J . Phys. Chem. 1990, 94, 189-194 Walker et al.,l6 and as can be seen, a reasonable fit is obtained.
189
IV. Conclusions This work describes a new method to treat reactive atom-diatom systems. The method is very similar to that we presented earlier, employing the time-dependent wave-packet approach except that now it is done within the time-independent framework. The main idea of these two methods (the present one and the previous time-dependent one) is that it is enough to solve a slightly extended nonreactive scattering process for the correct reactive probabilities to be obtained. This is done by employing short-range (absorbing) negative imaginary potentials at any exit from the reagents channel and in this way to convert any reactive system into a nonreactive one. The reactive transition probabilities are
obtained by calculating the outgoing flux at any exit channel just before entering the negative imaginary (optical) potential region. In contradiction to the time-dependent approach, the optical potentials have another vital role, in that they impose outgoing boundary conditions on both the inelastic and the reactive channels. This significantly reduces the numerical effort, because instead of calculating 2N independent solutions and then imposing the boundary conditions, one has to calculate only one solution for any given boundary condition. So far the method has been applied to a collinear reactive system and the correct results were obtained. However, no essential difficulties are expected in the extension of this method to three dimensions. In fact, the extension is now being done and we expect to have the first three-dimensional results in the near future.
(16) Walker, R. B.; Stechel, E. B.; Light, J. C. J. Chem. Phys. 1978, 69, 1922.
Acknowledgment. M.B. thanks Professor D. G. Truhlar for a discussion on this subject a few months ago.
Infrared and Tunneling Spectroscopy Study of AIN Films Prepared by Ion-Beam Deposition Ursula Mazur* and Ann Cuneo Cleary Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 (Received: April 27, 1989)
Aluminum nitride films ranging in thickness from 0.4 to 25 nm were prepared by reactive ion-beam sputter deposition, utilizing a pure aluminum metal target and both pure nitrogen and 25% hydrogen-enriched nitrogen plasma. The 0.4-1.5-nm AIN films were studied by inelastic electron tunneling spectroscopy. Complementary vibrational techniques, specular reflectance and absorbance FT-IR spectroscopy, were used to analyze the thicker insulating films. The principal component of the insulating layers is AIN. A1-H, AI-N2, and NH, surface species are also present as minor components. Tunneling spectroscopy showed superior sensitivity over the optica! methods in identifying various components of the AIN films. The quality of the tunneling barrier is enhanced by the presence of H2during the AIN deposition.
Introduction The purpose of this paper is two fold. First, we present a structural analysis of aluminum nitride thin films prepared by ion-beam sputter deposition onto aluminum. This analysis is based upon inelastic electron tunneling spectroscopy (IETS) and specular reflectance and absorbance FT-IR spectroscopy. The second motivation for this work is to demonstrate the potential utility of AIN as an insulating barrier for M-I-M devices used in IETS. The interest in AlN thin films stems from their desirable electrical, optical, dielectric, and acoustical properties.I-’ The chemical and thermal stability of AIN together with its high resistivity offers a potential application to insulating and passivating layers in semiconducting devices. The piezoelectric behavior of AIN makes it a viable new material for surface acoustic wave devices (SAW) and integrated SAW/silicon circuits. The ionbeam method has been used extensively for fabricating stoichiometric AIN films under mild experimental conditions. Here, layers of AIN are synthesized by employing a pure aluminum metal target and an energetic beam of ionized pure nitrogen gasE-10or ( I ) Cox, C. A.; Cummins, D. 0.; Kawaba, K.; Tredgold, R. H. J . Phys. Chem. Solids 1967, 28, 543. (2) Kubiak, C. J. G.; Aita, C. R.; Hinkernell, F. S.; Joseph, S. J. Marer. Res. Soc. Symp. Proc. 1985. 47, 75. (3) Netterfield, R. P.; Muller, K. H.; McKenzie, D. R.; Goonan, M. J.; Martin, P. J. J . Appl. Phys. 1988, 63, 760. (4) Hentzell, H. T. G.; Harper, J. M. E.; Cuomo, J. J. J. Appl. Phys. 1985,
58, 556. (5) Xinjiao, L.; Zechuan, X.; Ziyou, H.; Wuda, S.; Zhongcai, C.; Feng, 2.; Enguang, W. Thin Solid Films 1986, 139, 261. (6) Xinjiao, L.; Zechuan, X.; Ziyou, H.; Huazhe, C.; Wuda, S.; Zhongcai, C.; Feng, 2.; Li, S.IEEE 35th Electronic Components Conf. Proc. 1985, 442. (7) Fathimulla, A.; Lakhani, A. A. J. Appl. Phys. 1983, 54, 4586.
0022-3654/90/2094-0l89$02.50/0
of 1:I nitrogen-argon11,12 mixtures. Variable ratio hydrogennitrogen mixtures have also been utilized in the production of AlN film^.'^*'^-'^ Studies have determined that the presence of hydrogen ions in the plasma beam actually improves the bulk and the interface properties of A1N film^.'^,'^ The inclusion of hydrogen in these AIN films was substantiated by SIMS data,I4 but no hydride phases were observed in the TEM diffraction patterns of the film^.'^^^^ Thus, no hydrogen-containing molecular surface species were proposed in these work^.'^^^^-'^ Although there have been many studies addressing the electrical, optical, acoustical, and morphological properties’-’ of AlN thin films, the available structural results are usually based upon X-ray diffraction mea~urements.~-~ Spectroscopic techniques have been rarely employed for structure analysis of AIN films, and the reported vibrational data consist primarily of transmission spectra of films deposited onto KBr windows and of nitride films removed physically from a substratei6-I9and pressed into a KBr pellet.20 (8) Weissmantel, C. J . Vac. Sei. Technol. 1981, 18, 179. (9) Sibran, C.; Blanchet, R.; Garrigues, M.; Viktorovich, P. Thin Solid Films 1983, 103, 21 I . (IO) Bhat, S.; Ashok, S.; Fonash, S. J.; Tonason, - L. J . Electron. Mater. 1985, 14, 405. (1 I ) Weissmantel, C. Thin Solid Films 1981, 92, 55. (12) Grill, A. Vacuum 1983,33, 329. (13) Taylor, J. A,; Rabalais, J. W. J. Chem. Phys. 1981, 75, 1735. (14) Hudis, M. J . Appl. Phys. 1973, 44, 1489. (15) Liu, M. B.; Gruen, D. M.; Krauss, A. R.; Reis, A. H., Jr.; Peterson, S. W. High Temp. Sei. 1978, IO, 5 3 . (16) Demiryont, H.; Thompson, R. L.; Collins, G.J. J. Appl. Phys. 1986, 59 _37-45 _ ,
(17) Hasegawa, F.; Takahashi, T.; Kubo, K.; Nannichi, Y. Jpn. J . Appl. Phys., Part I 1987, 26, 1555. (18) Duchene, J. Thin Solid Films 1971, 8, 69. (19) Gerova, E. V.; Ivanov, N. A,; Kirov, K. I. Thin Solid Films 1981,81, 201.
0 1990 American Chemical Society
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These vibrational studies were principally concerned with locating the AI-N stretch. This stretching band was used as a means of chemical identification and as a measure of the stoichiometry of the deposited nitride films.’620 In this work, we analyze and compare the structure of A1N thin films, prepared both with pure nitrogen and 25% hydrogen-enriched nitrogen plasma, by examining their vibrational spectra in the 400-4000-~m-~region. For the first time, we present and interpret vibrational spectra of A1N films prepared with N2-H2 plasma. Two vibrational techniques, IETS and FT-IR, were employed in surface-structure analysis. Both IETS and FT-IR were used to study films up to 3 nm thick. Films thicker than 3 nm were analyzed by FT-IR only. To our knowledge, this is a first such detailed vibrational work on AIN thin films. IETS is unique among the presently available molecular spectroscopic tools in that it allows the observation of both allowed and silent molecular transitions.2’-26 Unfortunately, application of IETS to molecular spectroscopic studies has been limited. This limitation is imposed by the rather scant number of insulators of proven utility as substrates in the metal-insulator-metal, M-I-M, structures employed in IETS. To date, the most commonly used IET adsorbates are alumina and magnesia?’ Both are redox-active and thus inappropriate for the study of many molecular syst e m ~ . ~Several ~ - ~ native ~ metal oxide ~ u b s t r a t e s , ~other @ ~ ~than alumina or magnesia, as well as a few and non-oxide barrier^'^^^^ have been prepared. Most of the these adsorbates were prepared utilizing a combination of resistive-evaporation and plasma-discharge methods. None of them, however, have found general usefulness as IET adsorbates. In order to develop and exploit IETS as an all-purpose molecular tool, a broad spectrum of substrates with varying reactivity and electron-transfer properties must be found. We are presently exploring the applicability of different dielectric materials as possible tunneling substrates and different preparative methods for fabricating thin films of these insulators. Aluminum nitride is an excellent IET adsorbate candidate because of its wide band gap (6.3 V), good chemical and thermal stability, and high re~ i s t i v i t y .We ~ found ion-beam-deposition technology very well suited for fabricating tunnel barriers. This film synthetic method proved clean, easy to use, and provided very good film reproducibility. Ion-beam-synthesized AlN insulating barriers yielded satisfactory AI-AIN-Pb IET devices with low-noise tunneling spectra.
(20) Singh, A.; Lessard, R. A.; Knystautas, E. J. Thin Solid Films 1987, 138,19. (2 1) Hansma, P. K. Tunneling Spectroscopy: Capabilities, Applications, and New Techniques; Plenum: New York, 1982. (22) Hipps, K. W.; Mazur, U. J . Am. Chem. Soc. 1987, 109, 3861. (23) Hipps, K. W.; Mazur, U. J. Phys. Chem. 1987, 91, 5818. (24) Hipps, K. W.; Susla, B. Chem. Phys. Lett. 1987, 132, 507. (25) Mazur, U.; Hipps, K. W. Reu. Sci. Instrum. 1984, 55, 1120. (26) Hipps, K. W.; Dunkel, E. A.; Mazur, U. Langmuir 1986, 2, 528. (27) Koreman, C. S.; Coleman, R. V. Phys. Reu. 1977, 815, 1877. (28) Muka, G. M. J . Cafal. 1979, 58, 470. (29) Deflin, M.; Enltantawy, I. M.; Bavarez, M. J . Catal. 1978.54, 345. (30) Adler, J. G.;Chen, T. T. Solid State Commun. 1971, 9, 501. (31) Straw, J.; Adler, J. G. Solid. State Commun. 1974, 15, 1639. (32) Jaclevic, R. C.; Lambe, Phys. Reu. 1970, E2, 808. (33) Adane, A. Solid State Commun. 1975, 16, 1071. (34) Rochlin, G. 1.; Hansma, P. K. Phys. Rev. 1970, 82, 1460. (35) Amen. . . F.: Goldman. A. M. Crvopenics 1976. 12. 721. (36) Yanson, I. K.; Bogatina, N. I.; Cerkin, 9. I.; Shklyarevskii, 0. I. Zh. Eksp. Teor. Fiz. 1972, 62, 1023. (37) Phillips, W. A. Phisica 1984, 1278. 112. (38) Suzuki, M.; Mazur, U.; Hipps, K. W. Surf. Sci. 1985, 161, 156. (39) Mazur, U.; Hipps, K. W. Chem. Phys. Lett. 1981, 79, 54. (40) Mazur, U.; Hipps, K. W. J. Phys. Chem. 1981, 85, 2244. (41) Bell, L. D.; Coleman, R. V. Phys. Reu. 1984,830, 4120. (42) Hipps, K. W.; Mazur, U. Surf. Sci. 1989, 207, 385. (43) van Velzen, P. N. T.; Rass, M. C. Sur/. Sci. 1985, 161, L605. (44) Alexander, J. D.; Gent, A. N.; Henriken, P. N. J . Chem. Phys. 1985, 83, 5981. (45) Gauthier, S.; de Cheveigne, S.; Klein, J.; Belin, M. Surf. Sci. 1985, 155, 31. (46) Hipps, K. W.; Mazur, U. Rev. Sci. Instrum. 1989, 59, 1903. \
I
Mazur and Cleary
0
2000 Energy
4000
(cm-1)
Figure 1. Tunneling spectra of AIN films prepared with different gas mixtures: (a) film produced with 25% H2-enriched N2 plasma; (b) film made with pure N 2 plasma.
Experimental Section Film Deposition. Film fabrication was carried out in an all stainless steel deposition chamber of our own design. This chamber houses a low-power 2.5-cm Ion Tech, Inc., ion beam source as well as two resistive evaporation sources. The ion source is set up in the single-beam sputtering mode. In this configuration, a beam of ionized inert or reactive gas molecules or atoms impinges upon a target and material is sputtered from the target onto a substrate. The UHV chamber is also equipped with an Edward’s FTM4 film-thickness monitor, Edward’s residual gas analyzer (1-80 amu), and a cold cathode gauge. An 8-in. CTI Cryo-Torr pump is used to evacuate the chamber. This system operates with Torr (attainable in approximately a base pressure of C1 X 10-20 min). The A1N films were deposited on unheated substrates with pure N2 and 25% H2-enriched N2 gas (premixed) at 2 X lo4 Torr of total pressure. All gases used were ultrapure grade. The target employed was a 3-in. aluminum disk (99.999% pure) from Cerac, Inc. The experimental ion-beam voltage and current were 1000 eV and 9 mA, respectively. The average deposition rate achieved was 0.01 nm/s. Electron microprobe analysis indicates that these films are stoichiometric, 49.0 f 1.1 mol % AI and 5 1.O f 0.9 mol % N. IETS. The tunnel junctions reported here are A1-AlN-Pb structures. The incorporation of aluminum nitride films into tunnel junctions was accomplished as follows. First, a 1-mm-wide 200-nm-thick strip of aluminum base metal was resistively deposited on a precleaned Pyrex microscope slide at l X Torr of base pressure. The substrate was then rotated to the ionbeamdeposition station. Here, by utilizing experimental conditions described earlier, an AlN film of appropriate thickness was deposited (using either pure N2 or 25% H2-enriched N2 gas). The system was then evacuated to the original base pressure. A cross-strip (200-nm-thick and I-mm-wide) lead top electrode was resistivity deposited to complete the tunnel junctions. The junctions were attached to a sample rod, and electrical connections were made with indium solder. Tunneling spectra were measured at 4.2 K with a modulation voltage of 3 mV and the aluminum electrode biased negatively. A constant-resolution tunneling spectrometer, recently described in the literature,& was employed for data acquisition. Typical junction resistances ranged from 40 to 200 Q . In some cases, a low-order polynomial was used to remove the voltage-dependent elastic tunneling contribution from the spectral data. This procedure yields flatter base lines. FT-IR. Vibrational spectra were measured with an IR/98 FT-IR spectrometer. Specimens for the specular reflectance measurement were prepared by following the general procedure for fabricating tunnel junctions but omitting the top-metal deposition step. Thus, 10 nm of A1N film was deposited on a
Infrared and Tunneling Spectroscopy Study of A1N Films
The Journal of Physical Chemistry, Vof. 94, No. 1, 1990 191
TABLE I: Vibrational Spectral Data of Ion-Beam-Deposited AIN Films (cm-I)' . . 25% H2 in N2 IETS spr 640 727 sh 950b 870 914 1025 1202 1297 1300 1379 1400 1448 1516 1600 1597
> 4J 4
cn C al c,
C H
0
2000
Energy
4000
(cm-1)
Figure 2. Vibrational spectra of AIN films synthesized with 25% H2enriched N2: (a) absorbance spectrum of a 25-nm-thick film deposited on a KBr window (absorbance maximum, 0.062); (b) specular reflectance spectrum (at 80' incidence) of 8-nm-thick film deposited on an aluminum mirror; (c) IET spectrum of a -0.4-nm-thick film. Base-line correction was performed on the tunneling and the absorbance data.
200-nm-thick 1 in. X 2 in. aluminum mirror. Spectra were measured with 80° grazing incident radiation. The incident light was not polarized. Samples for the absorbance measurements were prepared by depositing 8-25 nm of aluminum nitride film directly onto a KBr window. FT-IR data were acquired at 8-cm-' resolution and are the result of 1024 scans. A helicoil source, a germanium-coated KBr beam splitter, and a nitrogen-cooled MCT detector were employed. All spectra were obtained at room temperature.
pure N2
FT-IR
*
abs 655
IETS 645 727 sh 950b 879 1030
FT-IR abs 652
assinnmt TO(AIN)C G(AIH)d U(AIO)~ 860sh LO(AIN)C 6(NH) in N H i a 6(NH) in NHIS 1304 1297 1280 6(NH) in NH38 1400 1404 sh 1400 6(NH) in [NH4+]8 6(NH) in [NH4+]8 1516 1505 1508 6(NH) in [NH '18 1589 1594 1608sh ~ ( N H in ) NH&, [NH3+]f,and [NH4+18 1730 sh 1730 sh G(NH) in [NH4']8 1815 1822 u(AIHY 2119 2125 2125 2126 2145 u(AINN)* 2220 2220 (2350) v(HAINN)' or u(CN)~ 2904 2901 (2900) 2904 u(CH)' 3283 3259 (3271) 3265 (3270) u(NH)fJ 3638 (3560) 3620 u(0H)' "Uncertainty in the tunneling values is 5 cm-I and in the optical values is 10 cm-I. Values in parentheses have an uncertainty of 20 cm'l. bExpected position of AI0 stretch. This band is not observed in our IET spectra. CReference 47-49. Reference 53-55. e Reference 21. /Reference 61 and 62. SReference 63, 64,66, and 67. *Reference 56-59. 'Reference 56 and 60.
Results Figure 1 displays the tunneling spectra of ion-beam-deposited aluminum nitride with different gas mixtures being utilized during the fabrication process. These are raw spectra without any base-line correction. Each spectrum is representative of tunneling data collected from 10 or more junctions. Both traces exhibit relatively broad very weak to medium intensity features built upon a steep elastic background. However, the IET spectra of AIN films prepared with pure nitrogen exhibit much weaker vibrational bands, relative to the elastic background, than the spectra of AlN films prepared with madded H2. Spectral resolution of these bands did not improve with lower modulation voltage. Spectra of junctions prepared with variable AIN film thickness (1 .O-2.0 nm) with pure Nz gas showed no apparent relationship between the observed band intensity and the insulator thickness. The general appearance of these spectra remained invariant with AlN film thickness. The junctions did, however, exhibit the expected trend in device resistance. Figure 2 compares the FT-IR and tunneling spectra of AIN films synthesized by use of 25% H2-enriched N z gas. The mean peak positions of the features in Figure 2 along with possible band assignments are given in Table I. The most striking attribute of this figure is the dramatic variation in the level of spectral sensitivity produced by the different vibrational techniques. As will be shown later, IETS is more sensitive to XH-type surface species. There are also some additional bands appearing in the IET spectrum that do not manifest any appreciable intensity in the FT-IR spectrum. These bands occur near 1030, 1820,2220, and 3640 cm-I. Below 1000 cm-', the tunneling spectrum exhibits both the LO and TO modes of A1N.47-49
Both the absorbance and the specular reflectance spectra appear quite similar above 1000 cm-' except for the increased intensity of the N H and the CH stretches appearing near 3250 and 2900 cm-l, respectively, in Figure 2b. Below 1000 cm-l, only the LO AlN mode can be observed in the reflectance spectrum. This result is expected for near grazing incidence reflectance data.s0,51 In trace 2a, the LO mode appears as a shoulder along with the prominent T O component near 640 Figure 3 displays segments of the tunneling and the specular reflectance spectrum
(47) Carlone, C.; Lakin, K. M.; Shanks, H. R. J. Appl. Phys. 1984, 55, 4010. (48) Sanjurjo, J. A.; Lopez-Cruz, E.; Vogl, P.; Cardona, M. Phys. Reu. 1983, B28. 4579.
(49) Collins, A. T.; Lightowlers, E. C.; Dean, P. J. Phys. Reu. 1967, 158, 833. (50) Greenler, R. G.; Rahn, R. R.; Schwartz, J. J . Coral. 1971, 23, 42. ( 5 1 ) Handke, H.; Paluszkiewcz, C. Infrared Phys. 1984, 24, 121.
> 4J .r(
cn C al 4J
C H
1000
1500 Energy
2000
2500
(cm-I)
Figure 3. Spectra of the 1000-2500-~m-~region selected from Figure 2. Trace 3a is an expanded fragment of the tunneling spectrum shown in Figure 2c. Trace 3b displays the equivalently expanded fragment of the specular reflectance spectrum shown in Figure 2b.
192
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990
m C a, LL
c
H
0
2000
4000
(cm-1) Figure 4. Vibrational spectra of AIN films fabricated with a pure N, plasma: (a) absorbance spectrum of a 8-nm film deposited on KBr (absorbance maximum, 0.036); (b) tunneling spectrum of a 1-nm-thick film. Base-line correction was performed on both IR and IETS data. Energy
-
of Figure 2 in the range 1000-2500 cm-’. The tunneling spectrum and the absorbance spectrum of AIN fabricated with a plasma composed of pure N 2 are presented in Figure 4. Peak positions of the observed bands are recorded in Table I. In general, the tunneling spectrum of AIN, trace 4b, appears very similar to trace c in Figure 2 with respect to the presence and the location of the observed transitions. However, the intensities of a number of bands present in both spectra are different. The absorbance spectrum, Figure 4a, displays weak and poorly resolved vibrational transitions except for one strong band near 640 cm-l.
Discussion A1N Films Fabricated with 25% H,-Enriched N2. The presence of hydrogen in the ion beam during AIN film production resulted in better quality AIN tunneling barriers. As Figure 1 demonstrates, the observed inelastic signal in the tunneling spectrum of AIN fabricated in a H2-Nz mixture is stronger than in the IET spectrum of AIN prepared with pure N,. Consequently, the features in trace l a are better developed than those in trace lb. The role of hydrogen during reactive ion-beam deposition of AIN films was examined in comparative studies that utilized both nitrogen and hydrogen-nitrogen b e a m ~ . ~AIN J ~ films fabricated in the presence of hydrogen ion in the plasma exhibited reduced conductivity and interface trap density along with an increase in dielectric strength and in the optical g a ~ . ~The , ’ ~surface hardness of metals was also shown to improve substantially when their surfaces were bombarded with hydrogen-nitrogen ion beams.I4J5 NH,+-type molecular ions and not nitrogen ions were proposed as the reactive nitrating species.14 Prior to this work, there exits no experimental evidence for hydrogen-containing molecular surface species. Identification of such surface species would greatly aid in the understanding of the surface-hardening mechanism of metals. In order to better examine the vibrational structure observed in the tunneling spectrum of AIN films fabricated with H,-enriched Nz plasma, we have amplified the spectral features in Figure la with a background subtraction procedure. The result is Figure 2c. This spectrum is very rich and appears much more complicated than the IET spectrum of AIN grown in an ammonia discharge as reported by Shklyarevskii and co-workers.52 Similarities and differences between the tunneling spectra resulting from the ion-beam-fabricated AIN insulator and the AIN barrier grown in ammonia discharge will be considered in subsequent paragraphs. The low-energy region of the spectrum is dominated by transitions due to the AIN. The bands near 640 and 870 cm-’ cor(52) Shklyarevskii, 0. I.; Yansen, I. K.; Zaporozhskii, V . D. Solid Stare Commun. 1974, 14. 327.
Mazur and Cleary respond to the TO and LO AIN modes, re~pectively.~’-~~ These two bands are also present in the IET spectrum reported by Shklyarevskiis2although the relative intensities of these bands are reversed. The reason for the apparent enhancement in the intensity of the LO(A1N) component in the tunneling spectrum in ref 52 is the contribution of the AI-0 stretching motion.21 That is, Shklyarevskii’s junctions had a significant oxide component. The tunneling vibrational bands in Figure 2c near 730 and 1820 cm-l are associated with AI-H motions. The AI-H stretching motion was first assigned by Igalson and Adler based on their tunneling work with hydrogen-implanted alumina barriers.s3 Thiry and co-workers assigned the AIH stretching vibration independently by examining the AI( 1 10) surface exposed to atomic ~ AI-H bending hydrogen with electron loss s p e c t r o s ~ o p y . ~The motion in the tunneling spectra of water-grown aluminum oxide barriers was assigned latter by Gauthier et aL5* The weak tunneling band at 2 120 cm-’ is assigned to motion arising from a surface species in which a nitrogen molecule is coordinatively bound to an aluminum atom or ion. The relative position of this band is in good agreement with the M-N2 stretching vibration found in metal-nitrogen complexess6as well as in N, absorbed on clean metal s ~ r f a c e s ~and ~ - ~metals ’ on oxide The high-energy shoulder appearing at 2220 cm-’ may be due to hydrogenated aluminum bearing an intact nitrogen molecule, -H-AI-Nz+. The existence of this type of species has been postulated based on experiments that involved the coadsorption of nitrogen and hydrogen gases on clean metal surfaces and bulk catalysts.57@An alternative surface species “responsible” for the 2220-cm-’ band could be a C N group. The source of this surface fragment may be the result of the reaction between the graphite grids and the ionized nitrogen gas. The tunneling spectra of AIN films prepared in 25% H2-enriched N, show substantial hydrocarbon contamination as evidenced by a rather intense C H stretching band near 2900 cm-l. A possible source of carbon is the graphite grids in the ion gun. Hydrocarbon fragments are produced and codeposited on the aluminum substrate during the film fabrication step when the grids react with the ionized hydrogen gas. The presence of the O H stretching band near 3640 cm-’ indicates the existence of residual water in the deposition system. Thus, some AIO, is probably formed during the AIN film fabrication. However, the concentration of AIO, in the AlN barrier is low, as indicated by the very weak AI-0 stretch and a very weak 0-H stretch. Previous studies Torr indicate that a residual partial pressure of H 2 0 of 5 X or less is necessary to produce oxide-free AlN films. At higher residual pressures, uncontrolled amounts of oxygen were incorporated in the film, mainly due to water vapor c o n t a m i n a t i ~ n . ~ ~ ~ The remaining tunneling vibrations in Figure 2c (or 3a) in the 1000-1600-~m-~ region, as well as the band near 3280 cm-I, are identified as nitrogen-hydrogen motions. The same interpretation applies to the bands located in similar regions of corresponding optical spectra, Figures 2a, 2b, or 3b. The positions of these bands are correlated with the presence of different NH, surface species. It is reasonable to presume the existence of such species since the nitrogen and hydrogen ions in the plasma are expected to react forming NH,’ ions.I3-I5 It was demonstrated that the most abundant ionic species in H2-N, mixed-ion-beam plasmas are the H+ and NH,+ fragments.I4 Minor plasma constituents include NH4+, NH2+,NH+, and N2+ions.14 While one or more of the bands in the 1000-1600-~m-~region may be due to species other (53) Igalson, J.; Adler, J . G. Phys. Reu. 1983, 828, 4970. (54) Thiry, P. A,; Pieaux, J. J.; Liehr, M.; Caudano, R. J . Vac. Sci. Technol. 1985, A3, 1439. (55) Gauthier, S.; de Cheveigne, S.; Klein, J.; Belin, M. Phys. Reu. 1984, 829, 1748. ( 5 6 ) Murray, R.;Smith, D. C. Coord. Chem. Rev. 1968, 3, 429. (57) Slavnikov, V. S.; Petukhov, P. A.; Trubitsyn, A. M.; Usynina, N. A. SOC.Phys. J . (Engl. Transl.) 1974, 17, 1189. (58) Oh-Kita, M.; Aika, K.; Urabe, K.; Ozaki, A. J . Catal. 1976, 44,460. (59) Oh-Kita, M.; Midorikawa, H.; Aika, K.; Ozaki, A. J . Catal. 1981, 70, 384. (60) Kinoshita, N.; Kido, K.; Domen, K.; Aika, K . ; Onishi, T. J . Chem. SOC..Farads-v Trans. 1 1986. 182, 2269.
Infrared and Tunneling Spectroscopy Study of AIN Films than those identified here, the present data does not justify a more complex assignment. We have based our assignments of the different plausible NH, surface species upon the existing infrared spectra of ammonia61,62 and NH4C163,64and upon the vibrational studies of ammonia adsorption on thin-film-al~mina~~ and bulk-alumina catalyst^.^^,^^ We note that the 1200-1 8 0 0 - ~ m -spectral ~ region in our optical data compares quite favorably with the infrared spectra of ammonia adsorbed on activated high surface area alumina catal y s t ~ . ~ ~A, definitive ~' assignment of the possible NH, species present in our AIN films is made difficult by the presence of only one unresolved broad band in the N H stretching region. This band spans approximately the 3 100-3400-cm-' range in the tunneling spectrum and the 3000-3500-~m-~region in the optical spectra. We limit our assignments to the three most probable surface candidates, namely, NH,, NH4+,and NH3+. We attribute bands near 1030, 1200, and 1600 cm-l to surface ammonia. In solid-state NH,, a band at 1060 cm-' is assigned as the Raman-active N H deformation while a band at 1585 cm-' is assigned as the IR-active N H deformation.62 The corresponding N H stretching vibrations are observed at 3223 and 3378 cm-' in the Raman and at 3369 and 3202 cm-I in the IR spectra.6z Bands near 1000, 1200, and 1600 cm-l are reported as combination bands for solid ammonia.62 The IET spectrum of AIN grown in an ammonia discharge shows a single N H stretching motion near 3300 cm-l and N H bending vibrations near 1600 and 1000 cm-1.52These bands are attributed to surface-bound NH, molecules. Infrared studies of ammonia adsorbed on high surface activated catalysts attributed bands near 1290, 1630, 3370, and 3410 cm-' to ammonia bound coordinatively to an aluminum ion. Our vibrational data in Figures 2 and 3 also display a band near 1300 cm-l. The vibrational transitions near 1400, 1450, and 1730 cm-' are assigned to an NH4+ surface ion. These agree well with infrared-active N H deformation modes reported for NH4Cl at 1403, 1445, and 1762 cm-1.63364 A Raman-active N H vibration in ~ ~ N H stretching bands in NH4CI is found at 1712 c ~ n - l .The NH4CI are observed at 3228, 3126, and 3090 cm-1.63 Infrared bands at 1410, 1450, 1680, 1700, 3 155, and 3050 cm-l were also reported for a NH4+surface ion formed upon absorption of NH, on bulk catalyst^.^^,^^ The tunneling spectrum of NH, adsorbed on alumina indicated the presence of two well-resolved N H stretching motions at 3260 and 3330 cm-' and one N H deformation motion near 1600 Shklyarevskii noted that the N H stretching modes in his tunneling spectra agreed well with the N H stretches assigned to a NH4+surface ion in the IR spectra of NH, chemisorbed on an aluminosilicate catalyst.65 We do not observe the well-defined doublet of the N H stretches reported by S h k l y a r e ~ s k iin i ~our ~ tunneling data possibly due to the presence of several NH, species. Infrared bands near 1500, 1630, 3370, and 3415 cm-l have been attributed to a NH,+ moiety produced from the adsorption of ammonia on activated bulk catalyst^.^,^^ The NH,+ species results from the coordination of an ammonia molecule to an aluminum surface As indicated earlier, the 1200-1 800-cm-' region of our optical data and the infrared spectra of N H 3 adsorbed on bulk catalyst^^^,^^ exhibit very close similarity. Because of this close spectral resemblance, it seems reasonable to assign the 15 10-cm-I band present in our data to the NH,+ surface species. It is puzzling, however, why this transition does not appear with any appreciable intensity in the tunneling spectra, Figure 2c and 3a. A 1505-cm-I band is present in the IET spectrum of AIN prepared in pure N2 plasma, Figure 4b. The 1448-cm-I transition is not visible in this figure. Perhaps, the 1448-cm-l band in Figure 2c (or 3a) is the same transition as the one appearing near 1500 (61) Reding, F. P.; Horning, D. F. J . Chem. Phys. 1951, 19, 594. (62) Reding, F. P.; Homing, D. F. J . Chem. Phys. 1954, 22, 1926. (63) Wagner, E. L.;Horning, D. F. J. Chem. Phys. 1950, 18, 296. (64) Vedder, W.; Horning, D. F. J . Chem. Phys. 1961, 35, 1560. (65) Shklyarevskii, 0. I.; Lysykh, A. A,; Yansen, 1. K. Sou. J . Low Temp. Phys. (Engl. Transl.) 1978, 4, 717. (66) Dunken, H.; Fink, P. Acta Chim.Acad. Sci. Hung.1967, 53, 179. (67) Sobalik, Z.; Pour, V . Collect. Czech. Chem. Commun. 1984,49, 355.
The Journal of Physical Chemistry, Vol. 94, No. I, I990
193
cm-l in Figure 4b but shifted to lower energy. Isotopic labeling experiments utilizing D, and I5N2plasma during AIN film fabrication are planned to improve spectral assignments. The optical spectra of AIN films presented in Figures 2 and 3 generally agree with the tunneling data. The dissimilarity in intensities of the features in the FT-IR spectra vs the IET spectra arises from the difference in selection rules governing the two spectroscopic techniques. Vibrational transitions common to FT-IR and tunneling spectra include the N H stretches and bends as well as the AIN and AI-N2 motions. The relative broadness of the 3260-cm-I N H stretching band in the specular reflectance spectrum is indicative of the existence of more then one NH, complex or of extensive hydrogen bonding. The bands near 1820, 2220, and 3640 cm-' associated with the AIH, H-A1-Nz, and O H surface species, respectively, are very weak in the FT-IR data. The near absence of these bands in the optical spectra can be related to the overall diminished intensity in X-H-related vibrations relative to the same bands appearing in the tunneling spectra. Considering the weakness of the C H stretch in the optical spectrum relative to the same mode in the IETS, it is not surprising that the AIH stretch appears to be absent in parts a and b of Figure 2. AlN Films Fabricated with Pure N2 Plasma. The tunneling spectrum of an AIN barrier prepared with pure nitrogen plasma, Figure 1b, displays very weak and broad bands. As mentioned before, the resolution of these bands did not improve with a variation in barrier thickness or with a decrease in modulation voltage during IET data collection. Because of the low inelastic tunneling signal in Figure lb, employment of a background subtraction procedure produced somewhat noisier results than when the same routine was applied to Figure la. Compare Figures 4b and 2c. Most of the features in the tunneling spectrum of the AIN barrier prepared in pure N2 plasma also appear in the IET spectrum of the barrier prepared with coadded hydrogen. The most pronounced differences between the two spectra include a lowering in the intensity of the band near 2900 cm-' and an increase in intensity and a shift to higher energy of the band near 880 cm-' in Figure 4b. The observed decrease in the intensity of the C H stretch band in Figure 4b corresponds to a decrease in hydrocarbon contamination that is a direct result of the absence of hydrogen in the beam plasma. The observed change in intensity and position of the band near 880 cm-l, assigned as LO AIN motion in Figure 4b, can be attributed to an increased contribution of the AIO, component in the barrier. (It should be pointed out that the relative positions and the intensities of the T O and LO aluminum nitride phonons in Figure 4b are in close agreement with the tunneling results reported by Shklyarevskii and cow o r k e r ~ . ~The ~ ) formation of alumina can be directly associated with the presence of residual water in the deposition system. It has been demonstrated that while residual oxygen was present in appreciable concentration when AI was sputtered in a pure nitrogen beam, its concentration actually dropped several orders of magnitude when the sputtering ion beam contained hydrogen4' This lowering of the oxygen content by the hydrogen ions present in the beam was referred to as an oxygen-gathering effect. No explanation for this phenomenon was given. The intensities of some of the low-energy N H deformation bands in the tunneling spectrum of the AIN barrier prepared in pure N, plasma (Figure 4b) appear to be different when compared to the relative intensities of the same transitions observed in the IET spectrum of the barrier prepared in Hz-N, plasma (Figures 2c and 3a). When one compares Figure 4b and Figure 2c, the former exhibits a substantial increase in the 1505-cm-' band and a comparable decrease in the 1448-cm-' band. One reason for this is that the 1448-cm-' band may have shifted to higher energy and now appears near 1500 cm-I. Alternatively, the observed decrease in the intensity of the 1448-cm-I transition may perhaps be related to a decrease in the concentration of the NH4+ surface ion in the AIN films prepared in pure N2 plasma. A substantial lowering in the concentration of all the NH, surface species is anticipated in the absence of hydrogen in the plasma beam.
J . Phys. Chem. 1990, 94, 194-199
194
During the deposition of AIN films utilizing pure N 2 plasma, the only expected source of hydrogen is residual water in the system. The absorption spectrum of an AIN film fabricated in a pure nitrogen plasma, Figure 4a, displays a strong AIN band and weak AI-N, and N H motions. This spectrum is of much poorer quality than the analogous spectrum obtained from an AIN film deposited in a hydrogen-nitrogen plasma. Visual inspection of the AIN absorption specimen prepared in pure N2 plasma revealed the presence of opalescence on the KBr disk, suggesting that the deposited material was quit rough. TEM and electron diffraction data indicate that ion-beam-fabricated AIN films are made up of crystallites ranging in size from 2 to 15 nm.3+4*8J3,20 We are now in the process of conducting morphological studies on our AIN films. Based on the above discussion, it is reasonable to assume that AIN tunneling barriers, fabricated under the conditions described in this work, are composed of a thin underlayer of alumina on which AIN is deposited. The electron tunneling process proceeds through both AIO, and the AI0,-AIN regions. The presence of hydrogen ions in the nitrogen plasma during the deposition of aluminum nitride clearly improves the quality of the tunneling barriers. It should also be pointed out that whereas the A1N junctions prepared in pure N, plasma required deposition of 1-2-nm (measured by the films-thickness monitor) thick insulating films in order to produce resistances of about 200 R, AIN diodes prepared in the hydrogen-enriched nitrogen plasma required barriers of only 0.4-nm apparent thickness to yield comparable resistances. It is not likely that tunnel barriers less than 1 nm thick will produce resistances of tens of ohms.2' It is likely that the H2-N2 beam reacts with the aluminum substrate during the early stages of the A1N deposition. The film-thickness monitor registers only the net amount of AIN deposited, while the amount of surface material formed due to beamsubstrate reaction is not measured. The measured thickness of A1N barriers is greater when pure N, plasma is employed because (1) the nitrogen beam reacts with
the AI substrate to a lesser extent than H2-N2 plasma and (2) more native oxide is formed in the absence of H2. We are presently evaluating the contribution of the plasma-substrate interaction to the AIN tunnel barriers.
Conclusion We have observed and identified for the first time surface species, other than AIN, in ion-beam-deposited aluminum nitride films. The structural analysis of AIN films based on tunneling data indicates the presence of AlN along with AI-H, AI-N,, H-AI-N2, and NH, surface species. Optical spectra of these AIN films corroborated most of the IETS results. IETS, however, proved to be a more sensitive surface-spectroscopic method than FT-IR techniques, yielding additional information about the composition of the AIN films. The possible existence of NH, surface species was suggested by others but has not been verified to date. These species were also linked with a possible mechanism for the ion-nitriding process. Our results provide a new insight into the structure of A1N films and are also relevant to the understanding of the mechanism of the ion-nitriding process. The AIN tunneling barrier prepared in a pure N2 plasma most probably consists of AIN aggregates scattered on an alumina layer that is formed due to the residual water in the deposition system. Thus, it is expected that electron tunneling occurs through both AIO, and AIN. The presence of hydrogen ions in the nitrogen plasma during the deposition of aluminum nitride yields smoother tunneling barriers with less oxygen. Morphological and reactivity studies of the ion-beam-fabricated AIN films are now in progress. Acknowledgment. We gratefully acknowledge the National Science Foundation and the Division of Chemistry for their support in the form of Grant CHE-8805612. We also thank Professor K. W. Hipps for many helpful discussions and valued assistance in acquiring the various reported spectra. Registry No. AI-N2, 24304-00-5; hydrogen, 1333-14-0; nitrogen, 7727-37-9.
Theoretical Study of the Vibrational Circular Dichroism of 1,&Dideuterioallene: Comparison of Methods A. Annamalai,t K. J. Jalkanen,f U. Narayanan,t M.-C. Tissot,+ T. A. Keiderling,*,t and P. J. Stephens*,* Department of Chemistry, University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680. and Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482 (Received: April 28, 1989)
Vibrational rotational strengths are calculated for allene-1,3-d2by using the fixed partial charge (FPC), localized molecular orbital (LMO), and atomic polar tensor (APT) models and the a priori theory of Stephens. The LMO model is implemented using semiempirical methods. The APT model and the a priori theory are implemented at the ab initio SCF level of approximation. Dipole strengths for allene-1,3-d2, -do, and -d4 are simultaneously calculated. Dipole strengths predicted a priori are in reasonably good agreement with experimental values; the LMO and FPC models give similar and very different results, respectively. Rotational strengths calculated from the FPC, LMO, and APT models overall bear little resemblance to those calculated from the a priori theory.
Introduction
Vibrational circular dichroism (VCD) has been measured for a wide variety of chiral molecules, and a number of structural applications have been reported.' Most of the latter have been based on empirical correlation of spectroscopic features among chemically similar molecules. A few have attempted to use one 'University of Illinois at Chicago. 'University of Southern California.
of the several theoretical formulations2of VCD to correlate spectra with structure. The latter approach requires a reliable theory, capable of practical implementation. In order to evaluate their reliability, the predictions of some of the proposed theories of VCD (1) Freedman, T. B.; Nafie, L. A. Top. Stereochem. 1987, 17, 113. Polavarapu, P. L. Vib. SpectraStruet. 1984,13, 103. Nafie, L. A. Ado. Infrared Raman Spectrosc. 1984, I I , 49. Keiderling, T. A. Appl. Spectrosc. Rev. 1981, 17, 189.
(2) Stephens, P. J.; Lowe, M. A. Annu. Rev. Phys. Chem. 1985, 36, 213.
0022-3654/90/2094-0194$02.5Q/O 0 1990 American Chemical Society
