J. Phys. Chem. 1993,97, 5673-5677
5673
Photodecomposition of Adsorbed Methoxy Species by UV Light and Formaldehyde Adsorption on Si(ll1) Studied by XPS and UPS Katsumi Tanaka,'lt Shigeo Matsuzaki,+Jand Isamu Toyoshima$vl Department of Electronics Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo, 182 Japan, and Catalysis Research Center, Hokkaido University, Sapporo, 060 Japan Received: November 5, 1992; In Final Form: March 9, I993
The adsorption of methanol and formaldehyde was studied on clean p-Si( 111) in an ultrahigh-vacuum chamber equipped with X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Methoxy species (-0CH3)were formed as evidenced by C 1s and 0 1s core levels of 287.0 and 532.8 eV, respectively, and features in the UP spectra at 4.3 and 9.0 eV with a shoulder at around 10 eV below the Fermi level. The methoxy species changed gradually during UPS measurements at 298 K resulting in a new species characterized by a broad UPS spectra centered at 7 eV. The C 1s binding energy (BE) value of the remaining species shifted to 284.6 eV, whereas the peak area of C 1s and 0 1s did not change appreciably. These results suggest that methoxy species are decomposed by He-I1 (40.8eV) UV source on Si(111). It was found that the stability of methoxy species for photodehydrogenation as well as photodecomposition was dependent on the coverage of the species formed after UV irradiation, Formaldehyde adsorption was carried out to elucidate the surface species obtained by thermal and photochemical decomposition of methoxy species on p-Si(ll1). It was found that formaldehyde is adsorbed dissociatively on Si( 111) at 298 K and the species remaining at the surface resembles that obtained in photodecomposition of methoxy species.
1. Introduction
indicates the presence of Si(OCH3), on Si( 111)7X7 surface.1° Heating the surface causes a desorption maximum of molecularly The interaction of various gases on semiconductor surfaces adsorbed methanol at 123K. While the adsorbed methoxy species has gathered much interest especially in Si device technology. remain unchanged on the surface up to -573 K, it begins to This is attributed to the possibility that decomposition of an decomposeat temperatures higher than 573 Kat which hydrogen adsorbate might produce surface species functioning as specific desorption starts due to decomposition of methoxy species.8 devices. First of all, adsorption of simple gases on Si surfaces has However, neither adsorption of formaldehyde nor its reaction widely been studied, for instance, adsorption of water,'" have been reported on Si single-crystal surfaces. alcoh~l,~-'O ammonia,' '-12 nitrogen monoxide,llJ3 dissociated Recently photochemical steps have been focused on the surface h y d r ~ g e n , ~and J ~ oxygen.'-2Js reactions from viewpoint of the application to chemical vapor In general, surface speciescontainingcarbon and oxygen atoms depositionand the development to optoelectronic devices. Ethoxy receive much attention because they may supply us fundamental species (-0CH2CHs) do not decompose on Si surface by Ar+ aspects in the catalytic reactions of carbon monoxide.16 For laser irradiation(257 nm).9 Whereas molybdenumhexacarbonyl instance, formyl (-CHO) species is consideredas the intermediate decomposes by KrF laser (248 nm)3o or Ar+ laser (257 nm) on formed in the reaction of CO and H2, however the species is also cleanSi surface," no photochemistry occurs by 5 14-nm radiation. known as the poison in the catalytic process simultaneously. Sufficient energy can be supplied to cause separation of electrons Methanol adsorption has been studied on a number of metals, and holes (Si bandgap 1.1 eV) with both wavenumbers.3' Ying and it is found predominantly to adsorb molecularly at low et al. investigated thoroughly the photolysis of Mo(CO)~in the temperaturesonNi(l11),17C~(100),18Pt(l11),19Pd(l11),20FeUV region on Cu(ll1) and Si(111)-7X7 and interestingly points (1 and Ni( 110).22 Since the first identification on Ni( 111) out that the photoyield increases substantially in the UV region, using EELS, methoxy specieshave been reported in many systems, and it extends to further the visible and infrared region.32 They forexample,onPt(l 11),~9Cu(110),Ag(110),2~Pd(l11),20~24R~conclude that photodissociation of Mo(CO)~on potassium free (001),25Rh(ll1),26 Pd(100),27 etc. Especially in some systems surfaces is dominated by direct electronic excitation of the methoxy species are stabilized in the presence of oxygen adsorbate via single-photon process and also observe a new at0ms.'~*~~J3-26 While, the possibility for successive dehydrophotodissociationchannel on potassium-preadsorbedsurfaces due genation of methoxy speciesto CO and H2 and whether adsorbed to interaction of photogenerated hot carrier with the a d ~ o r b a t e . ~ ~ formaldehydeis formed or not are both studied on polycrystalline Photochemistry at adsobate/metal interfaces is reviewed by Zhou Ag and ZnO single crystal^.^^^^^ et al.j4 In this paper, photochemical and thermal decomposition The adsorption and thermal reaction of methanol have been of methoxy species on Si(ll1) and the surface remaining species studied on Si( 111).7.8*'0Methoxy species (-0CH3) are dominant are studied using XPS and UPS. at exposures of methanol below 0.2 langmuir (1 langmuir = 1 X 10-6 Torr s) and at higher exposures methoxy species saturate 2. Experimental Section and compose a mixture of methoxy species and molecularly The experiments were carried out in the VG ESCA3 Mark I1 adsorbed methanol.8 A laser-induced thermal desorption also photoelectron spectrometer chamber. Spectra of X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron specUniversity of Electro-Communications. troscopy (UPS) were recorded using a VG dual-anode X-ray Hokkaido University. * To whom correspondence should be. addressed. source operating the Mg anode (hv = 1253.6 eV) and a He-I1 I Idemitsu Kosan Co., LTD. 1280Kami-izumi, Sodegaura-machi, Kimitsu(hv = 40.8 eV) UV source under the base pressure less than 1 gun, Chiba, 299-02 Japan. X 10-"JTorr(l.3 X 10-8Pa),respectively. Commerciallyavailable 1 Present address: Marubun Corporation, Dohkoh Building, 3-3-4 Minamisuna, Kohtoh-ku Tokyo, 136 Japan. boron doped p-type Si(ll1) wafers (1.6-2.1 fl cm, Komatsu +
0022-365419312097-5673$04.00/0
0 1993 American Chemical Society
Tanaka et al.
5674 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
Denshi Kinzoku, Japan), denoted hereafter p s i ( 11l), wereused. The silicon substrate was mounted on a sample holder made of Ta and the holder terminal was connected to sample transfer rod at which chromel-alumel thermocouple was contacted to estimate sample temperature. Normally the sampletemperature was about 30 K below the temperature measured. It is noted that under this experimentalconditionthere is no possibility for direct contact of silicon substrate with other metals except Ta. The silicon was cleaned by repeating an Ar ion bombardment at 298 K and resistive heating at 623K, then finally annealed at 1273 K for 30 s with a halogen lampin the preparationchamber. Thecleanliness of Si surface was ascertained by surveying oxygen 1s (0 1s) and carbon 1s (C 1s) XPS core level spectra. The sample can be cooled down to liquid Nz temperature and heated until 873 K with the resistive heating. Methanol (CH30H, Aldrich Chemical Co., 99+% in purity, packaged under nitrogen sure/seal bottle) was further purified by repeating freeze-pumpthaw cycles several times after transfering to Pyrex tubes with stop valves by the Shlenk te~hnique.3~ Methanol was vaporized at 298 K and the vapor was introduced to the system. Formaldehyde (HCHO) gas was obtained by heating the glass cylinder with a ribbon heater above 473 K, in which degassed paraformaldehyde powder had been filled about one-fifth of the volume. The evolved formaldehyde gas was purified by passing through a glass-wool filter, which had been cooled to 225 K in a CaCl2-ice mixture, to remove polymers formed by sublimation and polymerization of formaldehydemonomer and finally by drying through a column packed with P2O5. The purity of respective gases was checked by an Anelva AGA-100 quadrupole mass spectrometer. These gases were introduced to the UHV chamber through a variable leak valve. They were successively introduced in the analyzer chamber and the spectra were recorded in the intervals. It is noted that formaldehyde adsorptionat lower temperatures was unsuccessful due to condensing formaldehyde to sample transfer rod which bothered background pressure and heating Si. The pressure was monitored by a VG VIG 20 ionization gauge. Binding energy (BE) values of 0 1s and C 1s were all referred to the Si 2p BE value (99.5 eV).
Binding EnergyleV
a
.-c
e
t
I
.-8
L
3. Results and Discussion
0
d
Adsorption of Methanol a d Thermal Decomposition of Methoxy Species. Adsorption of methanol and the thermal decom-
position of species remaining at the Si surface were studied using XPS and data were elucidated in comparison with the previous EELS studies.* Results are shown in Figure 1. Here 0 1s and C 1s spectra are shown separately. The spectra in Figure 1 were recorded at the evacuating temperatures. When a clean p-Si(1 11) was exposed to 1 langmuir of methanol at liquid N2 temperature, large 0 1s and C 1s peaks were observed at 534.0 and 287.2 eV, respectively. These peaks are assigned to a multilayer condensed phase of methanol. According to the TDS studies of methanol on Si( 111)-7X7, the multilayer condensed phase is dominant at 0.75-langmuir exposure of methanol.8 Condensed methanol reveals C 1s at 287.2 eV and 0 1s at 533.6 eV on Zn0,29almost the same values as ours. Heating the Si sample to 123 K caused methanol desorption and the remaining species on the surface showed 0 1s peak at 532.8 eV and C 1s at 287.0 eV. However, these intensities diminished by a factor of 4. The sample was heated at 573 K, but C 1s and 0 1s spectra did not change drastically. These results can be explained as follows. At (or below) 123K, methanol is dissociativelyadsorbed on p s i ( 111) and forms methoxy and hydrogen species, which are thermally stable up to 573 K, which is consistent with EELS data.8 Methoxy species (-0CH3) is bonded to Si with the oxygen atom, consequently0 1s BE should be influenced more effectively than C 1s. This assumption will be probably correct because of the BE shift by 1.2 eV in 0 1s but almost negligible BE shift in
- ...._...-.
(b) (a) I
282
I
I
I
I
286 290 Binding EnergyIeV
I
I
29
Figure 1. XPS spectra of methanol adsorption on a clean psi(]11). (A) 0 Isspectra;(B)C lsspectra. (a)cleanp-Si(lIl)at77K(b) 1 langmuir of CH30H at 77 K. Following (b), evacuation at (c) 123 K, (d) 173 K, (e) 223 K, ( f ) 298 K, (g) 373 K, (h) 473 K, (i) 573 K, (j) 673 K, and (k) 773 K. Note that spectra c-h were recorded at the same temperatures as the evacuation.
C 1s at 123 K. BE values of 0 1s and C 1s of methoxy species are reported at 532.0 and 285.7 eV on Rh(l1 1)26and at 531.7532.2 eV and 286.2-286.4 eV on Ag,29 ZnO,s6and Cu.” From the UPS result mentioned later and the comparison of BE values, the species at 287.0 and 532.8 eV is assignable to methoxy species on Si(ll1). At 673 K new peaks were observed at 532.0 and 284.3 eV. The carbon peak was dominant at 773 K. Regarding an origin of oxygen species, not only methoxy species but also S i 4 reveal 0 1s at 532.8 eV.39 On cleaved Si( 111)-2X1 surface, dissociated OHisreportedat 532.4eVreferredtoSi2pat 99.5eV.4 However, no evidence is reported to OH on Si( 111). Even if it exists, the
Photodecomposition of Adsorbed Methoxy Species stability should be quite low at high temperature. Consequently it is concluded that 0 1s BE is probably attributed to oxygen atom. The C 1s value above 673 K (284.3 eV) is assigned to surfacecarbon. The formation of new peaks at 673 K is explained due to the decomposition of methoxy species on pSi(ll1). The relative atomic ratio of carbon to oxygen (C/O ratio) of the adsorbed species was almost unity in Figure 1. It is noted that small peaks were detected at 285.3 and 284.3 eV at 473 K. The coverage of the peaks was estimated to be about 0.2 monolayer. These species might be assigned to CH, species as observed on ZnO, 285.3 eV for CH3 and 284.4 eV for CH and/or CH2.29 If this is the case, small fraction of adsorbed methoxy species should decompose and at the same time oxygen species should be present on the surface. However, no CH, species is detected on an ordered Si( 111)-7X7 surface.* The annealing temperature in our experiments, 1273K, passes through the 7x71X 1 transition and cools quickly back to 298 K, so that it seems less possible to achieve a high degree of order. Decomposition of methoxy species and the formation of CH, species below 473 K probably occurs on defect centers due to unsuccessful ordered phase. Adsorptionof Formaldehyde. It is interesting to know whether adsorption of formaldehyde is molecular or dissociative at 77 K. The experiment might provide the molecule reference to XPS and UPS. However formaldehyde adsorption at low temperatures was unsuccessful and was carried out at 298 K. As shown in Figure 2, following 0.1 langmuir of formaldehyde dosage at 298 K, 0 1s and C 1s peaks were observed at 532.1 and 285,6eV,respectively (spectrumb). HowevertheC lsspectrum has a tail toward lower BE. It would be possible to deconvolute C 1s signal into two species peaked at 285.6 and 284.6 eV. No appreciable BE shift was detected in 0 1s spectrum up to 773 K (A; c-g). Here each spectrum was recorded after being maintained at the evacuation temperature. However, an intermediate C 1s BE shift was observed at 573 K (B; e) and the species peaked at 284.6 eV was dominant at 673 K (B; f ) . The same spectrum was observed at 773 K (g). C 1s BEvalue moved to 285.1 after 12 h at 298 K (B; i). Binding evergy values of C 1s and 0 1s of adsorbed formaldehyde species are reported to be 288.7 and 532.8 eV on 2110.2~ The C 1s at 285.6 eV is too low to assign to formaldehyde but reasonable to CH3 species or multi carbon-to-silicon geometries. These results imply that formaldehyde molecules are adsorbed dissociatively on Si( 111) surface at 298 K. However, dissociation of formaldehyde might be based on defect centers. Formaldehydewas exposed to Si( 111) without measuring pressure increase by ion gauge at 298 K. Dissociative adsorption was also observed in this case. Above 673 K C 1s BE shifted to 284.6 eV. This result implies that dehydrogenation of CH3 species occurs on Si( 111). The C 1s BE shift to an intermediate value between 284.6 and 285.6 eV upon recooling to 298 K indicates the reversibility of hydrogen and CH, species adsorbed on Si( 111) as follows: CH3*CH,
+ H*
CH
+ 2H*
C
+ 3H
At high temperature the equilibrium shifts to the right. When the temperature is dropped to 298 K, the equilibrium moves to the left. However the rate is probably quite slow and the number of surface hydrogen species are limited due to desorption during heating until 773 K. Photodecomposition of M e h x y Species. Photodecomposition of methoxy species was studied by UPS. A clean Si( 111) was exposed to 0.1 langmuir of methanol at 77 K and then heated at 298 K. The adsorbed species showed UP spectrum as shown in Figure 3a. Below Fermi level three peaks were observed at 2-3, 4.3, and 9.0 eV with a shoulder at around 10 eV. The peak at 2-3 eV is due to valence electrons of Si( 111). For methoxy species two UPS peaks are observed at around 4-6 eV and 9 eV.18329.39.40
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5675 HCHO/p-Si(lll)
526
280
A I
530 534 530 Binding Energy /eV
284 288 Binding Energy/eV
25 !
F i p e 2. XPS spectra of formaldehyde adsorption on a clean p s i ( 111). (A) 0 1s spectra; (B) C 1s spectra. (a) clean p S i ( l l 1 ) at 298 K, (b): 0.1 langmuir of H C H O a t 298 K. Following (b), evacuation at (c) 373 K, (d) 473 K, (e) 573 k, (f) 673 K and (g) 773 K, (h) coold to 298 K after (g). (i) left 12 h a t 298 K after (h). Note that spectra c-h were recorded at the same temperatures as the evacuation.
While adsorbed methanol reveals four peaks in UPS at 6.0,7.8, 10.4, and 12.7 eV which are assigned to 2a”, 7a’, la” 6at and 5a’ molecular orbitals of m e t h a n ~ l . The ~ ~ ,ionozation ~ potential of methoxy species is calculated, and the 3-fold center site is suggested for the methanol adsorption on Cu( 11l).41The data aresimultaneouslycompared to an experimentalres~lt.4~ It seems that 2e, le, and Sal orbitals of methoxy species are observed below Cu Fermi level at 5.3,9.2, and 9.7 eV, respectively. Both
+
Tanaka et al.
5676 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
I
i aI
A !6 530 534 531 Binding EnergyIeV
v
CH3OH,
I
111111111111(1111
0 2 4 6 8 IO 12 14 Io Binding Energy /eV
Figure 3. UPS spectra of adsorbed methanol and formaldehyde on psi(1 1 1). All spectra were recorded at 298 K. (a) 0.1 langmuir of CHlOH exposure to a clean pSi(ll1) at 298 K. (b) UV irradiation (He-11) for 45 min at 298 K, (c) 0.1 langmuir of HCHO on a clean psi(111) at 298 K, (d) evacuation at 773 K after (c).
2e and 5al orbitals are concentrated near the oxygen center of methoxy species. Consequently two peaks with the shoulder observed in our experiments can be assigned to methoxy species on Si(ll1). The UP spectrum of methoxy species changed with time considerably under UV irradiation at 298 K. After several minutes, the spectralchange was detectable despite no temperature change was detected. It took about 45 min to obtain the final spectrum as shown in (b) of Figure 3. Spectrum (b) did not change at all, and never returned to the original spectrum of methoxyspecies. Thisresult means that suchchange isirreversible and is attributed to the photodecomposition of methoxy species by He-I1 (40.8 eV = 30.4 nm). The UP spectrum was recorded after dissociative adsorption of formaldehyde to assign the photodecomposed species. When 0.1 langmuir of formaldehyde was exposed to clean Si( 111) at 298 K, two peaks were observed at 3.5 and 7.5 eV as shown in (c). Absorption spectra of formaldehyde molecule are observed at 7.97 eV (r- ?r* transition), 7.09 eV (n u*), 3.09 and 3.12 eV (n r*). The latter two absorptions are orbitally forbidden, however both observed but with small inten~ities.~~ As UPS data are based on photoionization to measure kineticenergiesof ejected electrons, they should be correlated with absorption spectra supported by electron transitions from occupied orbital to unoccupied one, which is so called yKoopmans’theorem”. These values are quite similar to the UPS data observed in formaldehyde adsorption on Si( 111). According to the reference, CH3, CH2, CH, and surface carbon are seen in UP spectra at 6.6, 5.6, 5.1, and 4.7 eV and OH at 9.5 and 11.9 eV.44 0 2p and Si valence band are typically seen between 6 and 8 eV in most extensive intensity in Si02, SO,, and adsorbed oxygen on S i . 3 ~The ~ ~UPS peak at 7.5 eV in our experiments can be reasonably assigned to mixing up of CH, species and adsorbed oxygen based on their BE value below Fermi level. The peak at 3.5 eV might be valence band density variation influenced by these adsorbates on Si( 111).
-
-
- (
B
p-Si(l I I) lCIS
I
A I
- (
(d’
3
I
284 288 Binding Energy/eV
2 2
Figure 4. XPS spectra of adsorbed species obtained after photodecom-
position and thermal decomposition of adsorbed methanol on a clean pSi(ll1). (A) 0 1s spectra; (B) C 1s spectra. (a) clean pSi(ll1). (b) 0.1 langmuir of CHsOH at 298 K. (c) 0.2 langmuir (total) of CHIOH at 298 K. Following (c), (d) UV irradiation (He-11) for 45 min at 298 K, (e) evacuation at 473 K and (f) evacuation at 673 K. Note that spectra were recorded at the same temperatures as the adsorption, irradiation and evacuation temperatures. The UP spectrum (d) in Figure 3 reveals a new peak at around 13 eV. The new peak may be assigned to 0 2p due to OH or formyl species. But OH and CHO species should be less stable at higher temperature so that further experiments should be necessary to the assignment. X-ray photoelectron spectra were recorded to gather the information both of species which remain after the photodecomposition and of the stability of methoxy species on Si( 11 1). As shown in Figure 4,0.2 langmuir of methanol dosage resulted in a few increase of 0 Is and C 1s peak intensities (A,B:c) compared to 0.1-langmuir methanol dosage (A,B:b) on p s i (1 11) at 298 K. Further methanol dosage did not cause any intensitychange. This result means that 0.2 langmuir of methanol dosage corresponds to the saturation coverage. A UV (He-11) irradiation for 45 min resulted in the formation of new C 1s peak at 285.4 eV (B-d), while the peak position and the full width at half maximum of 0 1s did not change at all (A-d). Here it is noted that methoxy species, which is observed at 287.0 eV, remained about two third after the irradiation. This should be compared to the result of Figure 3, in which all methoxy species are photodecomposed after 0.1 langmuir of methanol dosage at 298 K. The difference implies that submonolayer of methoxy
Photodecomposition of Adsorbed Methoxy Species species are quite reactive to photoactivation but monolayer of these species are less reactive on Si( 11 1). The C 1s peak at 285.4 eV shifted to 285.0 eV at 473 K and methoxy species still remained at 673 K. The C 1s peak at 285.3 eV can be assigned to CH3 species as refered to the study on Zn0.*9 The C 1s peak shift of 0.4 eV to lower BE value will suggest the dehydrogenation of CH3 species formed by the photodecomposition of methoxy species. The C 1s value (285.0 eV) implies the presence of hydrogen containing carbon species probably CH and/or CH2 is still stabilized on the surface. This should be compared to the thermal decomposition of methoxy species at 673 K as shown in Figure 1, in which C 1s BE was observed at 284.3 eV assigned to carbon. The difference can be interpreted due to either (a) the stability of adsorbed methoxy species influenced by the surface species remaining after UV irradiation or (b) block of dehydrogenation route associated with vacant sites, which are covered with adsorbed methoxy species at full coverage. The formation of thesurface remaining species should be coverage dependent. With increasing coverage of the surface species, both decomposition and dehydrogenation is less possible. The methoxy species is stable to 700 K and decomposes to atomic oxygen and carbon at higher temperatures and there is no proof for the formation of CH, species during thermal decomposition of methoxy species on Si( 111)-7X7.8 It is also suggested by Strocio et al., that the decomposition of methoxy species is ratelimited by the desorptionof hydrogen and dependent on the coverage of methoxy species on the clean Si surface, although an exposure of 0.2 langmuir CH30H is still below saturation and sites are available for thermal decomposition of methoxy species.8 In our UV irradiation mediated methoxy decomposition, the dehydrogenation seems to play an important role on the decomposition; however, CH3 species are formed by UV irradiation and the CH and/or CH2 species are reamined on the surface by subsequent thermal decomposition of fnethoxy species.
T‘he Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5677
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