Chem. Mater. 1990,2,263-268
263
Preparation of an Iron Nitride Film from a Molecular Tetrairon Nitrido Cluster T. P. Fehlner* and M. M. Amini Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556
W . F. Stickle Perkin-Elmer, Eden Praire, Minnesota 55344
0. A. Pringle and Gary J. Long Departments of Physics and Chemistry, University of Missouri-Rolla, Rolla, Missouri 65401
F. P. Fehlner R.D.&E. Division, Corning Glass Works, Corning, New York 14831 Received September 21, 1989 The transition metal-main-group atom cluster HFe4(C0)12Nhas been studied as a potential molecular precursor for the production of thin films that contain amorphous iron nitride phases. Films 300-500 A thick are formed by chemical vapor deposition at 160-180 "C on glass substrates. Analysis by X-ray photoelectron spectroscopy shows =lo% nitrogen and low oxygen and carbon impurities. X-ray diffraction shows the presence of an a-Fe phase. Mossbauer spectoscopy of the purest films confirms the presence of a-Fe and, in agreement with the atomic composition, shows an equally abundant nitride phase which is probably due to 7'-Fe4N. Mass spectrometric analysis of the gas-phase products shows the presence of CO, which is the major gaseous product in the deposition, but low levels of H2, NHs, and C02are also identified as pyrolysis products of HFe4(C0)12N.The iron nitride film appears to be very similar to that produced by reactive sputtering of iron in argon in the presence of N2 and H2 in a 1:l ratio. Introduction
Because of the wide application of iron nitride materials in industry, the nitridation of iron, titanium, and other metals has attracted substantial interest.'-' Several techniques have been employed previously to nitride iron, including radio-frequency sputtering of iron and nitrogen? pulsed excimer laser treatment under liquid ammonia? the reaction of iron carbonyls and ammonia,'O and nitrogen ion implantation."J2 The above techniques often lead to the formation of more than one phase. Alloy materials are important in advanced technology, and new methods of preparation are continuously sought. There are now several examples in the literature of the effective use of volatile molecular precursors for the preparation of binary materials.13 One method is to incorporate the iron and (1) Milic, M.; Milosavljevic, M.; Bibic, N. Popovic, N.; Bogdanov, Z. Thin Solid Films 1988, 163, 309. (2) Shrivastava, S.; Tarey, R. D.; Jain, A.; Chopra, K. L. Thin Solid Films 1988, 163, 359. (3) Jurik-Rajman, M.; Veprek, S. Surf. Sci. 1987,189, 221. (4) Ogale, S. B.; Polman, A.; Quentin, F. 0. P.; Roorda, S.; Saris, F. W. Appl. Phys. Lett. 1987,50, 138. (5) Kamar, N.; McGinn, J. T.; Pourrezaei, K.; Lee, B.; Douglas, E. C. J. Vac. Sci. Technol. 1986. A6. 1602. ~(6) Krishnaswamy, S. V.; Heter, W. A.; Szedon, J. R.; Francombe, M. H.; Driscoll, M. M. Thin Solid Films 1985, 125, 291. (7) Ostlingos, M.; Nygren, S.; Petersson, C. S.; Norstrom, H.; Buchta, R.; Blom, H.-0.; Berg, S. Thin Solid Films, 1986, 145, 81. (8)Lo, C.; Krishnaswamy, S. V.; Mulay, L. N.; Diffenbach, R. A. J. Appl. Phys., Part 2 1982,53, 2745. (9) O d e , S. B.; Patil, P. P.; Roorda, S.; Saris, F. W. A_ d_ . Phys. - Lett. v
~
1987,SO; 1802. (10) Saito, Y.; Matauda, S.; Mizumoto, M.; Sato, Y. Jpn. Kokai Tokkyo Koho JP 61/288071 A2 [86/288071], 18 Dec 1986; Chem. Abstr. 1987,107, p 32081~. (11) Chatterjee, P.; Batabyal, A. K. J.Non-Cryst. Solids 1988,103, 14. (12) Naeo, M.; Nagakubo, M.; Yamamoto, T. J.Appl. Phys. 1988,64, 5449.
nitrogen atoms into a single precursor that is then directly converted into iron nitride. Thus, we have explored the preparation of iron nitride films from a preassembled, volatile tetrairon carbonyl nitrido cluster by utilizing a low-pressure chemical vapor deposition technique. Experimental Section The HFe4(C0)12N cluster was prepared according to published procedures and purified by repeated crystallization from hexenes and methylene chloride.14 To prepare a film,the microcrystalline HFe4(C0)12Nwas sublimed at 50 "C in two chemical vapor deposition reactors. All information reported, except the measurement of volatile products, was obtained from depositions carried out in a single reactor (reactor l),which is shown in Figure 1. The position of the precursor sublimer relative to the substrate surface could be easily adjusted from outside the vacuum. For most depositions it was positioned 2-3 cm from the substrate (13) Aylett, B. J.; Colquhoun, H. M. J. Chem. SOC.,Dalton Trans. 1977,2058. Aylett, B. J.; Tannahill, A. A. Vacuum 1985,&5,435. Giro-
lami, G. S.; Jensen, J. A.; Pollina, D. M.; Williams, W. S.; Kaloyerso, A. E.; Allocca, C. M. J . Am. Chem. SOC. 1987,109, 1579. Kaloyeros, A. E.; Williams, W. S.; Allocca, C. M.; Pollina, D. M.; Girolami, G. S. Adu. Ceram. Matter. 1987,2,257. Jensen, J. A.; Gozum, J. E.; Pollina, D. M.; Girolami, G. S. J. Am. Chem. SOC. 1988, 110, 1643. Jefferies, P. M.; Girolami, G. S. Chem. Mater. 1989,1, 8. Czekaj, C. L.; Geoffroy, G. L. Inorg. Chem. 1988,27,8. Bochmann, M.; Hawkins, I.; Wilson, L. M. J. Chem. SOC., Chem. Commun. 1988,344. Wayda, A. L.; Schneemeyer, L. F.; Opila, R. L. Appl. Phys. Lett. 1988,53,361. Steigerwald, M. L.; Rice, C. E. J. Am. Chem. SOC. 1988, 110,4228. Steigerwald, M. L. Chem. Mater. 1989, I, 52. Boyd, D. C.; Haasch, R. T.; Mantell, D. R.; Schulze, R. K.; Evans, J. F.; Gladfelter, W. L. Chem. Mater. 1989,1,119. Cowley, A. H.; Benac, B. L.; Ekerdt, J. G.; Jones, R. A.; Kidd, K. B.; Lee, J. Y.; Miller, J. E. J.Am. Chem. SOC. 1988,110,6248. Gross, M. E., Jasinski, J. M., Yates, J. T., Jr., Eds.; Chemical Perspectiues of Microelectronic Materials: Materials Research Society Pittsburgh, PA, 1989. Interrante, L. V.; Carpenter, L. E., n; Whitmarsh, C.; Lee, W.; Garbauskas, M.; Slack, G. A. Mater. Res. Soc., Symp. Proc. 1986,73,359. Seyferth, D.; Wisemann, G. H. In Ultrastructure Processing of Ceramics, Glasses,and Composites; Hench, L. L., Ulrich, D. R., Eds.; Wiley, New York, 1984; p 265. (14) Fjare, D. E.; Gladfelter, W. L. Inorg. Chem. 1981,20, 3533.
0897-4756/90/ 2802-0263$02.50/0 0 1990 American Chemical Society
Fehlner et al.
264 Chem. Mater., Vol. 2, No. 3, 1990 valve
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surface. This gave films of relatively uniform thicknesses in reasonable times. The substrates were resistively heated to a temperature of 160-180 "C as monitored by an iron/constantan thermocoupleclamped to the outer face of the substrate surface. The temperature measured by the thermocouple increased 5-10 "C during the deposition of the highly reflective film.This turned out to be a convenient,if phenomenological, method of monitoring the onset and initial growth of the film. Reactor 2 differed from reactor 1 in three aspects. First, the precursor crucible was oriented at right angles to that shown for reactor 1in Figure 1. Hence, there is direct evaporation of the precursor onto the substrate in reactor 2. Second, the precursor crucible was heated by heating the small diameter outer vacuum chamber wall in reactor 1. In reactor 2 the precursor was sublimed from a resistance-heated probe, and the reactor walls about 5 cm from the probe remained at room temperature. Third, reactor 1 was evacuated with an oil diffusion pumped system with a base pressure of lo* Torr with low pumping speeds in the deposition region, whereas reactor 2 was pumped with a turbomolecularpump achieving a base pressure of 2 X Torr and high pumping speeds in the deposition region. Because of the greatly differing effective pumping speeds at the substrate, the effective deposition pressure is estimated to be about a order of magnitude higher in reactor 1 than reactor 2. Residual gases, as well as volatile products of the deposition reaction, were monitored in reactor 2 by utilizing a Balzers QMG 064 partial-pressure analyzer. This was plumbed off the main chamber and was separately pumped. No attempt was made to obtain the relative sensitivities for the molecular precursors of the ion signals observed. The glass substrates (Corning 7059) were cleaned with soap and rinsed with distilled water, acetone, and ethanol, followed by heating at 550 "C for several hours in air before deposition. Deposition was also carried out on aluminum foil and was also
observed on the ceramic,stainless steel and tantalum foil supports and shields in the deposition apparatus. Film thicknesses were measured with a Tencor profilometer. The extent of crystallinity was assayed with an X-ray powder diffraction system. Film compositionwas determined by X-ray photoelectron spectroscopy (XPS) and Mossbauer spectroscopy. The XPS profiles were obtained on a Perkin-Elmer 5400 ESCA spectrometer that used 4-keV Ar+ ions at a sputter rate of 30 A/min relative to Ta205. The multiplex windows, which make up the profile, were obtained with a spectrometer resolution of 0.7 eV relative to the Ag 3d5I2line by using monochromatic A1 X-rays. For comparativepurposes,the f h s obtained with reactor 2 were also analyzed but by Auger spectroscopy. Auger spectra were obtained on a Perkin-Elmer 660 scanningAuger multiprobe with 10-kV electrons after sputtering with 5-kV Ar+ to constant composition. The Mossbauer effect spectra were obtained at 85 K on a conventionalHarwell constant-accelerationspectrometer that utilized a room-temperaturerhodium matrix 67C0source and was calibrated at room temperature with natural-abundance a-iron foil. The spectra were fit with Lorentzian line-shape magnetic sextets by using standard least-squares computer minimization techniques. Because the foils were so thin, the Mijasbauer absorber was made up of four layers of the foil. However, despite the use of four layers, the absorber gives a very low percent-effect absorption, especially at 295 K.
Results Because the mass spectrum of the HFe4(CO)12Ncluster can be obtained in the gas phase, the solid compound vaporizes in a molecular form.14 Exposure in the chemical vapor deposition reactor of a glass substrate at 175 "C to Torr of HFe4(C0)12Npressure, produced by (1-5) X
Iron Nitride Film
Chem. Mater., Vol. 2, No. 3, 1990 265 loo0 metalhihide
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the sublimation of a 20-30 mg of powdered sample at 40-50 O C for 0.5-2 h, results in the formation of a film on the glass substrate with a metallic appearance and a smooth, mirrorlike surface. The film thickness did not increase appreciably after 15 min. The vaporization of two individual 20-mg charges of HFe4(C0)12Ndid not double the film thickness over that resulting from the evaporation of a single charge of 20 mg. Direct pyrolysis of the molecular beam emanating from the precursor crucible on the substrate surface was demonstrated for reactor 2 by noting decreasing film thickness with increasing crucible-to-substrate distance for the same deposition time and crucible temperature. Further, an identical substrate facing away from the beam exhibited films much thinner than those for a substrate facing the beam. The films adhere well to glass as judged by the Scotch tape test. Film thicknesses ranged from 300 to 600 A. The X-ray diffraction studies of the thin film deposited on glass shows only one weak peak at 44.7O in 219, which corresponds to the presence of a-iron as a crystalline phase in the film. Analysis of films on glass substrates prepared in reactor 1 were obtained by XPS. After removal of 25 A of the surface by light sputtering, there is no detectable carbon. As shown in the depth profile (Figure 2) after 4 0 A of the surface is removed, the oxygen level falls below the noise level. Survey spectra revealed no other impurities. Furthermore, the spectra show the atomic concentration of nitrogen in the bulk of the film of 7-13%. The chemical environments of the Fe and N atoms are also revealed by the XPS spectra. Figure 3 shows both the iron 2p3p ionization before and after (706.8 eV) sputtering and the nitrogen 1s ionization before and after (396.8 eV) sputtering. The iron 2pSJ2binding energy in authentic iron nitride is approximately the same as the bulk metal16 and cannot be used to distinguish a nitride from a-Fe. However, the N l s binding energy observed here is characteristic of an iron nitride phase, as is shown by a comparison of the core level binding energies of iron nitride films, prepared by various techniques, with our results in Table Analysis of films prepared in reactor 2, ostensibly by the same technique as that for reactor 1, by Auger spectroscopy with standard instrument sensitivities, gives a nitrogen content of 5 7 % and C and 0 impurities (15) Kothari, D. C.; Nair, M. R.; Rangwala, A. A.; Lal, K. B.; Parbhawalkar, P. D.; h o l e , P. M. Nucl Instrum. Methods 1985, B718,235. (16) Singer, I. L.; Murday, J. S. J. Vac. Sci. Technol. 1980, 17, 327. (17) Kishi, K.; Ikeda, S. Bull. Chem. SOC. Jpn. 1974,47, 2532. Vacuum 1987, 140, 137. (18) Arabczyk, W.; Mussig, H.-J.
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Figure 3. Top: Fe 2p spectra of an iron nitride film deposited on glass at 175 "C by LPCVD of HFe4(C0)12N.Bottom: N 1s spectra of the same film. The times refer to sputter times with the dashed spectra referring to the unsputtered surface of the film.
Table I. Comparison of XPS Core Binding Energies (eV) for Iron Nitride Phases Prepared by Various Techniques Fe 213312 N Is techniaue ref 706.9 397.2 nitrogen ion implantation 15 707.1 397.2 nitrogen ion implantation 16 396.6 reaction of iron and nitric oxide 17 397.0 nitrogen ion implantation 18 706.8 396.8 CVD this work
at the same levels. We do not know why these films prepared in a system with a better base vacuum were of lower purity but suspect that the higher steady-state precursor pressure in reactor 1 plays a role. As the Fe/N ratio of the films prepared in the two reactors are similar, we expect the identities of the gaseous products to be the same in both reactors. However, further discussion of the nature of the solid films itself is restricted to samples prepared in reactor 1. Mossbauer spectroscopy of films deposited in reactor 1 on glass permits the iron nitride phase in the film to be more precisely characteri~ed.'~*~~ The Mossbauer spectrum of an iron nitride film prepared by chemical vapor deposition of HFe4(C0)12Nis shown in Figure 4. The hyperfine parameters obtained for the thin film at 296 and (19) Longworth, G.; Hartley, N. E. W. Thin Solid Films 1978,48,95. (20) Ogale, S. B.; Patil, P. P.; Phase, D. M.; Bhandarkar, Y. V.; Kulkami, S. K.; Kulkami, S.: Ghaisas, S. V.; Kanetkar, A. M.; Bhide, V. G.; Guha, S. Phys. Rev. E. 1987,36,8237.
Fehlner et al.
266 Chem. Mater., Vol. 2, No. 3, 1990 R E L A T I V E ENERGY -2
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85 K, as well as literature hyperfine parameters for various possible constituents of the are given in Table 11. Because most of the literature data on related thin films are obtained by conversion electron or X-ray Mossbauer backscattering spectroscopy, they are all reported at room temperature. The observed spectra can be fit with two magnetic sextets of approximatelyequal area and a doublet of much smaller area. Both the hyperfine parameters and the temperature dependence of the absorption area for the doublet are characteristic of a superparamagnetic component in the thin foil, a not unreasonable component for foils prepared in the fashion employed in this work. One of the magnetic sextets, the outer component in Figure 4, is clearly a-iron, as is indicated by its hyperfine parameters given in Table 11. The presence of a-iron is also consistent with the X-ray results discussed above. The second magnetic sextet, with a substantially smaller hyperfine field and a higher isomer shift, also has a higher line width, which is typical of a small distribution of hyperfine fields for this component in the thin film. The hyperfine parameters for this magnetic sextet1%%indicate that it could consist of either 7’-Fe4N or c-FeZ+*N,or perhaps more likely a mixture of both. 7’-Fe4N exhibits three sextets, see Table 11, one of which has a hyperfine field slightly larger than a-iron, and two of which have very similar fields, fields very close to those observed in our thin film. By taking the appropriate averages of the known spectra for a-iron and 7’-Fe4N we can obtain a reasonable composite of the observed thin foil spectrum at both 296 and 85 K. Of course for the 85 K composite spectrum the hyperfine fields of 7’-Fe4N must be rescaled to those expected at 85 K. In this case the high-field component in 7’-Fe4N, which would comprise about 10% of the absorption area, is concealed within the more intense a-iron component. If we assume that the nitride phase present is predominantly 7’-Fe4N, then the Mossbauer spectra show that it and a-iron are present in a close to 1:l ratio. In this case, on the basis of the Mossbauer absorption areas, the calculated nitrogen content should be about (21) Eickel, K. H.; Pitach, W. Phys. Status Solidi 1970,39,121. (22) Lo,C.; e h n a s w a m y , S. V.; Messier, R.; Rao, K. R. P. M.; Mulay, L. N. J. Vac. SCC.Technol. 1981,18,313.
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lo%, a value quite similar to that observed by XPS. It is also possible that the second phase corresponds to t-Fez+*N,with 0.3 < x < 2.7. This material exhibits twoz1 or three23magnetic sextets with hyperfine fields of about 290,230, and 130 kOe, which correspond to one, two, and three near-neighbor nitrogen atoms, respectively. As the nitrogen content decreases, the relative areas of the three subspectra change, and their hyperfine fields decreasesz3 For t-Fea2N the hyperfine parameterslg corresponding to the predominant sextet, see Table 11, are very similar to those observed in our film. However, the a-iron and nitride phase are present in an approximately 1:l ratio. Thus, if c-Fe3N were the predominant nitride phase, the overall nitrogen content of the film would be about 14%. On the basis of the significant uncertainty in the composition measurements, the presence of the e-Fe3N phase cannot be ruled out. However, more iron-rich €-Fez+$/phases are not possible, because when x increases from 1 to 2.7, the hyperfine field decreases rapidly, and for x above 2.7 the Curie temperature falls below room temperature, in contradiction with the field we observe at 296 K. The thin films under study conduct electricity and exhibit resistivities at 22 “C of 40 f 15 pR cm as measured by the four-probe method. Because thin films generally have higher resistivities than bulk materials and consistent with the composition and state of the film described above, the resistivity of the film is less than the typical 150 p a cm value found in main group-transition metal amorphous alloys but is greater than that the 10 pi2 cm value for a-ir~n.~~ The formation of a-Fe suggests the loss of nitrogen by some process during the deposition, e.g., the formation of a volatile nitrogen-containing gas. One possibility is oxidation to form NO gas. An alternative explanation is the formation of a nitrogen hydride, e.g., NzH4. If all the hydrogen were lost in the latter form, the solid product would contain 10 at. % nitrogen as required by a 5050 (23) N. DeCristofaro, N.; R. Kaplow, R. Metal.Trans. 1977,8A, 425. (24) Johnson, W. L.; Tenhover, M. In Glassy Metal: Magnetic,
Chemical and Structural Properties; Haaegawa, R., Ed.; CRC Press: Boca Raton, FL, 1983: p 65.
An Iron Nitride Film 50 ..
Chem. Mater., Vol. 2, No. 3, 1990 267
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mixture of y'-Fe4N and a-Fe. Hence, we have examined the gases formed during deposition. These experiments were carried out in reactor 2; however, there is no reason to believe the volatile products would be qualitatively different in the two reactors. To discriminate against pyrolysis products from the heater elements, the position of the precursor crucible was used as an effective shutter, i.e., with the probe about 15 cm from the substrate all pyrolysis was from randomly scattered molecules, whereas with the probe about 2 cm from the substrate surface, a substantial pyrolysis fraction comes from molecules directly impinging on the surface. Hence, the difference in these signals is due to the direct pyrolysis. The signal from a trace water vapor impurity was used as an internal standard. There were no qualitative differences in composition between the ion spectra with the probe at 2 vs. 15 cm. The difference spectra, however, showed an apparent onset for decomposition of HFe4(C0)12Nat a higher temperature. Thus, some pyrolysis is taking place on the heater elements. Figure 5 shows selected ion current differences as a function of temperature over the range at which most depositions were studied. The onset of pyrolysis of HFe4(C0)12Noccurs at about 150 OC, as shown by the production of CO and Ha. A search was made for nitrogen-containinggaseous products by looking for their parent ions and prominent fragment ions, i.e., NH3 ( m / e 15 and 17),N2H4 ( m / e 32 and 30), NO ( m / e 30), N20 ( m / e 44 and 30), NO2 ( m / e 46 and 30), and N2 ( m / e 14). Only NH3 was positively identified as shown by the decrease in the ratio of m / e 18/17 as a function of temperature (Figure 6), where m / e 18 is due to residual H 2 0 in the system. At 200 "C the ion signals at m / e 2,17(corrected for H20),and 28 were in the approximate ratio of 1:2:100. If one assumes equal sensitivities,this accounts for nearly all the hydrogen but implies a Fe/N ratio in the solid of =5. It seems there must be another nitrogen-containing product. Most likely this product is N2; however, the molecular ion and fragment ion of this possible product are obscured by the molecular ion and doubly charged ion from of the large amounts of CO produced. If all the missing nitrogen appeared as N2, the m / e 14 ion signal would increase by a factor of 1.2 over the signal due to C02+ions. This is not outside of the error limits of our measurements. It should be noted that small amounts of C02 are also produced. Presumably C02 results from the disproportionation of two CO ligands with perhaps some of the carbon atoms remaining in the film as carbide. At 210 "C the ratio of CO to C02 is 100 and the ratio decreases significantly with increasing temperature. This level of C02 would result in carbide levels at most of 2-3%. Finally, although the mass range of the quadrupole mass spectrometer was insufficient to monitor the principal ions of HFe4(CO)12N,( m / e 56, Fe+),gave an approximate measure
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of the partial pressure of the precursor. From the change in this ion intensity it appeared that ~ 2 0 %of the cluster was decomposed at 200 "C in reactor 2.
Discussion Although the preparation of thin films from metal carbonyls often can lead to the formation of films with significant levels of oxygen and carbon ~ o n t a m i n a t i o n , ~ ~ the use of HF4(CO)12Nas a molecular precursor results in the deposition of a relatively clean film provided the substrate temperature remains below 200 "C. However, in contrast to the isoelectronicmolecular precursor for iron boride films, HFe4(C0)12BH2,26 the cluster stoichiometry is not preserved in the thermolysis and subsequent deposition process. Further, with the borides there is no limitation in film thickness, whereas here the thickness of the nitride films is restricted to about 400 A. We can only speculate on the mechanistic origin of this effect. Curiously, the same thickness FeN film could be grown on top of a Fe4B film; however, it is still possible film growth in this temperature regime is promoted by an unknown surface species present on both glass and coated substrates. Loss of the promoter on full surface coverage would terminate growth. Despite the unknown origin of the inhibition of the growth process, it is clear that this mechanistic problem greatly reduces the effectiveness of the molecular presursor approach in the formation of nitrides from HFe4(C0)12N. The observed Mossbauer spectrum is very similar to that obtained for an iron film sputtered in Ar, N2, and H2 mixtures when the N2/H2ratio was 1:1.22Note that this is the same ratio present in the HFe4(CO)12Ncluster. In this earlier work,22pure y'-Fe4N was observed at a N2 to H2 ratio of only 1:3. For ratios higher or lower than 1:3, a-Fe was a major constituent of the films. The substrate temperatures in the sputtering experiment were 200 "C and above. Thus, even at the low temperatures employed herein, we are obtaining a film whose composition appears to be determined predominantly by the N/H ratio of the cluster precursor. This suggests that although the exocluster ligands must be lost on the hot substrate surface rapidly with respect to film buildup, the integrity of the cluster core is lost at a rate comparable to the rate of (25) Stauf, G. T.;Dowben, P. A.; Boag, N. M.; Morales De La Garza, L.; Dowben, S. L. Thin Solid Films 1988,156,327. Jervis, T. R. J. Appl. Phys. 1985,58,1400. Vogt, G. J. J. Vac. Sci. Technol. 1982,20,1336. Green, M. L.; Gross, M. E.; Papa, L. E.; Schnoes, K. J.; Brasen, D. J . Electrochem. SOC.1985, 132, 2677. Gross, M. E. J. Vac. Sci. Technol. 1988, B6,1553. (26) Fehher, T. P.; Amini, M. M.; Zeller, M. V.; Stickle, W. F.; Pringle, 0. A.; Long, G. J.; Fehlner, F. P. In Chemical Perspectives of Microelectronic Materials; Gross, M. E., Jasinski, J. M., Yates, J. T., Jr., Eds.; Materials Research Society: Pittsburgh, PA, 1989; p 413.
Chem. Mater. 1990,2, 268-273
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cluster deposition. As the CO ligands are lost, the cluster collapses and condenses to a solid material similar to that produced in the sputtering experiments at the same N / H ratio. This is perhaps reasonable as exo-cluster ligand bonding is known to constitute the largest fraction of the disruption energy of metal carbonyl clusters." In addition, once one strips the ligands off from a cluster, the residual bonding of the remaining core is far from that of the stable solid. Hence, the question of whether or not a given cluster stoichiometry will serve to determine the material stoichiometry depends not only on the properties of the cluster itself but also on those of the material. In the case of borides where Fe,B clusters are thought to be elements of the amorphous structure, discrete cluster precursor (27) Connor, J. A. Top. Curr. Chem. 1977, 71, 71.
stoichiometry does appear to define the B:Fe ratio.26 In conclusion, we have demonstrated that a discrete nitrido cluster can be used in the preparation of thin iron nitride films by chemical vapor deposition at relatively low temperatures. The films have properties very similar to those formed by reactive sputtering in Ar/N2/H2 systems with the same N / H ratio as the discrete nitrido cluster. Acknowledgment. The support of the Army Research Office (Contract DAALO3-86-K-0136,M.M.A., T.P.F.) and the donors of the The Petroleum Research Fund, administered by the American Chemical Society (G.J.L.), is gratefully acknowledged. T.P.F. thanks Prof. M. Lagally, A. Lefkow, and Dr. Ngoc C. Tran of the Thin Film Institute of the Univeristy of Wisconsin for their assistance in carrying out part of this work and their hospitality during the tenure of a Guggenheim Fellowship.
In Situ Photodegradation of SnBrB R. Ken Force,+ Mebrahtu G. Fessehaie,f Robert P. Grosso,t John M. Laliberte,i Skye McClain,i William S. Willis,t and Steven L. Suib*Jis Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881; Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268; and Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06268 Received October 4, 1989 The in situ photodegradation of SnBrz has been studied under vacuum or under nitrogen with X-ray photoelectron spectroscopy,residual gas analysis, photogravimetric analysis, and thermogravimetric analysis. Our results suggest that the photodegradation process is quite complex and occurs through several pathways. The initial step involves the photoexcitation of SnBr2with evolved gaseous species containing SnBr(g) and Br(g) as major decomposition products. In addition, water molecules on the SnBr2form SnO(s)and HBr. Minor secondary pathways for the decomposition involve the dissociation of HBr and SnBr(g). In addition, some metallic Sn is formed during photolysis. Introduction Some semiconducting metal oxides have been shown to be reduced with light to the metallic state if the bandgap of the semiconducting metal oxide is greater in energy than the reverse of the heat of formation of the metal oxide.'-5 All of this type of research studied to date has concerned photoreduction of metal oxides in the powdered state. For example, Fleisch and Mains'" used X-ray photoelectron spectroscopy to study the photoreduction of CuO, PdO, PtO, W03, Moo3,and ZnO. They found that metallic Cu, Pd, and Pt could be formed via photolysis but that only partial reduction of the molybdenum and tungsten oxide systems occurred. ZnO was not photoreduced, and Fleisch and Mains suggested that this was because the bandgap of the ZnO was less energetic than the reverse of the heat of formation of ZnO. This theory concerning bandgaps and thermodynamic parameters has not been tested with semiconducting materials other than oxides. In addition, there are a number University of Rhode Island. *Department of Chemistry and Institute of Materials Science, University of Connecticut. Department of Chemical Engineering,University of Connecticut. * Author to whom correspondence should be addressed.
of interesting questions regarding this theory that could be tested experimentally. Some of the questions that were apparent to use are the following: (1)Can this theory be applied to semiconductors besides metal oxides? (2) Can fragments of metal oxide be deposited on various surfaces and photoreduced? (3) Are other factors such as the nature of surface functional groups important in this photoreduction process? This paper concerns the photolysis of a semiconducting metal halide complex, SnBr,. The band gap6 of SnBr, is 328.3 kJ/mol, and the heat of formation is -243.3 kJ/mol. According to the theory postulated by Fleisch and Mains: SnBr2 should be photoreduced to metallic Sn. We have investigated the photolytic behavior of SnBr, using several different spectroscopic and gravimetric methods. To monitor changes in the oxidation state of the SnBr2,in situ photolytic X-ray photoelectron spectroscopy measure(1) Fleisch, T.H.; Mains, G . J.2. Phys. Chem. 1986, 90,5317-5320. (2) Fleisch, T. H.; Mains, G . J. J. Chem. Phys. 1982, 76, 780-786. (3) Fleisch, T.H.; Zajak, G . W.; Schreiner, J. 0.;Mains, G . J. Appl. Surf. Sci. 1986, 26, 488-497. (4) Fleisch, T.H.; Mains, G . J. Appl. Surf. Sci. 1982, 10, 50. (5) Mains, G . J.; Schreiner,J. 0.;Flesich, T. H. J. Phys. Chem. 1981, 85,4084-4089. (6) Strehlow, W.H.; Cook, E. L. J. Phys. Chem. Ref. Data 1973, 2, 163-193.
0897-4756/90/2802-0268$02.50/00 1990 American Chemical Society
