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J. Phys. Chem. B 2000, 104, 6576-6583
Reaction of H2S with Si(100) M. Han, Y. Luo, N. Camillone, III, and R. M. Osgood, Jr.* Columbia Radiation Laboratory, Columbia UniVersity, New York, New York 10027 ReceiVed: January 20, 2000; In Final Form: May 4, 2000
The dissociative adsorption of H2S on Si(100) and the subsequent desorption of hydrogen from and diffusion of sulfur into the same surface have been investigated using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Desorption of hydrogen occurred at 546 °C from surfaces exposed to H2S at temperatures ranging from -145 to 425 °C; an additional H2 desorption channel at 433 °C was seen for the -145 °C case. A comparison of this behavior with desorption from atomic-hydrogen-dosed Si(100) indicated that these features are due to the desorption of surface monohydride and dihydride species, respectively. In the TPD studies, the surface was saturated by 0.5 ML of H2S for all substrate exposure temperatures. No desorption feature attributable to sulfur-related species was observed for any of the surface conditions. However, AES measurements revealed a sharp decrease in the concentration of sulfur at the surface over the temperature range of 525-625 °C, indicating that H2 desorption is accompanied by diffusion of sulfur into the Si crystal. The exponential decay of the sticking coefficient derived from the coverage dependence of the H2S adsorption at 25 °C is consistent with a two-step model for the adsorption kinetics.
I. Introduction The understanding and control of the interaction of atoms and molecules with silicon surfaces are of great importance both for fundamental science and for technological applications in silicon-based electronics and optoelectronics. Whereas the adsorption of water on Si(100) has been extensively studied both experimentally and theoretically,1-4 the surface chemistry of its sulfur homologue, H2S, has been investigated on the same surface far less extensively.5-7 These studies have, however, shown that the chemistry of both compounds involves a facile reaction with the Si(100) dangling bonds. In addition, the reaction of H2S with Si may be relevant in certain areas of advanced electronic device technology. For example, H2S has been used as a precursor for CVD growth of groups II-VI compound semiconductors such as ZnS and CdS.8,9 These sulfur-containing crystals have wide band gaps, e.g., 3.68 eV for ZnS compared to 1.12 eV for Si. ZnS is thus a good candidate for a barrier material for Si double-barrier resonanttunneling diodes (DB-RTDs).10,11 In such devices, the chemical bonding at the interface strongly affects the first few monolayers of the heteroepitaxial growth and, hence, influences the properties of these tunneling devices. Thus, the surface chemistry of H2S on Si(100) that occurs during CVD growth of such thin films will impact device fabrication and performance. As indicated above, relatively few studies of the adsorption of H2S on Si(100) have been undertaken.5-7 In the most recent of these, Rezaei et al. reported STM observations of the dissociation of individual D2S molecules on Si(100), induced both by electrons and by thermal excitation.5 Their results show that, at low coverage, D2S dissociates only partially on Si(100) at temperatures less than -125 °C, yielding a DS moiety bonded between two dimer rows and a lone deuterium atom bound to a dimer near the DS. Above -75 °C, the dissociation of DS occurs thermally at an appreciable rate, such that, at room * Corresponding author. E-mail:
[email protected]. Fax: (212)860-6182.
temperature, D2S is fully dissociated on the surface into one S and two D atoms. In contrast, electrons injected from the STM tip induce DS dissociation even at low temperature. These results are in accord with earlier UPS measurements made by Schro¨derBergen et al.6 Their results showed that, at -125 °C, saturation coverage is 0.5 ML of H2S, where 1 ML of atoms (or molecules) is defined as one atom (or molecule) per Si surface atom. In addition, the presence of a constant sticking coefficient at -125 °C indicated mobile precursor kinetics. At 275 °C, the adsorption kinetics were found to have changed, and an initial adsorption was observed, followed by a slower adsorption that exhibited no clear saturation.6 In the present paper, the dissociative adsorption of H2S on Si(100) and subsequent thermal processes have been investigated using temperature-programmed desorption and Auger electron spectroscopy. At room temperature, the initial adsorption is fast, but the sticking coefficient decreases rapidly with coverage. Saturation for H2S adsorption is reached at a coverage of 0.5 ML; the coverage is calibrated in separate experiments using an atomic-hydrogen-dosed surface. The same saturation coverage was observed for adsorption at -145 °C. However, in this latter case, an extra H2-desorption feature appears, suggesting that some of the chemisorbed species have a different arrangement on the surface than those adsorbed at room temperature. Combining these observations with LEED measurements allows us to propose possible adsorption and reaction mechanisms for the H2S/Si(100) system. II. Experimental Section The experiments were performed in a UHV chamber, which had a base pressure of 5 × 10-11 mbar, achieved with a 550 L s-1 turbomolecular pump and a titanium sublimation pump. The chamber was equipped with a differentially pumped quadruple mass spectrometer for temperature-programmed desorption (TPD), a hemispherical energy analyzer for Auger electron spectroscopy (AES), a low-energy electron diffraction (LEED) system, and an ion gun for sputter cleaning of the sample.
10.1021/jp0002446 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000
Reaction of H2S with Si(100)
Figure 1. Illustration of the Si sample mount used in this work. (a) The 0.025-mm thick Mo-foil heater was fabricated using the following procedure: (i) a pattern is printed directly on the front of the foil using a laser printer, (ii) the back of the foil is protected with a coat of VacSeal, (iii) Mo is etched in a HNO3:H2O ) 1:1 solution, and (iv) the laser printer toner is removed by acetone. (b) The sandwich-type mounting is achieved by using two tungsten clips to hold the Mo heater between two Si crystals. The thermocouple is spot-welded to a tab extending from the Mo heater.
The Si(100) samples (7.5 mm × 10.5 mm × 0.35 mm) were cut from p-type wafers (B-doped, with a resistivity of 0.010.02 Ω cm). A special sample-mounting procedure was designed for our manipulator, as illustrated in Figure 1. A patterned Mosheet heater (0.025 mm thick) was sandwiched tightly between two Si crystals held together by two tungsten clips. Uniform heating of the sample to high temperature (above 1200 °C) was achieved by passing current through the Mo heater, which has a high resistivity provided by the designed pattern. The sample temperature was measured by a K-type (chromel-alumel) thermocouple spot-welded to a tab near the middle of one side of the Mo heater. This configuration provided very good thermal contact and an accurate temperature reading. The sample was cleaned by successive sputter-anneal cycles until the well-known two-domain (2 × 1) Si(100) pattern was clearly observable at room temperature and no contamination features were seen in the Auger spectra. During routine measurements, before H2S or atomic H adsorption, the sample was subjected to 2-keV Ar+ bombardment for 15 min with an ion current of ∼4 µA and then annealed at 850 °C for 8 min and 900 °C for 3 min, followed by cooling at a rate of ∼2 °C s-1 to the desired temperatures. H2S (Matheson, 99.5%) was purified by repeated freeze-pump-thaw cycles. Two methods were used to introduce H2S onto the crystal surface. In one series of experiments, suitable for low exposures, the sample was exposed by backfilling the chamber through a leak valve that was not in the line of sight of the sample surface. In this case, values for the exposure are estimated simply by multiplying the exposure time by the overall chamber pressure (the uncorrected ion gauge reading), which was constant during dosing. In a second series of experiments, a dosing system was used to allow delivery of higher dosages within a reasonable time frame. In this case, a leak valve was used to meter the flow of H2S into a stainless steel tube. The outlet of the tube was positioned ∼1.5 cm from the sample surface. Exposures were monitored by measuring the background pressure with the ion gauge. However, an estimation of the exposure for comparison of this second series with the first series of measurements cannot be made directly from the ion gauge reading because the doser generates much higher local pressures in the vicinity of the surface than the ion gauge reading would indicate. Thus, calibration of the exposure achieved using the doser was made by cross checking the amount of adsorbed H2S, as measured by TPD, with that obtained by backfilling. This comparison of the two series of TPD measurements indicated that the actual exposure obtained with the doser at a given chamber pressure was 1 × 103 higher than that obtained by backfilling. For all exposures quoted below (in units of L, where 1 L ) 1 × 10-6
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6577
Figure 2. Hydrogen desorption spectra taken from the Si(100) surface after saturation doses of atomic H (dashed line) and H2S (solid line) at room temperature. Two features at 523 °C and 423 °C in the spectrum for H-covered surface correspond to β1-H2 and β2-H2 desorption, respectively. The single feature at 546 °C in the spectrum for the H2Scovered surface is attributed to β1-H2 desorption.
Torr s), this factor was taken into account. For dosing of atomic hydrogen, a W spiral filament at ∼1800 °C was used to generate atomic hydrogen in the chamber during H2 backfilling. The quadrupole mass spectrometer (QMS) was enclosed in a water-cooled shroud for TPD measurements. To ensure that only species desorbing from the central area of the prepared surface were detected, a 3-mm-diameter aperture was mounted on the shroud in front of ionizer. The sample was accurately and reproducibly placed ∼2 mm away from the aperture. A linear temperature ramp of 2 °C s-1 was used in all TPD measurements. Throughout our experiments, AES measurements and LEED observations were made after the sample was heated to specific temperatures. The procedure was, first, to heat using the linear, 2 °C s-1, heating rate. When the desired temperature was reached, the substrate heating was abruptly stopped, and the sample was immediately allowed to cool to room temperature. This heating procedure was followed to allow for a direct comparison between the surface composition and structure and the corresponding desorption process following a wellestablished thermal excursion. III. Results 1. TPD Measurements. A study of thermally activated desorption was made as a function of the H2S coverage and the temperature of the Si surface during H2S exposure. All possible desorption species, i.e., S, S2, SH, H2S, SiS, SiH, SiH2, SiH3, SiH4, and H2, were monitored during TPD measurements; however, only desorption of H2 was observed. In fact, desorbed H2 was measured over the full range of coverage and surfacedosing temperatures studied here, i.e., -145 to 425 °C. Figure 2 shows a comparison of the hydrogen desorption spectra, at room temperature, for clean Si(100) saturated with either H2S or atomic-H exposures. Because hydrogen adsorption on Si(100) has been widely studied both experimentally and theoretically,12-16 experiments with atomic-H exposure were undertaken to serve as a “calibration” in our studies and to provide a direct route for introducing one of the surface product species onto the surface. Specifically, Cheng et al.12 reported using temperature-programmed desorption mass spectrometry to differentiate the distinct binding states existing on a Si(100) surface and to determine the surface coverage after H adsorption. Their experiments showed two major desorption features in the
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Figure 3. Hydrogen desorption spectra taken from the surfaces after saturation with H2S at room temperature (dashed line) and -145 °C (solid line). Two features are visible in the spectrum recorded for the surface dosed at -145 °C. The major feature at 546 °C is located at the same temperature as the single desorption feature for a surface dosed at room temperature. The smaller low-temperature feature at 433 °C is attributed to β2-H2 desorption.
TPD spectra of atomic hydrogen on Si(100) chemisorbed at room temperature; these features were designated as β1-H2 and β2-H2 and were assigned to surface monohydride and dihydride, respectively. The β2-H2 desorption process at ∼425 °C involves the desorption of molecular hydrogen, 2SiH2(ads) f H2(g) + 2SiH(ads), and leaves a full monolayer of monohydride on the Si surface. Therefore, the area of the β1-H2 desorption peak at ∼525 °C serves as an absolute calibration of the amount of hydrogen on Si(100). In our experiments, only a single H2 desorption peak at ∼545 °C was observed for the surface exposed to H2S at room temperature, which implies that only a single desorption channel exists. Further, this feature has the same shape and area at saturation as the saturated β1-H2 feature, seen in the dashed line in Figure 2, indicating the presence of Si monohydride and a surface coverage of one monolayer of hydrogen from H2S, i.e., θsat ) 0.5 ML for H2S. The ∼20 °C shift to higher temperature (see Figure 2) is evidence of a perturbation of the Si-H bond energy due to the presence of adsorbed sulfur on the Si surface. The role of sulfur during the thermal process will be discussed further in connection with the AES measurements. To compare the adsorption behavior of H2S at low temperature with that at room temperature under our experimental conditions and to understand the adsorption mechanisms that had been observed in previous STM and UPS studies at low temperatures, TPD measurements were made for Si surfaces dosed with H2S at -145 °C. Again, desorption of H2 was seen in the TPD spectrum after dosing with H2S (Figure 3). The initial uptakes, i.e., coverage versus exposure, of H2S on Si(100) both at room temperature and -145 °C, as determined from H2 TPD measurements, were found to be similar. This result suggests that the initial sticking coefficients on the two surfaces are comparable. An earlier UPS6 study showed that the initial sticking coefficient at -125 °C is nearly unity. In addition, previous STM and UPS observations showed that, at this temperature, surface hydrogenation occurred.5,6 These results indicate a low activation barrier for dissociative adsorption of H2S throughout the temperature range examined here. In contrast to the H2 desorption spectrum recorded after dosing at room temperature, an additional feature was seen at ∼435 °C; this peak is similar to the β2-H2 desorption feature seen for a dihydrided Si surface. However, the major desorption channel still remains that at ∼545 °C. The total integrated area of these
Figure 4. Coverage of H2S, calibrated by a TPD spectrum of an atomic-H-covered surface, versus H2S exposure on Si(100) at room temperature. In the top panel (a), the coverage dependence is plotted on a linear scale. In the bottom panel (b), the data (•) are plotted on a semilog scale along with curves obtained by using a double secondorder adsorption kinetics model (solid) and a fit to the Elovich equation (dashed).
two desorption peaks is within about (10% of the integrated area for the single desorption feature observed for the roomtemperature-dosed surface, when both are exposed to their saturation doses of H2S. Further, the fact that the same amount of desorbed H2 is observed for both the -145 and the 25 °C samples indicates that Si(100) is also saturated by 0.5 ML of H2S at -145 °C. However, the observation of two TPD features after dosing at -145 °C suggests that the adsorption geometry and/or adsorbed species are different from those formed during room-temperature adsorption. The dependence of the peak intensity of the H2-desorption signal upon H2S exposure at room temperature is displayed in Figure 4. The data in Figure 4a show that the initial, rapid adsorption of H2S is followed by a much slower, coveragedependent adsorption phase. A saturation coverage of θsat ) 0.5 ML of H2S is eventually reached at relatively high H2S exposure (see Figure 4a). When coverage versus H2S exposure is plotted using a semilog scale (Figure 4b), the adsorption process is seen to scale linearly with exposure. This dependence on the logarithm of the exposure implies that the sticking coefficient decreases exponentially with H2S exposure. Such a curve cannot be interpreted via recourse to simple, single-step Langmuir adsorption kinetics (for more detail, see the Discussion Section). Note, however, that such a strong variation in the sticking coefficient differs from the adsorption kinetics of H2O on Si(100) at room temperature. In that case, a UPS study by Ranke showed that the dissociative adsorption of water at room temperature has a constant sticking probability up to saturation coverage, indicating that adsorption occurs via a mobile precursor state.1 The dependence of the integrated H2-desorption signal, from a saturated H2S-exposed surface, on the substrate temperature during dosing was also investigated by TPD. In all cases, desorption of H2 was found to occur at the same temperature,
Reaction of H2S with Si(100)
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Figure 5. Integrated area of the H2 desorption feature versus the temperature of the substrate during the H2S exposure. The data reveal that saturation coverage is essentially independent of dosing temperature. TPD spectra (data not shown) show that the desorption temperature is also independent of dosing temperature.
Figure 7. (a) Variation of the Si Auger feature position with the surface temperature for the H2S-covered surface. (b) Expanded view of the transition region showing a variation in the rate of change of the Auger feature position with annealing temperature.
Figure 6. Ratio of S (LMM) to Si (LMM) Auger peak intensity, H2 TPD spectrum on H2S-saturated surface, and surface H coverage derived from the TPD data are plotted versus the surface temperature. The surface H coverage is obtained from the integrated area of the H2 desorption peak and normalized by saturation coverage at 1.0 ML of H, as described in the text.
i.e., ∼545 °C, as that found after dosing at room temperature. Figure 5 shows that the amount of H2 desorption is constant for deposition temperatures between room temperature and 425 °C. The slight decrease of desorption at 425 °C can be explained by the fact that this temperature is near the desorption temperature for H2, thus reducing the absolute value of the coverage at saturation for ∼425 °C. 2. AES Measurements. AES was used to monitor the dependence of the surface sulfur concentration on temperature and thus to correlate sulfur concentration with hydrogen desorption. AES measurements were made after the crystal was heated to a specified temperature and cooled to room temperature, as described in the Experimental Section. Figure 6 shows the ratio of the S (LMM) and Si (LMM) Auger peak heights (large dots) versus temperature. On the same figure, the surface coverage of hydrogen (solid line), derived from TPD measurements (small dots), is plotted for comparison. The surface H coverage is easily extracted from the TPD data, as, at temperature T, the coverage is proportional to the integrated area under the H2 desorption curve from T to 800 °C. The proportionality constant used in Figure 6 was selected such that the H coverage corresponding to the integrated area under the entire H2 desorption curve is equal to 1.0 ML, as calibrated by the atomichydrogen-dosed H2 TPD measurements (Figure 2).
A careful examination reveals that both the S and the Si signals, as well as the S/Si ratio, slightly increased after heating to temperatures below ∼500 °C. The increase of both the Sisubstrate and the S-adsorbate Auger signal intensities can be explained by an improvement in surface order that occurs as a result of heating. An increase in surface order can result in an increase in the intensity of an Auger signal because of coherent interference effects between the outgoing emitted electron wave and the elastically scattered wave from atomic centers in the vicinity of the emitter.17 This slight increase is followed by a dramatic decrease in the S/Si ratio in the range from 525 to 625 °C. This behavior is very similar to that observed earlier for elemental sulfur deposited on Si(100) by thermal dissociation of MoS2 with a tungsten filament.18 Because no detectable S-related species was seen during our TPD measurements, the fact that sulfur disappeared from the Si surface indicates that the sulfur diffuses into the Si substrate. Furthermore, note that, in our experiments, the change in the surface-hydrogen coverage closely follows that of the sulfur. This behavior raises the question as to whether hydrogen desorbs first or sulfur diffuses into the bulk Si first. In this connection, Figure 6 shows that the sulfur decay curve was observed to lag behind the hydrogen desorption curve by ∼60-70 °C. However, we cannot confidently assert that hydrogen desorption precedes sulfur diffusion as sulfur can be detected to a depth of several monolayers during diffusion into the Si bulk; the mean free path of the sulfur Auger electron (LMM at 154 eV) is ∼2.5 monolayers. Without detailed information regarding the sulfur diffusion profile, we cannot rule out the possibility that the apparent lag is simply due to the sensitivity of Auger spectroscopy to subsurface sulfur. A reasonable interpretation of the data is that the two processes occur simultaneously. Finally, an increase in the Si Auger chemical shift was observed as the surface was heated to progressively higher temperatures. Figure 7a shows the energy shift of the differential Auger Si (LMM) peak versus the heating temperature of the Si substrate. The Si (LMM) was observed at 94.1 eV for the clean surface. The peak was observed to shift to 92.8 eV after the
6580 J. Phys. Chem. B, Vol. 104, No. 28, 2000 surface had been saturated with H2S at room temperature, consistent with the formation of Si-S bonds. As the surface was heated to successively higher temperatures, the Si (LMM) peak shifted back to the peak position observed for the clean surface. This process occurred at 500-600 °C. The data in the Figure 7b show that the rate of this energy shift varies as the temperature increases. Such a change in rate would be consistent with changes in the interaction of S with Si. For example, initially, the local environment of the sulfur may change as the S moves from the surface to the subsurface, followed by a slower change as sulfur diffuses into the Si bulk. Note that the temperature range over which the shift was observed is the same as that for hydrogen desorption and sulfur diffusion. There are several possible explanations for how the loss of surface hydrogen could correlate with the onset of S penetration into bulk. The kinetics of the desorption of H2 from the otherwise bare Si(100) have been carefully studied and are currently understood to be limited not by the diffusion of H but rather by the reaction of H atoms to form H2.13 It seems reasonable, then, that the presence of S interferes with this reaction, as would be expected given the proposed H2S adsorption geometries (see Discussion). Thus, desorption of H2 from an H2S-dosed surface might be limited by the diffusion of S away from key surface sites. 3. LEED Patterns. It is well-known that the LEED pattern for clean Si(100) shows a well-defined (2 × 1) reconstruction at room temperature; we observed such LEED patterns from bare Si(100) after surface preparation. At room temperature, the (2 × 1) periodicity remained after the surface was saturated with H2S. In fact, the pattern remained sharp and intense until the sample was flashed to the temperature at which hydrogen desorbed from the surface (∼545 °C). After the sample was heated to ∼545 °C, the diffraction spots decreased in intensity and became more diffuse; there also was a concomitant increase in the intensity of the background. This degradation of the LEED pattern is attributed to surface disorder resulting from hydrogen desorption and sulfur diffusion. Upon heating of the sample to ∼650 °C, the quality of the (2 × 1) LEED pattern reverted to that observed for the clean surface. IV. Discussion 1. Adsorbate Site and Chemical State. In this section, we attempt to relate the TPD and LEED data to a specific adsorbate geometry and site on Si(100). Before discussing the case of H2S on Si(100), however, we note that the adsorption and reaction of water on Si(100)-(2 × 1) are comparatively well studied and provide a useful reference for comparison. For H2O adsorption, there is general agreement that the dominant process below ∼125 °C is dissociation into OH and H, which saturates the two dangling bonds of one dimer;1 each dimer thus acts as one adsorption site. LEED measurements also have shown that the (2 × 1) periodicity remains for all coverages. At a surface temperature in excess of ∼225 °C, surface Si-OH decomposes rapidly to give Si-O-Si and Si-H. Above 400 °C, the surface loses hydrogen as H2, and above 600 °C, surface oxygen desorbs as SiO.19 In addition, experiments using deuterated water showed two desorption products, D2 and SiO, at 545 °C and 685 °C, respectively.3 Flowers et al. also found that the D2 desorption feature was similar in appearance to that for β1-D2 desorption from purely deuterated Si(100).3 The peak temperature for D2 desorption was also dependent on the initial water coverage and occurred ∼25-30 °C higher for the D2O-saturated surface than that for the D-covered surface. The increase in the activation energy for desorption via the β1-D2 channel was attributed to a cumulative interaction with adsorbed oxygen.
Han et al.
Figure 8. Schematic depiction of the proposed adsorbate geometry following adsorption of H2S on Si(100) for the following conditions: (a) e0.25 ML of H2S at room temperature, (b) >0.25 ML of H2S at room temperature or after the rearrangement of the adsorption site in panel c at a temperature just below desorption of surface hydrogen, (c) major adsorption site at -145 °C, (d) minor adsorption site at -145 °C, and (e) after the rearrangement of the adsorption site in panel d at a temperature just below desorption of surface hydrogen.
As shown in the previous section, the adsorption behavior of H2S on Si(100) differs from that of H2O in several important aspects. In the following discussion, we compare and contrast the behavior of H2O and H2S and suggest possible surface adsorption and desorption channels for H2S/Si(100). (i) Room-Temperature Adsorption. At room temperature, our observations, namely, saturation at 0.5 ML of H2S coverage and an exponential decrease in the sticking coefficient, suggest that the adsorption of H2S on Si(100) may involve two different dissociative pathways for low and high coverage. At low coverage (e0.25 ML of H2S), the dominant adsorption channel can be attributed to sulfur bridge-bonding between two adjacent Si dimers, with the two remaining dangling bonds of these dimers being saturated with two hydrogen atoms (Figure 8a). The dissociation products of each H2S molecule would then fully occupy the dangling bonds of two adjacent Si dimers, and the strong σ bond linking the two Si atoms in the dimers would remain intact. We favor this adsorption geometry, illustrated in Figure 8a, because it is similar to the geometry observed in the STM measurements of Rezaei et al. for very low coverage D2S on Si(100)-(2 × 1) at room temperature.5 This geometry is also consistent with the results of theoretical calculations by ab initio and MNDO methods, which suggest that one of the most energetically favorable geometries for dissociative H2S is S (bridge) + 2H (on-top).7 Note that the configuration sketched in Figure 8a would be expected to give a (4 × 1) LEED pattern at 0.25-ML coverage of H2S, whereas, in fact, we observed a (2 × 1) LEED pattern at this coverage. In fact, other reasonable choices of adsorption configurations also would not yield a (2 × 1) LEED pattern. However, one possible explanation for our observations is that the (2 × 1) reconstruction of the Si surface is preserved when the Si dangling bonds are capped with the dissociated H2S. This situation would occur if the adsorbate layer did not possess long-range order and the diffraction pattern reflected the (2 × 1) reconstruction of the substrate surface layer. A similar phenomenon has been observed in other cases, e.g., the (1 × 1) LEED pattern seen at high H coverages is due to the underlying Si(100) subsurface structure, as observed through a highly disordered surface.12,16 In contrast, our TPD measurements clearly indicate that, at saturation coverage, the H2S overlayer cannot consist of molecules adsorbed in the geometry of Figure 8a. Recall that, at H2S saturation coverage, our measurements of H2 desorption showed a full monolayer of hydrogen present, whereas from
Reaction of H2S with Si(100) Figure 8a, we would predict a saturation coverage of only 0.5 ML of hydrogen. Assuming that the H2S is fully dissociated, a coverage of one monolayer of hydrogen, i.e., one adsorbed hydrogen per surface Si, cannot be achieved without breaking the Si dimer bond. For this reason, we propose that, at high coverage (g0.25 ML of H2S), Si dimer bonds are broken by the addition of a second H2S to surface sites composed of one H2S molecule preadsorbed on two adjacent Si dimers (Figure 8a). Thus, the adsorption site, depicted schematically in Figure 8b, begins to be populated. Occupation of this site at saturation would yield 1 ML of H, as each Si bonds to one H and every two neighboring Si share one S. This adsorption geometry is also consistent with the (2 × 1) LEED pattern that we observed at saturation coverage, although again it is unclear whether we are merely probing the substrate surface order through a disordered upper layer. A similar occupation of the Si dimer bond by an adsorbate has been seen for Si(100) after atomic H adsorption: specifically, at first, hydrogen bonds to the dangling bond of the Si atoms, and then at high exposure, it cleaves the dimer bond.12 We expect the formation of this second site to be kinetically and thermodynamically less favorable than the formation of the first site, thus accounting for the observed exponential decrease in the sticking coefficient with coverage. In addition, note that our observation of a shift of ∼20 °C in the desorption of H2 from the H2S-covered surface compared to that observed from the H-saturated surface is very similar to the shift seen for the case of a D2O-covered surface. This similarity suggests that the adsorbed H2S and D2O may have an analogous dissociative geometry before the desorption of hydrogen. Recall that, for the H2O-covered Si, surface Si-H and Si-OH have been identified following H2O adsorption at room temperature. However, upon heating, the surface hydroxyl (Si-OH) decomposes to bridge-bonded oxygen (Si-O-Si) and an additional Si-H. This step is complete at ∼125 °C.19 These observations with H2O are consistent with our interpretation of the high-coverage H2S dissociation channel and support our proposed geometry, shown in Figure 8b, for high exposure of H2S on a 25 °C Si(100) surface. However, the similarity in the 20 °C shift does not necessarily indicate a similarity in binding geometry but may rather reflect the electron-withdrawing natures of both O and S. Note that the adsorption of H2S at temperatures higher than room temperature showed the same saturation and the same H2 desorption features as those at room temperature. This result suggests that the same adsorption mechanism is operable for temperatures ranging from 25 to 425 °C. (ii) Low-Temperature Adsorption. For adsorption of H2S on Si(100) at -145 °C, our H2-desorption measurements again showed a saturation coverage of ∼1 ML of hydrogen. This coverage is consistent with dissociation of H2S to yield one Si-H and one Si-SH per dimer, as shown in Figure 8c, and is thus analogous to the adsorption of water on Si(100) at room temperature.2 Furthermore, the STM studies by Rezaei et al. showed that H2S partially dissociates to form Si-H and SiSH below -125 °C.5 In our experiments, the major H2desorption feature was found to peak at the same temperature as the β1-H2 desorption channel from a surface dosed with H2S at room temperature. This result indicates that, at some point during the TPD ramp and prior to desorption of hydrogen, surface adsorbates may convert to the adsorption geometry found on a saturated surface at room temperature (see Figure 8b). Vibrational spectroscopies such as IR spectroscopy or HREELS would be required to unambiguously identify this transition by observation of Si-S-Si-related modes and the disappearance
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6581 of the S-H stretching mode. Such has been observed in the case of water on Si(100) by Weldon et al.,2 who showed that oxidation starts with the insertion of an oxygen into the surface dimer bond at approximately 350 °C from the homogeneous H2O-exposed surface, which consists of one H and one OH per dimer. As a result, insertion is an activated process requiring heating to relatively high temperature. We expect that the analogous activated insertion of S into the dimer bond occurs in the case of H2S/Si(100) at a somewhat lower temperature; Rezaei et al. observed the thermally activated full dissociation of H2S to occur at approximately -70 °C,5 much lower than the temperature at which H2O fully dissociates. The extra hydrogen-desorption feature seen in Figure 3, for adsorption of H2S at -145 °C, is located at a temperature similar to that of β2-H2 desorption for a heavily hydrogenated Si surface, Figure 2 (dashed line). The appearance of this feature most likely coincides with the formation of the surface dihydride species.12 The existence of dihydride on the surface may be due to the lack of long-range order in the arrangement of H and SH at low temperature, Figure 8d. A rearrangement of these adsorbates, such as that shown in Figure 8e, would be accompanied by a breaking of the dimer bond; this rearrangement could occur during the temperature increase. An adsorption geometry, such as that shown in Figure 8e, would likely result in H2 desorption through the β2-H2 desorption channel. In addition, note that the sum of the integrated areas for the H2 desorption signals from the H2S-saturated surface at low temperature is about equal to the integrated area for the single desorption feature observed for the room-temperature-dosed surface. The process sketched in Figure 8d and e would also be consistent with this result. 2. Adsorption Kinetics. Our room-temperature adsorption experiment, which spanned seven decades of exposure, reveals very interesting behavior: the quantity of adsorbed H2S scales linearly with the logarithm of the exposure (see Figure 4b). Such a linear dependence on the logarithm of the exposure indicates an exponential (or near-exponential) decrease in the sticking coefficient with coverage. Thus, it is apparent that chemisorption decreases the probability of adsorption at sites “over and beyond actual occupancy.”20 Such behavior, which follows the wellknown Elovich equation, has been observed for a number of systems21 and has been interpreted by Taylor and Thon in terms of a two-step process wherein an initial fast adsorption is responsible for the production of surface sites that decay at a bimolecular rate over the course of the second, slower chemisorpion step.20 The Elovich equation, in differential form, is
dθ/dl ) a exp(-Rθ) where θ is the coverage, l is the exposure, (dθ/dl) is, therefore, the sticking probability, and a and R determine the initial and asymptotic adsorption rates, respectivley. A fit of the integral form of the Elovich equation to our data is given in Figure 4b. The failure at large l is simply due to the fact that the Elovich equation does not account for complete consumption of all active sites at the surface. Alternative explanations for the data have been explored. Attempts to model the kinetics using other simple forms such as single-step first- or second-order Langmuir kinetics fail badly. Because, as shown in Figure 8a and b, dissociative adsorption of H2S at room temperature requires two adjacent dimer sites for both the low-coverage and high-coverage adsorption-site geometries, the coverage dependence of the sticking probability might be expected to be described by two second-order rate laws, dθ/dl ) S0(1 - θ/θS)2, where θS is the saturation coverage and S0 is the initial sticking probability. In fact, a reasonable
6582 J. Phys. Chem. B, Vol. 104, No. 28, 2000 simulation of our data can be obtained by including two adsorption processes, each with a second-order Langmuir isotherm; this simulation is shown by the solid line in Figure 4b. Our fit requires that the two initial sticking probabilities differ by approximately 3 orders of magnitude. The fast adsorption process corresponds to a process that dominates at coverages e0.25 ML of H2S. Near θ ) 0.25 ML, further adsorption via this first process decreases because the surface sites shown in Figure 8a are occupied. The slow adsorption process starts as soon as the adsorption sites formed by the first process are present on the surface and becomes dominant as the number of sites available to the first process vanish. This second process gradually leads to saturation at 0.5 ML of H2S. The Langmuir model used to generate the curve in Figure 4b is quite crude in that the two steps are considered separately. In reality, the second step should begin at the moment the first step produces appropriate reactive sites. A more realistic model that accounted for the simultaneous progress of the two steps should more closely approximate the observed linear trend. A two-step adsorption model is consistent with the conclusions of Schro¨der-Bergen et al. They suggested that, whereas a mobile precursor is important for dissociative H2S adsorption on Si(100) at -125 °C, the kinetics are different at 275 °C.6 At the higher temperature, there are indications of adsorption characterized by an initial fast adsorption with a sticking coefficient of unity, which transitions to a slower adsorption process at a coverage of approximately 1/3 ML. Saturation coverage is not reached even after exposures 20 times that required to saturate the surface at -125 °C.6 Thus, the results of Rezaei et al. (which show a low-coverage geometry corresponding to the adsorption of one H2S molecule for every two Si surface dimers), our determination of the saturation coverage (which corresponds to the adsorption of one H2S molecule per Si surface dimer), our LEED observations, and the adsorption kinetics at room temperature taken together form a consistent picture for the adsorption of H2S on Si(100) involving a twostep process that terminates in an ordered (2 × 1) overlayer, as suggested in Figure 8b. 3. Subsurface Migration by Sulfur. Our experiments have shown that, instead of desorbing from the surface, sulfur diffuses into the Si bulk during heating. In an earlier photoemission study of sulfur on Si(100), Weser et al. found that sulfur penetrated into the bulk crystal and formed a disordered silicon sulfide region.22 The same authors also investigated the behavior of S on Ge(100)-(2 × 1).23 They found that S adsorption removes the (2 × 1) reconstruction to produce a (1 × 1) “ideally” terminated S/Ge surface. A later study by Papageorgopoulos18 then showed that adsorption of S at room temperature caused a gradual change of the reconstructed Si(100)-(2 × 1) to its original bulk-terminated Si(100)-(1 × 1) surface. In this experiment, at low coverage, the S adsorbate initially forms 0.5 ML of an S (2 × 1) ordered layer on the Si(100)-(2 × 1) surface; at high coverage, it subsequently forms a (1 × 1) surface by breaking the Si dimer bond. Above 1 ML, sulfur is inserted into the Si bulk near the surface. The behavior of the S Auger signal as a function of the heating temperature of the Si substrate was the same over the same temperature range as we have observed in our experiments, as was the chemical shift of the Si (LMM) signal, which is essentially the same as the 1.3-eV shift measured in our work. These results indicate that the sulfur is bound to Si and is not obviously affected by the presence of coadsorbed H. Finally, note that, in the case of the D2O-dosed Si(100) surface, desorption of SiO occurred at ∼685 °C,3 a result that
Han et al. is similar to those obtained following oxygen adsorption on Si(100).24 The desorption behavior of SiO for a pure oxygencovered surface was simulated using first-order kinetics, and an activation energy of ∼75-80 kcal/mol was found.24 This value was attributed to the energy involved in breaking the Si back-bonds. The bond strength of Si-O (∼195 kcal/mol) is greater than that of Si-S (149 kcal/mol), and both are stronger than the Si-Si back-bond. Apparently, in the case of sulfur, the diffusion barrier is lower than the energy required to break the Si back-bonds. Therefore, the sulfur migrates into the Si bulk rather than desorbing as Si-S from the surface. V. Summary and Conclusions Our study has investigated the dissociative chemisorption of H2S on Si(100) at both low and high H2S coverage and at several surface temperatures. In all cases, only desorption of H2 is observed, via TPD measurements, from the H2S-covered surface. The single H2-desorption feature at 545 °C observed following adsorption at temperatures from 25 to 425 °C appears to be nearly identical to the β1-H2 desorption feature observed for a purely H-covered surface. This similarity is attributed to the formation of Si-H following H2S chemisorption. In addition to this dominant desorption channel at 546 °C, a smaller H2 desorption feature was found at 433 °C after adsorption of H2S at -145 °C. This feature resembles the β2-H2 desorption feature occurring from surface Si-H2 sites for a heavily atomichydrogen-dosed Si surface. Coverage-dependent measurements showed that saturation coverage for the range of substrate temperatures examined here is 0.5 ML of H2S. At a temperature range comparable to that of the β1 desorption feature, the sulfur AES signal (LMM) decreased rapidly at 525-625 °C. Because it was found that no S-related species desorbed from the surface, sulfur must diffuse into the bulk. Thus, the diffusion of sulfur and the desorption of H2 are believed to occur simultaneously. The H2S room-temperature adsorption isotherm shows that the sticking coefficient decreases exponentially with coverage. Thus, the adsorption kinetics are well-described by the Elovich equation. We propose a two step adsorption process, i.e. a fast and a slow adsorption step, corresponding to low coverage (e 0.25 ML of H2S) and high coverage (g 0.25 ML of H2S) adsorption regimes, respectively. In each adsorption step, two adjacent Si dimers are required for dissociation of H2S. Further measurements and more in-depth theoretical modeling is required before this adsorption process can be considered wellunderstood. From a more general perspective, our results show that the surface chemistries of H2S and H2O1-4 are, although similar in the behavior of the hydrogenated surface, different because of the distinct nature of the interaction of the Group VI atom with the Si surface. Specifically, these two molecules dissociate through similar pathways for low temperature and high temperature. Although the transition temperature from partial dissociation (e.g., H + SH) to full dissociation (e.g., 2H + S) is different (i.e., H2S starts to fully dissociate on Si(100) at approximately -75 °C compared to ∼125 °C for H2O), the H2 desorption channels for dosed surfaces are similar because desorption of hydrogen via β1-H2 desorption is observed for both cases. The most striking difference between the two systems is that, following H2 desorption at high temperature, sulfur becomes incorporated into the Si bulk whereas oxygen combines with surface Si to leave as SiO. Finally, understanding of the behavior and mechanism of adsorption and desorption of H2S on Si(100) also provides an insight into the limitations of this hydride precursor for the
Reaction of H2S with Si(100) growth of sulfur-containing Group II-VI materials at low temperatures. Specifically, the dissociative adsorption of H2S involves formation of Si-H bonds, and these H atoms passivate the surface against further adsorption. Thus, the removal of surface H by desorption becomes a major limit to material growth. This process makes it impossible to form 1.0 ML of surface sulfur without a separate means of H-atom removal. Acknowledgment. We gratefully acknowledge the support of this work by NSF through Grant DMR-96-32456 and instrumentation support by the Department of Energy through Grant DE-FG02-90ER14104. References and Notes (1) Ranke, W. Surf. Sci. 1996, 369, 137. (2) Weldon, M. K.; Stefanov, B. B.; Raghavachari, K.; Chabal, Y. J. Phys. ReV. Lett. 1997, 79 (15), 2851. (3) Flowers, M. C.; Johnathan, N. B. H.; Morris, A.; Wright, S. Surf. Sci. 1996, 351, 87. (4) Chander, M.; Li, Y. Z.; Patrin, J. C.; Weaver, J. H. Phys. ReV. B 1993, 48 (4), 2493. (5) Rezaei, M. A.; Stipe, B. C.; Ho, W. J. Chem. Phys. 1999, 110 (7), 3548. (6) Schro¨der-Bergen, E.; Ranke, W. Surf. Sci. 1990, 236, 103.
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