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Growth of aluminium nitride on porous silica by atomic layer chemical vapour deposition
Applied Surface Science 165 Ž2000. 193–202 www.elsevier.nlrlocaterapsusc
Growth of aluminium nitride on porous silica by atomic layer chemical vapour deposition R.L. Puurunen a,b,) , A. Root c , P. Sarv d , S. Haukka b,1, E.I. Iiskola b,2 , M. Lindblad e, A.O.I. Krause a a
c
Helsinki UniÕersity of Technology, Industrial Chemistry, P.O. Box 6100, FIN-02015 HUT, Finland b Microchemistry Ltd., P.O. Box 132, FIN-02631 Espoo, Finland Fortum Oil and Gas Scientific SerÕices, Technology Center, P.O. Box 310, FIN-06100 PorÕoo, Finland d Institute of Chemical Physics and Biophysics, Akadeemia 23, 12618 Tallinn, Estonia e Fortum Oil and Gas Oy, P.O. Box 110, FIN-00048 FORTUM, Finland Received 13 April 2000; accepted 18 May 2000
Abstract Aluminium nitride ŽAlN. was grown on porous silica by atomic layer chemical vapour deposition ŽALCVD. from trimethylaluminium ŽTMA. and ammonia precursors. The ALCVD growth is based on alternating, separated, saturating reactions of the gaseous precursors with the solid substrate. TMA and ammonia were reacted at 423 and 623 K, respectively, on silica which had been dehydroxylated at 1023 K and pretreated with ammonia at 823 K. The growth in three reaction cycles was investigated quantitatively by elemental analysis, and the surface reaction products were identified by IR and solid state 27Al and 29 Si NMR measurements. Steady growth of about 2 aluminium and 2 nitrogen atomsrnm2silicarreaction cycle was obtained. The growth mainly took place through Ži. the reaction of TMA which resulted in surface Al–Me and Si–Me groups, and Žii. the reaction of ammonia which replaced the aluminium-bonded methyl groups with amino groups. Ammonia also reacted in part with the silicon-bonded methyl groups formed in the dissociative reaction of TMA with siloxane bridges. TMA reacted with the amino groups, as it did with surface silanol groups and siloxane bridges. In general, the Al–N layer interacted strongly with the silica substrate, but in the third reaction cycle AlN-type sites may have formed. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.G Keywords: Aluminium nitride; Trimethylaluminium; Ammonia; ALCVD; IR; NMR
1. Introduction
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Corresponding author. Tel.: q358-9-451-2582; fax: q358-9451-2622. E-mail address: [email protected] ŽR.L. Puurunen.. 1 Present address: ASM Microchemistry. 2 Present address: Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland.
Aluminium nitride ŽAlN. w1x, silicon nitride w2x, and a hydrogen-containing Al–N layer, grafted on silica from liquid phase w3,4x, have been successfully studied as supports for metal catalysts in hydrogenation w1x, oxidation w2x, isomerisation w3x, and hydroformylation w4x. Although nitrides are potential catalyst supports, their catalyst applications have been
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 4 0 - 2
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hindered by the difficulty of obtaining them in a porous form suitable for catalyst applications. One approach to obtaining nitrides with high specific surface area is to coat porous oxide supports with a layer of the nitride. The layer should cover the whole surface area, and preferably be uniform in thickness. Atomic layer chemical vapour deposition ŽALCVD., also called atomic layer epitaxy ŽALE., is a technique based on well-separated saturating reactions of gas-phase precursors with a solid substrate. Its suitability for growing conformal metal nitride thin films on trenched surfaces has been demonstrated w5x, and it is also suitable for modifying porous oxides with other oxides in a controlled and reproducible way w6x. Thus, it has potential for coating of oxides with nitrides. Growth by ALCVD requires volatile precursors that do not condense and that are thermally stable under the applied reaction conditions. What must take place on the surface is chemisorption, not physisorption, of the precursors. Furthermore, the reactions must be saturating, so that sufficiently large precursor doses on similarly treated substrates always result in the same amount of chemisorption products. The saturation content of the chemisorption products is determined not only by the precursor reactivity but also by the availability of the bonding sites on the substrate and the size of the precursor molecule. For well-characterised growth, the bonding sites of the substrate and the surface species formed in each reaction step should be known. In general, suitable precursors for the ALCVD growth of AlN, which is our interest, are volatile aluminium compounds, such as alkyls, alkoxides, halides, and b-diketonates. Ammonia or hydrazine could be used as the nitrogen-containing precursor. We chose trimethylaluminium ŽTMA. and ammonia as precursors because their suitability has been shown for the manufacture of AlN both as a bulk material w7–10x and as a thin film w11–20x. Bulk AlN growth is based on the following types of reactions, in which one methyl group at a time combines with a hydrogen atom of the ammonia molecule or a surface amino group to form methane w7,9,10x: AlMe 3 q NH 3 Me 2 AlNH 2 q CH 4 Ž 1.
™ Me AlNH ™ MeAlNHq CH MeAlNH ™ AlN q CH . 2
2
4
4
Ž 2. Ž 3.
The above equations are idealised in the sense that, although reaction Ž3. mostly occurs by 700 K, the purity and crystallinity of AlN improves if the sample is heated under ammonia to about 1300 K w10x. The growth of AlN films from TMA and ammonia precursors has been studied on nonporous highsurface-area silica w21,22x and alumina w23–27x. Controlled growth has been achieved at 600 K by dosing TMA and ammonia separately w22,24,27x Žas in the ALCVD processing scheme.. To our knowledge, attempts have not yet been made to grow AlN on porous materials for catalyst purposes. The goal of this work was to study the applicability of the ALCVD technique for growing AlN on porous silica. Three reaction cycles of TMA and ammonia at 423 and 673 K, respectively, were carried out on silica dehydroxylated at 1023 K and pretreated with ammonia at 823 K. Quantitatively the growth was investigated with element determinations, and the surface species formed during the reactions were identified by diffuse reflectance IR spectroscopy ŽDRIFT. and solid state 29 Si and 27Al NMR spectroscopy.
2. Experimental 2.1. Materials Grace 432 silica of particle size 315–500 mm was used as substrate. The silica was heated in ambient air in a muffle furnace for 16 h at 1023 K to stabilise the number and type of bonding sites for the subsequent reactions. In addition, it was heated in the ALCVD reactor under vacuum for 3 h at 823 K to remove water physisorbed during transfer to the reactor. After this procedure, the specific surface area and pore volume of silica were 290 m2 gy1 and 1.0 cm3 gy1 , respectively. Nitrogen ŽOy Aga Ab, H 2 O - 3 ppm, O 2 - 3 ppm. was used as an inert carrier gas. Before use, it was purified with Messer Griesham Hydrosorb and Oxisorb filters, which yielded concentrations of water and oxygen less than 0.3 and 0.1 ppm, respectively. TMA ŽWitco GmbH, 97.5 wt.%, Cl - 0.01 wt.%. and ammonia ŽOy Aga Ab, 99.998%, H 2 O - 5 ppm. were used as received.
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2.2. Reaction procedure The reactions were carried out at a reduced pressure of 1–5 kPa in a modified flow-type ALCVD reactor ŽMicrochemistry, Finland. described in detail in Ref. w6x. Typically, 5–10 g of silica was loaded into the reactor. The reactants were kept at room temperature. During the reactions, they were transported in a flow of nitrogen downwards through a stationary silica bed. The reactions were started by pretreating silica with ammonia at 823 K to form primary siliconbonded amino groups ŽSi–NH 2 groups. on the surface. AlN was processed in one to three cycles of the saturating reactions of TMA and ammonia. One reaction cycle consisted of one TMA reaction, a purge with nitrogen, an ammonia reaction, and a purge with nitrogen. The nitrogen purges separated the TMA and ammonia reactions, and eliminated unwanted gas-phase reactions by removing unreacted precursors Žeither TMA or ammonia. and the gaseous reaction products. TMA was reacted with silica at 423 K. The reaction of ammonia was started at the reaction temperature of TMA Ž423 K., and then gradually elevated to 673 K under a flow of ammonia. Ammonia was fed during the temperature increase to avoid thermal decomposition of the TMA chemisorption products. Because the samples reacted easily with humidity, they were handled inertly in nitrogen. 2.3. Characterisation The aluminium contents of the samples were measured with a Siemens SRS303 X-ray fluorescence spectrometer, and the nitrogen and carbon contents with a LECO CHN analyser Žtreatment at 1223 K with oxygen.. The specific surface areas were measured according to the Brunauer–Emmett–Teller ŽBET. method with a Micrometrics Asap 2400 instrument. The aluminium, nitrogen, and carbon contents were calculated with respect to the original silica surface by subtracting the masses of aluminium, nitrogen, carbon, and the calculated mass of hydrogen from the mass of the sample. The mass of hydrogen was calculated assuming an H:C ratio of three Žall carbon in methyl groups. and an average H:N ratio of 1.5 in the amino groups. Because of the
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low molar mass of hydrogen, deviation of the hydrogen content from 3 and 1.5 in the assumed H:C and H:N ratios resulted in negligible error, on the order of 0.01 atomsrnm2silica Žnote that the unit used, atomsrnm 2silica , differs from surface density, atomsrnm2 ; the former counts all the atoms present in the sample, not just those on the outermost layer.. DRIFT spectra were measured with a Nicolet Impact 400 D diffuse reflectance spectrometer connected air-tightly to a nitrogen-filled glove-box. The samples were ground in an agate mortar and packed loosely in the sample holder. The spectra were recorded at room temperature in the wavenumber range 4000–650 cmy1 with a spectral resolution of 2 cmy1 . 29 Si cross-polarization magic-angle spinning ŽCPMAS. NMR spectra were acquired with a 270 MHz Chemagnetics CMX Infinity Spectrometer using a self-built 10-mm probe. The samples were transferred to the rotors under dry nitrogen. A 5-ms contact time, 33-kHz radio frequency field, 5-s recycle delay, and 4-kHz MAS speed were used in the experiments, and between 10,000 and 20,000 transients were acquired. The chemical shifts were referenced to 3-Žtrimethylsilyl.propanesulfonic acid sodium salt Žq1.6 ppm., which was itself referenced to tetramethylsilane ŽTMS. at 0 ppm. 27 Al MAS NMR spectra were recorded with a Bruker AMX500 spectrometer using a self-built 3.5mm probe. The spinning speed was 12 kHz and the radio-frequency field strength was 70 kHz. The spectra were acquired using a 108 excitation pulse, 0.2-s delay, and 20,000–40,000 transients. Delay of 5 s was tested, but it did not give additional signal intensity. KAlŽSO4 . 2 P 12H 2 O was used as an external chemical shift reference Ž0 ppm. and also as an intensity reference. The samples were loaded into rotors under dry nitrogen atmosphere ŽO 2 5 ppm, H 2 O 2 ppm..
3. Results The growth of AlN from TMA and ammonia precursors was studied on silica dehydroxylated at 1023 K that had been reacted with ammonia at 823 K. In an earlier work w28x, we found that about 0.4 of the 1.0 isolated hydroxyl groups rnm2silica present on
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1023 K silica exchanged with secondary amino groups ŽSi–NH 2 . during ammonia reaction. The ammonia did not dissociate on siloxane bridges. The present reactions with TMA and ammonia were carried out at 423 and 673 K, respectively. 3.1. Elemental analysis Fig. 1A shows the aluminium, carbon, and nitrogen contents on the modified silica substrate after one to three full reaction cycles. The amounts of aluminium and nitrogen on the surface increased steadily as a function of the number of reaction cycles. The average rates of increase of aluminium and nitrogen contents per reaction cycle were about 2 atomsrnm2silica . The carbon content stayed at a constant level of about 0.8rnm2silica . Fig. 1B presents the element contents separately after TMA and ammonia reaction steps during two
Fig. 2. ŽA. DRIFT spectra of Ža. the ammonia-modified silica, Žb. the ammonia-modified silica reacted with TMA at 423 K, Žc. the b-sample reacted with ammonia at 673 K, and Žd. the c-sample reacted with TMA at 423 K. ŽB. DRIFT spectra of ammonia-modified silica modified with Ža. one, Žb. two, and Žc. three reaction cycles of TMA and ammonia.
Fig. 1. ŽA. The aluminium, carbon and nitrogen contents on silica dehydroxylated at 1023 K and reacted with ammonia at 823 K, as a function of the number of reaction cycles of TMA and ammonia at 423 and 673 K, respectively. ŽB. The element contents after each reaction step during two reaction cycles. The ‘‘0.5’’ and ‘‘1.5’’ mean that the growth ended in TMA reaction.
reaction cycles. In the TMA reaction, the amounts of aluminium and carbon on the surface increased, and in the ammonia reaction, the amount of nitrogen increased and the amount of carbon decreased Žnot to
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the original level., while the amount of aluminium on the surface remained unaffected. These trends were similar during the two reaction cycles, with the exception that all the carbon attached to the surface in the TMA reaction of the second reaction cycle was removed in the subsequent ammonia reaction. 3.2. DRIFT spectroscopy The consumption and formation of methyl and amino groups during the TMA and ammonia reactions were investigated by DRIFT. Curve Ža. in Fig. 2A shows that the ammonia-modified silica substrate contained isolated hydroxyl groups Ž3745 cmy1 . and silicon-bonded primary amino groups Ž3537, 3451, 1551 and 932 cmy1 .. Both types of groups were consumed in the TMA reaction. Weak N–H stretching bands were left Ž3397, 3343 and 3280 cmy1 . and these were broad, probably due to hydrogen bonding. Simultaneously, aluminium-bonded methyl groups Ž2940 Žshoulder, sh., 2900 and 2830 cmy1 . and silicon-bonded methyl groups Ž2960 and 2900 cmy1 . were formed. These IR band assignments, and those reported later in this paper, are collected in Table 1, along with the references they are based on. As seen in Fig. 2A, the subsequent ammonia reaction consumed all the aluminium-bonded methyl groups, whereas methyl groups bonded to silicon were left Ž2973 and 2912 cmy1 .. New bands were observed in the NH stretching region; these have been previously w28x attributed to Al–NH 2 groups Ž3404 and 3343 cmy1 ., Si–NH 2 groups Ž3510 and 3434 cmy1 Žsh.., ŽAl– . 2 NH groups Ž3311 cmy1 ., Si–NH–M groups ŽM is Al or Si, 3378 cmy1 Žsh.., and associated molecular ammonia Ž3279, 3223 Žsh., 3201 cmy1 .. Furthermore, the N–H bending region reveals the presence of bridging Al–NH 2 groups Ž1513 cmy1 . and molecular ammonia Ž1620 cmy1 .. A band due to either or both terminal Al–NH 2 and Si–NH 2 groups appeared at 1550 cmy1 . However, no Si–N stretch of the Si–NH 2 groups at 932 cmy1 was seen. In addition to the changes in the peaks due to methyl and amino groups, changes were observed in other regions ŽFig. 2A.. Absorption bands were formed at 3771 and 3743 cmy1 , probably due to Al–OH groups, and a narrow, unidentified band was formed at 2091 cmy1 . Other groups have suggested that these types of bands are due to dinitrogen
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Table 1 IR assignments Wavenumber Žcmy1 .
Vibration
Assignment
3771 3745 3743 3510, 3537 3434 Žsh., 3451 3404 3378 3343 3311 3279 3223 3201 2960–2973 2940 2902–2912 2898 2830 1620 1550
n ŽOH. n ŽOH. n ŽOH. nas ŽNH. nsŽNH. n ŽNH. n ŽNH. n ŽNH. n ŽNH. n ŽNH.
1513 932
das ŽNH. n ŽSiN.
Al–OH a isolated Si–OH b Al–OH a Si–NH c2 Si–NH c2 Al–NH d2 Si–NH–M, M s Al or Si e Al–NH d2 Al–NH–Al f Al:NH 3g Al:NH h3 Al:NH 3g Si–CH i3 Al–CH i3 Si–CH i3 Al–CH i3 Al–CH i3 Al:NH 3j Si–NH 2k and terminal Al–NH 2l bridging Al–NH m 2 Si–NH c2
n ŽNH. nas ŽCH. nas ŽCH. nsŽCH. nsŽCH. das ŽNH. das ŽNH.
a
Refs. w28,40–42x. Refs. w28,43x. c Refs. w28,44x. d Refs. w21,22,25,28,45x. e Ref. w28x. f Refs. w10,28,31x. g Refs. w21,25,28,46x. h Refs. w21,28x. i Refs. w21,23–26,28x. j Refs. w23–26,28,46x. k Refs. w28,44,46x. l Refs. w24–28,46x. m Refs. w23–28,46x. b
species w25x or hydride species bonded on silicon w29x or aluminium w30x. A peak due to C[N bond could also be seen in the region, or perhaps it was formed from a combination of bands at other frequencies w31x. The changes in the DRIFT spectrum following the TMA reaction in the second reaction cycle were similar to those in the first cycle ŽFig. 2A.. The TMA reaction diminished the intensity of the bands due to amino groups; broad bands remained centred at 3381, 3325, and 3270 cmy1 . Absorption bands due to aluminium-bonded methyl groups appeared.
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However, the bands of Al–Me groups were more intense than the Si–Me bands, the reverse of what was observed during the first reaction cycle. The evolution of DRIFT spectra as recorded after one, two, and three ALCVD reaction cycles is shown in Fig. 2B. The methyl group region is similar in all three spectra: only silicon-bonded methyl groups are seen. This indicates that, during the second reaction cycle too, ammonia removed the methyl groups bonded to aluminium which were formed in the TMA reaction. Differences between the spectra can be seen in the amino group region, however, where the absorbance increases with the number of reaction cycles. This indicates that TMA does not consume all the hydrogen atoms of the amino groups and amino groups accumulate in the layer. Hydrogenbonding or some other type of interaction of the amino groups with neighbouring surface species may also shift and broaden the amino group peaks.
3.3.
29
Si CPMAS NMR spectroscopy
The presence of silicon-bonded methyl groups in the samples was examined by 29 Si CPMAS NMR. Curve Ža. of Fig. 3 shows the spectrum of ammoniamodified silica. Silicon-bonded amino groups were present in the form of O 3 SiNH 2 Žy87 ppm. and O 2 SiŽNH 2 .OH species Žy77 ppm. w28x. The TMA reaction Žcurve b. gave rise to new peaks at chemical shifts 24, 3, y10, and y61 ppm. On the basis of our previous study w28x, the peaks at 24, y10, and y61 ppm were attributed to OSiMe 3 , O 2 SiMe 2 , and O 3 SiMe species, respectively, and the peak at 3 ppm was identified as OSiŽMe 2 .NH x species. Curve Žc. shows the spectrum of the TMA-modified sample subsequently reacted with ammonia at 673 K. A peak due to the Si–NH 2 groups has reappeared at y83 ppm. No peaks of OSiMe 3 , OSiŽMe. 2 NH x , or O 2 SiMe 2 species are seen. The peak of the O 3 SiMe species remains, however, and to the left of it probably overlapping peaks are present with a maximum at about y44 ppm. These broad peaks could be due to species with one methyl group and one or two amino groups ŽOSiŽMe.ŽNH x . 2 and O 2 SiŽMe.NH x .. Species O 2 SiŽNH x . 2 and OSiŽNH x . 3 also may have formed, but their
Fig. 3. 29 Si CPMAS NMR spectra of Ža. the ammonia-modified silica, Žb. the ammonia-modified silica reacted with TMA at 423 K, Žc. the TMA-modified sample reacted with ammonia at 673 K, that is, modified with one ALCVD reaction cycle, Žd. the sample modified with two ALCVD reaction cycles, and Že. the sample modified with three ALCVD reaction cycles.
peaks would probably overlap the observed peaks. No significant differences are apparent between curves Žd. and Že. and curve Žc., that is, between the spectra of samples modified with two and three reaction cycles and the spectrum of the sample modified in one reaction cycle. Thus, the types of silicon-bonded methyl groups were not dependent on the number of ALCVD reaction cycles.
3.4. 27Al MAS NMR spectroscopy The state of the aluminium atoms in the layer formed in the TMA and ammonia reactions was
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studied by recording 27Al MAS NMR spectra. Fig. 4 shows the spectra of the samples modified with one, two, and three reaction cycles. The integrated intensities of the peaks showed the samples to contain 1.4, 2.8, and 2.1 aluminium atomsrnm2silica , respectively. These are considerably lower contents than were measured by X-ray fluorescence ŽFig. 1.. NMR-invisible aluminium sites were thus present in the samples. NMR-invisible sites are likely to be present when the electric field gradient around the aluminium atoms is asymmetric; typically these sites lie in grain boundaries that lack well-defined crystallinity w32x. Although the 27Al MAS NMR spectra did not provide reliable quantitative information about the samples, they gave valuable qualitative information. Curves Ža. and Žb. in Fig. 4, which show the spectra of the samples modified with one and two reaction cycles, respectively, look similar. The peaks centred at about 53, 35, and 5.5 ppm, respectively, indicate w32x the presence of 4-, 5-, and 6-coordinated aluminium. However, crystalline AlN sites, which have an isotropic chemical shift of 115 " 1 ppm and a
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Fig. 5. Simulation of Al sites with isotropic chemical shift of 100 ppm and a quadrupolar coupling constant of 11 MHz, superimposed on curve Žc. of Fig. 4.
quadrupolar coupling constant of 1.9 MHz w33x, are not seen. The spectrum of the sample modified with three reaction cycles Žcurve c. differs in both shape and position. The spectrum is a superposition of broad lines from different 6-, 5-, and 4-coordinated Al sites. The relative amount of 4-coordinated sites is considerably greater than in the samples modified with one and two reaction cycles. Considering the shape and width of the spectrum, we cannot exclude the presence of 4-coordinated Al sites with very strong quadrupolar coupling. Quadrupolar coupling induces a negative shift of the lines relative to their isotropic position. Fig. 5 shows a simulated spectrum of Al sites with an isotropic chemical shift of 100 ppm and a quadrupolar coupling constant of 11 MHz superimposed on curve Žc. of Fig. 4; clearly the curves coincide. Thus, the sample modified with three reaction cycles of TMA and ammonia may contain aluminium sites with isotropic chemical shift close to that of AlN.
4. Discussion 4.1. Reaction of TMA Fig. 4. 27Al MAS NMR spectra of ammonia-modified silica, modified with Ža. one, Žb. two, and Žc. three reaction cycles of TMA and ammonia.
TMA reacted with the hydroxyl and amino groups on ammonia-modified silica, as revealed by the de-
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creased intensity of the DRIFT peaks of these OH and NH 2 groups. The formation of silicon-bonded methyl groups, on the other hand, indicates that TMA also dissociated in the siloxane bridges. The reaction is similar to that observed previously for the TMA reaction on dehydroxylated silica w28x. Although investigation of the reactivity of the bonding sites of silica was not the primary aim of this study, our results show the bonding sites in a new light. The 29 Si CPMAS NMR results show that four types of species were formed in the reaction of TMA on ammonia-modified silica: OSiMe 3 , O 2 SiMe 2 , O 3 SiMe, and OSiMe 2 NH x Ž x may be 2, 1, or 0.. In the reaction of silica not first reacted with ammonia, OSiMe 3 , O 2 SiMe 2 , and O 3 SiMe species are formed w28,35x. The OSiMe 2 NH x species that were observed are thus a consequence of the modification of silica with ammonia before the TMA reaction. After the ammonia reaction on silica, single O 3 SiNH 2 and geminal O 2 SiŽNH 2 .OH species were present, which, in turn, had been formed from the O 3 SiOH and O 2 SiŽOH. 2 species, respectively w28x. For OSiMe 2 NH x species to form, two siloxane bridges attached to the same silicon atom as the amino group should have reacted with TMA. Thus, TMA can react with OH groups and siloxane bridges attached to the same silicon atom. The density of methyl groups on the surface after the TMA reactions of the first and the second AL. CVD reaction cycles was the same Ž5.3–5.5 nmy2 silica . Previously w28x, we observed the same value Ž5.5 . nmy2 silica after the TMA reaction on 1023 K silica. The finding of the same methyl group density on three different surface — silica, ammonia-modified silica, and silica modified with one TMA and ammonia reaction cycle — strongly suggests that the methyl group density defines the saturation level of the reaction. We reached a similar conclusion earlier w28x after obtaining a methyl group density of about 80% of the calculated maximum methyl group density Žassuming hexagonal packing on a planar surface.. The present results strongly support the previous conclusion. It is worth noting, however, that the reactant itself, in this case TMA, likely affects the saturation methyl group density. After saturation is achieved, the holes in the methyl group shroud covering the substrate are sufficiently small to inhibit reactant molecules Žin our case TMA. from interact-
ing with the surface. A smaller molecule than TMA, for example a molecule with just two methyl ligands, would probably fit in the free space present on the surface saturated with TMA, and make the methyl group shroud denser. 4.2. Growth of AlN on porous silica As indicated by the results of elemental analysis, growth of an aluminium- and nitrogen-containing layer took place on porous silica. The DRIFT results show that the growth was based on the adsorption of aluminium-bonded methyl groups onto the surface through the reaction of TMA with OH, NH 2 , and SiOSi groups, and the replacement of methyl groups by amino groups in the ammonia reaction. In addition, the 29 Si CPMAS NMR results show that silicon-bonded methyl groups were formed in the TMA reaction, and some of them took part in the ammonia reaction. The 27Al MAS NMR measurements indicate that, during the first two reaction cycles, 4-, 5-, and 6-coordinated aluminium atoms were present on the surface. This suggests that the Al–N layer formed on the surface interacted strongly with the silica substrate, since 5- and 6-coordinated aluminium are expected for oxides or oxynitrides of aluminium. Pure crystalline AlN contains mostly 4-coordinated Al sites with a characteristic chemical shift and quadrupolar coupling w33x. We did not observe such sites in any of the samples under study. However, the 27Al MAS NMR spectrum measured for the sample modified with three reaction cycles showed considerable increase in the relative amount of 4-coordinated sites, and the spectrum shifted to lower shielding, characteristic of the Al sites in AlN. This indicates that, although the layer formed on silica was not pure crystalline AlN Žand this is too much to expect considering the concentrations of Al and N., a shift towards AlN occurred in the distribution of aluminium sites after three reaction cycles. In general, our observations are in accord with the results of other groups w22,24,27x, though differences emerge in a detailed comparison. Bartram et al. w22x, for example, reported that only aluminium-bonded Žnot silicon-bonded. methyl groups react with ammonia at 600 K. There may be at least two reasons why we observed the reaction with silicon-bonded groups and Bartram et al. did not. First, our use of a
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higher reaction temperature Ž673 vs. 600 K. made the reaction more predominant in our case. Previously w28x, we have shown that the reaction more readily occurs at higher reaction temperature. Second, the 29 Si CPMAS NMR used in this study sensitively detects the different types of Si–Me species. In contrast, the infrared spectroscopy and X-ray photoelectron spectroscopy used by Bartram et al. are not very sensitive to changes in the types of Si–Me groups. However, our DRIFT measurements did reveal a shift of 13 cmy1 in the absorption band maxima of Si–Me groups due to the reaction of OSiMe 3 , O 2 SiMe 2 , and OSiMe 2 NH x species. Another difference relative to the studies of other groups w22,24,27x is that, in our study, secondary amino groups ŽŽM– . 2 NH, M s Al, Si. were observed as products of the ammonia reaction. The secondary amino groups were identified on the basis of their N–H stretching peak; bending absorptions were too weak to be seen w34x. The reason why secondary amino groups were identified in our study but not in other investigations may be that the N–H stretching region has not been sufficiently well resolved. At least in one study, absorption of hydrogen-bonded OH groups disturbed the observations w27x. In our case, the high dehydroxylation temperature of silica Ž1023 K. left the N–H stretching region undisturbed. However, the formation of secondary amino groups does not change the overall growth scheme because, like primary amino groups, they reacted with TMA. Wiberg et al. w36x, Seyferth et al. w37x and Boury and Seyferth w38x have found, too, that TMA reacts with the ŽSi– . 2 NH function of hexamethyldisilazane ŽMe 3 SiNHSiMe 3 . even at room temperature, releasing methane. Although the growth of aluminium- and nitrogen-containing layer took place in a steady manner, amino groups accumulated in the layer during the three reaction cycles. This was indicated by the DRIFT measurements, as the absorbance of the N–H stretching region increased with the number of reaction cycles. Ideal growth of the AlN layer would require ammonia to consume all the methyl groups and TMA to consume all the hydrogen atoms in the amino groups. Accumulation of hydrogen-containing groups Žhydroxyls. was earlier observed by Dillon et al. w39x when they deposited aluminium oxide from TMA and water on alumina. Dillon et al. carried out
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tens of reaction cycles, and to eliminate the extra hydroxyl groups they heated the sample periodically to 1000 K. Heating, in ammonia for example, probably would decrease the amino group content of our samples.
5. Conclusions The ALCVD technique can be applied for growing AlN on porous silica from TMA and ammonia precursors. Steady growth took place during three successive reaction cycles of TMA and ammonia carried out at 423 and 673 K, respectively. The growth was based on Ži. the ligand exchange reaction of TMA with hydrogen-containing groups Ži.e. OH and NH x groups. and the dissociation of TMA in siloxane bridges, which results in the formation of Al–Me and Si–Me groups on the surface, and Žii. the ligand exchange reaction of ammonia with all the aluminium-bonded and some silicon-bonded methyl groups, leading to the formation of amino groups on the surface. The amino groups served as new bonding sites for the following TMA reaction. As the aluminium and nitrogen-containing layer formed, it first interacted strongly with the substrate, but after the third reaction cycle, sites resembling AlN began to form. The steric hindrance caused by the methyl group shroud formed in the TMA reaction defined the saturation level. It is likely that the number of methyl groups on the surface Ži.e., the number of bonding sites. was determinant of the saturation of the following ammonia reaction. Thus, in the growth of AlN from TMA and ammonia by ALCVD, two limiting factors resulting in saturation were operative: Ži. steric hindrance by ligands on the surface; and Žii. number of bonding sites. Under the applied reaction conditions, methyl groups and, especially after the first reaction cycle, associated ammonia molecules were left on the surface as impurities. In addition, unreacted amino groups accumulated in the layer. To produce highquality AlN on silica that can be used as a catalyst support, the processing conditions need to be optimised and the number of reaction cycles should be increased beyond three. These improvements will be the subject of a future study.
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Acknowledgements w20x
We thank the Department of Chemistry, University of Joensuu, for providing the equipment to measure DRIFT spectra inertly. The Analytical Department of Fortum Oil and Gas is thanked for the elemental analysis and BET measurements.
w21x w22x w23x w24x w25x
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Growth of aluminium nitride on porous silica by atomic layer chemical vapour deposition R.L. Puurunen a,b,) , A. Root c , P. Sarv d , S. Haukka b,1, E.I. Iiskola b,2 , M. Lindblad e, A.O.I. Krause a a
c
Helsinki UniÕersity of Technology, Industrial Chemistry, P.O. Box 6100, FIN-02015 HUT, Finland b Microchemistry Ltd., P.O. Box 132, FIN-02631 Espoo, Finland Fortum Oil and Gas Scientific SerÕices, Technology Center, P.O. Box 310, FIN-06100 PorÕoo, Finland d Institute of Chemical Physics and Biophysics, Akadeemia 23, 12618 Tallinn, Estonia e Fortum Oil and Gas Oy, P.O. Box 110, FIN-00048 FORTUM, Finland Received 13 April 2000; accepted 18 May 2000
Abstract Aluminium nitride ŽAlN. was grown on porous silica by atomic layer chemical vapour deposition ŽALCVD. from trimethylaluminium ŽTMA. and ammonia precursors. The ALCVD growth is based on alternating, separated, saturating reactions of the gaseous precursors with the solid substrate. TMA and ammonia were reacted at 423 and 623 K, respectively, on silica which had been dehydroxylated at 1023 K and pretreated with ammonia at 823 K. The growth in three reaction cycles was investigated quantitatively by elemental analysis, and the surface reaction products were identified by IR and solid state 27Al and 29 Si NMR measurements. Steady growth of about 2 aluminium and 2 nitrogen atomsrnm2silicarreaction cycle was obtained. The growth mainly took place through Ži. the reaction of TMA which resulted in surface Al–Me and Si–Me groups, and Žii. the reaction of ammonia which replaced the aluminium-bonded methyl groups with amino groups. Ammonia also reacted in part with the silicon-bonded methyl groups formed in the dissociative reaction of TMA with siloxane bridges. TMA reacted with the amino groups, as it did with surface silanol groups and siloxane bridges. In general, the Al–N layer interacted strongly with the silica substrate, but in the third reaction cycle AlN-type sites may have formed. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.G Keywords: Aluminium nitride; Trimethylaluminium; Ammonia; ALCVD; IR; NMR
1. Introduction
)
Corresponding author. Tel.: q358-9-451-2582; fax: q358-9451-2622. E-mail address: [email protected] ŽR.L. Puurunen.. 1 Present address: ASM Microchemistry. 2 Present address: Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland.
Aluminium nitride ŽAlN. w1x, silicon nitride w2x, and a hydrogen-containing Al–N layer, grafted on silica from liquid phase w3,4x, have been successfully studied as supports for metal catalysts in hydrogenation w1x, oxidation w2x, isomerisation w3x, and hydroformylation w4x. Although nitrides are potential catalyst supports, their catalyst applications have been
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 4 0 - 2
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hindered by the difficulty of obtaining them in a porous form suitable for catalyst applications. One approach to obtaining nitrides with high specific surface area is to coat porous oxide supports with a layer of the nitride. The layer should cover the whole surface area, and preferably be uniform in thickness. Atomic layer chemical vapour deposition ŽALCVD., also called atomic layer epitaxy ŽALE., is a technique based on well-separated saturating reactions of gas-phase precursors with a solid substrate. Its suitability for growing conformal metal nitride thin films on trenched surfaces has been demonstrated w5x, and it is also suitable for modifying porous oxides with other oxides in a controlled and reproducible way w6x. Thus, it has potential for coating of oxides with nitrides. Growth by ALCVD requires volatile precursors that do not condense and that are thermally stable under the applied reaction conditions. What must take place on the surface is chemisorption, not physisorption, of the precursors. Furthermore, the reactions must be saturating, so that sufficiently large precursor doses on similarly treated substrates always result in the same amount of chemisorption products. The saturation content of the chemisorption products is determined not only by the precursor reactivity but also by the availability of the bonding sites on the substrate and the size of the precursor molecule. For well-characterised growth, the bonding sites of the substrate and the surface species formed in each reaction step should be known. In general, suitable precursors for the ALCVD growth of AlN, which is our interest, are volatile aluminium compounds, such as alkyls, alkoxides, halides, and b-diketonates. Ammonia or hydrazine could be used as the nitrogen-containing precursor. We chose trimethylaluminium ŽTMA. and ammonia as precursors because their suitability has been shown for the manufacture of AlN both as a bulk material w7–10x and as a thin film w11–20x. Bulk AlN growth is based on the following types of reactions, in which one methyl group at a time combines with a hydrogen atom of the ammonia molecule or a surface amino group to form methane w7,9,10x: AlMe 3 q NH 3 Me 2 AlNH 2 q CH 4 Ž 1.
™ Me AlNH ™ MeAlNHq CH MeAlNH ™ AlN q CH . 2
2
4
4
Ž 2. Ž 3.
The above equations are idealised in the sense that, although reaction Ž3. mostly occurs by 700 K, the purity and crystallinity of AlN improves if the sample is heated under ammonia to about 1300 K w10x. The growth of AlN films from TMA and ammonia precursors has been studied on nonporous highsurface-area silica w21,22x and alumina w23–27x. Controlled growth has been achieved at 600 K by dosing TMA and ammonia separately w22,24,27x Žas in the ALCVD processing scheme.. To our knowledge, attempts have not yet been made to grow AlN on porous materials for catalyst purposes. The goal of this work was to study the applicability of the ALCVD technique for growing AlN on porous silica. Three reaction cycles of TMA and ammonia at 423 and 673 K, respectively, were carried out on silica dehydroxylated at 1023 K and pretreated with ammonia at 823 K. Quantitatively the growth was investigated with element determinations, and the surface species formed during the reactions were identified by diffuse reflectance IR spectroscopy ŽDRIFT. and solid state 29 Si and 27Al NMR spectroscopy.
2. Experimental 2.1. Materials Grace 432 silica of particle size 315–500 mm was used as substrate. The silica was heated in ambient air in a muffle furnace for 16 h at 1023 K to stabilise the number and type of bonding sites for the subsequent reactions. In addition, it was heated in the ALCVD reactor under vacuum for 3 h at 823 K to remove water physisorbed during transfer to the reactor. After this procedure, the specific surface area and pore volume of silica were 290 m2 gy1 and 1.0 cm3 gy1 , respectively. Nitrogen ŽOy Aga Ab, H 2 O - 3 ppm, O 2 - 3 ppm. was used as an inert carrier gas. Before use, it was purified with Messer Griesham Hydrosorb and Oxisorb filters, which yielded concentrations of water and oxygen less than 0.3 and 0.1 ppm, respectively. TMA ŽWitco GmbH, 97.5 wt.%, Cl - 0.01 wt.%. and ammonia ŽOy Aga Ab, 99.998%, H 2 O - 5 ppm. were used as received.
R.L. Puurunen et al.r Applied Surface Science 165 (2000) 193–202
2.2. Reaction procedure The reactions were carried out at a reduced pressure of 1–5 kPa in a modified flow-type ALCVD reactor ŽMicrochemistry, Finland. described in detail in Ref. w6x. Typically, 5–10 g of silica was loaded into the reactor. The reactants were kept at room temperature. During the reactions, they were transported in a flow of nitrogen downwards through a stationary silica bed. The reactions were started by pretreating silica with ammonia at 823 K to form primary siliconbonded amino groups ŽSi–NH 2 groups. on the surface. AlN was processed in one to three cycles of the saturating reactions of TMA and ammonia. One reaction cycle consisted of one TMA reaction, a purge with nitrogen, an ammonia reaction, and a purge with nitrogen. The nitrogen purges separated the TMA and ammonia reactions, and eliminated unwanted gas-phase reactions by removing unreacted precursors Žeither TMA or ammonia. and the gaseous reaction products. TMA was reacted with silica at 423 K. The reaction of ammonia was started at the reaction temperature of TMA Ž423 K., and then gradually elevated to 673 K under a flow of ammonia. Ammonia was fed during the temperature increase to avoid thermal decomposition of the TMA chemisorption products. Because the samples reacted easily with humidity, they were handled inertly in nitrogen. 2.3. Characterisation The aluminium contents of the samples were measured with a Siemens SRS303 X-ray fluorescence spectrometer, and the nitrogen and carbon contents with a LECO CHN analyser Žtreatment at 1223 K with oxygen.. The specific surface areas were measured according to the Brunauer–Emmett–Teller ŽBET. method with a Micrometrics Asap 2400 instrument. The aluminium, nitrogen, and carbon contents were calculated with respect to the original silica surface by subtracting the masses of aluminium, nitrogen, carbon, and the calculated mass of hydrogen from the mass of the sample. The mass of hydrogen was calculated assuming an H:C ratio of three Žall carbon in methyl groups. and an average H:N ratio of 1.5 in the amino groups. Because of the
195
low molar mass of hydrogen, deviation of the hydrogen content from 3 and 1.5 in the assumed H:C and H:N ratios resulted in negligible error, on the order of 0.01 atomsrnm2silica Žnote that the unit used, atomsrnm 2silica , differs from surface density, atomsrnm2 ; the former counts all the atoms present in the sample, not just those on the outermost layer.. DRIFT spectra were measured with a Nicolet Impact 400 D diffuse reflectance spectrometer connected air-tightly to a nitrogen-filled glove-box. The samples were ground in an agate mortar and packed loosely in the sample holder. The spectra were recorded at room temperature in the wavenumber range 4000–650 cmy1 with a spectral resolution of 2 cmy1 . 29 Si cross-polarization magic-angle spinning ŽCPMAS. NMR spectra were acquired with a 270 MHz Chemagnetics CMX Infinity Spectrometer using a self-built 10-mm probe. The samples were transferred to the rotors under dry nitrogen. A 5-ms contact time, 33-kHz radio frequency field, 5-s recycle delay, and 4-kHz MAS speed were used in the experiments, and between 10,000 and 20,000 transients were acquired. The chemical shifts were referenced to 3-Žtrimethylsilyl.propanesulfonic acid sodium salt Žq1.6 ppm., which was itself referenced to tetramethylsilane ŽTMS. at 0 ppm. 27 Al MAS NMR spectra were recorded with a Bruker AMX500 spectrometer using a self-built 3.5mm probe. The spinning speed was 12 kHz and the radio-frequency field strength was 70 kHz. The spectra were acquired using a 108 excitation pulse, 0.2-s delay, and 20,000–40,000 transients. Delay of 5 s was tested, but it did not give additional signal intensity. KAlŽSO4 . 2 P 12H 2 O was used as an external chemical shift reference Ž0 ppm. and also as an intensity reference. The samples were loaded into rotors under dry nitrogen atmosphere ŽO 2 5 ppm, H 2 O 2 ppm..
3. Results The growth of AlN from TMA and ammonia precursors was studied on silica dehydroxylated at 1023 K that had been reacted with ammonia at 823 K. In an earlier work w28x, we found that about 0.4 of the 1.0 isolated hydroxyl groups rnm2silica present on
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1023 K silica exchanged with secondary amino groups ŽSi–NH 2 . during ammonia reaction. The ammonia did not dissociate on siloxane bridges. The present reactions with TMA and ammonia were carried out at 423 and 673 K, respectively. 3.1. Elemental analysis Fig. 1A shows the aluminium, carbon, and nitrogen contents on the modified silica substrate after one to three full reaction cycles. The amounts of aluminium and nitrogen on the surface increased steadily as a function of the number of reaction cycles. The average rates of increase of aluminium and nitrogen contents per reaction cycle were about 2 atomsrnm2silica . The carbon content stayed at a constant level of about 0.8rnm2silica . Fig. 1B presents the element contents separately after TMA and ammonia reaction steps during two
Fig. 2. ŽA. DRIFT spectra of Ža. the ammonia-modified silica, Žb. the ammonia-modified silica reacted with TMA at 423 K, Žc. the b-sample reacted with ammonia at 673 K, and Žd. the c-sample reacted with TMA at 423 K. ŽB. DRIFT spectra of ammonia-modified silica modified with Ža. one, Žb. two, and Žc. three reaction cycles of TMA and ammonia.
Fig. 1. ŽA. The aluminium, carbon and nitrogen contents on silica dehydroxylated at 1023 K and reacted with ammonia at 823 K, as a function of the number of reaction cycles of TMA and ammonia at 423 and 673 K, respectively. ŽB. The element contents after each reaction step during two reaction cycles. The ‘‘0.5’’ and ‘‘1.5’’ mean that the growth ended in TMA reaction.
reaction cycles. In the TMA reaction, the amounts of aluminium and carbon on the surface increased, and in the ammonia reaction, the amount of nitrogen increased and the amount of carbon decreased Žnot to
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the original level., while the amount of aluminium on the surface remained unaffected. These trends were similar during the two reaction cycles, with the exception that all the carbon attached to the surface in the TMA reaction of the second reaction cycle was removed in the subsequent ammonia reaction. 3.2. DRIFT spectroscopy The consumption and formation of methyl and amino groups during the TMA and ammonia reactions were investigated by DRIFT. Curve Ža. in Fig. 2A shows that the ammonia-modified silica substrate contained isolated hydroxyl groups Ž3745 cmy1 . and silicon-bonded primary amino groups Ž3537, 3451, 1551 and 932 cmy1 .. Both types of groups were consumed in the TMA reaction. Weak N–H stretching bands were left Ž3397, 3343 and 3280 cmy1 . and these were broad, probably due to hydrogen bonding. Simultaneously, aluminium-bonded methyl groups Ž2940 Žshoulder, sh., 2900 and 2830 cmy1 . and silicon-bonded methyl groups Ž2960 and 2900 cmy1 . were formed. These IR band assignments, and those reported later in this paper, are collected in Table 1, along with the references they are based on. As seen in Fig. 2A, the subsequent ammonia reaction consumed all the aluminium-bonded methyl groups, whereas methyl groups bonded to silicon were left Ž2973 and 2912 cmy1 .. New bands were observed in the NH stretching region; these have been previously w28x attributed to Al–NH 2 groups Ž3404 and 3343 cmy1 ., Si–NH 2 groups Ž3510 and 3434 cmy1 Žsh.., ŽAl– . 2 NH groups Ž3311 cmy1 ., Si–NH–M groups ŽM is Al or Si, 3378 cmy1 Žsh.., and associated molecular ammonia Ž3279, 3223 Žsh., 3201 cmy1 .. Furthermore, the N–H bending region reveals the presence of bridging Al–NH 2 groups Ž1513 cmy1 . and molecular ammonia Ž1620 cmy1 .. A band due to either or both terminal Al–NH 2 and Si–NH 2 groups appeared at 1550 cmy1 . However, no Si–N stretch of the Si–NH 2 groups at 932 cmy1 was seen. In addition to the changes in the peaks due to methyl and amino groups, changes were observed in other regions ŽFig. 2A.. Absorption bands were formed at 3771 and 3743 cmy1 , probably due to Al–OH groups, and a narrow, unidentified band was formed at 2091 cmy1 . Other groups have suggested that these types of bands are due to dinitrogen
197
Table 1 IR assignments Wavenumber Žcmy1 .
Vibration
Assignment
3771 3745 3743 3510, 3537 3434 Žsh., 3451 3404 3378 3343 3311 3279 3223 3201 2960–2973 2940 2902–2912 2898 2830 1620 1550
n ŽOH. n ŽOH. n ŽOH. nas ŽNH. nsŽNH. n ŽNH. n ŽNH. n ŽNH. n ŽNH. n ŽNH.
1513 932
das ŽNH. n ŽSiN.
Al–OH a isolated Si–OH b Al–OH a Si–NH c2 Si–NH c2 Al–NH d2 Si–NH–M, M s Al or Si e Al–NH d2 Al–NH–Al f Al:NH 3g Al:NH h3 Al:NH 3g Si–CH i3 Al–CH i3 Si–CH i3 Al–CH i3 Al–CH i3 Al:NH 3j Si–NH 2k and terminal Al–NH 2l bridging Al–NH m 2 Si–NH c2
n ŽNH. nas ŽCH. nas ŽCH. nsŽCH. nsŽCH. das ŽNH. das ŽNH.
a
Refs. w28,40–42x. Refs. w28,43x. c Refs. w28,44x. d Refs. w21,22,25,28,45x. e Ref. w28x. f Refs. w10,28,31x. g Refs. w21,25,28,46x. h Refs. w21,28x. i Refs. w21,23–26,28x. j Refs. w23–26,28,46x. k Refs. w28,44,46x. l Refs. w24–28,46x. m Refs. w23–28,46x. b
species w25x or hydride species bonded on silicon w29x or aluminium w30x. A peak due to C[N bond could also be seen in the region, or perhaps it was formed from a combination of bands at other frequencies w31x. The changes in the DRIFT spectrum following the TMA reaction in the second reaction cycle were similar to those in the first cycle ŽFig. 2A.. The TMA reaction diminished the intensity of the bands due to amino groups; broad bands remained centred at 3381, 3325, and 3270 cmy1 . Absorption bands due to aluminium-bonded methyl groups appeared.
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However, the bands of Al–Me groups were more intense than the Si–Me bands, the reverse of what was observed during the first reaction cycle. The evolution of DRIFT spectra as recorded after one, two, and three ALCVD reaction cycles is shown in Fig. 2B. The methyl group region is similar in all three spectra: only silicon-bonded methyl groups are seen. This indicates that, during the second reaction cycle too, ammonia removed the methyl groups bonded to aluminium which were formed in the TMA reaction. Differences between the spectra can be seen in the amino group region, however, where the absorbance increases with the number of reaction cycles. This indicates that TMA does not consume all the hydrogen atoms of the amino groups and amino groups accumulate in the layer. Hydrogenbonding or some other type of interaction of the amino groups with neighbouring surface species may also shift and broaden the amino group peaks.
3.3.
29
Si CPMAS NMR spectroscopy
The presence of silicon-bonded methyl groups in the samples was examined by 29 Si CPMAS NMR. Curve Ža. of Fig. 3 shows the spectrum of ammoniamodified silica. Silicon-bonded amino groups were present in the form of O 3 SiNH 2 Žy87 ppm. and O 2 SiŽNH 2 .OH species Žy77 ppm. w28x. The TMA reaction Žcurve b. gave rise to new peaks at chemical shifts 24, 3, y10, and y61 ppm. On the basis of our previous study w28x, the peaks at 24, y10, and y61 ppm were attributed to OSiMe 3 , O 2 SiMe 2 , and O 3 SiMe species, respectively, and the peak at 3 ppm was identified as OSiŽMe 2 .NH x species. Curve Žc. shows the spectrum of the TMA-modified sample subsequently reacted with ammonia at 673 K. A peak due to the Si–NH 2 groups has reappeared at y83 ppm. No peaks of OSiMe 3 , OSiŽMe. 2 NH x , or O 2 SiMe 2 species are seen. The peak of the O 3 SiMe species remains, however, and to the left of it probably overlapping peaks are present with a maximum at about y44 ppm. These broad peaks could be due to species with one methyl group and one or two amino groups ŽOSiŽMe.ŽNH x . 2 and O 2 SiŽMe.NH x .. Species O 2 SiŽNH x . 2 and OSiŽNH x . 3 also may have formed, but their
Fig. 3. 29 Si CPMAS NMR spectra of Ža. the ammonia-modified silica, Žb. the ammonia-modified silica reacted with TMA at 423 K, Žc. the TMA-modified sample reacted with ammonia at 673 K, that is, modified with one ALCVD reaction cycle, Žd. the sample modified with two ALCVD reaction cycles, and Že. the sample modified with three ALCVD reaction cycles.
peaks would probably overlap the observed peaks. No significant differences are apparent between curves Žd. and Že. and curve Žc., that is, between the spectra of samples modified with two and three reaction cycles and the spectrum of the sample modified in one reaction cycle. Thus, the types of silicon-bonded methyl groups were not dependent on the number of ALCVD reaction cycles.
3.4. 27Al MAS NMR spectroscopy The state of the aluminium atoms in the layer formed in the TMA and ammonia reactions was
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studied by recording 27Al MAS NMR spectra. Fig. 4 shows the spectra of the samples modified with one, two, and three reaction cycles. The integrated intensities of the peaks showed the samples to contain 1.4, 2.8, and 2.1 aluminium atomsrnm2silica , respectively. These are considerably lower contents than were measured by X-ray fluorescence ŽFig. 1.. NMR-invisible aluminium sites were thus present in the samples. NMR-invisible sites are likely to be present when the electric field gradient around the aluminium atoms is asymmetric; typically these sites lie in grain boundaries that lack well-defined crystallinity w32x. Although the 27Al MAS NMR spectra did not provide reliable quantitative information about the samples, they gave valuable qualitative information. Curves Ža. and Žb. in Fig. 4, which show the spectra of the samples modified with one and two reaction cycles, respectively, look similar. The peaks centred at about 53, 35, and 5.5 ppm, respectively, indicate w32x the presence of 4-, 5-, and 6-coordinated aluminium. However, crystalline AlN sites, which have an isotropic chemical shift of 115 " 1 ppm and a
199
Fig. 5. Simulation of Al sites with isotropic chemical shift of 100 ppm and a quadrupolar coupling constant of 11 MHz, superimposed on curve Žc. of Fig. 4.
quadrupolar coupling constant of 1.9 MHz w33x, are not seen. The spectrum of the sample modified with three reaction cycles Žcurve c. differs in both shape and position. The spectrum is a superposition of broad lines from different 6-, 5-, and 4-coordinated Al sites. The relative amount of 4-coordinated sites is considerably greater than in the samples modified with one and two reaction cycles. Considering the shape and width of the spectrum, we cannot exclude the presence of 4-coordinated Al sites with very strong quadrupolar coupling. Quadrupolar coupling induces a negative shift of the lines relative to their isotropic position. Fig. 5 shows a simulated spectrum of Al sites with an isotropic chemical shift of 100 ppm and a quadrupolar coupling constant of 11 MHz superimposed on curve Žc. of Fig. 4; clearly the curves coincide. Thus, the sample modified with three reaction cycles of TMA and ammonia may contain aluminium sites with isotropic chemical shift close to that of AlN.
4. Discussion 4.1. Reaction of TMA Fig. 4. 27Al MAS NMR spectra of ammonia-modified silica, modified with Ža. one, Žb. two, and Žc. three reaction cycles of TMA and ammonia.
TMA reacted with the hydroxyl and amino groups on ammonia-modified silica, as revealed by the de-
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creased intensity of the DRIFT peaks of these OH and NH 2 groups. The formation of silicon-bonded methyl groups, on the other hand, indicates that TMA also dissociated in the siloxane bridges. The reaction is similar to that observed previously for the TMA reaction on dehydroxylated silica w28x. Although investigation of the reactivity of the bonding sites of silica was not the primary aim of this study, our results show the bonding sites in a new light. The 29 Si CPMAS NMR results show that four types of species were formed in the reaction of TMA on ammonia-modified silica: OSiMe 3 , O 2 SiMe 2 , O 3 SiMe, and OSiMe 2 NH x Ž x may be 2, 1, or 0.. In the reaction of silica not first reacted with ammonia, OSiMe 3 , O 2 SiMe 2 , and O 3 SiMe species are formed w28,35x. The OSiMe 2 NH x species that were observed are thus a consequence of the modification of silica with ammonia before the TMA reaction. After the ammonia reaction on silica, single O 3 SiNH 2 and geminal O 2 SiŽNH 2 .OH species were present, which, in turn, had been formed from the O 3 SiOH and O 2 SiŽOH. 2 species, respectively w28x. For OSiMe 2 NH x species to form, two siloxane bridges attached to the same silicon atom as the amino group should have reacted with TMA. Thus, TMA can react with OH groups and siloxane bridges attached to the same silicon atom. The density of methyl groups on the surface after the TMA reactions of the first and the second AL. CVD reaction cycles was the same Ž5.3–5.5 nmy2 silica . Previously w28x, we observed the same value Ž5.5 . nmy2 silica after the TMA reaction on 1023 K silica. The finding of the same methyl group density on three different surface — silica, ammonia-modified silica, and silica modified with one TMA and ammonia reaction cycle — strongly suggests that the methyl group density defines the saturation level of the reaction. We reached a similar conclusion earlier w28x after obtaining a methyl group density of about 80% of the calculated maximum methyl group density Žassuming hexagonal packing on a planar surface.. The present results strongly support the previous conclusion. It is worth noting, however, that the reactant itself, in this case TMA, likely affects the saturation methyl group density. After saturation is achieved, the holes in the methyl group shroud covering the substrate are sufficiently small to inhibit reactant molecules Žin our case TMA. from interact-
ing with the surface. A smaller molecule than TMA, for example a molecule with just two methyl ligands, would probably fit in the free space present on the surface saturated with TMA, and make the methyl group shroud denser. 4.2. Growth of AlN on porous silica As indicated by the results of elemental analysis, growth of an aluminium- and nitrogen-containing layer took place on porous silica. The DRIFT results show that the growth was based on the adsorption of aluminium-bonded methyl groups onto the surface through the reaction of TMA with OH, NH 2 , and SiOSi groups, and the replacement of methyl groups by amino groups in the ammonia reaction. In addition, the 29 Si CPMAS NMR results show that silicon-bonded methyl groups were formed in the TMA reaction, and some of them took part in the ammonia reaction. The 27Al MAS NMR measurements indicate that, during the first two reaction cycles, 4-, 5-, and 6-coordinated aluminium atoms were present on the surface. This suggests that the Al–N layer formed on the surface interacted strongly with the silica substrate, since 5- and 6-coordinated aluminium are expected for oxides or oxynitrides of aluminium. Pure crystalline AlN contains mostly 4-coordinated Al sites with a characteristic chemical shift and quadrupolar coupling w33x. We did not observe such sites in any of the samples under study. However, the 27Al MAS NMR spectrum measured for the sample modified with three reaction cycles showed considerable increase in the relative amount of 4-coordinated sites, and the spectrum shifted to lower shielding, characteristic of the Al sites in AlN. This indicates that, although the layer formed on silica was not pure crystalline AlN Žand this is too much to expect considering the concentrations of Al and N., a shift towards AlN occurred in the distribution of aluminium sites after three reaction cycles. In general, our observations are in accord with the results of other groups w22,24,27x, though differences emerge in a detailed comparison. Bartram et al. w22x, for example, reported that only aluminium-bonded Žnot silicon-bonded. methyl groups react with ammonia at 600 K. There may be at least two reasons why we observed the reaction with silicon-bonded groups and Bartram et al. did not. First, our use of a
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higher reaction temperature Ž673 vs. 600 K. made the reaction more predominant in our case. Previously w28x, we have shown that the reaction more readily occurs at higher reaction temperature. Second, the 29 Si CPMAS NMR used in this study sensitively detects the different types of Si–Me species. In contrast, the infrared spectroscopy and X-ray photoelectron spectroscopy used by Bartram et al. are not very sensitive to changes in the types of Si–Me groups. However, our DRIFT measurements did reveal a shift of 13 cmy1 in the absorption band maxima of Si–Me groups due to the reaction of OSiMe 3 , O 2 SiMe 2 , and OSiMe 2 NH x species. Another difference relative to the studies of other groups w22,24,27x is that, in our study, secondary amino groups ŽŽM– . 2 NH, M s Al, Si. were observed as products of the ammonia reaction. The secondary amino groups were identified on the basis of their N–H stretching peak; bending absorptions were too weak to be seen w34x. The reason why secondary amino groups were identified in our study but not in other investigations may be that the N–H stretching region has not been sufficiently well resolved. At least in one study, absorption of hydrogen-bonded OH groups disturbed the observations w27x. In our case, the high dehydroxylation temperature of silica Ž1023 K. left the N–H stretching region undisturbed. However, the formation of secondary amino groups does not change the overall growth scheme because, like primary amino groups, they reacted with TMA. Wiberg et al. w36x, Seyferth et al. w37x and Boury and Seyferth w38x have found, too, that TMA reacts with the ŽSi– . 2 NH function of hexamethyldisilazane ŽMe 3 SiNHSiMe 3 . even at room temperature, releasing methane. Although the growth of aluminium- and nitrogen-containing layer took place in a steady manner, amino groups accumulated in the layer during the three reaction cycles. This was indicated by the DRIFT measurements, as the absorbance of the N–H stretching region increased with the number of reaction cycles. Ideal growth of the AlN layer would require ammonia to consume all the methyl groups and TMA to consume all the hydrogen atoms in the amino groups. Accumulation of hydrogen-containing groups Žhydroxyls. was earlier observed by Dillon et al. w39x when they deposited aluminium oxide from TMA and water on alumina. Dillon et al. carried out
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tens of reaction cycles, and to eliminate the extra hydroxyl groups they heated the sample periodically to 1000 K. Heating, in ammonia for example, probably would decrease the amino group content of our samples.
5. Conclusions The ALCVD technique can be applied for growing AlN on porous silica from TMA and ammonia precursors. Steady growth took place during three successive reaction cycles of TMA and ammonia carried out at 423 and 673 K, respectively. The growth was based on Ži. the ligand exchange reaction of TMA with hydrogen-containing groups Ži.e. OH and NH x groups. and the dissociation of TMA in siloxane bridges, which results in the formation of Al–Me and Si–Me groups on the surface, and Žii. the ligand exchange reaction of ammonia with all the aluminium-bonded and some silicon-bonded methyl groups, leading to the formation of amino groups on the surface. The amino groups served as new bonding sites for the following TMA reaction. As the aluminium and nitrogen-containing layer formed, it first interacted strongly with the substrate, but after the third reaction cycle, sites resembling AlN began to form. The steric hindrance caused by the methyl group shroud formed in the TMA reaction defined the saturation level. It is likely that the number of methyl groups on the surface Ži.e., the number of bonding sites. was determinant of the saturation of the following ammonia reaction. Thus, in the growth of AlN from TMA and ammonia by ALCVD, two limiting factors resulting in saturation were operative: Ži. steric hindrance by ligands on the surface; and Žii. number of bonding sites. Under the applied reaction conditions, methyl groups and, especially after the first reaction cycle, associated ammonia molecules were left on the surface as impurities. In addition, unreacted amino groups accumulated in the layer. To produce highquality AlN on silica that can be used as a catalyst support, the processing conditions need to be optimised and the number of reaction cycles should be increased beyond three. These improvements will be the subject of a future study.
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Acknowledgements w20x
We thank the Department of Chemistry, University of Joensuu, for providing the equipment to measure DRIFT spectra inertly. The Analytical Department of Fortum Oil and Gas is thanked for the elemental analysis and BET measurements.
w21x w22x w23x w24x w25x
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