pubs.acs.org/Langmuir © 2010 American Chemical Society
Effect of Substrate Surface Reconstruction on Interaction with Adsorbates: Pt on 6H-SiC(0001) Zhen Wang, Qiang Fu,* and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, PR China Received November 16, 2009. Revised Manuscript Received January 11, 2010 √ √ Three reconstructed 6H-SiC(0001) surfaces, including a Si-rich 3 3 surface, a C-rich 6 3 6 3 surface, and a graphitized SiC surface, were used as substrates for the deposition of Pt overlayers. The interaction between Pt and the SiC(0001) surfaces was studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Pt reacts readily with the 3 3 surface to form platinum silicide even at room temperature. On the graphitized SiC surface, metal particles with low lateral dispersion form and keep on aggregating √ √ upon annealing. In contrast, homogeneously distributed small Pt nanoclusters were grown on the C-rich 6 3 6 3 surface. The unique nanomesh surface structure helps to stabilize the Pt nanoclusters until 800 °C. Above 1000 °C, Pt tends to diffuse into the subsurface region, forming the C/Pt silicide/SiC(0001) interface structure. The different surface electronic structures of the three Pt/SiC(0001) systems were discussed as well. The present data show that surface reconstruction provides an effective route to control the growth of metal overlayers and the formation of metal/substrate interfaces.
1. Introduction The superior mechanical and thermal properties as well as high chemical stability of silicon carbide, SiC, enable this material to be an important support for metal catalysts, which are expected to be stable at high reaction temperature and for highly exothermic reactions.1-3 Furthermore, nanostructured carbon overlayers, such as carbon nanotubes4,5 and graphene,6 could be grown on a SiC surface. These novel carbon materials have large advantages over conventional ones and could be used in many carbon-related catalytic processes. SiC also presents potential applications in semiconductor devices. It could be a good candidate to replace Si, Ge, and GaAs semiconductor materials in high-temperature, high-speed, and high-power electronic devices because of its excellent thermal conductivity, large breakdown fields, and wide band gap.7-9 For all of these applications, the contact between metal and SiC is often critical to the performance. Thus, good control of the metal/SiC interface is of great importance. The substrate surface structure presents one of the most important factors influencing the metal/substrate interface structure and can be used to manipulate the interface formation.10 SiC has many polytypes, and 6H-SiC(0001) is the most used crystal. Recently, atomic control and understanding of *Corresponding author. Tel: þ86-411-84686637. Fax: þ86-411-84694447. E-mail:
[email protected] (Q.F.),
[email protected] (X.B.). (1) Ledoux, M. J.; PhamHuu, C.; Chianelli, R. R. Curr. Opin. Solid State Mater. Sci. 1996, 1, 96. (2) Ledoux, M. J.; Pham-Huu, C. Cattech 2001, 5, 226. (3) Nhut, J. M.; Vieira, R.; Pesant, L.; Tessonnier, J. P.; Keller, N.; Ehret, G.; Pham-Huu, C.; Ledoux, M. J. Catal. Today 2002, 76, 11. (4) Kusunoki, M.; Rokkaku, M.; Suzuki, T. Appl. Phys. Lett. 1997, 71, 2620. (5) Kusunoki, M.; Suzuki, T.; Hirayama, T.; Shibata, N.; Kaneko, K. Appl. Phys. Lett. 2000, 77, 531. (6) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191. (7) Morkoc, H.; Strite, S.; Gao, G. B.; Lin, M. E.; Sverdlov, B.; Burns, M. J. Appl. Phys. 1994, 76, 1363. (8) Casady, J. B.; Johnson, R. W. Solid-State Electron. 1996, 39, 1409. (9) Ruff, M.; Mitlehner, H.; Helbig, R. IEEE. Trans. Electron. Devices 1994, 41, 1040. (10) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431.
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6H-SiC(0001) surfaces have been achieved, opening up the possibility to investigate metal growth on well-defined SiC surfaces. It is well known that 6H-SiC(0001) √ √ can take √ various √ reconstructions, such as 3 3,11-16 3 3,13,17,18 6 3 6 3,12,19-21 and the graphitized SiC surface.22-24 On the various reconstructed surfaces, the atomic structure and chemical composition differ with each other dramatically. Thus, 6H-SiC(0001) can serve as an ideal substrate to explore the effect of the SiC surface structure on the metal growth and formation of metal/SiC interfaces. In the present work, Pt was deposited onto a 6H-SiC(0001) surface with various surface reconstructions. The growth and thermal stability of Pt overlayers on the 6H-SiC(0001) surfaces were studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The results show that the interaction of Pt with SiC is strongly affected by the substrate surface reconstruction, which demonstrates that engineering the substrate surface could be an effective way to control interface formation, for example, in metal/SiC systems. (11) Kulakov, M. A.; Henn, G.; Bullemer, B. Surf. Sci. 1996, 346, 49. (12) van Elsbergen, V.; Kampen, T. U.; Monch, W. Surf. Sci. 1996, 365, 443. (13) Kulakov, M. A.; Hoster, H.; Henn, G.; Bullemer, B. Mater. Sci. Eng., B 1997, 46, 227. (14) Reuter, K.; Bernhardt, J.; Wedler, H.; Schardt, J.; Starke, U.; Heinz, K. Phys. Rev. Lett. 1997, 79, 4818. (15) Starke, U.; Schardt, J.; Bernhardt, J.; Franke, M.; Reuter, K.; Wedler, H.; Heinz, K.; Furthmuller, J.; Kackell, P.; Bechstedt, F. Phys. Rev. Lett. 1998, 80, 758. (16) Schardt, J.; Bernhardt, J.; Starke, U.; Heinz, K. Phys. Rev. B 2000, 62, 10335. (17) Owman, F.; Martensson, P. Surf. Sci. 1995, 330, L639. (18) Johansson, L. I.; Owman, F.; Martensson, P. Phys. Rev. B 1996, 53, 13793. (19) Owman, F.; Martensson, P. Surf. Sci. 1996, 369, 126. (20) Li, L.; Tsong, I. S. T. Surf. Sci. 1996, 351, 141. (21) Chen, W.; Xu, H.; Liu, L.; Gao, X. Y.; Qi, D. C.; Peng, G. W.; Tan, S. C.; Feng, Y. P.; Loh, K. P.; Wee, A. T. S. Surf. Sci. 2005, 596, 176. (22) Starke, U.; Schardt, J.; Franke, M. Appl. Phys. A: Mater. Sci. Process. 1997, 65, 587. (23) Seyller, T.; Emtsev, K. V.; Gao, K.; Speck, F.; Ley, L.; Tadich, A.; Broekman, L.; Riley, J. D.; Leckey, R. C. G.; Rader, O.; Varykhalov, A.; Shikin, A. M. Surf. Sci. 2006, 600, 3906. (24) Varchon, F.; Feng, R.; Hass, J.; Li, X.; Nguyen, B. N.; Naud, C.; Mallet, P.; Veuillen, J. Y.; Berger, C.; Conrad, E. H.; Magaud, L. Phys. Rev. Lett. 2007, 99, 126805.
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2. Experiment Details All of the experiments were performed in an Omicron multiprobe ultrahigh vacuum (UHV) system with a base pressure of 3.0 10-10 mbar. The system is equipped with various surface analytical techniques, such as XPS, ultraviolet photoelectron spectroscopy (UPS), and STM (Omicron VT-STM).25,26 XPS spectra were acquired with Mg Ka radiation (hν = 1253.6 eV) using a pass energy of 30 eV. STM images were recorded in constant current mode at room temperature (RT) with a W tip. Commercial nitrogen-doped (n-type, ND = 1018 atoms/cm3) 6H-SiC(0001) (Si face) single-crystal wafers (TKHD Research, Inc.) were cut into 3.0 10.0 0.3 mm3 or 5.0 10.0 0.3 mm3 pieces. The crystals were dipped into a 20% hydrofluoric acid (HF) solution for 5 min to remove the surface oxide and then were rinsed with deionized water, acetone, and ethanol. After being loaded into the UHV chamber, each sample was degassed at around 650 °C overnight and annealed at 1000 °C for 1 h to remove any residual surface oxide. Si was deposited onto the clean surfaces at 900 °C for 2 min using a homemade Si source. Subsequently, the samples were stepwise annealed from 900 to 1400 °C in the absence of Si flux. Pt deposition was performed at RT by resistive heating of a tungsten filament wrapped with Pt wire (99.95% purity, Johnson Matthey). Pt overlayers with low and high coverage were obtained by depositing Pt for 1 and 15 min at RT, respectively. Sample heating was carried out by direct resistive heating of SiC crystals. The surface temperature was monitored by an infrared pyrometer with an emissivity of 0.63.
3. Results and Discussion 3.1. 6H-SiC(0001) Surface Reconstructions. Figure 1 displays STM images of six reconstructed 6H-SiC(0001) surfaces that were obtained by annealing the Si-dosed crystal in UHV between 900 and 1400 °C. First, annealing the surface at 900 °C for 5 min produces an ordered structure, which is well known as 3 3 reconstruction (Figure 1a). The atomic structure of the 3 3 surface has been well described by Starke’s model,14-16 which consists of three Si layers on top of bulk Si-C bilayers (i.e., the 1/9 ML Si adatoms, the 3/9 ML Si trimers, and the full Si adlayer). Every protrusion seen in Figure 1a corresponds to a single Si tetramer (adatom þ trimer) arranged in 3 3 periodicity of 9.3 A˚. Additionally, defects appearing as dark regions can be observed on the surface as well and are attributed to a deficiency in the Si tetramer.27 When √ the √ surface is annealed at 1050 °C for 5 min, a well-defined 3 3 structure can be imaged (Figure 1b). The atomic structure of this reconstruction was proposed to consist of 1/3 ML Si adatoms sitting at T4 positions of the top Si-C bilayers.17,20,28 Every Si adatom is exhibited as a bright dot in the STM image, and the surface is 5.4 A˚ (inset in √ periodicity √ Figure 1b). Both 3 3 and 3 3 structures are Si-terminated surfaces, but the former one is more Si-rich than the latter. With the annealing temperature above 1100 °C, a relatively complicated structure appears, which was reported to √ transitional √ have a 6 3 6 3-R30° low-energy√electron √ diffraction (LEED) pattern and was thus referred to as 6 3 6 3 reconstruction.18,19 When the SiC(0001) surface is annealed above 1100 °C, the surface is subject to evolution from Si-rich to C-rich and thus presents different atomic structures depending on the temperature. Figure 1c,d presents two typical STM images, which were (25) Zhang, Z.; Fu, Q.; Zhang, H.; Li, Y.; Yao, Y. X.; Tan, D.; Bao, X. H. J. Phys. Chem. C 2007, 111, 13524. (26) Yao, Y. X.; Liu, X.; Fu, Q.; Li, W. X.; Tan, D. L.; Bao, X. H. ChemPhysChem 2008, 9, 975. (27) Gasparov, V. A.; R.-C., M.; Schroter, B. Europhys. Lett. 2000, 51, 527. (28) Coati, A.; Sauvage-Simkin, M.; Garreau, Y.; Pinchaux, R.; Argunova, T.; Aid, K. Phys. Rev. B 1999, 59, 12224.
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Figure 1. STM images (50 nm 50 nm) of the 6H-SiC(0001) √ √ surface with different reconstructions. (a) 3 3; (b) 3 3 √ √ (the inset: 5 nm 5 nm); (c) 6 3 6 3; (d) carbon nanomesh (the inset: 10 nm 10 nm); (e) graphene/6H-SiC(0001) surface (the inset: 5 nm 5 nm); and (f) graphitized SiC surface (the inset: 5 nm 5 nm).
recorded on the surface after annealing at 1150 and 1200 °C, respectively. The former one is similar to the 6 6 superstructure reported previously,19 and the surface is composed of asteriskshaped, trimerlike, and some irregularly shaped units. The surface shown in Figure 1d displays a long-range-ordered honeycomb structure, which is called a carbon nanomesh by Chen et al.21,29 A high-resolution image shows that each mesh consists of √ three bright corners and one black hole (inset of Figure 1d). The 6 3 √ 6 3 reconstructed surface is enriched by carbon as shown below, which is, however, not yet in graphitic form.18,19 Chen et al.21 proposed a 6 6 model for the carbon nanomesh structure in which one-atom-layer-thick isolated carbon islands assemble to form the nanomesh structure and cover the entire surface. At 1250 °C, graphitized carbon begins to form on the SiC surface (Figure 1e). A typical atomic-resolution STM image indicates the appearance of monolayer graphene, which shows a fine honeycomb structure (inset of Figure 1e).30 When the 6HSiC(0001) sample was annealed to 1400 °C, thicker graphite layers formed. In contrast to monolayer graphene, the multilayer graphene surfaces present a triangular lattice with three C atoms of the hexagonal ring visible as protrusions30-32 (Figure 1f and inset). The stepwise annealing of the 6H-SiC(0001) surface at elevated temperatures was further investigated by XPS. Figures 2 and 3 show Si 2p and C 1s spectra from the 6H-SiC(0001) surface annealed at the indicated temperatures successively, which were recorded at takeoff angles of 90 and 30°, respectively. Between 900 and 1000 °C, the main peak of the Si 2p signal is located at 101.3 eV with a weak shoulder at 99.6 eV. The intensity of this shoulder peak increases at the more surface-sensitive takeoff angle of 30°, which suggests that this Si 2p component is from surface Si species, probably Si bonded with Si in the deposited Si adlayers.33 The intensity of the Si 2p shoulder peak decreases with increasing temperature, and the main Si 2p component remains at 101.3 eV. From 1050 to 1150 °C, the main peaks in the Si 2p spectra show a rigid, positive BE shift of 0.9 eV. (29) Chen, W.; Loh, K. P.; Xu, H.; Wee, A. T. S. Appl. Phys. Lett. 2004, 84, 281. (30) Mallet, P.; Varchon, F.; Naud, C.; Magaud, L.; Berger, C.; Veuillen, J. Y. Phys. Rev. B 2007, 76, 041403. (31) Lauffer, P.; Emtsev, K. V.; Graupner, R.; Seyller, T.; Ley, L.; Reshanov, S. A.; Weber, H. B. Phys. Rev. B 2008, 77, 155426. (32) Huang, H.; Chen, W.; Chen, S.; Wee, A. T. S. ACS Nano 2008, 2, 2513. (33) Amy, F.; Soukiassian, P.; Hwu, Y. K.; Brylinski, C. Surf. Sci. 2000, 464, L691.
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Figure 2. XPS Si 2p (a) and C 1s (b) spectra from the 6H-SiC(0001) surface annealed to the indicated temperatures. The spectra were recorded at a takeoff angle of 90°.
Figure 3. XPS Si 2p (a) and C 1s (b) spectra from the 6H-SiC(0001) surface annealed to the indicated temperatures. The spectra were recorded at a takeoff angle of 30°.
C 1s spectra show one main peak below 1050 °C. At 1100 °C, a new surface C 1s component at 285.8 eV was observed and its intensity increases upon annealing at 1150 °C. STM results already indicate that carbon √ starts√to accumulate on the surface to form a well-developed 6 3 6 3 surface at these temperatures. The angle-resolved XPS experiments confirm that the intensity of the new C 1s component becomes higher at smaller takeoff angles (Figures 2b and 3b). Therefore, the new C 1s component should originate from the surface carbon species.34,35 Above 1200 °C, the peak shifts to lower BE and is finally located at 285.0 eV, close to the C 1s BE from the graphite surface. It should be noted that a similar rigid, positive BE shift of 0.9 eV was also observed in the main peaks of the C 1s spectra between 1050 and 1150 °C. On the basis of the above results, the evolution of the 6HSiC(0001) surface with annealing temperature can be summarized as follows: (1) 900-1100 °C: Si-rich surface √ in√ the presence of surface Si adatoms, including 3 3 and 3 3 structures; (2) 1100-1200 °C: transformation of the Si-rich surface into a C-rich surface to form one-atom-layer-thick carbon islands; (3) above 1200 °C: graphitic carbon overlayers on SiC(0001). 3.2. Pt Deposition on 6H-SiC(0001) Surfaces. The abovementioned three typical 6H-SiC(0001) surfaces were chosen as substrates for Pt deposition: the Si-rich 3 3 surface, the C-rich (34) Johansson, L. I.; Owman, F.; Martensson, P.; Persson, C.; Lindefelt, U. Phys. Rev. B 1996, 53, 13803. (35) Martensson, P.; Owman, F.; Johansson, L. I. Phys. Status Solidi B 1997, 202, 501.
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Figure 4. XPS Pt 4f spectra recorded from the 3 3, carbon nanomesh, and graphitized SiC surfaces after Pt deposition times of 1 min (a) and 15 min (b) at RT.
carbon nanomesh surface, and the graphitized SiC surface. Figure 4 shows XPS Pt 4f spectra recorded from the surfaces after Pt deposition times of 1 and 15 min at RT. With 1 min of Pt growth, BEs of Pt 4f7/2 are 73.0, 72.3, and 71.8 eV on the 3 3, carbon nanomesh, and graphitized SiC surfaces, respectively. With high-coverage deposition (15 min for Pt), the Pt 4f7/2 BE shifts to 72.0 eV on the carbon nanomesh surface while still being located at 73.0 and 71.8 eV on the 3 3 and graphitized SiC surfaces (Figure 4b). The much higher Pt 4f7/2 BE of Pt overlayers on the 3 3 surface suggests the formation of Pt silicide.36 A similar silicide reaction was also observed in the case of Co growth on the Si-rich surfaces.29,37,38 On the carbon nanomesh and graphitized SiC surfaces, we infer that metallic Pt should form according to the Pt 4f BE values (Figure 4). Higher BEs than that from bulk Pt are due to the cluster size effect in Pt nanoparticles grown on the two surfaces (STM results below).39-41 It is known that at the given coverage a wetting metal overlayer would produce stronger photoemission signals than a nonwetting overlayer. Thus, the intensity of the XPS Pt 4f spectra can be used to identify Pt lateral dispersion on the three surfaces. At the same coverage, the strongest Pt 4f signal on the 3 3 surface indicates the highest lateral dispersion of the deposited Pt and the weakest intensity on the graphitized SiC surface demonstrates the aggregation of the Pt overlayers. Obviously, the strong interaction between Pt and the 3 3 surface (i.e., silicide formation) results in better wetting of Pt and higher Pt 4f intensity. STM images of the Pt/6H-SiC(0001) surfaces are displayed in Figure 5. After 1 min of Pt deposition onto the 3 3 surface, Pt silicide clusters, which are about 0.2 nm high and 2 nm in diameter, distribute homogeneously on the terraces. The 3 3 periodicity becomes diffuse, and only small patches with unperturbed 3 3 reconstruction can be observed, as shown in Figure 5b. At high coverage (15 min Pt), silicide clusters grow in both size and density such that the 3 3 arrangement has been completely destroyed (Figure 5c). After the Pt deposition, a weak, negative BE shift ( carbon nanomesh > graphitized SiC surface. 3.3. Stability of Pt Overlayers on the 6H-SiC(0001) Surfaces. To investigate the thermal stability of Pt overlayers on the three 6H-SiC(0001) surfaces, the 15 min Pt/6H-SiC(0001) surfaces were annealed at elevated temperatures and subsequently studied by XPS and STM. Annealing the Pt/6H-SiC(0001)-3 3 sample at 600 °C did not cause any obvious changes in either the peak position or the peak intensity of the Pt 4f, C 1s, and Si 2p XPS spectra (Figure 6). However, the Pt 4f intensity decreased to 50% after annealing at 800 °C. At the same time, the STM image recorded from this surface shows no aggregation of Pt silicide clusters but rather an ordered structure (Figure 6d). It presents a typical two-level of structure43,44 with a surface periodicity √ √ 1.2 nm, which is referred to as a silicide-induced 2 3 2 3 reconstruction.45 Accordingly, we suggest that the surface annealed at 800 °C should be subject to the inward diffusion of Pt, which thus results in the decrease in the Pt 4f signal. From 1000 to 1200 °C, the Pt 4f intensity, however, increases again. It has been shown that hightemperature annealing causes the evaporation of Si adatoms (43) Katayama, M.; Williams, R. S.; Kato, M.; Nomura, E.; Aono, M. Phys. Rev. Lett. 1991, 66, 2762. (44) Lander, J. J.; Morrison, J. Surf. Sci. 1964, 2, 553. (45) Hoshino, Y.; Matsubara, Y.; Nishimura, T.; Kido, Y. Phys. Rev. B 2005, 72, 235416.
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Figure 8. XPS Pt 4f (a), Si 2p (b), and C 1s (c) spectra of the Pt/graphitized SiC surface annealed at the indicated temperatures.
(section 3.1) on the Si-rich surface. The increase in the Pt 4f signal can be attributed to the removal of top surface Si and the weakened attenuation of Pt 4f photoelectrons by Si layers. At 1200 °C, a new C 1s component that peaked at 284.8 eV was observed, which is characteristic of the surface graphitic carbon. In comparison with the bare 3 3 surface (Figures 2 and 3), graphitic carbon forms on the Pt/6H-SiC(0001)-3 3 surface at 1200 °C whereas 1250 °C is necessary on the bare 3 3 surface. Thus, Pt induces earlier graphitization on 6H-SiC(0001). Annealing the Pt/6H-SiC(0001)-3 3 surface from 800 to 1200 °C did not lead to any further BE shifts. On the carbon nanomesh surface, the Pt 4f intensity decreases a little when annealing at 600 and 800 °C (Figure 7). The STM image of the surface annealed at 600 °C demonstrates that only weak agglomeration of Pt nanoclusters occurs (Figure 7d). Above 1000 °C, Pt 4f7/2 BE shifts to 72.7 eV, suggesting the formation of Pt silicide. At the same time, the Pt 4f intensity increases and is even larger than that of the as-deposited Pt overlayers. We infer that the grown Pt silicide should form a wetting layer between the surface carbon layer and the substrate surface. The transformation from 3D Pt nanoclusters to a 2D Pt silicide layer results in the increase in the XPS Pt signal. At the same time, Pt-induced earlier graphitization was also observed at the Pt/6H-SiC(0001)-carbon nanomesh surface. The graphitized C 1s signal at 284.8 eV was detected at an annealing temperature of 1000 °C. Finally, the main peaks of the Si 2p and C 1s spectra present a large, negative BE shift of 1.0 eV. On the Pt/graphitized SiC surface, there are no obvious changes in the line position and line shape in the Pt 4f, Si 2p, and C 1s spectra (Figure 8), which indicates that no further interaction occurs between Pt and the SiC substrate during annealing. However, the Pt 4f intensity keeps on decreasing in the investigated temperature range, which indicates that Pt on the graphitized SiC surface is subject to aggregation upon annealing. 3.4. Band Bending at the Pt/SiC(0001) Interfaces. As mentioned earlier, XPS Si 2p and C 1s spectra have been subject to rigid BE shifts upon various treatments. The phenomena can be explained by surface band bending at metal/semiconductor interfaces. To show the changes in the interfacial electronic structure, energy band diagrams of four 6H-SiC(0001) interfaces are shown in Figure 9. Strong upward band bending happens at the 3 3 surface (Figure 9a) because Si adatoms on the Si-rich SiC(0001) surface (46) Emtsev, K. V.; Seyller, T.; Ley, L.; Broekman, L.; Tadich, A.; Riley, J. D.; Leckey, R. G. C. Phys. Rev. B 2006, 73, 075412.
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Figure 9. Energy band diagrams showing the electronic structures of the 3 3 surface (a), the carbon nanomesh surface (b), the Ptsilicide/6H-SiC(0001)-3 3 interface (c), and the C/Pt-silicide/6HSiC(0001) interface (d). eV1, eV2, eV3, and eV4 represent the maximum band bending at these interfaces.
produce surface states pinning the Fermi level (EF).46,47 Successive UHV annealing of the 3 3 surface leads to surface Si evaporation, which eliminates the Si adatoms’ related surface states. It could lead to downward bending of the surface energy band back to a weak upward bending condition (Figure 9b).48 On the basis of the BE shift of the Si 2p and C 1s spectra observed in Figures 2 and 3, the difference in the band bending strength between the 3 3 surface and the carbon nanomesh surface is equal to 0.9 eV (eV1 - eV2 = 0.9 eV). (47) King, S. W.; Davis, R. F.; Nemanich, R. J. Surf. Sci. 2009, 603, 3104. (48) Seyller, T.; Emtsev, K. V.; Speck, F.; Gao, K. Y.; Ley, L. Appl. Phys. Lett. 2006, 88, 242103.
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After 15 min of Pt deposition on the 3 3 surface at RT, further weak upward band bending occurs, which could be inferred from the negative BE shift of 0.2 eV (eV3 - eV1 = 0.2 eV, Figure 9c). Because Pt silicide forms upon Pt deposition on the 3 3 surface at RT, the upward band bending can be attributed to Fermi level pinning by silicide-induced interface states. Pt deposition on the carbon nanomesh at RT does not produce any changes in the surface energy band of SiC(0001). However, annealing the 15 min Pt/6H-SiC(0001)-carbon nanomesh interface at 1000 °C causes the formation of Pt silicide and a -1.0 eV BE shift in the Si 2p and C 1s spectra (Figure 7). Like the Pt/6HSiC(0001)-3 3 interface, the electronic states produced by formed silicide lead to the strong upward band bending (eV4 eV2 = 1.0 eV, Figure 9d). The close Si 2p and C 1s core-level positions on the bare 3 3 surface and the Pt silicide/6HSiC(0001) surface suggest that the new electronic states related to Pt silicide have a similar energy level to that of the Si adatoms’ related states.
4. Conclusions Three reconstructed 6H-SiC(0001) surfaces were prepared by UHV annealing at elevated temperatures, √ √ which include the Sirich 3 3 surface, the C-rich 6 3 6 3 surface with carbon nanomesh structure, and the graphitized SiC surface. The growth and thermal stability of Pt overlayers show different behaviors on the three surfaces: (1) Pt silicide forms on the 3 3 surface, producing the Pt silicide/6H-SiC(0001) interface even at RT. The presence of Pt induces earlier surface graphitization and forms the C/Pt silicide/6H-SiC(0001) interface at 1200 °C. The high reactivity of the 3 3 surface
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(2)
(3)
(4)
should originate from the surface Si adatoms and the resulting surface electronic states. Highly dispersed Pt nanoclusters grow on the carbon nanomesh surface, which could be stable until 800 °C. The honeycomb nanomesh surface structure helps to anchor metal particles, suggesting that this surface could be an ideal template for the growth of metal nanoclusters. Above 1000 °C, the Pt nanoclusters tend to diffuse into the subsurface region, producing a wetting Pt silicide layer there. Pt nanoparticles with low lateral dispersion and poor spatial distribution grow on the graphitized SiC surface and aggregate gradually upon UHV annealing. The presence of Si adatoms and Pt silicide induces strong upward band bending at the bare 6H-SiC(0001)-3 3 surface and the Pt silicide/6H-SiC(0001) interface, respectively.
Accordingly, the interaction between the Pt and SiC(0001) surfaces follows the trend of 3 3 surface > carbon nanomesh > graphitized SiC surface. The present results demonstrate that the surface reconstruction presents an effective route to manipulate the interface structure between metal and the semiconductor substrate. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (nos. 20603037, 20733008, and 20873143), the Ministry of Science and Technology of China, and the Chinese Academy of Sciences (Bairen program).
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