6492
Langmuir 1996, 12, 6492-6500
Adsorption and Thermal Decomposition of Hydrazoic Acid on Al(111) John N. Russell, Jr.,*,† Victor M. Bermudez,‡ and Andre´s Leming† Chemistry Division and Electronics Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375-5000 Received May 22, 1996. In Final Form: September 16, 1996X The surface chemistry of hydrazoic acid (HN3) on Al(111) was investigated from 100 to 800 K with temperature-programmed desorption, infrared reflection-absorption spectroscopy (IRRAS), Auger electron spectroscopy, and low-energy electron diffraction (LEED). IRRAS suggests that the first monolayer dissociatively chemisorbs as N2(ads) and NH(ads) at 120 K. Subsequently, HN3 adsorbs molecularly in two phases, a physisorbed second layer and a condensed multilayer. In the former, the N3 chain of the HN3 is aligned normal to the substrate surface, while in the latter HN3 exhibits a random orientation. Molecular HN3 desorbs at 125 K, and two decomposition products, N2 and H2, desorb at 295 and 615 K, respectively. Mixed-isotope desorption experiments show that N2 is derived from an intact NN species, rather than from recombinative desorption of N(ads), and is consistent with the IRRAS observation of chemisorbed N2. IRRAS indicates that the NH species dissociates into N(ads) and H(ads) above 320 K, with H2 thermal desorption resulting from dissociation of the AlHx (1 e x e 3) moiety. After the sample was annealed at 800 K, N was observed with Auger spectroscopy. The partially nitrided Al(111) surface exhibits a concentric, double hexagonal LEED pattern indicating AlN(0001) islands. IRRAS indicates that the islands are Al-terminated and capped with H at temperatures as high as 700 K.
1. Introduction AlN is an attractive material for many applications due to its wide bandgap, high thermal stability, high thermal conductivity,1 and reported negative electron affinity.2 Unfortunately, nitridation of Al using ammonia is difficult3 due to the high N-H bond dissociation energy, which requires that AlN growth by metalorganic chemical vapor deposition (MOCVD) be performed at ∼1273 K. This has led to a search for alternate low-temperature methods and an interest in the surface chemistry of nitridation precursors. Successful methods for nitridation of Al metal have employed N-ion implantation,4,5 N2H4 adsorption,3,6 or N2 glow discharge.7,8 We recently demonstrated9 that hydrazoic acid, HN3, is an effective precursor for lowtemperature growth of ordered AlN(0001) thin films on Al(111), and the chemistry of HN3 on Si,10-13 Ge,14 GaAs,15 * To whom correspondence should be addressed: e-mail,
[email protected]. † Chemistry Division. ‡ Electronics Science and Technology Division. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Morkoc¸ , H.; Strite, S.; Gao, G. B.; Lin, M. E.; Sverdlov, B.; Burns, M. J. Appl. Phys. 1994, 76, 1363. (2) Benjamin, M. C.; Wang, C.; Davis, R. F.; Nemanich, R. J. Appl. Phys. Lett. 1994, 64, 3288. Benjamin, M. C.; Wang, C.; Kern, R. S.; Davis, R. F.; Nemanich, R. J. Mater. Res. Soc. Symp. Proc. 1994, 339, 81. (3) Johnson, D. W.; Roberts, M. W. J. Electron Spectrosc. Relat. Phenom. 1980, 19, 185. (4) Lieske, N.; Hezel, R. J. Appl. Phys. 1981, 52, 5806. (5) Gautier, M.; Duraud, J. P.; LeGressus, C. J. Appl. Phys. 1987, 61, 574. (6) Madden, H. H.; Goodman, D. W. Surf. Sci. 1985, 150, 39. (7) Aita, C. R.; Gawlak, C. J. J. Vac. Sci. Technol. A 1983, 1, 403. (8) Meletis, E. I.; Yan, S. J. Vac. Sci. Technol. A 1991, 9, 2279. (9) Russell, J. N. Jr., Bermudez, V. M.; Leming, A. J. Vac. Sci. Technol. A 1996, 14, 908. (10) Bu, Y.; Chu, J. C. S.; Lin, M. C. Surf. Sci. 1992, 264, L151. Chu, J. C. S.; Bu, Y.; Lin, M. C. Surf. Sci. 1993, 284, 281. (11) Jonathan, N. B. H.; Knight, P. J.; Morris, A. Surf. Sci. 1992, 275, L640. (12) Ishihara, R.; Kanoh, H.; Sugiura, O.; Matsumura, M. Jpn. J. Appl. Phys. 1992, 31, L74. (13) Bu, Y.; Lin, M. C. Surf. Sci. 1994, 301, 118. (14) Tindall, C.; Hemminger, J. C. Surf. Sci. 1995, 330, 67. (15) Bu, Y.; Lin, M. C. Surf. Sci. 1994, 317, 152.
S0743-7463(96)00505-7 CCC: $12.00
and diamond16 surfaces has also been reported. Several groups have used HN3 as a replacement for ammonia in MOCVD or gas-source molecular beam epitaxy of BN,17 GaN,18-20 and InN21 but not, to our knowledge, of AlN. In this paper we examine the adsorption and reaction of HN3 on an Al(111) substrate as part of an on-going study of the chemistry of low-temperature nitride growth using highly reactive nitriding agents.9,16 The principal techniques employed are temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRRAS), supplemented by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). 2. Experimental Details 2.1. Apparatus and Al(111) Crystals. The experiments were carried out in two different ultrahigh vacuum (UHV) chambers. Both were equipped with LEED optics, a single-pass cylindrical mirror analyzer for AES, an ion gun for sample cleaning, and dosers for exposing the sample to HN3. Auger data were recorded with a 2 eV modulation amplitude and an Ep ) 3 keV, ip ≈ 5 µA primary beam incident normal to the surface. All Auger and LEED data were taken at a 300 K sample temperature. One chamber (the “IRRAS chamber”) was equipped with a Fourier-transform IR spectrometer, a liquid-N2-cooled “wideband” HgxCd1-xTe detector, and a wire-grid polarizer. Data are given in the form of (δR/R)p, the fractional change in the p-polarized reflectance of the substrate caused by the adsorbate layer. KBr windows were positioned to give an angle of incidence of 86° with (4° spread, and the sample could be cooled to 120 K. The resolution was 8 cm-1, and triangle apodization with 4-fold zero filling was used in processing the interferograms. The second chamber (the “TPD chamber”) contained a differ(16) Thoms, B. D.; Russell, J. N., Jr. Surf. Sci. 1995, 337, L807. (17) Ishihara, R.; Sugiura, O.; Matsumura, M. Appl. Phys. Lett. 1992, 60, 3244. (18) Flowers, M. C.; Jonathan, N. B. H.; Laurie, A. B.; Morris, A.; Parker, G. J. J. Mater. Chem. 1992, 2, 365. (19) Bu, Y.; Lin, M. C.; Fu, L. P.; Chtchekine, D. G.; Gilliland, G. D.; Chen, Y.; Ralph, S. E.; Stock, S. R. Appl. Phys. Lett. 1995, 66, 2433. (20) Go¨tz, W.; Oberman, D. B.; Harris, J. S., Jr. Mater. Res. Soc. Symp. Proc. 1995, 378, 527. Oberman, D. B.; Lee, H.; Go¨tz, W. K.; Harris, J. S., Jr. J. Cryst. Growth 1995, 150, 912; Inst. Phys. Conf. Ser. 1995, 141, 131. (21) Bu, Y.; Ma, L.; Lin, M. C. J. Vac. Sci. Technol. A 1993, 11, 2931. Bu, Y.; Lin, M. C. Mat. Res. Soc. Symp. Proc. 1994, 335, 21.
© 1996 American Chemical Society
HN3 on Al(111)
Langmuir, Vol. 12, No. 26, 1996 6493 Table 1. Normal-Mode Frequencies for HN3 in an N2 Matrix and Physisorbed or Condensed on Al(111)a assignment ν1: ν2: ν3: ν4: ν5: ν6:
N-H str NNN asym str N-H bend NNN sym str NNN bend (a′) NNN bend (a′′)
N2 matrixb
physisorbedc
condensedd
3324 2150 1273 1168 527 588
3220 2160 1310 1200 e e
3120 2155 1277 1189 549 624
a Frequencies are in cm-1. All modes except ν involve displace6 ments within the plane of the molecule and transform as the a′ representation of the Cs point group. The out-of-plane bend, ν6, transforms as a′′. b Values for HN3 monomer from Pimentel et al. (ref 29). c Values are for a physisorbed molecular layer on Al(111) at ∼122 K (see text and Figure 2a). d Values are for a condensed multilayer at ∼122 K (see text and Figure 2b). e Not observed.
Figure 1. Auger and LEED data for the clean and partially nitrided Al(111) surfaces. The latter was obtained by dosing with HN3 at 100 K and annealing for 1 min at 800 K. The LEED patterns were obtained with 54 eV incident electron beam energy. The missing spots were obscured by the sample holder. entially-pumped quadrupole mass spectrometer for temperature-programmed desorption, and the sample could be cooled to 90 K. The Al crystal (∼10 and 18 mm diameter for the TPD and IRRAS chambers, respectively) was oriented to within 0.5° of (111) and polished with 1.0 µm diamond paste. The sample was mounted as described by Crowell et al.22 The temperature was measured with a chromel-alumel thermocouple, the bead of which was buried in the crystal. The sample was cleaned with repeated cycles of 0.65 keV (TPD) or 1.0 keV (IRRAS) Ne+ bombardment at 300 K followed by annealing at 800 K. For the clean surface, the only impurity seen in AES was oxygen whichsif present at allswas near the detection limit of ∼0.005 monolayers, and a low-background (111)-(1×1) LEED pattern was observed (Figure 1, top). 2.2. Hydrazoic Acid (HN3). Hydrazoic acid was synthesized23,24 on a Lexan-shielded, glass high-vacuum line by slowly heating a mixture of stearic acid (Aldrich)spreviously dried by heating to 323 K in vacuosand NaN3 (Aldrich) in a glass roundbottom flask to ∼373 K using a mineral oil bath. (The explosive properties of HN3 are discussed in ref 25). Water and CO2 were the major gaseous byproducts of the reaction. The gaseous products were separated as they were generated by passage through a series of 195, 133, and 77 K traps under a dynamic vacuum. Small amounts of H2O and CO2 were collected in the 195 and 77 K traps, respectively, and the bulk of the HN3 was trapped at 133 K. Additional drying was accomplished by passing the HN3 vapor through a tube containing P2O5. Mass spectrometry and gas-phase IR spectroscopy (5 Torr, 5 cm path length) were used to check the purity of the HN3. Although no impurities were detected by these means, a trace amount of H2O is evidenced by a low level of O contamination (see Figure 1) after HN3 adsorption and decomposition. The O (22) Crowell, J. E.; Chen, J. G.; Yates, J. T., Jr. Surf. Sci. 1986, 165, 37. (23) Krakow, B.; Lord, R. C.; Neeley, G. O. J. Mol. Spectrosc. 1968, 27, 148. (24) Schlie, L. A.; Wright, M. W. J. Chem. Phys. 1990, 92, 394. (25) Gray, P.; Waddington, T. C. Nature 1957, 179, 576.
concentration is higher after dosing at cryogenic temperatures, or after repeated dosing at any temperature, than after a single dose at g300 K. For the essential reaction data shown below, the O/N atomic ratio is e0.05, based on standard AES sensitivity factors. Of previous nitridation studies using HN3, those that specifically mention surface purity14,18,20 also report O contamination. A 1:1 mixture of H15NNN and HNN15N was prepared from NaNN15N (Cambridge Isotopes) using the above procedure. The isotopic ratio was confirmed by examining the Q branch of the NH stretching mode in the IR transmission spectrum of the vapor. Consistent with previous observations,26 each component involves two peaks of equal intensity, separated by ∼6.2 cm-1, due to the difference in the H-14N vs H-15N stretching frequency. Deuterated hydrazoic acid (DN3) was prepared by slowly dropping concentrated D3PO4 (Cambridge Isotopes) onto NaN3 in a glass round-bottom flask which was cooled in an ice water bath.27 The purification procedure was similar to that described above, except that P2O5 was not used. Each hydrazoic acid gas sample was stored in a 1-L glass bulb with a stainless steel valve. The final pressure at ambient temperature in the bulbs did not exceed ∼200 Torr. Using a stainless steel gas handling system, the Al(111) crystal was exposed to hydrazoic acid via a calibrated doser.28
3. Results and Discussion The results are naturally divided into the general categories: (a) molecularly adsorbed HN3 and (b) HN3 decomposition. Before describing our results, it is worthwhile to review the molecular structure and low-temperature IR spectroscopy of HN3. The left column of Table 1 summarizes IR results29 for the matrix-isolated molecule (see also ref 26). The diagram shown below represents schematically the two resonance structures26 for free HN3 and gives the ∠(HNN) angles and the signs of “partial” charges. Note that the bond lengths are not to scale and that the N atoms are lettered for easier reference.
The inequivalence of the three N(1s) binding energies in X-ray photoemission spectroscopy30 and the 10% larger bond length30 and ∼30% smaller stretching force constant26 (26) Moore, C. B.; Rosengren, K. J. Chem. Phys. 1966, 44, 4108. (27) Bendtsen, J.; Winnewisser, M. Chem. Phys. 1979, 40, 359. (28) Russell, J. N., Jr.; Sarvis, S. S.; Morris, R. E. Surf. Sci. 1995, 338, 189. (29) Pimentel, G. C.; Charles, S. W.; Rosengren, K. J. Chem. Phys. 1966, 44, 3029. Rosengren, K.; Pimentel, G. C. J. Chem. Phys. 1965, 43, 507.
6494 Langmuir, Vol. 12, No. 26, 1996
Figure 2. IRRAS data for molecular HN3 on Al(111) at ∼122 K in (a) physisorbed and (b) condensed layers. Spectrum b was recorded after a dose of ∼3 × 1016 HN3/cm2. The insets show parts of the spectra in detail, with features labeled “chemi” (chemisorbed), “physi” (physisorbed) or “cond” (condensed). The energies of the various “physi” and “cond” modes are listed in Table 1, while those of the “chemi” modes are given in the text. Note the difference in sensitivity scales for the two spectra.
for Na-Nb vs Nb-Nc indicate that resonance structure i is more heavily weighted. 3.1. HN3 Adsorption. The adsorption of HN3 below 130 K was investigated with IRRAS, with which we identify the adsorbate species and derive information about molecular orientation. Three distinct phases are found under conditions where multilayer adsorption is possible. We refer to these, in the order formed, as a “chemisorbed” monolayer, a “physisorbed” second layer, and a “condensed” multilayer. We describe first the molecularly-adsorbed second layer and multilayer, then the dissociative adsorption in the first monolayer. 3.1.1. HN3 Second Layer and Multilayer Adsorption. Figure 2a shows data recorded at ∼122 K under a steady-state flux of about 5 × 1012 HN3/(cm2 s). This temperature, the lowest attainable in the present IRRAS experiment, is close to that of the molecular HN3 desorption peak (see below). A steady-state flux was used in order to maintain a finite coverage of molecular HN3 during the ∼16 min required for data acquisition. Essentially the same results are obtained at 122 K for an appropriately-chosen dose followed by evacuation to UHV. Features in Figure 2a labeled ν1, ν3, and ν4 and the physisorbed second layer ν2 peak are identified with molecular HN3 through their disappearance in UHV at slightly higher temperature. The energies of these features are listed in Table 1, and the chemisorbed ν2 peak (as well as other, unlabeled, bands) is described later. The ν1 N-H stretch is broad, asymmetric, and ∼100 cm-1 lower than for HN3 monomers in an N2 matrix, suggesting (30) Lee, T. H.; Colton, R. J.; White, M. G.; Rabalais, J. W. J. Am. Chem. Soc. 1975, 97, 4845. Colton, R. J.; Rabalais, J. W. J. Chem. Phys. 1976, 64, 3481.
Russell et al.
H-bonding in the physisorbed second layer. The presence of ν1 and ν3 indicates that the N-H bond is intact. Likewise, the existence of ν2 and ν4 shows that the NNN chain is intact; however, the ν5 and ν6 NNN bending modes are not observed. It is noted that ν2 is near the range of the stretching mode of CO (a common UHV background contaminant) on metals (∼1800-2100 cm-1). However, CO has been found31,32 not to adsorb on Al(111) above 95 K, and none was seen in the present TPD data. Very different results are obtained for a condensed multilayer, Figure 2b, formed by a massive HN3 dose. All six fundamentals (see Table 1) are observed. All are relatively sharp, and some may be resolution-limited under the present conditions. The ν2 mode shows a third component, at ∼2155 cm-1, distinct from the chemi- and physisorbed features discussed above, and a condensedmultilayer ν4 mode appears at 1189 cm-1. One sees that ν1 for the condensed layer, although relatively sharp, is ∼200 cm-1 lower than in an N2 matrix, a much larger difference than for other fundamentals. This might indicate that ν1 is perturbed by Fermi resonance26 with, e.g., ν2 + ν4. The magnitude of the shift would depend on the exact values of the various fundamentals and on the strength of anharmonic coupling, and we have not investigated this effect in detail. 3.1.2. Adsorption Geometry and the Role of Symmetry. The appearance of an intense ν6 peak in the condensed multilayer, but not in the physisorbed second layer, is significant. Interpretation requires a brief discussion of symmetry effects33 in IRRAS, which will also be invoked elsewhere in this work. For a highly reflecting metal, a large IR optical electric field at the vacuummetal interface can be sustained only in p-polarization and at a high angle of incidence. This leads to a strict symmetry-based selection rule:33 an observable normal mode must belong to the same irreducible representation of the point-group describing the adsorbate symmetry as does the surface normal. Thus, for intact HN3 adsorbed normal to the surface, the appropriate point group is Cs, and all modes except ν6 are allowed. The same holds for an NNN chain parallel to the surface with the N-H bond lying in a plane normal to the surface. However, a completely flat structure (i.e., NNN chain and N-H bond parallel to the surface) or a structure with the plane of the molecule inclined away from the surface normal would possess no symmetry elements, and all six modes would be allowed. An allowed mode does not necessarily have detectable intensity. For example, ammonia chemisorbs on Al(111) via an Al-N bond normal to the surface.34 The νs symmetric N-H stretch is allowed in the C3v point group but is too weak to be detected,35 and the same is found for ammonia on other metals.36,37 This leads to a “physical selection rule”: only those symmetry-allowed modes having a significant surface-normal dynamic dipole are observed. Usually this means that the direction of atomic displacements must also lie near to the surface normal, but there are exceptions. For O2 lying down on a (111)33 (31) Chen, J. G.; Crowell, J. E.; Ng, L.; Basu, P.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 2574. (32) Jacobi, K.; Astaldi, C.; Geng, P.; Bertolo, M. Surf. Sci. 1989, 223, 569. (33) Fan, J.; Trenary, M. Langmuir 1994, 10, 3649. (34) Hermann, K.; Bagus, P. S.; Bauschlicher, C. W., Jr. Phys. Rev. B 1985, 31, 6371. (35) Kim, C.; Bermudez, V. M.; Russell, J. N., Jr. To be submitted for publication. (36) Rodriguez, J. A.; Kuhn, W. K.; Truong, C. M.; Goodman, D. W. Surf. Sci. 1992, 271, 333. (37) Dastoor, H. E.; Gardner, P.; King, D. A. Surf. Sci. 1993, 289, 279.
HN3 on Al(111)
Figure 3. Schematic representation of the three different adsorption layers observed in IRRAS: (a) dissociatively chemisorbed monolayer; (b) physisorbed molecular second layer; (c) condensed molecular multilayer. Layer c is intended to represent a randomly-oriented “glass”. The normal modes observed with each layer are noted on the left. See Table 1 for a description of the notation. In the chemisorbed layer, NH is depicted as a bridging species, but terminal (i.e., AldNH) groups are also possible. An alternative model for the chemisorbed layer is discussed in section 3.2.4.
or polycrystalline38 Pt surface at 80 K, the O-O stretch is easily detected in IRRAS. Here bonding involves charge transfer from Pt d-orbitals into the antibonding 2π* of O2, forming a π-bonded complex.38 This weakens the O-O bond, reducing the stretching frequency to 875 cm-1 from the gas-phase value of 1580 cm-1. The O-O stretch then modulates this charge transfer, leading to a surfacenormal dynamic dipole. This mode is symmetry-allowed33 for the C2v point group appropriate to “lying-down” O2. Returning to the subject at hand, we note that the energies of the physisorbed ν2 and ν4 modes (Table 1) differ only slightly from those of free HN3. Also, ν2 differs only slightly from the NN stretch (∼2200 cm-1) of N2 chemisorbed on Ni(110) in a surface-normal orientation.39 In contrast, N2 adsorbed as a π-bonded complex40 on Fe(111) (i.e., in a “lying down” geometry) exhibits an NN stretch at 1490 cm-1. This argues against the existence of such a complex for HN3 on Al(111) and, therefore, that the NNN axis must be nearly normal to the surface to account for both the large intensity of the ν2 band and the small difference in the ν2 and ν4 energies from those of free HN3. Hence, the data indicate that the physisorbed second layer of HN3 is in a surface-normal orientation, whereas the condensed multilayer resembles a randomly-oriented glass. The ν5 mode, although allowed, is undetectably weak in the physisorbed layer, probably due to a very small surface-normal dynamic dipole for the bending of a chain aligned along the surface normal. Finally, the presence in Figure 2a of ν1, at an energy not greatly different from that of free HN3, indicates that physisorption occurs via the N-end of the molecule and not the H-end. The two structures discussed above are shown schematically in Figure 3 with a list of the normal modes observed for each. The ν2 band (Figure 2b) shows three components, each associated with a different layer, appearing sequentially with HN3 exposure. The chemisorbed phase, which dominates at submonolayer coverage, will now be examined. (38) Canning, N. D. S.; Chesters, M. A. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 69. (39) Brubaker, M. E.; Trenary, M. J. Chem. Phys. 1986, 85, 6100; Brubaker, M. E.; Malik, I. J.; Trenary, M. J. Vac. Sci. Technol. A 1987, 5, 427. (40) Grunze, M.; Golze, M.; Hirschwald, W.; Freund, H.-J.; Pulm, H.; Seip, U.; Tsai, M. C.; Ertl, G.; Ku¨ppers, J. Phys. Rev. Lett. 1984, 53, 850.
Langmuir, Vol. 12, No. 26, 1996 6495
Figure 4. IRRAS data showing the transition from monolayer to physisorbed molecular second layer with increasing HN3 dose at 122 K. The total dose for each spectrum is given. The (δR/R)p scale is indicated in the lower-right corner. The “baseline” is obtained from two successive single-beam spectra at 122 K before dosing. Features labeled “physi” are molecular HN3 modes in the physisorbed layer and those labeled “ν(NN)” and “δ(NH)” are the stretching and bending modes, respectively, of the chemisorption products.
3.1.3. HN3 Monolayer Adsorption: Dissociative Chemisorption. Figure 4 gives a set of data, recorded at ∼122 K, showing the transition between chemisorbed and physisorbed HN3. The sloping background is probably due to long-term drift in the optical system and/or to changes in the Al optical properties caused by adsorption.41 The weak [(δR/R)p ≈ 10-3] upward-pointing peak at 1195 cm-1 is due to the δs symmetric deformation mode of a low coverage of ammonia adsorbed from the UHV background as the clean surface is cooled. The ammonia results from ion-pumping of HN3 during many dosing cycles and is displaced from the surface by the first HN3 dose, resulting in a “negative” absorption peak in the (δR/R)p difference spectrum. For intentional ammonia dosing, this peak appears as a normal, downward-pointing feature, shifting to 1175 cm-1 at the maximum attainable 125 K coverage. In the NN stretch region (2100-2200 cm-1), a band first appears at 2140 cm-1 and shifts to 2185 cm-1 with increasing coverage. This coverage-dependent shift results from the combined effects of (nonresonant) chemical and (resonant) dipole-dipole interactions and is a typical feature of adsorbate IRRAS, as discussed at length by Hollins and Pritchard.42 The ν4 NNN symmetric stretch of HN3 is conspicuously absent in the “chemisorbed” spectrum, Figure 4b. The onset of physisorption is signaled, in Figure 4c, by the simultaneous appearance of ν1 and ν4. The “ν(NN) + ν2(physi)” band is due to overlapping contributions from physi- and chemisorbed species, as in Figure 2a. The “ν3(physi)” label marks the position of this mode for physisorbed HN3, as in Figure 2a. Since there is very little difference between parts b and c of Figure 4 in this region, ν3(physi) actually makes only a small contribution to Figure 4c. A weak shoulder, not associated with molecular HN3, appears at ∼1308 cm-1, just below the δ(NH) band in Figure 4b. This, and other features, will be discussed in section 3.2.3. (41) Lin, K. C.; Tobin, R. G.; Dumas, P. Phys. Rev. B 1994, 49, 17273; Erratum Phys. Rev. B 1994, 50, 17760. (42) Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275.
6496 Langmuir, Vol. 12, No. 26, 1996
As implied by the labels, ν(NN) and δ(NH) in Figure 4b are assigned to chemisorbed NN and NH groups formed by dissociative adsorption of HN3. Support for the existence of chemisorbed NN (i.e., Al-NN) is found in the appearance of such species during photodissociation13 (at λ ) 308 nm) of HN3 on Si(111)-(7×7), for AlN grown43 by MOCVD on powdered γ-Al2O3 and for sputter-deposited AlN films.44-46 Using arguments given above, the frequency and intensity of the ν(NN) mode indicate adsorption with the NN axis normal to the surface. The Al-NN stretch (“frustrated translation”) is expected to be inaccessible in the present experiments, based on highresolution electron energy loss spectroscopy (HREELS) data47 for N2/Ni(110) showing this mode at 320 cm-1. The δ(NH) band appears at 1360 cm-1 (1190 cm-1 δ(ND)) and is distinct from the 1310 cm-1 ν3 mode of physisorbed HN3. The isotope shift (δ(NH)/δ(ND) ) 1.14) is somewhat larger than the matrix-isolation value26 ν3(HN3)/ν3(DN3) ) 1.06, indicating relatively more H (less N) motion in the adsorbed-NH normal coordinate. This feature corresponds well with δ(NH) on Ni(111) (1270 cm-1, ref 48), Ru(0001) (1360 cm-1, ref 49; 1340 cm-1, ref 50), and various Si surfaces (1290-1330 cm-1, ref 10). The N-H stretch is assumed to be undetectably weak, as in the case for chemisorbed ammonia discussed in section 3.1.2, due to a small projection of the dynamic dipole on the surface normal. Other evidence for these assignments and for dissociative chemisorption will be given below. 3.2. Thermal Decomposition of HN3 on Al(111). We employ two complementary techniques to develop a complete picture of the reaction mechanism of HN3 with the Al(111) surface. Desorption products are monitored using TPD, which also reveals the temperatures at which reaction events occur under decomposition-limited desorption conditions, and adsorbed decomposition products are detected using IRRAS. 3.2.1. Temperature-Programmed Desorption of HN3. For a DN3 multilayer, TPD (Figure 5) shows only three desorption species for a surface dosed at 100 K and heated to 800 K. Deuterated hydrazoic acid was used to eliminate the interference of background H2. The total dose was 2.5 × 1015 DN3/cm2, and the heating rate was 1 K/s. Molecular DN3 desorbs with a peak maximum at 129 K; N2 also shows a peak at 129 K, due to cracking of DN3 in the mass spectrometer. The N2 decomposition product desorbs in a broad feature peaked at about 295 K, and D2 desorption gives peaks at ∼475 K and 615 K. For comparison, clean Al(111) exhibits only physisorption when exposed to molecular N2 in the gas phase, with desorption occurring32 at ∼26 K. Desorption of H2 from H-saturated Al(111) follows zero-order kinetics51 with a peak maximum at 340 K. Desorption of AlH3 from (43) Liu, H.; Bertolet, D. C.; Rogers, J. W., Jr. Surf. Sci. 1994, 320, 145; Surf. Sci. 1995, 340, 88. (44) Mazur, U. Langmuir 1990, 6, 1331. Mazur, U.; Cleary, A. C. J. Phys. Chem. 1990, 94, 189. Wang, X.-D.; Hipps, K. W.; Mazur, U. J. Phys. Chem. 1992, 96, 8485; Langmuir 1992, 8, 1347. (45) Loretz, J. C.; Despax, B.; Marti, P.; Mazel, A. Thin Solid Films 1995, 265, 15. (46) Kumar, S.; Tansley, T. L. Jpn. J. Appl. Phys. 1995, 34, 4154. (47) Horn, K.; DiNardo, J.; Eberhardt, W.; Freund, H.-J.; Plummer, E. W. Surf. Sci. 1982, 118, 465. (48) Gland, J. L.; Fisher, G. B.; Mitchell, G. E. Chem. Phys. Lett. 1985, 119, 89. (49) Rauscher, H.; Kostov, K. L.; Menzel, D. Chem. Phys. 1993, 177, 473. (50) Shi, H.; Jacobi, K.; Ertl, G. J. Chem. Phys. 1995, 102, 1432. (51) Winkler, A.; Pozˇgainer, G.; Rendulic, K. D. Surf. Sci. 1991, 251/ 252, 886.
Russell et al.
Figure 5. TPD data following a 2.5 × 1015 DN3/cm2 dose at 100 K. The heating rate was 1 K/s.
Figure 6. TPD data (all on the same scale) showing the isotopic distribution of N2 desorption products from the reaction of a 1:1 mixture of H15NNN:HNN15N.
H/Al(111) has been reported52 but was not observed here, possibly due to the slow heating rate of 1 K/s. To investigate whether N2 desorption results from a recombination of adsorbed N atoms or from an intact NN unit, the surface was dosed with a 1:1 mixture of H15NNN and HNN15N. In Figure 6, the desorption of masses 28 (14N14N), 29 (14N15N), and 30 (15N15N) is monitored for a multilayer of the isotopic mixture. Equal amounts of masses 28 and 29, but no mass 30, were detected. The absence of isotopic scrambling shows that desorption of N2 is derived from an intact NN moiety. Recombinative desorption of adsorbed N is thus excluded. 3.2.2. IRRAS of HN3 Thermal Decomposition on Al(111). Figure 7 shows IRRAS data after a dose of 1.4 × 1015 HN3/cm2 at 130 K followed by annealing at temperatures spanning the range of thermal desorption (52) Kondoh, H.; Nishihara, C.; Nozoye, H.; Hara, M.; Domen, K. Chem. Phys. Lett. 1991, 187, 466. Kondoh, H.; Hara, M.; Domen, K.; Nozoye, H. Surf. Sci. 1993, 287/288, 74. Kondoh, H.; Nozoye, H. J. Chem. Phys. 1994, 101, 8087.
HN3 on Al(111)
Figure 7. IRRAS data showing the temperature dependence of the surface composition. The spectra were obtained after a dose of 1.4 × 1015 HN3/cm2 at 130 K followed by annealing at the indicated temperatures. In each case the sample was cooled to 130 K before acquiring the data.
(Figure 5). Each spectrum was acquired at 130 K after heating to the indicated temperature and covers the range where most of the change occurs. Spectrum a was obtained after dosing at 130 K and is similar to that obtained at the end of the adsorption series in Figure 4c. At 130 K, the main peaks are at 1178, 1375, and 2180 cm-1 corresponding, respectively, to the ν4(physi), δ(NH), and ν2(physi) + ν(NN) features in Figure 4c. Upon heating to 220 K, the peak at 1178 cm-1 disappears, the 1375 cm-1 peak shifts to 1360 cm-, and the mode at 2180 cm-1 loses some intensity and shifts to 2173 cm-1. One notes the similarity between this spectrum and that for a submonolayer HN3 dose at 122 K, Figure 4b. Thus, Figure 7 shows desorption of molecular HN3 above 130 K, with a chemisorbed layer remaining. This is consistent with the TPD data, Figure 5, in which the DN3 multilayer was completely desorbed by 140 K. We again use the presence of the ν4 mode as an indicator of molecularly-adsorbed HN3. Annealing to 320 K, just above the N2 desorption maximum, leads to a loss of intensity in the 2173 cm-1 mode and a shift to 2150 cm-1. In fact there is an approximately 2-fold decrease in the integrated intensity of this band with no significant effect on that of the 1360 cm-1 band. This indicates that the two bands belong to different species and supports the conclusion that HN3 chemisorption yields separate NN and NH species. Heating to 453 K results in the disappearance of the 2150 cm-1 NN and 1370 cm-1 NH modes and the appearance of a weak band at 1825 cm-1 (shown more clearly below) which we assign to H adsorbed on the Al surface. In similar experiments using DN3, this band appears at ∼1340 cm-1. Data given below will show that chemisorbed N atoms are formed simultaneously with chemisorbed H. The TPD data (Figure 5) show that, in the presence of co-adsorbed N, surface H is stable to temperatures in excess of 700 K. Further annealing to 795 K eliminates the 1825 cm-1 band. For H/Al(111), HREELS (ref 52), and IRRAS (ref 35) data show bands at about 1600 and 1890 cm-1, assigned to AlH2 and to monohydride (AlH) stretching modes,
Langmuir, Vol. 12, No. 26, 1996 6497
Figure 8. IRRAS data showing the temperature dependence of the Al-Hx modes. The spectra were obtained after a dose of 2.5 × 1015 HN3/cm2 at 410 K followed by annealing at the indicated temperatures. In each case the sample was cooled to 130 K before acquiring the data.
respectively. The corresponding bending modes at 800 and ∼1000 cm-1 are also seen in HREELS but not IRRAS. For H in a 2-fold bridge site, a much lower stretching frequency, ∼1200 cm-1, is found.35,52 Data for hydrogenated AlN films44 show an Al-Hx (1 e x e 3) stretch at 1820-1830 cm-1 and [(C2H5)(CH3)2]NAlH3 (dimethylethylamine alane) adsorbed on SiO2 exhibits Al-Hx stretching modes53 in the 1700-1800 cm-1 range. In Figure 8, the temperature dependence of the Al-Hx (1 e x e 3) stretching region is shown in detail for a surface dosed at 410 K with 2.5 × 1015 HN3/cm2. This yields a higher Al-Hx intensity than does annealing a surface dosed at low temperature, due to the desorption of lowtemperature decomposition products and the availability of more sites for species that are stable at elevated temperatures. Upon further annealing the band splits, giving a broad band at ∼1795 cm-1 and a sharper one at 1900 cm-1, both of which disappear between 750 and 790 K. For DN3 after a 700 K anneal (not shown), the narrow (broad) band appears at 1395 (∼1300) cm-1; although, the broad band is not well-defined. The results for the 700 K annealed surface are quite similar to those35 for H/Al(111) except that, in the absence of co-adsorbed N, H recombines and desorbs from Al(111) by 360 K. The source of the Al-Hx species in Figures 7 and 8 is the decomposition of chemisorbed NH groups. Dosing at 130 K followed by a series of anneals (Figure 7) indicates that Al-H formation begins above 320 K, while dosing at 300 K (not shown) yields little or no detectable Al-Hx intensity (i.e., little or no NH decomposition). The results further indicate that Al-Hx stabilization above ∼320 K is a feature of an Al(111) surface with co-adsorbed N. Upon dosing at 410 K, a single, relatively narrow and symmetric Al-N stretching band is seen at 840 cm-1 (Figure 9). Because it is the dominant Al-N feature at (53) Urisu, T.; Zhang, Y.; Nagasono, M.; Yoshigoe, A.; Imaizumi, Y.; Ohshima, H.; Hattori, T.; Sato, S. Jpn. J. Appl. Phys. 1994, 33, 7123.
6498 Langmuir, Vol. 12, No. 26, 1996
Figure 9. Similar to Figure 8 but showing the Al-N stretching region.
low coverage, it is assigned to a chemisorbed N atom derived from decomposition of chemisorbed NH. For a surface dosed at 130 K and annealed in steps from 220 to 400 K (not shown), the 840 cm-1 Al-N band gains intensity along with the ∼1825 cm-1 Al-Hx band (discussed above) as the 1360 cm-1 δ(NH) band loses intensity, showing again that NH decomposition leads to chemisorbed N and H. Cycles of dosing at 300-400 K with annealing at 790 K lead to a second peak, at ∼890 cm-1, which dominates at higher coverage9 and is associated with the A1 longitudinal optic (LO) phonon9,54 of wurtzite AlN. Annealing leads to a loss of intensity in the 840 cm-1 band, starting at g600 K (Figure 9), and the growth of a mode at ∼1010 cm-1. In principle, either recombinative desorption or indiffusion of N could account for this intensity loss. However, since the former has already been ruled out (Figure 6), indiffusion55 is considered the likely explanation. Auger data (Figure 1) for the annealed surface continue to show a high concentration of N (see below). 3.2.3. Assignment of Other IRRAS Features. An inhomogeneous surface results from the initial chemisorption and decomposition of HN3. The surface is only partially nitrided, as will be shown below, with a variety of potential adsorption sites and several, possibly interacting, adsorbates. Although, we have been able to follow the internal HN3 modes up to the point of complete decomposition and to correlate the IR and TPD results, some IRRAS features remain unassigned. Figure 9 shows a broad band at ∼740 cm-1, and a sharper one at 1010 cm-1, emerging as the 840 cm-1 Al-N stretch disappears. The former is thought to be due in (54) McNeil, L. E.; Grimsditch, M.; French, R. H. J. Am. Ceram. Soc. 1993, 76, 1132. (55) For a typical free-electron metal (e.g., Al) in the mid-IR, the very large dielectric constant means that the surface-normal electric field on the vacuum side of an ideal (i.e., infinitely sharp) vacuum-metal interface becomes tangential inside the metal. It is thus strongly attenuated, and a vibrating subsurface atom, in this ideal limit, would be practically undetectable.
Russell et al.
some way to N atoms which have diffused into, or just below, the terminating Al layer. The rising background toward lower energy also influences the shape of this band. In their HREELS study of AlN growth on NiAl(100) and (111) by decomposition of ammonia, Gassmann et al.56 also observed low-energy modes (at 600 and 830 cm-1) for the partially-nitrided surface prior to the appearance of the 865 cm-1 Fuchs-Kliewer mode of bulk AlN. In previous work,9 the 1010 cm-1 peak was almost undetectable in well-formed AlN thin films exhibiting a sharp, symmetric LO phonon band in IRRAS but was stronger in defective films. Hence we speculate that it is due to a local mode associated with some kind of defect in Ndeficient AlN. In the NH bending region for the physisorbed layer (Figure 2a) there are two peaks, one at 1310 cm-1 due to ν3 of HN3 and one at 1370 cm-1 which is assigned to chemisorbed NH. Examination of data for adsorption, Figure 4b,c, and annealing, Figure 7a-c, reveals the further presence of a shoulder, at ∼1308 cm-1, below the stronger 1370 cm-1 band. This feature persists, at T > 130 K, after desorption of molecular HN3 and so is distinct from ν3. We suggest that one of these two modes (i.e., at ∼1308 and ∼1370 cm-1) may be due to δ(NH) of a terminal species (AldNH) and the other to a bridging group, Al(NH)-Al. For physisorbed or condensed layers, Figures 2b, 4c, and 7a show weak satellites below and/or above the strong ν2 band. These may be due to HN3 dimers,29 which can exist in a variety of configurations, or they may instead be monomer combination modes (e.g., ν3 + ν4, ν4 + 2ν5, or 2ν4). Finally, Figure 7 shows a broad feature emerging in the 1200-1600 cm-1 range with continued annealing. Its origin is uncertain, but it may be related to the broad ∼740 cm-1 band discussed above, since it exhibits similar thermal behavior. One must also be aware that weak, broad features can arise in IRRAS annealing data as artifacts due to miscancellation of the single-beam spectra. This can result from minute changes in sample position during cycling between high and low temperature. 3.2.4. Competing Models for HN3 Chemisorption. The above discussion has been entirely in terms of dissociative chemisorption of HN3, at T g 130 K, to form adsorbed NN and NH species. To recapitulate, this interpretation is based on the following observations. (1) The ν4 symmetric stretch of the NNN chain in molecular HN3 is absent in the chemisorbed state, suggesting that such chains are also absent. (2) A stretching band, ν(NN), is seen at higher energy than ν2 of either physisorbed or condensed HN3, implying a stronger NN bond. Also, ν(NN) falls in the range (∼2185-2220 cm-1) reported for N2 chemisorbed in a surface-normal orientation on the (111) surfaces of other metals, e.g., Ni (ref 57) and Pt (ref 58). (3) Annealing a chemisorbed layer in the 130 < T < 330 K range causes a more rapid intensity loss for the ν(NN) band than for the NH bending mode, δ(NH), with little or no change in frequency for either. These observations suggest that the two bands belong to different species. (4) Vibrational data for thin AlN films formed by means other than HN3 decomposition also show a ν(NN) mode. This suggests that Al-NN groups are, in fact, stable if (56) Gassmann, P.; Schmitz, G.; Boysen, J.; Franchy, R. J. Vac. Sci. Technol. A 1996, 14, 813. (57) Fox, S. G.; Browne, V. M.; Hollins, P. J. Chem. Soc., Chem. Commun. 1989, 697. (58) Arumainayagam, C. R.; Tripa, C. E.; Xu, J.; Yates, J. T., Jr. Surf. Sci. 1996, 360, 121.
HN3 on Al(111)
Langmuir, Vol. 12, No. 26, 1996 6499
Figure 10. Competing models for the chemisorbed layer.
they can be formed, which will be further supported using molecular orbital calculations described below. The model assumed thus far for the chemisorbed layer is shown schematically in Figure 10 as “model A”. For simplicity, only bridging chemisorbed NH is shown, but terminal AldNH groups (discussed above) are also possible. There is, however, an alternative model (B) which does not invoke dissociative chemisorption. In model B, release of N2 into vacuum occurs via decomposition-limited desorption from the intact chemisorbed HN3, resulting in chemisorbed NH. Model A is motivated mainly by the need to account for the absence of a ν4 mode in the chemisorbed layer but leads to the unprecedented conclusion that stable chemisorbed NN species exist on the Al(111) surface. Model B avoids this conclusion but provides no obvious explanation for the absence of ν4. Both models can rationalize the higher energy of ν(NN), vs the molecular ν2, through formation of an “NtN-like” functional group. Although model B cannot be completely excluded on the basis of available data, there is reason to question its importance in the present case. In Figure 7, there is little or no change in the δ(NH) energy or intensity during annealing of a layer, formed at 130 K, through the point of extensive N2 desorption. Model B would then lead one to conclude that there is no spectroscopically-observable difference in δ(NH) between NH groups chemisorbed on the Al surface and those in the proposed chemisorbed HN3 molecule. On the other hand, model A can account for this result in that all NH seen above 130 K is in a chemisorbed form. Consequently, in view of the absence of a detectable ν4 mode and the annealing behavior of δ(NH), we conclude that model A better describes the available data. 3.3. Auger and LEED of Partially Nitrided Al(111). After the surface was dosed at 100 K and annealed for 1 min at 800 K, AES (Figure 1) reveals the deposition of N, which we refer to as “N/Al(111)”. The clean-surface LEED (Figure 1) has sharp spots in a hexagonal pattern, while that of N/Al(111) consists of two concentric hexagons9 with the outer spots corresponding to the clean Al(111) surface and the inner to AlN(0001). This indicates island growth on the partially nitrided surface, since a continuous, but incommensurate, overlayer would result in satellite spots,59 which were not observed. The distances between spots in the two patterns9 indicate a unit-cell ratio of 1.09, in excellent agreement with the crystallographic value of 1.086 for the corresponding AlN(0001)/Al(111) ratio. We have also shown9 that repeated HN3 exposure/anneal cycles leads to a continuous AlN(0001) film having the wurtzite structure with the c-axis normal to the surface and the basal plane rotationally ordered with respect to the Al(111) surface. Further insight into the structure of the N/Al(111) surface is provided by the H2 TPD data, which depend strongly on co-adsorbed N. For H/Al(111), H2 desorption35,51,52 peaks at 340 K, several hundred degrees below that seen in Figure 5. Figures 8 and 9 show that changes (59) Clarke, L. J. Surface CrystallographysAn Introduction to Low Energy Electron Diffraction; John Wiley & Sons: Chichester, 1985; Chapter 1.
Figure 11. Schematic diagram of the 10-atom Al cluster (Al10(7,3)) used in the molecular orbital calculations. The filled (open) circles are the seven surface (three underlayer) atoms on the (111) surface. A, B, and C indicate on-top, 3-fold occupied and 3-fold hollow sites, respectively.
in the AlHx IR spectrum occur concurrently with those in the Al-N spectrum. The final AlHx IR spectrum, just before complete H desorption, is very similar to that35 for H/Al(111) below 150 K. Considering the Auger data (which confirm the presence of N after heating to 800 K), the IRRAS data showing the disappearance of surface N and the IRRAS data indicating H on metallic-like Al sites up to 700 K, one is led to the conclusion that the H is chemisorbed on Al-terminated AlN(0001) islands. The AlN islands are presumably N-deficient, given the conditions under which they are formed, with an Al-rich terminating layer. 3.4. Cluster Molecular Orbital Calculations. Two chemical-bonding issues requiring further consideration are (1) the binding site of adsorbed atomic N and (2) the chemisorption of N2 on the Al surface. To explore these issues, semiempirical cluster molecular orbital calculations were done using the “Austin Model 1” (AM1) method of Dewar et al.60 as implemented in a publicly-available61 software package running on a DEC VAX 6310. The procedure was first checked against ab initio cluster results62 for O/Al(111) using the model shown in Figure 11. Following Broer et al.62 the Al-Al distances were fixed at the bulk values (e.g., surface relaxation was not considered), and an O atom was allowed to move along the surface normal above the high-symmetry sites A, B, and C. The stable adsorbed-O position was found at 0.89 Å above the surface plane, in reasonable agreement with the ab initio result of 0.76 Å for an Al10(7,3) cluster. The present calculations gave an identical result for 3-fold occupied (B) and hollow (C) sites. However, for an on-top (A) site, the minimum was 5.3 eV higher in energy and 1.70 Å above the surface. Similar calculations for atomic N gave analogous results. The stable position is at a B- or C-site, ∼0.33 Å above the surface plane, and is 5.7 eV lower in energy than the minimum-energy on-top position, 1.52 Å above the plane. (60) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (61) AMPAC, QCPE No. 506: Quantum Chemistry Program Exchange; Department of Chemistry; University of Indiana; Bloomington, IN 47405. (62) Broer, R.; Batra, I. P.; Bagus, P. S. Philos. Mag. B 1985, 51, 243. See also the review of O2 chemisorption on Al by: Batra, I. P.; Kleinman, L. J. Electron Spectrosc. Relat. Phenom. 1984, 33, 175.
6500 Langmuir, Vol. 12, No. 26, 1996
Figure 12. Surface reaction mechanism of HN3 on Al(111) leading to AlN growth.
For adsorption at a B- or C-site, bonding is with the three nearest-neighbor surface atoms with, in the former case, little interaction with the underlayer Al atom. A nearestneighbor Al-N distance of 1.68 Å is obtained, in reasonable agreement with the value of 1.85 Å computed63 for the planar hypermetallic molecule Al3N. Since Al(111) exposed to gas-phase N2 exhibits only physisorption,32 with desorption occurring at 26 K, the observation of chemisorbed NN resulting from HN3 decomposition was unexpected. Therefore, a calculation was done to determine whether AlN2 could exist as a free molecule. We emphasize that this is a preliminary treatment aimed at examining chemical bonding in a hypothetical AlN2 molecule and that these results say nothing about the energetic barrier to forming such a species in a chemisorbed state. As a check, the bondingorbital wave functions and binding energies for the free N2 molecule were first compared with those shown by Horn et al.47 The results predict a linear AlNN structure with a heat of formation of +62 kcal/mol (relative to the elements in their standard states) and Al-N and N-N bond lengths of 1.74 and 1.15 Å, respectively. The Al(3s) and (3p) orbitals undergo little or no mixing, and the Al(3py) remains essentially empty (where the x-axis is that of the molecule). A partial Al-N σ-bond forms by charge donation from N2 into the Al(3px) and a partial Al-N π-bond by back-donation from 3pz into the N2(π*). The bonding is thus somewhat like that of CO on a transition metal. Consequently, chemisorption of the NN fragment appears feasible. For simple exposure of Al(111) to an N2 ambient, there is, presumably, a high barrier to direct chemisorption. We speculate that decomposition of adsorbed HN3 occurs via a transition state which leaves the NN fragment in a favorable orbital configuration for bonding to a surface Al site. 3.5. Thermal Decomposition Mechanism. From the TPD and IRRAS results we have deduced the reaction mechanism shown in Figure 12. Molecular HN3 desorbs at 120-130 K from bi- or multilayers. The first monolayer dissociatively chemisorbs (but see below), at temperatures as low as 120 K, forming Al-NN and Al-NH groups. When the sample is heated, the first decomposition product to desorb is N2 with a peak maximum around 300 K. Using 15 N labeling, it was shown that all N2 desorption originates from a species with an intact N-N bond, rather than via recombinative desorption. Above 320 K, Al-NH decomposes to give chemisorbed N and H, exhibiting vibrational (63) Zakrzewski, V. G.; von Niessen, W.; Boldyrev, A. I.; Schleyer, P. von R. Chem. Phys. 1993, 174, 167.
Russell et al.
modes at 840 and 1825 cm-1, respectively. H2 desorption exhibits maxima at 450 and 615 K and is complete by 750 K. The IRRAS data indicate that most of the N(ads) has disappeared from the immediate surface by about 700 K. Auger data show that N remains in near-surface underlayer sites after a 1-min anneal at 800 K, and LEED indicates the presence of ordered AlN(0001) islands. These are terminated in metallic-like Al, as shown by the behavior of chemisorbed H in IRRAS and TPD. In Figure 12, the first step corresponds to model A, shown above. An alternative first step (model B), involving nondissociative chemisorption, cannot be completely excluded; although, arguments have been given (section 3.2.4) to support our inclination toward model A. In either case, it is the chemistry of the adsorbed NH which determines the initial stages of nitride growth, and the two models differ only in how the chemisorbed NH is produced. Decomposition of the NH species is key to nitridation of the Al surface. IRRAS (Figure 7) and isotopic-N TPD (Figure 6) confirm that the NH moiety is the precursor for nitridation. Since IRRAS indicates that the NH dissociates above 320 K (Figure 7), and the threshold for H2 thermal desorption is observed around 350 K (Figure 5), we conclude that NH begins to dissociate around 350 K. Consequently, the activation energy for NH bond scission is estimated to be 22 kcal/mol, with a prefactor of 1 × 1013 s-1, using the Redhead method64 and assuming first-order desorption kinetics for this decomposition-limited desorption process. The appearance of two maxima in H2 TPD is probably related to the opening of surface sites via diffusion of N into the bulk above 450 K (Figure 9) since the opening of sites, and the metallic enrichment of the surface, would accelerate H2 desorption. 4. Summary and Conclusions The reaction mechanism of HN3 with an Al(111) surface has been studied using TPD, IRRAS, Auger, and LEED, and the essential results are summarized in Figure 12 and the surrounding discussion. Some interesting observations have been made while following the conversion of chemisorbed HN3 to nascent AlN. One is that the true nitriding agent is NH, with HN3 being simply a means for delivering this species to the surface. Another is the possible stabilization of the chemisorbed NN fragment which, to our knowledge, has not been accomplished on a metallic Al surface other than by HN3 decomposition. We have also found that coadsorbed N stabilizes AlHx species at temperatures far in excess of those required to desorb H from clean Al(111). These observations suggest useful areas for further experiments and theory. Acknowledgment. We are grateful to the Office of Naval Research for support. J.N.R. is grateful to the Young Investigator Program, sponsored by the NRL Chemistry Division, for support. We appreciate the advice of A. Baronavski, in the preparation, and of A. D. Berry, in the purification, of HN3. We thank D. E. Ramaker for help with the AMPAC program and J. E. Butler and C. Kim for critical readings of the manuscript. LA960505W (64) Redhead, P. A. Vacuum 1963, 12, 203.
