Evidence for photochemical quenching by a surface: Laser induced decomposition of W(CO)6 on Si(111)-(7 × 7)

147

Surface Science 226 (1990) 147-168 North-Holland

EVIDENCE FOR PHOTOCHEMICAL LASER INDUCED DECOMPOSITION J.R. SWANSON, Department Received

F.A. FLITSCH

of Chemistry

Harvard

19 June 1989; accepted

QUENCHING BY A SURFACE: OF W(CO), ON Si(lll)-(7 X 7)

and C.M. FRIEND

University,

Cambridge,

for publication

MA 02138, USA

22 September

1989

The ultraviolet laser induced decomposition of W(CO), adsorbed on Si(lll)-(7 X 7) was investigated under ultrahigh vacuum conditions with temperature programmed desorption/reaction, multiple internal reflection Fourier transform infrared, laser induced desorption and Auger electron spectroscopies. Ultraviolet photodecomposition of W(CO), proceeds in a one photon process. Different photochemical activity is observed for multilayer W(CO), as compared to W(CO), directly in contact with the surface. At multilayer coverages, photodecomposition proceeds via a single step, and the yield of generated CO strongly depends on photolysis wavelength. This dependence is consistent with direct electronic excitation of the molecule being the primary mechanism leading to decomposition. At lower coverages, the photoprocess is significantly quenched and near constant levels of CO are produced during photolysis at all ultraviolet wavelengths. At multilayer coverages, coadsorbed CO contributes to the CO laser induced desorption signal generated during irradiation of W(CO), and interferes with the analysis of the resulting data. This coadsorbed CO is most likely introduced into the vacuum chamber by W(CO), decomposing in the doser. Ultraviolet photolysis generates adsorbed partially decarbonylated tungsten carbonyl fragments, which decompose further in postphotolysis temperature programmed reaction. The photoproducts, in multilayers, do not react with coadsorbed benzene or 13C0 and, when isolated, are insensitive to additional ultraviolet photolysis. Auger electron spectroscopy is shown to have limited usefulness for obtaining quantitative details regarding surface stoichiometries for W(CO), and its photoproducts.

1. Introduction The many recent investigations of photo-assisted surface processes are largely motivated by a desire to understand fundamental aspects of surface photochemistry and a technological interest in laser-assisted chemical vapor deposition (LCVD) processes [1,2]. The technological interest in photo-assisted metal deposition is driven by mask and interconnect repair and customized circuit metallization applications [3]. Refractory metal deposits are particularly desirable because of their good adhesion, low resistivity and high thermal stability [4]. Therefore, the group VI metal hexacarbonyls are frequently used as precursors for refractory metal photodeposition because of their relatively high volatility and known efficient gas phase photochemistry [3,5]. Deposition of refractory metals on a variety of substrates using metal hexacarbonyl precursors under conditions practical to device fabrication 0039-6028/90/$03.50 (North-Holland)

0 Elsevier

Science Publishers

B.V.

has been performed via pyrolysis [6-81, ultraviolet photolysis [3,5,9-151 and electron bombardment [16-181. Unfortunately, carbon and oxygen impurities in these films result in low electrical conductivity [4,10,13]. Because deposition is performed under relatively high background pressures of metal carbonyl in reaction vessels of less than ideal cleanness, both gas phase and surface reactions of precursor and contaminant molecules are possible. Therefore, the source of impurities is unknown and may result from incomplete removal of ligands, surface reactions of liberated CO, or incorporation of background gases present under practical LCVD conditions [14]. By performing investigations of W(CO), decomposition under ultrahigh vacuum conditions surface mediated processes are specifically studied; therefore, the origin of impurities can be readily identified and related to the mechanism. A goal of this work is to investigate how the photochemistry of the W(CO), is altered by ad-

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et al. / Photochemical

sorption on a surface. The photochemistry of W(CO), in the gas phase and in low temperature matrices has been extensively studied. W(CO), strongly absorbs radiation in the ultraviolet portion of the electromagnetic spectrum, exhibiting two intense metal-ligand charge transfer bands corresponding to the ‘A,, --, ’ T,$‘,’ and ‘A,, ---f ’ T,(:’ transitions near 35 000 and 44000 cm- ‘, respectively, [19] and several d-d transitions corresponding to excitation into the 3T,,, ‘Ti, and ‘TZg states [19]. The d-d transitions are thought to be induced by photolysis with the XeF (351 nm), XeCl (308 nm) and KrF (249 nm) lasers, respectively [20,21]. In the gas phase, it has been shown that the primary product of single photon absorption at 351 and 308 nm is W(CO),, while that at 249 nm is W(CO), [22]. Thus, the product distribution is strongly dependent on the energy of the absorbed photon. At higher ultraviolet laser fluences, multiphoton dissociation of gas phase metal carbonyls is also possible. Both neutral [23] and excited [24] bare metal atoms as well as ions [25] have been identified as major products following multiphoton dissociation/ionization of metal carbonyls. In addition, the products of laser photolysis in the gas phase are coordinatively unsaturated [22,26] and, therefore, highly reactive; thus, on a surface one might expect rapid reaction of such species with substrate atoms or neighboring adsorbates. The photodecomposition of W(CO), isolated in matrices has also been studied by a number of investigators [27,28]. The species W(CO),, W(CO), and W(CO), have been identified by infrared spectroscopy as products of ultraviolet photolysis [29-311. In addition, it has been determined that the ultraviolet/visible absorption spectra of W(CO), coordinated to a variety of matrix elements is shifted to energy lower than the parent W(CO),. The visible band in the spectra is red shifted by over 100 nm and its position is highly sensitive to the identity of the matrix material, while the ultraviolet band in the spectra is shifted to a much smaller extent and its position is relatively insensitive to the matrix element [32]. In this work, the ultraviolet laser induced decomposition of W(CO), adsorbed on Si(lll)-(7 X 7) at coverages ranging from submonolayer to

quenching

by a surfme

multilayers was studied and compared to results of previous investigations [33-391. The results rcported here demonstrate that the photodecomposition process for multilayer W(CO), is qualitatively different than the process for W(CO), in direct contact with the surface. Specifically, a strong wavelength dependence in the photoyield is only observed for multilayers of W(CO),; at lower coverages, essentially equal amounts of gaseous products are detected at all ultraviolet wavelengths. Thus, the surface perturbs the photochemistry of the adsorbed W(CO),, in contrast to the earlier proposal by others that the photochemistry of W(CO), is relatively unchanged upon adsorption [37] but analogous to the previously reported quenching of photochemistry of adsorbed CH,I, on Al,O, and Ag surfaces [40]. In addition, the results of this study show that photoproducts resulting from laser induced decomposition of when isolated on the Si(lll)-(7 x 7) W(CO),> surface, are insensitive to further ultraviolet photolysis and unreactive towards CO and C,H,, suggesting that rapid reaction with the surface or other W(CO), fragments occurs. The insensitivity of the photoproducts to undergo further ultraviolet photolysis may set a fundamental limit on the purity of tungsten films grown by photodecomposition of W(CO), on surfaces.

2. Experimental All experiments were performed in a two level, stainless steel ultrahigh vacuum system with a working base pressure of - 2 x lo- ‘a Torr. A long-travel, double-sided manipulator allows for movement of samples to various positions in the chamber. On one side the sample is fixed in place while on the other side it is possible to interchange samples in situ. The removable samples are mounted in copper and Macor holders and are interchanged on the manipulator. The assemblies not being used are stored on a rack inside the chamber until needed. The chamber is equipped with four-grid retarding field electron optics, used for low energy electron diffraction and Auger electron spectroscopy; a quadrupole mass spectrometer (UT1 lOOC), used in temperature pro-

J.R. Swanson et al. / Photochemical quenching by a surface

grammed desorption/reaction and electron and photon induced desorption experiments; a differentially pumped Ar+ sputtering gun; a directed cosine emitter with a flag for dosing; and the necessary optical mounts and windows for infrared and laser experiments. Two different types of silicon samples were used in this experiment. Both samples were rectangular in shape and - 25 mm long. Temperature programmed reaction experiments were primarily performed on silicon with a bulk resistivity of 0.04 Q cm, since its relatively high conductivity facilitated the programming of linear heating rates. For the multiple internal reflection Fourier transform infrared experiments, p-doped silicon with a resistivity of - 40 Q cm was used because of its ability to transmit infrared radiation. Crude temperature programmed reaction experiments could be performed on this crystal to confirm qualitative consistency with the results obtained on the higher conductivity material. The crystal used for infrared studies was always mounted on the fixed side of the manipulator. In both cases, the crystals are held in thermal contact with a liquid nitrogen cooled copper block by tantalum clips and they are both resistively heated. The crystal used for temperature programmed reaction was held at both ends by the clips, while the infrared crystal was held at the top and bottom. A small amount of indium metal was placed between the copper block and the backs of the substrates to improve the electrical characteristics of the contact. No evidence of indium migration onto the front faces of the crystals was observed by Auger electron spectroscopy. This arrangement makes it possible to obtain temperatures between 120 and 1400 K. The front surfaces of the silicon substrates were cleaned by Ar+ bombardment (3 kV, 80 PA) for - 10 min. During sputtering, the crystals were biased -67 V to improve sputtering efficiency. After sputtering the substrates were annealed to 1300 K and slowly cooled ( - 1 K/s) to 120 K. After cooling, a sharp, characteristic (7 x 7) pattern was observed by low energy electron diffraction, and the surface cleanness was established using Auger electron spectroscopy. The backs of the substrates were not cleaned and were, thus, covered with the native oxide.

149

Sample temperatures were measured with chromel-alumel thermocouples and calibrated with an optical pyrometer. Low temperature readings were calibrated by immersion, outside the chamber, of the sample into a series of standard low temperature baths. The thermocouple was held in mechanical contact with the back of the crystal used for temperature programmed reaction by a stainless steel spring. In contrast, the thermocouple for the infrared crystal was spot welded to one of the tantalum clips. Gaseous molecules are introduced into the chamber via a directed cosine emitter, consisting of a stainless steel tube with a 0.4 mm diameter pinhole at the end and connected to a variable leak valve. The doser is thermally isolated from the rest of the vacuum chamber by ceramic breaks and, thus, can be evenly heated for use with low volatility materials. The pinhole is covered with a movable flag, actuated from outside the chamber, so that exposures can be well controlled and are reproducible. The silicon substrates were biased -60 V during adsorption, laser irradiation, and temperature programmed reaction to prevent inadvertent electron bombardment because of the demonstrated ability of low energy electrons like those generated in vacuum chambers to decompose W(CO), [39]. The 13C-enriched W(CO), used in these experiments was provided by Professor D.J. Darensbourg, while W(CO), was obtained from Strem Chemicals, Inc. The metal carbonyl samples were transferred from their respective reagent bottles under dry nitrogen gas and purified by freezepump-thaw cycles. Both samples were stored under vacuum at room temperature in clear glass bottles wrapped in aluminum foil and were changed every few days. Analytical reagent grade C,H, and C,D, was purchased from Mallinckrodt, Inc. and MSD Isotopes Div. of Merck Frosst Canada, Inc., respectively; and both were purified by several freeze-pump-thaw cycles. Research grade 13C0 and Cl80 were obtained from MSD Isotopes and Cambridge Isotopes, Inc., respectively. Research grade CO and Xe was purchased from Matheson Gas Products, Inc.. All isotopic forms of CO were exposed from sample bottles cooled with liquid nitrogen to prevent any CO,

1.50

J. R. Swanson et al. / Pho.+ochemical quenching by a swface

present in the samples from entering the vacuum chamber. Xe was used without further purification. In all cases, the dosing lines were flushed and the reservoir behind the leak valve was filled immediately before adsorption to insure dose purity, as was confirmed mass spectrometrically. Temperature programmed desorption/reaction data were obtained with a UT1 1OOC quadrupole mass spectrometer. The multiplexed mass spectrometer 1411, capable of monitoring up to 10 masses in a single experiment, and voltage programmed power supply used for crystal heating are controlled by a personal computer. In a typical experiment, however, only CO (m/e 28) and W (m/e 182) were monitored. During temperature programmed desorption experiments, the crystal was spaced - 5 mm from the ionizer of the spectrometer. The ionizer is shielded with a stainless steel cap with a 6 mm aperture which allows selective detection of molecules desorbing from the crystal face and, thus, mini~zing contribution to the data from the crystal supports. Approximately constant heating rates ranging from 2 K to 10 K/s were used as noted. Laser photolysis was performed with a Lumonits model K-261 excimer laser. Wavelengths of 249, 308, 331, 351 and 720 nm were produced with the laser by using different gas mixtures. The excimer laser was run in a constant pulse energy mode. The laser fluence at the sample was maintained in the range of 0.02-2 mJ/cm’ by using neutral density filters. The pulse energy was continuously measured by splitting off a small fraction of the light and directing it to a Laser Precision energy meter and probe (models Rj 7100 and RjP 735, respectively). The laser light enters and exits the vacuum chamber via CaF, windows mounted on 22 inch outside diameter Conflat flanges. The laser light was unfocused and struck with the the crystal at a 45O angle of incidence reflected light exiting the vacuum chamber via another viewport at the complementary angle. A mask external to the chamber was used to limit the irradiation area to - 0.5 cm2 of the silicon crystal. The crystal was directly facing the mass spectrometer and was located - 2 cm from the mass spectrometer shield during photolysis to maximize detection of molecules desorbed normal

from the surface. The ultraviolet laser was aligned with a He-Ne laser to insure that the Iaser light was not striking the crystal supports or the vacuum chamber walls. Purposeful irradiation of the chamber walls produced no measurable CO (m/e 28) signal in the mass spectrometer. Gaseous products of the photolysis were detected mass spectrometrically with the laser, energy meter and multiplexed mass spectrometer all under the control of a personal computer. The computer fired the laser, collected mass spectral and energy meter data for that laser pulse, determined the magnitude of the evolution signal by subtracting off the background level of that species and then refired the laser once the background gas level had returned to normal. Typically, the laser pulse rate was programmed to be 1 Hz, even though it took less then 5 ms for the background level to return to normal after each pulse. Laser induced evolution spectra normalized to the laser pulse energy were obtained by assuming a one photon process (which is supported by our data) and then dividing the mass spectrometer signal by the pulse energy. Fluence dependence data for the photoprocess were collected in the following manner: first, the W(CO), sample was exposed to one pulse of unfiltered light, then a 10% transmission filter was placed in the beam path and the laser was pulsed again; next, the filter was removed for another pulse of light and finally a 50% transmission filter was inserted into the beam path for the last laser pulse. This process was repeated again and again until - 200 total laser pulses had been fired. All vibrational spectra were collected with a Nicolet model 7199 Fourier transform infrared spectrometer with an optical setup similar to that described previously [38]. Infrared radiation enters and exits the vacuum chamber via CaF, windows mounted on 2: inch outside diameter Conflat flanges. Unpolarized infrared radiation was used in all the experiments and the s and p components are assumed to be equal to within a factor of two. Adsorbates on both sides of the crystal are detected; however, the backside contribution is minimal because of the highly directed dosing used in these experiments. This was checked by performing a temperature programmed desorption

J.R. Swanson et al. / Photochemical quenching by a surface

experiment after a blank dose. The blank dose was performed by admitting W(CO), into the chamber for a time period and at a pressure typically used for dosing but not opening the flag separating the Si(lll)-(7 X 7) crystal from the doser. Typically, spectra were the average of 1024 scans, collected at 1 cm-’ resolution and required - 30 min. to obtain. Infrared spectra used in the figures were obtained by ratioing against the clean Si(lll)-(7 x 7) background spectrum.

3. Results 3.1. Adsorption of W(CO)), on Si(1 II)-(7 X 7) All adsorbed W(CO), desorbs from Si(lll)(7 X 7) in the range of 150-190 K during temperature programmed desorption, and none thermally decomposes. Temperature pro~~med desorption data, obtained for several coverages of W(CO), on Si(ll1) - (7 x 7), are shown for two mass spectral fragments of W(CO),, m/e 28 (CO) and m/e 182 (W), in fig 1. It is important to note that a small amount of background CO is coad-

Tenparetura(K ) Fig. 1. Temperature programmed desorption spectra for W(CO), on Si(lll)-(7x 7) as a function of coverage. Two mass spectral fragments of W(CO), are shown: m/e 28 (CO) and m/e 182 (W). (a) 5 s exposure (B = 0.02); (b) 10 s (0 = 0.06); (c) 15 s (0 = 0.17); (d) 20 s (8 = 0.25); (e) 30 s (0 = 0.49); (f) 40 s (r? = 0.66); and (g) 60 s (6 = 1). The heating rate was - 2 K/s and essentially constant in all cases.

151

sorbed with the W(CO),, as established by studies of 13C0 and W(CO), which are discussed in detail following. For all of the data shown in fig. 1, the crystal was annealed to 135 K to desorb background CO before collection of the W(CO), temperature programmed desorption data to preclude its disturbing the leading edge analysis. At low coverage, the relative intensity of the tungsten and CO fragments is invariant, and the lineshapes of the CO fragment peaks are essentially the same as those of the tungsten fra~ents. Therefore, the peak observed at low coverage, labelled as (Y,, is attributed solely to molecular desorption of W(CO), directly interacting with the Si(lll)-(7 x 7) surface. The peak is observed at - 149 K for the lowest coverage, shifting to higher temperature with increasing W(CO), coverage. At higher coverage, two partially resolved W(CO), desorption features are observed; however, as is evident in fig. 1, the relative intensity of the tungsten and CO fragments differ between the two features for unknown reasons. The second feature is defined as CQ and appears at - 170 K, its magnitude increasing indefinitely with W(CO), exposure. Although at high coverage the (Yepeak is clearly due to sublimation of W(CO), multilayers, the exact nature of its initial stages cannot be determined. While the (Ye state may be due entirely to sublimation of multilayers it is also possible that, initially, it is caused by lateral intermolecular interactions, much like those postulated in the case of N, on Ru(001) [42]. All W(CO), desorbing in the a, state must be in direct contact with the surface; therefore, for convenience the coverage where cyz is first observed is arbitrarily defined to be the monolayer saturation coverage (8 = 1). Attractive lateral interactions would result in this definition being an overestimate of the true W(CO), coverage; nevertheless, by using a wide coverage regime, results from submonolayer and multilayer coverages may be compared. A leading edge analysis was performed for the lower temperature, LY,, peak for both W(CO), mass spectral fragments, and the desorption energy was determined to be 12.2 c 0.7 kcal/mol at the 95% confidence level, indicative of weak inter-

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action between the Si(lll)-(7 X 7) surface and the molecule. The leading edges of the (at desorptions coincided for all coverages where cyz desorption is not observed, indicative of zero-order desorption kinetics [43]. The error limits placed on the desorption energy are derived from the spread in the data. Inclusion of the absolute error in temperature measurements (< 10 K) results in an error margin of rfr1.5 kcal/mol. This cy, desorption energy is comparable to the 11 + 1 kcal/mol heat of desorption determined previously [37], but significantly less than the heat of subhmation of W(CO), of 17.7 kcal/mol measured near room temperature f44f. The absence of features other than low temperature molecular desorption in the temperature programmed desorption spectra is evidence that W(O), does not thermally decompose on Si(lll)-(7 x 7) under the conditions of our experiment. Furthermore, no tungsten, carbon, or oxygen is detected in Auger electron spectra obtained subsequent to W(CO), temperature programmed desorption confirming that there are no detectable thermal reactions on the Si(lll)-(7 X 7) surface. Infrared spectra confirm that W(CO), weakly interacts with the Si(lll)-(7 X 7) surface. Spectra obtained as a function of coverage at 130 K, shown in fig. 2, are characterized by two symmetric bands: a strong absorption band at - 1940 cm-’ for all coverages and a weaker band near 2040 cm-l observed at higher coverages. These spectra were obtained from samples which were not annealed to - 135 K before data collection. The two infrared bands are only observed after adsorption onto clean, annealed Si(lll)-(7 x 7). On sputtered, but unannealed Si(l11) a single, broad, asymmetric absorption band is observed in the range of 2040 to 1920 cm-‘. In addition, the spectrum of 13C-enriched W(CO), adsorbed on the Si(lll)-(7 x 7) surface displays a strong band and a weaker band near 2000 near 1890 cm-’ For comparison, gas phase octahedral cm-‘. W(CO)6 exhibits a single band in the carbonyl stretching region [45], the T,, band, at 1997.6 and in argon matrices at 20 K [30], two cm-‘; bands at 1992.0 cm-’ and 1986.6 cm-‘. The dual bands observed in matrices are attributed to lowering of the molecular symmetry from oc-

2160

2080

Energy

2000 (an-' )

1320

1840

2. Multiple internal reflection infrared absorption spectra W(CO), on Si(lll)-(7 x7) at 125 K as a function of coverage: (a) 8 ~0.79: (b) 8 = 0.45; (c) B = 0.24; and (d) 0 = 0.11. W(CO), coverages were obtained by scaling the integrated dose to the larger infrared crystal size. The resolution was 1 cm-’ and 1024 scans were averaged. The data collection time was - 30 min.

tahedral W(CO), [30]. High resolution energy loss spectra 1371 of We adsorbed on Si(lll)-(7 x 7) at 90 K consist of a single, broad ( - 100 cm.-- ’ FWHM) loss at 2025 cm-‘. Since the infrared spectrum of adsorbed W(CO), is not greatly perturbed from that in the gas phase and matrices, the interaction between the surface and the W(CO), must be extremely weak, consistent with the experimentally determined low desorption energy. Infrared

spectra collected after annealing the overlayer were qualitatively different W(C% from those obtained from unannealed samples. For example, the infrared spectrum obtained after annealing a W(CO), layer (8 = 0.40) to - 735 is shown in fig. 3. No tungsten-containing fragment desorption is observed during annealing, only CO is desorbed. After the W(CO), layer is annealed, the infrared peak at - 1940 cm-t (see fig. 2) is resolved into two features ( - 1960 and - 1940 cm-‘). A comparison of the spectra obtained before and after annealing indicate that the two features in the annealed spectrum result not from creation of a new peak but from removal of intensity from the center of the original peak at - 1940

J. R. Swanson et al. / Photochemical

2160

2000

2080 Energy

1920

1 a40

( cm-l )

Fig. 3. Multiple internal reflection infrared absorption spectrum for W(CO), (0 = 0.40) on Si(lll)-(7 X 7) after annealing was 1 cm-‘, 1024 scans were to - 135 K. The resolution averaged and the data collection time was - 30 min.

cm-‘. The exact cause of this phenomenon is unknown, but it could be related to the background CO desorption observed during annealing to - 135 K or ordering of the overlayer similar to the cause of enhanced peak splitting in matrix isolation experiments [ 301. No evidence of long range order in W(CO), overlayers is obtained in low energy electron diffraction experiments. The characteristic (7 X 7) low energy electron diffraction pattern disappears, and no new features are evident after adsorption of one monolayer of W(CO), onto the clean surface. The (7 x 7) diffraction pattern reappears after the W(CO), overlayer is annealed to 300 K. Notably, a small amount of W(CO), decomposes during the low energy electron diffraction experiment as indicated by the observation of tungsten in Auger electron spectra obtained after heating above 300 K. This is consistent with previous studies showing that low energy electrons decompose W(CO), adsorbed on Si(lll)-(7 x 7) [39]. The reappearance of the (7 x 7) pattern further suggests that the remaining tungsten fragments formed during the low energy electron diffraction experiment do not form an ordered array. As anticipated based on earlier work, a substantial amount of electron induced decomposition occurs during collection of Auger electron

153

quenching by a surface

data of W(CO), overlayers, rendering this technique unreliable for quantitative coverage or compositional determinations of tungsten carbonyls in our experiments. A series of Auger spectra, collected at 1.5 minute intervals at the same spot on the crystal, are summarized in table 1. One monolayer of W(CO), was initially adsorbed; and data were collected with an incident electron energy of 1800 V, - 20 PA of emission current with - 1.25 min required for collection of a single Auger spectrum. The ratio of the tungsten (170 ev> integrated Auger peak intensity to that for the carbon (272 eV) peak was determined to be 1.089 during acquisition of the first spectrum subsequent to W(CO), adsorption. The tungsten to carbon Auger ratio remained constant, within experimental error, in the seven subsequent spectra resulting in an average ratio of 1.077 f 0.092. After annealing to 200 K to desorb intact W(CO),, the tungsten to carbon ratio was measured to be 1.089 + 0.046, identical, within experimental error, to the first spectrum. Recall that no tungsten or carbon was detectable after heating to 200 K if the adsorbed W(CO), is not exposed to an electron beam. Thus, observation of tungsten and carbon on the surface after annealing to 200 K is clear evidence that significant decomposition of W(CO), occurs during collection of a single Auger spectrum. Subsequent heating to 300 K, 450 K and 700 K yielded Auger spectra with average tungsten to carbon ratios of 1.033 k 0.066, 1.008 & 0.053 and 1.041 & 0.041 respectively; all equal to the tungsten to carbon ratio measured in the first spectrum ob-

Table 1 Summary adsorbed

of Auger electron data collected on Si(l1 l)-(7 x 7)

Temperature (K)

Tungsten to carbon ratio

Fresh 125 300 450 700 1150

1.089 1.077 1.033 1.008 1.041 1.501

for W(CO),

(0 I 1)

Uncertainty

* * f f

0.092 0.066 0.053 0.041 _

The tungsten to carbon ratio is defined as the integrated Auger electron peak area at 170 eV (tungsten) divided by the peak area at 272 eV (carbon).

J. R. Swanson et al. / Photochemical quenching b_va surface

154

tained after W(CO), adsorption within experimental uncertainty. After annealing the sample to 1150 K, the tungsten to carbon ratio was measured to be 1.501, the only appreciably different value obtained in this experiment. Clearly, a reliable measure of the tungsten to carbon Auger ratio as a standard of 1: 6 stoichiometry cannot be obtained with Auger electron spectroscopy; however, an upper bound of 1.077 for the tungsten to carbon Auger peak ratio for such a stoichiometry is obtained from these data since CO is known to desorb during electron bombardment. ,[a)

..: -

3.2. Photo&is

of W(CO),

on Si(lll)-(7

fj0

X 7)

120

180

240 300 360 Laser Pulse

420

480

180

;40-360 Laser

Photolysis of W(CO), adsorbed on Si(lll)(7 X 7) at 130 K with pulsed ultraviolet laser light produces gaseous CO and creates tungsten carbonyl fragments bound to the surface. The evolution of CO as a function of the number of laser pulses resulting from photolysis of W(CO), (0 = 1) with 249 nm light at a fluence of 1.12 mJ/cm* is shown in fig. 4. The data shown in fig. 4 was obtained from a W(CO), layer which was

60

120

540

Fig. 4. The CO (m/e 28) evolution intensity as a function of laser pulse number during photolysis of unannealed W(CO), (0 = 1) on si(lll)-(7 X 7). The zero point for the vertical axis is set at the level of the horizontal axis, so there is still measurable CO evolution even after 600 pulses. The laser wavelength and fluence were 249 nm and 1.12 mJ/cm*, respectively, and the repetition rate was - 1 Hz.

380

420

480

540

Pulse

Fig. 5. The CO (m/e 28) evolution signal as a function of laser pulse number during irradiation of W(CO), (8 = 1) with: (a) 720 nm, 0.086 mJ/cm2; (b) 337 nm, 0.24 mJ/cm’; (c) 308 nm, 1.43 mJfcm2; and (d) 249 nm, 1.41 mJ/cm2 laser light. The curves were offset for clarity and each (except h = 720 nm) approach a non-zero background. The solid lines are the best fit single exponential curves to the data, except at 720 nm, where a straight line was drawn to aid the eye. The samples were annealed to 135 K before photolysis, the evolution curves were normalized to the laser pulse energy and the repetition rate was - 1 Hz.

not annealed to 135 K before photolysis. Only CO production is observed; no tungsten-cont~~ng fragments are detected during photolysis. Qualitatively similar results are observed for photolysis at 308, 337 and 351 nm, although the shapes of the laser induced evolution curves vary substantially for multilayer coverages of W(CO),. In contrast, no gaseous CO is produced upon irradiation with visible laser light at 720 nm and a fluence of 0.086 mJ/cm2. The wavelength dependence of the CO yield during irradiation is shown in fig. 5 for irradiation of W(CO), (6’ 2: 1) with 720, 337, 308 and 249 nm light at fluences of 0.086, 0.24, 1.43 and 1.41 mJ/cm2, respectively (figs. 5a-5d, respectively). All the data shown in the figure were normalized to the laser pulse energy by assuming a one photon process; in addition, the samples were annealed to 135 K before laser irradiation to desorb molecular CO (see below), All of the CO evolution curves corresponding to ultraviolet wavelengths shown in fig. 5 approach a non-zero background.

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In contrast, the data for 720 nm visible light is at zero. The details of the laser induced CO evolution curves will be described below. There is a strong wavelength dependence in the initial CO yield from W(CO), photolysis, with the highest yields observed for 249 nm light. All three ultraviolet wavelengths have a long time decay that is essentially the same. The solid lines in fig. 5 (except for X = 720 nm) are the fits to the data generated by the analysis described in detail below. A series of experiments were performed with varied laser fluence at 249, 308, 337 and 351 nm, the wavelengths of light which induced reaction, to determine that the photodecomposition is a one photon process. The laser fluence was varied by using neutral density filters to attenuate the light intensity as described in the experimental section. One experiment generates four CO laser induced evolution curves which can be compared to extract the fluence dependence of the process. In a one photon process, the yield or mass spectrometer signal for photogenerated CO would vary linearly with the laser fluence. Thus, CO evolution curves collected at different laser fluences should parallel each other if all other variables, such as coverage, remain constant. In this analysis, we assume that

-4

-3 Logartthm

Fluence

Fig. 6. The logarithm of the laser fluence versus the logarithm of the initial CO (m/e 28) evolution peak height generated by photolysis of W(CO), (8 = 2) on Si(lll)-(7 X7) with 249 nm light at three separate fluences (0.11, 0.55 and 1.1 mJ/cm2). The slope of the least squares line drawn through the data is 0.97 with a 0.99 correlation.

Laser

Pulse

Fig. 7. The CO (m/e 28) evolution signal as a function of laser pulse number during photolysis of W(CO), on Si(lll)(7 X 7) with 249 nm light at an average fluence of 1.44 mJ/cm2 for coverages of: (a) 8 = 0.2; (b) 0 = 1.1; (c) 6’ = 1.8; and (d) 0 = 7.6. The zero point for the vertical axis is set at the level of the horizontal axis in all cases.

the W(CO), coverage is relatively constant from one laser pulse to the next because high fluence pulses were alternated with lower fluence pulses. Thus, the amount of adsorbed W(CO), was not rapidly depleted after only a few pulses, while other experimental variables, like temperature, should remain constant over the course of a single experiment. Therefore, a change in fluence dependence would manifest itself as a change in curve shape. For all four ultraviolet wavelengths, the CO production scales linearly with the laser fluence, consistent with a one photon decomposition process for fluences less than or equal to those in our experiment. Confirmation of the one photon nature of the process is shown in fig. 6, where the logarithm of the laser fluence is plotted versus the logarithm of the initial CO evolution peak height for the four curves generated by photolysis of W(CO), (@ = 2) with 249 nm light at an unfiltered fluence of 1.1 mJ/cm2. The least squares line drawn through the data has a slope of 0.97 with a 0.99 correlation; again, consistent with a one photon decomposition process. Shown in fig. 7 are the CO evolution curves resulting from photolysis of several coverages of from submonolayer to multiW(C0) 6 ranging layers with 249 laser light at an average fluence of

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J. R. Swrrnson et al. / Photochemical

i

IA 140

-

180

x3

lb)

xl0

(4

Y260

220 Temperature

I

300

(K)

Fig. 8. Temperature programmed reaction data for CO (m/e 28) obtained after photolysis of W(CO), on Si(lll)-(7 X7) with 249 nm light at coverages of: (a) 0 = 0.5, 200 pulses, 1.32 mJ/cm2; (b) B = 3, 200 pulses, 1.26 mJ/cm2; and (c) B = 10, 868 pulses, 1.42 mJ/cm2. For comparison, the temperature programmed reaction spectra for W (m/e 182) corresponding to (c) is shown in the inset. The heating rate was - 2 K/s and constant.

1.44 mJ/cm2. The curves shown in the figure were normalized to the laser pulse energy as described above and the zero point for the data corresponds to the horizontal axis in all cases. As is evident in fig. 7, only at coverages above one monolayer do the evolution curves contain the initial exponential-like decay: at low coverages, the CO evolution curves are nearly constant, but nonzero and reproducible. After many hundreds of laser pulses, the evolution curves all approach the same nonzero value, independent of initial W(CO), coverage. Temperature programmed reaction data obtained following ultraviolet photolysis shows that carbon monoxide remains bound to tungsten after photolysis. The temperature programmed reaction data shown in fig. 8c obtained after photolysis of W(CO), (6 = 10) with 868 pulses of light with wavelength 249 nm and fluence of 1.42 mJ/cm2 contains a new CO (m/e 28) peak near 250 K. Only CO evolution is observed at 250 K: no tungsten-containing fragments are observed at this temperature as is illustrated by the inset where the tungsten m/e 182 signal corresponding to fig. 8c is shown. The peak near 180 K for both masses is at the same temperature as molecular W(CO),

quenching by a surface

desorption and is not broadened; thus, it is attributed to unphotolyzed parent, Appreciable amounts of photoproduct are only observed at multilayer coverages of W(CO),, as indicated in fig. 8a and fig. 8b, which depict the CO temperature programmed reaction spectra collected after photolysis of a submonolayer coverage and - 3 layers of W(CO),, respectively. Although the lower coverages have been irradiated with fewer pulses (200 versus 86X), qualitative comparisons are still possible since - 50% of the total amount of CO produced during photolysis of the thick We layer (8 =e 10) was evolved during the first 200 pulses. In addition, Auger electron spectra collected after irradiation confirm that ultraviolet photolysis induces decomposition of the weakly adsorbed W(C0,); in general, more tungsten, carbon and oxygen are apparent in Auger spectra collected after photolysis and annealing the sample to 300 I( compared to unphotolyzed samples. A tungsten to carbon Auger peak ratio of 1.5 was measured after photolyzing W(CO), (0 = 2) with 249 nm light (662 pulses, 1.14 mJ/cm2) and annealing to 200 K to remove unphotolyzed W(CO),. In the same experiment, the tungsten to silicon (92 eV) Auger peak ratio was measured to be - 0.1. Although quantitative determination of the tungsten to CO ratio of the photoproduct is not possible because of electron induced decomposition during collection of Auger data, the higher tungsten to carbon Auger peak ratio after photolysis compared to electron beam damaged W(CO), demonstrates that decomposition in fact occurred during laser irradiation. CO does not coordinate to the surface products formed from ultaviolet photolysis of W(CO),. Large exposures of “CO ( - 600 L) at 120 K were used in an attempt to coordinate additional CO to the tungsten-containing photoproducts, but no enhancement of the 13C0 signal corresponding to W(CO), or a photoproduct was observed in any temperature pro~ammed reaction experiment. “CO was exposed to photoproducts generated by 249 nm laser light coexisting with unreacted parent, after annealing to 200 K to desorb residual W(CO), and after annealing to 300 K to decompose the photoproduct. In all cases, except for the “CO peak below 135 K (see below), no significant

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J. R. Swanson et al. / Phofochemical quenching by a surface

increase in 13C0 over its natural abundance was observed in temperature programmed reaction experiments. The CO peak below 135 K attributed to CO sticking to the surface, however, showed a - 200-fold increase in i3C0 over the natural level. Thus 13C0 was sticking to the surface, but was not mcorporated into adsorbed photoproducts. Qualitatively similar results were obtained with 308 nm and 351 nm laser radiation. Sequential adsorption of ‘3C-enriched W(CO), (f? = 1) after photolysis of W(CO), (8 = 1) demonstrates that the low temperature peak near 180 K is exclusively due to molecular W(CO), desorption. Immediately following 13C-enriched W(CO), adsorption, temperature programmed desorption was performed. The low temperature CO peak corresponding to molecular W(CO), desorption was compared for the two isotopes. Within experimental error, the peaks are superimposable suggesting that no photoproduct decomposition occurs below 200 K. The CO yield from photolysis of W(CO), multilayers is greater than for W(CO), bound directly to the surface as shown by first adsorbing W(CO), (8 = 1) followed by i3C-enriched W(CO), (B = 1) onto Si(lll)-(7 X 7) at 130 K. Both ‘*CO and 13C0 were observed during photolysis with 249 nm laser light at a fluence of - 1.0 mJ/cm*, however, - 10 times more 13C0 was produced than “CO during photolysis. The amount of 12C0 produced above 200 K during temperature programmed reaction of the photoproduct was - 65% of the amount of i3C0, suggesting that relatively more CO ligands are ejected from those photoproducts in direct contact with the surface. Infrared spectra obtained after ultraviolet photolysis of multilayer coverages of W(CO), and annealing were different from those collected prior to photolysis. Photolysis of a thick layer of W(CO), (0 = 7.6) with 829 pulses of 249 nm light at a fluence of 1.00 mJ/cm2 induced formation of a new peak in the infrared spectrum observed at - 1700 cm-i characteristic of bridging CO [46], as shown in fig. 9. In contrast, infrared spectra collected after photolysis of both submonolayer and monolayer coverages of W(CO), contain no new features in the carbonyl stretching region, Spectra were obtained for samples immediately following

IO

2120

1360

1600 Energy

( m-l

1640

ZT

1480

1

Fig. 9. Infrared absorption spectrum obtained after photolysis of multilayer W(CO), (6’~ 7.6) on %(111)-(7x7) with 829 pulses of 249 nm light at a fluence of 1.00 mJ/cm2 is shown in the carbonyl stretching region. The resolution was 1 cm-’ and 1024 scans were averaged.

irradiation and after annealing to isolate the photoproduct from the unphotolyzed parent. In both cases, no new stretching features were detected; only diminished intensity of the parent peak was observed. Recall that the infrared spectra shown in fig. 2 set a detection threshold of B = 0.1 for W(CO), on Si(lll)-(7 x 7). Thus, assuming constant infrared absorption cross sections, the inability to detect products generated by photolysis of low coverages of W(CO), is not due solely to poor sensitivity in the infrared experiment. No evidence for long range order of isolated photofragments was obtained by low energy electron diffraction after photolysis and annealing. For example, a (7 x 7) pattern with much higher diffuse background intensity was observed after photolysis of W(CO), (6’ = 1) with 249 nm light (600 pulses, 1.12 mJ/m*) and subsequent isolation of the photoproduct by heating to 200 K. Annealing the photoproduct sequentially to 340, 500 and 700 K did not improve the diffraction pattern. Only after annealing to 1150 K, did the diffuse background attenuate; although, the pattern was not as sharp as that for a freshly prepared clean surface. Irradiation of the Si(lll)-(7 x 7) crystal with 430 pulses of unfocused 249 nm radiation at a fluence of 1.04 mJ/cm2 had no affect on the

f.R. Swanson et al. / Photochemical quenching by a surface

158

observed diffraction pattern. Therefore, laser induced damage of the silicon surface causing the diffuse background in tow energy electron diffraction patterns can be ruled out in these experiments. 3.3. Isolated photoproduct

experiments

The isolated CO and tungsten-containing photoproducts, generated by ultraviolet laser induced decomposition of W(CO), on Si(lll)-(7 x 7), do not undergo subsequent ultraviolet photolysis. Products were isolated by photolyzing W(CO), (B = 1) with several hundred laser pulses and then annealing to 200 K to desorb any remaining W(CO),. The isolated products were irradiated with ultraviolet light, typically with the same wavelength and fluence as used to create them. Experiments were performed with 249, 308 and 3.51 nm laser light, at fluences of 0.93, 1.40 and 0.20 mJ/cm’, respectively. In one experiment, 308 nm laser light (0.86 mJ/cm’) was used to form the photoproducts which were subsequently irradiated with 337 nm light (0.19 mJ/cm2). No laser induced evolution of CO was detected in any of these experiments. Furthermore, temperature progrimed reaction spectra collected after irradiation of the photoproducts, were essentially the same as those obtained after photolysis of W(CO)6 except that the molecular W(CO), peak was absent. 3.4. Isotopically

1

I

labelled CO experiments

The photolysis of 13C0 adsorbed ‘on clean Si(lll)-(7 x 7) and W(CO), overlayers demonstrates that samples must be annealed to - 135 K to preclude the contribution of coadsorbed CO to the CO signal generated by photolysis of W(CO),. In particular, the effect of annealing the W(CO), samples to 135 K to desorb CO before photolysis is illustrated in figs. 1Oa and lob. We (6 * 1) was photolyzed with 308 nm light at a fluence of 1.0 mJ/cm2 and the CO evolution curves were normalized to the laser pulse energy in both cases; the only difference is that in fig. lOa, the sample was annealed to 135 IS before photolysis, while the sample in fig. lob was not annealed. Only CO

Laser

Pulse

Fig. 10. The CO (m/e 28) evolution signal as a function of laser pulse number obtained during photolysis of W(CO), (0 = 1) with 308 nm light at a fluence of 1.0 mJ/cm2 following: (a) annealing the sample to 135 to desorb background CO: and (b) no treatment. The solid lines are the best single exponential fits to the data. The ‘“CO (m/e 29) evolution signal as a function of number of laser pulses obtained during photolysis of pure ‘aC0 adsorbed on a Si(lll)-(7 X7) crystal with 308 nm light at a fluence of 1.22 mJ/cm2 is shown in (c). The difference between the two CO evolution signals depicted in figs, 10a and 1Ob are shown in (d). In all cases, the laser firing rate was - 1 Hz.

thermal desorption was observed during the annealing step. The solid lines drawn in figs. 10a and lob are the best fit single exponential curves to the data. Initially, photolysis of the unannealed sample produces relatively more CO. However, after - 50 laser pulses, the two CO evolution signals line up almost perfectly. This suggests, at least in the early stages, that coadsorbed CO contributes to the laser induced CO evolution curve resulting from photolysis of W(CO),. Laser induced desorption of 13C0 was also detected from clean Si(lll)-(7 X 7) surfaces exwere performed posed only to i3C0 . Experiments on well ordered Si(lll)-(7 X 7) surfaces only; the effect of surface disorder was not investigated. The Si(lll)-(7 x 7) surface was exposed to ‘“CO (- 600 L) at 120 K and “CO was found to have an extremely small sticking coefficient. Although CO adsorption may be occurring on defects, the adsorbed CO will clearly contribute to the yield

J. R. Swanson et al. / Photochemical

during photolysis and, therefore, must be accounted for or removed. Irradiation of a small spot on the crystal face with 308 nm laser light at a fluence of 1.22 mJ/cm* induced i3C0 evolution which decayed rapidly to zero as shown in fig. 1Oc *I. Unfortunately, the small amount of 13C0 adsorbed on Si(lll)-(7 x 7) was not detected by infrared spectroscopy and no carbon or oxygen was detected by Auger electron spectroscopy. However, the presence of adsorbed 13C0 was clearly demonstrated by temperature programmed desorption experiments in which 13C0 desorption is observed below 135 K. In order to probe the state of the CO evolved during irradiation, a mixture of 13C0 and Cl80 was exposed to the surface and irradiated with 351 nm laser light at a fluence of 0.27 mJ/cm*. No cross labelled ‘3C’80 was detected in either laser induced desorption or temperature programmed reaction experiments, indicative of a non-dissociative state for the CO. In addition, fig. 10d shows the difference between figs. 10a and lob, which should represent the contribution of coadsorbed CO to the photolysis data for unannealed W(CO),. A comparison indicates that this difference (fig. 10d) is almost superimposable on the evolution curve resulting from photolysis of i3C0 covered Si(lll)-(7 x 7) (fig. lOc), further evidence that coadsorbed CO is adding to the CO evolution curve caused by ultraviolet photolysis of W(CO), and that annealing to 135 K largely removes the adsorbed CO. The presence of 13C0 coadsorbed with W(CO), was observed in experiments in which i3C0 was adsorbed (- 600 L) either subsequent to or during W(CO), (6’ = 1) adsorption at 120 K. In both cases, photolysis of the W(CO), with 351 nm laser radiation at a fluence of 0.27 mJ/cm* produced CO and 13C0 evolution: the CO signal exhibits an initial exponential-like decay followed by a long, non-zero tail, while the i3C0 signal decays rapidly and is essentially zero after 20 laser pulses. In contrast, adsorption of “CO before adsorption of W(CO), produced no evidence for coadsorbed

#’ A He-Ne laser was used to align the ultraviolet laser beam and a purposely small spot ( - 0.05 cm*) on the Si(lll)-(7 X 7) crystal, well away from the sample holding clips, was irradiated.

quenching by a surface

159

13C0 in subsequent laser induced desorption or temperature programmed reaction experiments, presumably due to displacement by W(CO),. 3.5. Photolysis and xenon

of W(CO),

with coadsorbed benzene

The photochemistry of W(CO), trapped in C,H, adsorbed on the silicon surface was investigated to assess the contribution of possible laser induced thermal effects to the overall decomposition process. In temperature programmed desorption spectra, C,H, exhibits a molecular desorption feature near 290 K at low coverages and a lower temperature peak near 150 K at higher coverages. The high temperature feature is identified with the first layer of C,H6, while the lower temperature feature corresponds to multilayers. No buildup of carbon on the crystal surface is detected by Auger electron spectroscopy after temperature programmed desorption indicating that C,H, does not react on Si(lll)-(7 x 7). No gaseous products were detected during irradiation of C,D, at several coverages (h = 249 nm, average fluence of 1.44 mJ/cm2): no D,, C,D, or C,D, were evolved. C,D, was used in these photolysis experiments because the lower chamber background pressure at mass 4 (D2) as opposed to mass 2 (H,) increases the detection sensitivity. Temperature programmed desorption spectra collected after photolysis contain no new features: only molecular C,D, desorption is observed. Also, Auger electron spectroscopy detects no carbon on the surface after C,D, photolysis and temperature programmed desorption. Qualitatively similar results were obtained for photolysis with 351 nm laser radiation. Thus, C,H, neither thermally decomposes nor photolyzes on the Si(lll)-(7 X 7) surface. In addition, since no laser induced desorption of C,H, is observed at any of the fluences or wavelengths of light used in these investigations, an upper bound of - 30 K can be set for the maximum substrate temperature rise during our photolysis experiments since C,H, desorbs at 150 K during temperature programmed desorption. Photolysis of W(CO), coadsorbed with C,H, on the Si(lll)-(7 X 7) surface with 351 nm laser light produced solely gaseous CO evolution. Ad-

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J. R. Swanson et al. / Photochemical

sorption was performed by coadsorbing a mixture of W(CO), and C,H, with the amount of W(CO), corresponding to a submonolayer dose and the C,H, in large excess. The laser fluence was - 0.27 mJ/cm* in all experiments. No evidence for C,H, desorption during irradiation was obtained. After photolysis, temperature programmed reaction spectra were essentially the same as those observed for W(CO), and C,H, independently irradiated. Auger electron spectroscopy detected tungsten, carbon and oxygen on the surface after temperature programmed desorption. Ultraviolet photodecomposition of W(CO), trapped in benzene on the surface takes place in spite of the benzene and without evolving benzene; thus, the W(CO), photodecomposition can not be thermally driven. Photolysis of W(CO), adsorbed on a Xe covered Si(lll)-(7 x 7) surface also sets an upper bound of - 25 IS on the maximum surface temperature rise during irradiation. As expected, Xe itself weakly physisorbs onto Si(lll)-(7 X 7) and desorbs in a single broad feature near 140 K in temperature programmed desorption experiments (not shown). In contrast, temperature programmed desorption spectra collected after W(CO), adsorption onto a silicon crystal precovered with Xe exhibit two Xe features: a relatively large, broad peak near 150 K and a smaller one near 175 K, immediately following the W(CO), peak at 165 K. The second peak is probably Xe trapped below the W(CO),. A leading edge analysis of the W(CO), desorption when adsorbed on top of Xe yielded a desorption energy of 9.7 kcal/mol, considerably lower than the 12.2 kcal/mol for pure W(CO), adsorbed on Si(lll)-(7 x 7). Similarly, there is a single broad infrared peak near 1970 cm-’ for W(CO), adsorbed on Xe. Gently annealing this sample to - 140 K to desorb molecular CO induces a splitting of the original peak into two bands: one near 1960 cm-’ and another weaker feature near 2000 cm-‘. Photolysis of W(CO), (6 = 1) adsorbed onto a Xe precovered silicon surface with 249 nm laser light at a fluence of - 1.20 mJ/cm* produced gaseous CO, but no gaseous Xe. Data for annealed and unannealed overlayers were qualitatively the same. Infrared spectra collected after

quenching b_v(1 surface

Table 2 Curve fit summary for CO evolution signals generated photolysis of W(CO), (0 - 1) on Si(lll)-(7 x 7) Wavelength (nm)

Relative cross sections

249 308 331 249 ”

1.oo 0.24 0.13 1.96

by

‘) For W(CO), separated from the Si(lll)-(7 x 7) surface by a Xe multilayer.

photolysis contained no new features; only diminished intensity of the parent carbonyl stretching region is observed. After photolysis, temperature programmed reaction yields Xe desorption at 150 K and CO production in peaks similar to those obtained after photolysis of pure W(CO), on Si(lll)-(7 X 7). After the desorption experiment, Auger electron spectroscopy detects tungsten, carbon and oxygen on the crystal surface. Again, the absence of Xe evolution during photolysis rules out laser induced thermal processes as the cause of the decomposition. 3.6. Analysis

of laser induced CO evolution data

In order to quantify the relative efficiency of the photodecomposition of W(CO), adsorbed on Si(lll)-(7 x 7), the initial stages of the CO evolution curves were fit to an exponential decay. The fitting parameters that resulted are summarized in table 2. Only data for W(CO), which contain the initial exponential decay (6 = 1) were analyzed in the fashion described following. Furthermore, except where annealing was impractical (i.e. the Xe layer experiments), the data came exclusively from experiments in which the sample was annealed to remove coadsorbed CO. Our experiments indicate that the yield of CO from photolysis of W(CO), is actually described by two exponential features, a relatively rapid decaying exponential, due to the multilayers, riding on top of a very slowly decaying feature which is attributed to W(CO), on the surface. This is represented by the following model where 6N is the yield of CO, Z is the photon flux, 0, is the initial coverage of multilayer W(CO),, eb is the

J. R. Swanson et al. / Photochemical quenching by a surface

initial coverage of surface ub are the cross sections species, n, and n b are ejected by the two species number. FN = n,u,M,

bound W(CO),, a, and of reaction for the two the number of ligands and x is the laser pulse

exp( -1u,x)

+n,u,lB,

exp( -1u,x).

The first step in the analysis was to normalize the raw CO evolution data with the laser photon flux, This is based the fluence is consistent with a is necessary since to pulse around a in their decay characteristics, and to a of CO from surface a constant offset to the multilayer of the remaining

of reaction.

60

120

180

is related Fig. 11 shows

240 Laser

300

360

to

420

480

540

Pulse

Fig. 11. The straight line (slope = -!~.65XlO-~) generated when the analysis described in the text is applied to the CO evolution data obtained during photolysis of W(CO), with 308 nm laser light (fig. 10a).

161

which is generated when this single exponential/ constant background analysis is performed on the CO evolution curve depicted in fig. lOa, obtained from the photolysis of W(CO), with 308 nm light. In addition, it is possible to take the exponential of the straight line and regenerate the best fit exponential curve to the original CO evolution curve, as illustrated in figs. 10a and lob and fig. 5. The relative efficiencies of the process at the different ultraviolet wavelengths were obtained by ratioing the slopes of the fitted lines (fig. 5) against their respective average laser fluence and then ratioing against the result for 249 nm light. Photolysis of W(CO), multilayers is most efficient with 249 nm laser light and progressively less efficient with 308 and 337 nm radiation. In table 2, the relative cross sections are tabulated for photolysis with 249, 308 and 337 nm light of W(CO), (# = 1) adsorbed on Si(lll)-(7 x 7) (from fig. 5) and with 249 nm irradiation of W(CO), (8 = 1) on top of a Xe layer. The relative cross section for photolysis with 249 nm light of W(CO), on Xe is - 2 times greater than that for W(CO), on clean Si( ill)-(7 X 7) (1.960 versus 1); suggesting that the Si(lll)-(7 x 7) substrate is somehow quenching the excitation. The subtraction of a constant background from the data was chosen over an analysis in which both exponentials are fit because the relative amounts of parent molecules (8,, f3,) are unknown and unrelated as are the product yields of the two species (n,, nb). Furthermore, the signal to noise ratio of the later stages of the photolysis experiment, where the second exponential dominates, is inferior to earlier stages. Finally, due to the very slow decay characteristics of the second exponential it is difficult to obtain enough data for a unique fit in the time scale of our experiments. Nevertheless, as a check to the approximation, the data in fig. 10a were fit with two exponentials. This two exponential fit produced a cross section for multilayer W(CO), within - 10% of the cross section generated by the single exponential/constant background approximation. In confirmation of the approximation, the two exponential fit also gave a cross section for the multilayer W(CO), - 8 times greater than for W(CO), in direct contact with the surface.

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4. Discussion At least three possible mechanisms can be presented to account for the observed ultraviolet laser induced decomposition of W(CO), adsorbed on Si(lll)-(7 X 7). The first possibility is laser induced heating of the substrate which leads to simple thermal decomposition of the W(CO),. In the second mechanism, the laser light excites electron hole pairs in the silicon substrate which then react with the adsorbed W(CO),. The third possibility involves direct electronic excitation of the W(CO), to a repulsive state leading to decomposition. In this work, the observed decomposition is most consistent with electronic excitation of the W(CO), at multilayer coverage, since only in this regime is there strong wavelength dependence in the photoyield. In contrast, at lower W(CO)h little wavelength dependence is obcoverages, served. Thus, the Si(lll)-(7 x 7) surface clearly changes the photochemical reactivity of the adsorbed W(CO),. Laser induced heating of the silicon substrate can be excluded as the mechanism for the photodecomposition of W(CO), adsorbed on Si(lll)(7 x 7) at all coverages. There is no detectable d~omposition of W(CO), during heating at rates on the order of 2 K/s; rather, all W(CO), molecularly desorbs in two features below 200 K. Furthermore, no tungsten, carbon or oxygen was detected on the surface by Auger electron spectroscopy experiments performed after W(CO), desorption. The calculated local surface temperature rise for laser fluences used in this investigation is - 10 K [47], too small of a jump to induce thermal reaction or desorption. In addition, the absence of thermal reaction during temperature programmed reaction precludes laser heating as the mechanism for the photodecomposition of adsorbed W(CO),. Pulsed laser heating of the silicon surface is on a time scale comparable to the laser pulse width (- 10 ns) and, thus, extremely rapid 1481. Such exceedingly high heating rates favor molecular desorption over reaction, because of the difference in their pre-exponential factors, since the differences in energetics for the two competing processes are erased when the temperature is raised above that at which the two processes occur. Al-

quenchrng by a surface

though the pre-exponential factors are not known for either W(CO), desorption or reaction, typically those for desorption are substantially larger than those for reaction. For example, the pre-exponential for dissociation of CO on Mo(ll0) has been measured (493 to be - 10” s-l, while the pre-exponential for desorption is expected to be greater than 10” s-’ based on values measured on other transition metals [50]. Therefore, in the limit of rapid heating, desorption should be even more favored over reaction compared to the slower temperature programmed desorption experiment, and neither is observed. We can further place an upper bound on the temperature rise in our experiments based on the fact that no tungsten-containing fragments, nor coadsorbed benzene or Xe, desorb during photolysis. Therefore, laser induced heating of the Si(lll)-(7 X 7) substrate cannot be driving the photodecomposition of W(CO),. Having ruled out laser heating of the substrate, W(CO), decomposition must be due to electronic excitation of either the adsorbed W(CO), itself or the Si(lll)-(7 x 7) substrate. The interaction of the W(CO), with the Si(lll)-(7 x 7) is exceedingly weak so that the electronic absorption spectrum should be relatively unperturbed with respect to gas and/or condensed phase. A leading edge analysis for the IX, peak indicates the desorption energy is 12.2 f 0.7 kcal/mol under ultrahigh vacuum conditions. This is substantially less than the room temperature heat of sublimation [44], but comparable to the 11 + 1 kcal/mol measured by Ho et al. [37]. Also, the infrared spectrum of W(CO)6 adsorbed on the Si(lll)-(7 X 7) surface confirms the weak nature of the interaction. The infrared spectra exhibit a single absorption at - 1940 cm-’ for all coverages, which is not significantly perturbed from the gas phase value (1997.6 cm-‘) [45] or argon matrix values (1992.0 and 1986.6 cm-‘) [30], and a smaller band at - 2040 cm-’ at higher coverages. Furthermore, no ordering is observed for one monolayer coverages of W(CO), on Si(lll)-(7 X 7) by low energy electron diffraction, another indication of weak interaction between the adsorbate and substrate. Ultraviolet laser induced decomposition of multilayer W(CO), proceeds via a one photon process that is qualitatively different than that for

J. R. Swanson et al. / Photochemical quenching by a surface

W(CO), in direct contact with the Si(lll)-(7 X 7) surface. For unannealed W(CO),, the CO evolution signal generated by ultraviolet photolysis has three distinctive regions. The first region occurs during the first - 20 laser pulses and is a distinctive feature which decays very quickly. Next, there is broad exponential-like decay region which lasts for hundreds of laser pulses. In this second region, strong wavelength dependence is observed in the yield of photogenerated CO. This region is followed by a near constant, but non-zero CO evolution signal which does not decay to zero even after thousands of laser pulses. At submonolayer W(CO), coverages only this near constant CO evolution is observed with all ultraviolet wavelengths. This three component behavior is qualitatively similar to that observed by Ho et al., who investigated the continuous wave laser induced decomposition of W(CO), and Mo(CO), adsorbed on Si(lll)-(7 X 7) under ultrahigh vacuum conditions [37]. They propose that the photodecomposition involves a three step mechanism, starting with M(CO), (M = MO and W) and ending with M(CO),. Thus, in their analysis, laser induced desorption data resulting from photolysis was fit sequentially to three exponential curves, each corresponding to the disappearance of a different adsorbed species (M(CO),, x = 6, 5, 4). This model is proposed on the basis of postphotolysis temperature programmed reaction data, in which three new CO desorption features were observed (the three CO ligands remaining on the proposed M(CO), photoproduct) and Auger electron spectroscopy experiments which indicated that - 4 CO ligands were removed by photolysis. Notably, in our experiments, Auger electron spectroscopy is unreliable as a quantitative measure of surface stoichiometry because of electron induced decomposition of W(CO), which induces CO desorption. In our experiments, for multilayer W(CO), coverages, the presence of the first, rapidly decaying feature in the CO evolution data is entirely attributed to laser induced desorption of coadsorbed CO. The initial rapid decay of CO is eliminated from the laser induced evolution data when the sample is annealed to 135 K and CO is desorbed. Only CO desorption is observed during

163

annealing to 135 K: no tungsten-containing fragments are observed. In experiments in which 13C0 was either adsorbed on top of adsorbed W(CO), or coadsorbed with the W(CO),, laser induced evolution of 13C0 was observed indicating that CO coadsorbed with the W(CO), can contribute to the laser induced evolution signal. The source of the coadsorbed CO is most likely decomposition of W(CO), in the doser. Samples of W(CO), were repeatedly purified; yet, this did not remove the distinctive CO feature from the laser induced evolution curves, and a substantial increase in the CO background was observed during dosing W(CO),. Evidently, it is virtually impossible to dose pure W(CO),. The coadsorbed CO, observed during photolysis of unannealed samples, most likely is liberated by laser induced heating of the substrate, although the mechanism is unknown. The calculated temperature rise during ultraviolet laser irradiation is - 10 K [47], a sufficient temperature rise to desorb the coadsorbed CO. Recall that CO adsorbed on Si(lll)-(7 X 7) desorbs below 135 K in temperature programmed desorption and that ultraviolet laser irradiation induces desorption. Furthermore, the difference between the CO evolution signal with and without annealing the W(CO), overlayer to 135 K is essentially the same as the CO evolution curve generated by irradiation of CO adsorbed on Si(lll)-(7 X 7) (fig. 10). Lastly, there is no significant decrease in the molecular W(CO), peak during temperature programmed desorption experiments after - 20 laser pulses compared to overlayers with similar coverages not irradiated. Clearly, the first decay process is due to CO desorption, not photochemical decomposition of W(CO), as proposed previously 1371. The exponential-like decay feature in the second region of the CO evolution data is likewise only observed for multilayer coverages of W(CO),. For samples annealed to 135 K before photolysis, this exponential decay starts with the first laser pulse; thus, the resulting CO evolution signal actually contains only two regions: the exponential decay and the very slowly decaying background discussed more fully below. The efficiency of the one photon photodecomposition process is strongly wavelength dependent in this region. Photolysis

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with 249 nm laser light induces the most rapid decay, with a relative cross section defined to be 1.00. For comparison, 308 and 337 nm light produce progressively less efficient decay (relative cross sections of 0.24 and 0.13, respectively), and photolysis with 720 nm light evolved no CO and induced no changes in postphotolysis temperature programmed reaction experiments. Recall that these relative cross sections were obtained from samples which were annealed to 135 K before photolysis, precluding the possibility of coadsorbed CO interfering with the analysis. This trend qualitatively follows the ultraviolet/visible absorption spectrum of W(CO), in a hydrocarbon solvent at 77 K (optical densities of - 0.5, - 0.25 and - 0.1 at 249, 308 and 337 nm, respectively [19]. This is consistent with direct electronic excitation of the adsorbed W(CO), being the primary mechanism of photodecomposition, in agreement with the results of others for the same or similar systems [35,37,38]. This assumes that the CO yield is proportional to the ultraviolet/visible absorption of W(CO), and ignores possible trapping of the photoejected CO and changes in the product distribution as a function of photolysis wavelength which are known to occur in the gas phase [22]. Excitation of electron-hole pairs in the bulk silicon which then react with the adsorbed W(CO), is inconsistent with our data and, thus, can also be rejected as a primary driving force of the reaction at least for multilayers. Electron-hole pairs are created in the crystal bulk via interaction of the laser light with the silicon atoms and their affect should be strongest near the surface. In contrast, the presence of the substrate does not enhance the photoprocess, as evidenced by the results indicating that the process is approximately twice as efficient on a Xe covered surface as on clean Si(lll)-(7 x 7). In fact, the opposite is true: the Si(lll)-(7 x 7) surface partially quenches the excitation suggesting that laser induced electron hole pair production is not a primary mechanism in these studies. Also, we observed much more efficient reaction of multilayer W(CO),, which is separated from the surface, as compared to submonolayer coverages again incompatible with an electron-hole pair mechanism. Thus, electronhole pair production cannot be the primary driv-

quenching by a surface

ing force for the multilayer W(CO), photoreaction. The CO evolved in this exponential-like region must be associated with decomposition of W(CO), multilayers not only because it is only observed for multilayer coverages but also because significant decreases in the W(CO), desorption intensity and CO stretch in infrared experiments are only observed for multilayers. Thus, CO evolved in this region is evidently associated with photodecomposition of multilayer W(CO),. This second region can be fit to a single exponential decay curve with a correlation of 0.95 or better after performing a background subtraction. This suggests that CO is evolved in a single step yielding one or more partially decarbonylated tungsten carbonyl fragments which are insensitive to ultraviolet light. Furthermore, the fragments created by ultraviolet irradiation when isolated on the surface do not undergo further ultraviolet photolysis. Temperature programmed desorption experiments performed after adsorption of 13C-enriched W(CO), on top of photolyzed W(CO), indicate that the molecular W(CO), desorption peak is not broadened and cannot contain an unresolved photoproduct peak. Therefore, the tungsten-containing photoproducts yield CO as the only gaseous product. After ultraviolet photolysis of a thick layer of W(CO), (0 = lo), a new peak at - 1700 cm-’ appears in the infrared spectra, in contrast to previous results in which a new loss is observed at - 1490 cm-’ in the high resolution electron energy loss spectroscopy experiments after laser irradiation [37]. The reasons for the discrepancy between our results and Ho et al. [37] are unknown, but may be due to the different detectabilities in infrared and high resolution electron energy loss experiments. However, part of Ho’s argument is based on results of Auger electron spectroscopy experiments, and W(CO), is known to rapidly decompose upon exposure to electrons [39]. In addition, high resolution electron energy loss spectroscopy experiments employ low energy electrons which may damage the W(CO), overlayer. The third region in the laser induced CO evolution data consisting of nearly constant, but nonzero, production of CO during photolysis is attri-

J. R. Swanson et al. / Phorochemical quenching by a surface

buted to W(CO), directly interacting with Si(lll)(7 x 7). Roughly the same amount of CO is produced both in the latter stages of photolysis of multilayers and immediately during photolysis of submonolayer coverages of W(CO),. In contrast to the multilayer results, little wavelength dependence is observed in the photoyield in this region: near constant levels of CO production for all ultraviolet photolysis wavelengths. This is in stark contrast to the three step mechanism proposed by Ho et al. in which the second and third portions of the laser induced CO desorption curves are assigned to the decomposition of surface adsorbed photoproducts (M(CO), and M(CO),, respectively) and where there was no distinction made between submonolayer and multilayer coverages of W(CO),. In our data, the slowly decaying region is not attributed to loss of CO from a photoproduct because isolated photoproducts were shown to be insensitive to ultraviolet light based on laser induced desorption and temperature programmed desorption experiments. Also, this nearly constant production of CO cannot be due to CO trapped in a W(CO), matrix on the surface, because laser induced desorption experiments performed on coadsorbed i3C0 and W(CO), indicate that the 13C0 signal decays rapidly to zero and that a negligible amount of 13C0, consistent with natural abundance, is contained in the long time tail. In addition, this region cannot be attributed to repeated ejection and readsorption of background CO because laser irradiation experiments conducted on W(CO), samples exposed to 13C0 before photolysis demonstrate that the 13C0 signal decays rapidly to zero. Furthermore, we have no evidence for second product formation in postphotolysis temperature programmed desorption and infrared experiments. Thus, it seems highly unlikely that this flat region is associated with the photodecomposition of the product generated during the earlier stages of photolysis. W(CO), in direct contact with the surface has much less efficient photochemistry than multilayer W(CO),; therefore, it is impossible to rule out electron-hole pair production as the primary mechanism for decomposition of W(CO), touching the surface. The lack of wavelength specificity in the photoyield is consistent with an electron-

165

hole model since all of the wavelengths of light used in this investigation are more energetic than the indirect bandgap of silicon (1.1 eV). However, even 720 nm photons have more energy (1.7 eV) than the bandgap and we detected no photolysis at this wavelength; but, the low fluence at this wavelength coupled with its much greater penetration depth and, therefore, smaller carrier concentration at the surface renders quantitative comparisons impossible. Perhaps the electronic excitation of the W(CO), is completely quenched by the surface, and the observed photodecomposition is due to electron hole pairs. The results for photolysis of a W(CO), (6 = 1) on Xe indicate that even at this coverage some quenching by the surface occurs. Unfortunately, we have no method for differentiating the two mechanisms. The W(CO), decomposition mechanism is undoubtedly a primary factor governing the overall purity of tungsten films grown via photolysis of W(CO),. The isolated photoproducts generated by single photon ultraviolet laser decomposition of adsorbed W(CO), are remarkably insensitive to further photolysis at the wavelengths of light used in this study. Isolated photoproducts produced by photolysis of W(CO), with 249, 308 and 351 nm radiation followed by annealing to 200 K to desorb the remaining W(CO), did not decompose further when exposed to additional ultraviolet light of the same wavelength as evidenced by laser induced desorption and temperature programmed reaction experiments. Furthermore, photoproducts produced with 308 nm light were not sensitive to 337 nm light. Unfortunately, the exact nature of the photoproducts is unknown. The isolated photoproducts on the surface must be coordinatively saturated since we were unable to readsorb 13C0 onto them as determined by temperature programmed reaction and infrared experiments. It should be noted that the annealing step used to isolate the products may change them; therefore, direct comparison to unannealed photoproducts is difficult. However, this photochemical insensitivity contrasts with the results of Ho et al. in which the primary photodecomposition product of W(CO), is proposed to undergo further photolysis [37]; however, Ho reported no photolysis studies of isolated photoproducts similar to those dis-

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cussed here. Such insensitivity of the photoproduct to additional photolysis is not surprising if the isolated product is coordinated to the substrate. In this case, the substrate would be expected to quench any further excitation of the fragment on a very rapid time scale and, thus, prevent additional photolysis. The insensitivity of the isolated photoproducts to additional ultraviolet photolysis could be a contributing factor to the overall poor quality of tungsten films grown from W(CO),. At multilayer coverages of W(CO),, the products generated by photolysis react with each other or with other W(CO), most likely to form clusters. Evidence for cluster formation in multilayers is obtained from the infrared spectrum collected after photolysis of a thick (0 = 7.6) deposit of W(CO), with 308 nm light (fig. 9) which contains a new - 1700 cm-‘. carbonyl stretching feature at Carbonyl ligands bridging three metal atoms in cluster compounds typically have stretches in the range of 1600-1750 cm-’ [46], in good agreement with our results. Unfortunately, the identity of the clusters cannot be determined by the methods available in this work. Surprisingly, the photoproducts generated at multilayer coverages of W(CO), are unreactive towards i3C0 and C,H,. This contrasts gas phase results, in which the coordinatively unsaturated photoproducts are highly reactive [22]. The absence of reaction with coadsorbed ligands suggests that rapid reaction of photoproducts with either the surface or other coordinatively unsaturated tungsten-containing fragments predominates, consistent with the observed clustering in multilayers. This work suggests that the impurities remaining in tungsten films grown by photodecomposiunder practical conditions are tion of W(CO), intrinsically limited by the energetics for CO detachment, the insensitivity of the primary photoproduct to further photolysis and the thermal chemistry of the products. Our results indicate that it is impossible to remove all six CO ligands from W(CO), by purely photochemical means under the conditions used in these investigations. Finally, the remaining CO ligands are not completely detached in subsequent temperature programmed reaction experiments, rather partial CO

quenching

by a surfuce

dissociation occurs based on the observation of tungsten, carbon and oxygen on the Si(l1 l)-(7 x 7) surface by Auger electron spectroscopy #2. These carbon and oxygen impurities are not thermally labile and do not exhibit infrared features. They are, therefore, most likely in the form of carbides and oxides.

5. Conclusions Ultraviolet

photodecomposition Si(lll)-(7 x 7) surface proceeds via a one photon process. Significantly different photochemical behavior is observed for multilayer W(CO), than for W(CO), directly in contact with the surface. At multilayer coverages, the photodecomposition involves a single step, and the yield of evolved CO is highly wavelength dependent. This wavelength dependence is consistent with direct electronic excitation of the W(CO), being the primary mechanism. In contrast, the photodecomposition process for W(CO), directly interacting with the surface is highly quenched and no wavelength dependence is observed in the product yield. At multilayer W(CO), coverages, molecular CO coadsorbed with the W(CO), adds to the laser induced CO desorption signal via laser heating of the substrate. This coadsorbed CO is most likely introduced into the vacuum chamber by W(CO), decomposing in the doser. If the coadsorbed CO is not removed from the sample by annealing before photolysis, it interferes with the analysis of the resulting laser induced desorption data. The products of the photoreaction, when isolated on the surface, are photo-inactive. This inactivity prevents further removal of CO ligands and, thus, potentially limits the purity of tungsten films photolytically grown from W(CO), on surfaces. For multilayer coverages of W(CO),, the photofragments are unreactive towards i3C0 and C,H, yet form metal carbonyl cluster compounds, either amongst themselves or with unreacted W(CO),. Auger electron spectroscopy is shown to be useful ** The tungsten

to carbon and tungsten to silicon Auger peak ratios after ultraviolet photolysis and annealing to 200 K were 1.5 and - 0.1, respectively. in one experiment.

J. R. Swanson et al. / Photochemical

for

collecting only qualitative information: is rapidly decomposed during data W(CQ, acquisition; therefore, it is impossible to develop a standard for the 1 : 6 stoichiometry in W(CO), from the ratio of the tungsten and carbon Auger peak areas.

Acknowledgements We gratefully acknowledge and thank the National Science Foundation (CHE-83-09455 and CHE-84-51307) and the Harvard University Materials Research Laboratory (NSF DMR-83169790) for support of this work, Professor D.J. Darensbourg of Texas A&M University for proW(CO), and Dr. Y.J. viding the 13C-enriched Chabal of AT&T Laboratories for his help in setting up the infrared spectroscopy experiment.

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