Langmuir 1998, 14, 5845-5849
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Reaction between Silane and the Lattice Oxygen of Transition Metal Oxides Eli Ruckenstein* and Yun Hang Hu Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260 Received May 13, 1998. In Final Form: July 27, 1998 The reaction between SiH4 and the lattice oxygen of NiO, CoO, or Fe2O3 was investigated by pulse mass spectrometry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) techniques. The pulse reaction indicated that almost no homogeneous conversion of SiH4 took place until 400 °C; the conversion was, however, 19% over NiO, at the low temperature of 100 °C. The conversions over oxides followed the sequence CoO > Fe2O3 > NiO. The TEM and XPS investigations allowed one to conclude that SiH4 is converted into amorphous SiO2 and NiSiO3 over NiO and into amorphous SiHxOy over CoO and Fe2O3, with a value of y larger over Fe2O3. The bond strength between the lattice oxygen and the transition metal is mainly responsible for the above behavior; the weaker the bond strength, the easier the oxidation.
1. Introduction The pyrolysis of silane is widely employed to generate silicon thin films and the transition metals have a catalytic effect on the process.1 Indeed, Takahashi et al. noted the formation of platinum silicides when silane was passed over Pt at 250 °C.2 This reaction is initially very vigorous, due to the catalytic dehydrogenation activity of the Pt surface, but slows down rapidly as the surface is covered by the Si formed. In contrast, Westwater et al.3 found that Au is not poisoned by the silane dissociation and leads to the formation of nanoscale Si whiskers. The direct formation of SiO2 thin film from SiH4 or other Si compounds via its oxidative deposition on a substrate, at relatively low temperatures (below about 400 °C), is of particular interest in the electronic industry.4 Recently, Klaus et al.5 reported that films of silicon dioxide could be deposited at room temperature by means of the reaction: SiCl4 + 2H2O f SiO2 + 4HCl. The oxidation of Si thin film to SiO2 was also widely studied.6,7 Our investigation regarding the CH4 reaction with lattice oxygen8 as well as the very high reactivity of SiH4 with oxygen due to the high affinity of Si for oxygen9 suggested to us to generate a SiO2 thin film directly via the reaction of SiH4 with the lattice oxygen of a transition metal oxide. The goal of the present paper is to investigate the reaction between SiH4 and the lattice oxygen of NiO, CoO, or Fe2O3 by pulse mass spectrometry (pulse-MS), BraunauerEmmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).
Figure 1. SiH4 homogeneous conversion vs reaction temperature. The experiment was carried out in an empty tube under the conditions mentioned in the experimental section. Only silane was detected at the exit of the reactor.
* To whom correspondence should be addressed. (1) Harbke, G.; Krausbauer, L.; Steigmeier, E. F.; Widmer, A. E. J. Electrochem. Soc. 1984, 131, 675. (2) Takahashi, Y.; Ishii, H.; Murota, J. J. Appl. Phys. 1985, 58, 3190. (3) Westwater, J.; Gosain, D. P.; Yamauchi, K.; Usui, S. Materials Lett. 1995, 24, 109. (4) Tsu, D. V.; Parsons, G. N.; Lucovsky, G.; Watkins, M. W. SiO2 and Its Interfaces; Pantelides, S. T., Lucovsky, G., Eds.; Materials Research Society: Warrendale, PA, 1998; p 73. (5) Klaus, J. W.; Sneh, O.; George, S. M. Science 1997, 278, 1934. (6) Lewis, E. A.; Irene, E. A. J. Vac. Sci. Technol. 1986, A4, 916. (7) Rochet, F.; Rigo, S.; Froment, M.; Anterroches, C. D.; MGaillot, C.; Roulet, H.; Dufour, G. Adv. Phys. 1987, 35, 237. (8) Hu, Y. H.; Ruckenstein, E. Langmuir 1997, 13 (7), 2055. (9) Cannon Sneed, M.; Brasted, R. C. Comprehensive Inorganic Chemistry; D.Van Nostrand Company, Inc.: 1958; Vol. 7, p 59.
Figure 2. Relationship between SiH4 conversion and SiH4 pulse number over NiO: (a) 100; (b) 200; (c) 300 °C. The reaction was carried out as indicated in the experimental section. No coupling products (e.g., Si2H6 and Si2H4) were detected during reaction.
1. Experiment 1.1. Chemicals. NiO (325 mesh), CoO (325 mesh), and Fe2O3 ( Fe2O3 > NiO. To determine the effect of the surface area on the SiH4 reaction, the BET surface areas of the oxides were measured. They are listed in Table 1, which indicates that the surface areas are in the sequence CoO > Fe2O3 > NiO, which coincides with that of the SiH4 conversion. It is, however, worth noting that the total conversion after 80 pulses over CoO is 10 times higher than those over NiO or Fe2O3, while the surface area of CoO is only about 3 times larger than those of NiO and Fe2O3. This indicates that an additional factor, which will be identified later, affects the SiH4 conversion over the transition metal oxides.
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Figure 11. Electron diffraction patterns corresponding to Figure 10: (a) lighter colored material; (b) dark particle.
2.2. Analysis of the Surface Species. The compound(s) formed during the pulse reaction were identified via XPS analysis. Figures 4-7 present the spectra over the entire meaningful range of binding energies. As shown in Figure 4a, the spectrum exhibits over NiO a peak at 103.6 eV. Because the binding energy for Si(2p) in SiO2 is 103.3 eV,10 SiO2 can be one of the products of decomposition over NiO. Over CoO, there is a broad overlapped peak, which can be decomposed by peak fitting into two peaks at 101.6 and 103.1 eV (Figure 4b). Because the binding energy of Co(3s) is about 103.0 eV, the 103.1 eV peak can be most likely attributed to Co. This peak being broad, one should not, however, exclude the possibility that some SiO2 is also present. The binding energies of Si(2p) in SiO2 and Si are 103.3 and 99.7 eV, respectively.10 Consequently, the peak at 101.6 eV for the product of SiH4 decomposition cannot be attributed to SiO2 or Si. Because the binding energy of 101.6 eV is between the binding energies of Si(2p) for Si0 (99.7 eV) and for SiO2 (103.3 eV), it is likely that over CoO the product containing Si is only partially oxidized; i.e., the product can be represented as SiHxOy, with y < 2. Figure 4c shows that over Fe2O3 there is one peak at 102.8 eV. As for CoO, the product of decomposition can be represented for similar reasons by SiHxOy. However, as indicated by XPS, the Si binding energy in the compound SiHxOy over Fe2O3 is greater than that over CoO. Consequently, the oxidation state of Si is higher over Fe2O3 than over CoO and the amount of oxygen contained in the product is in the sequence NiO > Fe2O3 > CoO. The additional factor (besides the surface area) responsible for the sequence CoO > Fe2O3 > NiO in the SiH4 conversion is due to the amount of oxygen in their products. The formation of different Si compounds over the three metal oxides can be explained as follows: The reaction of SiH4 with the transition metal oxide depends on the (10) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley & Sons: New York, 1983; p 595.
Figure 12. TEM for Fe2O3 reacted with SiH4. The reaction was carried out at 200 °C by the pulse method; a succesion of pulses was employed until the SiH4 conversion became negligible.
binding strength of the lattice oxygen to the transition metal; the weaker the binding, the easier the Si oxidation by the lattice oxygen. One can observe that Ni has the largest (1.75), Co the intermediary (1.70), and Fe the lowest
Reaction between SiH4 and Lattice Oxygen
Figure 13. Electron diffraction patterns corresponding to Figure 12.
(1.64) electronegativity.11 This indicates that the binding of the lattice oxygen to Ni is the weakest. As a result, the surface lattice oxygen of NiO can completely oxidize SiH4 to SiO2, whereas Fe2O3 and CoO can only partially oxidize SiH4 to SiHxOy. Even though Co has a higher electronegativity than Fe, the higher valency of Fe in Fe2O3 and its facile reduction to FeO may explain why y in the formula SiHxOy is smaller over CoO than that over Fe2O3. For the NiO already used for SiH4 conversion, Figure 5 indicates a peak at 857.0 eV and a shake-up peak at 863.2 eV. Because the binding energy of Ni(2p3) in NiSiO3 (11) Porterfield, W. W. Inorganic Chemistry, 2nd ed.; Academic Press: San Diego, CA, 1993; p 1.
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is 856.9 eV,10 the peak at 857.0 eV should be attributed to NiSiO3. From Figures 4 and 5, one can conclude that there are two kinds of Si products over NiO, namely, SiO2 and NiSiO3. Figure 6 for CoO after reaction exhibits a peak at 780.7 eV, which should be attributed to CoO,10 and Figure 7 for Fe2O3 after reaction exhibits a peak at 711.5 eV, which should be attributed to Fe2O3.10 2.3. Morphology and Crystalline Structure of Products. As shown in the micrograph (Figure 8), the particle (dark color) is covered with a lighter colored layer. The electron diffraction patterns identified the dark particle as the cubic crystalline NiO and the covering material as being amorphous (Figure 9). The XPS results indicated that over NiO, SiO2 and NiSiO3 are the only products containing Si. Therefore, it is reasonable to conclude that amorphous SiO2 and NiSiO3 are deposited over NiO. Figures 10 and 11 refer to CoO and Figures 12 and 13 to Fe2O3. One can see from Figure 10 that a lighter colored layer covers dark particles and bridges two dark particles. The electron diffractions identified the dark particle as the cubic crystalline CoO and the lighter colored material as being amorphous (Figure 11). Combining the XPS and TEM information, one can conclude that the lighter colored material is amorphous SiHxOy. Figure 12 indicates the presence of a very thin lighter colored layer over a dark material, and the electron diffraction identified the dark particle as crystalline Fe2O3 with the corundum structure (Figure 13). It is reasonable to conclude that the lighter colored material is SiHxOy. 3. Conclusion The SiH4 reaction with the lattice oxygen of transition metals depends on the nature of the transition metal oxide. SiH4 was converted into amorphous SiO2 and NiSiO3 over NiO, whereas amorphous SiHxOy was generated over CoO and Fe2O3. However, the value of y in formula SiHxOy is larger over Fe2O3 than that over CoO. The SiH4 conversion is the highest over CoO, smaller over Fe2O3, and even smaller over NiO. This is a result of both the surface area of the oxide and of the bond strength between the lattice oxygen and the transition metal in the oxide. LA9805645
