Atomic structures of 6HSiC (0001) and (0001̄) surfaces

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Surface Science 351 (1996) 141-148

Atomic structures of 6H-SiC (0001) and (0001) surfaces L. Li, I.S.T. Tsong * Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287, USA Received 31 August 1995; accepted for publication 28 November 1995

Abstract

We have studied the reconstructions of the 6H-SiC (0001) and (0001) surfaces after annealing at 850-950°C under a Si flux using scanning tunneling microscopy (STM). On both the Si-terminated (0001) and C-terminated (0003) surfaces, we observed a (3 x 3) reconstruction after annealing at 850°C, which changed to a ( f x w/3) reconstruction after further annealing at 950°C. We propose a model for the (3 x3) surface with a 4/9 ML adatom-coverage. The ( f i x f ) surface has a 1/3 ML adatorn-coverage. These observations are consistent with previous LEED results. We also observed a new (9 x 9) reconstruction on the (0001) surface after further annealing the (3 x 3) surface at 900°C under a Si flux. When the (0001) surface was flashed to I150°C without a Si flux, a graphitized surface with a (6 x 6) reconstruction was formed. The empty-state and filled-state images showed a contrast reversal. When Si was deposited in the (6 x 6) surface, the polarity of the contrast reversal was reversed, confirming the validity of our previous electronic structure calculations for this surface.

Keywords: Scanning tunneling microscopy; Silicon carbide; Surface relaxation and reconstruction; Surface structure, morphology, roughness, and topography

1. Introduction

Because of its importance as a structural ceramic and a high-temperature wide-bandgap semiconductor, the surface structures and properties of various SiC polytypes have been the subject of numerous studies. For the hexagonal e-SiC single crystal, the ideal (0001) and (0001) faces are terminated by a layer of Si and C atoms respectively. (See, for example, fig. 1 of Ref. [1]). Comprehensive studies of the structures of these two surfaces, as well as the equivalent cubic t-SiC (111) surface, have been conducted by Kaplan and Parrill [2], Kaplan [3] and Bermudez [4] using low-energy electron diffraction (LEED), Auger * Corresponding author. Fax: + 1 602 965 7954;

e-mail: [email protected] 0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 1 3 5 5 - 5

electron spectroscopy (AES) and electron energyloss spectroscopy (ELS). I n addition, Bermudez [4] also gave a brief review of the LEED patterns observed by various workers over the past two decades. The surface structures derived from LEED appeared to be influenced by the method of surface preparation. Miyoshi and Buckley [5], using AES and X-ray photoelectron spectroscopy (XPS), and Muehlhoff et al. [1], using ELS, have shown that annealing the e-SiC (0001) surface at or above ~900°C quickly results in the termination of a graphite layer on the surface. Van Bommel et al. [6] showed a LEED pattern of (6x/~ x 6V/3) for such a graphitized surface. To overcome the problem of the graphite layer, Kaplan and Parrill [2] and Kaplan [3] cleaned the surfaces of their crystals in a Ga or Si flux at temperatures between

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850 and 950°C. The Si-terminated 6H-SiC (0001) and ¢/-SiC (111) faces showed (3 x 3) and !x/3x x/~) patterns while the C-terminated (000]) face showed (1 x 1) and (x~J x x/3) patterns. Bermudez [4] used essentially the same procedure of annealing the (0001) and (0001) surfaces at ~ 1000°C in a Si flux and observed (3 x 3) and (1 x 1) L E E D patterns for the Si- and C-faces respectively. For the Si-face, further annealing produced a ( x ~ x ,]-3) pattern. Our previous STM studies [7,8] have confirmed graphitization of the /%SIC (111) surface when annealed to ~ 1150°C: However, there have been reports of ordered surface structure without a graphitized layer when the e-SiC (0001) and (0001) surfaces were annealed in the absence of a Si flux. Nishimori et al. [9] and Nakamishi et al. [10] obtained a ( 3 x 3 ) L E E D pattern for the C-terminated (0001) face and a (x//3 x x~) pattern for the (0001) face, exactly opposite to the observations of Kaplan and Parrill [2], Kaplan [3] and Bermudez [4], when the surfaces were annealed at 1000°C in ultrahigh vacuum (UHV). Most recently, Owman and M~rtensson [11] have observed the (x/~ x x/~) structure in STM images on a 6H-SiC (0001) surface after annealing at ~ 900°C without the use of a Si flux. In the present work, we report STM studies of 6H-SiC (0001) and (0003) surfaces cleaned by annealing in the presence of a Si flux in order to investigate the surface structures to resolve some of the discrepancies in the previous studies.

This process removes oxygen contamination from the surface in the form of SiO, a volatile reaction product, while also preventing the formation of a graphite layer by reacting with non-carbidic C to form SiC [ 2 - 4 ] . A quartz crystal thin-film monitor was used to monitor the Si deposition rate which was in the range of 0.1-0.3 ML/min. After the cleaning treatment, the sample was transferred to the STM for imaging at room temperature. The base pressure in the chamber was < 1 x 10 -1° Torr.

3. Results and discussion 3.1. (6 x 6) reconstruction When the 6H-SiC (0001) surface was cleaned by flashing to ~ 1150°C in UHV, a (6 x 6) reconstruction was obtained, indicative of an incommensurate grown graphite layer. Simultaneous filledstate and empty-state scans (Fig. 1) taken on the same region of the surface show a honeycomb structure and a hexagonal structure respectively. The contrast reversal is identical to that previously observed graphite monolayer incommensurately grown on the (1 x 1) Si-terminated fl-SiC (111) surface [7,8]. This was interpreted by us as the difference in electronic contributions from graphite states of those C atoms with Si atoms directly or nearly directly underneath and those without [8]. The graphitized (0001) surface was heated to 900°C

2. Experimental The 6H-SiC (0001) and (0001) single crystals were supplied by Cree Research. They were nitrogen-doped n-type samples with a dopant concentration of ,-~ 10 is cm -3. The samples were cleaned in situ in the U H V chamber containing the STM and a heating stage. The sample under study was mounted on the heating stage and was resistively heated to 850-950°C by passing a DC current through the sample. A flux of Si vapor was directed onto the heated sample surface from a Si evaporator which consisted of a resistively heated small Si wafer positioned at 15 cm away from the sample.

(a)

(b)

Fig. 1. STM images taken on the same area of the graphitized 6H-SiC (0001) surface showing the ( 6 x 6 ) reconstruction. Contrast reversal is manifested as honeycombs in the filledstate image (a) and hexagons in the empty-state image (b). Sample b i a s = - 2 . 0 V in (a) and 2.0V in (b). Scan area= 300 A x 300 A.

L. Li, £S.T. Tsong/Surface Science 351 (1996) 141-148 and placed under a Si flux until ,-~ 1 M L of Si covered the surface. Re-examination by the STM at room temperature revealed simultaneous filledstate and empty-state images such as those shown in Fig. 2. The (6 x 6) reconstruction still remains, but the "polarity" of the contrast reversal is now reversed, with hexagons showing up in the filledstate and honeycombs in the empty-state. This observation is a complete confirmation of our electronic structure model proposed previously [ 8 ] for the graphitized surface. If we assume that the deposited Si atoms formed a hexagonal (1 x 1) structure on top of the graphite layer, then the direction of charge transfer from C to Si atoms is reversed compared with our previous case of graphite monolayer on the ( l x l ) Si-terminated surface (fig. 4 of Ref. [8]). Using the same symbols as in our previous calculations [8], we now have pl(E) the density of states of those C atoms having a Si atom directly above or close to them, and p2(E) the density of states of all other atoms. Then as before [8], p2(E) is unchanged and can be written as: P2(E) ,~ K (E - -

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charge-transfer amount, 6, indicates a change in direction of the charge transfer. Substituting Eqs. (1) and (2) into Eqs. (4) and (6) given in Ref. [8], the calculations give: AN + ~Ke2V25

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and A N - ~ - Ke2V2b

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where AN is the difference in the number of states sampled by the STM from the two kinds of C atoms on the surface; and AN + is for positive tip bias or Vt > 0 sampling filled states, and A N - for negative tip bias or Vt<0 sampling empty sates. Thus AN gives rise to the contrast observed in the STM images. The opposite signs in Eqs. (3) and (4) explain the contrast reversal observed in Figs. 2a and 2b. More importantly, however, the signs of AN + and A N - are exactly reversed from those determined previously for the graphite-terminated surface [8], which explains the "polarity" change in the contrast reversal in Figs. 1 and 2. We should mention that we used sample bias rather than tip bias in the present series of experiments to probe filled and empty states, but it makes no difference to our theoretical analysis.

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pl (E),,~ K ( E - E F - 6 ) 2.

Compared with the previous pl(E)~ K ( E - E F + O ) 2, the negative sign in front of the

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Fig. 2. STM images taken on the same area of the SiC (0001)-(6 x 6) reconstruction after deposition of a monolayer of Si on the graphitized surface. The polarity of the contrastreversal is reversed with hexagons appearing in the filled-state image (a) and honeycombs in empty-state image (b!. Sample bias = -2.0 V in (a) and 2.0 V in (b). Scan area= 150 A x 150 A.

3.2. (3 x 3) reconstruction The (3 x 3) reconstruction was observed after annealing the Si-terminated 6H-SiC (0001) surface at 850°C in a Si flux for 10 min, in agreement with previous L E E D results [-2-4]. STM images of the ( 3 x 3 ) surface are shown in Figs. 3 and 4. Annealing the C-terminated (0001) surface under the same conditions also produced the ( 3 x 3 ) surface. From his L E E D and ELS results, Kaplan proposed a (3 x 3) structural model based on the DAS model of the Si(7 x 7) [12]. Our STM images of the (3 x 3) structure, however, do not agree with the DAS model. Firstly, the very prominent corner holes of the DAS (3 x 3) model are absent in Figs. 3 and 4. Secondly, contrary to the filled-state images of apparent difference in height of adatoms on faulted and unfaulted halves of a DAS structure, the filled-state image of Fig. 4a shows no height difference in the bright spots. However, in Fig. 3,

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Fig. 3. STM image of the (3 x 3) reconstruction formed after annealing the Si-terminated (0001) surface to 850°C for 10 rain under a Si flux. The arrow indicates the location of a lessbright spot "A". The (3 x 3) unit cell is outlined. Sample bias = 2.0 V; scan area = 120 A x 120 ~,.

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Fig. 4. STM images taken on the same area of the (3 x 3) surface showing bright, less-bright and dark spots. Image (a) is filled-state image and (b) an empty-state image. Sample bias = - 1.6 V for (a) and 1.6 V for (b). Scan area=325 ~t x 270 ,~.

there are some spots that are less bright, giving the appearance of recessed " a t o m s " or "clusters"; while some are dark, resembling vacancies. Figs. 4a and 4b show filled-state and empty-state images taken sequentially on the same area of the (3 x 3) surface. While the majority of the bright spots are c o m m o n to b o t h images, hardly any of the lessbright or dark spots o c c u p y the same sites in the

two images. F r o m Fig. 3 and Fig. 4, the following sites are present on the (3 x 3) surface: those sites occupied by (i) bright spots, (ii) less-bright spots in filled state, (iii) dark spots in filled state, (iv) lessbright spots in e m p t y state, and (v) dark spots in e m p t y state. If a dark spot occupies the same site in b o t h filled and empty state images, then we consider it a vacancy. We propose a (3 x 3) structural model as shown in Fig. 5 based on our S T M images. The building block of hexagonal e-SiC is a tetrahedron of Si and C atoms as shown in Figs. 5a and 5b. In bulk a-SiC, these tetrahedra are stacked on top of one another such that a C a t o m is always surrounded by four Si atoms and similarly, a Si a t o m is always surrounded by four C atoms. The different polytypes in a-SiC are simply due to the sequence of h o w the different layers of the tetrahedra are stacked. In Fig. 5c, we show these tetrahedra distributed on the Si-terminated (0001) surface in a (3 x 3) geometry. W h e n b o n d e d to the surface in this manner, each a t o m of the tetrahedron has a single dangling b o n d which contributes to the tunnel current, and hence the brightness of the spots in the S T M images. The size of the bright spots in Figs. 3 and 4 is approximately 4 A across, r o u g h l y the size of a tetrahedron unit. The corrugation of the (3 x 3) bright spots is ~ 2 . 4 A, close to the height of the S i - C double-layer. Since the tetrahedron can be either of the two types shown in Figs. 5a and 5b, the surface can be Si or C terminated or both. Tetrahedra m a d e up entirely of Si atoms or C a t o m s also cannot be ruled out. The variety of sites observed in S T M images of Figs. 3 and 4 can be attributed to the different configurations of tetrahedra residing on the (3 x 3) surface. O u r model of the (3 x 3) surface has an a d a t o m coverage of 4/9 ML. The Si coverage is 1/9 M L for the type (a) tetrahedron, 1/3 M L for the type (b) tetrahedron and 4/9 M L if all four atoms of the tetrahedron are Si. 3.3. x / ~ x w / 3 s t r u c t u r e F u r t h e r annealing of the ( 0 0 0 1 ) - ( 3 x 3 ) and (0001)-(3 x 3) surfaces under a Si flux at a higher temperature of 950°C for 2 - 4 min p r o d u c e d a

L, LL I.S.T. Tsong/Surface Science 351 (1996) 141-148

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Fig. 6. STM image of the SiC(0001)-(x/3 x ,f3) reconstruction after further annealing the (3 x 3) surface at 950°C under a Si flux for 4 min. The (~f3 × ~ ) unit cell is outlined. Sample bias = 2.0 V; scan area = 100 A x 100 A.

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(c) Fig. 5. A structural model for the 6H-SiC (0001)-(3x3) reconstruction based on the STM images. It consists of (a) C-rich and (b) Si-rich tetrahedral clusters arranged in a (3 x 3) geometry as shown in (c). Both the (1 x 1) and the (3 x 3) unit cells are outlined in (c).

(~v/3 x x/J) structure as shown in Fig. 6. After taking STM images of the (x/r3 x x/~) surface at room t e m p e r a t u r e , r e a n n e a l i n g t h e surface at a l o w e r t e m p e r a t u r e , 850°C, in a Si flux c a u s e d t h e r e a p p e a r a n c e o f t h e (3 x 3) s t r u c t u r e . O c c a s i o n a l l y , w e o b s e r v e d t h e c o e x i s t e n c e o f b o t h (x/~ x x/~) a n d (3 x 3) s t r u c t u r e s as s h o w n in Fig. 7. S i n g l e line-

Fig. 7. STM image of the coexistence of (3 x 3) and ( ~ x ~/3) phases on the (0001) surface. The (3 x3) and (;,/3x,j~) unit cells are outlined. The (3 x 3) structure sits ~ 2 A higher than the° ( , ~ x J 3 ) structure. Sample bias=2.0V; scan area= 8 0 A x 80A.

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L. LL 1.S.T. Tsong/Surface Science 351 (1996) 141-148

scan profiles show a height difference of ~ 2 between the (3 x 3) and ( ~ x V~) domains, with the (3 x 3) being higher. The corrugation of the (wfJ x ~/J) spots is 0.2 A, very close to the 0.15 measured by O w m a n and Mg~rtensson [11]. In contrast with the work of O w m a n and Mgtrtensson [11], however, we never observed the (V~ x ,¢/3) reconstruction without a Si flux. The difference is probably due to the surface preparation prior to S T M imaging, which in their case, involves a standard RCA cleaning procedure followed by a N H 4 F : H F ( 7 : I ) oxide strip on all of their samples with some receiving an additional high-temperature sublimation etch [ 13] to remove surface scratches. The appearance of the (~v/3x ~/3) phase after further annealing of the (3 x 3) surface at a higher temperature, 950°C in our case, is consistent with previous L E E D results [3,4]. According to our model of the (3 x 3) surface, Si coverage from 1/9 to 4/9 M L coverage is possible, If we assume that annealing at a higher temperature results in a reduction of Si on the surface, then the coverage of the ( ~ J x ,¢f3) structure must be lower than 4/9 ML. The adsorption of G r o u p IV elements, such as Sn and Pb, on the Si (111) surface results in the appearance of the (V/-3x V/3) phase at 1/3 M L coverage [14,15], with the adsorbates occupying the T4 sites. It is therefore reasonable to assume that the (V/3 x V/3) structure of both the 6 H - S i C (0001) and (0001) surfaces occur at 1/3 M L Si coverage. Of course a C-covered (wfJ x x/3) surface cannot be excluded since it is difficult to use the STM to identify the a t o m species. However, it is unlikely because of the continuing exposure of the surface to the Si flux. For the C-terminated (0001) surface, Badziag [16] has performed serf-consistent total-energy calculations on the (x/~ x ff~) structure, suggesting that it is similar to the S i ( l l l ) - ( x / ~ x x/~)B structure with C in the Bs position and Si in the T 4 position as shown in Fig. 8. For the (0001) surface, O w m a n and Mgtrtensson [11] suggest the possibility of Si adatoms occupying the T4 sites. Our STM images of the (V~ x x/~) structure on both the (0001) and (0001) surfaces do not contradict with either of these models.

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Fig. 8. Schematic diagram of two structural models of the 6H-SiC (0001)-(x/3x ~,/3) surface. (a) The T 4 model with the Si adatom on the T4 site; and (b) the Bs model with the Si in the T4 site, and a C atom substituting for Si in the B5 site. 3.4. (9 x 9) structure

After the appearance of the (3 x 3) phase on the (0001) surface, further annealing at a slightly higher temperature, 900°C, for 5 rain in the Si flux produced a (9 x 9) reconstruction as shown in Fig. 9. Theo(9 x 9)oStructure has a unit cell dimension of (28 A x 28 A) as outlined in Fig. 9a, and it has three domains with equivalent orientations on the three-fold symmetric (0001) surface as shown in

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(a)

(b)

Fig. 9. STM image of the 6H-SiC (0001)-(9 x 9) reconstruction after further annealing at 900°C for 5 min under a Si flux. (a) The (9 x 9) unit cell is outlined. Scan area=300 A x 300 A. (b) Three domains of the (9 x 9) with equivalent orientations. Scan area =400 A x 400 A. Sample bias = 2.0 V for both (a) and (b). The rectangular shape of the spots is most likely due to tip irregularity.

L . LL I.S.T. Tsong/Surface Science 351 (1996) 141-148

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Fig. 10. A schematic diagram of the (9 x 9) surface showing a particular registration of the large and small bright spots in relation to the (1 x 1) layer of Si on the 6H-SiC (0001) surface. Fig. 9b. This structure has not been observed previously by LEED. It did not show any bias dependence in the filled-state and empty-state images. After similar annealing treatments on the C-terminated (0001) surface, the (9 x 9) reconstruction did not appear. This suggests that the structure may be due to a Si-rich phase. There are two different spot-sizes in the STM images of the (9 x 9 ) surface. The large spot is about 7 A in diameter while the small spot is about 3 A. With the information from STM images alone, it is not possible to determine what these spots consist of. The smaller spots are close to the size of the (3 x 3) spots and so there is a likelihood that they are units of tetrahedra as in the (3 x 3). However, a major difference in the two is that while some of the (3 x 3) spots show a bias dependence, none of the small spots in the (9 x 9) do. In view of the complexity of the (9 x 9) structure, we have merely noted a particular registration (out of many possibilities) of the bright spots in relation to the (1 x 1) Si first layer as shown in Fig. 10.

4. Conclusions We have determined various reconstructions on clean 6H-SiC (0001) and (0001) surfaces by annealing them in the temperature range 850-950°C under a Si flux. On the Si-terminated (0001) surface, a (3 x 3) phase appeared after annealing at

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850°C. Further annealing at 950°C produced a (x/~ x x/c3) structure. After imaging the (x/3 x v/3) surface by STM at room temperature, the (3 x 3) phase could be restored by reannealing the surface at 850°C under the Si flux. Sometimes both (3 x 3) and (x/~ x r ~ ) could be found to coexist on the surface. The (3 x 3) and (V~ x V~) phases were also observed on the C-terminated (0001) surface under the same annealing conditions. A (9 x 9) reconstruction was observed in the (0001) surface only, after further annealing the (3 x 3) surface at 900°C under the Si flux. We propose a model of the (3 x 3 ) surface, consisting of units of Si-C tetrahedra. This model is different from the DAS model previously proposed by Kaplan I-3] because we did not observe the DAS structure in our STM images of the (3 x 3). The STM images of the (w/3 x x/~) structure on both the (0001) and (0001) surfaces are consistent with the models proposed by Owman and Mgtrtensson [11] and Badziag [16] respectively for a 1/3 M L adatom coverage. We also studied the effect of depositing Si atoms on a graphitized surface obtained after annealing the Si terminated (0001) surface to 1150°C in the absence of a Si flux. The change in polarity of the contrast-reversal in the filled-state and empty-state images provides confirmation of our previous firstprinciples calculations of the electronic structure of the graphitized surface [8].

Acknowledegments This work was supported by the U.S. Army Research Office under contract number DAAL 03-92-G-0038. We thank U. Knipping for technical assistance and V.M. Bermudez of Naval Research Laboratory for helpful discussions.

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[5] K. Miyoshi and D.H. Buckley, Appl. Surf. Sci. 10 (1982) 357. [6] A.J. van Bommel, J.E, Crombeen and A. van Tooren, Surf. Sci. 48 (1975) 463. [7] C.S. Chang, I.S.T. Tsong, Y.C. Wang and R.F. Davis, Surf. Sci. 256 (1991) 354. [8] M.H. Tsai, C.S. Chang, J.D. Dow and I.S.T. Tsong, Phys. Rev. B 45 (1992) 1327. [9] K. Nishimori, H. Tokutaka, S. Nakanishi, S. Kishida and N. Ishihara, Jpn. J. Appl. Phys. 28 (1989) L1345. [10] S. Nakanishi, H. Tokutaka, K. Nishimori, S. Kishida and N. Ishihara, Appl. Surf. Sci. 41/42 (1989) 44.

[11"I F. Owman and P. Mgtrtensson, Suf. Sci. 330 (1995) L639. [12] K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502. [13] M.M. Anikin, A.A. Lebedev, S.N. Pyatko, A.M. Strel'chuk and A.L. Syrkin, Mat. Sci. Eng. B 11 (1992) 113. [14] J. Nogami, S.I. Park and C.F. Quate, J. Vac. Sci. Technol. A 7 (1988) 1919. [15] E. Ganz, I.S. Hwang, F. Xiong, S.K. Theiss and J. Golovchenko, Surf. Sci. 257 (1991) 259. E16] P. Badziag, Surf. Sci. 236 (1990) 48.