X-ray absorption and photoemission spectroscopy of 3C- and 4H-SiC

Surface Science 600 (2006) 3879–3883 www.elsevier.com/locate/susc

X-ray absorption and photoemission spectroscopy of 3C- and 4H-SiC M. Tallarida a

a,*

, D. Schmeisser a, F. Zheng b, F.J. Himpsel

b

Angewandte Physik-Sensorik, Brandenburgische Technische Universita¨t, Konrad Wachsmann Allee, 17, D-03046 Cottbus, Germany b Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, WI 53706, USA Available online 8 May 2006

Abstract We have studied the electronic properties of 3C- and 4H-SiC with X-ray absorption (XAS). Particular emphasis is placed on the conduction bands because they exhibit larger differences between the various SiC polytypes than valence bands. XAS spectra at the Si2p and C1s edges provide projections onto Si3d, 4s and C2p conduction band states. We explain the observed differences in the Si L2,3 XAS data to arise from transition into dispersive bands which occur at the M and K point of the hexagonal Brillouin zone. The XAS data are sensitive to a difference in the dispersion of the two lowest conduction bands. For 3C-SiC the dispersion is larger than for 4H-SiC in agreement with theory. We compare the XAS data at the Si L edge with CFS and CIS spectra and find that the SiLVV Auger is dominant.  2006 Elsevier B.V. All rights reserved. Keywords: NEXAFS; Silicon carbide; Photoelectron emission

1. Introduction SiC is a promising material for future high performance electronic devices. The technical problems that need to be solved, in order to spread out the use of SiC devices, is the low capability of producing good quality, defect-free, SiC wafers. Among the possible origins of defects, particularly interesting is the existence of many polytypes, the zinc-blende (cubic) 3C-SiC, the wurtzite (hexagonal) 2HSiC and other hexagonal modifications like the 4H- and 6H-SiC, which have similar energy of formation and may mix during the growth of thick wafers. This is the reason why bulk growth of cubic SiC is very difficult without an intrusion of other polytypes, so the usual way to obtain 3C-SiC samples is by epitaxy on silicon substrate. Different polytypes consist of different stacking sequences on the cubic (1 1 1) and the hexagonal (0 0 0 1) planes of the same Si-C bilayer. The bilayers are successively displaced sidewise and rotated of 60, and may occupy three different position: A, B or C. In 3C-SiC, the sequence is

*

Corresponding author. E-mail address: [email protected] (M. Tallarida).

0039-6028/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.096

ABCABC. . ., in 2H it is ABAB. . ., in 4H it is ABCBABCB. . . and in 6H it is ABCACBABCACB. . . Many physical properties, like the lattice constants, the bulk moduli and their pressure derivatives have small variation with polytype [1,2]. Also the ionicity of the Si-C bond, due to charge asymmetry, is similar for all the polytypes [1,3]. On the contrary, other important properties, like the indirect band gaps, the electronic mobility, and the location of the conduction band minima in k-space, vary strongly with crystal structure [1,2]. The indirect gap value, for example, is 2.4 eV in 3C-SiC, and 3.3 eV in the 4H modification. Much effort has been devoted in the past years to study the dependence of both ground state and electronic properties of SiC on polytype. Ab initio methods gave good results in calculating the electronic bands, the density of states (DOS) and the optical spectra of the various polytypes by implementation of various approximations [1–13]. Agreement has been found for the location of the valence band maximum (VBM) at the C point in all polytypes. The differences in the electronic structure are generally attributed to conduction band states. In the conduction band a competition between the two lowest band states is observed, which is responsible for the different location of the conduction band minimum (CBM) in

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the Brillouin zone of different polytypes. Calculations revealed the location of the CBM at the X point in 3C-SiC; at the M point in 4H-SiC, and at the K point in the 2HSiC, while for the 6H polytype the location of the CBM is still under debate [1,2,4]. In order to compare the band structures of the cubic and the hexagonal polytypes, it has been proposed to project the cubic Brillouin zone onto an hexagonal one. In this way the CBM for the 3C-SiC occurs near the M point [2,4,8]. Going from the 3C polytype to the 2H, the energy of the lowest conduction band located near the M point increases while the energy of the K conduction band state decreases. This shift of the lowest conduction bands result in their different energy separation and a different energy position of the CBM with respect to the VBM, i.e. a different band gap. The dependence of the indirect band gap on polytype has been predicted by ab initio calculations, although it is well-known that calculations based on DFT-LDA theory give underestimated band gap values. These are usually corrected by considering manybody quasiparticle effects to treat the exchange-correlation term during the excitation of electrons and holes in a better way [9], corresponding mainly to a rigid shift of the conduction bands. Partial density of states (PDOS) have been also calculated. Zhao and Bagayoko [6] found that the major contribution at the CBM arises from the Si (s + p + d) states; in contrast Bernstein et al. [7] found as most important contribution, near the conduction band edge, the C p-derived states and the Si s-derived states. Experimentally the bulk electronic bands have been studied by Lu¨ning et al. [14] who applied soft X-ray emission (SXE) and absorption spectroscopy to determine the bulk valence band dispersion of 3C-, 4H- and 6H-SiC polytypes, focussing their studies on the valence band dispersion and the variation of indirect gap with polytypes; and by Long et al. [15] who determined the structure of the c(2 · 2) reconstruction of 3C-SiC(0 0 1) from the C 1s surface–core exciton. The electronic properties of surface reconstructions on both cubic and hexagonal polytypes have been studied mainly through angle-resolved, direct and inverse, photoemission [16,17] and core level spectroscopy [18– 20], and their atomic structure through scanning tunneling microscopy [21,22] and LEED [23]. We show here a detailed X-ray absorption spectroscopy (XAS) investigation of 3C-SiC epitaxial films, grown on silicon substrate, and of 4H-SiC homoepitaxial films devoted to study their electronic properties by comparing the XAS spectra with the calculations found in literature. Different from a previous XAS study [14] we extend the energy range up to 20 eV above the absorption onset and compare the results with density of states calculations. XAS is a powerful technique to study the unoccupied density of states of compounds, being element specific [24]. XAS gives also the possibility to investigate the electronic character of the states responsible for the density of states, being the absorption process governed by the dipole selection rules which state the changing of orbital momentum l in the excited state of 1 quantum number consequently to the photon absorption.

Moreover, we use photoemission to assign the individual features in XAS spectra. 2. Experimental 3C-SiC sample were epitaxially grown ex situ by chemical vapor deposition (CVD) on Si(0 0 1) and characterized by means of XPS, XRD, and FT-IRAS [25]. 4H-SiC samples were homoepitaxial films grown on single crystalline samples of the same polytype at the IKZ institute, Berlin [28]. Both samples, initially covered with native oxide, were prepared by etching with HF ex situ and introduced to the measurement chamber. This procedure ensures a clean, oxide free, 1 · 1 bulk terminated surface in both polytypes [23]. XAS spectra were detected in the total electron yield (TEY) mode by measuring the sample photocurrent at the Synchrotron Radiation center (SRC) on the HERMON beamline (port 033) [26,27]. All spectra are normalized to the incoming photon intensity, and a linear background accounting for the valence absorption is subtracted. In order to account for the spin–orbit splitting of the Si 2p core level, the Si L2,3 edge spectra are decomposed by a numerical routine into two components which have 0.5 relative intensity and are shifted of 0.61 eV from each other. Photoemission data for 3C-SiC were recorded in Berlin at BESSY at the U49/2 beamline [29] with samples prepared by heating in UHV to 1100 C [30]. 3. Results Absorption spectra of the 3C- and 4H-SiC polytypes, for the Si L and the C K edges, are shown in Fig. 1, where the TEY intensity is plotted in function of photon energy. In Fig. 1a we show the absorption spectra of 3C- and 4HSiC at the Si 2p edge and in Fig. 1b those at the C 1s edge. One can easily distinguish two energy regimes where the absorption spectra of the two polytypes behave in different ways, defined by the maximum of absorption intensity E0. On the right side of E0, for the Si L3 and C K edge, respectively, the spectra are dominated by similar modulations in both 3C- and 4H-SiC, but for the 4H polytype they are sharper than for the 3C-SiC. Left from E0 the absorption spectra are different in the two SiC structural modifications. The absorption cross section of 4H-SiC increases very steeply, and the Si L2,3 spectrum has two relative maxima before E0, while in the 3C-SiC the absorption increases slowly up to the peak at E0. These findings are in agreement with XAS data measured in the more bulksensitive fluorescence mode [14], although those data were limited to smaller energy range (only 6 eV above the CBM), and our spectra seem to show a better resolution. In Fig. 2 we show a representative valence band spectrum of 3C-SiC measured above, but close to the C K edges. At 340 eV photon energy the valence band spectrum is dominated by three main features; we denote them with a, at about 3 eV; b at about 9 eV, and c at about 13 eV. This photon energy was chosen in order to com-

M. Tallarida et al. / Surface Science 600 (2006) 3879–3883

Si L3 edge

Si L2,3 edge

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3C-SiC

Total electron yield (a.u.)

3C-SiC 4H-SiC

AEY=92eV

AEY=80eV

E0 95

100

TEY

105

(a)

110

115

120

Photon energy (eV)

a

C K edge

b

CIS

Total electron yield (a.u.)

3C-SiC

280

4H-SiC 95

100

105

110

115

120

Photon energy (eV) Fig. 3. (Top) Comparison of TEY and AEY spectra of 3C-SiC at the Si L edge. The Auger yield were detected at 80 eV and 92 eV kinetic energy. The XAS spectrum is same as in Fig. 1a but without spin–orbit subtraction. All spectra were arbitrarily normalized to the peak at 105 eV for comparison. (bottom) CIS spectra. As initial states are chosen the two peak a and b shown in Fig. 2.

E0

285

290

295

300

305

Photon energy (eV)

(b)

Fig. 1. XAS spectra of 3C-SiC and 4H-SiC after normalization and background subtraction for (a) Si L edges after spin–orbit decomposition and (b) C K edges. With E0 are shown the absorption maxima at both edges, occurring, respectively at about 105 eV and 293 eV.

3C-SiC(001) Valence band hν=340 eV

(c)

(a)

(b)

pare our results with those on Ref. [14]. The SXE data from Ref. [14] indicated that the peak a is due to states with C p-character, while b and c are due to states with Si (s + d)-character, predominantly, while for the electronic character of the other states between a and b the SXE data could no give any indication. In Fig. 3 the comparison of TEY, Auger electron yield (AEY) and constant initial state (CIS) spectra is shown. The kinetic energies of the two AEY are, respectively, 92 eV and 80 eV, but they show very similar features. The TEY and AEY spectra were arbitrarily normalized to the peak at 105 eV for comparison. As initial states for CIS spectra are chosen the peaks a and b of Fig. 2. Here we see a different behaviour of the two spectra, with a resonance for peak b at about 101 eV and an almost flat behaviour of peak a. 4. Discussion

-20

-15

-10

-5

0

Binding energy (eV) Fig. 2. Valence band of 3C-SiC(001) measured at 340 eV, i.e. at the same energy as in Ref. [14]. Three peaks dominate the valence band spectrum: (a) at 3 eV; (b) at 9 eV; (c) at 13 eV.

First we note that in the XAS data the shift of the absorption onset is the signature of the particular polytype, and depends on the stacking sequence of the Si-C bilayer. In Fig. 1 we find a shift of about 0.9 eV, which is near the difference of indirect band gap between 3C- and 4HSiC [1–3]. This shift is due to the different energy position of the lowest conduction band in the two polytypes with respect to the valence band maximum which occurs, in a similar manner in all polytypes, at the C point of the Brillouin

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zone. The differences in the XAS spectra are directly related to the different stacking sequence of the two polytypes, and consequently to their different electronic properties. The region in the Si L2,3 on the left side of E0 is different for the two polytypes and, as anticipated in the introduction, is influenced by the larger dispersion of the lowest conduction band in 3C- with respect to 4H-SiC. One finds that in both polytypes the CBM are near the M point, but the energy separation between the states at M and at K points is smaller in the 4H-SiC than in the 3C polytype, when one compares the two polytypes in the same hexagonal Brillouin zone.1 Theoretical studies agree that the lowest states in the CB consist of two almost parabolic bands at the M and K points [1,2,4,6–14]. They reflect the hybridization of Si (s + p + d) and C (s + p) states along the Si C Si bonds of Si-C tetrahedra of adjacent SiC double layers. For the 3C-SiC polytype there is a larger dispersion of the two bands because of the cubic stacking of the tetrahedra and, therefore, this polytype has the lowest band gap. Accordingly, we explain the differences in the data of Fig. 1. The onset of the XAS absorption spectrum of 3C-SiC is rather unstructured as the band at the M point has a large dispersion and the next unoccupied states which appear at the K point are already close to the low dispersive d-states. In contrast, for the 4H polytype the two bands at the M and at the K points are much closer in energy as the bands have a smaller dispersion and the first band is shifted to higher energies and the second to lower energies, resulting in the higher band gap, hence. This is the result of the rotation of the Si-C tetrahedra to give the hexagonal ABCBA. . . stacking sequence. As a consequence, the orbital overlap is reduced and the dispersion of the two lowest bands become smaller. In the absorption process of the 4HSiC polytype we find the two transitions to the bottom of the bands at the M and at the K points which give the double peak structure shown in Fig. 1a. The C K edges show also the shift due to the different gap of 3C- and 4H-SiC, and also have different features near the absorption edges up to E0 (see Fig. 1b). Like in the Si L3 edge spectra, for energies larger than E0 similar modulation of the absorption cross-section in both polytypes, although the 3C-SiC has broader features. The dipole selection rules state that the transition of the core level due to the photon absorption may happen only to states with Dl = ±1, but for the states with Dl = +1 the transition probability is almost 50 times larger than for those with Dl = 1 [31]. Therefore, the observation of the peaks at 105 eV suggests the presence of mainly Si 3d-derived and, possibly, Si 4s-derived states. The interpretation of XAS spectra as a simple reproduction of the unoccupied density of states is not straightforward, at least near the absorption edge, where the excited electrons are in a bound state and their lifetime can be long enough to interact with the valence band states 1

For comparison with band structure calculations we suggest those which use the hexagonal Brillouin zone representation, in particular that of Ka¨ckell et al. of Ref. [2].

nearby, before decaying. Taking into account this possibility could mean considering the joint density of states (JDOS) instead of only the unoccupied DOS, and comparing the XAS with the calculated JDOS or with the imaginary part of the dielectric tensor Im{e(x)}, which is directly calculated by the electronic band structure and is related to the absorption of valence band states into the conduction band. We show in Fig. 4 the comparison for the 3C-SiC polytype with the Im{e(x)} calculated by Adolph et al. (c) and by Xu et al. (b), and complete the figure with the DOS calculated by Adolph et al. (a). The XAS results and the Im{e(x)} and DOS calculation are represented in the same scale, but the reference energies are different. The XAS curve is referred to the valence band maximum (VBM) as measured by photoemission; the DOS is referred to the corrected indirect band gap (2.4 eV); while the JDOS are referred to the direct band gap (3 eV), as calculated by Adolph et al. [12]. The curves (b) and (c) are very similar, and the agreement with the Si L3 edge is very good: the slope near the band edge is well reproduced and the separation of the two peaks also agrees with the XAS features. The absorption cross section decreases for energies larger than 5 eV above the absorption onset, but above this energy value, the final states obtained by the absorption are vacuum states and the interaction of the excited states with the local valence band states is just a weak Coulomb force. The onset of the Im{e(x)} is obviously shifted with respect to the XAS absorption edge, being due to a direct transition occurring at about 3 eV [12], anyway the coincidence of the two major peaks is very interesting. On the contrary the curve (a) has two peaks at 6.3 eV and 7.2 eV above the VBM. The energy position of the two peaks does not agree with the peaks measured by XAS, and even their energy separation does not coincide with our XAS results. This confirms that to compare the XAS with theoretical calculations of SiC, one has to take

(e) (d)

3C-SiC

Direct gap Indirect gap JDOS (a)

(b)

(c)

DOS

0

2

4

6

8

10

12

Energy relative to valence band maximum (eV) Fig. 4. Comparison of the XAS spectra (d = Si L edge and e = C K edge) of 3C-SiC with calculations of the imaginary part of the dielectric tensor of Xu et al. (b) (from Ref. [5]) and of Adolph et al. (c) (from Ref. [12]). DOS calculation from the same authors is also included (a) to underline the differences with the Im{e(x)} calculations. This curve is shift for clarity.

M. Tallarida et al. / Surface Science 600 (2006) 3879–3883

into account also the density of occupied states. We focus in Fig. 3 on the differences occurring between the XAS, Auger electron yield (AEY), and CIS data, respectively. There is a very good agreement in the features of the Auger yield curves and the XAS data. They agree in the position of the resonance energy E0, the energy of the weaker feature at about 108 eV, and of the more pronounced structure at about 116 eV. We learn from this agreement that the Si LVV Auger is the dominating mechanism at the Si2p resonance. Differences occur for the features appearing before the main resonance energy E0 at 104.8 eV. Here the features around 101 eV occur for the valence band state labelled b and are not observed in the XAS data. We find for this state a resonance energy which is below the XAS onset, concluding that this feature is indicative of gap states which are located in energy about 0.7 eV below the CBM. In summary, the XAS spectrum of 3C-SiC at the Si L edge is consistent with JDOS. However, also the Si LVV Auger is of importance already at E0. At present we are not able to decide to which content these two mechanisms contribute to the XAS data. 5. Conclusions We have measured XAS spectra of 3C- and 4H-SiC at the Si L2,3 and C K edges. Differences and similarities between the polytypes are interpreted considering the different stacking sequence of the two polytypes. The shift of 0.9 eV of the XAS onset is a signature of the different band gap of the two polytypes. The Si L2,3 edge in 3C-SiC is structureless near the onset the XAS spectrum and has two peaks in 4H-SiC. We attribute these two peaks to states of the conduction bands located near M and K points of the hexagonal Brillouin zone, respectively. The maximum of absorption intensity is at E0 = 105 eV and is mainly due to Si 3d states. The XAS Si L3 spectrum for the 3C-SiC polytype has similar features to those present in calculated joint density of states (JDOS), as observed in the calculated imaginary part of the dielectric tensor. In particular, the energy E0 of the Si L2,3 edge fits correctly with the JDOS calculations, while in DOS calculations the maximum appears shifted of 0.6 eV. From the comparison of the TEY with CFS data we show that at the Si L2,3 edge, even at energies below the vacuum level, the main contribution to XAS is given by the Si LVV Auger process. Acknowledgements We would like to thank D. Siche (IKZ, Berlin) and R. Sohal for discussions and for providing SiC samples. This

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work is supported by the Deutsche Forschungsgemeinschaft with the project DFG SCHM 745/9-2. References [1] C.H. Park, B.-H. Cheong, K.-H. Lee, K.J. Chang, Phys. Rev. B 49 (1994) 4485. [2] P. Ka¨ckell, B. Wenzien, F. Bechstedt, Phys. Rev. B 50 (1994) 1076. [3] J. Pollmann, P. Kru¨gerHandbook of Surface Science, vol. 2, Elsevier Science BV, 2000, p. 96. [4] W.R. Lambrecht, S. Limpijumnong, S.N. Rashkeev, B. Segall, Phys. Status Solidi (B) 202 (1997) 5. [5] P. Xu, C. Xie, H. Pan, F. Xu, J. El. Spec. 144–147 (2005) 593. [6] G.L. Zhao, D. Bagayoko, New J. Phys. 2 (2000) 16. [7] N. Bernstein, H.J. Gotsis, D.A. Papaconstantopoulos, M.J. Mehl, Phys. Rev. B 71 (2005) 075203. [8] G. Wellenhofer, U. Ro¨ssler, Phys. Status Solidi (B) 202 (1997) 107. [9] B. Wenzien, P. Ka¨ckell, F. Bechstedt, Phys. Rev. B 52 (1994) 10897. [10] G. Pennington, N. Goldman, Phys. Rev. B 64 (2001) 045104. [11] Z. Jiang, X. Xu, H. Wu, F. Zhang, Z. Jin, Solid State Commun. 123 (2002) 263. [12] B. Adolph, K. Tenelsen, V.I. Gavrilenko, F. Bechstedt, Phys. Rev. B 55 (1997) 1422. [13] G. Cubiotti, Yu.N. Kucherenko, V.N. Antonov, J. Phys.: Condens. Matter 9 (1997) 165. [14] J. Lu¨ning, S. Eisenbitt, J.E. Rubensson, C. Ellmers, W. Eberhardt, Phys. Rev. B 59 (1999) 10573. [15] J.P. Long, V.M. Bermudez, D.E. Ramaker, Phys. Rev. Lett. 76 (1996) 991. [16] P.-A. Glans, T. Balasubramanian, M. Syva¨ja¨rvi, R. Yakimova, L.I. Johansson, Surf. Sci. 470 (2001) 284. [17] R. Ostendorf, C. Benesch, M. Hagedorn, H. Merz, H. Zacharias, Phys. Rev. B 66 (2002) 245401. [18] V.Yu. Aristov, H. Enriquez, V. Derycke, P. Soukiassian, G. Le Lay, C. Grupp, A. Taleb-Ibrahimi, Phys. Rev. B 60 (1999) 16553. [19] H.W. Yeom, Y.-C. Chao, S. Terada, S. Hara, S. Yoshida, R.I.G. Uhrberg, Surf. Sci. 433 (1999) 392. [20] N. Sieber, Th. Seyller, R. Graupner, L. Ley, R. Mikalo, P. Hoffmann, D.R. Batchelor, D. Schmeisser, Appl. Surf. Sci. 184 (2001) 278. [21] P. Soukiassian, F. Semond, L. Douillard, A. Mayne, G. Dujardin, L. Pizzagalli, C. Joachim, Phys. Rev. Lett. 78 (1997) 907. [22] F. Owman, P. Ma˚rtensson, Surf. Sci. Lett. 330 (1995) L639. [23] U. Starke, Phys. Status Solidi (B) 202 (1997) 475. [24] J. Sto¨hr, NEXAFS Spectroscopy, in: R. Gomer (Ed.), Series of Surface Sciences, vol. 25, Springer-Verlag, Berlin, 1992. [25] A. Goryachko, Y. Yemorenko, K. Henkel, J. Wollweber, D. Schmeisser, Phys. Status Solidi (A) 201 (2004) 245. [26] Y.-Y. Luk, N.L. Abbott, J.N. Crain, F.J. Himpsel, J. Chem. Phys. 120 (2004) 10792. [27] R.D. Peters, P.F. Nearley, J.N. Crain, F.J. Himpsel, Langmuir 18 (2002) 1250. [28] H.-J. Rost, K. Irmscher, J. Doerschel, D. Siche, D. Schulz, J. Wollweber, Mater. Sci. Forum 433–436 (2003) 91. [29] D. Schmeisser, P. Hoffmann, G. Beuckert, in: Z. Ehrenfried, W. Caroline, M. Thomas (Eds.), Materials for Information Technology, Devices, Interconnects and Packaging, Series: Engineering Materials and Processes, Springer, 2005, ISBN 1-85233-941-1. [30] M. Tallarida, R. Sohal, D. Schmeisser, Surf. Sci., to be submitted. [31] B.K. Teo, P.A. Lee, J. Am. Chem. Soc. 101 (1979) 2815.