Langmuir 1995,11, 3446-3449
3446
Emergence of Basic Sites on a Si(ll1) Surface in the Initial Stage of Oxidation in Water Yuji C. Sasaki" and Munehisa Mitsuya"?' Advanced Research Laboratory, Hitachi Ltd., Hatoyama, Saitama, 350-03, Japan Received April 4, 1995. In Final Form: June 14, 1995@ The surface oxidation of hydrogen-terminated Si(ll1)single crystals in water has been studied by the adsorption of adenosinetriphosphate and chlorine from their aqueous solutions. The radioactive tracer (32P)method and Auger electron spectroscopy have revealed that these anions are chemisorbed on the Si surface before the surface is fully oxidized. The results indicate that the silicon surface is positively charged in nearly neutral solutions,and is suggestive of the emergenceofbasic sites in the initial oxidation stage. Structural change of the surface-oxidized layer has been discussed in relation to the oxidation of a clean silicon surface under low oxygen pressure in a vacuum.
Introduction Oxides are important in many areas of materials science and play many roles due to their chemical, mechanical, optical, and electronic properties. Among oxides, silicon dioxide (silica) is probably the most useful since it has many technological applications, including use as a catalyst, catalyst carrier, filler, and insulator. The adsorption and other properties of silica depend on the surface hydroxyl groups (silanols).l The distribution and the nature of silanols have been studied over the last 30 years using fluorescence decay,2 ~alorimetry,~ chromatography: nuclear magnetic re~onance,~ neutron reflection,6 infrared absorption spectro~copy,~ and smallangle neutron scattering.* These studies have included many forms of silica, such as silica gels, fine powders, and porous glasses, differing in production conditions, surface area, and pore size distribution. As with many other oxides, however, the characteristics of silica strongly depend on the way in which the sample is prepared. Surface impurities may also cause serious problems since they further alter the surface characteristics. For example, the presence of ethoxy groups was reported for a precipitated silica and was due to incompletehydr~lysis.~ We believe that studying a clean and well-defined surface will facilitate elucidation of the oxidation process. This will clarify our understanding of the oxide. A wet chemical oxidation of a silicon single crystal followed by hydrofluoric acid etching was reported to remove the surface oxide and leave behind a siliconsurface mainly covered with covalent Si-H b o n d ~ . ~The J ~extant
* Author to whom correspondence should be addressed. e-mail:
[email protected]. Abstract published in Advance ACS Abstracts, September 1, 1995. +
@
(1)Iler, R. K. The Chemistry of9iZicu;Wiley Interscience: New York, 1979. (2)Bauer, R. K.; de Mayo, P.; Ware W. R.; Wu, K. C. J. Phys. Chem. 1982,86,3781. (3)Partyka, S.;Lindheimer, M.; Zaini, S.;Keh, E.; Brun, B. Langmuir 1986,2,101. (4)FMi, G.; sz. Kovdts, E. Langmuir 1989,5 , 232. ( 5 ) Tuel, A.; Hommel, H.; Legrand, A. P.; sz. Kovdts, E. Langmuir
770.
1990.6. _ _ I - _ , . -.- -
(6)Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990,6,1031. (7)Burneau, A.; Barr&, 0.; Gallas, J. P.; Lavalley, J . C. Langmuir 1990,6,1364. (8)Cummins, P. G.;Staples, E.; Penfold, J. Phys. Chem. 1990,94,
37411 -.
(9)Kern, W. Semicond. Int. 1984 April, 94. (10)Yablonovitch, E.;Allara, D. L.; Chang, C. C.; Gmitter, T.; Bright, T. B. Phys. Rev. Lett. 1986,57,249.
Si-F bonds were minor species.ll Scanning tunneling microscopy yielded atomic-scale images showingthat the surface consisted of smooth terraces.12 The rate of oxide growth on the surface was much lower than that on a silicon surface obtained by fracture of a single crystal in avacuum.13 This stable and well-definedsurface, coupled with the usefulness of silica, makes it one of the best candidates for studying the oxidation process in water. In this study, [ ~ l - ~adenosine ~Pl 5'-triphosphate was used as a surface probe and radioisotope marker for the oxidation of a silicon surface in water. Chlorine was also used as an adsorbate. The radioactive tracer method and Auger electron spectroscopy have revealed that these anions are adsorbed from their aqueous solutions before the surface is fully oxidized. Silica is a typical solid acid14 with an isoelectric point of 1-3; thus, the surface becomes negatively charged in a neutral aqueous solution. While the adsorption of various cationic electrolytes or surfactants has already been examined,15we believe this is the first report of anion adsorption on an oxidized silicon surface. Experimental Section Polished n-type (111)oriented silicon wafers (10 x 10 mm) were oxidized in a boiling HCVHzOfi20 solution (molarproportion 1:1:13)and dipped in 1%HF to remove amorphous surface oxide. After several repetitions of this procedure, the oxidized silicon was again immersed in 1%HF. The final HF immersion step produced a silicon surface terminated with hydrogen atoms.9-12 The H-terminated silicon was then rinsed in flowing water (pH 7 and l8Mohm resistivity)for a definite interval to oxidize the surface. Immediately after rinsing, [a-32P] adenosine 5'triphosphate (ATP;Du PontNemours & Co. Inc.)was adsorbed from a 30,aL drop of 1.63x M solutionin a triethylammonium buffer (pH 7). After incubating for 24 h at room temperature and in an atmospheresaturatedwith water vapor,the substrates were washed with deionized water and air-dried at room temperature. The amount and the spatial distribution of adsorbed 32Pon the silicon surface were measured by scanning a diode laser on (11)Grundner, M.; Jacob, H. Appl. Phys. A 1986,39,73. (12)Bell, L. D.;Kaiser, W. J.; Hecht, M. H.; Grunthaner, F. J.AppZ. Phys. Lett. 1988,52,278. (13)Grtif, D.; Grundner, M.; Schulz, R. J. VUC.Sci. TechnoZ.A 1989, 7,808. (14)Parks, G. A. Chem. Rev. 1966,65,177. (15)See, for example: Trompette, J. L.; Zajac, J.; Keh, E.; Partyka, S.Langmuir 1994, 10, 812.Rutland, M. W.; Parker, J. L. Langmuir 1994,10,1110. Monticone,V.; Mannebach, M. H.; Treiner, C.Langmuir 1994, 10, 2395. Lajtar, L.; Narkiewicz-Michalek, J.; Rudzidski, W . Langmuir 1994,10, 3754.Krasnansky, R.;Thomas, J . K. Langmuir 1994,10, 4551 and references therein.
0743-7463/95/24l1-3446$09.00/00 1995 American Chemical Society
Emergence of Basic Sites on Si(ll1)
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Langmuir, Vol. 11, No. 9, 1995 3447
loo0
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Figure 1. The number density of 32patoms on the H-terminated Si(ll1) surface rinsed with water at room temperature before ATP adsorption. an imaging plate16(BAS-3000readout system, Fuji Film Co.) to which the sample was exposed for 12 h. For calibration, known amounts ofATP (1to 100 x mol) were allowed to permeate clean thin films and the films were also exposed to the imaging plate as references. The linear relationship between the intensity of the radiation from the 32Pand the number of ATP molecules was confirmed by these references. The specific radioactivity of the ATP was 1.11TBq/mmol. X-ray photoelectron spectra were measured using a VacuumGenerator ESCA-Lab MARK-2 equipped with a hemispherical analyzer and Mg Ka X-ray source (1253.6 eV). The vacuum system had a base pressure of Pa and the irradiation angle was 60" with respect to the surface normal. Binding energies were referenced to Ag 3Qz set at 367.9eV. Auger electron spectra were measured with a PHI CMA10-155. The primary beam energy was 2 keV, the modulation voltage was 3 eV, and the vacuum system had a base pressure of lo-' Pa.
Results and Discussion The density of 32Pin ATP adsorbed on the H-terminated and then water-rinsed surface is shown in Figure 1. The abscissa indicates duration of rinsing before exposure to the ATP solution. This figure summarizes the results obtained in three experiments repeated on different days with different reagents under the same conditions. This adsorption-time curve is characterized by an abrupt increase of ATP density, followed by an equally rapid decrease. For a rinsing time between 10 and 20 min the density of ATP bound is apparently about 5 times that for other times. We could, however, also say that the real ratio of ATP density is still higher. This point will be mentioned in the following paragraph. A two-dimensionaldistribution and profile of 32Pon the Si samples rinsed for various times are shown in Figure 2. Intensity is shown in a logarithm scale for both photographs and profiles. Radioactive 32Pappears as white spots in each photograph. The outline of the drop can be seen on the surfaces rinsed for 1 and 20 min, indicating that these surfaces are still hydrophobic. After rinsing for 150 min, on the other hand, the surface has become hydrophilic and the ATP solution has spread over the surface. Each profile contains some irregularly emphasized spots that can be ascribed to aggregatedATP molecules, presumably condensed by capillarity around dust. The radiation intensity of the surface rinsed for 20 min, except for these spots, is about 20 times that found after 1 or 150 min rinses. The extremely low ATP density after long rinses can be attributed to the Si surface with a thin oxide layer having the characteristics of bulk silica. Since the isoelectricpoint (iep) of silica14occurs between pH values of 1and 3, the (16)Miyahara, J.;Takahashi, K.; Amemiya, Y.; Kamiya, N.; Satow, Y . Nucl. Instrum. Methods A 1986,246,572.
surface exposed to the nearly neutral solution of ATP is negatively charged, repelling the ATP anions. This is the reason why cationic surfactants have been examined as adsorbates for ~i1ica.l~ Under the same experimental conditions, the density of ATP adsorbed onto Ni and Co with native oxide layers was about 10l2molecules/cm2. The ieps of hydroxides of Ni and Co occur at pH values of 11.1and 11.4, re~pective1y.l~ These results indicate that the Si surface is positively charged in the initial oxidation stage and that the surface charge dominates ATP adsorption. The effect of surface morphology on adsorption properties can be excluded as will be shown later. X-ray photoelectron spectrum of the H-terminated Si(ll1)surface changes with rinsing as shown in Figure 3. For rinsing times of 20 min or less the peak assigned to Si02 is not detectable in either the Si2p band (104 eV) or the 01s band (533 eV). The 01s peak (532 eV) for the unrinsed surface can be ascribed to physisorbed oxygen. For rinsing times of 10 and 20 min, the 01s peak is at a binding energy lower than that attributed to SiOz. This shows that oxygen atoms on these surfaces are bonded to one silicon atom, and the 01s peak for these surfaces can be ascribed to a silanol group. This indicates that the surface showing the highest ATP adsorption consists of silanol groups with no disiloxy bridges formed by the condensation of silanol pairs. We must consider the possibility that the result shown in Figures 1 and 2 is due to the change of the surface morphology caused by rinsing with water, since the textured surface makes the physically adsorbed molecules more resistant to removal by washing. The surface is smooth at the beginning. The roughness is increased by oxidation, and the surface may become smooth again as the thickness of the oxide increases. However, the XPS measurements have excluded the growth of the Si02 layer after rinsing for 20 min, and the geometric size of ATP molecules must far exceed the surface roughness at the beginning of oxidation. Consequently, we can conclude that the observed result is not due to the roughness effect but to the electrostatic interactions. Surface hydroxyl groups determine the acidic or basic properties of amphoteric oxides. For example, infrared study has revealed that two types of hydroxyl groups, acidic and basic, exist on the surface of titanium oxide.17 However, while there appears to be of more than two types of hydroxyl groups on the silica surface, they have not been explicitly classified, and a positive charge on the surface has not yet been reported. The acidic character of silica is due to the small ionic radius of Si (0.04 nm), which causes surface hydroxyl groups to be attracted toward the inner 1 a ~ e r s . lThe ~ weight loss curve of silica in a vacuum strongly suggests that each silicon atom in the second layer possesses a hydroxyl group4 The multicoordinated hydroxyl groups must thus be strongly polarized by the surrounding Si atoms, loosening their bond to hydrogen and resulting in an acidic character. Elimination of water from adjacent silanol pairs also creates Lewis acid sites on the exposed oxygen atoms. The acidic character must be much weaker for the singly coordinated hydroxyl groups in the outer layer, and one would therefore expect them to dissociate
as OH- ions.18 These silanols formed in the initial stage of oxidation must be predominantly basic in character and should be able to exchange OH- ions for other anions. At the interface between ATP solution and Si, both ATP (17)Primet, M.; Pichat, P.; Mathieu, M.-V. J.Phys. Chem. 1971,75, 1221. (18)Boehm, H.P.Discuss. Furaduy SOC.1971,52,264.
3448 Langmuir, Vol. 11, No. 9, 1995
Sasaki and Mitsuya
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Figure 2. Two-dimensional distribution and profiles of 32Pon Si(111) rinsed for various times. The sample area within the white lines is 10 x 10 mm2. Intensity is shown in logarithm scale for both photographs and profiles. I
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Figure 3. X-ray photoelectron spectra in the Si2p and 01s bands from a series of rinsed Si(111) surfaces. Rinsing times are shown in the figure.
anions and water molecules competitively attack the silanols and the latter simultaneously oxidize the surface to yield acidic sites. It is thought that chemisorbed ATP protects the surface from the attack of water molecules and that the actual density of basic silanols must be much higher than the result shown in Figure 1. This basic-sitemodel is also supportedby the adsorption of Na and C1 ions. The H-terminated and then waterrinsed Si substrates were first incubated for 10 min with the NaCl solution (0.163 moVL, pH 6.3) at room temperature. Then the substrates were washed with water and air-dried at room temperature. The Auger electron intensity from the Cl(LMM)transition was greatest for the Si surfaces rinsed for 10 and 20 min, as shown in Figure 4. These rinsing times correspond to those for which ATP adsorptionwas greatest (Figure 1). In contrast to the anion, the Auger electron from the Na(KLL) transition was detectable only for the surfaces rinsed for 1 and 40 min, as shown in Figure 4. One question still remains unresolved. The hydrogenterminated Si surface would be expected to yield basic
A
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Kinetic energy / eV Figure 4. Auger electron spectra of Cl(LMM) and Na(KLL) transitions from the Si(111) surface. Rinsing times before incubation in NaCl solution are shown in the figure.
sites upon contact with ATP solution. Why was not ATP adsorbed on surfaces rinsed for less than 10 min? This may be due to the presence of small amounts of surface Si-F bonds made by the HF treatment.ll The large electronegativity of F atoms could cause an electron shift in the neighboring atoms. This shift could weaken the silanol 0 - H bond and consequently increase surface acidity.lg The resulting negative charge on the surface is thought to repel the ATP anions. The oxidation of Si under vacuum conditionswas studied extensively to understand the unique electrophysical characteristics of the Si-Si02 interface. In-situ XPS investigation revealed that at high temperatures Si exhibits a layer-by-layer growth of oxide:20Si1+and Si2+ are first formed and then converted to Si3+and Si4+(the superscripts here refer to the number of oxygen atoms bonded to a central silicon atom). Our result is consistent with this study, since the surface Si atoms with basic (19)Chapman, I. D.; Hair, M. L. J. Cutul. 1963,2, 145. (20) Borman, V. D.; Gusev, E. P.; Lebedinskii, Yu. Yu.; Troyan, V. I. Phys. Rev. Lett. 1991, 67,2387.
Langmuir, Vol. 11, No. 9, 1995 3449
Emergence of Basic Sites on Si(ll1) hydroxyl groups could be ascribed to Si1+andlor Si2+states. The structure and nature of surfaces are known to depend on the preparation process. Still the behavior of oxygen atoms at the surface has nevertheless been found to be similar for thermal oxidation in vacuum and for roomtemperatureoxidation in water. The localization ofoxygen atoms on the surface and the subsequent diffusion into inner layers must be a specific characteristic of silicon. In this work, we examined the oxidation process of a clean silicon surface in water using adenosinetriphosphate and sodium chloride as surface probes. We have shown that the surface is positively charged at the initial stage of oxidation in water. In most cases, subtle structural
changes in surfaces can be detected as small shifts of physical properties such as wavenumbers in vibrational spectroscopy or binding energy in electron spectroscopy. Our results show that, while the surface physicalstructure underwent gradual change, the chemical characteristics were reversed abruptly.
Acknowledgment. The authors are grateful to Dr. Seiichi Iwata, Central Research Laboratory,Hitachi Ltd., for valuable suggestions. LA950272T
