densities of defects. A slightly broader distribution is observed for the case 282 of TAr = 1300◦ C for both n-type and Si, reflecting a slightly larger variation of the crystallite size. 283 Based on these results we can conclude that the temperature, at which Ar is introduced has a 284 determining role for the formation of the buffer layer in high-temperature sublimation in enclosed 285 environment independently on conductivity of the SiC substrate. As a result of an interplay 286 between oxidation and restructuring in Ar atmosphere, the formation of the buffer layer is shifted Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 9 of 16 Figure 6. Histograms of the GB band energy and the ratio of the GB and GB areas, ADB /AGB , for the buffer layers grown with TAr = 800◦ C (a - d) on n-type (a,b) and SI (c,d) 4H-SiC; and for the buffer layers grown with TAr = 1300◦ C (e - h) on n-type (e,f) and SI (g,h) 4H-SiC. The histograms are obtained over Raman maps of 3 µm by 3 µm. Three Lorentzian lineshapes centered around GB of 1585 - 1600 cm−1 , DB of 1330 - 1530 cm−1 and a band centered at 1340 cm−1 were used for the fitting. 287 to higher temperatures for increased TAr above 1300◦ C. Increasing TAr also leads to reduction of sp3 288 hybridization contents and densities of defects on n-type SiC. However, TAr has a less pronounced 289 effect for SI substrates, where ordered buffer layers form with similar structural properties. 290 3.3. Free charge carrier properties of MLG and QFS-MLG 291 It is well known that MLG on SiC is intrincially n-type doped [45–47]. However, exposure to 292 ambient can cause environmental doping of graphene via an acceptor redox reaction at the surface of 293 the graphene involving various environmental gases (O2 , H2 O, and CO2 ), which results in electron 294 withdrawal [48]. Consequently, MLG can exhibit p-type conductivity depending on sample history 295 [20,49]. We have previously shown that the THz OHE is an excellent tool to precisely determine free 296 charge carrier density and mobility parameters of graphene and monitor their in-situ variation under 297 the influence of different gases [20,31,34,50]. In order to determine the intrinsic properties of MLG 298 and QFS-MLG, prior to the measurements they were annealed in vacuum (10−6 mbar) at 1000 ◦ C 299 and 500 ◦ C,2 respectively. The samples were kept in dry N2 during the measurements and storage. 300 In addition, we have performed measurements after purging with dry N2 for several days and air 301 with RH of 45% for several hours. Both transient and static measurements were carried out. Finally, 302 the samples were measured after being stored in ambient conditions for several months. We have 303 selected for these investigations samples with the following TAr : i) TAr = 800◦ C, for which the surface 304 reconstruction happens entirely in Ar atmosphere and that shows completed buffer layer after heated 305 to 1600◦ C (0 s); ii) TAr = 1300◦ C, for which the surface reconstruction happens entirely in vacuum, and 306 which needed heating to 1800◦ C (0 s) for the buffer layer to form. Although no indications of surface 307 oxidation was observed for the buffer layer sample with TAr = 800◦ C, a nano-scale oxidation cannot be 308 excluded. Furthermore, the graphitization processis shifted to higher temperatures in comparison to 309 n-type substrate as pointed out above. We therefore, included in our investigation MLG and QFS-MLG 310 samples, for which the Ar was introduced at iii) TAr = 640◦ C. Heating to 1600◦ C for 0 s was employed 311 to produce the buffer layer sample in this case. The QFS-MLG samples were obtained by hydrogen 312 intercalation of the respective buffer layers as described in Ref.[24]. The MLG samples were fabricated 2 The annealing temperature was confirmed to not cause deintercalattion or any changes in the QFS-MLG structural properties by LEEM, AFM, and µ-LEED Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 10 of 16 Figure 7. Free charge carrier density (left panel) and mobility (right panel) of MLG (filled symbols) and quasi-freestanding MLG (open symbols) with TAr = 640◦ C (black circles), TAr = 800◦ C (red squares) and TAr = 1300◦ C (blue triangles) for different environmental conditions: after annealing in vacuum (Annealed), after being purged with dry N2 for several days (N2 RH 0%), after being purged with moist moist air (RH 45%) for several hours (Air RH 45%), and after being exposed to the ambient for several months (Ambient). 313 using our optimized conditions of TGr = 2000◦ C for zero growth time (0 s), which results in less than 314 1% bi-layer inclusions. The sample with TAr = 1300◦ C required longer growth time of 5 min for a 315 homogeneous MLG to form leading to increased bi-layer inclusions of 8%. 316 Figure 7 shows the free charge carrier density (left panel) and mobility3 (right panel) of MLG (filled 317 symbols) and quasi-freestanding MLG (open symbols) with different TAr for different environmental 318 conditions. The freshly annealed MLG samples show n-type conductivity, as expected, with values in 319 the range of 3.9×1012 cm−2 to 6.6×1012 cm−2 . Due to the semi-insulating nature of the substrates, the 320 MLG doping should be entirely governed by charge transfer due to surface donor states [51]. All three 321 free electron density values are below the saturation density of n-type doping of MLG of 1013 cm−2 322 [52], indicating successful efficient annealing of donors on and near the SiC surface. The observed 323 differences with TAr , albeit small, are significantly below the error bar of 0.3×1012 cm−2 . Since the 324 MLG with TAr = 1300◦ C was obtained for a considerably longer time (5 min as compared do 0 s) it is 325 tempting to speculate that the longer annealing may have a positive effect on reducing the interface 326 dangling bonds effectively reducing the density of the surface state and leading to a lower free electron 327 density. We have previously shown that purging with N2 (or inert gases) effectively removes the 328 ambient acceptor dopant, which may require up to several days of purging [31,34]. The free electron 329 densities in the MLG samples with TAr = 640◦ C and TAr = 800◦ C after purging in dry N2 for 9 - 10 330 days increased slightly to 5.1×1012 cm−2 and 7.0×1012 cm−2 , respectively, remaining below the the 331 saturation density of n-type doping. The electron mobility parameters in these two cases slightly 332 decreased in comparison to the freshly annealed MLG, most likely as a result of the slightly increased 333 charge density. The MLG with TAr = 1300◦ C shows the opposite behavior with slightly decreased 334 charge density and slightly increased mobility parameter. Overall the purging win dry N2 led to very 335 small changes in the MLG electron density and mobility, which can be considered as the intrinsic free 336 electron parameters of MLG. 3 The mobility parameters were found to be slightly anisotropic in accordance with our recent study [20]. The anisotropy, which is caused by the substrate step edges, does not have any baring on the results discussed in the current work. Consequently, for brevity we present here the averaged mobility between the parameters determined along and perpendicular to the step edge. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 11 of 16 337 As expected after purging with moist air (RH of 45%) the electron density in the MLG samples 338 decreased due to the acceptor redox reaction at the graphene surface. The samples with different TAr 339 show very similar electron density of ∼2×1012 cm−2 after ∼ 20h of purging. We have measured the 340 in − situ variations of free charge carrier properties and found that approximately 45 h are needed to 341 flip the conductivity of MLG from n-type to p-type with free hole density of 1.4×1012 cm−2 . Long-term 342 exposure in ambient conditions (several months) leads to only very small increase of free hole density 343 to ∼2×1012 cm−2 indicating saturation of p-type ambient doping in MLG. Again very similar free 344 hole densities are found for the samples with different TAr = 1300◦ . On the other hand, the free charge 345 carrier mobility of the ambient doped MLG with TAr = 1300◦ C is more than 50% larger than the 346 respective values of MLG with TAr = 800◦ C and TAr = 640◦ C. This is true for both the cases of free 347 electrons and free holes (see Fig.7 right panel results for Air RH 45% and Ambient). This finding is very 348 interesting considering that the samples with TAr = 640◦ C and 800◦ C have better MLG coverage of 349 99% and lower RMS' 0.4 nm, compared with the TAr = 1300◦ C sample, which has 92% MLG coverage 350 and RMS' 0.75 nm. It was previously suggested that dominant scattering mechanisms at room 351 temperature in graphene on SiC are the remote interface phonon scattering, as a result of coupling 352 to the polar modes in the substrate, and scattering by impurities [53–55]. Since the MLG samples are 353 grown at the same TMLG and have similar history we do not anticipate difference in impurity levels. 354 It is thus plausible to suggest that in the MLG with TAr = 1300◦ C the interface phonon scattering is 355 reduced as a result of different interface properties. We recall that the buffer layers grown at different 356 TAr on SI substrates exhibit very similar sp3 contents and defect densities (Fig.6). Furthermore, the 357 Raman scattering spectral features associated with the buffer layer in the respective MLG samples 358 with different TAr are practically identical. Hence, the reduced interface phonon scattering is likely 359 a result of a different interface between MLG and the buffer layer rather than between buffer layer 360 and SiC substrate. This suggestion is further supported by the similar free charge carrier density in 361 the ambient doped MLG with different TAr indicating similar surface states densities. To gain further 362 insight into the origin of the different interface properties between MLG and the buffer layers we turn 363 now out attention to the free charge carrier properties of the QFS-MLG samples. 364 In QFS-MLG the intercalated hydrogen saturates the Si dangling bonds passivating the interface 365 donor states. Consequently, QFS-MLG exhibits p-type doping induced by the spontaneous polarization 366 of the SiC substrate [24,56,57]. The resulting free hole density in QFS-MLG on SI 4H-SiC was reported 367 to be 8.6×1012 cm−2 as determined by angular resolved photoelectron spectroscopy (ARPES) [57]. 368 As expected our freshly annealed QFS-MLG samples show p-type conductivity (see Fig.7 left panel). 369 We find very similar free hole densities in the QFS-MLG with TAr = 640◦ C and TAr = 800◦ C of 370 1.2×1013 cm−2 and 1.5×1013 cm−2 , respectively. These values are slightly higher than the free hole 371 density expected from pure polarization doping [57]. It is possible that some residual ambient doping 372 is present as the annealing temperature for the QFS-MLG samples was relatively low in order to 373 prevent deintercalation. Purging in dry N2 for several days lead to small reduction of the free hole 374 density in these two samples to ∼1.0×1013 cm−2 , which is suggested to be the intrinsic value for our 375 QFS-MLG resulting from polarization doping. We consider this to be a good agreement with the 376 previously reported value of 8.6×1012 cm−2 [57] given the different experimental techniques used 377 in the two works and the various fitting parameters employed to deduce the free hole concentration 378 from ARPES. Both the freshly annealed and the dry N2 purged QFS-MLG with TAr = 1300◦ C show 379 significantly lower free hole density of 4.4×1012 cm−2 and 2.1×1012 cm−2 , respectively. According to 380 the polarization doping model the negative pseudo-polarization charge, which is a constant parameter 381 for the 4H-SiC is balanced by the free holes in the QFS-MLG and the positive space charge in the 382 substrate depletion layer [57]. Since the bulk doping in the SI substrate is the same for all three samples 383 leading to similar positive space charge in the substrate depletion layer, the observed lower free hole 384 density in QFS-MLG with TAr = 1300◦ C indicates the presence of donor surface states. As noted earlier, 385 the buffer layers grown at different TAr on SI substrates exhibit very similar sp3 contents and defect 386 densities (Fig.6). We also confirmed by µ-Raman scattering spectroscopy mapping that no structural Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 12 of 16 387 changes occur as a result of the intercalation process. Recall that in comparison to lower TAr the buffer 388 layer with TAr = 1300◦ C is incomplete. We speculate that this incomplete buffer layer formation is 389 the cause of the surface donor states, likely dangling bonds. Interestingly, purging with moist air 390 (RH 45%) for ∼18 h leads to small increase of free hole density in QFS-MLG with TAr = 800◦ C and 391 TAr = 640◦ C while for TAr = 1300◦ C the hole density remains unchanged. This can be potentially 392 explained by the above mentioned scenario since the purge with moist air has different effects: for the 393 polarization doped QFS-MLG it leads to chemical acceptor doping of graphene while for the sample 394 with TAr = 1300◦ C it leads to passivation of surface donor states. The two process will naturally have 395 different dynamics. This proposal is also consistent with the results for prolonged exposure to ambient. 396 The free hole density in QFS-MLG with TAr = 1300◦ C increases to 9.212 cm−2 nearing the intrinsic 397 polarization doping since most (all) surface donor states have been passivated. For TAr = 640◦ C and 398 TAr = 800◦ C the free hole densities increase to 2.3×1013 cm−2 and 1.9×1013 cm−2 , respectively, as a 399 result of chemical acceptor doping. In all cases, except for the freshly annealed samples, the largest 400 hole mobility parameters are found for the QFS-MLG with TAr = 1300◦ C . This is most likely related 401 to the generally lower free hole density parameters. Note that the free charge mobility (and density) 402 parameters represent average parameters obtained over the entire sample area of 10 mm×10 mm. 403 4. Conclusions 404 We have critically reviewed the processes in high-temperature sublimation of graphene in Ar 405 atmosphere using enclosed graphite crucible with emphasis on buffer layer formation and free charge 406 carrier properties of MLG and QFS-MLG on 4H-SiC. We have explored the effect of introducing Ar at 407 different temperatures, TAr , and have evaluated the impact of different gas exposures. We have found 408 that the buffer layer coverage anticorrelates with TAr with well developed buffer layer for TAr = 800◦ C 409 while for TAr = 1300◦ C the buffer layer is just beginning to form. The observed suppression of buffer 410 layer formation at higher TAr is accompanied by surface oxidation of the uncovered regions of the SiC 411 substrates. A scenario in which oxidation occurs during the annealing process is proposed to explain 412 the peculiar shift of the buffer layer formation to higher temperatures. The latter leads to reduced 413 sp3 hybridization content and defect densities in the buffer layer when grown on n-type conductive 414 substrates. Growth on SI substrates results in significantly improved structural properties of the buffer 415 layers, which is attributed to a slower graphitization process closer to equilibrium due to the reduced 416 thermal conductivity of the substrate. For SI substrate TAr plays a minor role for the sp3 hybridization 417 content and defect densities in the buffer layer. A comprehensive study of the free charge density and 418 mobility parameters of MLG and QFS-MLG with TAr = 640◦ C, TAr = 800◦ C and TAr = 1300◦ C and 419 four different environmental conditions: freshly annealed in vacuum, after purging with dry N2 (RH 420 0%) for ∼20 h, after purging with moist air (RH 45%) for ∼18 h and after ambient exposure for several 421 months allows us to draw the following conclusions: 422 i) successful efficient annealing of donors on and near the SiC surface can be inferred for MLG 423 grown at 2000◦ C independent of TAr ; 424 ii) approximately 45 h purging with moist air (RH 45%) are needed to flip the conductivity of 425 MLG from n-type to p-type and long term exposure to ambient leads to a saturation of the free hole 426 density to ∼2×1012 cm−2 ; 427 iii) the highest mobility of MLG is found for TAr = 1300◦ C in both intrinsically n-type and ambient 428 p-type doped situations. It is suggested that this is a result of reduced interface phonon scattering due 429 to improved interface between MLG and the buffer layer rather than between buffer layer and the SiC 430 substrate; 431 iv) A free hole density of ∼1.0×1013 cm−2 is suggested to be the intrinsic value for our 432 QFS-MLG resulting from polarization doping in good agreement with the previously reported value 433 of 8.6×1012 cm−2 [57]; Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 January 2021 doi:10.20944/preprints202101.0021.v1 Version December 31, 2020 submitted to Appl. Sci. 13 of 16 434 v) TAr is found to have a profound effect on the free hole parameters of QFS-MLG. A significantly 435 lower free hole density of ∼2×1012 cm−2 is found in intrinsic QFS-MLG with TAr = 1300◦ C, which is 436 attributed to additional surface donor states associated with incomplete buffer formation. 437 Our findings contribute to establishing comprehensive picture of high-temperature sublimation 438 growth and provide guidance for growth parameters optimization depending on the targeted 439 application of QFS-MLG and MLG. 440 Author Contributions: Individual contributions of the authors are as follows: Conceptualization, V.S. and V.D.; 441 methodology, N.A. and P.K.; software, V.S.; validation, V.S.; formal analysis, V.S. and V.D.; investigation, V.S., N.A., 442 A. Z., C.C., I.I., and C.B.; resources, R.Y and V.D.; writing–original draft preparation, V.S. and V.D; writing–review 443 and editing, V.S, R.Y., C.C. and V.D; visualization, V.S and V.D; supervision, V.D.; project administration, V.D.; 444 funding acquisition, R.Y and V.D. All authors have read and agreed to the published version of the manuscript. 445 Funding: The authors would like to acknowledge financial support from the Swedish Research Council (VR 446 Contract 2016-00889), the Swedish foundation for strategic research (SSF) under Grants No. FFL12-0181 and 447 No. RIF14-055, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at 448 Linköping University (Faculty Grant SFO Mat LiU No 2009 00971). RY is grateful for financial support by SSF via 449 grant RMA 15-0024. 450 Acknowledgments: We thank Dr. Valdas Jokobavicius for his help with annealing the MLG and QFS-MLG 451 samples in vacuum. 452 Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the 453 study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to 454 publish the results. 455 References 456 1. 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