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Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces
Surface Science 606 (2012) 1387–1392
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Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces Shuai Wang, Jinghui He, Yongping Zhang, Guo Qin Xu ⁎ Department of Chemistry, National University of Singapore, 3 Science Drive 3 117543, Republic of Singapore
a r t i c l e
i n f o
Article history: Received 15 February 2012 Accepted 25 April 2012 Available online 5 May 2012 Keywords: Silicon oxidation X-ray photoelectron spectroscopy Oxygen Carbon dioxide
a b s t r a c t The interaction of O2 and CO2 with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O2 and CO2 molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO2 has a lower O/Si ratio than that of O2. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the continuous miniaturization of silicon based transistors in the complementary metal oxide semiconductor (CMOS) technology, there is an increasing impetus to fabricate a thinner silicon oxide film as the gate dielectric layer. Motivated by this technological importance of the ultrathin silicon oxide films in semiconductor devices, the oxidation of silicon surfaces has been extensively studied over the last decades [1–3]. One of the most critical issues is to understand the initial stage of the silicon surface oxidation, which is the key to grow very thin and yet high quality oxide films on the silicon substrate [4–6]. O2 is the natural choice and widely studied in the silicon oxidation process. It was found that the oxidation of silicon surfaces by the ambient O2 molecules results in a thin oxide film with a very low O/Si ratio [7]. The activated oxygen species via different approaches, such as plasma enhanced oxidation [8,9], noble-gas ion beam enhanced oxidation [10,11], and electron beam enhanced oxidation [12,13], must be used to form the thick silicon oxide film. It was proposed that the oxidation reaction is essentially suppressed when the active adsorption sites on the top layers are exhausted. For further oxidation with low substrate temperatures, O2 molecules must be activated to penetrate the covalent bonded silicon oxide layers [14]. In the photoelectron spectroscopic study of the adsorption of O2 on the Si(111)-7×7 surface, no physisorbed O2 species was detected at 90 K, and only two different chemisorbed oxygen species were found and assigned to oxygen in the strained Si\O\Si species with a lower binding energy (O 1s=533 eV) and oxygen in the SiO4 unit with a higher value of O 1s=535 eV [15]. Moreover, SiO4 can be converted to the Si\O\Si with electronic or thermal activation. However, there is limited knowledge on the configurational difference
⁎ Corresponding author. E-mail address: [email protected] (G.Q. Xu). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.04.026
between the Si\O\Si species and the SiO4 species or the interconversion mechanism of these two surface species. A more perceptive understanding of the initial oxidation stage was provided by STM studies [16], where O2 molecules were found to dissociatively adsorb on the adatoms of the Si(111)-7× 7 surface at 30 K and 300 K to form dark and bright sites. The exchange between the dark and bright sites can be achieved through the manipulation of the STM tip. The bright sites were attributed to the Si adatoms with the O atoms inserted in the Si\Si back bonds while the dark sites were proposed as O atoms binding on-top to the dangling bond. However, the STM studies gave little information as to the nature of the adsorption and where the O atoms bind, and the assignments of the configurations are inconclusive. There are also some controversies for the assignments of the different silicon oxide species, due to the lack of correlation between spectroscopic results and STM images. Up to now, despite the large number of experimental studies and theoretical simulations, the adsorption of oxygen and its configurations on the Si(111)-7× 7 surface in the initial oxidation stage are still not clear. There have been very few studies on the interaction of CO2 with silicon crystals. The interaction between ambient CO2 and Si(100) was expected to be negligible unless under the influence of a high energy (2 kV) electron beam [17–19]. However, CO2 dissociative chemisorption activated with a very low translational energy (0.17 eV→ 1.6 eV) was observed on Si(100) and Si(111) surfaces in the molecular beam study [20,21]. On both surfaces, the reaction probability increases dramatically with the incident kinetic energy. The activated CO2 dissociates and adsorbs on the silicon surface as CO(a) and O(a) species, which offers compelling similarities and contrasts to that of O2. The activation energy for the CO2/Si(111) reaction was determined to be as low as 0.6 eV by this molecular beam study; however, few detailed studies of this adsorption process were carried out. In this work, we investigated the adsorption of O2 and CO2 on the Si(111)-7 × 7 surface at 110 K. The results showed that both molecules exhibit similar oxidation effects, forming two oxide species on
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the Si(111)-7× 7 surface. These two chemisorbed oxygen species show unique binding energies and distinct O/Si ratios, owing to their different chemical configurations. A thermally induced conversion between these two species was also detected during the annealing process. In comparison to the previous studies, the two oxide species can be identified as oxygen atoms inserted in Si\Si back bonds and oxygen atoms attached to the dangling bonds.
99.3
Si 2p
2. Experimental The experiments were performed in an UHV chamber equipped with facilities for X-ray photoelectron spectroscopy (XPS, VG), high resolution electron energy loss spectroscopy (HREELS, ELS2000, LK), and quadrupole mass spectrometry (QMS, HAL 301, Hiden). The base pressure of the chamber was 1×10− 10 Torr. The Si(111) samples (7×20 mm2 with 0.38 mm thickness) were cut from p-type boron-doped wafers with a resistivity of 1–30 Ωcm and a purity of ~99.999% (Goodfellow). One Ta foil heater (0.025 mm thick, Goodfellow) was sandwiched between two Si(111) samples held together by a set of Ta clips, and was then attached to the manipulator by two Ta support leads. The sample could be resistively heated to 1300 K and liquid nitrogen cooled to 110 K. The temperature of the sample was measured by a 0.003 in. C-type thermocouple (W-5%Re/W-26%Re) which was spot-welted to the Ta clip. The high temperature was emendated by an infrared pyrometer (TR-630, Minolta). The silicon samples were firstly degassed at 850 K overnight in the vacuum, and then cleaned through cycles of Ar ion bombardment (30 min at 1 keV) and annealing at 1300 K. The cleanliness of the surface was checked by XPS and HREELS measurements. O2 (99.9995%, Soxal) and CO2 (>99.8%, Soxal) were used and checked using a quadrupole mass spectrometer (QMS) in the UHV system. The gases were introduced onto the sample through a variable leak valve in a direct dosing mode. The gas dosages are expressed in Langmuir (1 L=1×10− 6 Torr×s), and the dosing pressures were corrected by the relative sensitivity factor for the ion gauge, as 1.01 (O2) and 1.42 (CO2). The relative concentrations of surface species were determined using XPS sensitivity factors and peak intensities according to N1 =N2 ¼ I1 =I2 S2 =S1 where N, I and S represent, respectively, the atomic concentration, the peak intensity and the atomic sensitivity factor (ASF) of a certain element (S: 0.711 for O 1s, and 0.339 for Si 2p). In order to determine the absolute surface coverage on the silicon surface, the adsorption of ethanol (CH3CH2OH) on the clean Si(111)-7×7 surface was investigated by XPS as a probe (not shown), which would adsorb dissociatively on the adatom and rest atom sites with the ideal ethanol/Si adsorption site ratio as 1:2 [22]. Then the ratio of O/Si should be 0.5 (R=NO/NSi =Nethanol/ NSi =1:2). However, due to the strong silicon signal from the subsurface layers, the ratio of O/Si from the XPS peak intensities, R′=(IO/ISi)×(SSi/ SO)=0.0335. Thus a correction factor (F=R/R′=14.9) was obtained and used for obtaining the absolute surface coverage. 3. Results and discussion 3.1. Adsorption of O2 on the Si(111)-7 × 7 surface The oxidation process on the Si(111)-7 × 7 surface by O2 was studied at 110 K using XPS. Fig. 1 shows the Si 2p photoemission features from the clean and 3.6 L O2 exposed Si(111)-7 × 7 surfaces. Only one silicon species can be detected at 99.3 eV, which is consistent with its bulk peak. There are no changes found in both binding energies and peak intensities, which implies the formation of a very thin silicon suboxide film during the exposure of O2. This is in agreement with the previous studies that the oxidation of Si by the ambient O2 reaches a maximum with a very low O/Si ratio [7,12,13]. The details of the
(b) 3.6 L O2
(a) clean
106
104
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100
98
96
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Binding Energy (eV) Fig. 1. The Si 2p XPS spectra for the clean Si(111)-7 × 7 surface (a), and after 3.6 L O2 exposure (b) at 110 K.
oxidation process are exhibited in O 1s XPS spectra. As shown in Fig. 2, two chemisorbed oxygen species are formed during the oxidation process, which can be assigned to a low binding energy species Ox at 532.1–532.5 eV and a high binding energy species Oy at 533.5– 533.7 eV. There is no physisorbed O2 species detected during O2 exposure, possibly due to the weak physisorption of O2 on silicon surfaces. In the forepart of the oxidation process (b0.36 L O2), the O 1s peak intensities increase remarkably, which indicates the fast increase of the oxygen concentration on the Si(111)-7 ×7 surface. The increase of surface oxygen concentration also results in the decrease of the electron density on the surface, and thus induces the blue shift in binding energies of the two oxygen species from 532.1 eV to 532.5 eV, and 533.5 eV to 533.7 eV, respectively. However, when the O2 exposure is larger than 0.36 L, there is little change in both binding energies and peak intensities for the two chemisorbed oxygen species. This suggests that O2 induced silicon oxidation reaction is suppressed when the limited surface adsorption sites are depleted. On the Si(111)-7 × 7 surface, adatoms and rest atoms are the most active adsorption sites, where the Si atom contains one dangling bond and three Si\Si backbonds (schematically shown in Fig. 3). In the previous STM studies, it was assumed that O2 molecules dissociate and adsorb at on-top sites with dangling bonds saturated, as well as on bridge sites between the first and second layers [23,24]. Thereby the two chemisorbed oxygen species can be reasonably assigned to the low binding energy Ox to the bridge site and high binding energy Oy to the on-top site, based on their different electronic and geometric structures. The proposed oxidation process of O2 on the Si(111)-7× 7 surface is displayed in Fig. 3, where O2 dissociates and adsorbs via two possible reaction pathways: (a) one oxygen atom adsorbs at the on-top site and the other on the bridge site; and (b) two oxygen atoms adsorb on bridge sites. Further oxidation may form a SiO4 subunit (c) where the Si atom is bonded to four oxygen atoms. Theoretically, the final O/Si ratio should be 4:1, and the ratios of Ox/Oy are 1:1 in (a), 2:0 in (b), and 3:1 in (c), respectively. In comparison with the experimental data, a clear image of the oxidation process on the Si(111)-7 ×7 surface is presented. To further explore the oxidation process, the ratios of chemisorbed oxygen species and the surface adsorption sites on the Si(111)-7×7 surface (the calculation of the surface adsorption site is shown in Experimental section, and the correction factor (F=14.9) is used for calculation) are calculated and demonstrated in Fig. 4. With the exposures up to ~0.1 L, the oxygen signal increases linearly and the O/Si ratio quickly reaches 3.4, which indicates that the O2 molecules dissociate and adsorb onto the topmost layer sites with low energy barriers. Further oxygen dosing
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532.5
O 1s 533.7 f) 3.6 L
e) 0.36 L
d) 0.054 L
c) 0.042 L
b) 0.002 L 533.5
a) clean
540
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536
532.1
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532
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Binding Energy (eV) Fig. 2. The O 1s XPS spectra from the Si(111)-7 × 7 surface at 110 K with various O2 exposures: (a) clean surface, (b) 0.002 L, (c) 0.042 L, (d) 0.054 L, (e) 0.36 L, and (f) 3.6 L. Upon peak fitting, two chemisorbed oxygen species are assigned to low binding energy species (SiOx) at 532.1–532.5 eV and high binding energy species (SiOy) at 533.5–533.7 eV.
causes the O/Si ratio gradually approaching 3.8, which indicates most of the surface is saturated and the incoming O2 molecules are presumably scattered back into the gas phase. This result suggests that the rate of adsorption drops rapidly with the increase in the concentration of surface oxygen. Thus, further oxidation is remarkably suppressed, and the experimental ratios of O/Si (3.8), Ox/Si (2.9), and Oy/Si (0.85) are less than their theoretical value (4, 3, and 1), respectively. It should be mentioned that only the Si adatom oxidation process is discussed in the previous STM studies of the Si(111)-7×7 surface, while little information of the Si rest
atoms was present, due to the limitation of STM resolution [16]. In a Si(111)-7 × 7 unit cell, the ratio of adatom and rest atom is 12:6, and if only adatoms are oxidized and bonded to four oxygen atoms, the O/Si ratio should be less than (12 × 4)/(12 + 6), which is approximately equal to 2.67; while the adatoms and rest atoms are both saturated by oxygen atoms, the O/Si ratio should be 4. Our XPS result shows a higher O/Si ratio as 3.8, which indicates that both adatoms and rest atoms are oxidized by O2 indistinguishably. The SiOx and SiOy 4.0 3.5
O/Si
Ratio of O/Si
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Ox/Si
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Oy/Si
0.5 0.0 0.0
0.1
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Exposure (L) Fig. 3. Schematic diagram showing the proposed oxidation process for O2 adsorption on the Si(111)-7 × 7 surface at 110 K. The big dark gray ball indicates the Si atom, and the small red ball represents the O atom. In the initial stage, the oxide species are assumed to be (a) one oxygen atom adsorbs at the on-top site and one oxygen atom at the bridge site, and (b) two oxygen atoms adsorb at the bridge sites. Further oxidation may form a SiO4 subunit (c) where the Si atom is bonded to four oxygen atoms.
Fig. 4. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of O2 exposure on the Si(111)-7 × 7 surface at 110 K, where R = 0.067 is used as the correction factor (for S, the details are shown in Experimental section). With increasing the O2 exposure, the O uptake can be characterized as a fast initial region (b 0.1 L) followed by a slow uptake region (> 0.1 L). The ratio of Ox/Oy is distinctly divided by the two regions as ~ 4.8 and ~ 3.4.
S. Wang et al. / Surface Science 606 (2012) 1387–1392
species were detected simultaneously in the initial oxidation stage, which proves that both oxidation paths (a) and (b) coexist. With the increase in O2 exposure, the ratios of Ox/Si and Oy/Si reach saturation values of 2.9 and 0.85, respectively. Furthermore, the ratio of Ox/Oy is distinctly divided by the two uptake regions as ~4.8:1 (b0.1 L) and ~3.4:1 (>0.1 L). As shown in Fig. 3 the Ox/Oy ratios in species (a) and (b) are 1:1 and 2:0, if the species (a) and (b) was 1:1, the Ox/Oy ratio should be 3:1. However, based on XPS data, the ratio between Ox and Oy changes from ~4.8:1 to ~3.4:1 during O2 adsorption, which indicates the pathway (b) is preferred and the O2 dissociative adsorption on the Si\Si bridge site is more favored at the initial stage. The decrease of the Ox/Oy ratio from 4.8 to 3.4 is probably due to the amount of unreacted on-top sites that is larger than the amount of unreacted Si\Si bridge sites in the slow uptake region. The SiOx and SiOy species have different chemical environments, and would show different thermal stabilities. Fig. 5 displays the evolution of the O 1s XPS spectra of the two oxide species during the annealing process. The SiOy species shows a poor thermal stability, and vanishes when the sample temperature is higher than 600 K. On the other hand, the SiOx species shows a high thermal stability, with increasing peak intensity and constant binding energy until the sample temperature reaches 900 K. All the surface oxide species are desorbed completely after annealing above 1200 K. To observe the details in the annealing process, the normalized XPS peak intensity ratios of O/Si, Ox/Si, and Oy/Si are plotted in Fig. 6, as a function of annealing temperatures. It is clearly shown that the SiOy species, where oxygen adsorbs at the on-top site, is only stable at low temperatures and its peak intensity decreases when T >300 K. However, the intensity of the SiOx species increases simultaneously and meanwhile the total intensity of the silicon oxide remains unchanged, which is indicative that the SiOy species undergoes a thermal induced conversion to the SiOx species via the diffusion of the Ox atom into the sublayer and the transfer of the Oy atom to the Si\Si bridge site. This result is consistent with the previous studies [15], where it was found that thermal activation or electron bombardment converts the high binding energy species to the low binding energy species in silicon oxide films.
O 1s
4.0
O/Si Ox/Si
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Oy/Si
3.0
Ratio of O/Si
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2.5 2.0 1.5 1.0 0.5 0.0 0
200
400
600
800
1000
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Temperature (K) Fig. 6. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of annealing temperatures on the Si(111)-7 × 7 surface after an exposure of 3.6 L O2, where R = 0.067 is used as the correction factor. The SiOy species is only stable at low temperatures, and is converted thermally to the SiOx species when T > 300 K and vanishes at T = 600 K. The SiOy species appears again as a precursor during the SiOx species desorption when T > 900 K.
Due to their different electronic configurations, the site with SiOy species (as Fig. 3(a)) should be dark, and the site with only SiOx species (as Fig. 3(b)) should be bright in STM images. Thus, the thermally induced conversion of the two chemisorbed oxygen species can also account for the exchange between the dark and bright sites observed in STM studies [16,25,26]. Moreover, it is interesting to note that the SiOy species appears again when the surface oxide desorption occurs. It is well known that SiO is the major desorbing species during oxide desorption from the silicon surfaces. From Fig. 6, it can be seen that the SiOy species
e) 1210 K 532.3 d) 920 K 532.5
c) 600 K
532.5 533.6 b) 300 K
532.5 533.7 a) 110 K
540
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Binding Energy (eV) Fig. 5. The O 1s XPS spectra of the Si(111)-7 × 7 surface exposed to 3.6 L O2 at 110 K (a) and subjected to isochronal annealing up to 300 K (b), 600 K (c), 920 K (d), and 1210 K (e).
S. Wang et al. / Surface Science 606 (2012) 1387–1392
recurs with the SiOx species decreases when the sample temperature is above 900 K, which indicates the SiOx species is converted to the SiOy species by O atom diffusion from the Si\Si bridge site to the Si on-top site. Thus, it is reasonable to propose the SiOy species as the precursor of desorption. Thus, it is clear that O2 dissociatively adsorbs on the Si(111)-7 × 7 surface at both adatom and rest atom sites at 110 K to form a thin silicon oxide film. The oxide film contains two chemisorbed oxygen species as oxygen adsorbs at the on-top site (SiOy) and the Si\Si bridge site (SiOx). These two oxide species reach saturation with a low O2 exposure, and then the rate of adsorption falls off rapidly due to the depletion of the surface adsorption sites. The SiOy species shows a high binding energy but poor thermal stability, which can be converted to the low binding energy species (SiOx) by thermal activation at T >300 K and disappears completely at T > 600 K. Moreover, this SiOy species is reproduced as the precursor during the surface oxide desorption at T > 900 K. 3.2. Adsorption of CO2 on the Si(111)-7 × 7 surface The adsorption of CO2 on the Si(111)-7 × 7 surface at 110 K was also investigated using XPS, and the XPS spectra are shown in Fig. 7. It is noticed that there is no carbon signal observed while oxygen peak increases remarkably during the CO2 exposure. Two chemisorbed oxygen species in the spectra are assigned to the low binding energy species (SiOx) at 532.2–532.4 eV and the high binding energy species (SiOy) at 533.4–533.5 eV, and the binding energies of these two oxygen species increase coincidently with the increase of CO2 exposure. This result indicates that CO2 has the similar oxidation effect as O2 on the Si(111)7 × 7 surface, where CO2 dissociates into CO and O(a) (the adsorbed oxygen species). However, due to the spontaneous desorption of CO during the CO2 dissociative adsorption, no physisorbed CO species can be observed during the exposure at 110 K. CO2 dissociation on clean sp-metal surfaces is widely recognized, because there is a strong thermodynamic driving force for the oxide formation which leads to
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dissociative chemisorption [27]. The silicon oxidation reaction by CO2 is also thermodynamically favorable at 110 K [28]. Meanwhile, it is well known that the uppermost two surface state bands of the Si(111)-7×7 surface are located respectively on the adatoms as S1 at ≈−0.15 eV (relative to EF) and the rest atoms as S2 at ≈−0.9 eV, which provides a nonzero density of states near the Fermi level and thus presents a metallic surface character [29,30]. Therefore, it can reasonably explain this special feature that CO2 dissociatively adsorbs on the Si(111)-7 × 7 surface to form a thin oxide film. Fig. 8 shows the normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of CO2 exposure on the Si(111)-7 × 7 surface at 110 K. It is clear that CO2 shows a similar oxidation effect as O2 on the Si(111)-7 × 7 surface. With the increase in CO2 exposure, the O uptake can be characterized as a fast uptake region (b0.1 L) followed by a saturated plateau region (>0.1 L). The O/Si ratio, which indicates the oxygen surface concentration, increases rapidly in the initial stage and reaches its maximum at 3.2. The ratio of Ox/Oy can also be divided into two regions with values of ~4.1 and ~3.6, which indicates that the formation of the SiOx species is preferred in the initial stage. It should be mentioned that the CO2 dissociative adsorption is terminated with a small value of O/Si ratio as 3.2, and there is no increase for any oxygen species even with an extremely high CO2 exposure. It is well known that CO2 adsorption on metal surfaces is strongly influenced by the surface electron density [27], and thus it is very probable that the CO2 dissociation on the Si(111)-7× 7 surface is critically suppressed by the increase in surface oxygen concentration, which rapidly decreases the electron density of the silicon surface [15]. The reaction pathway of CO2 on the Si(111)-7× 7 surface is proposed in Fig. 9. In the initial oxidation stage, CO2 can dissociatively adsorb on the silicon surface via different pathways to produce two chemisorbed oxygen species: binding on-top to the dangling bond (a) or bridge binding to Si\Si back bond (b). As the SiOx species is dominant in the initial stage, this proves that reaction pathway (b) is preferential. With further exposure, the surface adsorption sites are oxidized and saturated at the
532.4
C 1s
O 1s
533.5
f) 3.5 L
e) 0.35 L
d) 0.0085 L c) 0.0042 L b) 0.0014 L 533.4
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a) clean 292
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Binding Energy (eV) Fig. 7. The C 1s and O 1s XPS spectra from the Si(111)-7 × 7 surface at 110 K with various CO2 exposures: (a) clean surface, (b) 0.0014 L, (c) 0.042 L, (d) 0.0085 L, (e) 0.35 L, and (f) 3.5 L. No carbon species are found during the CO2 exposure. Upon peak fitting, two chemisorbed oxygen species are assigned to low binding energy species (SiOx) at 532.2–532.4 eV and high binding energy species (SiOy) at 533.4–533.5 eV.
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4.0
4. Conclusions
3.5
Ox/Si
We have studied the adsorptions of O2 and CO2 on the Si(111)-7 × 7 surface at 110 K. It was found that both O2 and CO2 molecules can significantly oxidize the Si(111)-7 × 7 surface and form thin oxide films. Two oxygen species in the oxide films were identified as oxygen atoms binding to on-top sites of adatom/rest atoms with the high binding energy, and oxygen on bridge sites of adatom/rest backbonds with the low binding energy. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film produced by CO2 has a lower O/Si ratio than that of O2. Above all, a clearer insight of the initial oxidation process on the Si(111)-7 × 7 surface is presented, and it is expected to contribute towards the controllable silicon oxide thin film growth.
Oy/Si
Acknowledgment
Ratio of O/Si
3.0
O/Si
2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.1
0.2
0.3
0.4
1
2
3
4
Exposure (L) Fig. 8. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of CO2 exposure on the Si(111)-7 × 7 surface at 110 K, where R = 0.067 is used as the correction factor. With increasing the CO2 exposure, the O uptake can be characterized as a fast uptake region (b0.1 L) followed by a saturation plateau region (>0.1 L).
O/Si ratio of 3.2. Thus the major final silicon oxide configurations are likely to be (c) one oxygen atom adsorbs at the on-top site and two oxygen atoms on the bridge sites, and (d) three oxygen atoms adsorb on the bridge sites, which both have the O/Si ratio as 3. As expected, the surface oxide species formed in CO2 dissociative adsorption show a similar thermal behavior to those found in O2 reaction. Based on the above analysis, the dissociative adsorption of CO2 on the Si(111)-7 × 7 surface at 110 K may take place as following. During the CO2 dissociative adsorption, the resulting CO desorbs immediately due to its weak interaction with the silicon surface, and only chemisorbed oxygen species were observed at the Si\Si bridge sites (SiOx) and the ontop sites (SiOy). The CO2 dissociation reaction is terminated at a low CO2 exposure with a low O/Si ratio, which is probably caused by the decrease of the surface electron density.
We thank the National University of Singapore for the financial support of this work (grant nos. R-143-000-377-112 and R-143-000-377462). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
Fig. 9. Schematic diagram showing the proposed oxidation process for CO2 adsorption on the Si(111)-7 × 7 surface at 110 K. The big dark gray ball indicates the Si atom, and the small red ball represents the O atom. In the initial oxidation stage, CO2 dissociatively adsorbs on the surface, leaving oxygen atom on-top to the dangling bond (a) or bridge binding to Si\Si back bond (b). With further exposure, the final configurations are proposed as (c) one oxygen atom adsorbs at the on-top site and two oxygen atoms at the bridge sites, and (d) three oxygen atoms adsorb at the bridge sites.
K. Kato, T. Uda, K. Terakura, Phys. Rev. Lett. 80 (1998) 2000. U. Hofer, P. Morgen, W. Wurth, E. Umbach, Phys. Rev. B 40 (1989) 1130. C. Silvestre, M. Shayegan, Phys. Rev. B 37 (1988) 10432. T. Hoshino, Y. Nishioka, Phys. Rev. B 61 (2000) 4705. F. Matsui, H.W. Yeom, K. Amemiya, K. Tono, T. Ohta, Phys. Rev. B 85 (2000) 630. K. Sakamoto, S. Doi, Y. Ushimi, K. Ohno, H.W. Yeom, T. Ohta, Phys. Rev. B. 60 (1999) R8465. J.Z. Xu, W.J. Choyke, J.T. Yates Jr., J. Appl. Phys. 82 (1997) 6289. C.H. Bjorkman, C.E. Shearon Jr., Y. Ma, T. Yasuda, G. Lucovsky, U. Emmerichs, C. Meyer, K. Leo, H. Kurz, J. Vac. Sci. Technol., A 11 (1993) 964. K. Kim, M.H. An, Y.G. Shin, M.S. Suh, C.J. Youn, Y.H. Lee, K.B. Lee, H.J. Lee, J. Vac. Sci. Technol., B 14 (1996) 2667. J.R. Engstron, D.J. Bonser, T. Engel, Surf. Sci. 268 (1992) 238. J.R. Engstron, D.J. Bonser, T. Engel, Appl. Phys. Lett. 55 (1989) 2202. R.E. Kirby, D. Lichtman, Surf. Sci. 4 (1974) 447. S.S. Todorov, E.R. Fossum, Appl. Phys. Lett. 52 (1988) 48. T. Engel, Surf. Sci. Rep. 18 (1993) 91. S. Ohno, J.T. Yates Jr., J. Vac. Sci. Technol., A 23 (2005) 475. A.J. Mayne, F. Rose, G. Comtet, L. Hellner, G. Dujardin, Surf. Sci. 528 (2003) 132. E. Ekwelundu, A. Ignatiev, Surf. Sci. 179 (1987) 119. N. Van Hieu, D. Lichtman, J. Vac. Sci. Technol., A 1 (1983) 1. P.W. Lorraine, B.D. Thoms, R.A. Machonkin, W. Ho, J. Chem. Phys. 96 (1992) 3285. K.A. Brown, D.Q. Hu, W. Ho, J. Chem. Phys. 104 (1996) 2385. P.W. Lorraine, B.D. Thoms, R.A. Machonkin, W. Ho, J. Chem. Phys. 96 (1992) 3285. M. Carbone, M.N. Piancastelli, J.J. Paggel, Chr. Weindel, K. Jorn, Surf. Sci. 412/413 (1998) 441. I.R. McNab, J.C. Polanyi, Chem. Rev. 106 (2006) 4321. K. Edamoto, Y. Kubota, H. Kobayashi, M. Ouchi, M. Nishijima, J. Chem. Phys. 83 (1985) 428. I.-S. Hwang, R.-L. Lo, T.T. Tsong, Phys. Rev. Lett. 78 (1997) 4797. I.-S. Hwang, R.-L. Lo, T.T. Tsong, Surf. Sci. 399 (1998) 173. H.-J. Freund, M.W. Roberts, Surf. Sci. Rep. 25 (1996) 225. 2CO2(g) + Si(s) → 2CO(g) + SiO2(s), ΔH = − 627.7 kJ mol− 1. The thermochemistry data are obtained from , http://webbook.nist.gov/. R.I.G. Uhrberg, T. Kaurila, Y.C. Chao, Phys. Rev. B 58 (1998) R1730. Ph. Avouris, J. Phys. Chem. 94 (1990) 2246.
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Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces Shuai Wang, Jinghui He, Yongping Zhang, Guo Qin Xu ⁎ Department of Chemistry, National University of Singapore, 3 Science Drive 3 117543, Republic of Singapore
a r t i c l e
i n f o
Article history: Received 15 February 2012 Accepted 25 April 2012 Available online 5 May 2012 Keywords: Silicon oxidation X-ray photoelectron spectroscopy Oxygen Carbon dioxide
a b s t r a c t The interaction of O2 and CO2 with the Si(111)-7 × 7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O2 and CO2 molecules can readily oxidize the Si(111)-7 × 7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~533 eV as well as to bridge sites of adatom/rest atom backbonds at ~ 532 eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO2 has a lower O/Si ratio than that of O2. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the continuous miniaturization of silicon based transistors in the complementary metal oxide semiconductor (CMOS) technology, there is an increasing impetus to fabricate a thinner silicon oxide film as the gate dielectric layer. Motivated by this technological importance of the ultrathin silicon oxide films in semiconductor devices, the oxidation of silicon surfaces has been extensively studied over the last decades [1–3]. One of the most critical issues is to understand the initial stage of the silicon surface oxidation, which is the key to grow very thin and yet high quality oxide films on the silicon substrate [4–6]. O2 is the natural choice and widely studied in the silicon oxidation process. It was found that the oxidation of silicon surfaces by the ambient O2 molecules results in a thin oxide film with a very low O/Si ratio [7]. The activated oxygen species via different approaches, such as plasma enhanced oxidation [8,9], noble-gas ion beam enhanced oxidation [10,11], and electron beam enhanced oxidation [12,13], must be used to form the thick silicon oxide film. It was proposed that the oxidation reaction is essentially suppressed when the active adsorption sites on the top layers are exhausted. For further oxidation with low substrate temperatures, O2 molecules must be activated to penetrate the covalent bonded silicon oxide layers [14]. In the photoelectron spectroscopic study of the adsorption of O2 on the Si(111)-7×7 surface, no physisorbed O2 species was detected at 90 K, and only two different chemisorbed oxygen species were found and assigned to oxygen in the strained Si\O\Si species with a lower binding energy (O 1s=533 eV) and oxygen in the SiO4 unit with a higher value of O 1s=535 eV [15]. Moreover, SiO4 can be converted to the Si\O\Si with electronic or thermal activation. However, there is limited knowledge on the configurational difference
⁎ Corresponding author. E-mail address: [email protected] (G.Q. Xu). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2012.04.026
between the Si\O\Si species and the SiO4 species or the interconversion mechanism of these two surface species. A more perceptive understanding of the initial oxidation stage was provided by STM studies [16], where O2 molecules were found to dissociatively adsorb on the adatoms of the Si(111)-7× 7 surface at 30 K and 300 K to form dark and bright sites. The exchange between the dark and bright sites can be achieved through the manipulation of the STM tip. The bright sites were attributed to the Si adatoms with the O atoms inserted in the Si\Si back bonds while the dark sites were proposed as O atoms binding on-top to the dangling bond. However, the STM studies gave little information as to the nature of the adsorption and where the O atoms bind, and the assignments of the configurations are inconclusive. There are also some controversies for the assignments of the different silicon oxide species, due to the lack of correlation between spectroscopic results and STM images. Up to now, despite the large number of experimental studies and theoretical simulations, the adsorption of oxygen and its configurations on the Si(111)-7× 7 surface in the initial oxidation stage are still not clear. There have been very few studies on the interaction of CO2 with silicon crystals. The interaction between ambient CO2 and Si(100) was expected to be negligible unless under the influence of a high energy (2 kV) electron beam [17–19]. However, CO2 dissociative chemisorption activated with a very low translational energy (0.17 eV→ 1.6 eV) was observed on Si(100) and Si(111) surfaces in the molecular beam study [20,21]. On both surfaces, the reaction probability increases dramatically with the incident kinetic energy. The activated CO2 dissociates and adsorbs on the silicon surface as CO(a) and O(a) species, which offers compelling similarities and contrasts to that of O2. The activation energy for the CO2/Si(111) reaction was determined to be as low as 0.6 eV by this molecular beam study; however, few detailed studies of this adsorption process were carried out. In this work, we investigated the adsorption of O2 and CO2 on the Si(111)-7 × 7 surface at 110 K. The results showed that both molecules exhibit similar oxidation effects, forming two oxide species on
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the Si(111)-7× 7 surface. These two chemisorbed oxygen species show unique binding energies and distinct O/Si ratios, owing to their different chemical configurations. A thermally induced conversion between these two species was also detected during the annealing process. In comparison to the previous studies, the two oxide species can be identified as oxygen atoms inserted in Si\Si back bonds and oxygen atoms attached to the dangling bonds.
99.3
Si 2p
2. Experimental The experiments were performed in an UHV chamber equipped with facilities for X-ray photoelectron spectroscopy (XPS, VG), high resolution electron energy loss spectroscopy (HREELS, ELS2000, LK), and quadrupole mass spectrometry (QMS, HAL 301, Hiden). The base pressure of the chamber was 1×10− 10 Torr. The Si(111) samples (7×20 mm2 with 0.38 mm thickness) were cut from p-type boron-doped wafers with a resistivity of 1–30 Ωcm and a purity of ~99.999% (Goodfellow). One Ta foil heater (0.025 mm thick, Goodfellow) was sandwiched between two Si(111) samples held together by a set of Ta clips, and was then attached to the manipulator by two Ta support leads. The sample could be resistively heated to 1300 K and liquid nitrogen cooled to 110 K. The temperature of the sample was measured by a 0.003 in. C-type thermocouple (W-5%Re/W-26%Re) which was spot-welted to the Ta clip. The high temperature was emendated by an infrared pyrometer (TR-630, Minolta). The silicon samples were firstly degassed at 850 K overnight in the vacuum, and then cleaned through cycles of Ar ion bombardment (30 min at 1 keV) and annealing at 1300 K. The cleanliness of the surface was checked by XPS and HREELS measurements. O2 (99.9995%, Soxal) and CO2 (>99.8%, Soxal) were used and checked using a quadrupole mass spectrometer (QMS) in the UHV system. The gases were introduced onto the sample through a variable leak valve in a direct dosing mode. The gas dosages are expressed in Langmuir (1 L=1×10− 6 Torr×s), and the dosing pressures were corrected by the relative sensitivity factor for the ion gauge, as 1.01 (O2) and 1.42 (CO2). The relative concentrations of surface species were determined using XPS sensitivity factors and peak intensities according to N1 =N2 ¼ I1 =I2 S2 =S1 where N, I and S represent, respectively, the atomic concentration, the peak intensity and the atomic sensitivity factor (ASF) of a certain element (S: 0.711 for O 1s, and 0.339 for Si 2p). In order to determine the absolute surface coverage on the silicon surface, the adsorption of ethanol (CH3CH2OH) on the clean Si(111)-7×7 surface was investigated by XPS as a probe (not shown), which would adsorb dissociatively on the adatom and rest atom sites with the ideal ethanol/Si adsorption site ratio as 1:2 [22]. Then the ratio of O/Si should be 0.5 (R=NO/NSi =Nethanol/ NSi =1:2). However, due to the strong silicon signal from the subsurface layers, the ratio of O/Si from the XPS peak intensities, R′=(IO/ISi)×(SSi/ SO)=0.0335. Thus a correction factor (F=R/R′=14.9) was obtained and used for obtaining the absolute surface coverage. 3. Results and discussion 3.1. Adsorption of O2 on the Si(111)-7 × 7 surface The oxidation process on the Si(111)-7 × 7 surface by O2 was studied at 110 K using XPS. Fig. 1 shows the Si 2p photoemission features from the clean and 3.6 L O2 exposed Si(111)-7 × 7 surfaces. Only one silicon species can be detected at 99.3 eV, which is consistent with its bulk peak. There are no changes found in both binding energies and peak intensities, which implies the formation of a very thin silicon suboxide film during the exposure of O2. This is in agreement with the previous studies that the oxidation of Si by the ambient O2 reaches a maximum with a very low O/Si ratio [7,12,13]. The details of the
(b) 3.6 L O2
(a) clean
106
104
102
100
98
96
94
Binding Energy (eV) Fig. 1. The Si 2p XPS spectra for the clean Si(111)-7 × 7 surface (a), and after 3.6 L O2 exposure (b) at 110 K.
oxidation process are exhibited in O 1s XPS spectra. As shown in Fig. 2, two chemisorbed oxygen species are formed during the oxidation process, which can be assigned to a low binding energy species Ox at 532.1–532.5 eV and a high binding energy species Oy at 533.5– 533.7 eV. There is no physisorbed O2 species detected during O2 exposure, possibly due to the weak physisorption of O2 on silicon surfaces. In the forepart of the oxidation process (b0.36 L O2), the O 1s peak intensities increase remarkably, which indicates the fast increase of the oxygen concentration on the Si(111)-7 ×7 surface. The increase of surface oxygen concentration also results in the decrease of the electron density on the surface, and thus induces the blue shift in binding energies of the two oxygen species from 532.1 eV to 532.5 eV, and 533.5 eV to 533.7 eV, respectively. However, when the O2 exposure is larger than 0.36 L, there is little change in both binding energies and peak intensities for the two chemisorbed oxygen species. This suggests that O2 induced silicon oxidation reaction is suppressed when the limited surface adsorption sites are depleted. On the Si(111)-7 × 7 surface, adatoms and rest atoms are the most active adsorption sites, where the Si atom contains one dangling bond and three Si\Si backbonds (schematically shown in Fig. 3). In the previous STM studies, it was assumed that O2 molecules dissociate and adsorb at on-top sites with dangling bonds saturated, as well as on bridge sites between the first and second layers [23,24]. Thereby the two chemisorbed oxygen species can be reasonably assigned to the low binding energy Ox to the bridge site and high binding energy Oy to the on-top site, based on their different electronic and geometric structures. The proposed oxidation process of O2 on the Si(111)-7× 7 surface is displayed in Fig. 3, where O2 dissociates and adsorbs via two possible reaction pathways: (a) one oxygen atom adsorbs at the on-top site and the other on the bridge site; and (b) two oxygen atoms adsorb on bridge sites. Further oxidation may form a SiO4 subunit (c) where the Si atom is bonded to four oxygen atoms. Theoretically, the final O/Si ratio should be 4:1, and the ratios of Ox/Oy are 1:1 in (a), 2:0 in (b), and 3:1 in (c), respectively. In comparison with the experimental data, a clear image of the oxidation process on the Si(111)-7 ×7 surface is presented. To further explore the oxidation process, the ratios of chemisorbed oxygen species and the surface adsorption sites on the Si(111)-7×7 surface (the calculation of the surface adsorption site is shown in Experimental section, and the correction factor (F=14.9) is used for calculation) are calculated and demonstrated in Fig. 4. With the exposures up to ~0.1 L, the oxygen signal increases linearly and the O/Si ratio quickly reaches 3.4, which indicates that the O2 molecules dissociate and adsorb onto the topmost layer sites with low energy barriers. Further oxygen dosing
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532.5
O 1s 533.7 f) 3.6 L
e) 0.36 L
d) 0.054 L
c) 0.042 L
b) 0.002 L 533.5
a) clean
540
538
536
532.1
534
532
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Binding Energy (eV) Fig. 2. The O 1s XPS spectra from the Si(111)-7 × 7 surface at 110 K with various O2 exposures: (a) clean surface, (b) 0.002 L, (c) 0.042 L, (d) 0.054 L, (e) 0.36 L, and (f) 3.6 L. Upon peak fitting, two chemisorbed oxygen species are assigned to low binding energy species (SiOx) at 532.1–532.5 eV and high binding energy species (SiOy) at 533.5–533.7 eV.
causes the O/Si ratio gradually approaching 3.8, which indicates most of the surface is saturated and the incoming O2 molecules are presumably scattered back into the gas phase. This result suggests that the rate of adsorption drops rapidly with the increase in the concentration of surface oxygen. Thus, further oxidation is remarkably suppressed, and the experimental ratios of O/Si (3.8), Ox/Si (2.9), and Oy/Si (0.85) are less than their theoretical value (4, 3, and 1), respectively. It should be mentioned that only the Si adatom oxidation process is discussed in the previous STM studies of the Si(111)-7×7 surface, while little information of the Si rest
atoms was present, due to the limitation of STM resolution [16]. In a Si(111)-7 × 7 unit cell, the ratio of adatom and rest atom is 12:6, and if only adatoms are oxidized and bonded to four oxygen atoms, the O/Si ratio should be less than (12 × 4)/(12 + 6), which is approximately equal to 2.67; while the adatoms and rest atoms are both saturated by oxygen atoms, the O/Si ratio should be 4. Our XPS result shows a higher O/Si ratio as 3.8, which indicates that both adatoms and rest atoms are oxidized by O2 indistinguishably. The SiOx and SiOy 4.0 3.5
O/Si
Ratio of O/Si
3.0
Ox/Si
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Oy/Si
0.5 0.0 0.0
0.1
0.2
0.3
0.4
1
2
3
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Exposure (L) Fig. 3. Schematic diagram showing the proposed oxidation process for O2 adsorption on the Si(111)-7 × 7 surface at 110 K. The big dark gray ball indicates the Si atom, and the small red ball represents the O atom. In the initial stage, the oxide species are assumed to be (a) one oxygen atom adsorbs at the on-top site and one oxygen atom at the bridge site, and (b) two oxygen atoms adsorb at the bridge sites. Further oxidation may form a SiO4 subunit (c) where the Si atom is bonded to four oxygen atoms.
Fig. 4. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of O2 exposure on the Si(111)-7 × 7 surface at 110 K, where R = 0.067 is used as the correction factor (for S, the details are shown in Experimental section). With increasing the O2 exposure, the O uptake can be characterized as a fast initial region (b 0.1 L) followed by a slow uptake region (> 0.1 L). The ratio of Ox/Oy is distinctly divided by the two regions as ~ 4.8 and ~ 3.4.
S. Wang et al. / Surface Science 606 (2012) 1387–1392
species were detected simultaneously in the initial oxidation stage, which proves that both oxidation paths (a) and (b) coexist. With the increase in O2 exposure, the ratios of Ox/Si and Oy/Si reach saturation values of 2.9 and 0.85, respectively. Furthermore, the ratio of Ox/Oy is distinctly divided by the two uptake regions as ~4.8:1 (b0.1 L) and ~3.4:1 (>0.1 L). As shown in Fig. 3 the Ox/Oy ratios in species (a) and (b) are 1:1 and 2:0, if the species (a) and (b) was 1:1, the Ox/Oy ratio should be 3:1. However, based on XPS data, the ratio between Ox and Oy changes from ~4.8:1 to ~3.4:1 during O2 adsorption, which indicates the pathway (b) is preferred and the O2 dissociative adsorption on the Si\Si bridge site is more favored at the initial stage. The decrease of the Ox/Oy ratio from 4.8 to 3.4 is probably due to the amount of unreacted on-top sites that is larger than the amount of unreacted Si\Si bridge sites in the slow uptake region. The SiOx and SiOy species have different chemical environments, and would show different thermal stabilities. Fig. 5 displays the evolution of the O 1s XPS spectra of the two oxide species during the annealing process. The SiOy species shows a poor thermal stability, and vanishes when the sample temperature is higher than 600 K. On the other hand, the SiOx species shows a high thermal stability, with increasing peak intensity and constant binding energy until the sample temperature reaches 900 K. All the surface oxide species are desorbed completely after annealing above 1200 K. To observe the details in the annealing process, the normalized XPS peak intensity ratios of O/Si, Ox/Si, and Oy/Si are plotted in Fig. 6, as a function of annealing temperatures. It is clearly shown that the SiOy species, where oxygen adsorbs at the on-top site, is only stable at low temperatures and its peak intensity decreases when T >300 K. However, the intensity of the SiOx species increases simultaneously and meanwhile the total intensity of the silicon oxide remains unchanged, which is indicative that the SiOy species undergoes a thermal induced conversion to the SiOx species via the diffusion of the Ox atom into the sublayer and the transfer of the Oy atom to the Si\Si bridge site. This result is consistent with the previous studies [15], where it was found that thermal activation or electron bombardment converts the high binding energy species to the low binding energy species in silicon oxide films.
O 1s
4.0
O/Si Ox/Si
3.5
Oy/Si
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Ratio of O/Si
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2.5 2.0 1.5 1.0 0.5 0.0 0
200
400
600
800
1000
1200
Temperature (K) Fig. 6. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of annealing temperatures on the Si(111)-7 × 7 surface after an exposure of 3.6 L O2, where R = 0.067 is used as the correction factor. The SiOy species is only stable at low temperatures, and is converted thermally to the SiOx species when T > 300 K and vanishes at T = 600 K. The SiOy species appears again as a precursor during the SiOx species desorption when T > 900 K.
Due to their different electronic configurations, the site with SiOy species (as Fig. 3(a)) should be dark, and the site with only SiOx species (as Fig. 3(b)) should be bright in STM images. Thus, the thermally induced conversion of the two chemisorbed oxygen species can also account for the exchange between the dark and bright sites observed in STM studies [16,25,26]. Moreover, it is interesting to note that the SiOy species appears again when the surface oxide desorption occurs. It is well known that SiO is the major desorbing species during oxide desorption from the silicon surfaces. From Fig. 6, it can be seen that the SiOy species
e) 1210 K 532.3 d) 920 K 532.5
c) 600 K
532.5 533.6 b) 300 K
532.5 533.7 a) 110 K
540
538
536
534
532
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Binding Energy (eV) Fig. 5. The O 1s XPS spectra of the Si(111)-7 × 7 surface exposed to 3.6 L O2 at 110 K (a) and subjected to isochronal annealing up to 300 K (b), 600 K (c), 920 K (d), and 1210 K (e).
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recurs with the SiOx species decreases when the sample temperature is above 900 K, which indicates the SiOx species is converted to the SiOy species by O atom diffusion from the Si\Si bridge site to the Si on-top site. Thus, it is reasonable to propose the SiOy species as the precursor of desorption. Thus, it is clear that O2 dissociatively adsorbs on the Si(111)-7 × 7 surface at both adatom and rest atom sites at 110 K to form a thin silicon oxide film. The oxide film contains two chemisorbed oxygen species as oxygen adsorbs at the on-top site (SiOy) and the Si\Si bridge site (SiOx). These two oxide species reach saturation with a low O2 exposure, and then the rate of adsorption falls off rapidly due to the depletion of the surface adsorption sites. The SiOy species shows a high binding energy but poor thermal stability, which can be converted to the low binding energy species (SiOx) by thermal activation at T >300 K and disappears completely at T > 600 K. Moreover, this SiOy species is reproduced as the precursor during the surface oxide desorption at T > 900 K. 3.2. Adsorption of CO2 on the Si(111)-7 × 7 surface The adsorption of CO2 on the Si(111)-7 × 7 surface at 110 K was also investigated using XPS, and the XPS spectra are shown in Fig. 7. It is noticed that there is no carbon signal observed while oxygen peak increases remarkably during the CO2 exposure. Two chemisorbed oxygen species in the spectra are assigned to the low binding energy species (SiOx) at 532.2–532.4 eV and the high binding energy species (SiOy) at 533.4–533.5 eV, and the binding energies of these two oxygen species increase coincidently with the increase of CO2 exposure. This result indicates that CO2 has the similar oxidation effect as O2 on the Si(111)7 × 7 surface, where CO2 dissociates into CO and O(a) (the adsorbed oxygen species). However, due to the spontaneous desorption of CO during the CO2 dissociative adsorption, no physisorbed CO species can be observed during the exposure at 110 K. CO2 dissociation on clean sp-metal surfaces is widely recognized, because there is a strong thermodynamic driving force for the oxide formation which leads to
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dissociative chemisorption [27]. The silicon oxidation reaction by CO2 is also thermodynamically favorable at 110 K [28]. Meanwhile, it is well known that the uppermost two surface state bands of the Si(111)-7×7 surface are located respectively on the adatoms as S1 at ≈−0.15 eV (relative to EF) and the rest atoms as S2 at ≈−0.9 eV, which provides a nonzero density of states near the Fermi level and thus presents a metallic surface character [29,30]. Therefore, it can reasonably explain this special feature that CO2 dissociatively adsorbs on the Si(111)-7 × 7 surface to form a thin oxide film. Fig. 8 shows the normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of CO2 exposure on the Si(111)-7 × 7 surface at 110 K. It is clear that CO2 shows a similar oxidation effect as O2 on the Si(111)-7 × 7 surface. With the increase in CO2 exposure, the O uptake can be characterized as a fast uptake region (b0.1 L) followed by a saturated plateau region (>0.1 L). The O/Si ratio, which indicates the oxygen surface concentration, increases rapidly in the initial stage and reaches its maximum at 3.2. The ratio of Ox/Oy can also be divided into two regions with values of ~4.1 and ~3.6, which indicates that the formation of the SiOx species is preferred in the initial stage. It should be mentioned that the CO2 dissociative adsorption is terminated with a small value of O/Si ratio as 3.2, and there is no increase for any oxygen species even with an extremely high CO2 exposure. It is well known that CO2 adsorption on metal surfaces is strongly influenced by the surface electron density [27], and thus it is very probable that the CO2 dissociation on the Si(111)-7× 7 surface is critically suppressed by the increase in surface oxygen concentration, which rapidly decreases the electron density of the silicon surface [15]. The reaction pathway of CO2 on the Si(111)-7× 7 surface is proposed in Fig. 9. In the initial oxidation stage, CO2 can dissociatively adsorb on the silicon surface via different pathways to produce two chemisorbed oxygen species: binding on-top to the dangling bond (a) or bridge binding to Si\Si back bond (b). As the SiOx species is dominant in the initial stage, this proves that reaction pathway (b) is preferential. With further exposure, the surface adsorption sites are oxidized and saturated at the
532.4
C 1s
O 1s
533.5
f) 3.5 L
e) 0.35 L
d) 0.0085 L c) 0.0042 L b) 0.0014 L 533.4
540
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530
a) clean 292
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284
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Binding Energy (eV) Fig. 7. The C 1s and O 1s XPS spectra from the Si(111)-7 × 7 surface at 110 K with various CO2 exposures: (a) clean surface, (b) 0.0014 L, (c) 0.042 L, (d) 0.0085 L, (e) 0.35 L, and (f) 3.5 L. No carbon species are found during the CO2 exposure. Upon peak fitting, two chemisorbed oxygen species are assigned to low binding energy species (SiOx) at 532.2–532.4 eV and high binding energy species (SiOy) at 533.4–533.5 eV.
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4.0
4. Conclusions
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Ox/Si
We have studied the adsorptions of O2 and CO2 on the Si(111)-7 × 7 surface at 110 K. It was found that both O2 and CO2 molecules can significantly oxidize the Si(111)-7 × 7 surface and form thin oxide films. Two oxygen species in the oxide films were identified as oxygen atoms binding to on-top sites of adatom/rest atoms with the high binding energy, and oxygen on bridge sites of adatom/rest backbonds with the low binding energy. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film produced by CO2 has a lower O/Si ratio than that of O2. Above all, a clearer insight of the initial oxidation process on the Si(111)-7 × 7 surface is presented, and it is expected to contribute towards the controllable silicon oxide thin film growth.
Oy/Si
Acknowledgment
Ratio of O/Si
3.0
O/Si
2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.1
0.2
0.3
0.4
1
2
3
4
Exposure (L) Fig. 8. The normalized XPS peak intensity ratio of O/Si, Ox/Si, and Oy/Si, as a function of CO2 exposure on the Si(111)-7 × 7 surface at 110 K, where R = 0.067 is used as the correction factor. With increasing the CO2 exposure, the O uptake can be characterized as a fast uptake region (b0.1 L) followed by a saturation plateau region (>0.1 L).
O/Si ratio of 3.2. Thus the major final silicon oxide configurations are likely to be (c) one oxygen atom adsorbs at the on-top site and two oxygen atoms on the bridge sites, and (d) three oxygen atoms adsorb on the bridge sites, which both have the O/Si ratio as 3. As expected, the surface oxide species formed in CO2 dissociative adsorption show a similar thermal behavior to those found in O2 reaction. Based on the above analysis, the dissociative adsorption of CO2 on the Si(111)-7 × 7 surface at 110 K may take place as following. During the CO2 dissociative adsorption, the resulting CO desorbs immediately due to its weak interaction with the silicon surface, and only chemisorbed oxygen species were observed at the Si\Si bridge sites (SiOx) and the ontop sites (SiOy). The CO2 dissociation reaction is terminated at a low CO2 exposure with a low O/Si ratio, which is probably caused by the decrease of the surface electron density.
We thank the National University of Singapore for the financial support of this work (grant nos. R-143-000-377-112 and R-143-000-377462). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
Fig. 9. Schematic diagram showing the proposed oxidation process for CO2 adsorption on the Si(111)-7 × 7 surface at 110 K. The big dark gray ball indicates the Si atom, and the small red ball represents the O atom. In the initial oxidation stage, CO2 dissociatively adsorbs on the surface, leaving oxygen atom on-top to the dangling bond (a) or bridge binding to Si\Si back bond (b). With further exposure, the final configurations are proposed as (c) one oxygen atom adsorbs at the on-top site and two oxygen atoms at the bridge sites, and (d) three oxygen atoms adsorb at the bridge sites.
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