Atomic-Scale Mapping of Layer-by-Layer Hydrogen Etching and

Article pubs.acs.org/JPCC

Atomic-Scale Mapping of Layer-by-Layer Hydrogen Etching and Passivation of SiC(0001) Substrates Stefan Glass,† Felix Reis,† Maximilian Bauernfeind,† Julian Aulbach,† Markus R. Scholz,† Florian Adler,† Lenart Dudy,† Gang Li,‡,§ Ralph Claessen,† and Jörg Schaf̈ er*,† †

Physikalisches Institut and Röntgen Research Center for Complex Material Systems, Universität Würzburg, 97074 Würzburg, Germany ‡ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, 97074 Würzburg, Germany § Institut für Festkörperphysik, Technische Universität Wien, 1040 Wien, Austria ABSTRACT: Preparation of SiC(0001) substrates is of high relevance to graphene growth. Yet, if only a smooth surface could be achieved, heteroepitaxy of many other two-dimensional materials comes into reach. Here we report a novel approach to hydrogen etching of SiC, based on stepwise ultrapure H exposure with slow substrate cooling rates. For the first time, the atomic evolution of the surface structure is witnessed by scanning tunneling microscopy. A detailed picture of the gas phase chemistry emerges, such as a zipper-like material desorption at step edges. The Si−C sheets are removed in layer-by-layer fashion, leading to large terraces with straight rims. The process ultimately results in an atomically smooth surface with complete H-passivation, with no detectable defect states in photoemission. The degree of perfection achieved suggests the use of this substrate as a versatile nanostructure template.

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INTRODUCTION

The essential prerequisite of such 2D electronic materials is the availability of a smooth and defect-free SiC(0001) substrate. However, to date all efforts show that this goal is very difficult to achieve, and almost nothing is known about the quality of the substrate material on the atomic level. Main reasons for the moderate tractability of SiC are its hardness and chemical inertness. Since one is usually dealing with the Si-face, it would be conceivable to use wet-chemical etching recipes that work well for Si(111). There the use of hydrofluoric acid (HF) leads to H-terminated surfaces.8,9 Thus, it has been attempted to treat SiC with HF,10,11 however, smooth surfaces have not been achieved, even if concentrated 50% HF was used. Also, it was found that the surface, contrary to the case of Si(111), is not left H-terminated, but with OH groups instead.10,11 Another problem is the tremendous surface roughness that remains. As a variant of the chemical approach, it was reported that the use of a platinum catalyst in front of the SiC surface in HF solution can lead to a cleaned surface.12 However, large-scale atomic order has again not been demonstrated. As an alternative route, gas-phase hydrogen etching has been developed over the years. In various attempts, high hydrogen pressures (somewhat below or even up to 1 bar) were used, in combination with a heated SiC substrate to create reactive atomic H, at temperatures ranging from 800 to 1450 °C.10,13,14 By such means it becomes possible to stabilize a H-terminated

Two-dimensional (2D) layered materials are a rapidly developing area of nanoscience research. A key representative is graphene on SiC(0001) substrates, which is relevant to electronic applications, because long-range ordered layers can be grown.1,2 Already here the role of the substrate becomes evident, as its defect density limits the mobility achieved in the graphene overlayer. Various attempts at controlling substrate disorder such as, e.g., off-axis substrates are being discussed.3 SiC has also gained technological importance as a growth substrate for GaN epitaxy, due to the close lattice match.4 Important for applications is that the SiC substrate, in contrast to refractory metal templates such as iridium, bears the advantage that the insulating support ensures that all electrical transport occurs in the overlayer only. In this context, deposition of materials other than carbon becomes desirable, as the electronic use of graphene is limited due to its lacking band gap. Of heightened interest is the fabrication of graphene analogs, i.e., 2D honeycomb lattices, made of group-IV elements with high atomic number Z. The increased spin−orbit coupling in these monolayer films is expected to induce substantial band gaps. Possible candidate systems are “silicene”, i.e., a Si honeycomb lattice, on a hydrogen-terminated SiC surface5 or “stanene” made from Sn atoms with an energy gap of up to 0.3 eV.6 Very recently it has been proposed that a bismuth-based honeycomb system (“bismuthene”) can be stabilized on SiC(0001) yielding an even larger band gap of ∼0.5 eV.7 © 2016 American Chemical Society

Received: February 12, 2016 Revised: April 13, 2016 Published: April 19, 2016 10361

DOI: 10.1021/acs.jpcc.6b01493 J. Phys. Chem. C 2016, 120, 10361−10367

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The Journal of Physical Chemistry C SiC surface. The existence of Si−H surface bonds after an Htreatment at 1100 °C was demonstrated by infrared spectroscopy.10 Yet, a major problem can be the occurrence of a silicate layer on the surface (regardless of the hydrogenation), due to oxidation from gas contaminants such as oxygen and water.15 A significant improvement was thus the implementation of hydrogen purif ication, realized by a palladium diffusion filter, which removes impurities down to a level of ∼10−8 rather than the typical H purity of ∼10−5. A direct proof for the removal of oxygen from the surface by etching with cleaned H2 was given by Sieber et al. by means of core-level photoemission.16 Such silicate-free SiC substrates have since then successfully been used for the fabrication of graphene, based on thermal decomposition of the topmost SiC layers.1,2 Now, with the perspective of utilizing this substrate for epitaxy of a large variety of 2D materials, the need to achieve an atomically smooth SiC(0001) surface, and to thus understand the underlying chemical processes, becomes eminent. While the general concept of gas phase etching seems well suited for this purpose, little is known regarding the formation process of this surface. Scanning tunneling microscopy (STM) images thus far have only been reported for the annealed surface after hydrogen desorption, which leaves behind a (√3 × √3)reconstruction ascribed to surplus Si adatoms.17−19 Surprisingly, no STM images of the (1 × 1) H-terminated surface after the various gas phase etching steps have been demonstrated up to now (to the best of our knowledge), so that nothing is known about the local atomic morphology. Aim of This Report. In this report we study the formation of a smooth SiC(0001) surface subjected to gas phase H etching, performed in a special design of cold-wall reactor, where ultrahigh vacuum (UHV) conditions are obeyed in all process details. For the first time, STM images of the effects of such hydrogen etching are presented. We find that redeposition of volatile compounds is an inherent tendency of the gas phase chemistry, which can be minimized at reduced substrate temperatures. For suitable process parameters, we show that the substrate material is removed in layerwise fashion, with etching action taking place at Si−C bilayer steps in a zipper-like fashion, which leads to astoundingly straight terrace edges. High-resolution STM images show that the terraces are free of etch pits and perfectly H-passivated, consistent with DFT simulations of H atoms attached to the SiC substrate. Angleresolved photoemission documents the quantitative absence of dangling bonds or other defects. Thus, SiC(0001) substrates prepared in this manner appear well suited for epitaxial nanostructure growth.

Figure 1. (a) Schematic of the hydrogen process setup, comprising a UHV cold-wall reactor, direct current sample heating, and a gas purifier. (b) STM image of SiC(0001) after HF etching only, leaving a rough surface. (c) STM after first gas phase etching cycle (1100 °C, 45 min), leading to large terraces.

however, is detrimental to the purity of the process. The underlying idea of our spot-heating approach is thus to minimize desorption of contaminants in the neighborhood of the sample with its reacting Si−H bonds in statu nascendi. The sample holder has electrical contacts, to perform direct current heating through the SiC wafer piece itself (typical lateral dimensions 12 × 3 mm and a wafer thickness of ∼0.33 mm), mounted above a molybdenum base plate. The substrate temperature is monitored through a top window with a pyrometer (detection range 1.1−1.7 μm, emissivity ε = 85%). Hydrogen (purity 6.0) is admitted through a reactive sorption filter cartridge which removes residual impurities to a level of 100 parts per billion, i.e., 10−7 with respect to the input level (supplied by SAES getters). The H2 pressure within the gas phase reactor of typically 950 mbar is monitored with an absolute pressure indicator. Samples and Analytical Methods. The SiC(0001) substrates used were of 4H type and n-doped with a specified conductivity of less than 0.5 Ohm cm. The surface termination was the Si-face, which was polished by the manufacturer to typical roughness standards (∼5 Å rms). After the etching in the hydrogen reactor, a portable UHV transport chamber with getter pump has been used to transfer the H-treated SiC samples from the reactor to the STM and angle-resolved photoemission (ARPES) setups, respectively. The sample quality was routinely inspected by low-energy electron diffraction (LEED) for the (1 × 1) H-reconstruction in either UHV system prior to analysis. STM was carried out in the cryostat chamber of an Omicron low-temperature instrument (measurements performed at RT and 77 K). For ARPES, He I radiation (21.2 eV) from a discharge lamp was used. The photoelectrons were collected with a Phoibos 100 imaging analyzer at 10 meV resolution.

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EXPERIMENTAL METHODS Description of the Novel Cold-Wall Reactor. The newly developed hydrogen etching apparatus consists of a small UHV chamber which is evacuated by a turbo pump at the bottom. The whole setup, as depicted in Figure 1a, is baked prior to use to achieve UHV conditions with a base pressure of ∼10−10 mbar. The sample is held in the center of the chamber, so that it can be installed and removed by a linear transfer. We use a novel concept, which is based on a cold-wall reactor with external air-stream cooling, where only the sample material itself is heated locally. This is in contrast to the furnace-type heating in glass tubes used thus far, where the reaction temperature was, e.g., achieved by external high-power heaters17,20 or induction coils10,14 targeted at the whole sample support. Such global heat supply, 10362


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RESULTS AND DISCUSSION

Pretreatment with HF Wet Chemistry. The SiC(0001) substrates have been cleaned and mildly wet-chemically etched ex situ, prior to loading into the vacuum chamber. The procedure includes a standard sequential degreasing treatment (acetone, propanol, and methanol). Subsequently the material has been etched in buffered HF solution (5% HF concentration) to remove debris particles from the wafer polishing process. Samples that have been given the wet-chemical preparation only, i.e., a 10 min dip in buffered HF as the last step, have been inspected with STM, as shown in Figure 1b. Regardless of the fact that in LEED a faint (1 × 1) pattern with high background is observed (not shown here), one finds in STM that the surface is highly disordered. In particular, it is covered with very small islands or clusters of a few nanometers in lateral extension. Overall, the surface exhibits a high local roughness, which is certainly not suitable as a growth substrate. Results after First Gas-Phase Etching Step. For a single hydrogen etching step, substrate temperatures between 1000 and 1250 °C have been utilized, all leading to similar results on a microscopic level, as shown in Figure 1c. The most significant quantitative difference of higher process temperatures was found in the etching speed, which increases with temperature in accordance with ref 14. The STM image Figure 1c, for a treatment at a substrate temperature of 1100 °C for 45 min, reflects three large terraces of bilayer height (2.51 Å), and of typically 100 to 200 nm extent, a major improvement compared to the HF treatment alone. A smoothing action of the hydrogen treatment with respect to the polishing damage had been noted earlier by lowresolution atomic force microscopy (residual roughness there was on a ∼10 nm scale).14 However, on each of the terraces there is a lot of debris, leading to a rough appearance of the terrace. We argue that this is due to redeposition from the gas phase, as detailed below. Very obviously, after one such etching step the surface has been etched in a layer-by-layer fashion, albeit that atomic order atop the terraces is still missing. Results after Second Gas-Phase Etching Step. In a refined recipe, the substrate is subjected to two subsequent Hetching steps at 1100 °C, each utilizing a new gas filling with a pure hydrogen atmosphere, and with durations of 40 and 4 min, respectively. For both treatments, we used particularly slow cool-down rates: from 1100 to 1000 °C in ∼3 min, and from 1000 °C to RT in ∼8 min. The results are shown in Figure 2a. Now the key difference is that one can resolve atomically well-defined layers on each terrace. Upon closer inspection, these represent a triangular lattice of bright spots that are ascribed to the H atoms adsorbed on the surface. The terraces itself are smooth and terminated by straight edges. At the same time, one notices a number of small deposits on top of the smooth surface layer, which do predominantly not exhibit any atomic substructure, and are analyzed in detail in Figure 2b,c. We detect, on one hand, small elevated features, with diameters of ∼1 nm or less. We ascribe these to remainders of the redeposition process of silane from the gas phase, as discussed below. On the other hand, in some areas, as in Figure 2b, we see some small triangular indentations, as if the etching process had attacked the topmost layer. We speculate that the reason for this behavior is an initial defect atom. It possibly already resided in the substrate as purchased and has served as a nucleation site for the observed local pit formation.

Figure 2. STM images showing the surface quality after the second H etching step on SiC(0001). (a) The terraces exhibit straight edges and smooth terraces, with some remaining imperfections. (b) A small number of minor defects are detected, including triangular indentations. (c) Most of the defects have the size of a few atoms, and some are indicative of subsurface defects (top right).

Another feature that we observe is shown in Figure 2c: Again, most of the surface imperfections look like one or a few disordered atoms. There are also locations where the triangular surface lattice is intact, but the overall STM intensity seems locally enhanced. This must be ascribed to subsurface defects (possibly already present in the SiC crystal) which locally alter the electronic density of states but leave the surface lattice otherwise intact. The detailed inspection of the step edges shows that the removal of material occurs in multiples of the Si−C bilayer (BL) height of 2.51 Å in (0001) direction. In the example of Figure 3a, a double step is observed, with the second step located close to the first one. We note in passing that occasionally we find indications that an edge exhibits some extra intensity, indicating a charge spill-out which is detected by the tunneling tip. Above all, these findings indicate very clearly that the etching process occurs in a layerwise fashion, removing BL by BL successively. Discussion of Gas-Phase Processes: Overview. For the etching process, we assume the following key mechanisms to play a role, as concluded from our observations. First, the H2 molecules are cracked at the hot surface into highly reactive atomic hydrogen. These atoms will preferentially attach to Si atoms at step edges where atomic coordination is low and hence additional dangling bonds are available for chemical bonding. The subsequent intermediate steps involve the formation of substoichiometric SiH2 and SiH3 molecules (for a review of the H-etching chemistry of Si surfaces see ref 21). Eventually the step edge Si atoms will be removed as a gaseous SiH4 molecules, see depiction in Figure 3b. 10363


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On the basis of these observations we argue that the redeposition from the gas phase, and the subsequent growth of amorphous clusters on the surface (believed to be formed predominantly by silicon atoms), is the major challenge in achieving a clean process and creating long-range ordered surfaces without such adsorbed impurities. Deposition from the gas phase using silane is known to result in amorphous hydrogenated silicon25 or even nanocrystalline Si.26 Likewise, methane decomposition can lead to amorphous carbon27 or nanocrystalline diamond.28 Step Edges and Layer-by-Layer Etching Mode. In looking at the terraces seen in Figure 2a,b as well as Figure 3a, one finds that the edges are very smooth, i.e., they are devoid of a noticeable number of defects or zigzag shapes. Instead, in the STM images one observes an intense and continuous signal along the terrace edges, which probably results from an accumulation of charge density at the edge. In turn, we do not find any indication for large hole-like defects anywhere on the surface plane of a given terrace, so-called etch pits, that would result, e.g., from locally missing etched Si or C atoms. Notably, this is very different from what has been observed for the (3 × 3) Si reconstruction of the strongly annealed SiC surface, where H leads to massive atomic hole formation in a random fashion.29 In other words, the aggressive action of atomic hydrogen, generated by thermal dissociation of H2 at the “as polished” substrate, does not attack the closed Si−C bilayers from within the plane, but only at the step edges. This can be understood when considering the number of saturated bonds of the atoms in the topmost bilayer of a Sifaced substrate. Away from the edges, C atoms are fully saturated with four bonds to surrounding Si atoms, while Si atoms have three backbonds into the lower-lying carbon layer. However, at an edge, as illustrated in Figure 3b, Si and C atoms with only two backbonds can be found, leaving two dangling bonds−thus increasing the possibilities for hydrogen as a bonding partner to attach. The situation gets even more extreme at the apex of a bilayer island. Here, only one backbond remains, leaving three bonds for interaction with atomic hydrogen. Ultimately this leads to the formation of SiH4 and CH4, which can go into the gas phase. Our data of the smooth terrace rims now suggest that the etching action occurs row by row in a zipper-like fashion, which appears very plausible since the highly mobile H atoms will be moving along the step edge. Atomic-Scale Ordering. We now address the question of long-range surface ordering achieved after the two-step Hetching process in more detail. First, as seen in Figure 4a, the LEED image shows extremely sharp diffraction peaks of the (1 × 1) H-terminated surface, with virtually no background. This is a direct representation of the high degree of perfection achieved by the gas phase process. Moreover, the STM overview image in Figure 4b displays large terrace sizes on the 100 nm-scale and larger (depending on initial polishing quality and etching time), which is promising for the deposition of monolayer materials for electronic applications. Regarding the situation at the atomic level, we have taken atomically resolved close-up STM images as in Figure 4c,d. One notices clearly the hexagonal lattice arrangement of the H atoms. The bright intensity spots relate to the orbitals of the H atoms (specifically the antibonding part, for which we achieve particularly good contrast in experiment). The atomic-scale view in Figure 4c taken at +3.0 V in the unoccupied states and at very high current (close approach to the surface) exhibits an

Figure 3. (a) Detailed analysis of a step edge. The line profile shows a two-bilayer step, i.e., half a unit cell of 4H-SiC. (b) Depiction of the chemical reactions at the surface: attack by hydrogen atoms will occur preferentially at step edges, where Si and C atoms have only one or two backbonds (BB). Atoms are released into the gas phase in the form of SiH 4 and CH 4 . Redeposition of Si occurs by SiH 4 decomposition.

Consequently, deeper-lying carbon atoms will be exposed, which form the lower lining of the Si−C bilayer. Likewise, the exposed C atoms at terrace edges and defects will be hydrogenated and finally released as CH4 gas phase species. In order to sustain this chain of reactions it is mandatory to supply sufficient thermal energy to dissociate the H2 molecules, H2 → 2H, with a large bond-dissociation energy of +436.0 kJ/ mol (+4.52 eV/molecule). Also, formation of silane (SiH4) is an endothermal reaction, with an enthalpy of formation of +34.3 kJ/mol (+0.36 eV/molecule).22 For this reason, the hydrogen etching process is usually operated at very high substrate temperatures in the range of roughly 1100 to 1500 °C.10,13,14 Very obviously, the substrate temperature has a concurrent influence on the etching speed. Redeposition Problem. A major process of relevance to the quality of the outcoming surface is the fact that substantial amounts of silane and methane are generated in close proximity to the newly exposed surface layer. Due to the high substrate temperature, it is, in principle, possible that the SiH4 and CH4 molecules, when backscattered to that hot surface, decompose again into elementary Si and C atoms, plus the surplus hydrogen. Thermal dissociation of silane and methane is well understood.23,24 Decomposition of SiH4 is far more likely than that of CH4, because even at RT SiH4 is thermodynamically unstable (positive formation enthalpy), while CH4 is more stable than its individual elements, with a formation enthalpy of −74.87 kJ/mol (−0.78 eV/molecule).22 A substantial decrease of the SiH4 content in the gas phase has been noted for Si(111) etching already above 400 °C.21 10364


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Figure 4. (a) LEED pattern of the double-treated SiC surface with very sharp (1 × 1) reflections. (b) Overview STM shows large terraces of order ∼50−100 nm (residual pits ascribed to initial wafer polishing damage). (c) STM with close tip approach (large current) reveals a regular triangular lattice of H atoms, with an alternating pattern of hollow and filled spaces in the triangles. (d) Even at large tip distances (low current), the appearance of the elevations remains circle-shaped, consistent with localized H orbitals.

Figure 5. (a) Structural model of the H-saturated 4H-SiC(0001) surface. (b) Simulated STM from DFT in constant height mode. For a close approach (1 Å), a triangular pattern is seen, connecting three H sites with either a hollow site or with intensity in-between, respectively, depending on the presence of a C atom in the second layer. (c) STM simulation for iso-charge contours (i.e., constant current), showing the same behavior.

alternating pattern of either dark or moderately bright intensity, respectively, between the central elevations (which we ascribe to the hydrogen atoms). The origin of this pattern will relate to the charge landscape of the underlying substrate, i.e., the directed orbitals, as becomes apparent from the STM simulations below. When increasing the tip−surface distance by operating at a lower tunneling current, these fine details of the STM image disappear, and only the circular intensity spots in triangular arrangement remain. Structure Model and STM Simulation. For a detailed understanding, the atomic and electronic structure of the hydrogen-terminated surface has been modeled using densityfunctional theory (DFT) and the generalized gradient approximation30 of the exchange correlation functional within the projector argument wave method31 as implemented in the Vienna ab initio simulation package (VASP).32 Specifically, we have treated the 4H-SiC(0001) system as a six-layer slab, terminated with hydrogen on top, to simulate the situation with H passivation (back side of the slab also H-saturated), see depiction in Figure 5a.33 Simulated STM images were then obtained by generating a constant-height contour, as shown in Figure 5b, or alternatively as “iso-charge” contour (i.e., isocurves of the local integrated density of states) that mimics the measurements at constant current, see Figure 5c. Importantly, in the simulated STM images, for most conditions one sees a circular shape of the H orbitals in the unoccupied states, arranged in a triangular lattice−as in experiment. For the simulations of close tip approaches in the upper panels of Figure 5b,c, the local intensity maxima (resulting from H atoms) are faintly connected via three tails in

120° symmetry. The resulting pattern is an alternating arrangement of dark hollow spaces (between three H atoms) and slightly intensity-f illed spaces, respectively. It is a fascinating fine detail that this is also resolved in the experimental data of Figure 4c at high tunneling current. In comparing to the structural model of SiC:H in Figure 5a, these intensity tails are clearly directed along the underlying three Si−C bonds (of which at +3.0 V the antibonding component is detected). More generally, with respect to large-scale passivation of the SiC surface, we take the well-resolved appearance of the circular intensity maxima in a triangular lattice in the experimental STM images as an indication for the perfection of the surface, and specifically for the absence of defects at the surface or in the layer below. ARPES and Band Structure: Saturated Bonds. The electronic bandstructure of the SiC(0001) surface should also reflect whether or not the system is hydrogen-passivated. For the band dispersions obtained from our DFT calculation (sixlayer slab), in the initial situation without hydrogen, one would expect Si dangling bond orbitals filled with one electron. This leads to a half-filled metallic band at the Fermi level EF, as shown in Figure 6a (the calculation does not include the effect of electron correlations, which can shift the electron energies somewhat, or might even open a Mott gap). Upon hydrogen passivation, the dangling bonds become saturated, and the 10365


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unsaturated SiC(0001) surface in (1 × 1) reconstruction would lead to states close to EF. While the DFT calculation arrives at a half-filled metallic band with a width of ∼0.5 eV, it is wellknown from numerous related examples that due to repulsive on-site Coulomb interaction a Mott-Hubbard insulator is formed, with the consequence that an energy gap opens and the occupied part of the band is shifted down in energy significantly. For SiC(0001), the DB state has been observed at 1.8 eV below EF.20,34 Considering the effect of H-saturation, we argue that in comparison to the earlier work, where still residual DB intensity on the few percent level was seen, in the current UHV hydrogenation process the saturation is so complete that in Figure 6d no trace of a DB signal (or other defect states) is found anymore. We ascribe this high-quality outcome to the use of an immaculate UHV environment and the refined twostep H exposure process.

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CONCLUSIONS In conclusion, the analysis of this ultrahigh purity etching process of SiC substrate surfaces in hydrogen, operating at atmospheric pressure with filtered source gas, shows that a challenge encountered, inherent to any etching process, is the tendency for redeposition of material from the gas phase, which we have minimized by working at moderate substrate temperatures and renewing the etchant gas (two-step process). Eventually, very large and atomically flat terraces are obtained. The edges of these terraces are straight lines, devoid of a noticeable number of defects or zigzag distortions. This suggests that the etching process occurs in a layer-by-layer fashion. Performing the etching process in this regime lies at the very basis of the obtained exceptionally smooth surfaces. In view of the substrate quality obtained and the virtually perfect hydrogen saturation, such SiC surfaces appear ideal for use as a template for epitaxial applications, such as deposition of atom lattices in hexagonal symmetry imprinted by the substrate. As an immediate application, it is technologically desirable to obtain ideal graphene layers, which, however, are prone to warping and defect formation at step edges.35 Moreover, there are the envisioned extensions of the graphene-like lattice, e.g., silicene on H-terminated SiC(0001)5 (which would present a weak van der Waals interaction with the substrate) or the potential growth of honeycomb bismuthene lattices directly on SiC(0001).7 Regarding metal adatom studies on SiC(0001), little has been done today, most probably owed to the difficulty of preparing good SiC(0001) surfaces in the past. Upon hydrogen desorption, be it thermally (at high temperatures leading to a Si-(√3 × √3) reconstruction17), by UV irradiation (resulting in a (1 × 1) DB situation34), or by combined thermal desorption in simultaneous presence of the atom species to be deposited, such adsorption studies may be performed. A recent example using the latter method is the growth of the (√3 × √3) structure of Sn atoms36 on H-etched SiC(0001). This encouraging result shows that the substrate quality achieved opens novel possibilities for adlayer studies.

Figure 6. (a) Electronic band structure from DFT for SiC(0001) with dangling bond (DB) band near the Fermi level. (b) Band structure for H-terminated state, where DB band is removed. (c) ARPES band map (hν = 21.2 eV) on H-treated surface, indicative of long-range order. (d) ARPES close-up in energy window near EF, where the DB state (subject to Coulomb correlations) is expected at 1.8 eV, yet is fully absent, indicating perfect H saturation.

corresponding band is fully occupied, located deep below EF. As a result, the surface exhibits a wide band gap of several eV that is even larger than the bulk gap, as seen in Figure 6b, which reflects that the H−Si bonds are rather strong. Therefore, to probe the degree of hydrogenation, and as a further test of the long-range order, we have performed ARPES of the SiC(0001):H system after the double-etch procedure. Experimentally, the valence band ARPES data have been mapped in the full (1 × 1) surface Brillouin zone, as shown in Figure 6c. A number of surface bands can be seen in the ARPES data, as well as the bulk bands (intersected at 21.2 eV photon energy) around the surface Γ-point. Most of the bands relate to the topmost bilayer formed equally by Si and C atoms. The hydrogen character, as guided by DFT, is only weakly pronounced in the energy window down to −4 eV. Nonetheless, these dispersing surface bands are a clear indication of the large real-space extent of the terraces. Moreover, since defects would lead to a k-broadening of the spectra in similar manner as small terraces, the data are also proof of a globally low defect density. A close-up of the energy bands near the Fermi level EF is shown in Figure 6d. The dangling bond (DB) state of an

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AUTHOR INFORMATION

Corresponding Author

*Tel. ++49-931-31-83483. Fax: ++49-931-31-84921. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 10366


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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant SCHA 1510/5-1 and by the DFG Collaborative Research Center SFB 1170 “ToCoTronics” in Würzburg. G. L. acknowledges the computing time granted at the Leibniz Supercomputing Centre (LRZ) in Munich.

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