Surface analysis of 6HSiC

surface science ELSEVIER

Surface Science 365 (1996) 443-452

Surface analysis of 6H-SiC V. v a n E l s b e r g e n *, T.U. K a m p e n , W. M 6 n c h Gerhard-Mercator-Universitiit Duisburg, Laboratorium fiir Festk6rperphysik, Lotharstrafle 1-21, D-47048 Duisburg, Germany Received 18 December 1995; accepted for publication 22 March 1996

Abstract

The composition of {0001 } surfaces of 6H-SiC samples was studied by using low-energy electron diffraction, Auger electron (AES), and X-ray photoelectron spectroscopy (XPS/SXPS). The samples were cleaned in ultrahigh vacuum by heating them either in the presence of a Si flux at different temperatures or by annealing at 1170 K for 10 min. Depending on the preparation method and temperature used four reconstructions were observed: (1 x 1), (3 x 3), (x/3 x ~/3)R30 °, and (6x/3 x 6~/3)R30 °. The compositions of the reconstructions and the chemical bonding of the surface atoms were characterized using AES and XPS/SXPS. Models for the reconstructions are proposed.

Keywords: Auger electron spectroscopy; Low energy electron diffraction (LEED); Silicon carbide; Soft X-ray photoelectron spectroscopy; Surface relaxation and reconstruction; X-ray photoelectron spectroscopy

1. Introduction

Silicon carbide is a semiconductor with a wide band gap, large electron mobility as well as high physical and chemical stability. These properties make SiC an interesting material for electronic devices in high-temperature, high-power and highfrequency applications. As with all tetrahedrally coordinated compounds, silicon carbide consists of double layers. The stacking sequences ABCABC and ABAB, where each letter represents one Si-C double layer, result in the cubic zincblende or 3C and the hexagonal or 2H structure, respectively. In the present study, 6H-SiC wafers were used which exhibit ABCACB stacking units along their c-axis. 6H-SiC has two polar surfaces. The (0001) surface

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is terminated by silicon atoms and the (000i) surface by carbon atoms. The composition and the structure of SiC surfaces depend on the preparation procedure in ultahigh vacuum (UHV) and are of importance with regard to the production of electronic devices based on SiC. As early as 1975, van Bommel et al. [ 1] obtained clean and well-ordered 6H-SiC surfaces by annealing in UHV. By using low-energy electron diffraction (LEED) they observed several surface reconstructions with increasing temperature. Annealing of Si-terminated (0001) surfaces at 520 K resulted in (x/3 x x/3)R30 ° patterns which transformed to (6x/3 x 6,]3)R30 ° at 1070 K. On C-terminated (0001) surfaces, (1 x 1) LEED patterns were observed at 520 K and (2 x 2), (3 x 3), and (4x4) patterns for annealing temperatures above 1070 K. Dayan [2] made use of the same method to prepare 3C-SiC samples. Nakanishi et al. [3] prepared Si- and C-terminated 6H-SiC

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V. van Elsbergen et al./Surface Science 365 (1996) 443-452

samples by etching in H F followed by annealing at 1270 K for 10 min in UHV. These surfaces displayed (3 x 3) or (x/3 x ~/3)R30 ° reconstructions. Similar surface reconstructions were observed by Starke et al. [4] who reported (1 x 1) patterns for annealing temperatures up to 1160 K and (,/3 x x/3)R30 ° LEED patterns above this temperature. All these studies showed that high temperatures above 1100 K are necessary to remove O contaminations. Auger electron spectroscopy (AES) measurements demonstrated that annealing treatments above 1300 K reduce the Si/C ratio at the surface due to a loss of Si [5]. At elevated temperatures such Si desorption finally leads to the formation of graphite layers at the surface. Already van Bommel et al. [ 1 ] have reported a graphitization of the surface for the (6x/3 x 6x/3)R30 ° phase. This finding was confirmed later by scanning tunneling microscopy (STM) studies [6]. Cleaning of SiC samples by ion bombardment and subsequent annealing results in severe surface damage [7]. An alternative preparation method was used by Kaplan and Parrill [8]. They annealed 61-1- and 3C-SiC samples for some min at 1250 K in a Ga beam in order to remove oxygen as Ga20. An improvement of this method was achieved by Kaplan [9] using a Si instead of a Ga beam to clean Si-terminated SiC samples. The surface oxide desorbs as SiO while volatized Si is continuously replenished. Moreover, C-contaminations are converted to SiC. By adjusting the temperature of the SiC samples or the intensity of the Si flux, additional Si adlayers may be deposited or even a graphite layer may be produced. Annealing of Si-terminated 6H-SiC samples at 1120 K in the presence of a Si flux resulted in the formation of (3 x3) LEED patterns. This reconstruction was found to transform to a (x/3 x x/3)R30 ° and a (1 x 1) pattern by increasing the annealing temperature to 1220 and 1270 K, respectively. Kaplan identified the (3 x 3) phase with an adsorbed Si bilayer on the Si-terminated surface. The appearance of a (x/3 x x/3)R30 ° pattern was assigned to the adsorption of one third of a Si adlayer or to an outer bilayer missing some of its Si atoms whereas the (1 x 1) pattern was attributed to bare SiC surfaces.

The following scheme summarizes these results: van Bommel et al. [ 1 ]: Si-term.: (1 x 1) 520 K (x/3 ×x/3)R30°

lo7oK

(6x/3 × 6~/3)R30 ° C-term.: (1 × 1) lo7oK (2 × 2),(3 x 3),(4 x 4) Nakanishi et al. [3]: Si-term.: (1 x 1) 127oK (,/3 × x/3)R30O,( 1 × l) C-term.: (1 × 1) 127oK (3 x 3),(1 x 1) Starke et al. [4]: Si-term.: (1 x i) 116o~ (x/3 x x/3)R30 ° Kaplan and Parrill [8]: Si-term.: ( 1 x 1) C-term.: (1 x 1)

Ga,1250 K

, (3x3)

Ga,1250 K

(x/3 x x/3)R30 °

Kaplan [9]: Si-term.: ( 1 x 1)

Si, ll20 K

, (3x3)

(~/3x~/3)R30 °

1270K

1120 K

,

(1 X 1)

The above compilation of experimental observations reveals some discrepancies with regard to the surface reconstructions on SiC samples prepared by annealing in UHV. Furthermore, no detailed study on Si-terminated surfaces annealed in a Si flux is available. The intention of our work is to study both aspects by using low-energy electron diffraction (LEED), together with X-ray photoelectron (XPS) and Auger electron spectroscopy (AES). The later technique was added due to its high surface sensitivity.

2. Experimental The experiments were performed in a two-chamber stainless steel UHV system with a base pressure below 10 -8 Pa. The analysis chamber was equipped with a four-grid LEED optics (Varian). For photoemission spectroscopy (XPS/SXPS) studies an X-ray source with an A1/Zr double anode was used. The O(ls), C(ls), and Si(2p) spectra were acquired using the A1 anode. The Z r ( M 0 radiation

V. van Elsbergen et aL /Surface Science 365 (1996) 443-452

of 151.2 eV was utilized to probe the Si(2p) core level with higher surface sensitivity. The analysis chamber also contained a 10-kV electron gun for Auger electron spectroscopy (AES). A primary beam energy of 3 keV was used. Energy distribution curves (EDCs) of Auger and photoemitted electrons were measured with a cylindrical mirror analyser (Riber, MAC2) by using the pulse-counting technique. The AES EDCs were numerically differentiated to obtain dN(E)/dE. The angle between the sample normals and the axis of the X-ray and electron source and of the CMA measured approximately 30° and 40 °, respectively. The preparation chamber contained a silicon sublimation source consisting of a 2 cm × 1 cm Si slice between two Mo contacts that was resistively heated to 1370 K. Both chambers were connected by a gate valve. A fast load lock was attached to the preparation chamber. Among the three chambers the samples were transferred by means of magnetically coupled rods. Samples measuring 6 mm × 6 mm were cut from Si-terminated (0001) and C-terminated (000i) 6H-SiC wafers (Cree Research, Inc.). The following cleaning procedures were applied. The Si-terminated samples were first dipped into concentrated HF (50%) for two min, which was diluted with buffered HF:NH4F:NHaOH solution with pH = 9, were then rinsed in deionised water, blown dry with N2 gas and eventually transferred into the UHV system. There they were either heated indirectly by electron bombardment to a temperature between 1170 and 1250 K and simultaneously exposed to a Si flux or they were annealed at 1170 K for 10 min in UHV. Temperatures were measured using a PtRh/Pt thermocouple. The C-terminated samples were immersed into diluted HF solution (25%) for 10 min, rinsed in deionised water, blown dry with N2 gas and then transferred into UHV. There they were annealed at 1170 K for 10 min. These procedures gave clean and wellordered surfaces as was monitored by using XPS, AES and LEED. Occasionally, several types of LEED patterns were observed in different areas on the same specimen surface. This can be attributed to inhomogeneous heating of the samples due to the geometrical arrangement of sample and filament used for electron bombardment. In order

445

to relate the AES and XPS measurements to the individual reconstructions the position of the sample was varied and annealing was continued with the same temperature until LEED showed one and the same pattern across the entire surface. These procedures and the experimental data will be discussed in the next section.

3. Preparation of clean surfaces AI(K++) radiation (XPS) was used to detect surface contaminations and to probe the surface composition of the sample itself. The escape depth of the electrons measures approximately 20-30 ~, for XPS and 6-8 ~. for SXPS and AES. Due to their high surface sensitivity, SXPS and AES were used to detect variations in the bonding configuration of surface atoms. The energy positions of the AES transitions were taken as the minimum of the highenergy wing of the numerically differentiated peak. Fig. 1 shows the O(ls), C(ls) and Si(2p) corelevel signals of a Si-terminated surface after four different preparational steps of one and the same sample. The development of the O(ls) intensity demonstrates that the HF dip reduces the oxygen coverage only slightly while annealing at 1170-1230 K in a Si flux removes the O contaminations to below the detection limit. The C(ls) line of the as-received sample is asymmetrically broad•

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V. van Elsbergen et aL tSurface Science 365 (1996) 443-452

446

ened and exhibits a shoulder on its low-energy side. This points to C atoms in different bonding configurations. The two further preparational steps reduce the line width and simultaneously the peak becomes symmetrical. Obviously the intensity of the component on the low-energy side diminishes. Annealing of Si-terminated surfaces at 1250 K results in a (6x/3x6x/3)R30 ° reconstruction. Correlated with this, the C(ls) line shifts by 1 eV towards lower kinetic energies and its line width increases. The Si(2p) peaks at a kinetic energy of 1381 eV show no shift but their line width reduces during the successive preparational steps. The Si(2p) line was recorded with increased surface sensitivity by using SXPS. Fig. 2 shows the Si(2p) peak as observed after the preparational steps indicated. The lines were fitted by two Gaussians each which have a line widths of 2.4 eV (FWHM) and are shifted by 1.7 eV in energy. The HF dip and the subsequent annealing treatment decreases the intensity of the low-energy component and, as a consequence of this, the Si(2p) line width is reduced. Fig. 3 shows the O(ls), C(ls), and Si(2p) corelevel signals of an as-received C-terminated surface and after subsequent preparational steps. The experimental results are similar to those obtained i

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with Si-terminated samples. Annealing at 1170 K for 10 min reduces the O contaminations to below the detection limit. Furthermore, the asymmetrically broadened C(ls) peak becomes symmetric and both the C(ls) and the Si(2p) core-level lines exhibit the smallest line width after this heat treatment. Raising the temperature to 1270 K shifts the C(ls) peak to smaller kinetic energies and increases its line width. These findings are similar to the observations correlated with the appearance of the (6x/3 × 6x/3)R30 ° reconstruction on Si-terminated surfaces. The Si(2p) core levels of C-terminated surfaces were also investigated with SXPS. Experimental results are displayed in Fig. 4. They are similar to the SXPS data recorded with a Si-terminated sample which are displayed in Fig. 2. A remarkable difference is that the portion of the low-energy component of the as-received surface is smaller for C-terminated than for Si-terminated samples. The low-energy components are, however, of the same magnitude after the HF dip irrespective of whether the samples are Si- or C-terminated. Fig. 5 displays Si(LVV) and C(KVV) signals recorded with HF-prepared and clean, i.e., oxygen free, (1 x 1) Si- and C-terminated SiC surfaces obtained by annealing at 1170 K. The Si(LVV) line of the HF-treated surface has an energy of 85.4 eV and is shifted by 1.6 eV to lower kinetic energies in comparison to the clean surface. The

F.. van Elsbergen et aL /Surface Science 365 (1996) 443-452 t

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Fig. 5. Si(LVV) and C(KVV) AFS spectra from Si- and C-terminated surfaces of 6H SiC as a function of the preparational step. C(KVV) transition, on the other hand, has the same peak position for both preparational steps. Our AES studies demonstrated that the energies of the C(KVV) and Si(LVV) transitions are identical on C- and Si-terminated surfaces for HF-treated and ( 1 x 1) surfaces prepared by annealing at 1170 K for 10 min in vacuum, within the limits of experimental accuracy. As was to be expected, the Si(LVV)/C(KVV) intensity ratio of

C-terminated. AES may thus be used to distinguish between both terminations of 6H-SiC surfaces. In order to characterize the progress in preparation, XPS measurements were carried out on as-received, HF- and Si-prepared samples and on surfaces annealed at 1170 K (Fig. 1 and Fig. 3). For quantitative analysis the area under the photoelectron peak after a linear background subtraction was used. The as-received 6H-SiC surfaces are covered by an oxide layer. O contaminations can be reduced to below the detection limit by heating of the samples in a Si atomic flux or annealing in vacuum. The oxygen coverages were estimated from the experimental Si/O XPS intensity ratios by using a layer model [10] and respective photoemission cross-sections [ 11 ]. For as-received Si-terminated surfaces an O coverage of 1.6+0.1 monolayers (ML) was obtained which is reduced to 0.6+0.1 M L by the H F treatment. The oxygen contamination of the C-terminated samples amounts to 1.2+0.1 M L for the as-received and to 0.6__+0.1 ML for the HF-prepared surfaces. This shows that the O contamination is smaller on C-terminated than on Si-terminated as-received surfaces. The O contaminations of HF-prepared Si- and C-terminated surfaces are identical. This result may be explained by two different adsorption sites of O atoms. The H F treatment then removes only one of these oxygen species. The decrease of adsorbed oxygen and the correlated reduction of the Si(2p) line width points to Si-O bonds. The decomposition of the Si(2p) peak recorded with Zr(M~) radiation (Fig. 2 and Fig. 4) reveals the existence of a component shifted by 1.7eV to lower kinetic energies. Its intensity decreases as the oxygen coverage is reduced and is no longer observed with clean surfaces. A chemical shift of 1.7 eV is a characteristic of Si atoms forming two bonds with two O atoms /0= Si~. O -

[12-14]. A shift of the Si(LVV) transition by 1.6 eV is observed by AES on HF-prepared and clean surfaces (Fig. 5). Bermudez 1-13] proposed a

448

V. van Elsbergen et al./Surface Science 365 (1996) 443-452

model for the oxygen adsorption on SIC(001) surfaces. He suggested oxygen insertion into backbonds S i - O - C and the formation of Si-O-Si bridges between surface Si atoms. Our results suggest that the H F treatment reduces the amount of O atoms bridgebonded between two Si atoms at the surface while backbonded O atoms can be reduced by a heat treatment only. From his AES results Dayan [2] concluded the existence of O-Si bonds only. Mizokawa [15] related this results to electron-beam damage effects in AES by comparison of AES and XPS measurements. Because of the limited resolution no statements can be made about the bonding configuration of C atoms at the surface for as-received and HF-prepared samples. The asymmetric broadening of the C(ls) peak to lower kinetic energies is indicative of C-contaminations with C-C, C-H, and C - O bonding [13,15-17] because carbon, oxygen, and hydrogen are more electronegative than silicon so that the C(ls) electrons are more tightly bound than in SiC. The C(ls) peaks of the Si-prepared and at 1170 K annealed surfaces show the smallest line width of all spectra during the preparation. Furthermore, the Si/C-AES ratios of Si- and C-terminated surfaces prepared by annealing only show a maximum for an annealing temperature of 1170 K. Annealing reduces not only O- but also C-contaminations so that the Si/Cratio increases. For temperatures above 1170 K, this ratio decreases due to Si loss so that a maximum in Si/C intensity ratio may be attributed to stoichiometric surfaces [3,18].

4. Reconstructions

LEED was carried out to gain information about the type and the quality of the surface reconstructions as a function of the preparational step. Fig. 6 shows observed LEED patterns achieved with beam energies varying between 71 and 126eV. After H F treatments Si- and C-terminated surfaces show slightly diffuse (1 x 1) LEED patterns. Three different surface phases were observed with Si-terminated surfaces heated to different temperatures in the presence of a Si flux. The (1 x 1)

pattern of the HF-treated surface is preserved up to temperatures of 1070 K but the spots become more diffuse. Annealing of the samples at 1170 K with the Si beam turned on results in sharp and intense (3 x 3) LEED patterns (Fig. 6a). Subsequent annealing in vacuum at 1230 K converts this pattern into a clear (x/3 x x/3)R30 ° reconstruction (Fig. 6b). At 1250 K the transition to a (6x/3 x6ff3)R30 ° phase is observed (Fig. 6c). In contrast to the experimental results of Kaplan [9], we observed no (1 x 1) LEED patterns at 1270 K. Therefore we explored a second preparation method. After an initial H F dip, Si- and C-terminated surfaces were annealed in vacuo at 1170 K for 10 min. Such treatments produce sharp (1 x 1) LEED patterns. This (1 x 1) reconstruction persists up to the highest temperatures used in this study of at least, i.e., 1270 K. As an example, Fig. 6d shows a pattern obtained with a C-terminated sample. The surface reconstructions observed in the sequence of the preparational step are summarized in the following scheme: Si-term.: (1 x 1)d Si,ll70 K (3 x 3) 123oK) (x/3 x x/3)R30 ° 125o (6x/3 x 6x/3)R30 ° Si-term.: ( 1 x 1)a C-term.: (1 x 1)e

1170 K

, ( 1 x 1)

1170 K

, (1 x 1)

The subscript d indicates that the LEED patterns were diffuse after the initial H F dip. The chemical compositions of the differently reconstructed surfaces were characterized by using AES and SXPS. Fig. 7 shows C(KVV) AES spectra of a Si-terminated surface after the successive preparational steps. The energy position of the C(KVV) transition is the same for the (1 x 1) surface, which was obtained by annealing of the sample in vacuum, as well as after an H F dip or heating in a Si beam to temperatures between 1170 and 1230 K. The transition energy amounts to 270eV. In contrast to this, the conversion of the (x/3 x x/3)R30 ° pattern to the (6x/3 x 6x/3)R30 ° reconstruction shifts the C(KVV) line by 1 eV to 269 eV. This points to different bonding configurations of carbon in the latter phase.

449

V. van Elsbergen et al./Surface Science 365 (1996) 443-452

(a)

(b)

(c)

(d)

Fig. 6. LEED-panerns for the following reconstructions and primary beam energies: (a) ~3 x 3), 105 eV; (b) (x/3 x ,/3) R30 ~. 71 eV; (c) (6V'3 x 6\/3) R 3 0 , 9 6 eV; (d) (1 x 1), 126 eV.

The simultaneously observed Si(LVV) transitions are displayed in Fig. 8. Two different line positions are to be distinguished. In the spectrum recorded with the (1 x 1) surface, the minimum appears at an energy of 87 eV but is shifted by 3 eV to 90 eV if the samples are heated in the Si flux. The Si(LVV) transition energy as well as the shape of the spectrum of the (3 x 3) reconstruction agrees with the one of clean Si(111) surfaces. This means that the chemical environment of the Si atoms are similar on both surfaces and, therefore, Si-Si bonds prevail on 6H-SiC(0001)-(3 x 3) surfaces. With increasing temperature, the ( 3 x 3 ) LEED pattern changes to a (~/3 x x/3)R30 ° and finally to a (6x/3 x 6x/3)R30 ° reconstruction. The AES spectra become asymmetrically broadened

towards lower energies. With prolonged annealing time at 1250 K (lower trace in Fig. 8) a second minimum develops at an energy of 87 eV. Its energy position equals the one observed with 6H-SiC(0001)-(1 x 1) surfaces prepared by annealing in vacuum. The experimentally observed Si(LVV)/C(KVV) peak-to-peak intensity ratios are displayed in Table 1. The data agree with ratios previously published by others for 3C-SiC(111) surfaces [9]. Fig. 9 displays the line position of the Si(2p) core-level as observed with differently prepared and reconstructed 6H-SiC surfaces. The core-level shifts reflect the differing chemical environments of the silicon atoms specific of the various surface reconstructions.

450

K van Elsbergen et al./Surface Science 365 (1996) 443-452 I

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Table 1 Comparison of Si(LVV)/C(KVV) AES intensity ratios with values reported previously by Kaplan [9] Si(LVV)/ C(KVV) (0001)-(3 x 3) (0001)-(x/3 x x/3)R30 ° (0001)-(6x/3 x 6x/3)R30 ° (0001)-(1 x 1) (0001)-(1 x 1)

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6'0'7'0' oo ',oo Kinetic energy (eV) Fig. 8. Si(LVV) AES spectra from Si-terminated (0001) surfaces of 6H-SiC as a function of the preparational step. Also shown for comparison the Si(LVV) line of a S i ( l l l ) surface.

The appearance of the (3 x 3) and (x/3 x x/3)R30 ° reconstructions of Si-terminated surfaces prepared in a Si beam as a function of the preparational step agrees with results previously published by Kaplan [9]. In contrast to his study,

however, heat treatments of the (x/3 x x/3)R30 °reconstructed surface at 1250 K showed no development of a (1 x 1) but of a (6x/3x6x/3)R30 ° phase. Annealing of Si- and C-terminated surfaces at 1170 K in vacuum results in a (1 x 1) reconstruction. Previous studies reported the formation of different L E E D patterns after thermal treatments at different temperatures [1,3,4]. Only little work was performed on 3 C - S i C ( I l l ) [19] or 6H-SiC surfaces [ 1,20-25]. Van Bommel et al. [1 ] carried out L E E D and AES experiments and explained the (6x/3x6x/3)R30 ° reconstruction as an ordered graphite overlayer. This finding is verified by our study. Fig. 1 and Fig. 7 show that both the C(ls)

V. van Elsbergen et al./Surface Science 365 (1996) 443-452

line and the C(KVV) peak recorded with (6x/3 × 6x/3)R30 ° surfaces are shifted by 1 eV to lower kinetic energies with regard to, for example, the (3 × 3) and the (~/3 × ~/3)R30 ° phases. This shift to higher binding energies was explained by the evaporation of silicon and formation of graphite [17,26], i.e., a replacement of Si-C bonds by C - C bonds. Carbon is more electronegative than silicon so that the C(ls) electrons are less tightly bound in SiC than in graphite. The same shift in C(ls) core-level energy is observed with Si- and C-terminated surfaces annealed at 1270 K. This finding demonstrates that such heat treatments result in a graphitization of the surface due to Si desorption. In contrast to other works [3,4] no changes in surface reconstruction, at least up to this temperature, were observed. The Si(LVV) AES spectra displayed in Fig. 8 illustrate the changes in chemical bonding of the Si atoms during the preparation of Si-terminated surfaces. The peak recorded with the (3 x 3) phase is indistinguishable with regard to line position and width from that recorded on a Si(111) surface. Therefore, the chemical bonding of the Si atoms within the information depth of AES is identical on both surfaces and typical of Si tetrahedrally bonded to Si. This demonstrates the existence of at least one or even more additional Si layers on Si-terminated surfaces. Continued annealing of the (3 x 3) surface at increasing temperatures results in the formation of (x/3 x x/3)R30 °- and (6~/3 × 6x/3)R30°-recon strutted surfaces. The Si(LVV) line broadens due to the development of a second peak. The initial one at 90 eV is typical of Si-Si bonds while the other line at 87 eV indicates the probing of Si-C bonds. This finding confirms previous results of van Bommel et al. [1] and Grant and Haas [27] who observed differences in Si(LVV) line positions of 3 eV and 2 eV for Si and SiC, respectively. The fact that the peak at 90 eV dominates the spectrum of the (x/3 x x/3)R30 ° surface definitely proves that this reconstruction cannot be explained by a Si loss from the outmost bilayer of SiC as was alternatively proposed by Kaplan [9]. Owman and Mfirtensson [22] studied the (~/3 × ~/3)R30 ° surface produced by annealing at 1170 K by scanning tunneling microscopy. They found a x/3 x ~/3 array

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of protrusions which they identified as adatoms. They were not able to determine whether these adatoms were Si or C atoms. Northrup and Neugebauer [25] identified the adatoms of the (~/3 x x/3)R30 ° phase to be Si atoms in T4 sites by first-principles total energy calculations. The surface-state band derived from dangling bonds on the Si adatoms was calculated to be 1.2 eV above the valence-band maximum and to be half filled. We found the Fermi level for clean (~/3 x x/3)R30 ° surfaces to pin at 1.2 eV above the valence-band maximum [28]. This result is in excellent agreement with the theoretical value and strongly supports the structural model. As was discussed already above, the appearance of the (6~/3 x 6~/3)R30 ° is attributed to a graphitization of the surface. The two minima of the Si(LVV) line show that there are still Si-Si bonds present due to adsorbed Si while at the same time the surface is already graphitized. Chang et al. [6] proposed the (6x/3 x 6x/3)R30 ° surface to consist of a monolayer of graphite on bulk truncated SiC. Northrup and Neugebauer [25] concluded from their results that the adatom model is very stable with respect to the ideal surface. Therefore, they explained the (6x/3 x 6x/3)R30 ° phase as a monolayer of graphite on the (x/3 x x/3)R30 ° surface. The fact that there are still Si-Si bonds present might support the latter model. Furthermore, this finding can explain why we observed the (~/3 x x/3)R30 ° reconstruction to change directly to a (6x/3 x 6~/3)R30 . The energy positions of the Si(LVV) and C(KVV) lines of the (1 x 1) phase obtained by annealing at 1170 K in vacuo without additional supply of Si amount to 87 and 270 eV, respectively. This indicates that the Si-C bonds are present up to the surface. Furthermore, this surface structure exhibits the maximum Si/C intensity ratio of all surface phases studied here, as was already mentioned above. Therefore, this reconstruction is attributed to stoichiometric surfaces. In addition to the AES line positions the ratio of the derivative peak-to-peak intensities of the Auger lines were evaluated (Table 1). As was to be expected, the Si/C-ratio of the Si-terminated surfaces prepared in the Si flux decreases during subsequent annealing at elevated temperatures in vacuo due to desorption of the Si adlayers or Si

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depletion of the surface. The table displays that the intensity ratios vary in a wide range although one and the same LEED pattern is observed. This indicates lateral inhomogeneities in composition on such surfaces. For quantitative analysis the Si/C AES intensity ratios were calculated using a layer model [ 10]. If stoichiometric Si- and C-terminated surfaces are proposed for the (1 × 1) phases the model predicts values of 1.05+0.05 and 0.73+0.05, respectively. They are in good agreement with the experimental data.

5. Conclusions

Si- and C-terminated 6H-SiC samples were studied by LEED, AES, SXPS and XPS. The samples were first dipped in HF. This preparational step reduced the O contaminations only partly. Clean surfaces were achieved by a subsequent preparational step in ultrahigh vacuum. The samples were either held at different temperatures in the presence of a Si flux or annealed at 1170 K. XPS measurements showed that O contaminations were reduced to below the detection limit and that C(Is) and Si(2p) lines had the smallest line widths for the clean surfaces. SXPS and AES results indicated that Si was bonded to two O atoms. The existence of two different adsorption sites of O atoms was suggested. The clean samples showed four different LEED reconstructions, namely: (1 x 1), (3 x 3), (x/3 x x/3)R30 °, and (6x/3 x 6x/3)R30 °. The bonding configuration of surface atoms as a function of the reconstruction was resolved by AES and XPS/SXPS. By heating of SiC surfaces at different temperatures in the presence of a Si flux we obtained several surface compositions ranging from Si-rich to Si-deficient. The (3 x3) and (x/3×x/3)R30 ° surfaces showed additional Si adsorbed on the Si-terminated surface. Our results support that the (x/3 × x/3)R30°-reconstructed surface consists of Si adatoms occupying T 4 sites on a Si-terminated surface. The (6x/3x6x/3)R30 ° LEED pattern was indicative of a graphitization of the samples. There is experimental evidence that

the (6x/3 × 6x/3)R30°-reconstructed surface arises from a monolayer of graphite above the (x/3 × ~/3)R30 ° phase. Annealing of Si- as well as C-terminated 6H-SiC surfaces at 1070 K in UHV resulted in stoichiometric (1 x 1) surfaces.

References [1] A.J. van Bommel, J.E. Crombeen and A. van Tooren, Surf. Sci. 48 (1975) 463. [2] M. Dayan, J. Vac. Sci. Technol. A 4 (1986) 38. [3] S. Nakanishi, H. Tokutaka, K. Nishimori, S. Kishida and N. Ishihara, Appl. Surf. Sci. 41/42 (1989) 44. [4] U. Starke, Ch. Bram, P.-R. Steiner, W. Hartner, L. Hammer, K. Heinz and K. Mailer, Appl. Surf. Sci. 89 (1995) 175. [5] J.J. Bellina Jr., J. Ferrante and M.V. Zeller, J. Vac. Sci. Technol. A 4 (1986) 1692. [6] C.S. Chang, I.S.T. Tsong, Y.C. Wang and R.F. Davis, Surf. Sci. 256 (1991) 354. [7] R. Kaplan, J. Appl. Phys. 56 (1984) 1636. [8] R. Kaplan and T.M. Parrill, Surf. Sci. 165 (1986) L45. [9] R. Kaplan, Surf. Sci. 215 (1989) 111. [10] R. Memeo, F. Ciccacci, C. Mariani and S. Ossicini, Thin Solid Films 109 (1983) 159. [11] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [12] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff and G. Hollinger, Phys. Rev. B 38 (1988) 6084. [13] V.M. Bermudez, J. Appl. Phys. 66 (1989) 6084. [14] J.M. Powers and G.A. Somorjai, Surf. Sci. 244 (1991) 39. [15] Y. Mizokawa, S. Nakanishi, O. Komoda, S. Miyase, H.S. Diang, C.-H. Wang, N. Li and C. Jiang, J. Appl. Phys. 67 (1990) 264. [16] K.L. Smith and K.M. Black, J. Vac. Sci. Technol. A 2 (1984) 744. [17] L. Muehlhoff, W.J. Choyke, M.J. Bozack and J.T. Yates, J. Appl. Phys. 60 (1986) 2842. [18] T.M. Parrill and Y.W. Chung, Surf. Sci. 243 (1991) 96. [19] B. Wenzien, P. K~ickell and F. Bechstedt, Surf. Sci. 331 333 (1995) 1105. [20] P. Badziag, Surf. Sci. 236 (1990) 48. [21] J. Schardt, Ch. Bram, S. MOiler, U. Starke, K. Heinz and K. Miiller, Surf. Sci. 337 (1995) 232. [22] F. Owman and P. M~rtensson, Surf. Sci. 330 (1995) L639. [23] M.A. Kulakov, P. Heuell, V.F. Tsvetkov and B. Bullemer, Surf. Sci. 315 (1994) 248. [24] P. Badziag, Surf. Sci. 337 (1995) 1. [25] J.E. Northrup and J. Neugebauer, Phys. Rev. B 52 (1995) R17001. [26] K. Miyoshi and D.H. Buckley, Appl. Surf. Sci. 10 (1982) 357. [27] J.T. Grant and T.W. Haas, Surf. Sci. 23 (1970) 347. [28] V. van Elsbergen, T.U. Kampen and W. MOnch, J. Appl. Phys. 79 (1996) 316.