Low temperature oxidation of silicon (111) 7 × 7 surfaces

Surface Science 157 (1985) 273-296 North-Holland, Amsterdam

273

LOW TEMPERATURE OXIDATION OF SILICON (111) 7 x 7 SURFACES A.J. SCHELL-SOROKIN and J.E. DEMUTH IBM ThomasJ. Watson Research Center, P.0. Box 218, Yorktown

Heights, New York 10598, USA

Received 26 November 1984; accepted for publication 6 March 1985

The coverage dependent interaction of molecular oxygen with the Si (111) 7 X 7 surface at 20 K has been studied with high resolution electron energy loss spectroscopy, ultraviolet photoelectron spectroscopy, and low energy electron diffraction. These results provide a new view of the initial stages of oxidation of silicon. In addition to physisorbed molecular oxygen, two other oxide species occur at monolayer coverages: diatomic-like and bulk-like forms of silicon monoxide. Formation of the diatomic-like monoxide begins at the lowest coverages while the appearance of the bulk-like monoxide is delayed. The diatomic silicon monoxide is stable to a temperature between 475 and 575 K, and the bulk-Iike monoxide is stable to approximately 950 K. This latter oxide completely desorbs at approximately 975 K. The sticking coefficient for d~om~sition of molecular oxygen was found to be much greater at 20 K than at room temperature. This effect can be rationalized if it is assumed that a mobile precursor state, differing from those of metal surfaces, has a major role in the oxidation reaction. We also present evidence that the detailed vibrational spectra frequently observed for oxygen absorbed on Si (111) is complicated by the presence of a very small amount of hydroxyl contamination.

Many techniques, including photoelectron spectroscopy [l-l l] (PES), Auger electron spectroscopy [12-191 (AES), ultraviolet photoelectron spectroscopy [3,4,20-233 (UPS), surface soft X-ray absorption spectroscopy [7] (SSXAS), surface extended X-ray absorption fine structure [24,25] (EXAFS), electronic energy loss spectroscopy [21,22,26-303 (ELS), and high resolution electron energy loss spectroscopy [2,11,31-361 (EELS), have been applied to the study of monolayer adsorption and reaction of molecular oxygen on Si (111) surfaces. Despite this effort, there is little consensus regarding not only the precise nature of adsorbed oxygen but even whether Oz chemisorbs dissociativdy or molecularly. Several models of both types of adsorption, dissociative and molecular, have been proposed [7,28,34,36,37-441. Most relevant to this study is the recent comprehensive EELS study by Ibach et al. [34] of coverage dependence of 0, adsorption on Si (111) 7 x 7 surfaces at 100, 300, and 700 K which concluded that both a mofecular species, the peroxy radical or superoxide 141,421, and a dissociative one, a monoxide bridge 1341are formed.

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We report EELS, LEED (low energy electron diffraction), and UPS results for 0, adsorption on the Si (111) 7 x 7 surface at low temperature (20 K) which support dissociative chemisorption involving two surface oxides and which demonstrate that the chemical nature of the surface and its oxides change dramatically in a narrow range of coverage. This interpretation is based on the coverage dependence of the EEL and UP spectra and the thermal stabilities of the surface oxides. One of the oxides we identify is a diatomic monoxide. Evidence for its presence is the correspondence of observed vibrational and valence electronic levels with those of gas phase diatomic SiO. The second surface oxide we observe has loss features which correspond, in both spectral positions and relative intensities, to those of bulk silicon monoxide and also has a UP spectrum which is very similar to that of bulk silicon monoxide [3,45]. Since the vibrational and valence electronic spectra of the second surface oxide so closely resemble those of bulk silicon monoxide, the local structure of this surface oxide is probably very similar to that of bulk silicon monoxide. Therefore, we will refer to this second surface oxide as bulk-like surface monoxide. The appearance of this surface oxide is delayed; it does not begin to form until 0.15 or 0.2 L. In the exposure range below 0.1 L, all of the adsorbed molecular oxygen reacts to form the diatomic silicon monoxide and a precursor to the bulk-like surface monoxide. No weakly perturbed molecular or physisorbed molecular species forms below this exposure on the Si (111) 7 X 7 surface at 20 K. By 0.2 L, formation begins of both physisorbed molecular oxygen and the bulk-like surface monoxide. Up to about a langmuir exposure, the diatomic and bulk-like surface monoxides continue to form; above this exposure, the formation of the two monoxides ceases and molecular oxygen accumulates. If the surface with a langmuir exposure is warmed, the physisorbed molecular oxygen reacts to form more of the bulk-like monoxide. Silicon monoxides have been proposed in previous silicon oxidation studies which employed other measurement techniques. In a surface soft X-ray absorption study, Bianconi and Bauer (71 observed that a surface oxide (- 5 A thickness) with the local structure of bulk silicon monoxide was the only product of the 10 L O2 adsorption on cleaved Si (111) at 973 + 50 K, a temperature at which the cleaved surface is known to undergo a 7 X 7 reconstruction. Earlier in 1977, Hollinger et al. [3] and Hollinger et al. [45] had noted the similarity of the ultraviolet photoelectron spectra of 30 L of 0, adsorbed on Si (111) at 1025 K to that of a silicon monoxide film evaporated onto a 300 K substrate. Finally Ludeke and Koma [28] in 1975 noted correlations between the peaks of the electronic energy loss spectra of the room temperature Si (111) surface with monolayer 0, coverage and the visible and ultraviolet absorption spectra of the gas phase diatomic silicon monoxide molecule.

A.J. Shell-Sorokin,

2. Experimental

J.E. Demuth

/ Low temperature

oxidation of Si (Ill)

7X 7

215

apparatus

The experiments discussed in this paper were performed on a single crystal of silicon in an ultrahigh vacuum chamber which has a base pressure of 4 x 10-i’ Torr. The principal analytical tool used to investigate surface properties was EELS. Auxiliary analytical tools used in situ were ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). Details of the experimental apparatus have been described before [46]. The width of the elastic peak of the EELS beam reflected from the Si (111) 7 x 7 surface at 20 K was typically 13 meV FWHM for this study. The boron doped sample (1.3 X lOI atoms/cm3), 6 mm in diameter, was ultrasonically machined, mechanically and chemically polished, and clamped to the sample support by a tantalum retaining ring. The sample could be indirectly caoled or heated between 20 and 1400 K as measured with a chromel-alumel thermocouple spot welded to the retaining ring. The exposures of the silicon surface to molecular oxygen were made with the ion gauge ion. Initial sample cleaning required lengthy and repeated oxidation and ion sputtering treatments, performed daily over a period of several weeks, to remove both carbon and other surface impurities. A second and milder sample cleaning treatment which was occasionally employed was the following procedure. The sample was sputtered at room temperature in 10d6 Torr of argon with 500 eV argon ions for 10 min during which time the sample was moved so that the argon beam was rastered over the entire sample. The argon was then pumped out of the chamber and the sample was then heated sufficiently to drive out the buried argon atoms as evidenced by a pressure burst. The above sputtering procedure was repeated, but this time for a duration of 5 min. Using a gas doser, the sample was then repeatedly exposed to lop6 Torr of 0, for 3 to 4 min and heated to 1250 K. High quality 7 x 7 reconstructed surfaces could be produced reliably by first slowly heating the surface to 1250 K, holding this temperature for 2 min, and then cooling it to room temperature at a rate of 5 K/min. We found hydroxyl contamination of the surface to be a serious problem in this study. Si-OH has a very intense EELS band near 100 meV, and a fraction of a monolayer, as small as l%, can cause serious distortions of the loss spectra of the surface [34] which can be misinterpreted and attributed to oxidation. While we reliably were able to produce clean Si (111) 7 X 7 surfaces, free of any visible trace of the 100 meV Si-OH vibration, and to maintain these surfaces in vacuum for at least an hour, we almost invariably contaminated the surface with some OH upon exposure of the sample to 0,. This OH does not appear to be a background contaminant of the gas but is probably generated by 0, exposure to the chamber walls or to the ion gauge filament. The presence of OH as a contaminant on the oxygen exposed surface was revealed by the

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/ LAW temperature oxidation

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presence of the feature due to the OH stretching vibration at 462 meV which is about ten times weaker in intensity than that at 100 meV in our loss spectra. This sensitivity in the spectral range of the O-H stretch is greater than in most previous EELS studies because of the higher beam energies (12 to 15 eV) and the larger spectrometer acceptance angle (3”) employed in this study. Because the sticking coefficient of oxygen at room temperature [47] is much smaller than that at 20 K, the OH contamination problem was considerably worse for room temperature exposures.

3. Results Using EELS, we have studied the low coverage (submonolayer to few monolayer), low temperature (20 K), molecular oxygen adsorption on the Si (111) 7 X 7 surface. A sequence of loss spectra of the surface as the 0, exposure is increased from 0.1 to 3.7 L is shown in fig. 1. The four loss peaks characteristic of the higher coverage surface are 55, 85, 128, and 196 meV. The 196 meV peak (figs. lb-le) can be assigned immediately to physisorbed molecular oxygen. The absence of this peak in fig. la demonstrates the absence of any weakly perturbed molecular species at 0.1 L; all of the adsorbed molecular oxygen decomposes on the surface. The presence of the 196 meV peak in fig. lb shows that this high reactivity of the surface toward molecular oxygen abates at relatively low exposures, 0.2 L. With increasing exposure, more molecular oxygen accumulates. The progressive changes in the vibrational features of the surface oxides with increasing O2 exposure are shown in fig. 1, and the exposure dependence of the intensities of some selected features is shown in fig. 2. At the lowest exposure shown, 0.1 L, loss features are observed at 60, 95, 115, 130, and 149 meV. This spectrum was consistently reproducible, and the spectrum of a still lower exposure, 0.05 L, shows the same features though not as distinctly. The features at 60 and 149 meV are more prominent than those at 90 and 130 meV. When the 0, exposure is doubled (0.2 L), the overall shape of the vibrational spectra changes markedly; now features are observed at 55, 85, 128, and 152 meV. At this coverage the features at 55 and 152 meV are less intense than those at 85 and 128 meV. As the exposure increases from 0.2 L to 3.7 L, the most obvious change is that the 152 meV peak, prominent at low coverages, is obscured continuously with increasing coverage by its neighboring peak; it appears only weakly as a shoulder on the 128 meV peak at 3.7 L. This feature, the 152 meV peak, exhibits an exposure dependent shift; at the lowest exposures it occurs at 149 meV and shifts to its higher coverage position by 0.2 L. The shift of the second peak at 85 meV, seen in fig. Id for example, is not consistently reproducible and, as shown later, is an artifact caused by the presence of hydroxyl contamination. For example, fig. le, which is from a experimental run differ-

A.J. Schell-Sorokin,

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J. E. Demuth / Low temperature

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Fig. 1. Electron energy loss spectra of 0, adsorbed on the Si (111) 7 X7 surface (b) 0.2 L; (c) 0.4 L; (d) 1.3 L; (e) 3.7 L. Incident electron energy is 15 eV.

at 20 K: (a) 0.1 L;

from that of fig. Id, shows the spectra of a surface with greater 0, coverage and with no shift of the 85 meV peak from its lower exposure (0.2 L) position. The spectra of fig. 1 at intermediate and higher exposures consistently show an additional feature at 113 meV, a low frequency shoulder on the 128 meV peak. Off specular measurements of the loss spectra of the 20 K surface shown in fig. 1 reveal that the 85, 128, and 152 meV peaks are produced by dipole scattering; the 55 meV feature is produced by impact scattering since its relative intensity is unattenuated in the off specular observation mode. On surfaces with an exposure of 0.8 L or greater, LEED shows a weak 1 X 1

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Fig. 2. Exposure dependence of the intensities of the loss features at 152, 128, and 196 meV divided by the elastic peak intensity. Open circles mark the dependence of the 152 meV feature; solid circles and crosses, the 128 and 196 meV features, respectively. The left-hand scale applies to the 152 meV feature, and the right-hand scale to the 128 and 196 meV features. Data shown are for an incident electron energy of 15 eV. Using an exposure calibration based on our rare gas adsorption measurements [46], we estimate that at one langmuir twenty-five oxygen molecules have struck each 7 X 7 surface unit.

pattern and a highly intense diffuse background. Such a pattern indicates that surface order is largely disrupted. As mentioned, the presence of a very small amount of hydroxyl contamination causes an apparent shift of the second oxide loss feature. This feature, located at 85 meV in the absence of contamination, can overlap with the very strong 100 meV feature of the Si-OH group. To illustrate this point, we show in fig. 3 the loss spectra of 0.5 L of 0, adsorbed on a Si (111) 7 X 7 surface at 20 K and subsequently heated for a few seconds to successively higher temperatures. As the temperature to which the surface is heated increases, the increase of the intensity of the OH vibrational band at 462 meV is correlated with the gradual shift of the 85 meV loss feature to 100 meV, the position of the Si-OH vibration. This correlation is demonstrated more directly in fig. 4. Here the position of the second loss feature is plotted versus the ratio of the intensity of the 462 meV feature to that at 128 meV. In addition to the spectra of unheated surfaces, this plot includes spectra of surfaces which were heated up to 875 K and which had exposures, prior to heating, varying between 0.2 and 3.7 L. The three points on the plot which do not show this correlation (open circles) represent spectra of 0.2 L (such as fig. lb) where the 128 meV feature is relatively much less intense than it is at higher exposures. The apparent shift in the 85 meV peak disappears when the oxide is heated sufficiently to reduce the hydroxyl group, as shown in fig. 5. In fact. as shown

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Fig. 3. Electron energy loss spectra of 0.5 L of 0, adsorbed on the Si (111) 7X7 surface at 20 K and subsequently heated for a few seconds at the following temperatures: (a) not heated; (b) heated to 65 K; (c) heated to 160 K; (d) heated to 580 K; and (e) heated to 780 K. All spectra were taken after the surface had cooled to 20 K. Incident electron energy is 13 eV. The increased intensity of the loss features is caused by the additional exposure which occurs when oxygen desorbs from the sample holder during heating.

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Fig. 4. Position of the second loss peak as a function of the ratio of the intensity of the feature at 462 meV to that at 128-132 meV. Data shown are for an incident electron energy of 15 eV.

in fig. 6, the complete reduction of H,O produces essentially the same vibrational frequencies at 55, 85, and 127 meV we observe with the intermediate oxygen exposures shown, for example, in figs. lc, 3b, and 5~. (No evidence of the 152 meV feature is observed in this system.) Ibach et al. [34] also have observed that H,O adsorbed on a Si(100) surface is chemically reduced by heating to produce silicon oxides having an EEL spectrum similar to that shown by 0, adsorbed on this surface. When the shift of the 85 meV peak is discounted, then the spectrum of the silicon oxide created by 0, adsorption at 20 K undergoes very little change when the surface is heated to 950 K. Apart from changes in peak intensities attributable to the additional O2 exposure which occurs during heating, the series of loss spectra shown in figs. 3 and 7 exhibit only two actual changes in the spectra caused by heating the silicon surface. One change is that the loss spectra of the surface exhibits a small 4 meV shift in the 128 meV loss feature to 132 meV as seen in fig. 3 or in fig. 7 where much smaller OH contamination occurs. The actual temperature at which this shift occurs depends on the initial oxygen exposure: at 0.5 L (fig. 3b) a temperature above 160 K shifts this peak while at 1.3 L (fig. 7) only a temperature of 50 K is needed. Concurrent with this shift is an increase in the intensity of the 128-132 meV feature (relative to the elastic peak) as well as an increase the intensity of the oxide contribution to the UP spectrum. We conclude that the surface oxide responsible for the 55, 85, and 128 meV peaks formed at low coverages on low temperature silicon surfaces is stable to approximately 950 K, near the temperature at which it

A.J. Schell-Sorokin,

J.E. Demuth

/ Low temperature

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Fig. 5. Electron energy loss spectra of 0.2 L of 0, adsorbed on the Si (111) 7 X 7 surface at 20 K and subsequently heated for a few seconak (a) not heated; (b) heated to room temperature; (c) heated to 875 K. Loss spectra (b) and (c) were taken at room temperature. Incident electron energy is 14.5 eV. The increase in the 462 meV peak seen in (b) indicates that when this surface is warmed to room temperature, the hydroxyl contamination increases. After this sample is heated in (c), the 85 meV peak is only slightly shifted and a weak band at 462 meV persists in the spectrum of this sample.

completely desorbs. The other change is that the 152 meV (1230 cm-‘) loss feature is absent in spectra of the surface which had been heated above 575 K as shown in figs. 3c, 3d, and 3e. In fig. 8, the 152 meV feature is present in the loss spectra of a surface exposed at 20 K to 0.15 L of O,, and then heated to 475 K. We conclude that the surface oxide responsible for the 152 meV peak is stable below 475 K but not above 575 K. In fig. 9 we show the He I ultraviolet photoelectron spectra for the same coverages which were studied with EELS. At the lowest oxygen exposure of 0.1 L (fig. 9b), the valence bands of the clean surface (fig. 9a) are only slightly modified and the surface state nearest E, disappears. The UP spectrum shown

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Fig. 7. Electron energy Ioss spectra of 1.3 L of 0, adsorbed on the Si (311) 7x7 surface at 20 K and subsequently heated: (a) not heated; (b) heated for a Sew seconds to 50 K. Incident electron energy is 15 eV. The X5 meV feature appears at 90 meV due to OH contamination.

A.J. Schell-Sorokin, J.E. Demuth / iLowtemperature oxidation

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Fig. 8. Electron energy loss spectra of 0.15 L of 0, adsorbed on the Si (111) 7 X7 surface at 20 K and subsequently heated for a few seconds: (a) not heated; (b) heated to 475 K. Loss spectrum (b) was taken while surface was at 130 K. The incident electron energy is 13.5 eV. In both spectra a small amount of OH contaminant has shifted the 85 meV peak.

in fig. 9b is of- the same surface as the loss spectrum of fig. la; thus this UP spectrum contains that of the species responsible for the 152 meV loss feature. The loss spectrum of the surface which has the UP spectrum shown in fig. 9c has features at 55, 90, 129, and 152 meV; and that of fig. 9d has loss features at 55, 85, and 130 meV as shown in fig. 5c. The marked changes in the electron energy loss spectra which are observed as the 0, coverage is increased from 0.1 to 3.7 L are not reflected in the ultraviolet photoelectron spectra of similarly dosed surfaces. For comparison we show spectra characteristic of molecular oxygen (fig. 9e) and silicon dioxide (fig. 9f) formed by laser annealing the oxide of fig. 9d. It is obvious that the ultraviolet photoelectron spectra of figs. 9b--9d are not similar to that of either condensed molecular oxygen or silicon dioxide.

4. Discussion Examination of our thermal and spectroscopic results reveals that two surface oxides having different thermal stabilities and different coverage dependences are formed by a langmuir exposure of oxygen on the Si (111) 7 x 7 surface at low temperature. The loss feature at 152 meV is attributed to

284

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Fig. 9. Ultraviolet photoelectron spectra of 0, adsorbed on the Si (111) 7 x 7 surface at 20 K and subsequently heated for a few seconds: (a) clean surface; (b) 0.1 L adsorbed and not heated; (c) 0.4 L adsorbed and heated to 50 K; (d) 0.2 L adsorbed and heated to 875 K; (e) 0.9 L adsorbed and not heated; (f) silicon dioxide produced by laser annealing oxide of (d). Spectra (a)-(c) and (e) were taken at 20 K; spectra (d) and (f) were taken at room temperature. The much larger oxide contribution to the valence band spectra of (c) and (d) is caused by the reaction of the silicon surface with the molecular oxygen which desorbed from the sample holder during heating.

A.J. &hell-Sorokin,

J.E. Demuth

/ L.ow temperature

oxidation of Si (111) 7 x 7

285

one oxide, and the features at 55, 85, and 128 meV are attributed to a second oxide. Independent of coverage, the 152 meV feature is absent in the loss spectra of the surface which had been heated to above 575 K while the three other features are observed for the surface heated to as high a temperature as 950 K. Although it could be argued that this difference is an artifact caused by the effects of disorder or reorientation concurrent with increased temperature, on no experimental run did we see any trace of the 152 meV peak in the loss spectrum of a surface heated to above 575 K, yet it was routinely observed on the surface which had not been heated to more than 475 K and which had an exposure of less than 0.5 L. The second observation which indicates that the origin of the 149-152 meV peak is different from that of the 55, 85, and 128 meV peaks is the sequential appearance of these two groupings of the features. The 149 meV feature is present at the lowest exposure, 0.1 L (fig. la), and clearly has a counterpart, the 152 meV feature, in the spectra at 0.2 L and above. The second grouping of features does not appear until 0.15-0.2 L; below this exposure interval, the overall shape of the spectra, exclusive of the 149 meV feature, is markedly different. At about one langmuir exposure, another difference is observed between the coverage dependence of the 152 meV feature and that of the three lower frequency features at 55, 85, and 128 meV; this difference is that, at just above a langmuir exposure, the intensity of the 152 meV feature largely decreases while the intensities of the three lower frequency features hold their maximum values. These differences in the coverage dependence of the relative intensities are shown in fig. 2 for the 152 and 128 meV features. In their EEL spectra of O2 adsorbed on the Si (111) 7 X 7 surface at 100 K, Ibach et al. [34] observe a similar coverage dependence below one langmuir and propose a two oxide interpretation which involves the peroxy radical as the low coverage oxide and a monoxide bridge as the higher coverage one. Besides indicating an origin distinct from that of the 149-152 meV peak, the delayed appearance of the set of features of the second oxide also implies that the formation of this surface oxide involves some intermediate or precursor oxide. In addition to the loss spectra and its coverage dependence, our interpretation also takes into consideration the coverage dependence of the UP spectra and the thermal stabilities of the two oxide species. Together these results strongly indicate that both surface oxides are produced by dissociative chemisorption. Fig. 10 shows the result of the subtraction of the photoelectron spectra of the valence band of the clean surface (fig. 9a) from those of some of the oxidized surfaces of fig. 9. To investigate the surface with the lowest oxygen exposure (figs. 9b and lOa), 0.1 L, which exhibits the 149 meV peak and the other low coverage vibrational features in EELS, we have averaged three difference curves due to the smallness of the oxygen contribution to the valence band spectrum. As noted before, the surface which has the subtraction spectrum shown in fig. lob has the loss features of both surface oxides (55, 90,

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Fig. 10. Difference curves of the He I ultraviolet photoelectron spectra from fig. 9 with the spectrum of &he clean surface (fig. 9a) subtracted: (a) fig. 9b; (b) fig. 9~; (c) fig. 9d. Here a work function [lo) of 5.3 eV is assumed for all oxygen on silicon phases. The three vertical ionization potentials at 11.6. 12.2, and 14.8 eV, observed in the He 1 photoelectron spectrum [Sl] of gas phase diatomic silicon monoxide, are indicated by the dashed vertical lines.

130. and 152 meV); that of fig. 10~ has been heated to remove the species responsible for the 152 meV peak and has loss features of only the second oxide as shown in fig. 5c. Since the UP spectra show the same oxide peak positions but different relative intensities for both the lowest and near monolayer coverages, we argue that the two oxide species must be either both dissociative or both molecular chemisorption products. (To emphasize that this degree of similarity is not caused by the presence of condensed molecular oxygen, the UPS levels of a monolayer of oxygen condensed on the oxidized silicon surface are indicated in fig. 10.) These two types of chemisorption are clearly distinguished in the UP spectra of 0, on platinum where the peroxy species exists below 170 K and a dissociative species prevails above that temperature [48,49]. That the second oxide species is stable to a temperature as

A.J. Schell-Sorokin, J.E. Demuth / Low temperaiure oxidation of Si (I 1 I) 7 X 7

287

high as 950 K strongly indicates that it is formed by dissociative, rather than molecular, chemisorption [50]. Moreover, the two broad features shown in figs. lOa-10c roughly coincide with the three bands observed in the He I photoelectron spectrum [51] of gas phase diatomic silicon monoxide, as indicated in fig. 10; further, the UP spectra of 0, adsorbed of the Si (111) surface is known to be very similar to that of bulk silicon monoxide. As mentioned in the introduction, Hollinger et al. [3] and Hollinger et al. [45] noted the similarity of the UP spectrum of a film of silicon monoxide [45] evaporated onto a 300 K substrate with that of a monolayer of oxygen on the Si (111) surface. Shown in fig. 3a of ref. [45], this spectrum is also very similar to our figs. 9b and 9c. These spectra are essentially identical also to fig. 4 (curve 1) of a monolayer of oxygen on Si (111) reported by Ibach and Rowe [21]. Further support for the two phase interpretation is demonstrated by the exposure dependence of the intensity of the larger UP peak; as shown in fig. 11, the slope of the intensity of the peak at 11.6 eV (with respect to the vacuum) increases at about the same exposure that the bulk-like monoxide is first observed in EELS, 0.15 L. (A similar plot for the smaller UP peak cannot be made because of its near overlap with a feature of molecular oxygen.) Because of the near equality of the vibrational frequency of the 149-152 meV feature to that of gas phase diatomic silicon monoxide as well as the general similarity of the respective UP spectra, we propose that this loss feature is due to the diatomic SiO molecule adsorbed on the surface. Comparison of the vibrational frequency of this loss feature with that of diatomic silicon monoxide, both in the gas phase [52] and in matrix isolation [53-551 is made in table 1. At low exposure, 0.1 L, this peak is observed at 149 meV, a value which is 30 cm- ’ less than the band origin of the gas phase infrared absorption. It is well known that such a reduction of the vibrational frequency can be caused by ligancy or coordination of a molecule to a metal surface [56]. At higher coverages, this molecule-surface ligancy effect is compensated by a dipole-dipole interaction with neighboring silicon oxides, an effect which is known to increase the vibrational frequency [56]. A similar compensating shift has been observed for the coverage dependent spectra of carbon monoxide adsorbed on metal surfaces 156,571. Although little is known about diatomic silicon monoxide adsorbed on either metals or semiconductors, it is isoelectronic to carbon monoxide, a very well studied adsorbate [56]. If the adsorption of silicon monoxide is similar to that of carbon monoxide on metals, then the small frequency shift from that of the gas phase would indicate that the silicon monoxide molecule resides on the surface in a top (or terminal) site with a silicon-silicon singly bonded ligancy. Although we do not observe a loss feature attributable to the silicon-silicon stretching vibration, it is likely that its frequency is less than 40 meV, and therefore, not observable in our spectra due to the width of the elastic peak. Ibach et al. [34] also observed the 149-152 meV peak in their spectra of the surface at 100 K and 300 K and noted that

A.J. Schelf-Sorokin,

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J. E. Demuth

/ Low temperature oxidation of Si (I I I) 7 x 7

0 0

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Fig. 11. The intensity of the ultraviolet photoelectron spectra at 11.6 eV (with respect vacuum) of the Si (111) 7x7 surface at 20 K as a function of 0, exposure. The intensity clean surface has been subtracted. The intensity is not plotted above 0.5 L because significant contribution of physisorbed molecular oxygen to the UP intensity at 11.6 eV at exposures.

to the of the of the higher

this vibrational feature is essentially unshifted from the gas phase infrared absorption peak of diatomic silicon monoxide. However, they considered its assignment not to the adsorbed diatomic SiO molecule but to a three back bonded sp3 hybridized silicon surface atom singly bonded to an oxygen atom. They discounted their suggestion because such an Si-0 species would be unlikely to have the same vibrational frequency as that of diatomic multiply bonded SiO molecules, and instead, they favored a silicon peroxy radical assignment since the several other vibrations which appear with the 152 meV peak can be attributed to other modes of this one species. We attribute these other vibrations which occur below 0.1 L to a precursor to the second oxide species. We propose that the several other loss features which we observe at 55, 85, and 128-132 meV above 0.2 L are due to a second surface oxide. As in the Table 1 Observed

vibrational

frequencies

of diatomic

silicon monoxide

Environment

Frequency

Gas phase

1230 (band origin)

Matrix

isolated in argon

Matrix isolated Matrix

in nitrogen

isolated in neon

1225.9 1226.0 1223.9 1224.4 1228.5

(cm-‘)

Ref. ~521 1531 [541 1531 1551 [541

A.J. &hell-Sorokin,J.E. Demuth / Low temperature oxidation of Si (I1 1) 7 X 7

289

case of the 152 meV loss feature, both the direction and magnitude of the shift of the 128 meV loss feature to higher energy, 131-132 meV, are characteristic of the interaction between dipoles [56,57]. The absence of an observable shift in the positions of the two lowest frequency peaks is expected since such a shift depends on the magnitude of the vibrational polarizability which is proportional to the intensity of the loss feature [57]. Ibach et al. [34] also observed these three vibrational features as well as the coverage dependent shift of the third peak. They assigned these features to a monoxide bridge formed by oxygen atom insertion in bonds between silicon atoms. We find that the positions of these three loss features are predicted very well by the formula for the loss spectrum of the surface of a bulk dielectric [58] when the dielectric function [59] of silicon monoxide is employed. The intensity, I, of the inelastically reflected electron beam as a function of energy loss, hv, is related to the bulk dielectric constant, C(V) = et(v) + ic2(v) by [58] Ia

Im{ -l/[l

+ E(V)]}.

In fig. 12 we compare this simple calculated loss function (dashed line), with its peaks at 53, 85, and 132 meV, to the spectra of surfaces with greater than monolayer coverages which have (fig. 7b) and have not undergone a heat treatment (fig. le). It is noteworthy that this calculated loss function also predicts a shoulder on the low frequency side of the 132 meV peak similar to the one observed experimentally as well as the relative intensity of the two higher energy peaks. The observed intensity of the lowest energy peak at 55 meV, which is produced by impact scattering, is much less than that predicted by eq. (1). The negligible dipole activity of this vibration implies that this bulk-like monoxide species has some preferential orientation as compared to the bulk monoxide. The spectra of the surface with less than monolayer coverage, shown for example in fig. lb, is similar to that with greater than monolayer coverage; the most significant difference is the different relative intensities of the three loss features. For purposes of comparison, we also calculated the loss spectra of vitreous silicon dioxide from eq. (1) using its dielectric function [60,61]. This calculation yielded peaks at 60, 100 and 145 meV, in good agreement with the loss spectra of a thin layer of silicon dioxide on bulk silicon calculated by Ibach et al. [34] who used a dielectric layer theory. Because the vibrational and valence electronic spectra of the second surface oxide closely resemble those of bulk silicon monoxide, the local structure of this surface oxide is probably very similar to bulk silicon monoxide. In a study of 0, adsorption on Si (111) at 973 K which employed different experimental techniques, Bianconi and Bauer [7] drew a similar conclusion. We therefore refer to the second surface oxide as bulk-like surface monoxide. Although our two oxide interpretation can be questioned, we will show that it gives a view of the initial stages of oxidation of the Si (111) 7 x 7 which is consistent with the results of several other workers.

b

200

100 ELECTRON

ENERGY

LOSS

(meV)

Fig. 12. Comparison of the experimental electron energy loss spectra (solid line) of the surface oxide with that predicted by eq. (1) using the dielectric function of silicon monoxide from ref. 159) (dashed line): (a) 3.7 L of 0, adsorbed at 20 K (fig. le); (b) 1.3 L of O2 adsorbed at 20 K and heated to SO K (fig. 7b).

While the nature of the bulk silicon monoxide is still controversial, some early [62,63] and some recent studies [7,64] have concluded that this material is a uniform amorphous chemical compound, rather than either a microscopic mixture [65,66] of silicon and silicon dioxide or a randomly bonded network [67] of silicon and oxygen. Yasaitis and Kaplow [64] and Bianconi and Bauer (71 concluded that their X-ray diffraction data was consistent with the interpretation that the chemical structure of silicon monoxide is that of a heterocyclic ring molecule (SiO),. Yasaitis and Kaplow 1641 also noted the similarity of the infrared spectra of bulk silicon monoxide and the heterocyclic molecule (SiO), , isoelectronic to benzene, which has been observed in matrix isolation experiments [53-55,68,69]. The three observed infrared absorption bands of (SiO),, all vibrations in the plane of the molecule, occur at 39, 78, and 121 meV. A larger polymer, either the tetramer or the pentamer, observed in the same

A.J. Schell-Sorokin, J.E. Demuth / LAW temperature oxidation of Si (111) 7 X 7

291

experiments, has its strongest infrared absorption band at 125 meV; it is thought that this polymer may also be a ring molecule [54]. We speculate that the three loss features of the surface silicon monoxide also correspond to in plane vibrations of still larger heterocyclic polymers on the surface. Such ring structures may be considered a special class of monoxide bridge. Although one might doubt the existence of such complex amorphous structures forming on the surface at 20 K, it is reasonable that the large exothermicity of the silicon surface oxidation reaction [44] permits substantial diffusion and rearrangement of surface species. That the surface oxide layer is indeed amorphous is indicated by the highly intense diffuse background and the complete disappearance of the 7 X 7 pattern observed by LEED for the surface dosed with 0.8 L of 0,. In addition to the behavior observed by LEED, many other aspects of this oxidation reaction, such as the occurrence and coverage dependence of multiple phases, also indicate its complexity; therefore, we do not propose a detailed structural model of the silicon-surface oxide interface. A striking observation of our low temperature adsorption study of 0, on Si (111) 7 x 7 surface is that a langmuir exposure of molecular oxygen reacts to form a considerably quantiy of surface oxides at 20 K while at room temperature a similar dose of oxygen is known to produce very little. We have found using UPS, for example, that a 0.5 L exposure at 20 K produces as much oxide as at least a 20 to 100 L exposure at room temperature. This behavior is quite different from that of many metals [70,71] where about as much of the langmuir exposure of oxygen is adsorbed on the room temperature surface as on the low temperature one. Although completely passivated by a langmuir exposure at 20 K, the silicon surface initially (up to about 0.1 L) has such a high reactivity toward adsorbed molecular oxygen that silicon oxides only and no physisorbed molecular oxygen are observed as is shown in fig. 2. This implies a large impediment at room temperature to the surface chemical reaction process which is overcome at low temperature, and therefore, the effective activation energy barrier of this reaction is negligibly small. At room temperature, the low reactivity of the surface may be due to steric or kinetic factors which become significant when the molecule has a short residence time on the surface. The high reactivity of the surface at low temperature may be due to high lateral mobility of the adsorbed molecular oxygen. Some evidence for the occurrence of some mobile intermediate or metastable species is demonstrated in fig. 2: the relative intensities of the 152 and 128 meV features grow linearly with exposure up to about a langmuir where the peak intensities are maximum. The linearity of this coverage dependent behavior is suggestive of that found for cesium adsorbed on tungsten where the sticking coefficient is constant up to a monolayer coverage [72]; such behavior, commonly observed in low temperature systems, motivated the development of mobile intermediate or precursor theories, recently reviewed by Morris, Bowker, and King [73]. Although the many mobile precursor theories differ in detail, several [74-781

292

A. J. &hell-Sorokin,

J. E. Demuth / Low temperarure oxidaiion of Si (I I I) 7 X 7

predict analogous expressions for the temperature (zero coverage limit) sticking coefficient, sa, SO =akc+s(T)/[kdis(T)

+k,es(‘)]

T

where (Yis the probability of adsorption state and k,,,(T) and k,,,(T) are the from the precursor state, respectively. temperature independence below 300 K kdes ( T )/kdls ( T) = ( Z+JZdis

dependence

of the initial

(2)

from the gas phase into the precursor rates of dissociation and desorption In the context of these models, the of so for metal surfaces implies

> exp [ - ( &es - Edis)/R ~1 +I 1.

(3)

The presence of the strong temperature dependence of so for the silicon surface implies that Edes- Edis is small; if reasonable values are substituted into eq. (2) then a plausible temperature dependence for the sticking coefficient results. For example, if Zdes/Zdis = 14 and Edes- Edis= 500 cal/mol, then so/a is 1.0 at 20 K, 0.5 at 100 K, and 0.15 at 300 K. This latter value is equal to the room temperature low coverage sticking coefficient measured by Carosella and Comas [47]. The conclusion drawn from the application of this simple theory is that the difference between the activation energies of desorption and dissociation from the precursor state is smaller for silicon than for many metal surfaces. While our study is of 0, adsorption on the silicon surface at 20 K, we find evidence in the results of previous studies that our conclusions for near monolayer coverages may be valid over a large range of surface temperatures; we suggest that on the Si (111) 7 X 7 surface below 475 or 575 K only the diatomic and the bulk-like surface monoxides occur and that between this temperature and approximately 975 K only the bulk-like monoxide occurs. Recent AES studies by Fiori [18] of 0, adsorbed on the Si (111) 7 X 7 surface at room temperature show two oxides with desorption temperatures of 570 and 949 K (as measured with a thermocouple). Further support may be gained upon examination of the loss spectra of Ibach et al. [34] for low coverage of 0, on Si (111) 7 x 7 surfaces where two surface oxides are distinguished at 100 and 300 K and one at 700 K. Although their results were interpreted in a very different way, our observations are identical to theirs; our loss spectra show nearly the same peak positions with the same relative intensities and coverage dependence. Finally, we note again the conclusion of Bianconi and Bauer [7] that a surface oxide with the local structure of bulk silicon monoxide forms on Si (111) with low coverages of 0, at 973 K. A survey of the results of other workers also provides evidence that a bulk-like surface monoxide is the oxide formed on the cleaved Si (111) 2 X 1 at room temperature at low coverages of molecular oxygen. In the EXAFS study of St&r et al. [24,25], the silicon to oxygen bond length for 0, adsorbed on the cleaved Si (111) surface at room temperature was measured to be 1.65 + 0.03 A. This value is in good agreement with the 1.64 A bond length reported by

A.J. Schell-Sorokin,

J.E. Demuth

/ LAW temperature

oxidation of Si (1 II) 7 X 7

293

Table 2 Silicon oxide bond lengths Si-0 Bond

Bond length (A)

Ref.

SiO (diatomic) SiO (bulk)

1.51 1.64 1.63 1.62 1.607, 1.611 1.601, 1.608 1.61 1.65

179,801 i64] -

SiO, (vitreous) SiO, (a-quartz) SiO, (a-cristobalite) SiO, (P-cristobalite) 0, adsorbed on Si (111)

WI W,W [WW W,851 [82,861 [24,251

Yasaitis and Kaplow [64] in their X-ray diffraction study of a well characterized powdered sample of silicon monoxide, and is in fair agreement with the 1.63 A bond length reported by Ueno et al. [81] in their neutron diffraction study. Table 2, which shows the silicon to oxygen bond lengths for several bulk oxides of silicon, indicates that silicon monoxide has a bond length which is closest to that reported by Stohr et al. [24,25]. Further evidence for the formation of the bulk-like surface monoxide by 0, adsorption on the room temperature cleaved Si (111) surface is provided by the EELS studies of Ibach and Rowe [31] and Ibach et al. [33]. When exposed to molecular oxygen, the cleaved Si (111) 2 X 1 surface at room temperature exhibits nearly the same loss peaks, except for the 152 meV feature, as the 7 X 7 surface. Although Ibach et al. [34] ascribe the absence of the 152 meV feature to the lower resolution (14 meV) of the earlier studies [31,33], we see this peak clearly with our comparable resolution. We suggest that the 152 meV peak is unique to the 7 x 7 surface and is perhaps associated with the availability of specific sites that allow the formation of diatomic SiO and its bonding to silicon atoms on the surface. We note that silicon adatoms [87] on the 7 X 7 surface would provide an appropriate site to produce and bond such SiO groups.

5. Conclusion We have presented evidence that the apparent variability of position silicon oxide by the adsorption 7 surface is due to the of this loss with that of the hydroxyl contaminant which frequently appears features can be identified silicon monoxides, diatomic molecule adsorbed onto the and a bulk-like surface which has the local structure of bulk monoxide. We have presented a comparison between vibrational spectra and calculated function which

294

A.J. Schell-Sorokln,

J.E. Demuth / Law temperature

oxidatron

of Si (111)

7x 7

support the identification of the latter monoxide. From our low temperature adsorption studies we construct the following view of the initial stages of oxidation of silicon. At exposures of about 0.1 L and less, molecular oxygen dissociatively chemisorbs to form diatomic silicon monoxide and a precursor to the bulk-like surface monoxide. At somewhat higher exposures, 0.2 L and above, molecular oxygen is physisorbed as well; the features of the bulk-like monoxide are observed and no remnant of the precursor is apparent in the spectra. Both monoxides continue to form up to about a langmuir exposure where the surface ceases to react with molecular oxygen. At this coverage the 7 X 7 reconstruction has disappeared, and the surface is disordered. Finally, at an exposure of a few langmuirs, the spectrum of the diatomic molecule is obscured and only the spectra of the bulk-like monoxide and physisorbed molecular oxygen are seen. While the surface reactivity towards molecular oxygen is negligible above an exposure of about a langmuir, heating the surface to a temperature as low as 50 K produces some further reaction to form more of the bulk-like monoxide, possibly by allowing diffusion of molecular oxygen through the surface layer. The two silicon monoxides formed by the low temperature adsorption of 0, at low coverages are surprisingly stable. The diatomic silicon monoxide is stable to a temperature between 475 and 575 K, and the bulk monoxide is stable to approximately 950 K. This oxide completely desorbs at approximately 975 K. These observations of high temperature stability and high desorption temperature of a thin layer of the surface oxide formed by 0, adsorption on Si (111) agree with those of Bianconi and Bauer [7]. Upon examination of the literature, we suggest that the diatomic and bulk-like monoxides are the only oxides produced by adsorption of near monolayer coverage of molecular oxygen on the Si (111) 7 X 7 surface.

Acknowledgement It is a pleasure graphical dielectric

to thank data.

Arthur

Appel

for the sonic

digitization

of the

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