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Photoemission study of the initial oxidation of 6H-SiC(0001¯)-(2×2)C
Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44
¯ Photoemission study of the initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C C.W. Zou a,b , B. Sun b , Y.Y. Wu b , P.S. Xu a,b,∗ , H.B. Pan b , F.Q. Xu b a
Structure Research Laboratory, University of Science and Technology of China, Academia Sinica, Hefei 230026, China b NSRL, University of Science and Technology of China, Hefei 230029, China Received 16 June 2005; received in revised form 21 October 2005; accepted 21 October 2005 Available online 28 November 2005
Abstract ¯ Initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C surface was studied at different conditions by photoelectron spectroscopy using synchrotron radiation (SRPES). Oxidation process is performed at room temperature and at 800 ◦ C under different oxygen pressures. It was found that oxygen atoms prefer bonding with the bilayer silicon atoms located below the top surface, while the topmost adatoms are hardly oxidized at room temperature even at very high oxygen exposure. However, by elevating the temperatures and oxygen pressures, the SiC surface was oxidized and the thickness of the oxide layer was estimated, taking the refraction effect of low energy photoelectrons into account. © 2005 Elsevier B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Oxidation; Silicon carbide; Crystal surface
1. Introduction Silicon carbide is a promising semiconductor material for high power electronics in harsh environment due to a combination of remarkable properties such as high melting point, high breakdown fields, wide band gap and large electron mobility. Thus, it is regarded as the first candidate of the substrate material for the metal-oxide-semiconductor field-effect transistor (MOS-FET) of the next generation [1,2]. The (0 0 0 1) surface of hexagonal SiC crystals of the 4H (and 6H) poly-type has been considered the most suitable for device applications [3,4]. Now large size SiC substrates of different poly-types with high quality are commercially available, but a good SiO2 /SiC interface with low defect density is not realized yet, which is of crucial importance for the performance of electronic devices. Now people have tried different approaches to form oxides/SiC interface [5–8]. While good SiO2 /SiC interface of hexagonal SiC is not easily formed due to drastic surface reconstructions and different surface structures induced by different surface treatments. For example, the clean surface of Si-terminated 6H-SiC(0 0 0 √ 1) shows √ four √ kinds √ of surface reconstructed structures: 3 × 3, 3 × 3, 6 3 × 6 3 and 6 × 6 with different annealing temperatures after Si-flux deposition in
∗
Corresponding author. Tel.: +86 551 3650821; fax: +86 551 5141078. E-mail address: [email protected] (P.S. Xu).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.10.006
ultra-high vacuum (UHV) [9,10]. In contrast, the C-terminated ¯ surface gives 3 × 3, (2 × 2)C and (1 × 1)graphite 6H-SiC(0 0 0 1) reconstructions by annealing at different temperatures [11–13], and the (2 × 2)Si structure by annealing in Si-flux [14]. Atomic configurations of above reconstructed surfaces are frequently studied by STM and LEED intensity curves. For the 3 × 3 reconstructed surface, it is regarded that silicon adatoms are supported by trimeric√silicon √ adlayers located on the first Si–C bilayer [15]. The 3 × 3-SiC (0 0 0 1) and (2 × 2)C ¯ have much similar atomic configurations. The forSiC(0 0 0 1) mer consists of Si adatoms of 1/3 monolayer located at T4 site on the Si–C bilayer [16], while the latter is formed by Si adatoms on the H3 position of the Si–C bilayer [17]. For different surface reconstructions, the surface oxidation has recently been investigated by experimental spectroscopic techniques. Hoshino √et al.√[18] investigated the initial oxidation of 6H-SiC(0 0 0 1)- 3 × 3 surface by SRPES and showed all suboxide states (Si+ , Si2+ , Si3+ and Si4+ ) from Si 2p spectra. Amy et al. [19] investigated the SiC (0 0 0 1)-3 × 3 surface oxidation from 25 to 650 ◦ C to reveal the initial oxide SiC interface. In their experiment they also found all suboxide states of silicon atoms. For the same oxidation process, however,√Virojanadara √ and Johansson reported different results. For the 3 × 3 surface [20,21] they claimed that they only recognized the Si+ and ¯ ×3 Si4+ states, and for the carbon terminated 6H-SiC(0 0 0 1)-3 surface [22], they found only one suboxide component during the oxidation experiments. To explain the mechanism of the
C.W. Zou et al. / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44
oxidation process on the SiC surface, many related models have been proposed. Jernigan et al. [23] suggested a mechanism for both Si-face and C-face of the 6H-SiC(0 0 0 1) surface. They indicated that there was a transition layer between the SiC substrate and the oxide, which contained Si Si bonds. This is related to the high interfacial trap density and may be responsible for the low channel mobility in SiC. Thermal oxidation of 6H-SiC was investigated [24] by means of the isotope tracing and narrow nuclear resonant reaction profiling techniques. The results showed that oxygen atoms were incorporated in the bulk and interface oxide regions during thermal oxidation of SiC. In this present work, we investigate the oxidation of ¯ 6H-SiC(0 0 0 1)-(2 × 2)C surface at different temperatures and oxygen exposures by using the synchrotron radiation photoemission spectroscopy (SRPES). According to the curve fitting for Si 2p spectra, all of the silicon suboxides and surface related components are observed during the oxidation process. From the Si 2p relative intensity ratio of each suboxides, we estimate the thickness of each oxide and clarify the kinetics of the initial oxidation at RT and high temperature. 2. Experiment The experiment is performed at the surface physics station of National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. This experimental station is mainly composed of a threechamber VG multi-technique UHV system. It consists of a deposition chamber, a pre-treatment chamber with a fast entry lock and an analysis chamber equipped with ARUPS10 hemispherical analyzer. Sample can be cleaned by cycles of Ar+ ions etching and e-beam annealing in the pre-treatment chamber and transferred to the analysis chamber for SRPES. The XPS, LEED, RHEED and AES are also equipped and all these systems work under UHV conditions (better than 2 × 10−10 mbar). The beamline covers the energy range from 10 to 300 eV and the energy resolution (E/E) is better than 1000. More details of the experiment station and the related beamline are described elsewhere [25]. The samples used in this experiment are Si-terminated 6HSiC(0 0 0 1) single crystal from Cree. Research Corp. To obtain clean surface, the following processes are operated: (1) rinsed in acetone for 10 min with an ultrasonic cleaner, (2) dipped in the solution of HF and ethanol (5% HF) for several minutes, (3) dipped into acetone and finally cleaned by de-ionized water. After these treatments, the sample was immediately introduced into the UHV chamber and degassed by heated at 550 ◦ C for about 5 h with e-beam heater. Then we can observe LEED spots, But the XPS spectra shows that there is still a small mount of oxygen components on the surface. Cycles of slight Ar+ ions etching and annealing at 950 ◦ C in the UHV chamber are performed until no oxygen signal exists in XPS measurement. Now clear 2 × 2 LEED pattern is observed in Fig. 1. Annealing at higher temperature induces much sharper 2 × 2 LEED pattern just as shown in the insert. The obtained LEED pattern is quite consis¯ tent with the other reports from C-terminated 6H-SiC(0 0 0 1) [11,26]. Then the prepared 2 × 2 surface is oxidized at RT and 800 ◦ C in different oxygen pressures (purity ∼ 99.99%).
41
After each oxidation, the Si 2p core level was recorded with SRPES at emission angle of 45◦ related to the surface normal direction. 3. Results and discussion ¯ surface 3.1. The clean 6H-SiC(0 0 0 1) In Fig. 1, we can observe a clear 2 × 2 LEED pattern, indicating that our surface treatment method induces the formation ¯ of 6H-SiC(0 0 0 1)-(2 × 2)C reconstruction from the original Siterminated 6H-SiC(0 0 0 1) surface. It is reasonable because the terminated Si atoms of 6H-SiC(0 0 0 1) surface are almost removed by hydrogen fluoride acid and Ar+ ions etching, and further annealing at high temperature is prone to form the (2 × 2)C surface reconstruction. ¯ The Si 2p core level spectrum from the 6H-SiC(0 0 0 1)(2 × 2)C clean surface with photon energy of 150 eV is shown in Fig. 2. The background has been subtracted by the Shirley method. Then the original spectra can be fitted by three components, where the bulk Si 2p spin–orbit splitting doublet peaks are simulated by Gaussian shapes with a branching ratio of 0.5 and a spin–orbit splitting of 0.63 eV. The other two components marked with S1 (100.82 eV) and S2 (100.03 eV) are due to the ¯ surface contributions. Fig. 3 shows the 6H-SiC(0 0 0 1)-(2 × 2)C surface model of SiC [16], where a silicon adatom is located on the H3 site on the Si–C bilayer in a 2 × 2 unit cell. Similar to the earlier report [27], the S1 and S2 surface components are assigned to the Si atoms of the topmost Si–C bilayer and the silicon adatoms, respectively. 3.2. Oxidation at different conditions ¯ During the oxidation process, the 6H-SiC(0 0 0 1)-(2 × 2)C clean surface is oxidized for 5 min at RT with oxygen pressures
Fig. 1. The 6H-SiC(0 0 0 1)-2 × 2 LEED pattern. The inset is the LEED pattern when sample is annealing at higher temperature in the UHV chamber.
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C.W. Zou et al. / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44
Fig. 2. Si 2p core level observed for 150 eVphoton incidence on the 6HSiC(0 0 0 1)-2 × 2 clean surface at emission angle of 45◦ related to the surface normal direction.
of 1.2 × 10−8 , 1.6 × 10−7 and 1.4 × 10−6 Torr, respectively. Fig. 4(a) shows the Si 2p core level spectrum at photon energy of 150 eV for the surface oxidized at RT. The oxygen exposure is about 3.6 L (1 L = 1 × 10−6 Torr s). The observed spectrum is decomposed into seven components: two surface components (S1 and S2), bulk component (101.50 eV) and four oxidation states (Si+ , Si2+ , Si3+ and Si4+ ) located at the binding energy of 102.08, 102.92, 103.4 and 104.12 eV, respectively. The Gaussian width (full width at a half maximum, FWHW) for each component is about 0.7 eV besides the Si4+ peak, which stands by a single broad peak with larger FWHW. The Si 2p related binding energy (BE) shifts referred to the SiC bulk state are shown in Table 1. Our result of Si4+ state BE shift is quite close to the ear¯ surface [28]. The oxide-related lier report for 4H-SiC(0 0 0 1) Si2p BE shifts for some other structures are also listed in the table [29–31]. The peaks due to the surface states still remain,
Fig. 4. The Si 2p core level recorded after the SiC clean surface has been oxidized at RT with different oxygen pressure: (a) 1.2 × 10−8 Torr for 5 min; (b) 1.6 × 10−7 Torr for 5 min; (c) 1.4 × 10−6 Torr for 5 min.
¯ Fig. 3. The surface model for 6H-SiC(0 0 0 1)-(2 × 2)C unit cell structure in side view (upper panel) and top view (lower panel).
although the intensity is a little decreased comparing with those of the clean surface in Fig. 2. Our result is quite similar to the oxidation behavior of 6H-SiC(0 0 0 1)-3 × 3 surface investigated by Amy et al. [29]. They found that the oxygen prefers reacting
C.W. Zou et al. / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44 Table 1 BE shifts relative to the bulk state BE
Table 2 ˚ calculated from peak decomposition and Fractions of Si-oxides thickness (A) attenuation principle
Si 2p related BE shifts (eV)
Present (2 × 2)C ¯ × 3 [20] 4H-SiC(0 0 0 1)-3 6H-SiC(0 0 0 1)-3 × 3 [21] √ √ 6H-SiC(0 0 0 1)- 3 × 3 [22] Si(1 1 1) [23]
Si+
Si2+
Si3+
Si4+
0.58
1.42 1.5 1.36 1.07 1.75
1.9
2.62 2.7
0.25 0.5 0.95
2.48 1.63 2.42
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2.2 3.30
with silicon atoms located well below the surface, but not the top surface atoms under very low oxygen exposure regime. Thus, at low oxygen exposure, the surface states can still be observed in the Si 2p spectrum. Fig. 4(b) shows the Si 2p spectrum after the oxidation process of oxygen exposure of 1.6 × 10−7 Torr for 5 min (about 48 L). The Si4+ peak grows rapidly, indicating that sizable SiO2 domain has been formed after this process. Note that the surface related peaks still appeared, although their intensities are reduced. It means that silicon atoms on top of surface have been partly oxidized. Fig. 4(c) shows the Si 2p spectrum after further oxygen exposure (about 420 L). The spectral shape in Fig 4(c) has little difference with that in Fig. 4(b). This means that the surface has been saturated after the second oxidation process. The saturated stage might be reached by oxygen exposure less than 48 L, but further careful experiment is necessary to determine the critical point. What is the reason for this oxidation saturation behavior? One possibility could be the Mott-Cabrera mechanism, as it has been discussed for the oxidation of 6H-SiC(0 0 0 1) by van Eisbergen et al. [32]. Fig. 5 shows the Si 2p spectrum after annealing at 800 ◦ C in oxygen with a pressure of 1.5 × 10−6 Torr for 20 min (1800 L oxygen exposure). According to our curve fitting procedure, the surface related peaks S1 and S2 almost disappeared though the S1 peak still has small intensity as shown in the figure. This indicates that at high oxygen exposure and high oxidation temperature, the topmost silicon adatoms are almost oxidized. The
420 L oxygen exposure at RT 1800 L oxygen exposure at 800 ◦ C
Si+
Si2+
Si3+
Si4+
Total thickness
1.8 1.23
0.65 1.9
0.92 1.78
2.3 5.6
5.67 10.51
oxidized states related peak intensity significantly increases. It means that the temperature of sample induced more oxidation of silicon atoms. Finally, a saturation of oxidation probably occurs by accumulated strain-induced distortion, which leads to a close-packed structure at the top surface and suppresses further oxidation. From relative intensity ratio of Si 2p spectral peaks we can estimate the respective thickness of silicon oxides by using the photoelectron attenuation layer method [33]. Here we let elec˚ according to the earlier report tron attenuation length equal 4.2 A [34]. Another point should be noticed is that the photon energy in our experiment is 150 eV. Since the excited photoelectrons have low kinetic energy, we must account for surface refraction. This means that the internal emission angle ϕ and the external angle θ are different and their relation can be described by EK sin ϕ = sin θ EK + V 0 where V0 is the inner potential (about 15 eV for silicon carbide) and EK is the kinetic energy of photoelectrons in vacuum. After considering the surface refraction due to the low photon energy, we obtain the final result for each oxidation layer from the integrated intensities of the Si 2p peak decomposition. The detailed values are shown in Table 2. It gives thickness for each sub˚ oxide. At 420 L oxygen exposure, the SiO2 thickness is 2.3 A and the ratio to total mount of oxides is 40%, while at 800 ◦ C with 1800 L oxygen exposure, the ratio increases to 53%. This means that at higher oxygen exposure and higher temperature, the oxidation process becomes much more pronounced. 4. Conclusion
Fig. 5. The Si 2p core level recorded after oxidation in 1.5 × 10−6 Torr for 20 min at 800 ◦ C.
¯ The initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C surface is analyzed in situ by SRPES. The (2 × 2)C surface structure is prepared directly from the Si-terminated 6H-SiC(0 0 0 1) by chemical treatment, cyclic Ar+ sputtering and annealing in UHV chamber. The oxidation was performed at RT and 800 ◦ C under different oxygen pressures. During the oxidation process at RT the two surface components always existed even at high oxygen exposure. Our result shows that at RT the oxidation behavior ¯ of 6H-SiC(0 0 0 1)-(2 × 2)C surface is quite similar to the oxidation model for 6H-SiC(0 0 0 1)-3 × 3 surface at very low oxygen exposure. Furthermore, this model is also right to describe the oxidation behavior for our sample at higher oxygen exposure. While at high temperature and much higher oxygen pressure, the surface oxidation process becomes very pronounced and the surface components related peaks almost disappear, indicating that the surface adatoms are fully oxidized. Considering low
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energy photoelectrons surface refraction, the thickness of each suboxides was calculated. Acknowledgement This work is supported by the Project of Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20030358054). References [1] G. Harris, Properties of Silicon Carbide, EMIS Data-review Series, vol. 13, INSPEC, London, 1995. [2] V.M. Aroutiounian, V.V. Bouniatian, P. Soukiassian, Solid State Electron. 43 (1999) 343. [3] H. Morkoc, S. Srite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [4] H.F. Li, S. Dimitrijev, D. Sweatman, H.B. Harrison, P. Tanner, B. Feil, J. Appl. Phys. 86 (1999) 4316. [5] V.M. Bermudez, Appl. Surf. Sci. 84 (1995) 45. [6] L. Simon, L. Kubler, A. Ermolieff, T. Billon, Phys. Rev. B 60 (1999) 5673. [7] V.R. Vathulya, D.N. Wang, M.H. White, Appl. Phys. Lett. 73 (1998) 2161. [8] H.F. Li, S. Dimitrijev, H.B. Harrison, D. Sweatman, Appl. Phys. Lett. 70 (1997) 2028. [9] L. Li, I.S.T. Tsong, Surf. Sci. 351 (1996) 141. [10] F. Owman, P. Martensson, Surf. Sci. 369 (1996) 126. [11] J. Bernhardt, A. Seubert, M. Nerding, U. Starke, K. Heinz, Mater. Sci. Forum 338–342 (2000) 345. [12] L.I. Johansson, P.A. Glans, N. Hellgren, Surf. Sci. 405 (1998) 288. [13] M. Hollering, B. Mattern, F. Maier, L. Ley, A.P.J. Stamp, J. Xue, J.D. Riley, R.C.G. Leckey, Mater. Sci. Forum. 264–268 (1998) 331. [14] I. Forbeaux, J.M. Themlin, J.M. Debever, Surf. Sci. 442 (1999) 9.
[15] U. Stark, J. Schardt, J. Bernhardr, M. Franke, Phys. Rev. Lett. 80 (1998) 758. [16] A. Coati, M.S. Simkin, Y. Garreau, R. Pinchaux, T. Argunova, K. Aid, Phys. Rev. B 59 (1999) 12224. [17] A. Seubert, J. Bernhardt, M. Nerding, U. Starke, K. Heinz, Surf. Sci. 454–456 (2000) 45. [18] Y. Hoshino, T. Nishimura, T. Yoneda, K. Ogawa, H. Namba, Y. Kido, Surf. Sci. 505 (2002) 234. [19] F. Amy, P. Soukiassian, Y.K. Hwu, C. Brylinski, Phys. Rev. B 65 (2002) 165323. [20] C. Virojanadara, L.I. Johansson, Surf. Sci. 505 (2002) 358. [21] C. Virojanadara, L.I. Johansson, Surf. Sci. 519 (2002) 73. [22] C. Virojanadara, L.I. Johansson, J. Phys.: Condens. Matter 16 (2004) S1783. [23] G.G. Jernigan, R.E. Stahlbush, N.S. Saks, Appl. Phys. Lett. 77 (2000) 1437. [24] C. Radtke, I.J.R. Baumvol, B.C. Ferrera, F.C. Stedile, Appl. Phys. Lett. 85 (2004) 3402. [25] C. Zou, B. Sun, W. Zhang, G. Wang, Nucl. Instrum. Methods A 548 (2005) 574. [26] A. Seubert, D.K. Saldin, J. Bernhardt, U. Starke, K. Heinz, J. Phys.: Condens. Matter 12 (2000) 5527. [27] L.I. Johansson, F. Owman, P. Martensson, Phys. Rev. B 53 (1996) 13793. [28] C. Virojanadara, L.I. Johansson, Surf. Sci. 505 (2002) 358. [29] F. Amy, H. Enriquez, P. Soukiassian, P.-F. Storino, Y.J. Chabal, A.J. Mayne, G. Dujardin, Y.K. Hwu, C. Brylinski, Phys. Rev. Lett. 86 (2001) 4342. [30] Y. Hoshino, T. Nishimura, T. Yoneda, K. Ogawa, H. Namba, Y. Kido, Surf. Sci. 505 (2000) 234. [31] T. Nishimura, Y. Hoshino, H. Namba, Y. Kido, Surf. Sci. 461 (2000) 146. [32] V. van Eisbergen, O. Janzen, W. M¨onch, Mater. Sci. Eng. B 46 (1997) 366. [33] F.J. Himpsel, F.R. McFrrly, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys. Rev. B 38 (1988) 6084. [34] P.J. Cumpson, M.P. Seah, Surf. Int. Anal. 25 (1997) 430.
¯ Photoemission study of the initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C C.W. Zou a,b , B. Sun b , Y.Y. Wu b , P.S. Xu a,b,∗ , H.B. Pan b , F.Q. Xu b a
Structure Research Laboratory, University of Science and Technology of China, Academia Sinica, Hefei 230026, China b NSRL, University of Science and Technology of China, Hefei 230029, China Received 16 June 2005; received in revised form 21 October 2005; accepted 21 October 2005 Available online 28 November 2005
Abstract ¯ Initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C surface was studied at different conditions by photoelectron spectroscopy using synchrotron radiation (SRPES). Oxidation process is performed at room temperature and at 800 ◦ C under different oxygen pressures. It was found that oxygen atoms prefer bonding with the bilayer silicon atoms located below the top surface, while the topmost adatoms are hardly oxidized at room temperature even at very high oxygen exposure. However, by elevating the temperatures and oxygen pressures, the SiC surface was oxidized and the thickness of the oxide layer was estimated, taking the refraction effect of low energy photoelectrons into account. © 2005 Elsevier B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Oxidation; Silicon carbide; Crystal surface
1. Introduction Silicon carbide is a promising semiconductor material for high power electronics in harsh environment due to a combination of remarkable properties such as high melting point, high breakdown fields, wide band gap and large electron mobility. Thus, it is regarded as the first candidate of the substrate material for the metal-oxide-semiconductor field-effect transistor (MOS-FET) of the next generation [1,2]. The (0 0 0 1) surface of hexagonal SiC crystals of the 4H (and 6H) poly-type has been considered the most suitable for device applications [3,4]. Now large size SiC substrates of different poly-types with high quality are commercially available, but a good SiO2 /SiC interface with low defect density is not realized yet, which is of crucial importance for the performance of electronic devices. Now people have tried different approaches to form oxides/SiC interface [5–8]. While good SiO2 /SiC interface of hexagonal SiC is not easily formed due to drastic surface reconstructions and different surface structures induced by different surface treatments. For example, the clean surface of Si-terminated 6H-SiC(0 0 0 √ 1) shows √ four √ kinds √ of surface reconstructed structures: 3 × 3, 3 × 3, 6 3 × 6 3 and 6 × 6 with different annealing temperatures after Si-flux deposition in
∗
Corresponding author. Tel.: +86 551 3650821; fax: +86 551 5141078. E-mail address: [email protected] (P.S. Xu).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.10.006
ultra-high vacuum (UHV) [9,10]. In contrast, the C-terminated ¯ surface gives 3 × 3, (2 × 2)C and (1 × 1)graphite 6H-SiC(0 0 0 1) reconstructions by annealing at different temperatures [11–13], and the (2 × 2)Si structure by annealing in Si-flux [14]. Atomic configurations of above reconstructed surfaces are frequently studied by STM and LEED intensity curves. For the 3 × 3 reconstructed surface, it is regarded that silicon adatoms are supported by trimeric√silicon √ adlayers located on the first Si–C bilayer [15]. The 3 × 3-SiC (0 0 0 1) and (2 × 2)C ¯ have much similar atomic configurations. The forSiC(0 0 0 1) mer consists of Si adatoms of 1/3 monolayer located at T4 site on the Si–C bilayer [16], while the latter is formed by Si adatoms on the H3 position of the Si–C bilayer [17]. For different surface reconstructions, the surface oxidation has recently been investigated by experimental spectroscopic techniques. Hoshino √et al.√[18] investigated the initial oxidation of 6H-SiC(0 0 0 1)- 3 × 3 surface by SRPES and showed all suboxide states (Si+ , Si2+ , Si3+ and Si4+ ) from Si 2p spectra. Amy et al. [19] investigated the SiC (0 0 0 1)-3 × 3 surface oxidation from 25 to 650 ◦ C to reveal the initial oxide SiC interface. In their experiment they also found all suboxide states of silicon atoms. For the same oxidation process, however,√Virojanadara √ and Johansson reported different results. For the 3 × 3 surface [20,21] they claimed that they only recognized the Si+ and ¯ ×3 Si4+ states, and for the carbon terminated 6H-SiC(0 0 0 1)-3 surface [22], they found only one suboxide component during the oxidation experiments. To explain the mechanism of the
C.W. Zou et al. / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44
oxidation process on the SiC surface, many related models have been proposed. Jernigan et al. [23] suggested a mechanism for both Si-face and C-face of the 6H-SiC(0 0 0 1) surface. They indicated that there was a transition layer between the SiC substrate and the oxide, which contained Si Si bonds. This is related to the high interfacial trap density and may be responsible for the low channel mobility in SiC. Thermal oxidation of 6H-SiC was investigated [24] by means of the isotope tracing and narrow nuclear resonant reaction profiling techniques. The results showed that oxygen atoms were incorporated in the bulk and interface oxide regions during thermal oxidation of SiC. In this present work, we investigate the oxidation of ¯ 6H-SiC(0 0 0 1)-(2 × 2)C surface at different temperatures and oxygen exposures by using the synchrotron radiation photoemission spectroscopy (SRPES). According to the curve fitting for Si 2p spectra, all of the silicon suboxides and surface related components are observed during the oxidation process. From the Si 2p relative intensity ratio of each suboxides, we estimate the thickness of each oxide and clarify the kinetics of the initial oxidation at RT and high temperature. 2. Experiment The experiment is performed at the surface physics station of National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. This experimental station is mainly composed of a threechamber VG multi-technique UHV system. It consists of a deposition chamber, a pre-treatment chamber with a fast entry lock and an analysis chamber equipped with ARUPS10 hemispherical analyzer. Sample can be cleaned by cycles of Ar+ ions etching and e-beam annealing in the pre-treatment chamber and transferred to the analysis chamber for SRPES. The XPS, LEED, RHEED and AES are also equipped and all these systems work under UHV conditions (better than 2 × 10−10 mbar). The beamline covers the energy range from 10 to 300 eV and the energy resolution (E/E) is better than 1000. More details of the experiment station and the related beamline are described elsewhere [25]. The samples used in this experiment are Si-terminated 6HSiC(0 0 0 1) single crystal from Cree. Research Corp. To obtain clean surface, the following processes are operated: (1) rinsed in acetone for 10 min with an ultrasonic cleaner, (2) dipped in the solution of HF and ethanol (5% HF) for several minutes, (3) dipped into acetone and finally cleaned by de-ionized water. After these treatments, the sample was immediately introduced into the UHV chamber and degassed by heated at 550 ◦ C for about 5 h with e-beam heater. Then we can observe LEED spots, But the XPS spectra shows that there is still a small mount of oxygen components on the surface. Cycles of slight Ar+ ions etching and annealing at 950 ◦ C in the UHV chamber are performed until no oxygen signal exists in XPS measurement. Now clear 2 × 2 LEED pattern is observed in Fig. 1. Annealing at higher temperature induces much sharper 2 × 2 LEED pattern just as shown in the insert. The obtained LEED pattern is quite consis¯ tent with the other reports from C-terminated 6H-SiC(0 0 0 1) [11,26]. Then the prepared 2 × 2 surface is oxidized at RT and 800 ◦ C in different oxygen pressures (purity ∼ 99.99%).
41
After each oxidation, the Si 2p core level was recorded with SRPES at emission angle of 45◦ related to the surface normal direction. 3. Results and discussion ¯ surface 3.1. The clean 6H-SiC(0 0 0 1) In Fig. 1, we can observe a clear 2 × 2 LEED pattern, indicating that our surface treatment method induces the formation ¯ of 6H-SiC(0 0 0 1)-(2 × 2)C reconstruction from the original Siterminated 6H-SiC(0 0 0 1) surface. It is reasonable because the terminated Si atoms of 6H-SiC(0 0 0 1) surface are almost removed by hydrogen fluoride acid and Ar+ ions etching, and further annealing at high temperature is prone to form the (2 × 2)C surface reconstruction. ¯ The Si 2p core level spectrum from the 6H-SiC(0 0 0 1)(2 × 2)C clean surface with photon energy of 150 eV is shown in Fig. 2. The background has been subtracted by the Shirley method. Then the original spectra can be fitted by three components, where the bulk Si 2p spin–orbit splitting doublet peaks are simulated by Gaussian shapes with a branching ratio of 0.5 and a spin–orbit splitting of 0.63 eV. The other two components marked with S1 (100.82 eV) and S2 (100.03 eV) are due to the ¯ surface contributions. Fig. 3 shows the 6H-SiC(0 0 0 1)-(2 × 2)C surface model of SiC [16], where a silicon adatom is located on the H3 site on the Si–C bilayer in a 2 × 2 unit cell. Similar to the earlier report [27], the S1 and S2 surface components are assigned to the Si atoms of the topmost Si–C bilayer and the silicon adatoms, respectively. 3.2. Oxidation at different conditions ¯ During the oxidation process, the 6H-SiC(0 0 0 1)-(2 × 2)C clean surface is oxidized for 5 min at RT with oxygen pressures
Fig. 1. The 6H-SiC(0 0 0 1)-2 × 2 LEED pattern. The inset is the LEED pattern when sample is annealing at higher temperature in the UHV chamber.
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Fig. 2. Si 2p core level observed for 150 eVphoton incidence on the 6HSiC(0 0 0 1)-2 × 2 clean surface at emission angle of 45◦ related to the surface normal direction.
of 1.2 × 10−8 , 1.6 × 10−7 and 1.4 × 10−6 Torr, respectively. Fig. 4(a) shows the Si 2p core level spectrum at photon energy of 150 eV for the surface oxidized at RT. The oxygen exposure is about 3.6 L (1 L = 1 × 10−6 Torr s). The observed spectrum is decomposed into seven components: two surface components (S1 and S2), bulk component (101.50 eV) and four oxidation states (Si+ , Si2+ , Si3+ and Si4+ ) located at the binding energy of 102.08, 102.92, 103.4 and 104.12 eV, respectively. The Gaussian width (full width at a half maximum, FWHW) for each component is about 0.7 eV besides the Si4+ peak, which stands by a single broad peak with larger FWHW. The Si 2p related binding energy (BE) shifts referred to the SiC bulk state are shown in Table 1. Our result of Si4+ state BE shift is quite close to the ear¯ surface [28]. The oxide-related lier report for 4H-SiC(0 0 0 1) Si2p BE shifts for some other structures are also listed in the table [29–31]. The peaks due to the surface states still remain,
Fig. 4. The Si 2p core level recorded after the SiC clean surface has been oxidized at RT with different oxygen pressure: (a) 1.2 × 10−8 Torr for 5 min; (b) 1.6 × 10−7 Torr for 5 min; (c) 1.4 × 10−6 Torr for 5 min.
¯ Fig. 3. The surface model for 6H-SiC(0 0 0 1)-(2 × 2)C unit cell structure in side view (upper panel) and top view (lower panel).
although the intensity is a little decreased comparing with those of the clean surface in Fig. 2. Our result is quite similar to the oxidation behavior of 6H-SiC(0 0 0 1)-3 × 3 surface investigated by Amy et al. [29]. They found that the oxygen prefers reacting
C.W. Zou et al. / Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 40–44 Table 1 BE shifts relative to the bulk state BE
Table 2 ˚ calculated from peak decomposition and Fractions of Si-oxides thickness (A) attenuation principle
Si 2p related BE shifts (eV)
Present (2 × 2)C ¯ × 3 [20] 4H-SiC(0 0 0 1)-3 6H-SiC(0 0 0 1)-3 × 3 [21] √ √ 6H-SiC(0 0 0 1)- 3 × 3 [22] Si(1 1 1) [23]
Si+
Si2+
Si3+
Si4+
0.58
1.42 1.5 1.36 1.07 1.75
1.9
2.62 2.7
0.25 0.5 0.95
2.48 1.63 2.42
43
2.2 3.30
with silicon atoms located well below the surface, but not the top surface atoms under very low oxygen exposure regime. Thus, at low oxygen exposure, the surface states can still be observed in the Si 2p spectrum. Fig. 4(b) shows the Si 2p spectrum after the oxidation process of oxygen exposure of 1.6 × 10−7 Torr for 5 min (about 48 L). The Si4+ peak grows rapidly, indicating that sizable SiO2 domain has been formed after this process. Note that the surface related peaks still appeared, although their intensities are reduced. It means that silicon atoms on top of surface have been partly oxidized. Fig. 4(c) shows the Si 2p spectrum after further oxygen exposure (about 420 L). The spectral shape in Fig 4(c) has little difference with that in Fig. 4(b). This means that the surface has been saturated after the second oxidation process. The saturated stage might be reached by oxygen exposure less than 48 L, but further careful experiment is necessary to determine the critical point. What is the reason for this oxidation saturation behavior? One possibility could be the Mott-Cabrera mechanism, as it has been discussed for the oxidation of 6H-SiC(0 0 0 1) by van Eisbergen et al. [32]. Fig. 5 shows the Si 2p spectrum after annealing at 800 ◦ C in oxygen with a pressure of 1.5 × 10−6 Torr for 20 min (1800 L oxygen exposure). According to our curve fitting procedure, the surface related peaks S1 and S2 almost disappeared though the S1 peak still has small intensity as shown in the figure. This indicates that at high oxygen exposure and high oxidation temperature, the topmost silicon adatoms are almost oxidized. The
420 L oxygen exposure at RT 1800 L oxygen exposure at 800 ◦ C
Si+
Si2+
Si3+
Si4+
Total thickness
1.8 1.23
0.65 1.9
0.92 1.78
2.3 5.6
5.67 10.51
oxidized states related peak intensity significantly increases. It means that the temperature of sample induced more oxidation of silicon atoms. Finally, a saturation of oxidation probably occurs by accumulated strain-induced distortion, which leads to a close-packed structure at the top surface and suppresses further oxidation. From relative intensity ratio of Si 2p spectral peaks we can estimate the respective thickness of silicon oxides by using the photoelectron attenuation layer method [33]. Here we let elec˚ according to the earlier report tron attenuation length equal 4.2 A [34]. Another point should be noticed is that the photon energy in our experiment is 150 eV. Since the excited photoelectrons have low kinetic energy, we must account for surface refraction. This means that the internal emission angle ϕ and the external angle θ are different and their relation can be described by EK sin ϕ = sin θ EK + V 0 where V0 is the inner potential (about 15 eV for silicon carbide) and EK is the kinetic energy of photoelectrons in vacuum. After considering the surface refraction due to the low photon energy, we obtain the final result for each oxidation layer from the integrated intensities of the Si 2p peak decomposition. The detailed values are shown in Table 2. It gives thickness for each sub˚ oxide. At 420 L oxygen exposure, the SiO2 thickness is 2.3 A and the ratio to total mount of oxides is 40%, while at 800 ◦ C with 1800 L oxygen exposure, the ratio increases to 53%. This means that at higher oxygen exposure and higher temperature, the oxidation process becomes much more pronounced. 4. Conclusion
Fig. 5. The Si 2p core level recorded after oxidation in 1.5 × 10−6 Torr for 20 min at 800 ◦ C.
¯ The initial oxidation of 6H-SiC(0 0 0 1)-(2 × 2)C surface is analyzed in situ by SRPES. The (2 × 2)C surface structure is prepared directly from the Si-terminated 6H-SiC(0 0 0 1) by chemical treatment, cyclic Ar+ sputtering and annealing in UHV chamber. The oxidation was performed at RT and 800 ◦ C under different oxygen pressures. During the oxidation process at RT the two surface components always existed even at high oxygen exposure. Our result shows that at RT the oxidation behavior ¯ of 6H-SiC(0 0 0 1)-(2 × 2)C surface is quite similar to the oxidation model for 6H-SiC(0 0 0 1)-3 × 3 surface at very low oxygen exposure. Furthermore, this model is also right to describe the oxidation behavior for our sample at higher oxygen exposure. While at high temperature and much higher oxygen pressure, the surface oxidation process becomes very pronounced and the surface components related peaks almost disappear, indicating that the surface adatoms are fully oxidized. Considering low
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