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Adsorption and decomposition of SiH4 on Pd(1 1 1)
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Surface Science 602 (2008) 693–701 www.elsevier.com/locate/susc
Adsorption and decomposition of SiH4 on Pd(1 1 1) Dylan C. Kershner, J. Will Medlin * Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA Received 1 July 2007; accepted for publication 29 November 2007 Available online 26 December 2007
Abstract SiH4 adsorption and decomposition on Pd(1 1 1) has been studied using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). SiH4 was found to adsorb dissociatively on Pd(1 1 1) at 150 K, resulting in SiHX species populating the Pd(1 1 1) surface. Spectroscopic data indicate that the primary SiHX species on the surface is SiH3, possibly mixed with smaller amounts of SiH2. HREELS data show the majority of surface SiHX species dissociate by approximately 250 K. TPD experiments show only H2 desorption; however, the kinetics of H2 desorption are clearly affected by SiH4 coverage. AES confirms the presence of Si on the Pd(1 1 1) surface above 250 K and shows complete diffusion of Si into the Pd bulk above 950 K. A slight broadening of the Si AES peak may indicate the presence of a Pd silicide above 500 K. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Metal–semiconductor interfaces; Metal–insulator interfaces; Silicide; Palladium; Silane; Electron spectroscopy; Temperature programmed desorption
1. Introduction Fundamental studies of SiH4 adsorption on metal surfaces can assist in understanding the chemical interactions between metals (such as Pd) and Si at key interfaces. Metal–silicon interfaces are frequently used in the semiconductor industry as ohmic contacts and Schottky barriers. They are also potentially important in applications in supported catalysis where bonding between (for example) Pd and Si as well as Pd and O atoms form the basis of metal–support interactions. As an example of this, Pd catalysts have been observed to form a silicide compound between the Pd particles and the silica support [1–5]. This metal–support interaction has been observed to change both catalytic activity and selectivity [3–5]. This report describes a multi-technique investigation of the adsorption and reaction of SiH4 on Pd(1 1 1). Similar studies have been performed on SiH4 adsorption on the Pd(1 0 0) surface [6,7]. These studies found SiH4 physisorp-
*
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0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.11.023
tion at temperatures below 78 K. Above this temperature, physisorbed SiH4 was reported to dissociate to SiHX surface species. Above 200 K all Si–H bonds were found to have dissociated, depositing Si atoms on the surface [6]. At room temperature surface Si atoms form an amorphous Pd3Si silicide compound that was made crystalline by annealing to 650 K [7]. In another study, electron-beam evaporation of Si was employed to study the chemistry of Si-covered Pd(1 1 0) and Pd(1 0 0) surfaces [8,9]. Below 140 K amorphous Si was grown and above this temperature a silicide with stoichiometry Pd2Si was formed. All of the studies found that the Pd silicide dissipated and Si diffused into the Pd bulk upon annealing at high temperatures [6–9]. SiH4 adsorption has been studied on a variety of transition metals besides Pd. Adsorption of SiH4 on Pt(1 0 0) and Pt(1 1 1) has also been studied [10,11]. On Pt(1 1 1) a silicide forms in ordered overlayers at higher temperatures with two stoichiometry regimes, Pt2Si and Pt3Si. Si diffuses into the bulk at higher temperatures [11]. On Pt(1 0 0), ordered silicide overlayers are also formed. It was noted that saturation exposures of SiH4 were higher for Pt(1 0 0) than Pt(1 1 1), indicating a dependence on surface structure
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[10]. SiH4 adsorption on Cu surfaces and Cu(1 1 1) in particular has been studied more extensively than other transition metal surfaces [12–20]. These studies find the same general trend of dissociative adsorption forming stable SiHX surface groups, which dissociate completely between approximately 200 and 400 K. A silicide phase having a stoichiometry of Cu2Si forms at higher temperatures on Cu(1 1 1). Surface and subsurface Si diffuse into the Cu bulk at higher temperatures. SiH4 adsorption at 118 K on Au(1 1 0) occurs dissociatively, resulting in species assigned as SiH2 and possibly SiH3, which dissociate further to SiH at room temperature [21]. Complete dissociation occurs above 383 K, and Si diffusion into the Au bulk occurs above 623 K. SiH4 adsorbs dissociatively on W(1 1 0) at 120 K, leading to a surface species assigned as SiH3, with complete dissociation occurring between 200 and 300 K [22]. At 1050 K surface Si diffuses into the subsurface, forming a silicide with a stoichiometry WSi2. On Ni(1 0 0), SiH4 dissociates to SiH3 upon adsorption at low temperatures, and dissociates completely above 250 K [23]. Above 500 K a Ni silicide is formed. Adsorption on Ni(1 1 1) also resulted in formation of surface SiHX groups, which completely dissociated above 473 K [24]. In this study, high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) were used to study SiH4 adsorption on the close-packed Pd(1 1 1) surface. TPD experiments were used to determine desorption products, yields, and temperatures. AES experiments were used to monitor accumulation of Si on the Pd surface and assess whether silicide formation occurs [25]. Finally, HREELS was used to obtain a vibrational fingerprint of surface species resulting from SiH4 adsorption and decomposition. The SiH4 adsorption and decomposition pathway on Pd(1 1 1) is then compared to studies of SiH4 chemistry on other relevant surfaces. 2. Experimental Experimental work was performed in two separate ultra-high vacuum (UHV) chambers. One chamber had a base pressure below 1 10 10 Torr and was equipped for TPD, low energy electron diffraction (LEED), and AES. The chamber is pumped by an ion pump (Low Profile 100 L, Gamma Vacuum) and a turbomolecular pump (TMU 261, Pfeiffer Vaccum). An ion gun (model NGI3000, LK Technologies) is mounted on the chamber for sample cleaning through Ar+ sputtering. The chamber is equipped with a quadrupole mass spectrometer (QMS) (smart IQ+, ThermoElectron Corporation) for TPD experiments and a combined LEED/AES apparatus (Series RVL2000, LK Technologies) that utilized a reverse view LEED screen and an electron gun mounted at a right angle to a cylindrical mirror analyzer (CMA) (Model CMA 2000, LK Technologies), which was used to detect Auger electrons. The LEED screen and electron gun are a self-contained unit, while the CMA relies on the electron gun
from the LEED unit to bombard the sample. The QMS is equipped with a stainless steel shroud over the detector. Gas is backdosed through a leak valve. The chamber is equipped with a load-lock system to facilitate sample loading. The sample is manipulated through the use of a translation stage moveable in the x, y, z, and rotational directions. It is heated resistively and cooled through indirect contact with a liquid nitrogen reservoir. Temperature measurement is achieved through a thermocouple welded to a location near the sample on the sample mount. Accurate calibration of readings from this thermocouple is achieved through comparison of simultaneous data taken from this thermocouple and a thermocouple temporarily welded to a loaded sample. Accuracy of temperature ramp data was ensured by comparing calibrated temperature readings to known TPD desorption temperatures of CO and O2 from CO- and oxygen-dosed surfaces [26,27]. The second chamber also has a base pressure below 1 10 10 Torr and was used only for HREELS. Pumping was achieved through an ion pump (Low Profile 300L, Gamma Vacuum) and a turbomolecular pump (TMU 261, Pfeiffer Vacuum). The chamber consists of two levels; the top level is equipped with a sputter gun (Model 9812043, Varian) for sample cleaning with Ar ions and a leak valve used to dose gases. The leak valve is attached to a 1/ 8-in. diameter tube leading into the vacuum chamber to enable more directed dosing when desired. The bottom level of the chamber houses the HREEL spectrometer (ELS 3000, LK Technologies). This chamber is also equipped with a load-lock system to aid in loading samples. The sample is manipulated through the use of stage moveable in the x, y, z, and rotational directions. It is heated resistively and cooled through indirect contact with a liquid nitrogen reservoir. Temperature measurement is achieved through a thermocouple welded to a location near the sample on the sample mount. TPD experiments were run with the Pd sample positioned as close to the opening in the QMS shroud as possible without touching the sample to the shroud. A heating rate of 1.2 K/s was used in all experiments. H2 and CO TPD peak areas were calculated by integrating the peaks after subtracting away a baseline signal. The baseline value was determined to be the lowest QMS signal intensity observed for the measured species during the course of the TPD experiment. Surface coverages of H2 and CO were calculated based on experimentally determined standard TPD peak areas. These standard peak areas were found by running TPD experiments with the Pd(1 1 1) surface covered by known amounts of CO or H2, based on data found in the literature [27–29]. The polished Pd(1 1 1) single crystal used in all experiments was obtained from Princeton Scientific Corp. (Princeton, NJ). Sample cleaning was done by repeated cycles of Ar+ sputtering, annealing at 950 K in vacuum, and annealing at 800 K in 4 10 7 Torr O2. The method of determining surface cleanliness was different between the two chambers because of the different experimental
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techniques available in each. In the chamber equipped for TPD and LEED/AES, AES was used to detect surface impurities. However, C and Pd AES features overlap, making carbon contamination difficult to detect; O2 TPDs were therefore employed to verify cleanliness. In the chamber equipped for HREELS, HREELS scans were run to identify surface contaminants. O2 and Ar gases used in sample cleaning were ultra-high purity grade, obtained from Matheson Tri Gas Inc. The SiH4 gas used was a 0.5% mixture of SiH4 in a balance of helium and was obtained from Airgas. The purpose of this low concentration SiH4 gas mixture was to minimize explosion risk. All gas dosing was done by backfilling the vacuum systems. Gas exposure measurements in Langmuirs (L) were based on dose durations and gas partial pressures as monitored by the QMS faraday cup, except where otherwise noted. Gas exposures for SiH4 are given in terms of total exposure to SiH4, but total SiH4 mixture exposure was much higher given the dilute mixture. 3. Results 3.1. HREELS studies The Pd(1 1 1) sample was exposed to 2 L of SiH4 at 150 K and studied with HREELS. In previous studies, SiH4 adsorption at this temperature had been reported to result in chemisorbed SiHX species [6–11,13,14,20,22,23]. Such adsorbed species have been observed by vibrational spectroscopy in the past, characterized by bends, scissor motions and wags in the 600–1200 cm 1 range and Si–H stretches in the 1900–2200 cm 1 range [6,13,23,30–32]. Results are shown in Fig. 1a, showing loss peaks in both of the expected regions. There is also a peak at 320 cm 1. In the range of 600–1200 cm 1, there is one primary peak at 925 cm 1, which has a small shoulder located at 880 cm 1. There are also several broad bands centered at 556, 780, and 1120 cm 1. A single peak in the higher-fre-
Fig. 1. HREEL spectra of Pd sample exposed to 2 L SiH4 at 150 K: (a) scan at adsorption temperature of 150 K; (b) scan after flash to 200 K; (c) scan after flash to 250 K; (d) HREEL spectrum of CO adsorbed on Si atom covered Pd surface. Sample was exposed to 2 L of SiH4 at 150 K, annealed at 250 K for 5 min, cooled to 200 K, and exposed to 10 L CO.
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quency range is at 2105 cm 1. There is also a shoulder on this peak at 2150 cm 1. Previous studies of SiH4 adsorption on transition metal surfaces had shown complete dissociation of surface SiHX groups below 250 K [6–11,13,14,20,22,23]. To determine if this is also the case with Pd(1 1 1), the SiH4-dosed sample was flashed to higher temperatures by increments of 50 K, scanning the sample with HREELS after each flash. The results are shown in Fig. 1, where the spectra have been offset for clarity. Spectrum (a) was taken just after adsorption at 150 K. Spectrum (b) was taken after flashing to 200 K and shows slight differences to the spectrum taken at 150 K. The 2150 cm 1 shoulder peak is attenuated, causing a decrease in the apparent width of the 2105 cm 1 peak. The feature associated with the 2105 cm 1 peak has a full width half maximum (FWHM) of 98 cm 1 at 150 K and 84 cm 1 at 200 K and also becomes more intense by a factor of 1.2 relative to the elastic peak. (The resolutions of the elastic peaks in spectra (a) and (b) are comparable.) The peak at 925 cm 1 becomes less intense by a factor of 0.6, and the peak at 320 cm 1 slightly less intense by a factor of 0.9. The next flash/scan cycle resulted in the elimination of all three major loss peaks (spectrum (c)). However, minor peaks of lesser intensity remain on the surface. These small remaining peaks are likely due to remaining Si surface structures and/or contamination. Annealing at high temperatures (950 K) removes these features. As will be discussed later, the peak at 2105 cm 1 could be assigned to several vibrations including SiHX stretches and CO stretches [6–11,13,14,20,22,23,31,33,34]. To help in the mode assignments, the following experiment was performed: the sample was exposed to SiH4, annealed to 250 K to completely remove all SiHX species and create a Si-covered surface, and exposed to CO at temperatures below 200 K. The resulting spectrum can be seen in Fig. 1d. The frequency for the CO vibration peak is 2105 cm 1, and the peak has a FWHM of 78 cm 1. The FWHM of the elastic peak was 33 cm 1, which is 3% larger than the FWHM for the elastic peak of the spectra in Fig. 1a. Because some of the lower-frequency features in Fig. 1a cannot be attributed to CO, we suggest that both CO and SiHX species are present on the surface after large SiH4 exposures. Furthermore, it appears that the onset temperature for CO desorption may be slightly higher than the temperature for complete SiHX disappearance, as evidenced by the apparently earlier onset of disappearance (at 200 K) of features corresponding to SiHX discussed above. Coverage-dependent scans of the Pd(1 1 1) crystal were taken after sequential doses of 0.5 L SiH4 at 150 K. These results are summarized in Fig. 2, showing spectra for the first and second SiH4 exposures. Further exposure to SiH4 showed no further changes. The primary difference between these two spectra is the intensity of the peaks at 320, 925 and 2105 cm 1. These peaks became more intense by factors of 1.4, 2.0, and 1.5, respectively. The similar growth seen in the peaks at 320 and 2105 cm 1 could indicate the same adsorbate is responsible for both peaks. The
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Fig. 2. HREEL spectra of Pd sample exposed to sequential doses of 0.5 L SiH4 at 150 K. The spectra are offset for clarity. Elastic peak intensities have been made equivalent to emphasize changes in peak intensity. Two scans are shown, bottom: (a) scan after first dose of 0.5 L; (b) scan after second dose of 0.5 L (1.0 L total).
peak at 925 cm adsorbate.
1
is likely a vibration from a different
3.2. TPD studies The adsorption and reaction of SiH4 was also studied using TPD to observe desorption products, temperatures, and yields. The only desorbing species observed was H2, along with significant quantities of CO from contamination after higher exposures. SiH4 exposures were done at 120 K and varied from very low doses of approximately 0.15 L to larger doses of 10 L. Fig. 3 shows the H2 desorption spectra after five exposures of SiH4: 0.15, 0.25, 0.40, 1.0 and 10 L. Primary desorption peaks for 0.15, 0.25, and 0.40 L dosed surfaces yielded desorption temperatures of 390, 380, and 382 K, respectively, and are consistent with H2 desorption from clean Pd(1 1 1) [28]. This shift in peak temperature and the peak shape is indicative of second-order hydrogen recombination kinetics. The 0.40 L exposure spectrum also has two small desorption features at 240 and 480 K. Both the 1 and 10 L exposure spectra are significantly different than the other three, being characterized by sharp peaks
Fig. 3. Hydrogen TPD spectra (m/e = 2) following SiH4 exposures to Pd(1 1 1) at 120 K: (a) 0.15 L; (b) 0.25 L; (c) 0.40 L; (d) 1 L; (e) 10 L.
Fig. 4. TPD experiment H atom desorption yields. A line to guide the eye is included.
at 390 and 285 K and a small broad peak at 200 K. TPDs performed using the same experimental conditions as these desorption spectra without the Pd(1 1 1) sample present in the chamber were also performed. These TPDs showed that the initial spike in m/e = 2 signal at 135 K was due to and/or indistinguishable from desorption from the sample holder. Desorption yield as a function of SiH4 exposure for H atoms is reported in Fig. 4. H atom desorption yields saturated at approximately 4 ML after ca. 2 L SiH4 exposures. The scatter in the desorption yields is due to uncertainties in subtracting contributions from background H2 (i.e., the large ‘‘peak” at low temperatures). One interesting result of the TPD experiments was the location of the CO desorption peak, shown in Fig. 5a, for SiH4 exposures approximately 2 L or greater. The CO comes from the background; due to the large dilution of the SiH4 gas, higher than normal dosing pressures were used resulting in a higher background pressure. There are CO desorption peaks at 380 and 250 K; the higher-temperature peak seems consistent with first-order CO desorption (though at a lower-temperature than on clean Pd(1 1 1)), whereas the low-temperature peak is quite broad. CO
Fig. 5. (a) CO (m/e = 28) TPD spectrum after 2 L SiH4 exposure to Pd(1 1 1) at 120 K (coverage = 0.13 ML). (b) CO (m/e = 28) TPD spectrum after 0.05 L CO exposure to clean Pd(1 1 1) at 260 K (coverage = 0.12 ML).
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desorption from clean Pd(1 1 1) was found to have a single desorption peak around 500 K (Fig. 5b), consistent with results from other researchers [27]. The dramatically different spectrum in Fig. 5a shows that the CO adsorption state is significantly perturbed (destabilized) on the SiH4-dosed surface. 3.3. AES studies AES was used to determine surface composition and the chemical state of surface Si. In a representative set of experiments, the Pd sample was exposed to 1.5 L of SiH4 at 150 K and then annealed at 500 K for 5 min. Based on HREELS and TPD results, this temperature should have been high enough to completely dissociate SiH4 adsorbates. Fig. 6a shows the resulting AE spectrum. Si is clearly evident on the surface, shown by the Si AES feature at 92 eV [35]. No splitting was directly observed in the negative lobe of the peak; such splitting would clearly indicate the presence of a surface silicide compound [25]. The referenced study found that the examined Pd silicide (Pd2Si) consisted of three AES peaks at 84, 89, and 93 eV rather than a solitary peak at 78 eV shown by Pd or a solitary peak at 91 eV shown by Si. The surface studied was a Pd-covered Si(1 1 1) crystal in which the Pd was being sputtered away to observe the silicide. As the Pd is nearly sputtered away, the splitting becomes less pronounced, appearing as a slightly wider peak at 90 eV. The FWHM of the negative lobe of the Si AES peak observed in this work shows an increase from 8 eV at 150 K to 9 eV at 500 K and above, as shown in Fig. 7. This slight broadening can also be observed in the differences between AE spectra 6a and 6b. Spectrum (b) was for a surface annealed at 350 K rather than 500 K. The consistent peak broadening evident at temperatures above 500 K may indicate the convolution of ‘‘split” peaks separated by a small energy difference, as observed in previous studies [1,36,37]. Previous studies of SiH4 adsorption on Pd surfaces have shown silicide formation [6,7], and the increase in peak width may indicate silicide formation. Deconvolution of this peak is difficult due to the proximity of Pd AES features to Si
Fig. 7. Full width half maximum (FWHM) of the negative lob of the Si AES LVV peak as a function of anneal temperature. The initial SiH4 dose was 1.5 L at 150 K. Each anneal was held for 5 min and scanned immediately or after cooling to 450 K. A line to guide the eye is included.
Fig. 8. Si AES LVV peak-to-peak magnitude as a function of temperature. The initial SiH4 dose was 1.5 L at 150 K. Each anneal was held for 5 min and scanned immediately or after cooling to 450 K. A line to guide the eye is included.
AES features both from elemental Si and Si in a silicide compound. Previous studies of SiH4 decomposition on metals have shown that deposited Si atoms diffuse into the metal bulk upon heating [6,11,13,20]. This was tested using the same procedure as above: dosing SiH4 at 150 K, annealing, and then conducting AES measurements. The sample was annealed at successively higher temperatures until the Si AES feature disappeared. Fig. 8 shows the results of this study. A decrease in Si peak-to-peak magnitude was observed until the feature disappeared at approximately 950 K. Along the entire temperature range, no variation in the Si peak energy was observed. 4. Discussion
Fig. 6. AE spectra of Pd(1 1 1) exposed to 1.5 L SiH4 at 150 K and (a) (dashed line) annealed at 500 K for 5 min; (b) (solid line) annealed at 350 K for 5 min.
The objective of this discussion section is to combine the results of the various characterization techniques (HREELS, TPD, AES) to obtain a more cohesive understanding of SiH4 adsorption and reaction on Pd(1 1 1). The discussion section that follows is therefore centered on three major issues:
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(1) identification of the surface species observed using HREELS after low-temperature SiH4 exposures; (2) determining the mechanisms by which H2 is produced during TPD after SiH4 exposures; (3) comparing the SiH4 surface chemistry observed here with that from previous work on other surfaces. 4.1. HREELS studies: assignment of vibrational modes HREELS investigations can provide detailed adsorbate structure information, although peak assignment is in practice difficult. Scans of the Pd(1 1 1) sample exposed to SiH4 at 150 K revealed several peaks of interest, particularly sharp peaks at 320, 925, and 2105 cm 1. Vibrations at 320 cm 1 can be attributed to metal–adsorbate bonding [23,33]; vibrations near 925 cm 1 are due to bending, wagging, and scissoring of SiHX species [13,20,23,30,32]; vibrations around 2105 cm 1 can be attributed to a number of vibrations, including stretches of CO bound to either Pd or Si and stretches of SiHX species [13,20,23,30,33]. There were also two broad structures centered at 550 cm 1, ranging from 450 to 650 cm 1; and at 1115 cm 1, ranging from 1025 to 1250 cm 1. Small peaks at 780 and 880 cm 1 were also observed. Below, we consider in detail possible assignments for the most important modes observed in our experiments. Previous relevant mode assignments are summarized in Table 1. Identification of the higher-frequency peak around 2105 cm 1 is difficult due to the number of stretching Table 1 Summary of peak frequencies and assignments found in previous work Frequency (cm 1)
Vibration
Mode
Surface
Reference
340
Pd–CO
Stretch
Pd(1 1 1)
370 387 411 435
M–Si Si–Si Si–CO Si–Si
Stretch Stretch Stretch Stretch
625 637 640 688 862 880 910 923 930 940 2080 2091 2097 2105
SiH SiH SiH2 SiH3 SiH3 SiH2 SiH2 SiH3 SiH4 SiH4 CO SiH2 CO CO
Wag Wag Wag Rocking Deformation Scissor Scissor Deformation Bend Bend Stretch Stretches Stretch Stretch
2126 2120–2150
SiH2 SiH3
Stretches Stretches
2173
SiH4
Stretches
Ni(1 0 0) Si(1 0 0) Si(1 0 0) Gas phase disilane Porous Si Si(1 1 1) Amorphous Si Porous Si Porous Si Si(1 1 1) Porous Si Porous Si Ni(1 0 0) Ni(1 1 1) Si(1 0 0) Cu(1 1 1) Pd(1 1 1) SiH4 exposed Pd(1 1 1) Porous Si Cu(1 1 1), porous Si Crystalline film
This work [23] [39] [33] [39] [30] [32] [41] [30] [30] [32] [30] [30] [23] [23] [33] [13] [34] This work [30,31] [13,30,31] [38]
modes that coincide at this frequency. CO bound to Si has been assigned to stretching modes at 2080 cm 1 [33], and adsorbed SiHX (X = 1–4) species have been assigned to stretching frequencies varying from 2080 to 2173 cm 1 [13,30,31,38]. CO adsorption on atop sites of Pd(1 1 1) has also been observed to occur near 2100 cm 1 [34]. The peak observed extends to 2190 cm 1 and appears to have a contribution from a sub-peak at 2150 cm 1. We propose that the primary contribution to the 2105 cm 1 peak is from adsorbed CO, and that SiHX species are responsible for the shoulder feature at 2150 cm 1. Most of the major loss peaks decrease in intensity upon annealing at 200 K, suggesting the feature at 2105 cm 1 can be assigned to a species different from that responsible for the other losses. CO is likely because the loss at 2105 cm 1 shown in Fig. 1d develops after the exposure of the Si-covered Pd(1 1 1) surface to CO. As discussed above, it is known from TPD experiments that significant CO is adsorbed (ca. 0.13 ML at the 2 L exposures used for Fig. 1) after SiH4 exposures, making background adsorption of CO a likely reason for the peak at 2105 cm 1 increasing in intensity. Flashing the surface above 250 K resulted in the loss of this peak, and CO desorption from Si has previously been observed to occur between 200 and 250 K [6,7,13,14,33]. Also, the C–O stretching frequency observed for CO adsorbed on Si(1 0 0) is 2081 cm 1 [33]. However, it is also consistent with CO adsorbed in atop sites on Pd(1 1 1) (2097 cm 1) [34]. It is possible that on the Si-covered Pd(1 1 1) surface, Si blocks hollow and bridge sites from CO, leaving atop sites for adsorption. Adsorption at hollow and bridge sites is more favorable than atop sites, but characteristic C–O stretching frequencies in the 1820–1850 cm 1 and 1900– 2000 cm 1 ranges for hollow and bridge sites, respectively, are not observed [27,34]. CO adsorbed to atop sites may also be consistent with the disappearance of the loss feature above 250 K, due to the lower binding energy of CO bound to atop sites compared to hollow or bridge sites [27]. The higher-frequency peak at 2150 cm 1, which was reduced in intensity after annealing to 200 K, is most likely attributable to one or more SiHX species. Stretching modes from adsorbed SiH3 have been observed between 2120 and 2150 cm 1 [13,30,31], consistent with the observed peak. Stretching modes at both 2126 and 2091 cm 1 have been assigned to SiH2 [13,30,31], and SiH has been assigned stretching modes at both 2115 and 2090 cm 1. These frequencies are also close to the range of the observed peak [13,30,31]. It is difficult to identify the precise SiHX species present on the surface based purely on the 2150 cm 1 shoulder, although agreement appears best for SiH3 stretching modes. Analysis of lower-frequency modes is needed to better understand the likely species present. The lowest energy loss at 320 cm 1 can reasonably be attributed to several chemisorbed species, including vibrations resulting from Pd–CO, Si–CO, Si–Si, and Si–Pd bonding [23,33,39]. HREELS scans after CO adsorption on clean Pd(1 1 1) show a low-frequency peak at 340 cm 1
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assigned to Pd–CO vibrations [40], which is very near the peak seen on the SiH4 exposed surface. Adsorption of CO on atop sites is consistent with the observed low-temperature desorption [27]. This feature could also be due to Si–CO vibrations, where CO is bound to a Si atom deposited on the surface. CO adsorption on Si(1 0 0) has a Si–CO vibration at 411 cm 1, somewhat higher than the peak seen in this work. Si–Si bonds are also located in this low-frequency range, but are higher than 320 cm 1; gas phase disilane has a Si–Si stretching vibration at 435 cm 1; disilane adsorbed on Si(1 0 0) has a Si– Si vibration at 387 cm 1 [39]. Assignment to Pd–Si stretch would be consistent with a previous study that assigned Ni–Si stretching at 370 cm 1 [23]. It should be noted that the peak at 320 cm 1 disappears above 250 K, while AES studies clearly show that Si persisted on the Pd surface well above 250 K; however, the state of the adsorbed Si is expected to change upon complete dissociation of all Si–H bonds. Assignment to CO vibrations may be the most reasonable in light of results shown in Fig. 2, representing two sequential doses of SiH4 onto the Pd(1 1 1) surface. The vibration at 2105 cm 1 has at least a strong contribution from CO vibrations, and as shown in Fig. 2, the peak at 320 cm 1 had an increase in intensity of approximately the same amount. Experiments in which the SiH4-exposed Pd surface was flashed above the SiHX dissociation temperature, cooled to below 200 K and exposed to CO had poor resolution, but appear to show a shoulder on the elastic peak at approximately 325 cm 1 apparently due to Pd– CO vibrations (Fig. 1d). The 500–1200 cm 1 range is the expected frequency range for bending, wagging, and scissoring motions of SiHX adsorbates [6,13,23,30–32]. HREELS scans at 150 K have one primary peak at 925 cm 1. This peak does not appear to be the result of vibrations from either SiH or SiH4. The wagging vibrational modes of SiH are found around 637 cm 1 on Si(1 1 1) and 625 cm 1 on porous Si, ruling SiH out as the likely source of this peak [30,32]. However, this vibrational mode could be contributing to the broad peak centered at 550 cm 1. Molecularly adsorbed SiH4 on Ni(1 0 0) has been observed to have a vibration at 930 cm 1, and molecularly adsorbed SiH4 on Ni(1 1 1) a vibration at 940 cm 1 [23]. However, the temperatures used in this study were likely high enough that SiH4 would not remain intact on the surface. Previous studies on Pd(1 0 0) and Pt(1 1 1) have observed Si–H bond breakage by 80 and 110 K, respectively [6,11]. Table 2 Summary of tentative peak assignments from this work Frequency (cm 1)
Tentative assignment
320 880 925 2105 2150 (shoulder)
M–CO, M–Si stretches SiH2 scissor, SiH3 deformation SiH3 deformation CO stretches SiH3 stretches
699
Assigning this peak to either SiH2 or SiH3 is more reasonable. A scissor vibrational mode of adsorbed SiH2 has been assigned at 910 cm 1 on porous Si and at 880 cm 1 on Si(1 1 1) in previous work [30,32]. These frequencies may be too low to be the source of the vibration at 925 cm 1, but there is a smaller peak at 880 cm 1 that is more consistent with this scissor vibrational mode of SiH2. SiH2 adsorbed on amorphous Si has been observed to have a wagging vibrational mode at 640 cm 1, which could be contributing to the broad peak centered at 550 cm 1 [41]. Adsorbed SiH3 species have been observed to have vibrations at 923 and 862 cm 1 from asymmetric and symmetric deformation modes and at 688 cm 1 from a rocking mode [30,32]. The peak in our spectrum at 925 cm 1 (along with the smaller feature at 880 cm 1) is thus most consistent with a SiH3 deformation mode and the rocking modes may contribute to the broad feature centered at 550 cm 1. The large growth of the peaks at 925 and 880 cm 1 (as well as the feature near 600 cm 1, which may be tentatively assigned as a SiH3 rocking mode), as shown in Fig. 2, points to a coverage-dependent accumulation of SiH3. The relatively minimal growth of features near 500 and 780 cm 1 in Fig. 2 suggests that low exposures may result in isolation of a different SiHX species, but larger exposures would seem to result in addition of SiH3 (and possibly SiH2) species on the surface. The possibility that large exposures could result in reorientation of the intermediate so that the intensities of particular loss features become prominent relative to others cannot be ruled out; unfortunately the CO contamination issues prevent drawing clear conclusions in this regard. Experiments conducted using pure (rather than highly dilute) SiH4 for exposures could help in fully resolving these details. Flashing to temperatures greater than 250 K appears to result in complete decomposition of all SiHX species. SiHX dissociation has previously been observed to occur between 200 and 250 K [6,7,13,14,33]. Table 2 summarizes the mode assignments for the spectrum observed in Fig. 2a. 4.2. TPD studies: mechanism for H2 production TPD experiments confirm many of the results of the HREELS studies and provide some interesting insights. H2 is the only observed desorption product. H2 desorption for low SiH4 exposures exhibit desorption temperatures in the 390–380 K range, following the same patterns as TPD experiments in our lab with similarly low coverages of hydrogen after H2 exposures on clean Pd(1 1 1), and also comparable to H2 desorption temperatures observed in a previous study of H2 adsorption on otherwise uncovered Pd(1 1 1) [28]. This indicates that low Si coverages (below ca. 0.25 ML) resulting from exposures below ca. 0.40 L do not significantly affect hydrogen desorption kinetics. However, H2 desorption peaks resulting from larger SiH4 exposures (1 L and above as shown in Fig. 3) occurred at approximately 390 K, higher than the clean surface case of H atom saturation [28]. Note that H2 desorption
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observed after 0.25 L SiH4 exposure exhibited a desorption temperature of 380 K. This spectrum observed for a presumably lower H atom coverage has a lower desorption temperature than the higher H atom coverages resulting from higher SiH4 exposures. This is behavior contrary to typical second-order desorption. Since all SiHX appeared to have dissociated by 250 K in HREELS experiments— even at high exposures—the shift in peak temperature does not seem to be related to desorption being limited by SiHX dissociation. One possibility is that coverages of Si approaching saturation (0.75 ML Si at 1 L exposure SiH4, Fig. 4) increases the binding strength for atomic hydrogen; we note that H2 desorbs from Si surfaces between 600 and 800 K [42–44]. Another possibility is that substantial coverages of Si, SiHX adsorbates, CO, and H adatoms forces H atoms into the Pd subsurface, causing the increase in desorption temperature. Such a phenomenon has previously been observed following exposure of a H-covered surface with CO [45,46]. These studies suggested that exposing CO to a Pd(1 1 1) crystal pretreated with H2 forced H atoms into the Pd bulk. Finally, it is worth commenting on the result of desorption yield analysis. H atom desorption yield saturated at ca. 4 ML, which would correspond to 1 ML of Si atoms ultimately deposited on the Pd surface. Considering that there is a significant amount of Si deposited on the surface (as well as contaminating CO), these numbers show a ‘‘crowded” surface at high exposures, one where H atoms may be forced into the subsurface of the Pd. CO adsorption from the background was significant enough to detect substantial CO desorption. For 2 L SiH4 exposures (equivalent to SiH4 exposures in the reported HREELS experiments), the highest-temperature CO desorption peak was shifted down in temperature from approximately 500 to 380 K, and a large fraction of the CO desorbed in a broad low-temperature peak (Fig. 5). This is a large shift in desorption temperature that may originate from strong repulsive interactions with coadsorbates. Such a shift is consistent with HREELS results, where the presence of a single C–O stretching mode at 2105 cm 1 suggests that CO has been displaced from more favorable hollow and bridge sites into atop or possibly Si-modified sites [34]. The lower desorption temperatures associated with CO adsorption onto atop sites could also explain why CO loss features would have been greatly attenuated after a 5 min anneal at 250 K in HREELS experiments, as observed in our experiments, since the lower binding energy of atop sites would allow lower temperature desorption [27]. Of note, in a previous study, it was observed that the presence of the Pt silicide formed through SiH4 chemical vapor deposition altered the chemisorption properties of CO on Pt(1 1 1) [13]. TPD experiments performed in the referenced study showed a decrease in CO desorption temperature to 315 K for low coverages of CO and 305 K for higher coverages. This compares to desorption at 430 K for low CO coverages and 395 K for higher coverages on otherwise clean Pt(1 1 1) surface at the same ramp rate.
4.3. Comparison with previous studies of SiH4 chemistry on metals The observed chemistry in this work is similar to chemistry seen in past work on SiH4 adsorption on transition metal surfaces. Adsorption was found to occur dissociatively with the formation of SiHX species. This is consistent with studies of SiH4 adsorption on Pd(1 0 0), Au(1 1 0), W(1 1 0), Ni(1 0 0), Ni(1 1 1), Cu(1 1 1), and Cu(1 1 0) [6,11,13–15,19–24]. The tentative identification of the SiHX species as being primarily SiH3 is consistent with the work done on Ni(1 0 0) and W(1 1 0) [22,23]. Additionally, one study of SiH4 adsorption on Cu(1 1 1) also identified the primary surface SiHX species as SiH3 [20]. There may have been small amounts of SiH2 were present on the Pd(1 1 1) surface during experiments in this work, which is consistent with a number of studies that identified a mixture of SiH2 and SiH3 on the Cu(1 1 1) surface [13,19,47]. However, the study by Wiegand et al. showed SiH2 as the primary surface moiety [13]. Complete dissociation of SiHX species was observed to occur between 200 and 250 K, which agrees well with previous studies of SiH4 adsorption on W, Ni, Cu, and Pd surfaces [6,13–15,19,20,22,23]. No SiH4 desorption from Pd(1 1 1) was detected during TPD experiments, which is consistent with the description of SiH4 surface reaction on Pd(1 0 0) and Pt(1 1 1) [6,7,10]. In all of the studies discussed, Si diffused into the metal bulk at high temperatures [6,11,13–15,19–24]. Finally, studies on Cu, Ni, Pt, and W have shown the formation of a silicide compound on the metal surface [6,11,13– 15,19,20,22–24]. In this study, no splitting in the Si AES peak was observed, but a FWHM analysis of the Si AES peak suggest that some change in state (perhaps corresponding to silicide formation) occurs by 500 K. This is a higher-temperature than identified in previous work on SiH4 adsorption on Pd(1 0 0), where a silicide formed immediately after dissociation of all Si–H bonds [7]. SiH4 adsorption studies on Pt(1 1 1) and Cu(1 1 1) also reported the formation of metal-silicides immediately following dissociation of all Si–H bonds which happens at or below room temperature in both cases [10,13]. Because of the limited resolution of AES measurements in the current work, the formation of a surface silicide at lower temperatures cannot be ruled out; however, it seems reasonable to speculate that some change in binding state of the Si occurs between 250 and 500 K. 5. Conclusions SiH4 adsorption and decomposition on Pd(1 1 1) was studied using AES, TPD, and HREELS. HREELS studies indicate that large exposures of SiH4 lead to isolation of surface SiHX groups that primarily consist of SiH3. Upon heating, surface SiHX groups dissociate into surface Si and H atoms. H2 is the only species observed to arise from SiH4 reaction during TPD. Hydrogen desorption yield analysis indicates that the amount of deposited Si saturates
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at approximately 1 ML. H2 and CO desorption from the Si-covered Pd(1 1 1) surface show altered chemistry from otherwise clean Pd(1 1 1), with the CO desorption temperature being shifted downward and the H2 desorption temperature being increased. AES studies confirm the presence of deposited Si at temperatures <950 K, possibly existing as a silicide. The deposited Si atoms diffuse into the Pd bulk upon heating. Acknowledgments We gratefully acknowledge research funding from the National Science Foundation through grant CBET0347658. The HREELS system used in this work was acquired through an NSF MRI award (CBET-0420693). D.C.K. received partial support from the US Department of Education through the GAANN program. References [1] B.K. Min, A.K. Santra, D.W. Goodman, J. Vac. Sci. Technol. B 21 (2003) 2319. [2] B.K. Min, A.K. Santra, D.W. Goodman, Catal. Today 85 (2003) 113. [3] W. Juszczyk, Z. Karpinski, J. Catal. 117 (1989) 519. [4] W. Juszczyk, Z. Karpinski, D. Lomot, J. Pielaszek, J. Catal. 220 (2003) 299. [5] W. Juszczyk, D. Lomot, J. Pielaszek, Z. Karpinski, Catal. Lett. 78 (2002) 95. [6] C.J. Ennis, S.A. Morton, L. Sun, S.P. Tear, E.M. McCash, Chem. Phys. Lett. 304 (1999) 217. [7] C.J. Ennis, D.J. Spence, S.P. Tear, E.M. McCash, Phys. Rev. B 61 (2000) 8443. [8] E. Kampshoff, N. Walchli, K. Kern, Surf. Sci. 406 (1998) 117. [9] E. Kampshoff, N. Walchli, K. Kern, Surf. Sci. 406 (1998) 103. [10] J.C. Bondos, N.E. Drummer, A.A. Gewirth, R.G. Nuzzo, J. Am. Chem. Soc. 121 (1999) 2498. [11] M.S. Nashner, J.C. Bondos, M.J. Hostetler, A.A. Gewirth, R.G. Nuzzo, J. Phys. Chem. B 102 (1998) 6202. [12] A.P. Graham, W. Allison, E.M. McCash, Surf. Sci. 270 (1992) 394. [13] B.C. Wiegand, S.P. Lohokare, R.G. Nuzzo, J. Phys. Chem. 97 (1993) 11553. [14] I.G. Shuttleworth, W. Allison, Surf. Rev. Lett. 8 (2001) 613. [15] N.J. Curson, H.G. Bullman, J.R. Buckland, W. Allison, Phys. Rev. B 55 (1997) 10819.
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Surface Science 602 (2008) 693–701 www.elsevier.com/locate/susc
Adsorption and decomposition of SiH4 on Pd(1 1 1) Dylan C. Kershner, J. Will Medlin * Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA Received 1 July 2007; accepted for publication 29 November 2007 Available online 26 December 2007
Abstract SiH4 adsorption and decomposition on Pd(1 1 1) has been studied using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). SiH4 was found to adsorb dissociatively on Pd(1 1 1) at 150 K, resulting in SiHX species populating the Pd(1 1 1) surface. Spectroscopic data indicate that the primary SiHX species on the surface is SiH3, possibly mixed with smaller amounts of SiH2. HREELS data show the majority of surface SiHX species dissociate by approximately 250 K. TPD experiments show only H2 desorption; however, the kinetics of H2 desorption are clearly affected by SiH4 coverage. AES confirms the presence of Si on the Pd(1 1 1) surface above 250 K and shows complete diffusion of Si into the Pd bulk above 950 K. A slight broadening of the Si AES peak may indicate the presence of a Pd silicide above 500 K. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Metal–semiconductor interfaces; Metal–insulator interfaces; Silicide; Palladium; Silane; Electron spectroscopy; Temperature programmed desorption
1. Introduction Fundamental studies of SiH4 adsorption on metal surfaces can assist in understanding the chemical interactions between metals (such as Pd) and Si at key interfaces. Metal–silicon interfaces are frequently used in the semiconductor industry as ohmic contacts and Schottky barriers. They are also potentially important in applications in supported catalysis where bonding between (for example) Pd and Si as well as Pd and O atoms form the basis of metal–support interactions. As an example of this, Pd catalysts have been observed to form a silicide compound between the Pd particles and the silica support [1–5]. This metal–support interaction has been observed to change both catalytic activity and selectivity [3–5]. This report describes a multi-technique investigation of the adsorption and reaction of SiH4 on Pd(1 1 1). Similar studies have been performed on SiH4 adsorption on the Pd(1 0 0) surface [6,7]. These studies found SiH4 physisorp-
*
Corresponding author. Tel.: +1 303 492 2418; fax: +1 303 492 4341. E-mail address: [email protected] (J.W. Medlin).
0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.11.023
tion at temperatures below 78 K. Above this temperature, physisorbed SiH4 was reported to dissociate to SiHX surface species. Above 200 K all Si–H bonds were found to have dissociated, depositing Si atoms on the surface [6]. At room temperature surface Si atoms form an amorphous Pd3Si silicide compound that was made crystalline by annealing to 650 K [7]. In another study, electron-beam evaporation of Si was employed to study the chemistry of Si-covered Pd(1 1 0) and Pd(1 0 0) surfaces [8,9]. Below 140 K amorphous Si was grown and above this temperature a silicide with stoichiometry Pd2Si was formed. All of the studies found that the Pd silicide dissipated and Si diffused into the Pd bulk upon annealing at high temperatures [6–9]. SiH4 adsorption has been studied on a variety of transition metals besides Pd. Adsorption of SiH4 on Pt(1 0 0) and Pt(1 1 1) has also been studied [10,11]. On Pt(1 1 1) a silicide forms in ordered overlayers at higher temperatures with two stoichiometry regimes, Pt2Si and Pt3Si. Si diffuses into the bulk at higher temperatures [11]. On Pt(1 0 0), ordered silicide overlayers are also formed. It was noted that saturation exposures of SiH4 were higher for Pt(1 0 0) than Pt(1 1 1), indicating a dependence on surface structure
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[10]. SiH4 adsorption on Cu surfaces and Cu(1 1 1) in particular has been studied more extensively than other transition metal surfaces [12–20]. These studies find the same general trend of dissociative adsorption forming stable SiHX surface groups, which dissociate completely between approximately 200 and 400 K. A silicide phase having a stoichiometry of Cu2Si forms at higher temperatures on Cu(1 1 1). Surface and subsurface Si diffuse into the Cu bulk at higher temperatures. SiH4 adsorption at 118 K on Au(1 1 0) occurs dissociatively, resulting in species assigned as SiH2 and possibly SiH3, which dissociate further to SiH at room temperature [21]. Complete dissociation occurs above 383 K, and Si diffusion into the Au bulk occurs above 623 K. SiH4 adsorbs dissociatively on W(1 1 0) at 120 K, leading to a surface species assigned as SiH3, with complete dissociation occurring between 200 and 300 K [22]. At 1050 K surface Si diffuses into the subsurface, forming a silicide with a stoichiometry WSi2. On Ni(1 0 0), SiH4 dissociates to SiH3 upon adsorption at low temperatures, and dissociates completely above 250 K [23]. Above 500 K a Ni silicide is formed. Adsorption on Ni(1 1 1) also resulted in formation of surface SiHX groups, which completely dissociated above 473 K [24]. In this study, high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD) were used to study SiH4 adsorption on the close-packed Pd(1 1 1) surface. TPD experiments were used to determine desorption products, yields, and temperatures. AES experiments were used to monitor accumulation of Si on the Pd surface and assess whether silicide formation occurs [25]. Finally, HREELS was used to obtain a vibrational fingerprint of surface species resulting from SiH4 adsorption and decomposition. The SiH4 adsorption and decomposition pathway on Pd(1 1 1) is then compared to studies of SiH4 chemistry on other relevant surfaces. 2. Experimental Experimental work was performed in two separate ultra-high vacuum (UHV) chambers. One chamber had a base pressure below 1 10 10 Torr and was equipped for TPD, low energy electron diffraction (LEED), and AES. The chamber is pumped by an ion pump (Low Profile 100 L, Gamma Vacuum) and a turbomolecular pump (TMU 261, Pfeiffer Vaccum). An ion gun (model NGI3000, LK Technologies) is mounted on the chamber for sample cleaning through Ar+ sputtering. The chamber is equipped with a quadrupole mass spectrometer (QMS) (smart IQ+, ThermoElectron Corporation) for TPD experiments and a combined LEED/AES apparatus (Series RVL2000, LK Technologies) that utilized a reverse view LEED screen and an electron gun mounted at a right angle to a cylindrical mirror analyzer (CMA) (Model CMA 2000, LK Technologies), which was used to detect Auger electrons. The LEED screen and electron gun are a self-contained unit, while the CMA relies on the electron gun
from the LEED unit to bombard the sample. The QMS is equipped with a stainless steel shroud over the detector. Gas is backdosed through a leak valve. The chamber is equipped with a load-lock system to facilitate sample loading. The sample is manipulated through the use of a translation stage moveable in the x, y, z, and rotational directions. It is heated resistively and cooled through indirect contact with a liquid nitrogen reservoir. Temperature measurement is achieved through a thermocouple welded to a location near the sample on the sample mount. Accurate calibration of readings from this thermocouple is achieved through comparison of simultaneous data taken from this thermocouple and a thermocouple temporarily welded to a loaded sample. Accuracy of temperature ramp data was ensured by comparing calibrated temperature readings to known TPD desorption temperatures of CO and O2 from CO- and oxygen-dosed surfaces [26,27]. The second chamber also has a base pressure below 1 10 10 Torr and was used only for HREELS. Pumping was achieved through an ion pump (Low Profile 300L, Gamma Vacuum) and a turbomolecular pump (TMU 261, Pfeiffer Vacuum). The chamber consists of two levels; the top level is equipped with a sputter gun (Model 9812043, Varian) for sample cleaning with Ar ions and a leak valve used to dose gases. The leak valve is attached to a 1/ 8-in. diameter tube leading into the vacuum chamber to enable more directed dosing when desired. The bottom level of the chamber houses the HREEL spectrometer (ELS 3000, LK Technologies). This chamber is also equipped with a load-lock system to aid in loading samples. The sample is manipulated through the use of stage moveable in the x, y, z, and rotational directions. It is heated resistively and cooled through indirect contact with a liquid nitrogen reservoir. Temperature measurement is achieved through a thermocouple welded to a location near the sample on the sample mount. TPD experiments were run with the Pd sample positioned as close to the opening in the QMS shroud as possible without touching the sample to the shroud. A heating rate of 1.2 K/s was used in all experiments. H2 and CO TPD peak areas were calculated by integrating the peaks after subtracting away a baseline signal. The baseline value was determined to be the lowest QMS signal intensity observed for the measured species during the course of the TPD experiment. Surface coverages of H2 and CO were calculated based on experimentally determined standard TPD peak areas. These standard peak areas were found by running TPD experiments with the Pd(1 1 1) surface covered by known amounts of CO or H2, based on data found in the literature [27–29]. The polished Pd(1 1 1) single crystal used in all experiments was obtained from Princeton Scientific Corp. (Princeton, NJ). Sample cleaning was done by repeated cycles of Ar+ sputtering, annealing at 950 K in vacuum, and annealing at 800 K in 4 10 7 Torr O2. The method of determining surface cleanliness was different between the two chambers because of the different experimental
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techniques available in each. In the chamber equipped for TPD and LEED/AES, AES was used to detect surface impurities. However, C and Pd AES features overlap, making carbon contamination difficult to detect; O2 TPDs were therefore employed to verify cleanliness. In the chamber equipped for HREELS, HREELS scans were run to identify surface contaminants. O2 and Ar gases used in sample cleaning were ultra-high purity grade, obtained from Matheson Tri Gas Inc. The SiH4 gas used was a 0.5% mixture of SiH4 in a balance of helium and was obtained from Airgas. The purpose of this low concentration SiH4 gas mixture was to minimize explosion risk. All gas dosing was done by backfilling the vacuum systems. Gas exposure measurements in Langmuirs (L) were based on dose durations and gas partial pressures as monitored by the QMS faraday cup, except where otherwise noted. Gas exposures for SiH4 are given in terms of total exposure to SiH4, but total SiH4 mixture exposure was much higher given the dilute mixture. 3. Results 3.1. HREELS studies The Pd(1 1 1) sample was exposed to 2 L of SiH4 at 150 K and studied with HREELS. In previous studies, SiH4 adsorption at this temperature had been reported to result in chemisorbed SiHX species [6–11,13,14,20,22,23]. Such adsorbed species have been observed by vibrational spectroscopy in the past, characterized by bends, scissor motions and wags in the 600–1200 cm 1 range and Si–H stretches in the 1900–2200 cm 1 range [6,13,23,30–32]. Results are shown in Fig. 1a, showing loss peaks in both of the expected regions. There is also a peak at 320 cm 1. In the range of 600–1200 cm 1, there is one primary peak at 925 cm 1, which has a small shoulder located at 880 cm 1. There are also several broad bands centered at 556, 780, and 1120 cm 1. A single peak in the higher-fre-
Fig. 1. HREEL spectra of Pd sample exposed to 2 L SiH4 at 150 K: (a) scan at adsorption temperature of 150 K; (b) scan after flash to 200 K; (c) scan after flash to 250 K; (d) HREEL spectrum of CO adsorbed on Si atom covered Pd surface. Sample was exposed to 2 L of SiH4 at 150 K, annealed at 250 K for 5 min, cooled to 200 K, and exposed to 10 L CO.
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quency range is at 2105 cm 1. There is also a shoulder on this peak at 2150 cm 1. Previous studies of SiH4 adsorption on transition metal surfaces had shown complete dissociation of surface SiHX groups below 250 K [6–11,13,14,20,22,23]. To determine if this is also the case with Pd(1 1 1), the SiH4-dosed sample was flashed to higher temperatures by increments of 50 K, scanning the sample with HREELS after each flash. The results are shown in Fig. 1, where the spectra have been offset for clarity. Spectrum (a) was taken just after adsorption at 150 K. Spectrum (b) was taken after flashing to 200 K and shows slight differences to the spectrum taken at 150 K. The 2150 cm 1 shoulder peak is attenuated, causing a decrease in the apparent width of the 2105 cm 1 peak. The feature associated with the 2105 cm 1 peak has a full width half maximum (FWHM) of 98 cm 1 at 150 K and 84 cm 1 at 200 K and also becomes more intense by a factor of 1.2 relative to the elastic peak. (The resolutions of the elastic peaks in spectra (a) and (b) are comparable.) The peak at 925 cm 1 becomes less intense by a factor of 0.6, and the peak at 320 cm 1 slightly less intense by a factor of 0.9. The next flash/scan cycle resulted in the elimination of all three major loss peaks (spectrum (c)). However, minor peaks of lesser intensity remain on the surface. These small remaining peaks are likely due to remaining Si surface structures and/or contamination. Annealing at high temperatures (950 K) removes these features. As will be discussed later, the peak at 2105 cm 1 could be assigned to several vibrations including SiHX stretches and CO stretches [6–11,13,14,20,22,23,31,33,34]. To help in the mode assignments, the following experiment was performed: the sample was exposed to SiH4, annealed to 250 K to completely remove all SiHX species and create a Si-covered surface, and exposed to CO at temperatures below 200 K. The resulting spectrum can be seen in Fig. 1d. The frequency for the CO vibration peak is 2105 cm 1, and the peak has a FWHM of 78 cm 1. The FWHM of the elastic peak was 33 cm 1, which is 3% larger than the FWHM for the elastic peak of the spectra in Fig. 1a. Because some of the lower-frequency features in Fig. 1a cannot be attributed to CO, we suggest that both CO and SiHX species are present on the surface after large SiH4 exposures. Furthermore, it appears that the onset temperature for CO desorption may be slightly higher than the temperature for complete SiHX disappearance, as evidenced by the apparently earlier onset of disappearance (at 200 K) of features corresponding to SiHX discussed above. Coverage-dependent scans of the Pd(1 1 1) crystal were taken after sequential doses of 0.5 L SiH4 at 150 K. These results are summarized in Fig. 2, showing spectra for the first and second SiH4 exposures. Further exposure to SiH4 showed no further changes. The primary difference between these two spectra is the intensity of the peaks at 320, 925 and 2105 cm 1. These peaks became more intense by factors of 1.4, 2.0, and 1.5, respectively. The similar growth seen in the peaks at 320 and 2105 cm 1 could indicate the same adsorbate is responsible for both peaks. The
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Fig. 2. HREEL spectra of Pd sample exposed to sequential doses of 0.5 L SiH4 at 150 K. The spectra are offset for clarity. Elastic peak intensities have been made equivalent to emphasize changes in peak intensity. Two scans are shown, bottom: (a) scan after first dose of 0.5 L; (b) scan after second dose of 0.5 L (1.0 L total).
peak at 925 cm adsorbate.
1
is likely a vibration from a different
3.2. TPD studies The adsorption and reaction of SiH4 was also studied using TPD to observe desorption products, temperatures, and yields. The only desorbing species observed was H2, along with significant quantities of CO from contamination after higher exposures. SiH4 exposures were done at 120 K and varied from very low doses of approximately 0.15 L to larger doses of 10 L. Fig. 3 shows the H2 desorption spectra after five exposures of SiH4: 0.15, 0.25, 0.40, 1.0 and 10 L. Primary desorption peaks for 0.15, 0.25, and 0.40 L dosed surfaces yielded desorption temperatures of 390, 380, and 382 K, respectively, and are consistent with H2 desorption from clean Pd(1 1 1) [28]. This shift in peak temperature and the peak shape is indicative of second-order hydrogen recombination kinetics. The 0.40 L exposure spectrum also has two small desorption features at 240 and 480 K. Both the 1 and 10 L exposure spectra are significantly different than the other three, being characterized by sharp peaks
Fig. 3. Hydrogen TPD spectra (m/e = 2) following SiH4 exposures to Pd(1 1 1) at 120 K: (a) 0.15 L; (b) 0.25 L; (c) 0.40 L; (d) 1 L; (e) 10 L.
Fig. 4. TPD experiment H atom desorption yields. A line to guide the eye is included.
at 390 and 285 K and a small broad peak at 200 K. TPDs performed using the same experimental conditions as these desorption spectra without the Pd(1 1 1) sample present in the chamber were also performed. These TPDs showed that the initial spike in m/e = 2 signal at 135 K was due to and/or indistinguishable from desorption from the sample holder. Desorption yield as a function of SiH4 exposure for H atoms is reported in Fig. 4. H atom desorption yields saturated at approximately 4 ML after ca. 2 L SiH4 exposures. The scatter in the desorption yields is due to uncertainties in subtracting contributions from background H2 (i.e., the large ‘‘peak” at low temperatures). One interesting result of the TPD experiments was the location of the CO desorption peak, shown in Fig. 5a, for SiH4 exposures approximately 2 L or greater. The CO comes from the background; due to the large dilution of the SiH4 gas, higher than normal dosing pressures were used resulting in a higher background pressure. There are CO desorption peaks at 380 and 250 K; the higher-temperature peak seems consistent with first-order CO desorption (though at a lower-temperature than on clean Pd(1 1 1)), whereas the low-temperature peak is quite broad. CO
Fig. 5. (a) CO (m/e = 28) TPD spectrum after 2 L SiH4 exposure to Pd(1 1 1) at 120 K (coverage = 0.13 ML). (b) CO (m/e = 28) TPD spectrum after 0.05 L CO exposure to clean Pd(1 1 1) at 260 K (coverage = 0.12 ML).
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desorption from clean Pd(1 1 1) was found to have a single desorption peak around 500 K (Fig. 5b), consistent with results from other researchers [27]. The dramatically different spectrum in Fig. 5a shows that the CO adsorption state is significantly perturbed (destabilized) on the SiH4-dosed surface. 3.3. AES studies AES was used to determine surface composition and the chemical state of surface Si. In a representative set of experiments, the Pd sample was exposed to 1.5 L of SiH4 at 150 K and then annealed at 500 K for 5 min. Based on HREELS and TPD results, this temperature should have been high enough to completely dissociate SiH4 adsorbates. Fig. 6a shows the resulting AE spectrum. Si is clearly evident on the surface, shown by the Si AES feature at 92 eV [35]. No splitting was directly observed in the negative lobe of the peak; such splitting would clearly indicate the presence of a surface silicide compound [25]. The referenced study found that the examined Pd silicide (Pd2Si) consisted of three AES peaks at 84, 89, and 93 eV rather than a solitary peak at 78 eV shown by Pd or a solitary peak at 91 eV shown by Si. The surface studied was a Pd-covered Si(1 1 1) crystal in which the Pd was being sputtered away to observe the silicide. As the Pd is nearly sputtered away, the splitting becomes less pronounced, appearing as a slightly wider peak at 90 eV. The FWHM of the negative lobe of the Si AES peak observed in this work shows an increase from 8 eV at 150 K to 9 eV at 500 K and above, as shown in Fig. 7. This slight broadening can also be observed in the differences between AE spectra 6a and 6b. Spectrum (b) was for a surface annealed at 350 K rather than 500 K. The consistent peak broadening evident at temperatures above 500 K may indicate the convolution of ‘‘split” peaks separated by a small energy difference, as observed in previous studies [1,36,37]. Previous studies of SiH4 adsorption on Pd surfaces have shown silicide formation [6,7], and the increase in peak width may indicate silicide formation. Deconvolution of this peak is difficult due to the proximity of Pd AES features to Si
Fig. 7. Full width half maximum (FWHM) of the negative lob of the Si AES LVV peak as a function of anneal temperature. The initial SiH4 dose was 1.5 L at 150 K. Each anneal was held for 5 min and scanned immediately or after cooling to 450 K. A line to guide the eye is included.
Fig. 8. Si AES LVV peak-to-peak magnitude as a function of temperature. The initial SiH4 dose was 1.5 L at 150 K. Each anneal was held for 5 min and scanned immediately or after cooling to 450 K. A line to guide the eye is included.
AES features both from elemental Si and Si in a silicide compound. Previous studies of SiH4 decomposition on metals have shown that deposited Si atoms diffuse into the metal bulk upon heating [6,11,13,20]. This was tested using the same procedure as above: dosing SiH4 at 150 K, annealing, and then conducting AES measurements. The sample was annealed at successively higher temperatures until the Si AES feature disappeared. Fig. 8 shows the results of this study. A decrease in Si peak-to-peak magnitude was observed until the feature disappeared at approximately 950 K. Along the entire temperature range, no variation in the Si peak energy was observed. 4. Discussion
Fig. 6. AE spectra of Pd(1 1 1) exposed to 1.5 L SiH4 at 150 K and (a) (dashed line) annealed at 500 K for 5 min; (b) (solid line) annealed at 350 K for 5 min.
The objective of this discussion section is to combine the results of the various characterization techniques (HREELS, TPD, AES) to obtain a more cohesive understanding of SiH4 adsorption and reaction on Pd(1 1 1). The discussion section that follows is therefore centered on three major issues:
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(1) identification of the surface species observed using HREELS after low-temperature SiH4 exposures; (2) determining the mechanisms by which H2 is produced during TPD after SiH4 exposures; (3) comparing the SiH4 surface chemistry observed here with that from previous work on other surfaces. 4.1. HREELS studies: assignment of vibrational modes HREELS investigations can provide detailed adsorbate structure information, although peak assignment is in practice difficult. Scans of the Pd(1 1 1) sample exposed to SiH4 at 150 K revealed several peaks of interest, particularly sharp peaks at 320, 925, and 2105 cm 1. Vibrations at 320 cm 1 can be attributed to metal–adsorbate bonding [23,33]; vibrations near 925 cm 1 are due to bending, wagging, and scissoring of SiHX species [13,20,23,30,32]; vibrations around 2105 cm 1 can be attributed to a number of vibrations, including stretches of CO bound to either Pd or Si and stretches of SiHX species [13,20,23,30,33]. There were also two broad structures centered at 550 cm 1, ranging from 450 to 650 cm 1; and at 1115 cm 1, ranging from 1025 to 1250 cm 1. Small peaks at 780 and 880 cm 1 were also observed. Below, we consider in detail possible assignments for the most important modes observed in our experiments. Previous relevant mode assignments are summarized in Table 1. Identification of the higher-frequency peak around 2105 cm 1 is difficult due to the number of stretching Table 1 Summary of peak frequencies and assignments found in previous work Frequency (cm 1)
Vibration
Mode
Surface
Reference
340
Pd–CO
Stretch
Pd(1 1 1)
370 387 411 435
M–Si Si–Si Si–CO Si–Si
Stretch Stretch Stretch Stretch
625 637 640 688 862 880 910 923 930 940 2080 2091 2097 2105
SiH SiH SiH2 SiH3 SiH3 SiH2 SiH2 SiH3 SiH4 SiH4 CO SiH2 CO CO
Wag Wag Wag Rocking Deformation Scissor Scissor Deformation Bend Bend Stretch Stretches Stretch Stretch
2126 2120–2150
SiH2 SiH3
Stretches Stretches
2173
SiH4
Stretches
Ni(1 0 0) Si(1 0 0) Si(1 0 0) Gas phase disilane Porous Si Si(1 1 1) Amorphous Si Porous Si Porous Si Si(1 1 1) Porous Si Porous Si Ni(1 0 0) Ni(1 1 1) Si(1 0 0) Cu(1 1 1) Pd(1 1 1) SiH4 exposed Pd(1 1 1) Porous Si Cu(1 1 1), porous Si Crystalline film
This work [23] [39] [33] [39] [30] [32] [41] [30] [30] [32] [30] [30] [23] [23] [33] [13] [34] This work [30,31] [13,30,31] [38]
modes that coincide at this frequency. CO bound to Si has been assigned to stretching modes at 2080 cm 1 [33], and adsorbed SiHX (X = 1–4) species have been assigned to stretching frequencies varying from 2080 to 2173 cm 1 [13,30,31,38]. CO adsorption on atop sites of Pd(1 1 1) has also been observed to occur near 2100 cm 1 [34]. The peak observed extends to 2190 cm 1 and appears to have a contribution from a sub-peak at 2150 cm 1. We propose that the primary contribution to the 2105 cm 1 peak is from adsorbed CO, and that SiHX species are responsible for the shoulder feature at 2150 cm 1. Most of the major loss peaks decrease in intensity upon annealing at 200 K, suggesting the feature at 2105 cm 1 can be assigned to a species different from that responsible for the other losses. CO is likely because the loss at 2105 cm 1 shown in Fig. 1d develops after the exposure of the Si-covered Pd(1 1 1) surface to CO. As discussed above, it is known from TPD experiments that significant CO is adsorbed (ca. 0.13 ML at the 2 L exposures used for Fig. 1) after SiH4 exposures, making background adsorption of CO a likely reason for the peak at 2105 cm 1 increasing in intensity. Flashing the surface above 250 K resulted in the loss of this peak, and CO desorption from Si has previously been observed to occur between 200 and 250 K [6,7,13,14,33]. Also, the C–O stretching frequency observed for CO adsorbed on Si(1 0 0) is 2081 cm 1 [33]. However, it is also consistent with CO adsorbed in atop sites on Pd(1 1 1) (2097 cm 1) [34]. It is possible that on the Si-covered Pd(1 1 1) surface, Si blocks hollow and bridge sites from CO, leaving atop sites for adsorption. Adsorption at hollow and bridge sites is more favorable than atop sites, but characteristic C–O stretching frequencies in the 1820–1850 cm 1 and 1900– 2000 cm 1 ranges for hollow and bridge sites, respectively, are not observed [27,34]. CO adsorbed to atop sites may also be consistent with the disappearance of the loss feature above 250 K, due to the lower binding energy of CO bound to atop sites compared to hollow or bridge sites [27]. The higher-frequency peak at 2150 cm 1, which was reduced in intensity after annealing to 200 K, is most likely attributable to one or more SiHX species. Stretching modes from adsorbed SiH3 have been observed between 2120 and 2150 cm 1 [13,30,31], consistent with the observed peak. Stretching modes at both 2126 and 2091 cm 1 have been assigned to SiH2 [13,30,31], and SiH has been assigned stretching modes at both 2115 and 2090 cm 1. These frequencies are also close to the range of the observed peak [13,30,31]. It is difficult to identify the precise SiHX species present on the surface based purely on the 2150 cm 1 shoulder, although agreement appears best for SiH3 stretching modes. Analysis of lower-frequency modes is needed to better understand the likely species present. The lowest energy loss at 320 cm 1 can reasonably be attributed to several chemisorbed species, including vibrations resulting from Pd–CO, Si–CO, Si–Si, and Si–Pd bonding [23,33,39]. HREELS scans after CO adsorption on clean Pd(1 1 1) show a low-frequency peak at 340 cm 1
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assigned to Pd–CO vibrations [40], which is very near the peak seen on the SiH4 exposed surface. Adsorption of CO on atop sites is consistent with the observed low-temperature desorption [27]. This feature could also be due to Si–CO vibrations, where CO is bound to a Si atom deposited on the surface. CO adsorption on Si(1 0 0) has a Si–CO vibration at 411 cm 1, somewhat higher than the peak seen in this work. Si–Si bonds are also located in this low-frequency range, but are higher than 320 cm 1; gas phase disilane has a Si–Si stretching vibration at 435 cm 1; disilane adsorbed on Si(1 0 0) has a Si– Si vibration at 387 cm 1 [39]. Assignment to Pd–Si stretch would be consistent with a previous study that assigned Ni–Si stretching at 370 cm 1 [23]. It should be noted that the peak at 320 cm 1 disappears above 250 K, while AES studies clearly show that Si persisted on the Pd surface well above 250 K; however, the state of the adsorbed Si is expected to change upon complete dissociation of all Si–H bonds. Assignment to CO vibrations may be the most reasonable in light of results shown in Fig. 2, representing two sequential doses of SiH4 onto the Pd(1 1 1) surface. The vibration at 2105 cm 1 has at least a strong contribution from CO vibrations, and as shown in Fig. 2, the peak at 320 cm 1 had an increase in intensity of approximately the same amount. Experiments in which the SiH4-exposed Pd surface was flashed above the SiHX dissociation temperature, cooled to below 200 K and exposed to CO had poor resolution, but appear to show a shoulder on the elastic peak at approximately 325 cm 1 apparently due to Pd– CO vibrations (Fig. 1d). The 500–1200 cm 1 range is the expected frequency range for bending, wagging, and scissoring motions of SiHX adsorbates [6,13,23,30–32]. HREELS scans at 150 K have one primary peak at 925 cm 1. This peak does not appear to be the result of vibrations from either SiH or SiH4. The wagging vibrational modes of SiH are found around 637 cm 1 on Si(1 1 1) and 625 cm 1 on porous Si, ruling SiH out as the likely source of this peak [30,32]. However, this vibrational mode could be contributing to the broad peak centered at 550 cm 1. Molecularly adsorbed SiH4 on Ni(1 0 0) has been observed to have a vibration at 930 cm 1, and molecularly adsorbed SiH4 on Ni(1 1 1) a vibration at 940 cm 1 [23]. However, the temperatures used in this study were likely high enough that SiH4 would not remain intact on the surface. Previous studies on Pd(1 0 0) and Pt(1 1 1) have observed Si–H bond breakage by 80 and 110 K, respectively [6,11]. Table 2 Summary of tentative peak assignments from this work Frequency (cm 1)
Tentative assignment
320 880 925 2105 2150 (shoulder)
M–CO, M–Si stretches SiH2 scissor, SiH3 deformation SiH3 deformation CO stretches SiH3 stretches
699
Assigning this peak to either SiH2 or SiH3 is more reasonable. A scissor vibrational mode of adsorbed SiH2 has been assigned at 910 cm 1 on porous Si and at 880 cm 1 on Si(1 1 1) in previous work [30,32]. These frequencies may be too low to be the source of the vibration at 925 cm 1, but there is a smaller peak at 880 cm 1 that is more consistent with this scissor vibrational mode of SiH2. SiH2 adsorbed on amorphous Si has been observed to have a wagging vibrational mode at 640 cm 1, which could be contributing to the broad peak centered at 550 cm 1 [41]. Adsorbed SiH3 species have been observed to have vibrations at 923 and 862 cm 1 from asymmetric and symmetric deformation modes and at 688 cm 1 from a rocking mode [30,32]. The peak in our spectrum at 925 cm 1 (along with the smaller feature at 880 cm 1) is thus most consistent with a SiH3 deformation mode and the rocking modes may contribute to the broad feature centered at 550 cm 1. The large growth of the peaks at 925 and 880 cm 1 (as well as the feature near 600 cm 1, which may be tentatively assigned as a SiH3 rocking mode), as shown in Fig. 2, points to a coverage-dependent accumulation of SiH3. The relatively minimal growth of features near 500 and 780 cm 1 in Fig. 2 suggests that low exposures may result in isolation of a different SiHX species, but larger exposures would seem to result in addition of SiH3 (and possibly SiH2) species on the surface. The possibility that large exposures could result in reorientation of the intermediate so that the intensities of particular loss features become prominent relative to others cannot be ruled out; unfortunately the CO contamination issues prevent drawing clear conclusions in this regard. Experiments conducted using pure (rather than highly dilute) SiH4 for exposures could help in fully resolving these details. Flashing to temperatures greater than 250 K appears to result in complete decomposition of all SiHX species. SiHX dissociation has previously been observed to occur between 200 and 250 K [6,7,13,14,33]. Table 2 summarizes the mode assignments for the spectrum observed in Fig. 2a. 4.2. TPD studies: mechanism for H2 production TPD experiments confirm many of the results of the HREELS studies and provide some interesting insights. H2 is the only observed desorption product. H2 desorption for low SiH4 exposures exhibit desorption temperatures in the 390–380 K range, following the same patterns as TPD experiments in our lab with similarly low coverages of hydrogen after H2 exposures on clean Pd(1 1 1), and also comparable to H2 desorption temperatures observed in a previous study of H2 adsorption on otherwise uncovered Pd(1 1 1) [28]. This indicates that low Si coverages (below ca. 0.25 ML) resulting from exposures below ca. 0.40 L do not significantly affect hydrogen desorption kinetics. However, H2 desorption peaks resulting from larger SiH4 exposures (1 L and above as shown in Fig. 3) occurred at approximately 390 K, higher than the clean surface case of H atom saturation [28]. Note that H2 desorption
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observed after 0.25 L SiH4 exposure exhibited a desorption temperature of 380 K. This spectrum observed for a presumably lower H atom coverage has a lower desorption temperature than the higher H atom coverages resulting from higher SiH4 exposures. This is behavior contrary to typical second-order desorption. Since all SiHX appeared to have dissociated by 250 K in HREELS experiments— even at high exposures—the shift in peak temperature does not seem to be related to desorption being limited by SiHX dissociation. One possibility is that coverages of Si approaching saturation (0.75 ML Si at 1 L exposure SiH4, Fig. 4) increases the binding strength for atomic hydrogen; we note that H2 desorbs from Si surfaces between 600 and 800 K [42–44]. Another possibility is that substantial coverages of Si, SiHX adsorbates, CO, and H adatoms forces H atoms into the Pd subsurface, causing the increase in desorption temperature. Such a phenomenon has previously been observed following exposure of a H-covered surface with CO [45,46]. These studies suggested that exposing CO to a Pd(1 1 1) crystal pretreated with H2 forced H atoms into the Pd bulk. Finally, it is worth commenting on the result of desorption yield analysis. H atom desorption yield saturated at ca. 4 ML, which would correspond to 1 ML of Si atoms ultimately deposited on the Pd surface. Considering that there is a significant amount of Si deposited on the surface (as well as contaminating CO), these numbers show a ‘‘crowded” surface at high exposures, one where H atoms may be forced into the subsurface of the Pd. CO adsorption from the background was significant enough to detect substantial CO desorption. For 2 L SiH4 exposures (equivalent to SiH4 exposures in the reported HREELS experiments), the highest-temperature CO desorption peak was shifted down in temperature from approximately 500 to 380 K, and a large fraction of the CO desorbed in a broad low-temperature peak (Fig. 5). This is a large shift in desorption temperature that may originate from strong repulsive interactions with coadsorbates. Such a shift is consistent with HREELS results, where the presence of a single C–O stretching mode at 2105 cm 1 suggests that CO has been displaced from more favorable hollow and bridge sites into atop or possibly Si-modified sites [34]. The lower desorption temperatures associated with CO adsorption onto atop sites could also explain why CO loss features would have been greatly attenuated after a 5 min anneal at 250 K in HREELS experiments, as observed in our experiments, since the lower binding energy of atop sites would allow lower temperature desorption [27]. Of note, in a previous study, it was observed that the presence of the Pt silicide formed through SiH4 chemical vapor deposition altered the chemisorption properties of CO on Pt(1 1 1) [13]. TPD experiments performed in the referenced study showed a decrease in CO desorption temperature to 315 K for low coverages of CO and 305 K for higher coverages. This compares to desorption at 430 K for low CO coverages and 395 K for higher coverages on otherwise clean Pt(1 1 1) surface at the same ramp rate.
4.3. Comparison with previous studies of SiH4 chemistry on metals The observed chemistry in this work is similar to chemistry seen in past work on SiH4 adsorption on transition metal surfaces. Adsorption was found to occur dissociatively with the formation of SiHX species. This is consistent with studies of SiH4 adsorption on Pd(1 0 0), Au(1 1 0), W(1 1 0), Ni(1 0 0), Ni(1 1 1), Cu(1 1 1), and Cu(1 1 0) [6,11,13–15,19–24]. The tentative identification of the SiHX species as being primarily SiH3 is consistent with the work done on Ni(1 0 0) and W(1 1 0) [22,23]. Additionally, one study of SiH4 adsorption on Cu(1 1 1) also identified the primary surface SiHX species as SiH3 [20]. There may have been small amounts of SiH2 were present on the Pd(1 1 1) surface during experiments in this work, which is consistent with a number of studies that identified a mixture of SiH2 and SiH3 on the Cu(1 1 1) surface [13,19,47]. However, the study by Wiegand et al. showed SiH2 as the primary surface moiety [13]. Complete dissociation of SiHX species was observed to occur between 200 and 250 K, which agrees well with previous studies of SiH4 adsorption on W, Ni, Cu, and Pd surfaces [6,13–15,19,20,22,23]. No SiH4 desorption from Pd(1 1 1) was detected during TPD experiments, which is consistent with the description of SiH4 surface reaction on Pd(1 0 0) and Pt(1 1 1) [6,7,10]. In all of the studies discussed, Si diffused into the metal bulk at high temperatures [6,11,13–15,19–24]. Finally, studies on Cu, Ni, Pt, and W have shown the formation of a silicide compound on the metal surface [6,11,13– 15,19,20,22–24]. In this study, no splitting in the Si AES peak was observed, but a FWHM analysis of the Si AES peak suggest that some change in state (perhaps corresponding to silicide formation) occurs by 500 K. This is a higher-temperature than identified in previous work on SiH4 adsorption on Pd(1 0 0), where a silicide formed immediately after dissociation of all Si–H bonds [7]. SiH4 adsorption studies on Pt(1 1 1) and Cu(1 1 1) also reported the formation of metal-silicides immediately following dissociation of all Si–H bonds which happens at or below room temperature in both cases [10,13]. Because of the limited resolution of AES measurements in the current work, the formation of a surface silicide at lower temperatures cannot be ruled out; however, it seems reasonable to speculate that some change in binding state of the Si occurs between 250 and 500 K. 5. Conclusions SiH4 adsorption and decomposition on Pd(1 1 1) was studied using AES, TPD, and HREELS. HREELS studies indicate that large exposures of SiH4 lead to isolation of surface SiHX groups that primarily consist of SiH3. Upon heating, surface SiHX groups dissociate into surface Si and H atoms. H2 is the only species observed to arise from SiH4 reaction during TPD. Hydrogen desorption yield analysis indicates that the amount of deposited Si saturates
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at approximately 1 ML. H2 and CO desorption from the Si-covered Pd(1 1 1) surface show altered chemistry from otherwise clean Pd(1 1 1), with the CO desorption temperature being shifted downward and the H2 desorption temperature being increased. AES studies confirm the presence of deposited Si at temperatures <950 K, possibly existing as a silicide. The deposited Si atoms diffuse into the Pd bulk upon heating. Acknowledgments We gratefully acknowledge research funding from the National Science Foundation through grant CBET0347658. The HREELS system used in this work was acquired through an NSF MRI award (CBET-0420693). D.C.K. received partial support from the US Department of Education through the GAANN program. References [1] B.K. Min, A.K. Santra, D.W. Goodman, J. Vac. Sci. Technol. B 21 (2003) 2319. [2] B.K. Min, A.K. Santra, D.W. Goodman, Catal. Today 85 (2003) 113. [3] W. Juszczyk, Z. Karpinski, J. Catal. 117 (1989) 519. [4] W. Juszczyk, Z. Karpinski, D. Lomot, J. Pielaszek, J. Catal. 220 (2003) 299. [5] W. Juszczyk, D. Lomot, J. Pielaszek, Z. Karpinski, Catal. Lett. 78 (2002) 95. [6] C.J. Ennis, S.A. Morton, L. Sun, S.P. Tear, E.M. McCash, Chem. Phys. Lett. 304 (1999) 217. [7] C.J. Ennis, D.J. Spence, S.P. Tear, E.M. McCash, Phys. Rev. B 61 (2000) 8443. [8] E. Kampshoff, N. Walchli, K. Kern, Surf. Sci. 406 (1998) 117. [9] E. Kampshoff, N. Walchli, K. Kern, Surf. Sci. 406 (1998) 103. [10] J.C. Bondos, N.E. Drummer, A.A. Gewirth, R.G. Nuzzo, J. Am. Chem. Soc. 121 (1999) 2498. [11] M.S. Nashner, J.C. Bondos, M.J. Hostetler, A.A. Gewirth, R.G. Nuzzo, J. Phys. Chem. B 102 (1998) 6202. [12] A.P. Graham, W. Allison, E.M. McCash, Surf. Sci. 270 (1992) 394. [13] B.C. Wiegand, S.P. Lohokare, R.G. Nuzzo, J. Phys. Chem. 97 (1993) 11553. [14] I.G. Shuttleworth, W. Allison, Surf. Rev. Lett. 8 (2001) 613. [15] N.J. Curson, H.G. Bullman, J.R. Buckland, W. Allison, Phys. Rev. B 55 (1997) 10819.
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