J. Phys. Chem. B 2000, 104, 6599-6609
6599
IR and NMR Study of the Chemisorption of Ammonia on Trimethylaluminum-Modified Silica Riikka L. Puurunen,*,†,‡ Andrew Root,§ Suvi Haukka,‡,| Eero I. Iiskola,‡,∇ Marina Lindblad,⊥ and A. Outi I. Krause† Helsinki UniVersity of Technology, Industrial Chemistry, P. O. Box 6100, FIN-02015 HUT, Finland, Microchemistry Ltd., P. O. Box 132, FIN-02631 Espoo, Finland, Fortum Oil and Gas Scientific SerVices, Technology Center, P. O. Box 310, FIN-06100 PorVoo, Finland, and Fortum Oil and Gas Oy, P. O. Box 110, FIN-00048 Fortum, Finland ReceiVed: February 4, 2000
The saturating reaction of ammonia was studied on trimethylaluminum (TMA)-modified porous silica. This reaction step completes a reaction cycle of TMA and ammonia in aluminum nitride growth by atomic layer chemical vapor deposition (ALCVD), a technique based on well-separated saturating gas-solid reactions. The reaction was studied from 423 to 823 K. In addition, the separate reactions of TMA at 423 K and ammonia at 823 K were studied on silica dehydroxylated at 1023 K. The reaction products on the surface were identified by IR and 29Si, 13C, and 1H NMR spectroscopy, and they were quantified by element determinations and 1H NMR. In the reaction of TMA on silica, methyl groups were attached to the surface indirectly through aluminum and through direct bonding to silicon. In the subsequent ammonia reaction, ligand exchange of ammonia with the methyl groups occurred at all reaction temperatures, resulting in primary amino groups and the release of methane. Also, secondary amino groups were found on the surface, and quantitative determinations indicated the presence of tertiary amino groups, especially at high reaction temperatures. In addition, especially at low reaction temperatures, ammonia chemisorbed associatively on the TMA-modified silica. All of the methyl groups bonded to aluminum were removed with ammonia at 573-623 K, and about 80% of the methyl groups bonded to silicon were removed at 823 K; amino groups bonded to both aluminum and silicon were left behind. The higher the reaction temperature, the smaller was the average number of hydrogen atoms (x) in the amino groups (NHx).
Introduction Both coadsorption and separated reactions of trimethylaluminum (TMA, Al(CH3)3) and ammonia (NH3) have been used for low-temperature ( O2Si(CH3)2 > O3SiCH3. Part of the methyl groups in the OSi(CH3)3 species reacted with ammonia even at 423 K, and all of them had reacted by 623 K, probably leaving OSi(CH3)(NHx)2 species on the surface. After ammonia reaction at 623 K, considerable amounts of other species with one or two methyl groups (O3SiCH3, O2Si(CH3)2, and O2Si(CH3)NHx) were also present. After the
Chemisorption of Ammonia on TMA-Modified Silica
Figure 13. The average H/N ratio and the total number of hydrogen atoms in the groups as calculated from the element determinations, as a function of reaction temperature.
reaction at 723 K, there were no O2Si(CH3)2 species present, while species with one methyl group (O3SiCH3, O2Si(CH3)NHx, and OSi(CH3)(NHx)2) or no methyl groups (O3SiNHx, O2Si(NHx)2, and OSi(NHx)3) were left. The amount of carbon continued to decrease when the reaction temperature was elevated to 823 K; about 80% of the original Si-CH3 groups were removed with ammonia at 823 K. This indicates that part of the species with single methyl groups also reacted with ammonia. These results are comparable with those of Lakomaa et al.,23 who showed that the OSi(CH3)3 groups react with water even at 393 K. Thus, the reactivity of the O4-nSi(CH3)n species with water and ammonia seems to be similar: the species with more methyl groups react at lower temperatures. Hydrogen Content of the Amino Groups. Knowledge of the average H/N ratio in the amino groups (M-NH2, (M-)2NH, and (M-)3N) helps in determining the types of amino groups present after the ammonia reaction at various temperatures on TMA-modified silica. The total number of hydrogen atoms in the amino groups, on the other hand, gives valuable information on the state of the surface, for example, for the subsequent ALCVD growth of AlN. In general, if an ammonia molecule (or a surface amino group, i.e., (M-)3-xNHx) loses hydrogen when it reacts with a surface methyl group to form methane and a M-NH2 group (or (M-)3-(x-1)NHx-1), the equation [H] ) 3[N] - ∆[C] should be valid. The [H] and [N] denote, respectively, the amounts of hydrogen and nitrogen in the amino groups, and ∆[C] denotes the change in the carbon (i.e., methyl group) content when TMA-modified silica is reacted with ammonia. The average H/N ratio and the total number of hydrogen atoms in the amino groups calculated on the basis of the element determinations are shown in Figure 13. The total hydrogen content in the amino groups after ammonia reaction at 823 K was also directly measured by 1H MAS NMR, and that value (1.5 nm-2) is, within analytical accuracy, equal to the value calculated from the element determinations (1.7 nm-2). The average H:N ratio in the amino groups of the sample reacted with ammonia at 823 K can also be calculated from the 1H MAS NMR measurements. From the measurements of ∆[C] and [H] (5.5 and 1.5 nm-2, respectively), a value of 2.3 nm-2 was calculated for the nitrogen content, and 0.6 was obtained as the average H/N ratio in the amino groups. Both the nitrogen content and the H/N ratio are close to the values obtained on the basis of element determinations (2.3 nm-2 and 0.7, respectively). Amino Groups Formed in Ammonia Reaction on TMAModified Silica. The predominant types of amino groups present after ammonia reaction at various temperatures could be
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6607 determined on the basis of the calculated average H/N ratio in the amino groups and the spectroscopic evidence. A simplified scheme of the amino groups present after ammonia reaction at 423, 623, and 823 K is presented in Figure 12. After the ammonia reaction at 423 K, the average H/N ratio in the amino groups was close to 2 (Figure 13). This indicates that molecular ammonia (Al:NH3), primary amino groups (MNH2), and secondary amino groups ((M-)2NH) could be present. Molecular ammonia and bridging Al-NH2 groups were, indeed, seen in the DRIFT spectrum. A small band due to SiNH-M groups was also seen. (Secondary (Al-)2NH groups were not observed on the surface, but they were seen when the reaction temperature was elevated to 473 K.) There was no evidence of (M-)3N groups. Thus, bridging Al-NH2 groups and Al:NH3 were concluded to be the main types of groups present in the 423 K sample. The average H/N ratio in the amino groups in the sample reacted with ammonia at 623 K was about 1. DRIFT measurements showed that molecular ammonia, bridging and terminal Al-NH2 groups, and some Si-NH2 were present. Secondary (Al-)2NH and Si-NH-M groups were also observed; the number of (Al-)2NH groups was probably maximized at this reaction temperature. The presence of Al:NH3 and M-NH2 groups requires that a considerable number of (M-)3N groups be present to compensate them in the H/N ratio. (The metal atoms in the (M-)3N groups were most likely both aluminum and silicon.) This result is in agreement with the observation of Sauls et al.37 that the (Al-)2NH groups, formed in the reaction of TMA and ammonia in the absence of a surface, transform to AlN by about 473-673 K. In conclusion, TMA-modified silica reacted with ammonia at 623 K was a highly heterogeneous surface. After ammonia reaction at 823 K, the average H/N ratio in the amino groups was about 0.7. This suggests the presence of a considerable number of (M-)3N and (M-)2NH groups, and much fewer M-NH2 groups and ammonia. DRIFT showed no molecular ammonia at this reaction temperature. Terminal and in addition some bridging Al-NH2 groups were seen, along with Si-NH2 groups. Some (Al-)2NH groups were still present, whereas almost all of the Si-NH-M groups had disappeared. The 1H MAS NMR results indicated that about half of the hydrogen atoms were present in M-NH2 and the other half in (M-)2NH groups. Because of the considerable amount of M-NH2 and (M-)2NH groups, many tertiary (M-)3N groups should be present. Bulk nitride crystals evidently were not formed, since an X-ray diffractogram showed no nitride peaks. In conclusion, (M-)3N should be most abundant group in this sample, but Al-NH2, Si-NH2, and (Al-)2NH groups are also expected. The changes effected by the ammonia reaction in the surface methyl and amino groups can be explained in terms of reactions releasing methane and reactions releasing ammonia. The proposed reactions are shown in Figure 14. It is noteworthy that the secondary and tertiary amino groups can be produced on the surface by two types of reactions: reactions between methyl and amino groups (reactions 2 and 3), and reactions between two amino groups (reactions 4 and 5). Since ammonia can open Al-N bonds,37 reactions 4 and 5 are probably reversible. High reaction temperatures should, however, favor the forward reactions. In addition to the amino groups, Al-OH groups were formed in the reaction of ammonia on TMA-modified silica. These AlOH groups could have been formed through the dissociation of ammonia in Si-O-Al bridges on the TMA-modified silica,
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Puurunen et al. surfaces can be produced by ammonia reaction at about 623 and 823 K; further studies are needed to determine which surface is better for the subsequent TMA reaction in ALCVD growth of AlN. The present results should assist in producing highquality AlN on porous silica. Acknowledgment. We are grateful to the Department of Chemistry, University of Joensuu, for providing the equipment for measuring the DRIFT spectra inertly. The Analytical Department of Fortum Oil and Gas Oy is thanked for the element determinations and the BET and XRD measurements. We are grateful to Mirja Rissanen for teaching R.L.P. to use the ALCVD reactor, Anita Palukka for recording the DRIFT spectrum of hexamethyldisilazane, and to Eeva-Liisa Lakomaa, Tuomo Suntola, and Marko Tuominen for valuable discussions.
Figure 14. Proposed reactions resulting in loss of methyl groups and formation of amino groups.
forming a pair of either Si-NH2 and Al-OH groups or SiOH and Al-NH2 groups. In addition, Al-NH2 groups may have been replaced with Al-OH through the reaction of residual H2O in the reactor. This exchange reaction due to residual water is probable because a partial pressure of water of 7 × 10-6 Pa or less is a prerequisite for producing oxygen-free AlN thin films.44 Conclusions In the TMA reaction at 423 K on silica dehydroxylated at 1023 K, about six methyl groups per nm2 were attached to the surface, 60% in the form of Al-CH3 and 40% in the form of Si-CH3 groups. The Al-CH3 groups were present in both OAl(CH3)2 and O2AlCH3 species, and the Si-CH3 groups in O3SiCH3, O2Si(CH3)2, and OSi(CH3)3 species. A reaction mechanism for TMA on dehydroxylated silica is proposed: TMA reacts through ligand exchange with all of the sterically available OH groups on the surface, and the reaction proceeds with siloxane bridges through dissociation of TMA. Formation of a shroud of methyl groups blocks the rest of the surface from reacting and defines the saturation conditions of the reaction. When ammonia reacted with TMA-modified silica, the methyl groups bonded to aluminum and silicon were replaced by amino groups, with release of methane. Ammonia did not remove chemisorbed aluminum from the surface. When the reaction temperature was 573-623 K, ammonia reacted with all of the Al-CH3 groups. Furthermore, the Si-CH3 groups began to react with ammonia at 423 K, and when the reaction temperature was elevated to 823 K, over 80% of them were removed. Probably the number of Si-CH3 groups could be reduced even more by carrying out the reaction at temperatures higher than 823 K. Among the methyl groups in different O4-nSi(CH3)n species, those in OSi(CH3)3 species were the most reactive and those in O3SiCH3 the least reactive. Depending on the reaction temperature of the ammonia, various amounts of coordinatively bonded molecular ammonia (Al:NH3), primary amino groups (M-NH2), secondary amino groups ((M-)2NH), and tertiary amino groups ((M-)3N) were present on the surface. As a general trend, the amount of hydrogen in the amino groups decreased when the reaction temperature was elevated. Considering the total hydrogen content in the amino groups, two types of surfaces were produced: at reaction temperatures of 573-623 K about 2.5 hydrogens per nm2 were present, whereas at temperatures of 723-823 K, the density was only about 1.5 hydrogens per nm2. In conclusion, the amount of residual carbon can be decreased by elevating the ammonia reaction temperature. Regarding the hydrogen content of the amino groups, two different types of
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