Electronic and geometric structure of Si(111)-(7 × 7) and Si(001) surfaces

346

Surface

Science 1X1 (19X7) W--355 worth-Holland, Amsterdam

ELECTRONIC AND GEOMETRIC STRUCTURE OF Si(lll)-(7 AND Si(OO1)SURFACES R.J. HAMERS,

R.M. TROMP

X

7)

and J.E. DEMUTH

IRM T.J. WU~.VOI~ Rrswrch Center. P.O. Ro.r _?I& Yorh toxw Heighrr. NY i(l.tW. i,:.W Received

15 July 1986; accepted

for publication

30 July 1986

The atomic origins of the intrinsic surface states of the Si(lll)-(7x7) and Si(OO1) s~rfaceh have been identified using the recently developed method of current imaging tunneling \pcctroscopy (CITS). On Si(lll)-(7 X7) three filled and two empty surface states arc found and directly identified with atomic features of the dimer-adatom-stacking fault model. On Si(OO1) one filled and one empty state are observed and identified with atomic features of a dimcr model. The STM images of Si(OO1) are shown to be dominated by the surface electronic structure rather than geometric structure.

1. In~uctiun The real-space observation of the Si(lll)-(7 X 7) reconstruction by Binnig et al. [l] represented the first great triumph of scanning tunneling microscopy (STM). More recently, is has been recognized that both the atomic positions and the local electronic structure state contribute to STM images [2-S]. While in some cases STM images appear to mainly reflect the atomic geometry [6.9], in other cases [5,6,10-121 the local electronic structure completely dominates the images. Since the electronic structure is determined by the atomic positions, it is necessary to understand how both the geometric structure und the electronic structure contribute to STM images in order to be able to interpret the results. Previous attempts to study the electronic structure of surfaces on an atom-by-atom basis have been plagued by a number of experimental probiems. One approach f4] relied on a modulation technique to obtain images of dl/dV as a function of the average bias voltage. However, at each voltage the tip follows a different contour so that the resultant images contain contributions from both the geometric and electronic structure and are difficult to interpret. “Topographic” images obtained by switching the bias voltage between two values on alternate line scans can be interpreted in terms of atomic-scale spectroscopic features [II], but the procedure requires repetitive measurements and becomes tedious if several surface states are present. 0039-~028/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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Furthermore, neither of the above methods can provide spectroscopic information on electronic or defect states lying close to the Fermi level on materials with bandgaps of the intrinsic surface states. We have developed a new method, current imaging tunneling spectroscopy (CITS) [2], which overcomes the limitations of the previous techniques and allows us to obtain directlyinterpretable images of the surface electronic structure. We have applied this technique to the Si(lll)-(7 x 7) and Si(OO1) surfaces and have identified the various electronic states previously observed in photoemission and inverse photoemission and have established their geometric origin. These studies provide the first direct, real-space identification of intrinsic surface states and their atomic origins. In the CITS technique, a constant-sep~ation 1-Y curve is measured along with the topography at each point along a raster scan by gating the feedback loop so that it is only active about 10% of the time; during the remaining time the applied bias is ramped and the current is measured at up to 48 different bias voltages. On the Si(lll)-(7 x 7) and Si(OO1)surfaces, the bias voltage for stabilization of the feedback loop and the “ topography” scan can be chosen to give very close agreement between the STM images and total charge density calculations for the dimer-adatom-stacking fault (DAS) model [13] and for dimer models [14], respectively. Under these conditions, the tip approximately follows the atomic corrugations of the surface, so as to maintain a nearly constant tunnel barrier thickness along the surface.

2. Si(lll)-(7

X 7)

As first noted by Binnig et al. [l], STM images of the Si(lll)-(7 X 7) surface are characterized by twelve protrusions (generally interpreted as adatoms) and by a deep corner hole. Yet, the images vary substantially depending on the magnitude and polarity of the bias voltage applied to the sample [6]. When a negative bias is applied to the sample (while the tip is held at virtual ground), tunneling occurs from the filled electronic states of the sample to the empty states of the tip, and the STM images reveal a marked asymmetry with respect to the short diagonal of the unit cell. The adatoms in one half (the faulted half according to the the DAS model) appear higher than those in the other half, and in each half the six adatoms adjacent to a corner hole appear higher than the other six. At most positive sample voltages (tunneling from tip to sample), the unit cell appears to be symmetric. Yet, at some positive bias voltages the unit cell is again asymmetric [4], but with the “high” and “low” halves reversed from those observed at negative sample bias. We have found that at sample bias voltages near +2 V, STM images of Si(lll)-(7 x 7) are in excellent agreement with total charge density calculations for the DAS model of Takayanagi et al. [13] and appear to reflect the

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R.J. Humers et ~1. / Eiectrontc ond geometnc structure of St(I 11)-(7X 7) and Si(OO1)

geometric positions of the atoms. We have obtained CITS images of the tunneling current of this surface, choosing a +2 V bias for the feedback loop stabilization so that the tip follows this geometric contour. In figs. lA-IN we show the spatial distribution of the tunneling current measured at various bias voltages between +2 and -2 V. At the smallest negative bias voltages (lA, -0.15 V and lB, -0.25 V) the adatoms in the faulted half of the unit cell give rise to more tunneling current than those in the unfaulted half. In each half, the three adatoms which are closest to the corner hole appear to be higher than the other three. As the bias voltage is increased to near - 0.65 V (fig. lC), the tunneling current seems to fan toward the center and at -0.75 V (fig. 1D) six small regions of high current density appear in each unit cell. These points grow more intense at higher voltages (lE, -0.95 v> and still dominate the image at 1.55 V (lF, - 1.55 V). These high current regions are located exactly where the DAS model predicts dangling bonds on the “rest” atoms in the first full atomic layer. At still higher voltages (lG, -2.0 V), additional current density arises from the regions between the adatoms where backbonds between the adatoms and the first full atomic layer as well as other Si-Si bonds are exposed to the vacuum. At positive sample bias, the changes are more subtle. At the lowest positive bias (lH, 0.15 V), there is an asymmetry between the two halves of the unit cell which is the same as that observed at low negative bias. This similarity suggests that the images at the lowest positive and negative biases involved tunneling through the same electronic state. As the voltage is increased, the asymmetry becomes less pronounced (11, 0.25 V). At biases of 0.65 (fig. 1J) and 0.75 V (fig. lK), the unit cell appears rather symmetric, although a slight three-fold symmetry can be detected with the adatoms in the unfaulted half of the unit cell giving rise to more tunneling current that those in the faulted half. By 0.95 V (fig. lL), the asymmetry is more pronounced. Near 1.2 V, there is a uniform increase in the tunneling current from the entire unfaulted half of the unit cell, and near 1.4 V a similar increase is observed in the faulted half. A pronounced asymmetry results from the different onset energies in the two halves, which can be observed in the tunneling current images near 1.55 V (fig. 1M). This asymmetry disappears from both the current images and topographic images at higher bias voltages. At +2V (fig. lN), a uniform gray current image is obtained because this is the voltage chosen for stabilization of the feedback loop. We earlier reported [2] detailed conductance-voltage (u- V) measurements of various 3 A diameter areas within the (7 x 7) unit cell along with the spatial averaged characteristics. These measurements revealed step-like “onsets” in the conductance at sample voltages of - 1.7, -0.8, -0.4, + 0.5, and + 1.3 V. Furthermore, the energies of these onsets agree remarkably well with the energies of the peaks in the surface density-of-states (DOS) as determined from photoemission and inverse photoemission studies [15,16]. Fig. 1 shows

R.J. Hamers et al. / Electronic and geometric structure of Si(I I l)-( 7X 7) and Si(OO1)

A

H

B

I

c

J

D

K

E

L

F

G

349

M

N

Fig. 1. Spatial distribution of the tunneling current of Si(lll)-(7X7) with the feedback loop stabi Iized at + 2 V sample bias. Images shown at applied bias voltages of -0.15 V (A); -0.25 V -0.75 V(D); -0.95 V (E); -1.55 V(F); -2.0 V(G); +0.15 V(H); +0.25 V (IQ; -0.65 V(c); (I); to.65 V (J); + 0.75 V (K); +0.95 V (L); + 1.55 V (M); + 2.0 V (N).

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that these tunneling current increases are localized to small regions within the Si(lll)-(7 x 7) unit cell and give rise to the changes in the spatiul distribution of the tunneling current. These spatially resolved images also allow us to discriminate between the electronic structure of the sample and that of the tip. For example, energy-dependent variations in the DOS of the tip might also produce structure in the 1-V curves, but such structure should be uniform throughout the unit cell. In contrast, we can directly examine images such as fig. 1 and see that each “onset” is accompanied by a change in the sputial distribution of the tunneling current and so must be associated with the electronic structure of the sample. By subtracting images of the tunneling current made at slightly different bias voltages, the voltage-dependent changes in the spatial distribution of the tunneling current can be identified more clearly. When the voltages straddle an “onset” voltage the difference image can be represented as an image of the electronic state giving rise to that onset. Fig. 2 shows such difference images for the filled Si(lll)-(7 x 7) surface states. Fig. 2A shows a topograph taken with + 2 V applied to the sample. A difference image centered at - 0.35 V (fig. 2B) reveals an electronic state which is localized on the 12 adatoms but which has a different DOS on each of the four types of adatoms. The state is stronger in the faulted than in the unfaulted half of the cell, and in each half the state is stronger on the three adatoms adjacent to a corner hole than on the three. It is this state which gives rise to the asymmetry always observed at negative sample bias. Fig. 2C shows a difference image centered around -0.8 V. This state is localized to three spots within each half of the unit cell, exactly where the DAS model [13] predicts dangling bonds on the “rest atoms” in the first full atomic layer. When the STM tip is its sharpest (i.e., when we observe the largest corrugations), we also observe tunneling from the corner hole, where there is another dangling bond. A third filled state is observed near - 1.7 V as in fig. 2D. This state is characterized by an increase in tunneling current from the regions surrounding the adatoms as well as the corner hole, where Si-Si backbonds and additional Si-si bonds are exposed to the vacuum. Difference images for the empty surface states are not presented here, but the empty state producing the onset at + 0.5 V is localized on the adatoms and appears similar to the current images in fig. 11 and 1J. Another empty state is observed which is localized between the adatoms and has different onset energies in the unfaulted (1.2 V) and faulted (1.4 V) of the unit cell as described earlier. The difference in onset energies demonstrates that this state is intimately related to the presence of the stacking fault. We note that if the surface rearrangement was not accompanied by any charge transfer, the 19 dangling bonds (12 adatom, 6 rest atom, and one corner hole) would all be half-filled and would lie at the Fermi level. Yet, we find that the rest-atom dangling bonds are at -0.8 eV and must therefore be completely filled, while the adatom dangling bonds straddle the Fermi level

R.J. Hamers et al. / Electronic and geomeiric structure of Si(l I I)-( 7 X 7) and Si(OO1)

351

Fig. 2. CITS difference images of occupied Si(lll)-(7 X 7) surface states. Topographic image (A); adatom state at - 0.35 V (B); dangling bond state at -0.8 V (C); backbond state at - 1.7 V (D).

and give rise to the “metallic” nature of the (7 X 7) surface at room temperature. Our experimental identification of the filled (7 X 7) surface states is also in agreement with theoretical results of Northrup [17] who calculated the energies and spatially resolved density of states for adatom dangling bond, rest atom dangling bonds, and backbond states in a (2 X 2) geometry.

3. Si(OO1) We have also studied the geometric and electronic structure of the Si(OO1) surface. At negative sample bias, the STM images reveal rows of oblong protrusions, while at positive sample bias only very small corrugations (- 0.2

352

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et al. / Electronrc end geometnc

structure of St(l I I)-(

7 X 7) und

S1(001)

A) are observed. The magnitude (- 0.6 and 0.15 A) and shape of the negative-bias STM corrugations agree well with the corrugations expected for a dimer-type reconstruction but disagree with those expected for chain- and vacancy-type reconstructions [9]. A dimer-type reconstruction is strongly supported by various other experimental and theoretical studies. The Si(OO1) surface has a much simpler electronic structure than the Si(lll)-(7 x 7) surf.ace. On Si(OOl)-(2 x l), there are two surface states within 2 V of the Fermi level [18] - one state comprising the Si-Si dimer bond, and one state comprising the two dangling bonds remaining on each dimer. However, buckling the dimers can also produce regions of (2 X l), c(4 X 2), and p(2 x 2) symmetry, with resultant changes in the local electronic structure [19]. In particular, buckling the dimers is predicted to split the two surface states, producing a filled dimer-bond state and an empty dangling bond state. In our STM images of Si(OOl), we have observed both non-buckled and buckled dimers. Based on the location of buckled dimers near defects, we have argued [9] that the buckled dimers observed with the STM at room temperature are stabilized by defects. However, we are unable to ascertain whether the symmetric-looking dimers are truly symmetric or whether we are only sensitive to the time-average position of dimers which are rapidly switching between the two buckling directions. In this case defects would prohibit this time-averaged flipping of the buckled dimers. Fig. 3 shows the CITS results for the Si(OO1) surface. At -2 V bias, the STM tip follows the corrugation shown in fig. 3A, which is in close agreement with total charge density calculations for a symmetric dimer model. The image consists of rows of oblong protrusions, each of which is a dimer. In fig. 3A we also observe both symmetric dimers near the center and buckled dimers in the upper right corner. When the dimers are buckled, the buckling direction

Fig. 3. Topographic image of Si(oO1)taken with - 2 V applied bias (A) and simultaneously-acquired current image at + 2 V (B).

R.J. Humers et at. / Electronic and geometrrc structure of Si(ll I)-(7x 7) and Si(Qi.?l)

353

always alternates from dimer to dimer along a row, giving rise to the zig-zag patterns in fig. 3A. When we perform CITS measurements while the tip follows this contour, we find two electronic states separated by a surface-state bandgap of 0.5 V. Within the bandgap, only defects states are observed. At positive sampie bias, the spatial distribution of the tunneling current is shown in fig. 3B. Along the center of the dimer rows, where the tunneling probability is highest in fig. 3A, the positive-bias tunneling current image in fig. 3B shows a minimum. A minimum is also observed between the dimer rows just as in fig. 3A. Thus, we conclude that tunneling at negative sample bias occurs exclusively through the Si-Si dimer bond, while that at positive sample bias occurs into the empty dangling bonds, Fig. 3A then represents an image of the filled Si-Si dimer state, while fig. 3B is an image of the empty dangling bond state. When the dimers are buckled, we find different electronic structure on each of the two atoms. The atom which appears highest at negative bias gives rise to the least tunneling current at positive bias, and vice versa. This is direct evidence for a transfer of charge from the lower atom to the upper atom upon buckling, since the single filled state is localized on the upper atom while the single empty state is localized on the lower atom. This also suggests that for buckled dimers, the magnitude of the buckling in fig. 2A is larger than the true geometric buckling due to the accompanying change in the local electronic structure. Since we observe only two surface states in this energy range, we can check for self-consistency by scanning with a positive sample bias and examining the spatial variations in the negative-bias tunneling current. Fig. 4A shows the topography measured with a + 2 V sample bias, and fig. 4B shows an image of the tunneling current obtained when the feedback loop is gated and the bias is

Fig. 4. Topographic

image of Si(OO1) taken with + 2 V applied bias (A) and simultaneously-acquired current image at - 2 V (B).

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ofS(lIl)-(7x 7) md

S(OOI)

switched to -2 V. The results are complementary to and completely consistent with those shown in fig. 3. The positive-bias topography in fig. 4A looks like the positive-bias current image of fig. 3B, and the negative-bias topography of fig. 3A looks like the negative-bias current image of fig. 4B. This shows that the same surface electronic structure can be obtained under either set of biasing conditions and also demonstrates that under both positive- and negative-bias conditions, the “topographic” images of Si(OO1) obtained in constant-current STM arise primarily from the electronic structure of the surface rather than the geometric structure. Thus, the agreement between STM images obtained at negative sample bias and the results of total charge density calculations can be considered fortuitous and is a result of the spatial distribution of electrons in the Si-Si dimer bond.

4. Conclusions Current imaging tunneling spectroscopy has been used to directly identify the atomic origins of the intrinsic surface states of the Si(lll)-(7 X 7) and Si(OO1) surfaces. We find three filled and two empty states on Si(lll)-(7 x 7), the energies of which are in excellent agreement with results of photoemission and inverse photoemission studies. On Si(OO1) we have identified the Si-Si dimer bond and the dimer dangling bonds and have shown that the images obtained in the STM reflect the surface electronic structure rather than the surface geometric structure.

Acknowledgements The authors would like to acknowledge Peter Schroer and Jordan for developing the data acquisition software and hardware.

Becker

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]

G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Letters 50 (1983) 120. R.J. Hamers, R.M. Tromp and J.E. Demuth, Phys. Rev. Letters 56 (1986) 18 J. Tersoff. to be published. R.S. Becker, J.A. Golovchenko, D.R. Hamann and B.S. Schwartzentruber, Phys. Rev. Letters 55 (1985) 2032. A. Selloni, P. Camevali, E. Tossati and CD. Chen, Phys. Rev. B31 (1985) 2602. R.M. Tromp, R.J. Hamers and J.E. Demuth, to be published. W.J. Kaiser and R.C. Jaklevic, to be published. A.L. de Lozanne, S.A. Elrod and C.F. Quate, Phys. Rev. Letters 54 (1985) 22. R.J. Hamers, R.M. Tromp, and J.E. Demuth, to be published. R.V. Coleman, B. Drake, P.K. Hansma and G. Slough, Phys. Rev. Letters 55 (1985) 4.

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I 1)-f 7 x 7) and SifOOLj

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[ll] J.A. Stroscio, R.M. Feenstra and A.P. Fein, to be pub~shed. [12] R.N. Feenstra, W.A. Thompson and A.P. Fein, Phys. Rev. Letters 56 (1986) 608. [13] K. Takayanagi, Y. Tan&ire, M. Takabashi and S. Takabisbi, J. Vacuum Sci. Technol. A3 (1985) 1502. [14] J.J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 4. [15] F.J. Himpsel and Th. Fauster, .I. Vacuum Sci. Technol. A2 (1984) 815. [16] R.I.G. Uhrberg, G.V. Hansson, J.M. Nicholls, P.E.S. Persson and S.A. Flodstram, Phys. Rev. B32 (1985) 3805. [17] J.D. Northrup, to be published. [18] J.A. Appelbaum, G.A. Baraff and D.R. Hamann, Phys. Rev. Letters 35 (1975) 11. [19] D.J. Chadi, Phys. Rev. Letters 43 (1979) 43.