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Electronic structure of the nitride layers formed on a Si(111) surface: angle-resolved photoemission study
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ELSEVIER
Surface
Science 317 (1994) 143-151
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Electronic structure of the nitride layers formed on a Si( 111) surface: angle-resolved photoemission study Shuzo Tokumitsu a, Toshihisa Anazawa a, Ken-ichi Ozawa a, Eizo Miyazaki a, Kazuyuki Edamoto ap*,Hiroo Kato b aDepartment of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan b Photon Factory, National Laboratory for High Energy Physics, Tsukuba-shi, Ibaraki 305, Japan
Received 13 January 1994;accepted for publication 10 June 1994
Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been applied to study the electronic structure of the nitride overlayers formed on a Si(ll1) surface. As the Si(ll1) surface exposed to NO at saturation coverage is heated to 9Oo”C,the (8 X 81-N overlayer is formed. The N 2p, non-bonding, N 2px,Y+ Si 3p bonding, and Si3s-derived bonding bands are identified in the ARPE spectra using the polarization-dependent measurements. As the surface is further heated to lOSO”C, the (8 x 8)-N overlayer is changed to the quadruplet-N overlayer, however, the spectral profile for the quadruplet-N overlayer is similar to that for the (8 x 8)-N overlayer. The effect of the “surface umklapp” process arising from the lattice mismatch between nitride overlayers and Si(ll1) (1 x 1) substrate are investigated with off-normal-emission measurements.
1. Introduction Nitridation of the Si surface is of considerable interest not only from the scientific viewpoint but also for the technological applications of silicon nitride (high-temperature structural ceramics, diffusion-resist passivating material, etc.). Nitridation of the Si(ll1) surface has been studied using various gases such as N [l-61, NH, [5-91, HN, [lo], N, [ll] and NO [12,13]. These investigations have shown that the “(8 X 81-N” or “quadruplet-
N” nitride layers are formed on the Si(ll1) surface depending on the heating temperature of the
* Corresponding
author.
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0039-6028/94/$07.00 0 1994 Elsevier s.SD10039-6028(94)00339-B
Science
Si(ll1) surface, reactant gas pressure and carbon contamination. In the previous studies on the structure of these nitride layers was proposed that each N atom sits on the three-fold site forming the NSi, bond for both surfaces, and that the unit vectors of a triangular lattice are 8/11 and 3/4 the length of the Si(lll)(l x 1) unit vectors for the (8 X 81-N and quadruplet-N surfaces, respectively, and that the lattice vectors are rotated fB1 or fez with respect to those of silicon substrate, where 8, = $9, = 5” for the quadruplet-N domains [2-4,11,13]. In this work, the electronic structure of the (8 X 81-N and quadruplet-N surfaces formed on the Si(ll1) surface is investigated with angle-re-
solved photoemission spectroscopy (ARPES) utilizing synchrotron radiation. In this work, the
B.V. All rights reserved
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S. Tokumitsu et al. /Surface
Science 317 (1994) 143-151
nitride surfaces are formed using NO as a reactant gas.
normal emission
h Y =32.5eV
2. Experimental The ARPES measurements were conducted on Beam Line 11D of the Photon Factory, National Laboratory for High Energy Physics, using a constant deviation monochromator and an electron energy analyzer of 150”, spherical-sector type, with an acceptance angle of f 1”. The total experimental resolution was - 0.2 eV. The radiation was linearly polarized in the horizontal plane of incidence and the photoelectrons were detected in this incidence plane for all measurements. The base pressure in the vacuum system was 1 X 10-l” Torr. The sample used was a SXlll) wafer which was nearly intrinsic (p-type, boron-doped, - 5 s2. cm). The Si sample was mounted on the sample manipulator such that the (2ii) Az was in the incidence plane of the light. The (7 X 7) clean surface was prepared by flashing to about 1300°C. No impurities were observed on the (7 X 7) surface thus prepared within the detection limit of AES. The Si sample was heated by electron bombardment from the rear and the temperature was measured with a W-3%Re/W-25%Re thermocouple. In this paper, the incidence angle of the light (ei) and the detection angle of the photoelectron (0,) are given relative to the surface normal. In the ARPE spectra presented below, the electron binding energies are referenced to zero at the Fermi level, which is determined from the spectra for the Ta plate.
3. Results and discussion Fig. 1 shows the normal-emission spectra for the SKlll) surface exposed to NO, and for the surface subsequently heated to high temperatures. The photon energy is 32.5 eV and the incidence angle of the light is 60”. As the clean surface is exposed to NO at room temperature, a broad band developes at N 7 eV together with
)I
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12
10
8
(8 1
6
4
BINDING ENERGY
2
0 (=E,)
(eV)
Fig. 1. Normal-emission spectra: (a) the Si(ll1) (7X 7) clean surface, (b) after 100 L NO exposure, (c) after heating the Si(ll1) surface exposed to 100 L NO at 9OOT, (d) after heating at 1050°C.
smaller bands at - 4 and - 11 eV. Similar results have been obtained in the previous UPS study of the NO/Si(lll) system [14]. Fig. lb shows the spectra for the surface exposed to 100 L (1 L = 1 X lo-” Torr . s) NO, which is fully saturated. For 100 L exposure, all the LEED spots disappear and only the diffuse background is observed in the LEED pattern. Nishijima et al. have proposed that, using electron-energy loss spectroscopy (EELS), NO is adsorbed dissociatively on the SK1111 surface at room temperature, forming a disordered phase [13]. Previous ultraviolet photoemission spectroscopy (UPS) studies on the oxygen/SK1111 system have shown that the dissociatively adsorbed oxygen induces an 0 2pderived band at - 7 eV together with smaller bands at - 4 and - 10 eV in the valence band spectra [15,16]. The UPS study for the Si(100) surface exposed to atomic nitrogen has also shown that the adsorbed nitrogen induces a N2p-derived band at - 7 eV together with smaller bands at 4 and 11 eV [61. Thus, the NO-induced bands shown in Fig. lb can be interpreted as the sum of
S. Tokumitsuet al./Surface Science 317 (1994) 143-1.51
the emissions from both the N 2p- and 0 2p-derived bands due to the diss~iatively adsorbed N and 0 atoms. As the Si(ll1) surface exposed to 100 L NO is heated at 9OO”C, oxygen atoms are removed from the surface which is confirmed from the disappearance of the 0 IUL signal in the AES spectra. By heating at 9OO”C,the diffuse LEED pattern is changed to an (8 x 8) pattern. As the surface is further heated to 105O”C, the (8 X 8) spots disappear and a quadruplet LEED pattern is observed overlapping with the (7 X 7) pattern. These results are in good agreement with the previous study for the Si(lll)-NO system 1131, Figs. lc and Id show the nodal-emission spectra for the (8 X 81-N and quadruplet-N surfaces, respectively. The spectra for both surfaces are very similar, however, the features are somewhat sharpened for the quadruplet-N surface. In the following sections, the valence electronic states of the (8 X 81-N and the quadruplet-N surfaces are investigated with angle-resolved measurements. It is noted that &dependent measurements of the ARPES show that all peak energies are independent on the photon energy for both surfaces,
145
indicating that the nitride layers are not multilayers in these preparation conditions of nitride surfaces. 3.1. Siflll)(S
x8)-N
It is known that the polarization-dependent study is useful to identify the symmetry of the electronic states observed in ARPE spectra. Fig. 2 shows the 8i dependence of the normal-emission spectra for the Si(lllX8 X 8)-N surface taken at two different incidence angles (30” and 60”). The &-dependent spectra are taken at two different photon energies (hv = 32.5 and 45 eV> since it is difficult to observe all peaks cleanly in a single spectrum. Since the incident light is linearly polarized in the incidence plane, the A,, component (parallel to the surface) and A ,_ component (normal to the surface) of the polarization vector A are relatively dominant at Bi = 30” and 60”, respectively. The polarization-dependent spectra measured at two different photon energies show that the band at 4-8 eV is composed of 4.5 and 5.6 eV peaks together with the band at 6.5-7.6 eV which seems to be composed Si(ll1) @x6)-N
Sift 11) @x8)-N
~~
i 14
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10
8
6
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2
0 (=E,)
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12
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10
e
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6
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2
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BINDING ENERGY (eV)
BINDING ENERGY (eV)
Fig. 2. Comparison of the normal-emission spectra of the Si(lllX8 The photon energies are 32.5 eV (a) and 45 eV (b).
I.
14
X
81-N surface measured
at 6, =
30” and 60” (A,,
(di>).
146
S. Tokum~tsu et al. /Surface Science 317 (1994) 143-151
of two unresolved peaks. The band at N 12 eV is also resolved into two peaks (11.2 and 12.4 eV). The peaks are indicated by vertical bars. Fig. 2 shows that the features at 4.5, 5.6 and 12.4 eV are relatively enhanced at Bi = 60”, and those at 6.57.6 eV and 11.2 eV are relatively enhanced at 0, = 30”. When the nitrogen atom in the (8 X 8)-N phase forms a NSi, bond of C, symmetry [4,131, the N2p orbitals will form two types of molecular orbitals with Si orbitals at the T point; the a,-state constructed from the 2p, orbital and the e-state constructed from the 2p,,, orbitals. In the normal-emission collection, the A L(A,,) component of the polarization vector of the incident light can excite only an a,-(e-jsymmetry initial state. Previous photoemission study of silicon nitride has shown that the bulk valence band spectra of the Si,N, consist of three bands at 4.9, 7.5 and 12.4 eV which are assigned to the N2p, non-bonding, N 2p,,, + Si3p bonding, and N~P~,~ -t Si3s bonding state, respectively [17]. Thus, the peaks at 4.5 and 5.6 eV in Fig. 2 can be attributed to the emission from the N 2p, non-bonding state, which is confirmed from the determined symmetry (a,) Si(ll1)
j
of these states since the N2pr state should be formed from the N2p, orbital in this model. The band at 6.5-7.6 is attributed to the emissions from the N~P~,~ + Si3p states, which is also confirmed from the determined symmetry. The peaks at 11.2 and 12.4 eV are in the N2pX,+ + Si3s band region. The N 2pX,Y+ Si 3s band should have e-symmetry in this model. The polarization-dependent study shows that the state at 11.2 eV is an e-state, thus the state is we11 ascribed to the N2p,,+ t Si3s state, however, the state at 12.4 eV is observed to be an al-state. The state at 12.4 eV is considered to be also attributed to the Si3s-derived band, and thus the above result suggests that the N2p, orbital may have some contribution to the hybridization with the Si3s state in the (8 x 8)-N nitride surface. The (8 X 8)-N-induced features in the normalemission spectra are rather complicated as compared to those in the spectra for the silicon nitride [ 171,i.e. the N 2p,- and Si 3sderived bands are observed as doublet for the (8 x 81-N surface, and, probably the N 2p_ + Si 3p band should be composed of two unresolved peaks. The lattice constant of the (8 X 81-N surface does not coin-
(8x8)-N
Si(l1 1
off-normal emission
i
I 14
12
IO
8
6
4
2
o (=E,)
BINDING ENERGY (eV)
Fig. 3. Off-normal-emission direction (A,, along (211)).
I) (8x8)-N
spectra of the Si(lllX8
x
I., , 14
12
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8
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BINDING ENERGY (eV)
81-N surface measured at hv = 32.5 eV and Bi = 60” in the (Zii> (i%%)
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0.4 ,
3.5
0.6 ,
1.2
1
,,,~,:
, 7,
F,,
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iz
Fig. 4. Measured dispersion E(k,,) %fLhe N2p, band for the Si(lllXS x8)-N surface along the TM direction. Schematic illustration of the surface umklapp process in the (8X81-N and Si(lllX1 x 1) substrate system along the (2E> direction (reciprocal lattice vectors are G,, and + G,,, respectively) is also shown in the inset.
tide with the Si(ll1) substrate, thus, the “surface umkiapp” process should have considerabIe influence on the ARPE spectra as reported in the
Si(l11)
quadruplet-N
studies for the rare-gas adsorption systems 118,191. Fig. 3 shows off-normal-emission spectra for the (8 x 81-N surface taken at various detection angles @J along the (271) direction of the SiOllXl x 1) substrate. The spectra are taken at hu = 32.5 eV and Bi = 60’. The peak positions of the N~P~,~ + Si 3p- and Si 3s-derived bonding bands are not clear in this experimental condition, thus we only trace the dispersion of the N2p, states (indicated by vertical bars). Fig. 3 shows that the 5.6 eV level at the F point disperses towards a lower binding energy side with the increase of O,, as is expected for the N2p,derived state in the tight-binding scheme. The level overlaps with the upper level (4.5 eV at the ?; point) at 8, = 15”. The measured energies of the NZp,-derived states are plotted in an E versus k,, diagram via kj, = [ (2m/h2)E,i”]
1’2 sin O,j,
where Ekin is the measured kinetic energy. The results are summarized in Fig. 4. Since the unit vectors of a triangular Iattice of the (8 x 81-N layer is 8/11 the length of the Si(lllX1 x 1) unit
Si(ll1) quadruplet-N r@----j
~_
14
12
10 8 6 4 2 BINDING ENERGY (eV)
0 (=E,)
Fig. 5. Comparison of the normal-emission spectra of the Sitlll) The photon energies are 30 eV (a) and 45 eV (b). (zii)).
14
12
10
0
6
4
5lNDlNG ENERGY (t:
quadruplet-N
2
0 (=E,)
)
surface measured at Bi = 30” and 60” (A,, along
148
S. Tokumitsu et al. /Surface Science 317 (1994) 143-151
vectors, the surface reciprocal-lattice vectors of G,, and & G,, (where the G,, is the reciprocallattice vector of the (8 x 8)-N layer) may both contribute to the photoemission process. Thus, the photoelectrons scattered from the points in the surface Brillouin zone (SBZ) of k G,, and _+fi G,, are both included in the normal-emission spectra. In this case, a subband should be observed in the normal-emission spectra due to the latter photoelectrons, when the band has some dispersion. This process, called “surface umklapp” [18,19], is schematically shown in the inset of Fig. 4. The dispersion of the N2p, band shown in Fig. 4 strongly suggests that the band at 4.5 eV near the r point is caused by the scattered photoelectrons from the + A G,, points. Such an effect has already been observed for the valence states of rare gas adsorption layers which are also incommensurate to the substrate lattice [18,19]. The band for the N2p,,, + Si3p state cannot be well resolved into the peaks in this experimental condition, however, the same effect may be operative for this state, which results in the observa-
Si(l11) quadruplet-N
tion of broad band in the N~P~,~ + Si 3p band region in the normal-emission spectrum (Fig. 2). 3.2. Si(ll1)
Fig. 5 shows the ei-dependence of the normalemission spectra for the SK1111 quadruplet-N surface measured at hv = 30 and 45 eV. The spectra are taken at two different incidence angles (30” and 60”). The polarization-dependent spectra measured at two different photon energies show that, similar to the case for those of (8 x 81-N surface, the band at 4-8 is composed of 4.5 and 5.6 eV peaks together with the band at 6.8-7.8 eV. The broad peak is also found at 12.7 eV. The peaks are indicated by vertical bars. Previous studies on the structure of the quadruplet-N surface have concordantly proposed that a N atom is bonded to three Si atoms forming the NSi, bond of C,, symmetry [4,11,13]. Fig. 5 shows that the 4.5 and 5.6 eV peaks are relatively enhanced at ~9~ = 60”, and the features at 6.8-7.8 and at 12.7 eV are relatively enhanced at Bi = 30”. According to the symmetry selection
Si(ll1)
quadruplet-N
1.8-d
14
12
10
6
6
4
2
0 (=E,)
BINDING ENERGY (eV)
Fig. 6. Off-normal-emission spectra of the Si(ll1) quadruplet-N direction (A,, along (211)).
quadruplet-N
14
I.,.‘.,
12
,1,
10 6 6 4 2 BINDING ENERGY (eV)
,.
0 (=E,)
-surface measured at hv = 30 eV and 8, = 60” in the (2x1) (TM)
S. Tokumitsu et al./Sutface
k, (A")
0 4
0.5
1
1.5
2
""1""I""""' :
Fig. 7. Measured dispersion E(k,,) of the N2p, band for the Si(ll1) quadruplet-N system along the (2ii) direction of the Si(lllX1 x 1) substrate.
rules (described in the previous section) and the photoemission study for silicon nitrides [17], the 4.5 and 5.6 eV peaks, the band at 6.8-7.8 eV and the 12.7 eV peak are attributed to the emissions from the N 2p, non-bonding, N 2px,Y + Si 3p bonding, and N2p,,, + Si3s bonding state, re-
Science 317 (1994) 143-151
149
spectively. The a,-symmetry component in the Si3s bonding region is not found for the quadruplet-N surface. In the normal-emission spectra for the quadruplet-N surface, two weak peaks are observed above the N2p band (0.9 and 3.2 eV peaks), which are much smaller in the spectra for the (8 X 8)-N surface. The 0.9 eV peak is ascribed to the emissions from the Si(lllX7 x 7) clean surface region (S, surface state [20]), since the peak is removed by further NO exposure as discussed later (Fig. 8). It is noted that the quadruplet LEED pattern is observed with the weak (7 X 7) pattern, thus the (7 x 7) clean surface region is coexistent with the quadruplet-N region in this surface preparation condition. The origin of the 3.2 eV peak is unknown at present and will be discussed briefly later. Fig. 6 shows off-normal-emission spectra for the quadruplet-N surface taken at various detection angles along the (211) direction of the Si(lllX1 x 1) substrate. Similarly to the case for the (8 x 81-N surface, we can only trace the dispersion of the N2p,-derived states (indicated by vertical bars). Fig. 6 shows that the 5.6 eV level at ,““““““““““““‘,“““‘I
normal emission
14
12
10
t
6
BINDING
6
4
2
ENERGY (ev)
0 (=E,)
normal emlsslon
14
12
10
1
6
6
4
2
0 (=t,J
BINDING ENERGY (eV)
Fig. 8. Normal-emission spectra at hv = 30 and 45 eV: (a) the Si(ll1) quadruplet-N surface, (b) after 10 L NO exposure, (c) after heating the surface at 105O”C,(d) after 10 L NO exposure, (e) after heating the surface at lOSOT, (f) after 10 L NO exposure, (g) after heating at 1050°C.
the i? point disperses towards a lower binding energy side with the increase of 0,, as is expected for the N2p, non-bonding states in the tightbinding scheme. The dispersive level overlaps with the non-dispersing level at 4.5 eV at 0, = 15”. The measured energies of the N 2p,-derived states are plotted in an E versus k,, diagram (Fig. 7). For the quadruplet-N surface, the measured direction is not along the high-symmetry direction of the surface nitride layer and, furthermore, four domains whose lattice vectors are rotated by i 5” or f 30” with respect to those of the Si(lll)(l X 1) substrate are coexistent. Thus, it is rather difficult to analyze the results shown in Fig. 7, however, it is qualitatively shown in Fig. 7 that the 4.5 eV peak around the T point is caused by the “surface umklapp” effect as in the case for the (8 X 8)-N surface. 3.3. Multilayer formation Growth of the nitride layers on the Si surface is of interest, in particular from the technological viewpoint, and we will discuss the nitride multilayer growth on the quadruplet-N surface in this section. Fig. 8 shows the normal-emission spectra for the quadruplet-N surface and the surface treated with several cycles of successive nitridation processes (10 L NO exposure and subsequent heating at 1050°C). The spectra are taken at hv = 30 and 45 eV. As the quadruplet-N surface is exposed to NO at room temperature, the 0.9 eV peak is removed and broad emission at N 7 eV, which is ascribed to the N 2p- and 0 2pderived states of the dissociatively adsorbed N and 0 atoms, appears overlapping with the quadruplet-N emissions. As the surface is subsequently heated at 105O”C, the broad emission at N 7 eV is removed and the quadruplet-N emissions are increased in intensity. The 0.9 eV peak reappears but the intensity is weak relative to the original quadruplet-N surface. With the accumulation of the nitridation cycles, the quadrupIet-N emission develops the 0.9 eV peak becomes weak, and the emission at N 7 eV induced by NO exposure becomes weak. The 0.9 eV emission is attenuated by the NO adsorption and this is characteristic for the (7 x 7) clean surface in this
energy region (O-2 eV) DO], thus this is attributed to the emission from the clean surface region coexistent with the quadruplet-N region. Fig. 8 shows that, as the 0.9 eV emission becomes weak, the NO-induced features after successive NO adsorption are weakened, indicating that NO is adsorbed only on the (7 x 7) clean surface region and does not react with the quadruplet-N surface at room temperature. The peak at 3.2 eV, which is not found for the silicon nitride [17J, is not well developed with the accumulation of the nitridation cycles (Fig. 81, indicating that this is not ascribed to the emission from the nitridation layer itself. The origin of the 3.2 eV peak is unknown, however, we tentatively ascribed this to the emission from the state localized at the interface between the nitride layer and the SK1111 substrate, since such a state does not exist for the silicon nitride Fig. 8 shows that, after two cycles of nitridation process, the binding energies of each peak in the spectra taken at hv = 30 and 45 eV become different from each other, indicating that three-dimensional band dispersions of each state are formed. This result suggests that a multilayer region is formed even after two cycles of nitridation process, though the clean (7 X 7) surface region is still existent in this stage.
4. Summary Angle-resolved photoemission measurements have been performed on the eIectronic structure of the (8 X81-N and quad~plet nitride layers formed on the Si(lll) surface. For the (8 X 81-N surface the N 2p, non-bonding, N 2~,,~ + Si 3p bonding, and Si3s-derived bonding bands are observed at 4.5 and 5.6, 6.5-7.6, and 11.2 and 12.4 eV, respectively, in the normal-emission spectra. For the quadruplet-N surface, the spectral profile is similar to that for the (8 X 8)-N surface. The off-normal-emission measurements suggest that the “surface umklapp” process due to the lattice mismatch between the nitride over-layers and the Si(lll) substrate is operative in the ARPE spectra for both surfaces. The NO adsorption study on the quadruplet-N surface indicates that the quadruplet-N surface region is inactive for the
S. Tokumitsu et al. /Surface
NO adsorption and NO is adsorbed only on the coexistent clean surface region. The nitride multilayer can be formed by NO adsorption-heating (1050°C) cycles and a three-dimensional band is formed after two cycles of this process.
Acknowledgements
We are pleased to thank the staff of Photon Factory, National Laboratory for High Energy Physics for their excellent support. This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 91-143). This work was supported in part by the Grants-in-Aid for Scientific Research from the Ministry of Education.
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ELSEVIER
Surface
Science 317 (1994) 143-151
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Electronic structure of the nitride layers formed on a Si( 111) surface: angle-resolved photoemission study Shuzo Tokumitsu a, Toshihisa Anazawa a, Ken-ichi Ozawa a, Eizo Miyazaki a, Kazuyuki Edamoto ap*,Hiroo Kato b aDepartment of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan b Photon Factory, National Laboratory for High Energy Physics, Tsukuba-shi, Ibaraki 305, Japan
Received 13 January 1994;accepted for publication 10 June 1994
Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been applied to study the electronic structure of the nitride overlayers formed on a Si(ll1) surface. As the Si(ll1) surface exposed to NO at saturation coverage is heated to 9Oo”C,the (8 X 81-N overlayer is formed. The N 2p, non-bonding, N 2px,Y+ Si 3p bonding, and Si3s-derived bonding bands are identified in the ARPE spectra using the polarization-dependent measurements. As the surface is further heated to lOSO”C, the (8 x 8)-N overlayer is changed to the quadruplet-N overlayer, however, the spectral profile for the quadruplet-N overlayer is similar to that for the (8 x 8)-N overlayer. The effect of the “surface umklapp” process arising from the lattice mismatch between nitride overlayers and Si(ll1) (1 x 1) substrate are investigated with off-normal-emission measurements.
1. Introduction Nitridation of the Si surface is of considerable interest not only from the scientific viewpoint but also for the technological applications of silicon nitride (high-temperature structural ceramics, diffusion-resist passivating material, etc.). Nitridation of the Si(ll1) surface has been studied using various gases such as N [l-61, NH, [5-91, HN, [lo], N, [ll] and NO [12,13]. These investigations have shown that the “(8 X 81-N” or “quadruplet-
N” nitride layers are formed on the Si(ll1) surface depending on the heating temperature of the
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author.
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Si(ll1) surface, reactant gas pressure and carbon contamination. In the previous studies on the structure of these nitride layers was proposed that each N atom sits on the three-fold site forming the NSi, bond for both surfaces, and that the unit vectors of a triangular lattice are 8/11 and 3/4 the length of the Si(lll)(l x 1) unit vectors for the (8 X 81-N and quadruplet-N surfaces, respectively, and that the lattice vectors are rotated fB1 or fez with respect to those of silicon substrate, where 8, = $9, = 5” for the quadruplet-N domains [2-4,11,13]. In this work, the electronic structure of the (8 X 81-N and quadruplet-N surfaces formed on the Si(ll1) surface is investigated with angle-re-
solved photoemission spectroscopy (ARPES) utilizing synchrotron radiation. In this work, the
B.V. All rights reserved
144
S. Tokumitsu et al. /Surface
Science 317 (1994) 143-151
nitride surfaces are formed using NO as a reactant gas.
normal emission
h Y =32.5eV
2. Experimental The ARPES measurements were conducted on Beam Line 11D of the Photon Factory, National Laboratory for High Energy Physics, using a constant deviation monochromator and an electron energy analyzer of 150”, spherical-sector type, with an acceptance angle of f 1”. The total experimental resolution was - 0.2 eV. The radiation was linearly polarized in the horizontal plane of incidence and the photoelectrons were detected in this incidence plane for all measurements. The base pressure in the vacuum system was 1 X 10-l” Torr. The sample used was a SXlll) wafer which was nearly intrinsic (p-type, boron-doped, - 5 s2. cm). The Si sample was mounted on the sample manipulator such that the (2ii) Az was in the incidence plane of the light. The (7 X 7) clean surface was prepared by flashing to about 1300°C. No impurities were observed on the (7 X 7) surface thus prepared within the detection limit of AES. The Si sample was heated by electron bombardment from the rear and the temperature was measured with a W-3%Re/W-25%Re thermocouple. In this paper, the incidence angle of the light (ei) and the detection angle of the photoelectron (0,) are given relative to the surface normal. In the ARPE spectra presented below, the electron binding energies are referenced to zero at the Fermi level, which is determined from the spectra for the Ta plate.
3. Results and discussion Fig. 1 shows the normal-emission spectra for the SKlll) surface exposed to NO, and for the surface subsequently heated to high temperatures. The photon energy is 32.5 eV and the incidence angle of the light is 60”. As the clean surface is exposed to NO at room temperature, a broad band developes at N 7 eV together with
)I
I14
_-e----_
12
10
8
(8 1
6
4
BINDING ENERGY
2
0 (=E,)
(eV)
Fig. 1. Normal-emission spectra: (a) the Si(ll1) (7X 7) clean surface, (b) after 100 L NO exposure, (c) after heating the Si(ll1) surface exposed to 100 L NO at 9OOT, (d) after heating at 1050°C.
smaller bands at - 4 and - 11 eV. Similar results have been obtained in the previous UPS study of the NO/Si(lll) system [14]. Fig. lb shows the spectra for the surface exposed to 100 L (1 L = 1 X lo-” Torr . s) NO, which is fully saturated. For 100 L exposure, all the LEED spots disappear and only the diffuse background is observed in the LEED pattern. Nishijima et al. have proposed that, using electron-energy loss spectroscopy (EELS), NO is adsorbed dissociatively on the SK1111 surface at room temperature, forming a disordered phase [13]. Previous ultraviolet photoemission spectroscopy (UPS) studies on the oxygen/SK1111 system have shown that the dissociatively adsorbed oxygen induces an 0 2pderived band at - 7 eV together with smaller bands at - 4 and - 10 eV in the valence band spectra [15,16]. The UPS study for the Si(100) surface exposed to atomic nitrogen has also shown that the adsorbed nitrogen induces a N2p-derived band at - 7 eV together with smaller bands at 4 and 11 eV [61. Thus, the NO-induced bands shown in Fig. lb can be interpreted as the sum of
S. Tokumitsuet al./Surface Science 317 (1994) 143-1.51
the emissions from both the N 2p- and 0 2p-derived bands due to the diss~iatively adsorbed N and 0 atoms. As the Si(ll1) surface exposed to 100 L NO is heated at 9OO”C, oxygen atoms are removed from the surface which is confirmed from the disappearance of the 0 IUL signal in the AES spectra. By heating at 9OO”C,the diffuse LEED pattern is changed to an (8 x 8) pattern. As the surface is further heated to 105O”C, the (8 X 8) spots disappear and a quadruplet LEED pattern is observed overlapping with the (7 X 7) pattern. These results are in good agreement with the previous study for the Si(lll)-NO system 1131, Figs. lc and Id show the nodal-emission spectra for the (8 X 81-N and quadruplet-N surfaces, respectively. The spectra for both surfaces are very similar, however, the features are somewhat sharpened for the quadruplet-N surface. In the following sections, the valence electronic states of the (8 X 81-N and the quadruplet-N surfaces are investigated with angle-resolved measurements. It is noted that &dependent measurements of the ARPES show that all peak energies are independent on the photon energy for both surfaces,
145
indicating that the nitride layers are not multilayers in these preparation conditions of nitride surfaces. 3.1. Siflll)(S
x8)-N
It is known that the polarization-dependent study is useful to identify the symmetry of the electronic states observed in ARPE spectra. Fig. 2 shows the 8i dependence of the normal-emission spectra for the Si(lllX8 X 8)-N surface taken at two different incidence angles (30” and 60”). The &-dependent spectra are taken at two different photon energies (hv = 32.5 and 45 eV> since it is difficult to observe all peaks cleanly in a single spectrum. Since the incident light is linearly polarized in the incidence plane, the A,, component (parallel to the surface) and A ,_ component (normal to the surface) of the polarization vector A are relatively dominant at Bi = 30” and 60”, respectively. The polarization-dependent spectra measured at two different photon energies show that the band at 4-8 eV is composed of 4.5 and 5.6 eV peaks together with the band at 6.5-7.6 eV which seems to be composed Si(ll1) @x6)-N
Sift 11) @x8)-N
~~
i 14
12
10
8
6
4
2
0 (=E,)
.I...‘...,
12
,.I.
10
e
.I
6
.1
,.I...‘.
4
2
o (=E,)
BINDING ENERGY (eV)
BINDING ENERGY (eV)
Fig. 2. Comparison of the normal-emission spectra of the Si(lllX8 The photon energies are 32.5 eV (a) and 45 eV (b).
I.
14
X
81-N surface measured
at 6, =
30” and 60” (A,,
(di>).
146
S. Tokum~tsu et al. /Surface Science 317 (1994) 143-151
of two unresolved peaks. The band at N 12 eV is also resolved into two peaks (11.2 and 12.4 eV). The peaks are indicated by vertical bars. Fig. 2 shows that the features at 4.5, 5.6 and 12.4 eV are relatively enhanced at Bi = 60”, and those at 6.57.6 eV and 11.2 eV are relatively enhanced at 0, = 30”. When the nitrogen atom in the (8 X 8)-N phase forms a NSi, bond of C, symmetry [4,131, the N2p orbitals will form two types of molecular orbitals with Si orbitals at the T point; the a,-state constructed from the 2p, orbital and the e-state constructed from the 2p,,, orbitals. In the normal-emission collection, the A L(A,,) component of the polarization vector of the incident light can excite only an a,-(e-jsymmetry initial state. Previous photoemission study of silicon nitride has shown that the bulk valence band spectra of the Si,N, consist of three bands at 4.9, 7.5 and 12.4 eV which are assigned to the N2p, non-bonding, N 2p,,, + Si3p bonding, and N~P~,~ -t Si3s bonding state, respectively [17]. Thus, the peaks at 4.5 and 5.6 eV in Fig. 2 can be attributed to the emission from the N 2p, non-bonding state, which is confirmed from the determined symmetry (a,) Si(ll1)
j
of these states since the N2pr state should be formed from the N2p, orbital in this model. The band at 6.5-7.6 is attributed to the emissions from the N~P~,~ + Si3p states, which is also confirmed from the determined symmetry. The peaks at 11.2 and 12.4 eV are in the N2pX,+ + Si3s band region. The N 2pX,Y+ Si 3s band should have e-symmetry in this model. The polarization-dependent study shows that the state at 11.2 eV is an e-state, thus the state is we11 ascribed to the N2p,,+ t Si3s state, however, the state at 12.4 eV is observed to be an al-state. The state at 12.4 eV is considered to be also attributed to the Si3s-derived band, and thus the above result suggests that the N2p, orbital may have some contribution to the hybridization with the Si3s state in the (8 x 8)-N nitride surface. The (8 X 8)-N-induced features in the normalemission spectra are rather complicated as compared to those in the spectra for the silicon nitride [ 171,i.e. the N 2p,- and Si 3sderived bands are observed as doublet for the (8 x 81-N surface, and, probably the N 2p_ + Si 3p band should be composed of two unresolved peaks. The lattice constant of the (8 X 81-N surface does not coin-
(8x8)-N
Si(l1 1
off-normal emission
i
I 14
12
IO
8
6
4
2
o (=E,)
BINDING ENERGY (eV)
Fig. 3. Off-normal-emission direction (A,, along (211)).
I) (8x8)-N
spectra of the Si(lllX8
x
I., , 14
12
/.../...I,...,../..
IO
.i
8
6
4
2
0 (=E,)
BINDING ENERGY (eV)
81-N surface measured at hv = 32.5 eV and Bi = 60” in the (Zii> (i%%)
(A-*>
k,
0
0.4 ,
3.5
0.6 ,
1.2
1
,,,~,:
, 7,
F,,
T 6
1.6
“‘I iii
T
iz
Fig. 4. Measured dispersion E(k,,) %fLhe N2p, band for the Si(lllXS x8)-N surface along the TM direction. Schematic illustration of the surface umklapp process in the (8X81-N and Si(lllX1 x 1) substrate system along the (2E> direction (reciprocal lattice vectors are G,, and + G,,, respectively) is also shown in the inset.
tide with the Si(ll1) substrate, thus, the “surface umkiapp” process should have considerabIe influence on the ARPE spectra as reported in the
Si(l11)
quadruplet-N
studies for the rare-gas adsorption systems 118,191. Fig. 3 shows off-normal-emission spectra for the (8 x 81-N surface taken at various detection angles @J along the (271) direction of the SiOllXl x 1) substrate. The spectra are taken at hu = 32.5 eV and Bi = 60’. The peak positions of the N~P~,~ + Si 3p- and Si 3s-derived bonding bands are not clear in this experimental condition, thus we only trace the dispersion of the N2p, states (indicated by vertical bars). Fig. 3 shows that the 5.6 eV level at the F point disperses towards a lower binding energy side with the increase of O,, as is expected for the N2p,derived state in the tight-binding scheme. The level overlaps with the upper level (4.5 eV at the ?; point) at 8, = 15”. The measured energies of the NZp,-derived states are plotted in an E versus k,, diagram via kj, = [ (2m/h2)E,i”]
1’2 sin O,j,
where Ekin is the measured kinetic energy. The results are summarized in Fig. 4. Since the unit vectors of a triangular Iattice of the (8 x 81-N layer is 8/11 the length of the Si(lllX1 x 1) unit
Si(ll1) quadruplet-N r@----j
~_
14
12
10 8 6 4 2 BINDING ENERGY (eV)
0 (=E,)
Fig. 5. Comparison of the normal-emission spectra of the Sitlll) The photon energies are 30 eV (a) and 45 eV (b). (zii)).
14
12
10
0
6
4
5lNDlNG ENERGY (t:
quadruplet-N
2
0 (=E,)
)
surface measured at Bi = 30” and 60” (A,, along
148
S. Tokumitsu et al. /Surface Science 317 (1994) 143-151
vectors, the surface reciprocal-lattice vectors of G,, and & G,, (where the G,, is the reciprocallattice vector of the (8 x 8)-N layer) may both contribute to the photoemission process. Thus, the photoelectrons scattered from the points in the surface Brillouin zone (SBZ) of k G,, and _+fi G,, are both included in the normal-emission spectra. In this case, a subband should be observed in the normal-emission spectra due to the latter photoelectrons, when the band has some dispersion. This process, called “surface umklapp” [18,19], is schematically shown in the inset of Fig. 4. The dispersion of the N2p, band shown in Fig. 4 strongly suggests that the band at 4.5 eV near the r point is caused by the scattered photoelectrons from the + A G,, points. Such an effect has already been observed for the valence states of rare gas adsorption layers which are also incommensurate to the substrate lattice [18,19]. The band for the N2p,,, + Si3p state cannot be well resolved into the peaks in this experimental condition, however, the same effect may be operative for this state, which results in the observa-
Si(l11) quadruplet-N
tion of broad band in the N~P~,~ + Si 3p band region in the normal-emission spectrum (Fig. 2). 3.2. Si(ll1)
Fig. 5 shows the ei-dependence of the normalemission spectra for the SK1111 quadruplet-N surface measured at hv = 30 and 45 eV. The spectra are taken at two different incidence angles (30” and 60”). The polarization-dependent spectra measured at two different photon energies show that, similar to the case for those of (8 x 81-N surface, the band at 4-8 is composed of 4.5 and 5.6 eV peaks together with the band at 6.8-7.8 eV. The broad peak is also found at 12.7 eV. The peaks are indicated by vertical bars. Previous studies on the structure of the quadruplet-N surface have concordantly proposed that a N atom is bonded to three Si atoms forming the NSi, bond of C,, symmetry [4,11,13]. Fig. 5 shows that the 4.5 and 5.6 eV peaks are relatively enhanced at ~9~ = 60”, and the features at 6.8-7.8 and at 12.7 eV are relatively enhanced at Bi = 30”. According to the symmetry selection
Si(ll1)
quadruplet-N
1.8-d
14
12
10
6
6
4
2
0 (=E,)
BINDING ENERGY (eV)
Fig. 6. Off-normal-emission spectra of the Si(ll1) quadruplet-N direction (A,, along (211)).
quadruplet-N
14
I.,.‘.,
12
,1,
10 6 6 4 2 BINDING ENERGY (eV)
,.
0 (=E,)
-surface measured at hv = 30 eV and 8, = 60” in the (2x1) (TM)
S. Tokumitsu et al./Sutface
k, (A")
0 4
0.5
1
1.5
2
""1""I""""' :
Fig. 7. Measured dispersion E(k,,) of the N2p, band for the Si(ll1) quadruplet-N system along the (2ii) direction of the Si(lllX1 x 1) substrate.
rules (described in the previous section) and the photoemission study for silicon nitrides [17], the 4.5 and 5.6 eV peaks, the band at 6.8-7.8 eV and the 12.7 eV peak are attributed to the emissions from the N 2p, non-bonding, N 2px,Y + Si 3p bonding, and N2p,,, + Si3s bonding state, re-
Science 317 (1994) 143-151
149
spectively. The a,-symmetry component in the Si3s bonding region is not found for the quadruplet-N surface. In the normal-emission spectra for the quadruplet-N surface, two weak peaks are observed above the N2p band (0.9 and 3.2 eV peaks), which are much smaller in the spectra for the (8 X 8)-N surface. The 0.9 eV peak is ascribed to the emissions from the Si(lllX7 x 7) clean surface region (S, surface state [20]), since the peak is removed by further NO exposure as discussed later (Fig. 8). It is noted that the quadruplet LEED pattern is observed with the weak (7 X 7) pattern, thus the (7 x 7) clean surface region is coexistent with the quadruplet-N region in this surface preparation condition. The origin of the 3.2 eV peak is unknown at present and will be discussed briefly later. Fig. 6 shows off-normal-emission spectra for the quadruplet-N surface taken at various detection angles along the (211) direction of the Si(lllX1 x 1) substrate. Similarly to the case for the (8 x 81-N surface, we can only trace the dispersion of the N2p,-derived states (indicated by vertical bars). Fig. 6 shows that the 5.6 eV level at ,““““““““““““‘,“““‘I
normal emission
14
12
10
t
6
BINDING
6
4
2
ENERGY (ev)
0 (=E,)
normal emlsslon
14
12
10
1
6
6
4
2
0 (=t,J
BINDING ENERGY (eV)
Fig. 8. Normal-emission spectra at hv = 30 and 45 eV: (a) the Si(ll1) quadruplet-N surface, (b) after 10 L NO exposure, (c) after heating the surface at 105O”C,(d) after 10 L NO exposure, (e) after heating the surface at lOSOT, (f) after 10 L NO exposure, (g) after heating at 1050°C.
the i? point disperses towards a lower binding energy side with the increase of 0,, as is expected for the N2p, non-bonding states in the tightbinding scheme. The dispersive level overlaps with the non-dispersing level at 4.5 eV at 0, = 15”. The measured energies of the N 2p,-derived states are plotted in an E versus k,, diagram (Fig. 7). For the quadruplet-N surface, the measured direction is not along the high-symmetry direction of the surface nitride layer and, furthermore, four domains whose lattice vectors are rotated by i 5” or f 30” with respect to those of the Si(lll)(l X 1) substrate are coexistent. Thus, it is rather difficult to analyze the results shown in Fig. 7, however, it is qualitatively shown in Fig. 7 that the 4.5 eV peak around the T point is caused by the “surface umklapp” effect as in the case for the (8 X 8)-N surface. 3.3. Multilayer formation Growth of the nitride layers on the Si surface is of interest, in particular from the technological viewpoint, and we will discuss the nitride multilayer growth on the quadruplet-N surface in this section. Fig. 8 shows the normal-emission spectra for the quadruplet-N surface and the surface treated with several cycles of successive nitridation processes (10 L NO exposure and subsequent heating at 1050°C). The spectra are taken at hv = 30 and 45 eV. As the quadruplet-N surface is exposed to NO at room temperature, the 0.9 eV peak is removed and broad emission at N 7 eV, which is ascribed to the N 2p- and 0 2pderived states of the dissociatively adsorbed N and 0 atoms, appears overlapping with the quadruplet-N emissions. As the surface is subsequently heated at 105O”C, the broad emission at N 7 eV is removed and the quadruplet-N emissions are increased in intensity. The 0.9 eV peak reappears but the intensity is weak relative to the original quadruplet-N surface. With the accumulation of the nitridation cycles, the quadrupIet-N emission develops the 0.9 eV peak becomes weak, and the emission at N 7 eV induced by NO exposure becomes weak. The 0.9 eV emission is attenuated by the NO adsorption and this is characteristic for the (7 x 7) clean surface in this
energy region (O-2 eV) DO], thus this is attributed to the emission from the clean surface region coexistent with the quadruplet-N region. Fig. 8 shows that, as the 0.9 eV emission becomes weak, the NO-induced features after successive NO adsorption are weakened, indicating that NO is adsorbed only on the (7 x 7) clean surface region and does not react with the quadruplet-N surface at room temperature. The peak at 3.2 eV, which is not found for the silicon nitride [17J, is not well developed with the accumulation of the nitridation cycles (Fig. 81, indicating that this is not ascribed to the emission from the nitridation layer itself. The origin of the 3.2 eV peak is unknown, however, we tentatively ascribed this to the emission from the state localized at the interface between the nitride layer and the SK1111 substrate, since such a state does not exist for the silicon nitride Fig. 8 shows that, after two cycles of nitridation process, the binding energies of each peak in the spectra taken at hv = 30 and 45 eV become different from each other, indicating that three-dimensional band dispersions of each state are formed. This result suggests that a multilayer region is formed even after two cycles of nitridation process, though the clean (7 X 7) surface region is still existent in this stage.
4. Summary Angle-resolved photoemission measurements have been performed on the eIectronic structure of the (8 X81-N and quad~plet nitride layers formed on the Si(lll) surface. For the (8 X 81-N surface the N 2p, non-bonding, N 2~,,~ + Si 3p bonding, and Si3s-derived bonding bands are observed at 4.5 and 5.6, 6.5-7.6, and 11.2 and 12.4 eV, respectively, in the normal-emission spectra. For the quadruplet-N surface, the spectral profile is similar to that for the (8 X 8)-N surface. The off-normal-emission measurements suggest that the “surface umklapp” process due to the lattice mismatch between the nitride over-layers and the Si(lll) substrate is operative in the ARPE spectra for both surfaces. The NO adsorption study on the quadruplet-N surface indicates that the quadruplet-N surface region is inactive for the
S. Tokumitsu et al. /Surface
NO adsorption and NO is adsorbed only on the coexistent clean surface region. The nitride multilayer can be formed by NO adsorption-heating (1050°C) cycles and a three-dimensional band is formed after two cycles of this process.
Acknowledgements
We are pleased to thank the staff of Photon Factory, National Laboratory for High Energy Physics for their excellent support. This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 91-143). This work was supported in part by the Grants-in-Aid for Scientific Research from the Ministry of Education.
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