J . Phys. Chem. 1992, 96, 334-341
334
as berlinite. Therefore, the free energy of reaction 4 for AlP04-n molecular sieves can be estimated according to the available free energies of formation of dense AlP04 and SiOz phases, e.g., berlinite (AlP04) and quartz (SO2), which are structural analogues. For free energy calculations, we assume that AlC13 and PCls in reaction 4 are both gas-phase species since experimental reaction conditions employ high temperatures where these chlorides will be gas-phase species. As shown in Table IV, the free energy for the reaction of berlinite with SiCl, to form quartz at a temperature of 298 K is 301.5 kJ/mol. This value is very high compared to that observed for the reaction of an aluminosilicate zeolite with SiC14. The thermochemical properties of AlP04 berlinite at higher temperatures are not available to us. However, thermochemical data for the components in reaction 4 at different temperatures are available if we allow the AlP04 not to be berlinite.22 Using these data, it can be calculated that in a large temperature range the free energies do not significantly change (22) Barin, 1.; Knacke, 0. Thermochemical Properties of Inorganic Substances; Springer-Verlag: 1973. Barin, 1.; Knacke, 0.;Kubaschewski, 0. Thermochemical Properties of Inorganic Substances (Supplement); Spring er-Verlag: Berlin, 1973.
for the reaction of a dense AP04 phase with Sic&to form a dense SiOz phase (Table IV). Therefore, the free energy of reaction 4 should not change significantly at elevated temperatures. In a gross approximation, we may conclude that the reaction of AlP04-n molecular sieves with SiCl, is not thermodynamically favorable. Indeed, the experimental results show that substitution of A1 and P in A1Po4-5 framework by Si with SiC14 at elevated temperatures is unlikely. A very small amount of silicon observed in the treated AlP04-5 may be due to the silicon incorporated into defect sites (where hydroxyl groups were present). Silanation can take place when SiC14 contacts hydroxyl groups. This may be the reason for the initial temperature rise of 3-5 K when Alp04-5 is contacted with SiC14vapor. Silicon chloride species may reside in the channel of the molecular sieve and upon washing be hydrolyzed to form amorphous silica. These types of species are certainly capable of decreasing the argon adsorption capacity of the SiCl,-treated samples.
Acknowledgment. Support of this work was provided by Akzo America, Inc. Registry No. SAPO-5, 12736-95-7; SiCI,, 10026-04-7; AIP04-5, 7784-30-7.
Molecular Precursors to Boron Nitride Thin Films. 1. Adsorption of Diborane on Ru(0001), NH,/Ru(0001), and O/Ru(0001) Surfaces Josi A. Rodriguez, Charles M. Truong, Jason S. Comeille, and D. Wayne Goodman* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 (Received: April 8, 1991)
The adsorption of diborane (B2H6) on clean Ru(0001) and on surfaces precovered by NH, and oxygen has been studied by means of thermal desorption mass spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and Fourier-transform infrared reflectance absorption spectroscopy. At low coverages, the B2& molecules completely dissociated on Ru(0001), producing atomic hydrogen that desorbed as H2 between 250 and 450 K and boron adatoms that stayed on the surface up to temperatures above 1400 K. At large B2H6 exposures, desorption of the B2H6multilayer was observed at 150 K, with a broad feature at temperatures between 250 and 400 K due to desorptionof chemisorbed Bz&. The presence of boron adatoms enhanced the adsorption energy of NH3 on Ru(0001). Boron-nitrogen adlayers were formed by exposing BfRu(0001) surfaces to high pressures (5-10 Torr) of NH, or by coadsorbmg NH3 and B& at 90 K with sukquent annealing to 600 K. These adlayers were rich in boron and decomposed at temperatures well above 1100 K. On O/Ru(0001) surfaces, diborane was adsorbed dissociatively, forming strong B-O bonds. Adsorbed atomic oxygen largely reduced the stability of boron-nitrogen adlayers on Ru(0001).
-
I. Introduction (HREELS)? electron-stimulated desorption ion angular distribution (ESDIAD)? and work function measurements? Ammonia Boron nitride (BN) films are extensively used in modern is adsorbed on Ru(0001) molecularly, via the N end with the H technology as high-temperature thermal and electrical insulators.14 atoms pointing away from the surface. It desorbs without disTheir high melting point, good mechanical properties, and high sociation a t temperatures of 110 (multilayer), 140 (second resistivity make them ideal for use in the electronic industry. In monolayer), 180, and 290 K (monolayer). To the best of our recent years, a large amount of effort has been focused on developing routes to synthesize boron nitride in thin-film f ~ r m . ~ . ~ knowledge no study has appeared that examines the adsorption of diborane on Ru(0001). The present work adds to the few Diborane (B&) and ammonia ( N H J have been used as mostudies' in which the interaction between BzH, and a metal surface lecular precursors in the synthesis of BN films prepared by thermal and plasma-enhanced chemical vapor depositi~n.~?~ In the present work we investigate the interaction between NH3 and B2H6 on (1) Kouvetakis, J.; Patel, V. V.; Miller, C. W.; Beach, P. B. J . Vuc. Sci. Ru(0001) using thermal desorption mass spectroscopy (TDS), Technol. A 1990. 8. 3929. X-ray photoelectron spectroscopy (XPS), Auger electron spec(2) Weissmank; C.-Bewilogua, K.; Breuer, K.; Dietrich, D.; Ebersbach, troscopy (AES), and Fourier-transform infrared reflectance abV.; Erler, H. J.; Rau, 9.; Reisse, G. Thin Solid Films 1982, 96, 31. sorption spectroscopy (FT-IRAS). (3) Arya, S. P. S.; DAmico, A. Thin Solids Films 1988, 157, 267 and references therein. The adsorption of NH3 on Ru(0001) has been previously in(4) Weissmantel, C. Thin Films from Free Atoms and Particles; Klavestigated by means of TDS,5,6low-energy electron diffraction bunde, K. J., Ed.;Academic Press: New York, 1985; Chapter 4. (LEED),5*6high-resolution electron energy loss spectroscopy (5) Benndorf, C.; Madey, T. E. Surf. Sci. 1983, 135, 164.
-
* Author to whom correspondence should be addressed.
(6) Zhou, Y.; Akhter, S.; White, J. M. Surf. Sci. 1988, 202, 357. (7) Fryberger, T. B.; Grant, J. L.; Stair, P. C. Langmuir 1987, 3, 1015.
0022-3654/92/2096-334%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 335
NH3 and BzH6 on Ru(0001)
U ai
Ill
Q,
2 ,
;Ek&Y
0.25 L
100
200
300
400
500
600
0.25 L 700
TEMPERATURE, K Figure 2. Representative thermal desorption spectra showing the evolution of H2 following various B2H6exposures to clean Ru(0001) at a temperature of -90 K. Heating rate 7 K/s.
c n ,
i
B TDS
~
‘
1 EXPOSURE, 90 K 6L 2.5 L I1.3L 0.6 L I
’
ZI 700
I
800
900
1000 I100 1200 1300
TEMPERATURE, K Figure 3. Representative thermal desorption spectra showing the evolution of B following various B2H6exposures to clean Ru(0001)at a temperature of -90 K. The dashed spectra was acquired after saturating the surface with boron (0, = 1.1 ML) by dosing B2H6 at 500 K. Heating rate 7 K/s.
450 K and can be assigned to monolayer desorption. This region saturates a t an exposure of -2.0 langmuir and contains contributions from a t least two adsorption states. The final region to be filled desorbs at 150 K. It can be assigned to the desorption of the B2H6 multilayer since it grows continuously with exposure without saturation. In previous studies,’ a temperature of 160 K was reported for B2H6 multilayer desorption from Mo(100). The only boron-hydrogen species desorbing from the surface in the TDS experiments was B2H6. Signals observed for other boron-hydrogen species were part of the cracking pattern of diborane in the mass spectrometer. The thermal desorption spectra for H2evolution (Figure 2) are characterized by the presence of three distinct peaks at approximately 150, 280, and 380 K. In all the cases investigated, no desorption of hydrogen was observed at temperatures higher than 500 K. The desorption features above 230 K are a product of the decomposition of chemisorbed B2H6. At B2H6exposures below 1.5 langmuir, the hydrogen desorption peak is rate-limited probably by the associative desorption of adsorbed hydrogen atoms (2H, H2,J rather than by B-H bond scission, since this peak is very similar in temperature and shape to that for H2 desorption from hydrogen adatoms (deposited with H2 gas) on Ru(0001).” For
-
(8) (a) Campbell, R. A.; Rodriguez, J. A.; Goodman, D. W. Surf.Sci. 1990, 240, 71. (b) Campbell, R. A.; Goodman, D. W. Reu. Scientific In-
strumentation. in o m s .
(9) Houston, J.’E.;Peden, C. H. F.; Feibelman, P.J.; Hamann, D. R. SurJ Sci. 1987, 192, 457.
(IO) Houston, J. E.; Peden, C. H. F.; Blair, D. S.; Goodman, D. W. Surf. Sci. 1986, 167, 427.
-
( 1 1 ) (a) Peden, C. H. F.; Goodman, D. W.; Houston, J. E.; Yates, J. T. Surf.Sci. 1986, 194, 92. (b) Feulner, P.; Menzel, D. Surf. Sci. 1985, 154,
465.
336 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 B2H6 exposures greater than 2 langmuir, the TDS spectra for H2 evolution show a new line shape. At these large exposures significant desorption of B2H6 is also observed (see Figure 1). In these highaverage cases, the lack of a sufficient number of empty sites on the surface prevents full dehydrogenation of diborane. In Figure 2 the states above 230 K saturate at -2.5 langmuir whereas no saturation was observed for the H2-TDS peak at 150 K. This 150 K feature appears simultaneously with B2H6 multilayer desorption and is probably a consequence of the cracking of B2H6 in the mass spectrometer. TDS spectra of boron from B2&/Ru(OO01) are shown in Figure 3 as a function of diborane exposure. The m / e = 11 peaks are a consequence of the desorption of a boron species. No B2, B&,, BH3, BH2, or BH was detected in the mass spectrometer at the high-temperature conditions of Figure 3. For low B2H6 exposures (X0.6 langmuir), no desorption of B was observed at temperatures below 1450 K. The desorption features for boron are very broad, covering a temperature range between 1050 and 1250 K. Saturation of these features is observed at a B2H6 exposure of -2.5 langmuir. AES and XPS analyses showed that an appreciable amount of boron was still present on Ru(0001) after annealing the surface to 1450 K. These strongly bound boron atoms could be removed by oxidation in O2 and annealing to 1650 K. The amount of B2H6 that decomposed on Ru(0001) was determined by comparing the area in the spectra of Figure 2 (B& exposures of 0.25,0.60, and 1.10 langmuir) with the H2-TDS area measured after a saturation H2 exposure to Ru(0001). This exposure is known to give an absolute coverage of 1 hydrogen atom per Ru surface atom.12 These results were used to calibrate the average integrated B( ls)/Ru(3d) XPS intensity ratios. This procedure allowed the determination of the maximum amount of that reacts with Ru(0001) at 90 K (eB1H6 = 0.24 ML (monolayer)), the maximum amount of B2H6 that desorbs above 230 K (eB2H6 = 0.03 ML) and the maximum amount of diborane that completely dehydrogenates (eB2H6 = 0.21 ML) and remains as boron residue (0, = 0.42 ML) after flashing to 5 0 0 K. Using the van der Waals radius of diborane (-2.7 AI6) one can estimate the area occupied for a B2H6 molecule adsorbed with its B atoms parallel to the surface: -2.3 X cm2. Based on this, the saturation coverage of a close-packed layer of B2H6 on Ru(0001) should be close to 0.25 ML. This value is very close to the maximum amount of diborane that reacted with Ru(0001) at 90 K in our experiments: 0.24 B2H6 molecule per Ru surface atom. Upon heating, most of the adsorbed diborane completely dissociated producing boron and hydrogen atoms. The fact that diborane decomposes extensively on Ru(0001) is not surprising, if one takes into consideration the low energy necessary for B2H6 2BH3 (-38 kcal/molI8) and BH B + H (79 kcal/molI9) in the gas phase. These energies can be easily compensated by the formation of Ru-B (binding energy > 70 kcal/mol”) and Ru-H bonds (binding energy -67 kcal/mollib).
-
-
(12) Sun, Y.-K.; Weinberg, W. H. Surf. Sci. 1989, 214, L246. (13) Duncan, J. L.; McKean, D. C.; Torto, I.; Nivellini, G.D. J . Molec. Spectrosc. 1981,85, 198I . (14) (a) Kiessling, R. Acta Chem. Scand. 1950,4,209. (b) Joyner, D. J.; Johnson, 0.; Hercules, D. M. J . Am. Chem. SOC.1980, 102, 1910. (c) Joyner, D. J.; Johnson, 0.; Hercules, D. M.; Bullet, D. W.; Weaver, J. H. Phys. Reo. B 1981, 24, 3122. (15) (a) Rodriguez, J. A.; Truong, C. M.; Goodman, D. W. J . Vac. Sci. Technol., in press. (b) Rodriguez, J. A. Surf. Sci. 1990, 234, 421 and references therein. (16) (a) Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, 1986; D-188.(b) The van der Waals radius of diborane was calculated by using the as phase geometry of the molecule13and the van der Waals radius of H.’t *! (17) This lower limit for the bondin energy of B on Ru(0001) was estimated by using the Redhead algorithmq1 for first-order desorption kinetics, with a desorption temperature of 1150 K (see Figure 3) and a preexponential factor of IOl3 s-l. A much larger bonding energy can be expected for the B atoms that are still adsorbed on the surface after annealing to 1450 K. (18) Page, M.; Adams, G.F.; BinMey, J. S.; Melius, C. F. J . h y s . Chem. i9~,91,2675. (19) Jolly, W. L. Modern Inorganic Chemisfry;McGraw-Hill: New York, 1984; p 56.
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Rodriguez et al. 82HdRu(0001)
FT-IRAS SPECTRA
, I
2500
1500
2000
1000
WAVENUMBERS Figure 4. FT-IRAS spectra for a 3-layer B2H6film. Diborane was dosed a t 90 K to the Ru(0001) surface. The spectra were acquired after annealing the surface to the temperatures indicated in the right side of the figure. B(ls) XPS B2H6/Ru(0001)
202
200
188
186
184
BINDING ENERGY, eV
figure 5. B(1s) spectra taken after dosing 4 langmuir of B2H6to Ru(0001) at 100 K. Before acquiring the spectra, the surface was heated to the temperatures indicated in the right side of the figure. Annealing from 100 to 230 K induced desorption of the B2H6 multilayer. Above 450 K, only atomic B was on the surface.
The TDS results support the following reaction scheme for diborane on Ru(0001): low B& exposures (0, I0.15 ML)
250-400
high exposures
2H, (0,
H2
> 0.25 ML)
90 K
B2H6
K
BxHy,a + Ha + B2H6.physisorbcd
(3)
150 K
B2H6
B2H6,physisorbd 250-400
BxHy,a
K
B2H6
+ H2 + Ba
(4) (5)
1050-1250 K
Ba Brl (6) The exact identity of the BxHyspecies is unknown at the present time. During the desorption experiments, neither BH3 nor BH2 were detected with the mass spectrometer as desorption products from the surface. Figure 4 gives FT-IRAS results for a saturation coverage of diborane on Ru(0001). The spectra correspond to a condensed film, which is estimated to be -3 layers thick. O n l y minor
The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 337
NH3 and BzH6on Ru(0001) 1
1
1
I
N (15) XPS NH3/B/Ru(0001)
S(1S) XPS
I
B/Ru(0001)
n
I
a i; K
a1
1
\
406
404
402
400
398
396
406
404
402
400
398
396
BINDING ENERGY, eV
I
192
,
I
190
I
I
100
.
l
I
106
I
184
BINDING ENERGY, eV Figure 6. Effect of coverage upon the B(1s) XPS spectrum of atomic boron on Ru(0001). The B adlayers were formed by dosing B2Hs to the Ru(0001) surface at 500 K.
changes are observed in the IRAS spectra when the surface temperature is increased from 90 to 130 K. Above 150 K (desorption temperature of B2H6 multilayers), no feature that could be attributed to an adsorbed boron-hydrogen species was observed. This is probably a consequence of the low cross-section of the vibrational modes associated with an adsorbed BxHvspecies. The spectra of Figure 4 are in good agreement with infrared spectra for diborane in gas and crystal phases.13 The bands at 2600 and 2490 cm-I have been assigned to the stretching modes of the terminal B-H bonds.I3 Figure 5 shows B( 1s) XPS spectra acquired after saturating the Ru(0001) surface with B2H6. The peak observed at 100 K is very broad and is the product of electrons emitted from the BzH6 multilayer (region toward higher binding energy) and the adsorbed B a y species (region toward lower binding energy). Upon heating to 230 K, the B2H6 multilayer desorbs and a B( 1s) peak centered at 187.4 eV remains. Further heating to 500 K leaves only atomic boron on the surface and a B(1s) peak centered now at 187.55 eV. Finally, heating the surface to 1250 K induces a large decrease in the intensity of the B( 1s) signal probably as a consequence of boron desorption (see Figure 3). B. Adsorption at 500 K. The adsorption of diborane on Ru(0001) was also investigated at 500 K. At this high temperature, the BZH6 molecule totally decomposed on the surface, producing hydrogen that evolved into the gas phase as H2 and leaving adsorbed atomic boron on Ru(0001). Under these conditions a saturation coverage of 1.15 ML was observed for atomic boron. At OB = 1.15 ML, a fraction of the boron desorbed between 1000 and 1200 K (dashed line in Figure 3), with a portion of the adlayer (e, 7 0.2 ML) remaining on the surface at 1450 K. Figure 6 shows the effect of B coverage on the B(1s) XPS spectrum of B/Ru(0001). The peak position increases by -0.6 eV when the boron coverage increases from 0.13 to 1.O ML. An identical trend was observed for B adlayers on Mo( in which case strong chemical interactions between adsorbed boron atoms were responsible for the large B( 1s) shift with the ~ v e r a g e .The ~ same explanation is likely valid for the results in Figure 6. It is consistent with the tendency of boron to form boron-boron bonds in transition metal borides.I4 This has been corroborated by quantum chemical calculations that show appreciable bonding interactions between boron atoms separated by 2.71 A (rnetalmetal distance) on R u ( O O O ~ ) . ~ ~ Recently, we have investigated the interaction between atomic boron and Ru(0001) at a molecular orbital 1e~el.I~ The quantum chemical calculations were based on the INDO method.'5b
-
Figure 7. N(1s) spectra for ammonia adsorbed on clean Ru(0001) (part a) and over a surface precovered with 0.35 ML of boron (part b). Ammonia was dosed at a surface temperature of 90 K. The spectra were taken after annealing the surfaces to the temperatures indicated in the figure.
Clusters containing 15-18 metal atoms were used to model the Ru(0001) surface. Boron was adsorbed on 3-fold hollow sites and OB was varied between 0.1 and 0.3 ML. The calculations suggest that boron is a moderate electron acceptor when bonded to Ru(OOOl).15aCharges varying from -0.3Oe to -0.35e were calculated for the B adatoms. The direction of charge transfer predicted by INDO for the B/Ru(0001) system agrees quite well with experimental measurements that show an increase in the work function of Mo(100) when B is a d ~ o r b e d .Adsorption ~ of B on Ru(OOO1) induced a strong hybridization between the B(2s) and B(2p) orbitals.'" In general, the results of the INDO calculations indicated Ru-B bonds dominated by interactions between the B(2p) and Ru(4d) levels. The percentage of the adsorption bond in which each type of Ru orbital participated was approximately as 5s, 1 5 % 5p, 25%; and 4d,60%. From the viewpoint of the adsorbate, the B(2s) orbitals were involved in only 30% of the chemisorption bond, whereas the B(2p) orbitals participated in as much as 70%.'5a These trends are in good agreement with photoemission experiments and ab initio calculations for bulk iron borides (Fe2B and FeB) that show F e B bonds which are mainly a consequence of mixing the B(2p) with the iron 3d valence le~eL3.l~ III.2 A"in Adsorption on Clean Ru(0001). The interaction of NH3 with Ru(0001) was found to be quite similar to published results for this systema5q6The clean and well-ordered Ru(0001) surface showed only molecularly adsorbed NH3, with no decomposition products observable in surface analysis or TDS. At low coverages (&H, < 0.15 ML), ammonia desorbed from Ru(0001) with a TDS peak maximum at 300 K. Saturated Ru(0001) surfaces showed NH3 features at 110 (multilayer desorption), 135 (second monolayer), 190, and 285 K (monolayer). Figure 7a shows a N( 1s) XPS spectrum acquired after adsorbing 4-5 ML of ammonia on Ru(0001) at 90 K. The peak position is centered at -400.9 eV. Increasing the temperature to 150 K induced desorption of the ammonia multilayers, leaving only the chemisorbed species. This surface gave a N( 1s) XPS peak centered at -400.4 eV. The shift in peak position seen in Figure 7a after desorption of the multilayers is very similar to that observed for other NH3/metal systems.20 IU.3 Coadsorptionof Ammonia and Diborane on Ru(0001). A. Ammonia on B/Ru(0001). The effect of atomic boron on the decomposition of ammonia on Ru(0001) was investigated at temperatures between 90 and 300 K. The boron adlayers were generated by dosing diborane to Ru(0001) at 500 K. Under ultrahigh vacuum conditions the amount of ammonia that dissociated on the B/Ru(0001) surfaces was negligible. In a set of experiments ammonia was dosed at 90 K to Ru(0001) surfaces with B coverages between 0.15 and 1.1 ML.
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(20) Grunze, M.; Brundle, C. R.;Tomanek, D.Surf: Sci. 1982, f19, 133.
Rodriguez et al.
338 The Journal of Physical Chemistry, Vol. 96, No. I, 1992
1 B(1s)XPS
TDS NH3 FROM NH3iS/Ru(0001
A
NH3/B/Ru(0001)
-
195 193 191 189 187 185 183
'9 407 405 403 401 399 397 395
BINDING ENERGY, eV
0.58
100
200
300
400
500
TEMPERATURE, K Figure 8. Effect of B precoverage on the desorption of NH3 from Ru(Oool). The spectra were acquired after dosing 5 langmuir of ammonia to the B/Ru(0001) surfaces at 90 K. Heating rate 7 K/s.
Figure 8 shows NH3-TDS spectra acquired after dosing 5 langmuir of ammonia to different B/Ru(0001) surfaces. This large NH3 exposure saturates the chemisorbed states of the molecule. In all the cases investigated, the adsorbed ammonia desorbed at temperatures below 450 K. Above this temperature, the AES and XPS spectra showed only very small coverages of nitrogen (KO.05 ML). No significant evolution of H2 or N2 was detected during the TDS experiments. It appears that under ultrahigh vacuum conditions the interaction between boron adatoms and ammonia is not strong enough to produce boron-nitrogen films. However, atomic boron affects the behavior of ammonia on Ru(0001). In Figure 8, a new NH3 desorption state at -350 K is observed in the presence of boron. This state is at a higher temperature than those found on clean Ru(0001). Assuming first-order desorption and a typical preexponential factor of loi3s-', a standard Redhead analysis2' yields a desorption activation energy of -21 kcal/mol for the new NH3 adsorption state induced by boron, which can be compared with the value of 18 kcal/mol observed at low coverages of ammonia on clean Ru(0o01) (desorption temperature -300 K). Thus, for 6~ < 0.5 ML, the B-NH3 interaction enhances the adsorption energy of ammonia on Ru(0001). An identical phenomenon has been observed when atomic 0 and NH3 were coadsorbed on Ru(0001).22 In contrast, the presence of B adatoms reduced the bonding energy of CO on Ru(OOO~).~~ Figure 7b displays N( 1s) XPS spectra for ammonia adsorbed on a Ru(0001) surface with 0.35 ML of B. An increase in the surface temperature from 90 to 150 K induced desorption of the NH3 multilayers, producing a shift in peak position from -400.9 to -400.6 eV. A similar trend was observed for ammonia on clean Ru(0001) (see Figure 7a), with the peak position for the monolayer at -400.4 eV. Annealing of the NH3/B/Ru(0001) surface to
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(21) Redhead, P. A. Vacuum 1962, 12, 203. (22) Benndorf, C.; Madey, T. E. Chem. Phys. Left. 1983, 101, 59. (23) (a) Rodriguez, J. A.; Truong, C. M.; Kuhn, W. K.; Goodman, D. W. J . Chem. Phys., in press. (b) Truong, C. M.; Rodriguez, J. A,; Goodman, D. W. J . Phys. Chem. 1991, following paper in this issue. (24) Feulner, P.; Kulkarni, S . ; Umbach, E.; Menzel, D. Surf. Sci. 1980, 99, 489. (25) (a) McKee, M. L. Inorg. Chem. 1988, 27,4241. (b) Umeyama, H.; Morokuma, K. J . Am. Chem. SOC.1976, 98, 7208. (c) Carpenter, J. D.; Auk, B. S. J . Phys. Chem. 1991, 95, 3502.
Figure 9. B(1s) and N(1s) XPS spectra for boron-nitrogen adlayers on Ru(0001) formed by reaction between atomic boron and NH3: (a) 0.6 ML of atomic boron on Ru(0001) at 300 K; (b) after exposure of the surface to 10 Torr of NH3during 10 min at 300 K in the high-pressure cell; (c) and (d) after annealing the surface that gave spectra b to 600 and 1000 K, respectively.
400 K led to desorption of the chemisorbed ammonia, leaving a N( Is) signal within the noise level of our instrument (6, < 0.03 ML). The corresponding B(1s) XPS spectra showed a decrease of -0.25 eV in the peak position after adsorption of ammonia on the B/Ru(0001) surface at 90 K. This small NH3-induced shift is consistent with the TDS results of Figure 8 that show moderate interactions between the coadsorbed NH3 and B species. The effects of NH3 adsorption upon the B(ls) binding energy of the B adatoms were reversible. The B( Is) peak position observed after desorption of NH3 was identical to that seen before adsorption of the molecule. Recent molecular orbital calculations for NH3/Ru(0001)i5show charges on ammonia of -+0.4e and +0Se when the molecule is adsorbed at a-top and 3-fold hollow sites of the surface, respectively. On metal surfaces and in inorganic compounds, ammonia (a Lewis base) usually behaves as an electron donor g r o ~ p . ' ~On . ~ ~Ru(0001), most of the charge transferred from ammonia comes from the 3al orbital (N lone pair).i5a The RuNH3 chemisorption bond is dominated by the interaction between the NH3(3al) orbital and the Ru(Sp,4d) orbitals. The contribution of the metal orbitals to the chemisorption bond is as follows:'5a 5s, 17%; Sp, -43%; and 4d, -40%. The TDS results presented above indicate that the B-NH3 interaction is attractive. This phenomenon can be explained by means of a simple model: adsorbed boron atoms create Ru sites on the surface with partial positive charge, which facilitates electron donation from ammonia to these metal sites. In addition, there is a stabilizing throughspace interaction between the partial positive charge on the ammonia molecules. Our model agrees well with the shifts observed in XPS core-level binding energies when B and NH3 are coadsorbed (see above). The type of interaction observed for the coadsorbed species in the NH3/B/Ru(0001) system is totally contrary to that seen in the CO/B/Ru(0001) system.z3 We found that the B-CO interaction is repulsive due to the electron-acceptor nature of both adsorbatesz3 Experiments were carried out to investigate the interaction between high pressures of NH3 and B/Ru(0001) surfaces. In these experiments, the B adlayers (6, = 0.4-1.1 ML) were prepared in the UHV chamber, and then the crystal was transferred into the high-pressure cell and exposed to 5-10 Torr of NH3 for 10 min at -300 K. Post-reaction surface analysis in the UHV chamber with AES and XPS showed the presence of B and N. For these surfaces, an increase in the temperature from 300 to
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(26) (a) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; Freeman: New York, 1990 Chapter 1 1 . (b) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 2nd 4.; Wiley: New York, 1987; Chapter 12.
The Journal of Physical Chemistry, Vol. 96, No. I, 1992 339 v)
t z
'c-
TDS
a
g
N
H
3
NH3 x3
Nz
v)
z W
BzHs/Nl-b/Ru(0001)
NHa/BzH6/Ru(0001)
3
Hz 8
100
200
400
300
500
600
700
800
TEMPERATURE, K
Figure 11. Thermal desorption spectra acquired after coadsorbing 2 ML of ammonia (dosed first) and -1.5 ML of diborane (dosed second) on Ru(0001) at 90 K. Heating rate 7 K/s.
monia were adsorbed on the surface before dosing B&. Figure 11 shows thermal desorption spectra taken after coadsorbing 2 ML of ammonia and 1.5 ML of diborane on Ru(OOO1). These results indicate that upon heating, a large fraction of the diborane molecules was able to diffuse through the NH3film reaching the Ru(0001) substrate. The B2H6-TDS spectrum is similar to those observed in Figure 1 for Ru(0001) surfaces saturated with BzH6. In contrast, the Hz- and NH,-TDS spectra show features not observed before for the NH3/Ru(0001) and BzH6/Ru(0001) systems. For the BzH6/NH3/Ru(0001) surface, ammonia evolution was seen at temperatures as high as 450 K. In Figure 11, the H,-TDS peak at -500 K is rate-limited probably by the scission of B-H or N-H bonds, since it occurs well above the normal hydrogen desorption temperature on Ru(0001) (250-400 K"). The desorption peak for Nz appears at a temperature close to that found for recombination and desorption of nitrogen atoms (2N, N ) on Ru(OO01).'*24 The fact that there is simultaneous evolution H z and Nz at 500 K suggests the decomposition of a NH, species on the surface. AES and XPS spectra showed the presence of 0.35 ML of B and 0.2 ML of N on the Ru(0001) substrate after the TDS experiment. These data indicate that there was a reaction between B2H6 and NH3. In gas phase the reaction 2NH3 --c 2H3NBH3 is well known (AH -25 kcal/ However, no H3NBH3was detected in our thermal desorption experiment. In a similar way FT-IRAS spectra taken after coadsorbing multilayers of B2H6 and NH3 at 90-1 10 K showed only bands that could be attributed to NH3 or Bz&. These bands disappeared after flashing the surface to 200 K. Figure 12 displays B(1s) and N(1s) spectra acquired during an experiment similar to that described in the previous paragraph. The peak positions observed at 90 K are in very good agreement with the corresponding values for multilayers of NH, and B2H6. Heating to 200 K produced desorption of the physisorbed NH3 and BzH6 (see Figure 11). The XPS spectra observed at these conditions are very different from those observed for chemisorption of ammonia (Figure 7a) or diborane (Figure 5) on clean Ru(0001). In Figure 12, the N(1s) peak at 200 K (-401.5 eV) appears at higher binding energy than those for physisorbed (-401.0 eV) and chemisorbed (-400.4 eV) ammonia. On the other hand, the B( 1s) spectrum at 200 K shows features at lower binding energy than the spectrum for chemisorbed diborane. These differences are consistent with the formation of a dative bond between NH3 and BH3. In this bond, electron density will flow from ammonia (a Lewis base) into BH3 (a Lewis acid) making a H3N BH3 adduct.z5v26 Dramatic changes are observed in Figure 12 after annealing the surface from 200 to 600 K. For the N( 1s) features there is a decrease in peak intensity (N, and NH3 desorption, see Figure 11) and a large shift toward lower binding energy. Further heating to 1000 K results in a shift of the B(1s) features toward higher binding energy. This shift is probably a consequence of a change
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The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Rodriguez et al.
B(1s) XPS BzH6/NHdRu(0001)
d. 1000 K
I
BzH6/O/RU(0001)
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I I
195 193 191 189 187 185 183
k
I
I
I
I
I
I I
409 407 405 403 401 399 397 395
BINDtNG ENERGY, eV Figure 12. B(1s) and N(1s) XPS spectra for boron-nitrogen adlayers on Ru(0001) formed by reaction between ammonia (3 ML, dosed first) and diborane (- 1.5 ML, dosed second): (a) at 90 K after dosing B2H6 and NH,, (b) at 200 K after desorbing the N H , and B& multilayers, (c) after flashing to 600 K to desorb B&, NH,, H2, and N2, and (d) after annealing to 1000 K.
in the morphology of the boron-nitrogen adlayer. At 10oO K the Ru(0001) surface was covered by 0.38 ML of B and 0.22 ML of N. The fact that nitrogen was still on the surface suggests some kind of bond between the N and B adatoms. The peak positions seen at these conditions (B(ls), -189.4 eV; N(ls), -399.0 eV) are very different from the corresponding values for atomic boron (B(1s) = 187.5 eV, Figure 6) and nitrogen (N(1s) = 397.3 eV23) on Ru(0001). However, the N(1s) and B(1s) binding energies at lo00 K are close to those reported for boron-nitride films that are rich in boron.' 111.4 Coadsorption with Oxygen. Oxygen appears as a major contaminant in the synthesis of boron nitride films from NH3and B2H6, modifying the thermal, electrical, and mechanical properties of the coatings.24 We investigated the effects of atomic oxygen on the surface chemistry of B2H6 on Ru(0001), and the interaction of O2with B and BxNy adlayers. In a set of experiments diborane was dosed to O/Ru(0001) surfaces a t 90 K. The precoverage of oxygen was varied from 0.1 to 0.5 ML (the maximum Bo achieved on a clean Ru(0001) ~ u r f a c e ~ ' - ~ ~For ) . Be,", < 0.2 ML, the only species detected in the TDS experiments at temperatures below 1000 K were H2 (200-400 K) and very small amounts of H 2 0 (200-250 K). Figure 13 shows O(1s) and B(ls) XPS spectra acquired after dosing 0.1 ML of B2H6to a Ru(0001) surface precovered with 0.4 ML of atomic oxygen. Adsorption of B2H6induced a shift of > 1 eV toward higher binding energy in the O( 1s) features. The peak position of 192.5 eV seen for the B( 1s) spectrum is much higher (-5.5 eV) than that observed for the B2H6/Ru(0001) system and is close to those found for compounds with B-O bonds.30 This indicates that there is direct bonding between the oxygen and boron atoms on the surface, a phenomenon that is not surprising if one takes into consideration the large dissociation energy of the BO diatomic molecule in gas phase (187 kcal/mo13') and the heat of formation of B2O3 (-304 kcal/mol3I). Figure 14a shows spectra acquired after dosing different amounts of oxygen to a surface precovered with 0.9 ML of B. The presence of oxygen atoms induced the appearance of a second peak
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(27) Doering, D. L.; Madey, T. E. Surf. Sci. 1982, 123, 305. (28) Kiskinova, M.; Rangelov, G.; Surnev, L. Surf. Sci. 1986, 172, 57. (29) Pfnur, H.; Held, G.; Lindroos, M.; Menzel, D. Surf. Sci. 1989, 220,
43. (30) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1979; p 36. (31) Lange's Handbook of Chemistry, 13th ed.; Dean, J . A,, Ed.; McGraw-Hill: New York, 1985; pp 3-128 and 9-11.
194
192
190
188
186
184
BINDING ENERGY, eV
Figure 13. O(1s) and B(1s) XPS spectra acquired at 90 K after dosing 0.1 ML of B2H6 to a Ru(0001) surface precovered with 0.4 ML of atomic oxygen. For comparison, the spectra for oxygen on Ru(0001), 60 = 0.4 ML, and diborane on Ru(0001), OstHs = 0.1 ML, are also shown.
194 193 192 191 190 189 188 187 186
538 538 534 532 530 528 526 524
BINDING ENERGY, eV
Figure 14. (a): Effect of atomic oxygen upon the B(1s) XPS spectrum of 0.90ML of B on Ru(0001). Oxygen was dosed to the B/Ru(0001) surface at a temperature of 350 K. (b): O(1s) spectra for a Ru(0001) surface saturated with atomic oxygen, Bo = 0.5 ML, and for a O / B / Ru(0001) surface with 0.78 ML of oxygen and 0.90 ML of boron.
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in the B( 1s) region. The peak position of 192 eV is close to that reported for B203.30The O(1s) spectrum seen in Figure 14b for the O/B/Ru(0001) surface is at higher binding energy (-2 eV) than that found for O/Ru(0001). The boron adatoms enhanced the ability of the Ru(0001) to chemisorb oxygen. Oxygen coverages in excess of 0.5 ML were easily obtained when 0 and B were coadsorbed. 80 = 0.5 ML is the saturation coverage on clean R u ( O O O ~ ) . ~Thermal ~ - ~ ~ desorption spectra of a Ru(0001) surface with 0.78 ML of 0 and 0.90 ML of B showed appreciable signals for m / e = 11 (B), 16 (0),and 27 (probably BO) at temperatures between 1000 and 1250 K. 0 and B were still on the surface at 1400 K. A clean surface was observed after flashing to 1650 K. At room temperature, boron-nitrogen adlayers with boron and nitrogen coverages below 0.5 ML showed a large capacity to adsorb O2dissociatively. In all the cases examined, adsorbed atomic oxygen reduced the stability of boron-nitrogen adlayers on Ru(0001). Figure 15 displays B(1s) and N(ls) XPS spectra taken after dosing 0.4 ML of oxygen to a boron-nitrogen overlayer with 0.33 ML of B and 0 . 1 8 ML of N. The boron-nitrogen adlayer was formed by dosing NH3 and B2H6 at 90 K and subsequent annealing to 1000 K (see above). Drastic changes in the XPS spectra appeared upon adsorption of oxygen, indicating that there was a direct reaction of this element with the boron-nitrogen adlayer. In gas phase, the bond energy between B and 0 is -95 kcal/mol larger than that between B and NS3' The presence of
341
J. Phys. Chem. 1992,96, 341-347
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on Ru(0001), producing boron adatoms and H2 gas. The saturation coverage of B on Ru(0001) is 1.1 ML. At this coverage, part of the B adlayer can be removed from the surface by heating to 1250 K, with a large fraction of the B atoms still adsorbed at 1450 K.
UJ
t d. 1000 K
w
196 194 192 190 188 186 184 406 404 402 400 398 396 394 392
BINDING ENERGY, eV
Figure 15. Effect of atomic oxygen upon the B(1s) and N(1s) XF'S spectra for boron-nitrogen adlayers on Ru(0001): (a) after dosing NH3 (first) and B2H6(second) at 90 K and subsequent annealing to 1000 K (OB = 0.33 ML and ON = 0.18 ML); (b) after dosing 0.4 ML of 0 at 300 K (c) and (d) after annealing the surface that gave spectra b to 800 and 1000 K (6, = 0.33 ML, ON = 0.08 ML, and Bo = 0.4 ML).
oxygen reduced the stability of the boron-nitrogen adlayer by weakening the B-N bonds. Annealing of the O/B/N/Ru(0001) surface from 300 K to 800 and 1000 K induced an appreciable loss of nitrogen atoms (see Figure 15), a phenomenon that was not observed for the B/N/Ru(0001) surface. The B(1s) peak position seen in Figure 15 for spectrum d is very close to those displayed in Figures 13 and 14 for the boron-oxygen adlayers. The results of this section indicate quite clearly that diborane will react preferentially with oxygen impurities during the preparation of boron-nitride films from B2H6 + NH,, producing coatings with a low level of thermal stability.
IV. 'Conclusions (1) At a temperature of 500 K, diborane dissociates completely
(2) BzH6thermal desorption spectra acquired after adsorption of diborane on Ru(0001) at 90 K showed a sharp peak at 150 K (multilayer desorption) and a broad feature between 250 and 400 K (monolayer desorption). Evolution of H2was observed from 250 to 400 K. Above 400 K, only atomic boron was left on the surface. The maximum coverage of boron atoms produced during these experiments was 0.42 ML. (3) On the clean, well-ordered Ru(0001) surface, ammonia adsorbed molecularly. Saturated Ru(0001) surfaces showed NH, features at 110 (multilayer desorption), 135 (second monolayer), 190, and 285 K (first monolayer). Under ultrahigh vacuum conditions the amount of ammonia that dissociated on B/Ru(0001) surfaces was negligible. For BB < 0.5 ML, the B-.NH3 interaction enhanced the adsorption energy of ammonia on Ru(0001). A new NH3 desorption state a t -350 K was observed in the presence of boron adatoms. (4) Boron-nitrogen adlayers were formed: (a) by exposing B/Ru(0001) surfaces at 600 K to high pressures (5-10 Torr) of NH,; and (b) after coadsorption of NH3 (dosed first) and B2H6 (dosed second) at 90 K, and subsequent annealing to temperatures above 600 K. The boron-nitrogen overlayers were rich in boron and decomposed at temperatures above 1100 K. ( 5 ) On O/Ru(0001) surfaces, diborane was adsorbed dissociatively forming B-O bonds. Adsorbed atomic oxygen reduced the stability of the boron-nitrogen adlayers on Ru(0001).
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Acknowledgment. We acknowledge with pleasure the support of this work by the Texas Advanced Research Programs under Grant No. 160769. Registry NO.BZH6, 19287-45-7;Ru, 7440-18-8; NH,, 7664-41-7; 0, 17778-80-2; BN, 10043-11-5.
Molecular Precursors to Boron Nitride Thin Films. 2. Coadsorption and Reaction of Hydrazine and Diborane on Ru(0001) Charles M. Truong, Josd A. Rodriguez, and D. Wayne Goodman* Department of Chemistry, Texas A& M University, College Station, Texas 77843- 3255 (Received: May 30, 1991)
The adsorption of hydrazine (NzH4) on clean Ru(0001) and its coadsorption and reaction with diborane (B&) have been studied by means of thermal desorption mass spectroscopy, X-ray photoelectron spectroscopy,Auger electron spectroscopy, and Fourier-transform infrared reflectance absorption spectroscopy. On clean Ru(0001), desorption of N2H4was observed at temperatures of 185 (multilayer state) and 280 K (monolayer state, desorption energy -17 kcal/mol). Adsorbed hydrazine decomposed extensively into NH3, Nz, N, and H. TDS spectra from N/Ru(0001) surfaces showed N2 evolution in a sharp peak at -510 K and in a broad feature between 600 and 800 K. Boron-nitrogen adlayers with stoichiometry close to 1:l were formed: (a) by simultaneous dosing of B2H6 and N2H4 at 450 K, and (b) after coadsorption of B& and N2H4at 90 K and subsequent heating to 450 K. These adlayers decomposed at temperatures above 1100 K.
I. Introduction
BN in thin film forms.I-" In the preceding article in this journal,
inertness. These physical and chemical pioperties k a k e boron nitride useful for thermal insulation, as a die wash material, and as a lubrication and pressure-transmission m e d i ~ m . l - ~Considerable effort has been focused on developing routes to synthesize 'Author to whom correspondence should be addressed.
0022-3654/92/2096-341$03.00/0
S.;DAmico, A. Thin Solid Films 1988, 157, 261. (2) Weissmantel, C. In Thin Filmsfrom Free Aroms and Particles; Klabunde, K. J,, Ed,; Academic Press: New York, 1985;Chapter 4. (3) Kouvetakis, J.; Patel, V. V.; Miller, C. W.; Beach, D. B. J. Vac. Sci. Technol. A 1990, 8, 3929. (4) Rodriguez, J. A.; Truong, C. M.; Corneille, J. S.; Goodman, D. W. J. Phys. Chem.: preceding paper-in this journal. (1) Arya, S. P.
0 1992 American Chemical Society
