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Photoelectron and inverse photoelectron spectroscopy studies of the Si(111)3 × 3-Sb surface
Surface Science 204 (1988) 405-414 North-Holland, Amsterdam
405
PHOTOELECTRON AND INVERSE PHOTOELECTRON SPECTROSCOPY STUDIES OF THE Si(lll)fi X a-Sb T. KINOSHITA and S. KONO Department
Received
*, Y. ENTA,
H. OHTA,
Y. YAEGASHI,
SURFACE S. SUZUKI
of Physics, Faculty of Science, Tohoku University, Sendai, Japan 980 19 May 1988; accepted
for publication
1 June 1988
Angle-resolved ultraviolet photoelectron spectra and momentum (k)-resolved inverse photoelectron spectra have been measured for the Si(lll)fi X fi-Sb surface. It has been found that at least two filled-surface-state bands exist on the 6 xfi-Sb surface and that the fi X6-Sb surface is semiconducting with a large band gap between the filled- and empty-surface-state bands. The absence of dangling bonds in this surface is suggested from results of hydrogen exposure. These results are discussed in connection with the surface electronic structures expected for a recently proposed trimer model.
1. Introduction It is known that the fi X fi ordered surface often appears when submonolayer metals are adsorbed on a Si(ll1) substrate. The atomic and electronic structures of the Si(lll)o x &?-column-III (Al, Ga, In) and -IV (Sn) metal overlayer systems are understood rather well [l]. Although some experimental studies [2-lo] have been done for the Si(lll)fi X &-column-V metal surfaces, the understanding of the fi x fi-column-V systems is not yet thorough. Kawazu et al. [2] and Saito et al. [3] proposed a l/3 ML (1 ML (monolayer) being the atomic density of a truncated 1 x 1 surface) model for the Si(lll)fi x a-Bi surface from the results of low energy electron diffraction (LEED) and thermal desorption experiments. On the other hand, it is shown in refs. [5-71 that a trimer of three Bi adatoms exists on every &? X 16 site of the Si(ll1) surface. The coverage of Bi atoms for the trimer model is 1 ML. Because of the large spin-orbit splitting of the Bi 6p valence electrons (- 1.6 eV for a free atom [ll]), the electronic structures of the fi X a-Bi surface could not be understood easily even if the atomic arrangement of the fi X fi-Bi surface were established. In fact, the results of angle-resolved ultra* Present address: Synchrotron University of Tokyo, Tanashi,
Radiation Laboratory, Tokyo, Japan 188.
The
0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Institute
B.V.
for Solid
State
Physics,
406
T. Kmoshita et al. / PES and IPES of Si(1 I l)\ii
X \ii -Sh
violet photoelectron spectroscopy (ARUPS) for the fi X a-Bi surface were too complicated to definitely determine the surface-state (SS) dispersion [4]. Recently, Park et al. performed LEED and X-ray photoelectron spectroscopy studies of the formation of submonolayer interfaces of the Sb/Si(l 11) system [8]. It was found in ref. [8] that a relatively well ordered fi x fi structure [9] appeared for this system at nominal Sb coverage of 1 ML. More recently, Abukawa et al. [lo] measured X-ray photoelectron diffraction patterns for the 6 x a-Sb surface and proposed a trimer model as a plausible structure. From consideration of covalent bondings among Sb and Si atoms, they inferred that the so-called “milk stool” 1121 may be present for the fi x fi-Sb surface as illustrated in fig. 1. Abukawa et al. also argued that this can be the case for the fi X \/5-Bi surface. Since the spin-orbit splitting of the Sb 5p valence electrons is smaller than that of the Bi 6p valence electrons [ll], the fi X fi-Sb surface will be a good target for the full understanding of the electronic structures of the Si(lll)fi X a-column-V metal surfaces. In the present paper, we report ARUPS and momentum (k)-resolved inverse photoelectron spectroscopy (KRIPS) studies of the Si(lll)fi X fi-Sb surface. These techniques are most suited for investigating filled and empty electronic structures of surfaces. We discuss the experimental results in connection with the electronic structures expected for the trimer model shown in fig. 1.
Fig. 1. Schematic view of “milk-stool” type trimer model for the Si(lll)& Shaded circles represent Sb atoms.
X
a-Sb
surface.
T. Kinoshrta et al. / PES and IPES of Si(l I I)0
x fi-Sb
407
2. Experimental The ARUPS and KRIPS apparatus used in this study have been described previously [l]. The base pressure of the two UHV chambers was - 5 x lop” Torr. Each of the apparatus has a cylindrical mirror analyzer (CMA) Auger electron spectrometer, an Ar ion gun, a LEED or a reflection high energy electron diffraction (RHEED) system and a deposition unit. For the ARUPS measurements, a rotatable 150 o hemispherical electrostatic electron energy analyzer and a He1 discharge lamp (hv = 21.2 eV) were used. The energy- and angle-resolutions of the ARUPS spectrometer were - 0.1 eV and - 3”, respectively. For the KRIPS measurements, we used an improved incident electron gun [13] together with a I,/SrF, Geiger-Miiller-type photon detector (hv = 9.5 eV) [l]. The angle of electron incidence was changed by rotating the sample about an axis parallel to the surface. The outcoming light was focused onto the GM counter by a concave mirror, the surface of which was coated with Al and MgF, in order to raise the light-reflectivity. The KRIPS spectra were normalized by the incident electron current impinging onto the sample (- l-3 PA). The energy- and angle-resolutions of the KRIPS spectrometer were estimated to be - 0.35 eV and 3”-5”, respectively [13]. The substrate preparation was performed in the same way as in previous works [1,4]. Mirror-polished and pre-treated [14] Si(ll1) wafers (phosphorous doped, - lo-15 a cm: 0.15 X 4 X 20 mm3 for ARUPS, 0.15 X 8 X 20 mm3 for KRIPS) whose surface normal was off by - 2.2’ from the (111) direction were used in the experiments. Several times of Ar+ bombardment followed by annealing at - 900 o C was applied for the in-situ cleaning of the sample, giving a clear 7 x 7 LEED or RHEED pattern. 99.9999% pure Sb was deposited from an alumina crucible onto a *temperature controlled 7x 7 Torr. LEED or RHEED patterns of substrate under a pressure of - 5 X lo-” the fi X fi, diffuse 2 X 2, 1 X 1, 56 X 56 and/or 70 X 70 superstructures were observed under the same condition in ref. [8]. The well-ordered fi x fi-Sb surface was prepared by Sb deposition onto a - 630 o C substrate. The Auger electron spectra showed no contamination before and after the experiments. In order to characterize SS bands, we also measured the ARUPS spectra for a sample exposed to atomic hydrogen. The hydrogen exposures were made with the sample facing to a tungsten filament (T 2 1500” C) for dissociating H, molecules. 3. Results 3.1. AR UPS spectra Fig.
2 shows
representative
ARUPS
spectra
of the
Si(lll)fi
X
a-Sb
408
Fig. 2. ARUPS spectra of the Si(lll)@ X &?-Sb surface. The polar angle, 8, of electron emission -is changed along the (110) direction which corresponds to the T-M-r direction of the fi x fi SBZ. Unpolarized He1 light is incident at 6 = -45”. Symmetric points of the fi x fi SBZ are indicated. The origin of the binding energy is the Fermi level determined from that of a Ta holder. The surface state bands are marked with bars. -I_-
surface measured along the Y-M-T direction of the fi x J? surface Brillouin zone (SBZ). One can notice in the spectra that no electronic states exist at the Fermi level (EF), thus the 6 X &-Sb surface is semiconducting. In figs. 3a and 3b, we show fine structures of the ARUPS spectra for binding energy ----E, > - 3 eV along the T-M--T and T-K-M directions, respectively. The SS peaks are marked with bars in figs. 2 and 3 as explained --- below. Peak X (Eh = - 3 eV) which appears at B = 16”-40* along the T-M-T direction is broad and weak, and exists in the theoretical projected bulk bands [15,16]. However, as shown later (cf. fig. 5a), peak X was sensitively affected when the surface changed to the 1 X J structure upon hydrogen exposure. Therefore, it is likely that this structure is due to the SS band. Peaks A, and A1 are SS bands which are clearly observed around the g point and are situated in the gap of the projected bulk bands 115,161 in the outer region of the SBZ. These peaks are also affected by H exposure (cf. fig. 5a). In figs. 4a and 4b, the diagrams of the binding energy (E,,) plotted against the electron wave vector (k,,) parallel to the surface for the 6 X v’%-Sb along
409
(a)
Si(lll)~xfl-Sb(1101
j
/
I
u--I
!
Er BINDiNG ENERGY (eV) 2
1
(b) Si(lll)J5xJ3-Sb(llZ)
‘” 3 2 BINDING
1 EF ENERGY (eV)
Fig. 3. (a) Detailed ARUPS spectra for --the same polar angles in fig. 2. (b) As in (a) but along the T-K-M direction. ---
the p-M-P and I-K-M directions respectively are shown. These are obtained from the actual spectra in figs. 2 and 3 and others not shown here. The diagrams are shown for 0 5 E, 2 8 eV. The E,-k,, diagrams for E, >, 2 eV except for structure X are similar to those for other Si(ll1) reconstructed surfaces such as 7 X 7, fi X G-Al, -Ga, -In, -Sn and -Bi surfaces [1,4,17]. This implies that the electronic structures below - 2 eV are mostly bulk in origin. The diagrams for E, 5 2 eV are typical for the 6 X a-Sb surface. However, the dispersion of the SS bands is not uniquely determined from the diagrams because of the broadness of the SS peaks. It is also possible that these SS bands contain more than the identified number of SS bands. Next we turn to the effect of hydrogen exposure on the ARUPS spectra. The sensitivities of the Si(Ill)fi X v’?-Sb and clean 7 X 7 [17] surfaces to hydrogen exposure are compared in fig. 5. The spectra were measured at B = 16” along the (110) direction for the 6 X a-Sb surface and @= 12” along the (ll?} direction for the 7 X 7 surface. For these directions, all the SS peaks A,, AZ (and X) for the 6 X fi-Sb and the well-known three SS peaks for the 7 X 7 are clearly observable. One can notice in fig. 5a that the spectral profiles do not change very much upto an exposure of 500 L. The SS peaks A,, A, (and X) for the fi X fi-Sb surface disappear only when the surface converts to the 1 x 1 structure at 10000 L. On the other hand in fig. 5b, it is observed that the swift disappearance of the SS peaks of the 7 x 7 surface
POLAR
WAVE
ANGLE
VECTOR
(8)
!a”,
POLAR
WAVE
ANGLE
(0)
VECTOR
(k’,
Fig. 4. (a) E,-kll diagram for the filled hands of the Si(lll)fi x6-Sb surface along the --T-M-T direction. Circles are strong or sharp peaks and triangles are weak or broad structures in the actual ARUPS spectra. (b) As in (a) hut along the T-K-M direction.
begins at 50 L. These may be related to the presence or absence of dangling bonds on the surface. Namely, it may be said that the dangling bonds are absent for the fi x fi-Sb surface while it is known that 19 dangling bonds per unit cell are present for the dimer-adatom-stackingfault (DAS) structure [19] of the 7 x 7 surface. The disappearance of the SS peaks for the J? X fi-Sb surface at high exposure of 10000 L may be due to the breaking of the covalent bondings on the 6 x v%-Sb surface. Recently, the absence of dangling bonds was reported for the Si(Ill)l x I-As (which is a column-v element) surface by Uhrberg et al. [20]. They also found a decrease in intensity of the SS peak for 1 X I-As upon hydrogen exposures of - 10000 L. This was also ascribed to As-Si bond breaking and the replacement with As-H or Si-H bonding. This kind of bond breaking might occur on the fi X J?-Sb surface at large H exposures. 3.2. KRIPS
spectru
Some of the KRIPS spectra for the Si(lll)fi X fi-Sb surface are shown in figs. 6a --and 6b, where the polar angle of electron incidence is changed along --the F-M-T and along the T-K-M direction of the fi X fi SBZ, respectively. The selniconducting nature of the fi X y5-Sb surface is confirmed in
(5)
BINOING
ENERGY
S~(lll)
Ce’d)
7x7 *H (112)
BINOlNG
Q=lZ’
ENERGY
I eV j
Fig. 5. (a) Effect on the ARUPS spectra of exposure of the Si(lll)& x \i’S-Sb ---surface to atomic hydrogen (I L = 1 x 10v6 Tom *s). The spectra are at the R pain: along the F-M-F direction. After au exposure of 10000 L hydrogen, the surface converted to the 1 X 1 structure. (b) As in (a) but the clean Si(lll)7 x 7 surface is exposed to atomic hydrogen (ref. [17]). Alter an exposure of more than 200 L, the RWEED pattern changed to the so-called 6-7 X7 structure of ref. [t8]. (a)
Si(lllff?xfl-Sb(110)
(lll)flxfl-Sbill”L
(b)
I ’ ’ ’ ’ ’
L_
-M-r1 ;; 1i :
‘“,,.,, ‘“_.
F -.0=50’
I
1
L Energy
dative
to EF CeV)
Energy
relative
to EF (eV)
Fig. 6. (a) KRIPS spectra of the Si(llt)fi x fi;-St, surfme. The pofar angle, 8. of electron incidence is changed along the r--R-r direction of the 6 x&f? SBZ. (b) As in (a) but along the -7 F-k-B direction.
412
T. Kinoshita el al. / PES and IPES of Si(1 II)0 (a)
x J3 -Sh
(b) POLAR
ANGLE
(6)
POLAR
ANGLE
0’ 10’ 20’3O”LO’
0 WAVE
05 10 VECTOR
(8) 60”
1.5
(2,
WAVE
VECTOR
(h’,
Fig. 7. (a) Experimental empty band structures of the Si(lll)fi x J3 -Sb (solid symbols) and fi X fi-In [l] (open symbols) surfaces along the r-n-r direction. Circles are strong or sharp peaks and triangles are weak or broad structures in actuai KRIPS spectra. (b) As in (a) but along --the T-K-M direction.
the KRIPS spectra since no structures appear at E,. Fig. 7 shows E,-l\,: diagrams of empty states of the fi X a-Sb surface (solid symbols), which are obtained from the actual KRIPS spectra in fig. 6 and others not shown here. The E,-k,, diagrams for the Si(lll)fi x v%-In surface (open symbols) [l] are also shown for comparison. The E,-k,, diagrams for the fi x fi-Sb surface are rather different from those for the Si(lll)fi x G-In and -Sn surfaces [l]. - 1.7 eV above E, were assigned to be In previous works [I], the structures due to the bulk conduction bands. In the present KRIPS spectra for the fi x a-Sb surface, these bulk features seem hidden by new peaks, Y and Z. These peaks are therefore suggested to be SS peaks. Because these peaks are broad, the dispersion is not clearly determined from fig. 7. However. it can be said from the present results that the position of the empty SS peaks for the fi x fi-Sb surface is well above the bottom of the bulk conduction bands [15,16]. 4. Discussion From the results in section 3, the following findings can be deduced: (1) The Si(lll)fi X fi-Sb surface is semiconducting with a large band gap between the filled and empty SS bands. (2) At least two filled SS bands exist on the fi x fi-Sb surface. (3) It is likely that the dangling bonds are absent for the fi X fi-Sb surface.
T. Kinoshita et al. / PES and IPES of Si(l I I) I/? x fi -Sb
413
Complete understanding of surface atomic and electronic structures needs detailed theoretical calculations. Unfortunately, no other models except the trimer model [lo] have been proposed and no theoretical calculations have been reported so far for the fi X a-Sb surface. In this section, we assume the electronic structure of the trimer model in fig. 1 and compare this with the above findings. It can be considered that the two 5s and three 5p electrons of the Sb atom are related to the origin of the SS bands. In the “milk-stool” trimer model in fig. 1, it is probably mostly the p,, p.,.-like orbitals of the three Sb atoms that make the trimer with bond angles of 60”. The p, orbital of each Sb atom would couple with the otherwise dangling bond of the Si(ll1) substrate in order to situate the trimer on every fi X fi site. The s-like orbital of each Sb atom might not be associated with bonding to neighbors but forms a lone-pair bonding as illustrated with paired dots in fig. 1. It is supposed from these considerations that the dangling bond is absent for this trimer structure of the fi x fi-Sb surface, which is in accordance with finding (3). The s-like lone-pair state is expected to be a deep-lying state in the bulk valence bands. The Sb 5p states and Si dangling bonds are expected to form low lying SS bands in which 12 electrons (3 x 3 (Sb 5p) + 3 (Si dangling bond)) occupies six filled SS bands for the trimer model. Therefore, the surface electronic structure of the trimer model can be semiconducting. These results seem to be consistent with findings (1) and (2). Parts of the low lying SS bands may correspond to the A,, A, (and X) bands in the present experiment. 5. Conclusion Filled and empty electronic structures of the Si(lll)& X a-Sb surface have been investigated by angle-resolved ultraviolet photoelectron spectroscopy and k-resolved inverse photoelectron spectroscopy. It has been found that the fi x fi surface is semiconducting and that at least two filled SS bands exist on the fi x fi-Sb surface. The band gap between the filled and empty SS bands is larger than that of Si bulk bands. The absence of dangling bonds for this surface is suggested since the fi X fi-Sb surface is found to be rather stable upon hydrogen exposure. These findings are consistent with the electronic structures expected for the trimer model shown in fig. 1. However, theoretical calculations are needed for further study. Acknowledgements Our experimental apparatus were constructed under the guidance of Professor Takasi Sagawa who died on September 27th, 1986. We would like to thank Mr. Hiromasa Ishii and Mr. Hisao Ohsawa for their cooperation in constructing the KRIPS spectrometer.
414
T. Kinoshlta et al. / PES and IPES
of WI I I) f? x J? -Sh
References (11 T. Kinoshita, H. Ohta, Y. Enta, Y. Yaegashi. S. Suzuki and S. Kono, J. Phya. Sot. Japan 56 (1987) 4015, and references therein. PI A. Kawazu, Y. Saito. N. Ogiwara. T. Otsuki and G. Tominaga, Surface Sci. 86 (1979) 108. PI Y. Saito, A. Kawazu and G. Tominaga. Surface Sci. 103 (1981) 563. J. Phys. Sot. Japan 56 t 1987) 251 I. 141 T. Kinoshita, S. Kono and H. Nagayoshi, K. Izumi, T. Ishikawa and S. Kikuta. Surface Sci. 183 (1987) L302. [51 T. Takahashi, S. Nakatani, T. Ishikawa and S. Kikuta, Surface Sci. 191 (1987) LX25. [61 T. Takahashi, and S. Kono. Japan. J. Appl. Phys. 26 (1987) L1355. [71 C.Y. Park, T. Abukawa. K. Higashiyama PI C.Y. Park, T. Abukawa, T. Kinoshita, Y. Enta and S. Kono, Japan. J. Appl. Phys,. 27 (1988) 147. xfi-Sb structure was suggested by P. Martensson. private [91 The formation of the Si(lll)fi communication. PO1 T. Abukawa, C.Y. Park and S. Kono. Surface Sci. 201 (1988) L513. Spectroscopy (Academic Press, New York, 1969) p. 67. 1111 M. Cardona, Modulation and J.W. Moskowitz, J. Vacuum Sci. Technol. 16 (1979) 1266. [I21 L.C. Snyder, Z. Wasserman [I], we used a Pierce-type electron gun with a W 1131 In the previous KRIPS measurements filament and a three-element lens. The energy- and angle-resolutions of this electron gun were rather poor. In the present study, we used a new one which was composed of a BaO dispenser cathode (T = 850 o C) and a four-element lens. With the new electron gun, higher energy- and angle-resolutions were achieved (see text). The diameter of electron beams was estimated tc> be -3mm. J. Electrochem. Sot. 119 (1972) 772. [I41 J. Henderson, (151 H.I. Zhang and M. Schltiter, Phys. Rev. B 18 (1978) 1923. Phys. Rev. B 22 (1980) 4610. [161 J. Ihm, M.L. Cohen and J.R. Chelicowsky, PhD Thesis. Tohoku University, Sendai, Japan, 1983 (in Japanese. unpub[I71 T. Yokotsuka. lished). [I81 H. Daimon and S. Ino. Surface Sci. 164 (1985) 320. Y. Tanishiro, M. Takahashi and S. Takahashi. J. Vacuum Sci. Technol. A 3 P91 K. Takayanagi. (1985) 1502. R.Z. Bachrach and J.E. Northrup, Phys. PO1 R.I.G. Uhrberg. R.D. Bringans, M.A. Olmstead. Rev. B 35 (1987) 3945.
405
PHOTOELECTRON AND INVERSE PHOTOELECTRON SPECTROSCOPY STUDIES OF THE Si(lll)fi X a-Sb T. KINOSHITA and S. KONO Department
Received
*, Y. ENTA,
H. OHTA,
Y. YAEGASHI,
SURFACE S. SUZUKI
of Physics, Faculty of Science, Tohoku University, Sendai, Japan 980 19 May 1988; accepted
for publication
1 June 1988
Angle-resolved ultraviolet photoelectron spectra and momentum (k)-resolved inverse photoelectron spectra have been measured for the Si(lll)fi X fi-Sb surface. It has been found that at least two filled-surface-state bands exist on the 6 xfi-Sb surface and that the fi X6-Sb surface is semiconducting with a large band gap between the filled- and empty-surface-state bands. The absence of dangling bonds in this surface is suggested from results of hydrogen exposure. These results are discussed in connection with the surface electronic structures expected for a recently proposed trimer model.
1. Introduction It is known that the fi X fi ordered surface often appears when submonolayer metals are adsorbed on a Si(ll1) substrate. The atomic and electronic structures of the Si(lll)o x &?-column-III (Al, Ga, In) and -IV (Sn) metal overlayer systems are understood rather well [l]. Although some experimental studies [2-lo] have been done for the Si(lll)fi X &-column-V metal surfaces, the understanding of the fi x fi-column-V systems is not yet thorough. Kawazu et al. [2] and Saito et al. [3] proposed a l/3 ML (1 ML (monolayer) being the atomic density of a truncated 1 x 1 surface) model for the Si(lll)fi x a-Bi surface from the results of low energy electron diffraction (LEED) and thermal desorption experiments. On the other hand, it is shown in refs. [5-71 that a trimer of three Bi adatoms exists on every &? X 16 site of the Si(ll1) surface. The coverage of Bi atoms for the trimer model is 1 ML. Because of the large spin-orbit splitting of the Bi 6p valence electrons (- 1.6 eV for a free atom [ll]), the electronic structures of the fi X a-Bi surface could not be understood easily even if the atomic arrangement of the fi X fi-Bi surface were established. In fact, the results of angle-resolved ultra* Present address: Synchrotron University of Tokyo, Tanashi,
Radiation Laboratory, Tokyo, Japan 188.
The
0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Institute
B.V.
for Solid
State
Physics,
406
T. Kmoshita et al. / PES and IPES of Si(1 I l)\ii
X \ii -Sh
violet photoelectron spectroscopy (ARUPS) for the fi X a-Bi surface were too complicated to definitely determine the surface-state (SS) dispersion [4]. Recently, Park et al. performed LEED and X-ray photoelectron spectroscopy studies of the formation of submonolayer interfaces of the Sb/Si(l 11) system [8]. It was found in ref. [8] that a relatively well ordered fi x fi structure [9] appeared for this system at nominal Sb coverage of 1 ML. More recently, Abukawa et al. [lo] measured X-ray photoelectron diffraction patterns for the 6 x a-Sb surface and proposed a trimer model as a plausible structure. From consideration of covalent bondings among Sb and Si atoms, they inferred that the so-called “milk stool” 1121 may be present for the fi x fi-Sb surface as illustrated in fig. 1. Abukawa et al. also argued that this can be the case for the fi X \/5-Bi surface. Since the spin-orbit splitting of the Sb 5p valence electrons is smaller than that of the Bi 6p valence electrons [ll], the fi X fi-Sb surface will be a good target for the full understanding of the electronic structures of the Si(lll)fi X a-column-V metal surfaces. In the present paper, we report ARUPS and momentum (k)-resolved inverse photoelectron spectroscopy (KRIPS) studies of the Si(lll)fi X fi-Sb surface. These techniques are most suited for investigating filled and empty electronic structures of surfaces. We discuss the experimental results in connection with the electronic structures expected for the trimer model shown in fig. 1.
Fig. 1. Schematic view of “milk-stool” type trimer model for the Si(lll)& Shaded circles represent Sb atoms.
X
a-Sb
surface.
T. Kinoshrta et al. / PES and IPES of Si(l I I)0
x fi-Sb
407
2. Experimental The ARUPS and KRIPS apparatus used in this study have been described previously [l]. The base pressure of the two UHV chambers was - 5 x lop” Torr. Each of the apparatus has a cylindrical mirror analyzer (CMA) Auger electron spectrometer, an Ar ion gun, a LEED or a reflection high energy electron diffraction (RHEED) system and a deposition unit. For the ARUPS measurements, a rotatable 150 o hemispherical electrostatic electron energy analyzer and a He1 discharge lamp (hv = 21.2 eV) were used. The energy- and angle-resolutions of the ARUPS spectrometer were - 0.1 eV and - 3”, respectively. For the KRIPS measurements, we used an improved incident electron gun [13] together with a I,/SrF, Geiger-Miiller-type photon detector (hv = 9.5 eV) [l]. The angle of electron incidence was changed by rotating the sample about an axis parallel to the surface. The outcoming light was focused onto the GM counter by a concave mirror, the surface of which was coated with Al and MgF, in order to raise the light-reflectivity. The KRIPS spectra were normalized by the incident electron current impinging onto the sample (- l-3 PA). The energy- and angle-resolutions of the KRIPS spectrometer were estimated to be - 0.35 eV and 3”-5”, respectively [13]. The substrate preparation was performed in the same way as in previous works [1,4]. Mirror-polished and pre-treated [14] Si(ll1) wafers (phosphorous doped, - lo-15 a cm: 0.15 X 4 X 20 mm3 for ARUPS, 0.15 X 8 X 20 mm3 for KRIPS) whose surface normal was off by - 2.2’ from the (111) direction were used in the experiments. Several times of Ar+ bombardment followed by annealing at - 900 o C was applied for the in-situ cleaning of the sample, giving a clear 7 x 7 LEED or RHEED pattern. 99.9999% pure Sb was deposited from an alumina crucible onto a *temperature controlled 7x 7 Torr. LEED or RHEED patterns of substrate under a pressure of - 5 X lo-” the fi X fi, diffuse 2 X 2, 1 X 1, 56 X 56 and/or 70 X 70 superstructures were observed under the same condition in ref. [8]. The well-ordered fi x fi-Sb surface was prepared by Sb deposition onto a - 630 o C substrate. The Auger electron spectra showed no contamination before and after the experiments. In order to characterize SS bands, we also measured the ARUPS spectra for a sample exposed to atomic hydrogen. The hydrogen exposures were made with the sample facing to a tungsten filament (T 2 1500” C) for dissociating H, molecules. 3. Results 3.1. AR UPS spectra Fig.
2 shows
representative
ARUPS
spectra
of the
Si(lll)fi
X
a-Sb
408
Fig. 2. ARUPS spectra of the Si(lll)@ X &?-Sb surface. The polar angle, 8, of electron emission -is changed along the (110) direction which corresponds to the T-M-r direction of the fi x fi SBZ. Unpolarized He1 light is incident at 6 = -45”. Symmetric points of the fi x fi SBZ are indicated. The origin of the binding energy is the Fermi level determined from that of a Ta holder. The surface state bands are marked with bars. -I_-
surface measured along the Y-M-T direction of the fi x J? surface Brillouin zone (SBZ). One can notice in the spectra that no electronic states exist at the Fermi level (EF), thus the 6 X &-Sb surface is semiconducting. In figs. 3a and 3b, we show fine structures of the ARUPS spectra for binding energy ----E, > - 3 eV along the T-M--T and T-K-M directions, respectively. The SS peaks are marked with bars in figs. 2 and 3 as explained --- below. Peak X (Eh = - 3 eV) which appears at B = 16”-40* along the T-M-T direction is broad and weak, and exists in the theoretical projected bulk bands [15,16]. However, as shown later (cf. fig. 5a), peak X was sensitively affected when the surface changed to the 1 X J structure upon hydrogen exposure. Therefore, it is likely that this structure is due to the SS band. Peaks A, and A1 are SS bands which are clearly observed around the g point and are situated in the gap of the projected bulk bands 115,161 in the outer region of the SBZ. These peaks are also affected by H exposure (cf. fig. 5a). In figs. 4a and 4b, the diagrams of the binding energy (E,,) plotted against the electron wave vector (k,,) parallel to the surface for the 6 X v’%-Sb along
409
(a)
Si(lll)~xfl-Sb(1101
j
/
I
u--I
!
Er BINDiNG ENERGY (eV) 2
1
(b) Si(lll)J5xJ3-Sb(llZ)
‘” 3 2 BINDING
1 EF ENERGY (eV)
Fig. 3. (a) Detailed ARUPS spectra for --the same polar angles in fig. 2. (b) As in (a) but along the T-K-M direction. ---
the p-M-P and I-K-M directions respectively are shown. These are obtained from the actual spectra in figs. 2 and 3 and others not shown here. The diagrams are shown for 0 5 E, 2 8 eV. The E,-k,, diagrams for E, >, 2 eV except for structure X are similar to those for other Si(ll1) reconstructed surfaces such as 7 X 7, fi X G-Al, -Ga, -In, -Sn and -Bi surfaces [1,4,17]. This implies that the electronic structures below - 2 eV are mostly bulk in origin. The diagrams for E, 5 2 eV are typical for the 6 X a-Sb surface. However, the dispersion of the SS bands is not uniquely determined from the diagrams because of the broadness of the SS peaks. It is also possible that these SS bands contain more than the identified number of SS bands. Next we turn to the effect of hydrogen exposure on the ARUPS spectra. The sensitivities of the Si(Ill)fi X v’?-Sb and clean 7 X 7 [17] surfaces to hydrogen exposure are compared in fig. 5. The spectra were measured at B = 16” along the (110) direction for the 6 X a-Sb surface and @= 12” along the (ll?} direction for the 7 X 7 surface. For these directions, all the SS peaks A,, AZ (and X) for the 6 X fi-Sb and the well-known three SS peaks for the 7 X 7 are clearly observable. One can notice in fig. 5a that the spectral profiles do not change very much upto an exposure of 500 L. The SS peaks A,, A, (and X) for the fi X fi-Sb surface disappear only when the surface converts to the 1 x 1 structure at 10000 L. On the other hand in fig. 5b, it is observed that the swift disappearance of the SS peaks of the 7 x 7 surface
POLAR
WAVE
ANGLE
VECTOR
(8)
!a”,
POLAR
WAVE
ANGLE
(0)
VECTOR
(k’,
Fig. 4. (a) E,-kll diagram for the filled hands of the Si(lll)fi x6-Sb surface along the --T-M-T direction. Circles are strong or sharp peaks and triangles are weak or broad structures in the actual ARUPS spectra. (b) As in (a) hut along the T-K-M direction.
begins at 50 L. These may be related to the presence or absence of dangling bonds on the surface. Namely, it may be said that the dangling bonds are absent for the fi x fi-Sb surface while it is known that 19 dangling bonds per unit cell are present for the dimer-adatom-stackingfault (DAS) structure [19] of the 7 x 7 surface. The disappearance of the SS peaks for the J? X fi-Sb surface at high exposure of 10000 L may be due to the breaking of the covalent bondings on the 6 x v%-Sb surface. Recently, the absence of dangling bonds was reported for the Si(Ill)l x I-As (which is a column-v element) surface by Uhrberg et al. [20]. They also found a decrease in intensity of the SS peak for 1 X I-As upon hydrogen exposures of - 10000 L. This was also ascribed to As-Si bond breaking and the replacement with As-H or Si-H bonding. This kind of bond breaking might occur on the fi X J?-Sb surface at large H exposures. 3.2. KRIPS
spectru
Some of the KRIPS spectra for the Si(lll)fi X fi-Sb surface are shown in figs. 6a --and 6b, where the polar angle of electron incidence is changed along --the F-M-T and along the T-K-M direction of the fi X fi SBZ, respectively. The selniconducting nature of the fi X y5-Sb surface is confirmed in
(5)
BINOING
ENERGY
S~(lll)
Ce’d)
7x7 *H (112)
BINOlNG
Q=lZ’
ENERGY
I eV j
Fig. 5. (a) Effect on the ARUPS spectra of exposure of the Si(lll)& x \i’S-Sb ---surface to atomic hydrogen (I L = 1 x 10v6 Tom *s). The spectra are at the R pain: along the F-M-F direction. After au exposure of 10000 L hydrogen, the surface converted to the 1 X 1 structure. (b) As in (a) but the clean Si(lll)7 x 7 surface is exposed to atomic hydrogen (ref. [17]). Alter an exposure of more than 200 L, the RWEED pattern changed to the so-called 6-7 X7 structure of ref. [t8]. (a)
Si(lllff?xfl-Sb(110)
(lll)flxfl-Sbill”L
(b)
I ’ ’ ’ ’ ’
L_
-M-r1 ;; 1i :
‘“,,.,, ‘“_.
F -.0=50’
I
1
L Energy
dative
to EF CeV)
Energy
relative
to EF (eV)
Fig. 6. (a) KRIPS spectra of the Si(llt)fi x fi;-St, surfme. The pofar angle, 8. of electron incidence is changed along the r--R-r direction of the 6 x&f? SBZ. (b) As in (a) but along the -7 F-k-B direction.
412
T. Kinoshita el al. / PES and IPES of Si(1 II)0 (a)
x J3 -Sh
(b) POLAR
ANGLE
(6)
POLAR
ANGLE
0’ 10’ 20’3O”LO’
0 WAVE
05 10 VECTOR
(8) 60”
1.5
(2,
WAVE
VECTOR
(h’,
Fig. 7. (a) Experimental empty band structures of the Si(lll)fi x J3 -Sb (solid symbols) and fi X fi-In [l] (open symbols) surfaces along the r-n-r direction. Circles are strong or sharp peaks and triangles are weak or broad structures in actuai KRIPS spectra. (b) As in (a) but along --the T-K-M direction.
the KRIPS spectra since no structures appear at E,. Fig. 7 shows E,-l\,: diagrams of empty states of the fi X a-Sb surface (solid symbols), which are obtained from the actual KRIPS spectra in fig. 6 and others not shown here. The E,-k,, diagrams for the Si(lll)fi x v%-In surface (open symbols) [l] are also shown for comparison. The E,-k,, diagrams for the fi x fi-Sb surface are rather different from those for the Si(lll)fi x G-In and -Sn surfaces [l]. - 1.7 eV above E, were assigned to be In previous works [I], the structures due to the bulk conduction bands. In the present KRIPS spectra for the fi x a-Sb surface, these bulk features seem hidden by new peaks, Y and Z. These peaks are therefore suggested to be SS peaks. Because these peaks are broad, the dispersion is not clearly determined from fig. 7. However. it can be said from the present results that the position of the empty SS peaks for the fi x fi-Sb surface is well above the bottom of the bulk conduction bands [15,16]. 4. Discussion From the results in section 3, the following findings can be deduced: (1) The Si(lll)fi X fi-Sb surface is semiconducting with a large band gap between the filled and empty SS bands. (2) At least two filled SS bands exist on the fi x fi-Sb surface. (3) It is likely that the dangling bonds are absent for the fi X fi-Sb surface.
T. Kinoshita et al. / PES and IPES of Si(l I I) I/? x fi -Sb
413
Complete understanding of surface atomic and electronic structures needs detailed theoretical calculations. Unfortunately, no other models except the trimer model [lo] have been proposed and no theoretical calculations have been reported so far for the fi X a-Sb surface. In this section, we assume the electronic structure of the trimer model in fig. 1 and compare this with the above findings. It can be considered that the two 5s and three 5p electrons of the Sb atom are related to the origin of the SS bands. In the “milk-stool” trimer model in fig. 1, it is probably mostly the p,, p.,.-like orbitals of the three Sb atoms that make the trimer with bond angles of 60”. The p, orbital of each Sb atom would couple with the otherwise dangling bond of the Si(ll1) substrate in order to situate the trimer on every fi X fi site. The s-like orbital of each Sb atom might not be associated with bonding to neighbors but forms a lone-pair bonding as illustrated with paired dots in fig. 1. It is supposed from these considerations that the dangling bond is absent for this trimer structure of the fi x fi-Sb surface, which is in accordance with finding (3). The s-like lone-pair state is expected to be a deep-lying state in the bulk valence bands. The Sb 5p states and Si dangling bonds are expected to form low lying SS bands in which 12 electrons (3 x 3 (Sb 5p) + 3 (Si dangling bond)) occupies six filled SS bands for the trimer model. Therefore, the surface electronic structure of the trimer model can be semiconducting. These results seem to be consistent with findings (1) and (2). Parts of the low lying SS bands may correspond to the A,, A, (and X) bands in the present experiment. 5. Conclusion Filled and empty electronic structures of the Si(lll)& X a-Sb surface have been investigated by angle-resolved ultraviolet photoelectron spectroscopy and k-resolved inverse photoelectron spectroscopy. It has been found that the fi x fi surface is semiconducting and that at least two filled SS bands exist on the fi x fi-Sb surface. The band gap between the filled and empty SS bands is larger than that of Si bulk bands. The absence of dangling bonds for this surface is suggested since the fi X fi-Sb surface is found to be rather stable upon hydrogen exposure. These findings are consistent with the electronic structures expected for the trimer model shown in fig. 1. However, theoretical calculations are needed for further study. Acknowledgements Our experimental apparatus were constructed under the guidance of Professor Takasi Sagawa who died on September 27th, 1986. We would like to thank Mr. Hiromasa Ishii and Mr. Hisao Ohsawa for their cooperation in constructing the KRIPS spectrometer.
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of WI I I) f? x J? -Sh
References (11 T. Kinoshita, H. Ohta, Y. Enta, Y. Yaegashi. S. Suzuki and S. Kono, J. Phya. Sot. Japan 56 (1987) 4015, and references therein. PI A. Kawazu, Y. Saito. N. Ogiwara. T. Otsuki and G. Tominaga, Surface Sci. 86 (1979) 108. PI Y. Saito, A. Kawazu and G. Tominaga. Surface Sci. 103 (1981) 563. J. Phys. Sot. Japan 56 t 1987) 251 I. 141 T. Kinoshita, S. Kono and H. Nagayoshi, K. Izumi, T. Ishikawa and S. Kikuta. Surface Sci. 183 (1987) L302. [51 T. Takahashi, S. Nakatani, T. Ishikawa and S. Kikuta, Surface Sci. 191 (1987) LX25. [61 T. Takahashi, and S. Kono. Japan. J. Appl. Phys. 26 (1987) L1355. [71 C.Y. Park, T. Abukawa. K. Higashiyama PI C.Y. Park, T. Abukawa, T. Kinoshita, Y. Enta and S. Kono, Japan. J. Appl. Phys,. 27 (1988) 147. xfi-Sb structure was suggested by P. Martensson. private [91 The formation of the Si(lll)fi communication. PO1 T. Abukawa, C.Y. Park and S. Kono. Surface Sci. 201 (1988) L513. Spectroscopy (Academic Press, New York, 1969) p. 67. 1111 M. Cardona, Modulation and J.W. Moskowitz, J. Vacuum Sci. Technol. 16 (1979) 1266. [I21 L.C. Snyder, Z. Wasserman [I], we used a Pierce-type electron gun with a W 1131 In the previous KRIPS measurements filament and a three-element lens. The energy- and angle-resolutions of this electron gun were rather poor. In the present study, we used a new one which was composed of a BaO dispenser cathode (T = 850 o C) and a four-element lens. With the new electron gun, higher energy- and angle-resolutions were achieved (see text). The diameter of electron beams was estimated tc> be -3mm. J. Electrochem. Sot. 119 (1972) 772. [I41 J. Henderson, (151 H.I. Zhang and M. Schltiter, Phys. Rev. B 18 (1978) 1923. Phys. Rev. B 22 (1980) 4610. [161 J. Ihm, M.L. Cohen and J.R. Chelicowsky, PhD Thesis. Tohoku University, Sendai, Japan, 1983 (in Japanese. unpub[I71 T. Yokotsuka. lished). [I81 H. Daimon and S. Ino. Surface Sci. 164 (1985) 320. Y. Tanishiro, M. Takahashi and S. Takahashi. J. Vacuum Sci. Technol. A 3 P91 K. Takayanagi. (1985) 1502. R.Z. Bachrach and J.E. Northrup, Phys. PO1 R.I.G. Uhrberg. R.D. Bringans, M.A. Olmstead. Rev. B 35 (1987) 3945.