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Ultraviolet photoemission spectroscopy of HCl on Si(111) surfaces
Solid State Communications, Vol.36, pp. 241—243. Pergamon Press Ltd. 1980. Printed in Great Britain. ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF HC1 ON Si (111) SURFACES K. Fujiwara Central Research Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo, Japan (Received 15 April 1980 byA. A. Maradudin) Chemisorption of FtC! on thermally cleaned Si (111) surfaces has been studied by ultraviolet photoemission spectroscopy. HCI chernisorption, both at 300 K and 850 K, induces two main peaks in the photoemission spectrum which are attributed Cl lone pairThis p~,pstrongly ~,orbitals and that 3—Cl: p~bonding orbital,torespectively. suggests Si: spmolecules HQ dissociate on Si(l 11) surfaces already at room temperature. THERE HAS BEEN a controversy about the adsorption state of HC1 on Si(l 11) surfaces. Gas-volumetric measurements [1] indicated that t-lCl molecules decompose on the the silicon surface, and that one molecule occupies each two surface Si atoms, compensating two dangling bonds. On the other hand, electron-impact desorption (EID) study [21 showed the existence of at least two adsorbed states on Si(l 11) surfaces, whereas recent electron energy-loss spectroscopy (ELS), Auger electron spectroscopy (AES) and EID results [3] suggested that HCI molecules arc adsorbed associatively at room temperature before spontaneous dissociation. In this paper, chemisorption of l-ICI on thermally cleaned Si(1 11) surfaces has been studied by ultraviolet photoemission spectroscopy (UPS). UPS has an advantage to investigate this chemically active adsorbate because it provides a lesser destructive method than those employing electrons as an excitation source. From comparison between the UPS spectra and the theoretical local density of states of the chemisorbed hydrogen [41 and chlorine [5—71 atoms, the experimental results are interpreted to show that FICI molecules predominantly decompose on the Si(1 11) surfaces already at room ternperature to form Si—Cl and possibly Si—H covalent bonds. The symmetry properties of the orbital states [5, 7] has also been utilized to identify the nature of the chemisorption bonds. UPS—AES measurements were performed in an ultrahigh-vacuum (UI-IV) system [8]8Pa). withUPS the base measurepressure of 2 x l0~°Torr ( 3 x lO ments were made at photon energy t~= 21.2 eV, using He I radiation. Kinetic energy distribution curves EDC’s of the photoelectrons were obtained in an angle-
sample geometry for the UPS measurements. The analyzer axis was in the plane of incidence of the photons and perpendicular to the photon beam line. The angle of incidence of the photons to the sample surface normal O~,was varied by the sample rotation. Most of the UPS spectra were taken at O~= 45°. An atomically clean surface was obtained by mdirectly heating the Si(ll l)sarpple around 1500 K in UHV with the same method as described previously [81.Cleanness of the thermally cleaned surface was checked In-situ with AES and UPS measurements. The Auger spectra showed that there were no detectable impurities on the surface. For gas—surface reaction study, commercially available research grade HC1 gas was introduced into the chamber by the controlled amount through a bakable leak valve. The coverage of the adsorbates was monitored with AES. After HC1 gas exposure to the clean Si(1 11) surface, the coverage of the adsorbate was determined by momtoring the intensity of C1—LMM Auger peak at 181 eV. Saturation was reached at roughly I L(l06lorrsec) exposure. Figure 2 shows the UPS spectra for the clean Si(1 11) surface (a) and the surfaces after 1 L I-IC1 exposure at 300 K (b) and 850 K (c), respectively. For the clean surface, two characteristic peaks of bulk density of states of silicon are observed at 2.9eV (14) and 7.4 eV (L 1) below Fermi level EF in agreement with the theoretical results [9]. Enhancement of the emission intensity dueof tothe thevalence surface band. states After [101 is near the top 1 Lalso HC1observed exposure at room temperature, the emission intensity due to the surface states decreases, while new peaks appear at E EVAC = 11.1 eV (P 1), 13.2 eV (P2) and 16.2 eV (P3). Energy half width of the main peak F1 is estimated to be 1.6 eV. Changes are also observed in the upper region of the valence band, resulting in a relatively broad peak at 8.1 eV (F4). Work function change of + 0.2eV is obtained from the shift of the low energy cutoff of the EDC. No time-dependent variation —
—
—
averaged form with a spherical sector type analyzer and pulse counting system. EDC’s of the UPS spectra were measured relative to vacuum level EVAC, and the binding energy EB was determined, assuming that the work function ~ of thermally cleaned Si(l I I) surfaces is 4.7 eV [8]. Figure 1 shows the schematics of the
241
—
—
242
ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF HG ON Sm(l 11) SURFACES Vol. 36, No.3 He LAMP
p
OF NC(E~’CE)
Si(111)-HCI 1L at 21.2eV 300°K
SAMPLE ~C~1~L (Z-AXIS) -j
4 0 U)
R~X!S SAMftE ~TATION
ANALYZE
4
~LE
Fig. 1. Schematic diagram of the sample geometry.
4 300
__________________________
U
z
Si(lll)-HC(
150
__________
450
b
b(c)1Lt~oK
-
~
BINDING
/.
6
ENERGY, E
8 (eV) Fig. spectra N(lf) ofofhG-saturated Si(l 11) surface 3. at UPS 300 K as a function photon incidence angle O~. According to the gas-volumetric measurements by Boonstra [11,the room temperature adsorption state of IICI on Si surfaces becomes unstable with increasing temperature. Above 800K all hydrogen desorbs as
Z~~~/~alean
I
molecules while Cl atoms remain on the surface. Therefore, thermal desorption process of l-lCl proceeds as two
I
I
—18 —16 -14 -12 -10 BINDING
8 -6
I
18-16 -14 -12 -10 -8 -6 —4 -2 EF 2
I
I
steps, and chlorine atoms desorb at higher temperature.
I
4 -2 EF 2 4
6
ENERGY, E8 (eV)
Fig. 2. UPS spectra N(E) of Si(l 11) surface before and
after HCI chemisorption. Clean surface spectrum shown by curve (a) and HCI-saturated spectra at 300 and 850 K by curves (b) and (c), respectively. Angle of incidence of He I photons 0, was kept at 45°from the surface normal, was observed of theafter LiPS Ispectrum for at least 30 mm.to On the other hand, L HCI exposure at 850K the clean Si(l 11) surface, peaks are observed at — 11.3 and — 13.5 eVin the UPS spectrum, while the work function changes by + 0.5 eV, as shown in Fig. 2(c).
This implies that the UPS spectrum Fig. 2(c) conesponds to the surface covered with chlorine atoms. Residue of chlorine atoms on the surface could be confirmed by Auger measurements. Thus the peaks at — 11.3 and — 13.5 eV in Fig. 2(c) are attributed to the atomic chlorine chemisorption state. Comparison between the tiPS spectrum with the theoretical local
density of states for CI atoms cheinisorbed on the Si(l 11)1 x I surface [5—71 suggests that these peaks 3 are due to the Cl(a)lone pair Px p,, (ir) orbitals and Siof:sp Cl:p~bonding orbital, respectively, as a result Si—Cl covalent bond formation. The experimental ,
results are also in excellent agreement with those for the monochloride phase found by Cl
2 chemisorption on the In addition, two shoulders are identified at —7.5 and
significant changes in comparison with those of Fig. 2(b)
Si(l II) surfaces [5, 7]. Two shoulders at —7.5 and — 8.4 eV are tentatively assigned to the Cl-induced bulk states (Si back bond states, b) [6, 111. The UPS results for the room temperature UCI adsorption state are interpreted by the model, in which
except the i;tcrease of the emission intensity.
HO molecules predominantly decompose on Si(l II)
8.4eV in the upper region of the valence band. It should be noted that the binding energy and the —
energy half width of the main peaks do not show any
—
Vol. 36, No. 3 ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF HG ON Si(l 11) SURFACES surfaces, as discussed in the following. It is experimentally found that the binding energy of the main peaks P, and P2 has exactly the same value for the in and a orbitals, respectively, in the monochioride phase formed at 850 K. These facts give strong evidence for the Si—C! covalent bond formation on the HCI-chemisorbed Si(lll)surface at room temperature, provided that the based on the for concept of the case. local Itbond [5] is aapproach good approximation the present is not surprising that characteristic UPS peaks at 9.8 and 12.0eV of hydrogen chemisorbed on the Si( 111) 7 x 7 surface (monohydride phase) [121 were not observed, because they are easily masked by the Clinduced big peak at 11.1 eV. The assignment of the broad peaks P 3 and P4 located in the lower and upper region of the valence band is not clear now but these are tentatively attributed to the Si bulk-like states induced by the Si—H and Si—Cl bond formation [4, 61. Further evidence of the orbital assignments of the peaks P1 and P2 can be obtained from the dependence of the UPS spectra on the photon incidence angle 0~. The result is shown in Fig. 3. It should be noted that only the ratio of the relative peak intensity is valid in this case because the detected solid angle of the photoelectron decreases for 0. <45°. At normal incidence (0, = 0°),a purely s-polarized light is obtained, and electrons from ape-type orbital can not be excited. With increasing the photon incidence angle 0,, the corn—
—
the Si—Cl bonding orbital. Similar changes were also observed of the emission intensity ratio of the a and in orbitals ~ in the monochloride phase formed at 850K in agreement with the previous study [5]. As a summary, it has been shown that HG molecules dissociatively adsorb on thermally cleaned Si(hll) surfaces at room temperature, 3forming orbital the of Si covalent bond betweenwhich the dangling and the Cl p~orbital, is similarspto that in the monochionide phase. This means that a picure of the local bonding also gives a good description for HG on Si( 111) surfaces. REFERENCES
—
ponent of the electric vector normal to the surface increases, and the emission intensity from ape-type orbital can be expected to increase. This is the case for the peak P2 which is mainly due to the ps-type orbital. Figure 3 demonstrates that the peak intensity ratio Ip/Ip, considerably increases with increasing 0,. This gives further evidence that the peak P2 is due to
243
1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12.
A.H. Boonstra, Philips Res. Rept. SuppL 3, (1968). M. Nishijima, K. Fujiwara & T. Murotani,J. Appl. Phys. 46, 3089 (1975). M. Miyamura, Y. Sakisaka. M. Nishijima & M. Onchi, Surf Sci 72, 243 (1978). J.A. Appelbaum & DR. Hamann, Phys~Rev. Lett. 34,806 (1975);K.C. Pandey,Phys. Rev. Bl4, 1557 (1976). K.C. T. (1977). Sakurai & [ID. Hagstrum, Phys. Rev. Pandey, B16, 3648 K. Mednick & CC. Lin, Phys. Rev. B17, 4807 (1978). M. Schiuter, i.E. Rowe, G. Margaritondo, K.M. [ho & M.L. Cohen,Phys. Rev. Lett. 37,1632(1976). K. Fujiwara & II. Ogata, Surf Sci. 86, 700(1979). J. Chelikowsky, D.J. Chadi & M.L. Cohen,Phys. Rev. B8, 2786 (1973). J.E. Rowe & II. Ibach,Phys. Rev. Lett. 32,421 (1974). P.K. Larsen, N.y. Smith, M. Schluter, 11.11. Farrell, K.M. Ito & M.L. Cohen, Phys. Rev. B17, 2612 (1978). 1. Sakurai & hl.D. Ilagstrum,Phys. Rev. Bl2, 5349 (1975).
sample geometry for the UPS measurements. The analyzer axis was in the plane of incidence of the photons and perpendicular to the photon beam line. The angle of incidence of the photons to the sample surface normal O~,was varied by the sample rotation. Most of the UPS spectra were taken at O~= 45°. An atomically clean surface was obtained by mdirectly heating the Si(ll l)sarpple around 1500 K in UHV with the same method as described previously [81.Cleanness of the thermally cleaned surface was checked In-situ with AES and UPS measurements. The Auger spectra showed that there were no detectable impurities on the surface. For gas—surface reaction study, commercially available research grade HC1 gas was introduced into the chamber by the controlled amount through a bakable leak valve. The coverage of the adsorbates was monitored with AES. After HC1 gas exposure to the clean Si(1 11) surface, the coverage of the adsorbate was determined by momtoring the intensity of C1—LMM Auger peak at 181 eV. Saturation was reached at roughly I L(l06lorrsec) exposure. Figure 2 shows the UPS spectra for the clean Si(1 11) surface (a) and the surfaces after 1 L I-IC1 exposure at 300 K (b) and 850 K (c), respectively. For the clean surface, two characteristic peaks of bulk density of states of silicon are observed at 2.9eV (14) and 7.4 eV (L 1) below Fermi level EF in agreement with the theoretical results [9]. Enhancement of the emission intensity dueof tothe thevalence surface band. states After [101 is near the top 1 Lalso HC1observed exposure at room temperature, the emission intensity due to the surface states decreases, while new peaks appear at E EVAC = 11.1 eV (P 1), 13.2 eV (P2) and 16.2 eV (P3). Energy half width of the main peak F1 is estimated to be 1.6 eV. Changes are also observed in the upper region of the valence band, resulting in a relatively broad peak at 8.1 eV (F4). Work function change of + 0.2eV is obtained from the shift of the low energy cutoff of the EDC. No time-dependent variation —
—
—
averaged form with a spherical sector type analyzer and pulse counting system. EDC’s of the UPS spectra were measured relative to vacuum level EVAC, and the binding energy EB was determined, assuming that the work function ~ of thermally cleaned Si(l I I) surfaces is 4.7 eV [8]. Figure 1 shows the schematics of the
241
—
—
242
ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF HG ON Sm(l 11) SURFACES Vol. 36, No.3 He LAMP
p
OF NC(E~’CE)
Si(111)-HCI 1L at 21.2eV 300°K
SAMPLE ~C~1~L (Z-AXIS) -j
4 0 U)
R~X!S SAMftE ~TATION
ANALYZE
4
~LE
Fig. 1. Schematic diagram of the sample geometry.
4 300
__________________________
U
z
Si(lll)-HC(
150
__________
450
b
b(c)1Lt~oK
-
~
BINDING
/.
6
ENERGY, E
8 (eV) Fig. spectra N(lf) ofofhG-saturated Si(l 11) surface 3. at UPS 300 K as a function photon incidence angle O~. According to the gas-volumetric measurements by Boonstra [11,the room temperature adsorption state of IICI on Si surfaces becomes unstable with increasing temperature. Above 800K all hydrogen desorbs as
Z~~~/~alean
I
molecules while Cl atoms remain on the surface. Therefore, thermal desorption process of l-lCl proceeds as two
I
I
—18 —16 -14 -12 -10 BINDING
8 -6
I
18-16 -14 -12 -10 -8 -6 —4 -2 EF 2
I
I
steps, and chlorine atoms desorb at higher temperature.
I
4 -2 EF 2 4
6
ENERGY, E8 (eV)
Fig. 2. UPS spectra N(E) of Si(l 11) surface before and
after HCI chemisorption. Clean surface spectrum shown by curve (a) and HCI-saturated spectra at 300 and 850 K by curves (b) and (c), respectively. Angle of incidence of He I photons 0, was kept at 45°from the surface normal, was observed of theafter LiPS Ispectrum for at least 30 mm.to On the other hand, L HCI exposure at 850K the clean Si(l 11) surface, peaks are observed at — 11.3 and — 13.5 eVin the UPS spectrum, while the work function changes by + 0.5 eV, as shown in Fig. 2(c).
This implies that the UPS spectrum Fig. 2(c) conesponds to the surface covered with chlorine atoms. Residue of chlorine atoms on the surface could be confirmed by Auger measurements. Thus the peaks at — 11.3 and — 13.5 eV in Fig. 2(c) are attributed to the atomic chlorine chemisorption state. Comparison between the tiPS spectrum with the theoretical local
density of states for CI atoms cheinisorbed on the Si(l 11)1 x I surface [5—71 suggests that these peaks 3 are due to the Cl(a)lone pair Px p,, (ir) orbitals and Siof:sp Cl:p~bonding orbital, respectively, as a result Si—Cl covalent bond formation. The experimental ,
results are also in excellent agreement with those for the monochloride phase found by Cl
2 chemisorption on the In addition, two shoulders are identified at —7.5 and
significant changes in comparison with those of Fig. 2(b)
Si(l II) surfaces [5, 7]. Two shoulders at —7.5 and — 8.4 eV are tentatively assigned to the Cl-induced bulk states (Si back bond states, b) [6, 111. The UPS results for the room temperature UCI adsorption state are interpreted by the model, in which
except the i;tcrease of the emission intensity.
HO molecules predominantly decompose on Si(l II)
8.4eV in the upper region of the valence band. It should be noted that the binding energy and the —
energy half width of the main peaks do not show any
—
Vol. 36, No. 3 ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF HG ON Si(l 11) SURFACES surfaces, as discussed in the following. It is experimentally found that the binding energy of the main peaks P, and P2 has exactly the same value for the in and a orbitals, respectively, in the monochioride phase formed at 850 K. These facts give strong evidence for the Si—C! covalent bond formation on the HCI-chemisorbed Si(lll)surface at room temperature, provided that the based on the for concept of the case. local Itbond [5] is aapproach good approximation the present is not surprising that characteristic UPS peaks at 9.8 and 12.0eV of hydrogen chemisorbed on the Si( 111) 7 x 7 surface (monohydride phase) [121 were not observed, because they are easily masked by the Clinduced big peak at 11.1 eV. The assignment of the broad peaks P 3 and P4 located in the lower and upper region of the valence band is not clear now but these are tentatively attributed to the Si bulk-like states induced by the Si—H and Si—Cl bond formation [4, 61. Further evidence of the orbital assignments of the peaks P1 and P2 can be obtained from the dependence of the UPS spectra on the photon incidence angle 0~. The result is shown in Fig. 3. It should be noted that only the ratio of the relative peak intensity is valid in this case because the detected solid angle of the photoelectron decreases for 0. <45°. At normal incidence (0, = 0°),a purely s-polarized light is obtained, and electrons from ape-type orbital can not be excited. With increasing the photon incidence angle 0,, the corn—
—
the Si—Cl bonding orbital. Similar changes were also observed of the emission intensity ratio of the a and in orbitals ~ in the monochloride phase formed at 850K in agreement with the previous study [5]. As a summary, it has been shown that HG molecules dissociatively adsorb on thermally cleaned Si(hll) surfaces at room temperature, 3forming orbital the of Si covalent bond betweenwhich the dangling and the Cl p~orbital, is similarspto that in the monochionide phase. This means that a picure of the local bonding also gives a good description for HG on Si( 111) surfaces. REFERENCES
—
ponent of the electric vector normal to the surface increases, and the emission intensity from ape-type orbital can be expected to increase. This is the case for the peak P2 which is mainly due to the ps-type orbital. Figure 3 demonstrates that the peak intensity ratio Ip/Ip, considerably increases with increasing 0,. This gives further evidence that the peak P2 is due to
243
1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12.
A.H. Boonstra, Philips Res. Rept. SuppL 3, (1968). M. Nishijima, K. Fujiwara & T. Murotani,J. Appl. Phys. 46, 3089 (1975). M. Miyamura, Y. Sakisaka. M. Nishijima & M. Onchi, Surf Sci 72, 243 (1978). J.A. Appelbaum & DR. Hamann, Phys~Rev. Lett. 34,806 (1975);K.C. Pandey,Phys. Rev. Bl4, 1557 (1976). K.C. T. (1977). Sakurai & [ID. Hagstrum, Phys. Rev. Pandey, B16, 3648 K. Mednick & CC. Lin, Phys. Rev. B17, 4807 (1978). M. Schiuter, i.E. Rowe, G. Margaritondo, K.M. [ho & M.L. Cohen,Phys. Rev. Lett. 37,1632(1976). K. Fujiwara & II. Ogata, Surf Sci. 86, 700(1979). J. Chelikowsky, D.J. Chadi & M.L. Cohen,Phys. Rev. B8, 2786 (1973). J.E. Rowe & II. Ibach,Phys. Rev. Lett. 32,421 (1974). P.K. Larsen, N.y. Smith, M. Schluter, 11.11. Farrell, K.M. Ito & M.L. Cohen, Phys. Rev. B17, 2612 (1978). 1. Sakurai & hl.D. Ilagstrum,Phys. Rev. Bl2, 5349 (1975).