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Aqueous Etching Produces Si(100) Surfaces of Near-Atomic Flatness: Strain Minimization Does Not Predict Surface Morphology Ian T. Clark, Brandon S. Aldinger, Ankush Gupta, and Melissa A. Hines* Department of Chemistry and Chemical Biology, Cornell UniVersity, Ithaca, New York 14853 ReceiVed: September 3, 2009; ReVised Manuscript ReceiVed: October 28, 2009
A simple, room temperature processsetching of Si(100) surfaces in 40% NH4F(aq) solutionssproduces H-terminated surfaces of near-atomic smoothness over large areas (>1000 × 1000 Å2). The etched surface is primarily terminated by long alternating rows of strained and unstrained silicon dihydrides; no microfaceting or etching-induced surface roughness is observed. The Cartesian components of the infrared absorption spectrum of flat and vicinal etched surfaces show that the surface is almost entirely dihydride-terminated. This analysis disproves previous assignments of the infrared spectrum of NH4F-etched Si(100) which suggested that the etched surface was very rough and terminated by a variety of mono-, di-, and trihydride species. Although the steady-state etch morphology has lower interadsorbate strain than bulk-terminated H/Si(100), this morphology does not minimize interadsorbate strain as previously postulated. The relatively low reactivity of the strained dihydrides kinetically blocks this pathway. 1. Introduction The chemical control of surfaces at nanoscale dimensions is becoming increasingly important both for the advancement of fundamental science as well as for industrial applications. For example, the production of commercial microelectronic devices places stringent demands on both surface cleanliness and surface morphology. Industrially Si(100) wafers, the substrate of choice, are cleaned with highly aggressive aqueous solutions that slowly etch the surface as they remove contaminants. Although these solutions produce atomically clean surfaces, they also roughen the wafer, thereby decreasing the performance of subsequently fabricated transistors.1 This problem, and others like it, have sparked an intense search for chemical etchants that selectively remove atomicscale roughness while chemically passivating Si(100). From the standpoint of chemistry, the production of atomically flat surfaces would seem straightforward, requiring only an etchant that selectively removes defect sites much faster than sites on the ideal, close-packed surface. Although a number of etchants have been found that smooth Si(111) surfaces by step-flow etching,2–4 the many attempts to chemically smooth the industrially important Si(100) face have been unsuccessful. The general consensus5 is that the most studied (HF-based) etchants, which produce atomically smooth Si(111) surfaces [e.g., NH4F(aq)], progressively roughen Si(100) faces,6–8 in part by producing hillocks terminated by close-packed (111) microfacets.6,9–11 This well-accepted conclusion is, we show, wrong. The purported production of microfaceted Si(100) surfaces has been rationalized in terms of chemical strain minimization. HF-based etchants produce H-terminated Si surfaces.12 An ideal, bulk-terminated H/Si(100) surface would be dihydride-terminated; however, H atoms on adjacent dihydrides would be spaced by 1.5 Åsfar below the 2.4 Å van der Waals diameter of H. To partially relieve this strain, numerous simulations have predicted a spontaneous canting of the dihydrides,13–16 as sketched in Figure 1. This structure has never been observed * To whom correspondence
[email protected].
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Figure 1. Top and side view of the predicted structure of the ideal H-terminated Si(100) surface. Simulations predict the spontaneous canting (red arrows) of adjacent dihydrides to partially relieve interadsorbate strain. In spite of this relaxation, almost all of the H atoms on this surface are highly strained.
over significant areas, likely because of the high residual strain. In contrast, atomically flat H/Si(111)2 and H/Si(110)17 surfaces, which are monohydride-terminated and easily produced by NH4F(aq) etching, are essentially unstrained. Although microfaceting of the Si(100) surface into {111} (or {110})-terminated hillocks during etching would relieve interadsorbate strain, as shown in Figure 2, our experiments show that microfaceting is kinetically blocked. We use scanning tunneling microscopy (STM) and infrared spectroscopy to show that NH4F(aq) solutions selectively etch and passivate Si(100) surfaces, producing a final, H-terminated surface of near-atomic smoothness. The chemical structure of the etched surface was extracted from a new type of vibrational spectroscopy analysis that yields increased information about
10.1021/jp908527e 2010 American Chemical Society Published on Web 12/07/2009
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Figure 2. Microfaceting of the H-terminated Si(100) surface, such as that shown here (top and side views), could potentially relieve almost all interadsorbate strain. Other geometries (e.g., 4-fold symmetric pyramids or etch pits with either {111} or {110} microfacets) are also possible. The color scheme is described in Figure 1.
bond orientation. On this basis, we show that the etched surface is much less complicated than previously postulated6 and dominated by strained and unstrained silicon dihydride species. No evidence of the previously postulated (111) microfacets or silicon trihydride species was observed. This morphology is not produced by step-flow etching, but rather by a more complex mechanism that is driven by interadsorbate strain. Although the steady-state etch morphology has approximately one-half the interadsorbate strain of the bulk-terminated surface, the morphology does not minimize chemical strain. 2. Experimental Section For STM investigations, silicon samples were diced from P-doped, 0.5-10 Ω cm wafers cut to within 0.9° of the (100) orientation or miscut by 3.5((0.1)° from the (100) orientation toward the [011] direction. For spectroscopic analysis, wafers cut to within either 0.9° of the (100) orientation or miscut by 5((0.9)° from the (100) orientation toward the [001] direction were diced into 1.5 × 3.8 cm2 samples from >1000 Ω cm, floatzone wafers. The short ends of the samples were beveled at 45° for analysis in the multiple-internal-reflection geometry. The samples were thermally oxidized and annealed. Before etching, the samples were chemically cleaned as follows. All samples were thoroughly cleaned with sequential baths of warm trichloroethylene, acetone, methanol, and ultrapure water (Millipore Milli-Q). Immediately prior to the experiment, all labware, which was made of either glass or Teflon, was immersed in a solution composed of 1:1:5 by volume of 30% H2O2(aq):30% NH4OH(aq):H2O (SC-1) at 80 °C for at least 10 min, rinsed in ultrapure water, then immersed for at least 10 min in a solution of 1:1:5 by volume of 30% H2O2(aq):38% HCl(aq):H2O (SC-2) at 80 °C. The sample was then cleaned in a fresh SC-1 bath for 10 min at 80 °C, rinsed, and cleaned in a fresh SC-2 bath for 10 min at 80 °C. The oxide layer was removed by a 2 min immersion in a room temperature mixture of 5:1 by volume 50% hydrofluoric acid:40% NH4F(aq) (5:1 Buffered Oxide Etch, J. T. Baker, CMOS grade) and rinsed.
Clark et al. These procedures produced atomically rough, but extremely clean, H-terminated Si(100) surfaces.5 The reported surface morphologies were completely determined by the final step of processingsan etch in room temperature, 40% NH4F(aq) solution (J. T. Baker, CMOS grade) followed by a 15 s H2O (Millipore Milli-Q) rinse. During the etch, the sample was pulled through the air/solution interface every 15 s to remove H2 bubbles generated by the etching reaction. (Similar results were obtained when the bubbles were removed with ultrasonic agitation.) The potentially deleterious effects of bubbles on NH4F(aq) etching of silicon have been recognized for at least 18 years.18 No attempt was made to remove dissolved O2 in the etchant. After etching, the samples were rinsed in ultrapure water and loaded into an ultrahigh vacuum STM or a Fourier-transform infrared (FTIR) spectrometer for further analysis. Vibrational spectra were obtained with a dry-air-purged Nicolet 670 FTIR spectrometer equipped with a mercurycadmium-telluride detector and a ZnSe grid polarizer (Molectron). The infrared radiation underwent ∼75 total internal reflections within the sample. After spectra of the etched sample were obtained with both s- and p-polarized radiation, the sample was oxidized in situ with a 20 min exposure to O3 generated by a Hg pen lamp. Reference spectra were then collected from the oxidized sample with both polarizations of radiation. Interference fringes in the spectra were removed computationally.19 Both the surface morphology and the infrared spectrum of the etched surface were invariant with etch time over the range of 30 s to 45 min. This corresponds to the removal of 45-4000 monolayers of silicon, as determined by gravimetric measurements of the etch rate. 3. Results As shown by the STM images in Figure 3, the atomic-scale morphology of NH4F-etched Si(100) surfaces is characterized by long rows that are ∼0.9 Å in height and preferentially separated by ∼7.7 Åstwice the 3.84 Å that would separate adjacent Si atoms on a bulk-terminated surface. As seen in Figure 3d, the rows display elliptical protrusions spaced by ∼3.8 Å with the long axis of the ellipse being perpendicular to the row direction. The orientations of the long rows and the elliptical protrusions rotate by 90° on adjacent terraces. Over longer length scales, the etched surface is remarkably smooth. For example, 74% of the atoms in the 950 Å × 1000 Å image in Figure 3a are confined to two adjacent atomic planes, as calculated from the areas of the best-fit Gaussian distributions (dotted lines) to the histogram in Figure 3b. This morphology has a root-meansquare surface roughness of 0.57 Å. The observed periodicity is consistent with many different atomic-scale structures. Four possibilities can be generated by removing every other row of Si atoms (one or two atoms deep) from the bulk-terminated surface to generate long rows of dihydrides with the plane of the SiH2 units being either perpendicular or parallel to the long-row direction. (Two-atomdeep missing rows form atomic-height Si(111) microfacets.) According to simulations, dihydrides should appear elongated in STM images, with their long axes being in the dihydride plane.15 This argues against the parallel geometry. Another possibility is the well-known 2 × 1 H/Si(100) surface20 often formed in ultrahigh vacuum experiments by the reaction of dimerized, clean Si(100) with H atoms. The chemical structure of the etched surface was determined from the vibrational spectrum. Raw s- and p-polarized spectra
Atomically Flat Etched Si(100)
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Figure 4. The Si-H stretch region of the infrared absorption spectrum of NH4F-etched Si(100) taken with (a) s-polarized radiation and (b) p-polarized radiation. Other regions of the spectrum were featureless, including the regions corresponding to Si-O and Si-OH vibrations.
Figure 3. (a, c, d) STM images of Si(100) surface etched in room temperature NH4F(aq) show a near-atomically flat morphology. The root-mean-square surface roughness calculated from part a is of 0.57 Å. (b) Height distribution of part a. The dashed lines represent Gaussian fits.
of the etched surface are shown in Figure 4. These spectra are nearly identical with spectra reported by Dumas et al.6 more than 15 years ago. Multiple intense absorption bands were observed in the Si-H stretch region (i.e., 2050-2165 cm-1); no oxide or hydroxyl bands were seen. The spectrum was entirely inconsistent with that of a 2 × 1 H/Si(100) surface;21 this structure was therefore ruled out. As noted by Dumas et al., the raw polarized spectra are difficult to assign uniquely, as the absorption bands are strongly overlapping, and the s- and p-polarized spectra are not dramatically different. On the basis of molecular simulations and the weak polarization of the raw spectra, they assigned the spectral features to a mixture of surface monohydride (∼2085 cm-1), dihydride (2104-2112 cm-1), and trihydride (2025-2040 cm-1) species; however, a simpler explanation is offered below. The vibrational spectrum was significantly simplified using two techniquessone computational and one experimental. In the first, the polarized spectra were computationally transformed from the experimental reference frame, which was defined by
the direction of the surface electric field during irradiation by s- and p-polarized light, into a more appropriate Cartesian reference frame defined by the surface normal and the plane of incidence.22 In essence, this technique measures the Cartesian components of the squared transition dipole momentsµx2, µy2, and µz2 where z is the surface normalsfrom s- and p-polarized spectra taken in two orthogonal propagation directions. If the Cartesian reference frame is aligned with natural high-symmetry directions, this computational transformation can lead to significant spectral simplification (vide infra). Importantly, µz2 is screened by the z-component of the (assumed diagonal) adsorbate dielectric tensor, εz, a typically unknown quantity. As a result, the absolute orientation of the transition dipoles cannot be measured. The Cartesian components of the Si-H stretch region of the infrared spectrum of a nominally flat NH4F-etched Si(100) sample are shown in Figure 5a,b. Macroscopically, the flat Si(100) surface has 4-fold rotational symmetry, as successive terraces are rotated by 90°. Thus, µx2 and µy2 are identical by symmetry and are referred to as µ|2. Five distinct vibrational transitions, labeled by red lines in Figure 5, were resolved. Three transitions were primarily in-plane polarized, whereas the other two were nominally perpendicular transitions. These data improve the previous analysis in two ways. First, they contradict the original assignment of the 2084.9 cm-1 transition to H/Si(111) microfacets, as the observed transition is polarized entirely in the plane of the surface. On a (100) surface, all Si-H bonds are ∼55° from the surface normal, so vibrational transitions associated with monohydride species must have significant perpendicular character. Experimentally, this has been confirmed for both (111) and (110) microfacets on H/Si(100) surfaces.19 Second, a new highly z-polarized transition at 2142.3 cm-1 was resolved. The vibrational spectrum was further simplified by the opportune observation of spontaneous symmetry breaking. When Si(100) surfaces were miscut toward the [011] direction by a few degrees, NH4F etching spontaneously broke the expected macroscopic symmetry of the surface. For example, the STM image of a 3.5° miscut surface in Figure 6 shows that etching preferentially aligns the long rows perpendicular to the miscut direction (x in Figure 6). The mechanism for this alignment is
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Figure 5. The Cartesian components of the squared transition dipole moment in the Si-H stretch region extracted from infrared absorption spectra of NH4F-etched Si(100): (a, b) flat surface and (c, d) surface miscut by 5° toward [011]. The Cartesian axes are defined in Figure 6.
Figure 6. STM image of NH4F-etched Si(100) miscut by 3.5° toward [011] shows the preferential production of one terrace orientation. Assuming complete step doubling, this miscut corresponds to an average terrace width of 44 Å.
under investigation. Although the energetics of step doubling drive a similar transition on clean vicinal Si(100) surfaces,23,24 we have found some HF-based etchants that produce Hterminated surfaces do not induce symmetry breaking. Thus, this is likely a kinetic, not thermodynamic, effect. This symmetry breaking was used to probe the orientation of Si-H bonds in the plane of the surface. The x- and y-components of the infrared spectrum (as defined by the axes in Figure 6) of a 5° miscut NH4F-etched Si(100) sample are shown in Figure 5c,d. [The z-component was nearly identical with Figure 5a and is not reproduced.] As expected, the x- and y-components of the transition dipole moment are very different,
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Figure 7. Ideal structure of NH4F-etched H/Si(100) surface extracted from the analysis of STM images and the vibrational spectrum. This structure is obtained by removing every other row of silicon atoms from the bulk-terminated H/Si(100) surface (Figure 1), thereby reducing interadsorbate strain by approximately one-half. Although not directly observable in experiment, the remaining strained dihydrides are likely canted as predicted by simulations.13,14 The color scheme is described in Figure 1.
confirming the symmetry breaking seen in the STM image in Figure 6. Although the same five transitions were observed in the vicinal spectrum, the 2084.9 and 2112.2 cm-1 transitions were primarily y- and x-polarized, respectively. No entirely new features due to step-absorption modes were observed. Consistent with this, the spectra in Figure 5 obey the relation 2µ|2 ≈ µx2 + µy2. Both the STM images and the vibrational spectra of the etched surfaces are consistent with the atomic-scale geometry sketched in Figure 7. The flat surfaces display an alternating terrace structure, whereas vicinal surfaces are dominated by the “parallel row” terraces. The surface morphology is dominated by singleatom-wide rows of unstrained dihydrides (yellow balls). In agreement with Dumas et al.,6 the transitions at 2103.7 (xpolarized) and 2112.2 cm-1 (z-polarized) were assigned to the antisymmetric and symmetric vibrations, respectively, of unstrained dihydrides. The observed 8.5 cm-1 splitting is in excellent agreement with the 9-11 cm-1 splittings predicted by simulations.25,26 The missing rows were not microfaceted; they were singleatom-deep and terminated by highly strained dihydrides (orange balls) that were rotated by 90° with respect to their unstrained neighbors above. As shown in Figure 7, the strained dihydrides are likely canted to relieve steric interactions between neighboring H atoms, as predicted by simulations.13,14 The measured ∼0.9 Å missing row corrugation is comparable to the ∼1.1 Å corrugation predicted by simulation.14 The broad transitions at 2084.9 (y-polarized) and 2142.3 (z-polarized) cm-1 are assigned to vibrations of the lower and upper Si-H bonds, respectively. Accurate calculations of these modes have proved difficult (see ref 16). The observed ∼60 cm-1 splitting is comparable to the g100 cm-1 splitting predicted for an infinite array of canted dihydrides by recent simulations,15 and the experimental and simulated polarizations are in good agreement. The discrepancy in the splittings may be partially due to the finite length and one-dimensional nature of the strained rows. Importantly, the missing rows cannot be microfaceted (i.e., two atoms deep), as the highly strained monohydrides at the bottom of the microfacets (pink balls in Figure 8) would lead to an x-polarized absorption, not the observed y-polarized transition.
Atomically Flat Etched Si(100)
Figure 8. Structure of an imperfect NH4F-etched Si(100) surface showing the strained monohydride species (dark green balls) that (usually) terminate rows of unstrained dihydrides. Removal of more than one row of silicon at a time would produce {111}-microfaceted structures with highly strained monohydrides (pink), such as the one sketched in the upper left. The pink, highly strained monohydrides can be definitively ruled out based on the polarization dependence of the vibrational spectrum.
Finally, an imperfect surface, such as the one sketched in Figure 8, will invariably include a variety of strained monohydride species (e.g., ends of some unstrained dimer rows, dark green balls). We tentatively assigned the vibration of these atoms (possibly coupled to the adjacent dihydride) to the 2125.7 cm-1 mode (|-polarization, weak z-component) while noting that the frequency of this mode is unusually high for a strained monohydride. Although the modes at 2125.7 and 2142.3 cm-1 were previously assigned to the symmetric and asymmetric vibrations of trihydrides,27 no tall protrusions consistent with trihydrides were observed in the STM images. A definitive assignment of this mode will require accurate molecular simulations of the defective surface, which are under way. 4. Discussion The etching of Si(100) by NH4F is a well-studied system. Why, then, is this the first study to demonstrate the production of near-atomically flat Si(100) surfaces? A number of factorssboth experimental and scientificshindered previous investigations. First, Dumas et al.6 observed vibrational spectra that are nearly identical with those reported here, which suggests that ultraflat Si(100) surfaces may have been produced decades ago. As mentioned earlier, the complexity of the vibrational spectrum hindered the original spectral assignments; however, a more subtle factorsvibrational line widthssalso provided misdirection. Interestingly, the first observation of atomically flat etched silicon surfaces did not involve a scanned probe microscope. Instead, atomic flatness was inferred from the extremely narrow H-Si(111) stretch resonance in the vibrational spectrum of NH4F-etched Si(111) surfaces.2 At room temperature, the line width of this resonance is 0.9 cm-1, decreasing to 0.1 cm-1 at 130 K.28 The narrow spectral line width implies low heterogeneous broadening and thus the production of an extremely homogeneous etch morphology. On the basis of this observation, the (reasonable) expectation was that atomically, or near-
J. Phys. Chem. C, Vol. 114, No. 1, 2010 427 atomically, flat H/Si(100) surfaces would be characterized by similarly narrow vibrational resonances. In hindsight, this reasoning is overly simplistic. On the etched H/Si(100) surfaces studied here, a point defect (e.g., a missing atom) in a row of strained dihydrides will presumably lead to (partial) local relaxation of the canting and thus local frequency shifts in the vibrational resonances. We hypothesize that point inhomogeneities of this type lead to the relatively broad line widths observed on NH4F-etched Si(100) surfaces. The second complication is experimental. The etching reaction evolves significant quantities of H2. If left undisturbed, the etch morphology is significantly perturbed by H2 bubbles that nucleate and grow on the etching surface. Indeed, samples etched for tens of minutes in a quiescent solution are rough to the naked eye. The bubble-induced roughening mechanism is complex and will be discussed in a later publication. Importantly, near-atomically flat surfaces are only produced when bubble formation is suppressed, in our case by drawing the surface through the air-solution interface every 15 s or by ultrasonic agitation. Finally, silicon etchants are highly susceptible to contamination, particularly by dissolved metals. In some cases, contaminants affect the etching surface directly (e.g., by nucleating etch pits). In other cases, they collect at the air-solution interface and are deposited on the surface as the etched sample is withdrawn from solution. As a result, semiconductor-grade chemicals, ultrapure water, and strict attention to cleaning procedures play a crucial, but often unappreciated, role in the production of chemically and morphologically controlled silicon surfaces. In addition to its intrinsic value, the etch morphology also provides insight into the mechanism of NH4F(aq) etching. First, the preferential production of long chains of unstrained dihydrides implies that the strained monohydride-dihydride unit at the end of the chain is significantly more reactive than the unstrained dihydrides in the middle. Indeed, kinetic Monte Carlo simulations suggest that this is the fastest etching site on the surface. Interestingly, this site has the same geometry as the fastest etching (kink) site on H/Si(111)29 in spite of the differences in strain on the two surfaces. This observation underscores the close parallels between the chemistry of the two surfaces. Second, the high concentration of strained dihydrides implies that these sites are relatively unreactive. (If they were highly reactive, they would have been preferentially etched to form opposing strained monohydrides in the layer below.) Although strain often weakens bonds and accelerates chemical reactions, the relative unreactivity of the strained dihydride suggests that steric hindrance during the etching reaction plays an important role. Third, the pronounced (although not perfect) periodicity of the missing rows shows that strain has significant long-range effects on chemical reactivity. The range of these effects is under investigation using kinetic Monte Carlo simulations. Perhaps most importantly, the etch morphology invalidates previous hypotheses regarding the steady-state etch morphology. In analogy with equilibrium crystal shapes, which minimize surface free energy, researchers have long sought a simple predictor of etch morphology. As discussed earlier, the common wisdom that Si(100) etching acts to minimize surface strain is overly simplistic. Although the steady-state morphology has approximately one-half as many strained sites as the bulkterminated surface, the formation of {111}-terminated (or {110}terminated) microfacets would further lower strain. Similarly, etching does not produce the surface of minimal etch rate, which
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would also be a {111}-microfaceted surface. Instead, the final morphology appears to be determined by the site-specific rates of etching of surface speciessrates that are sensitive to, but not solely determined by, interadsorbate stress. 5. Conclusions Near-atomically flat, H-terminated Si(100) surfaces were produced by etching in a room temperature, 40% NH4F(aq) solution, but only when care was taken to prevent evolved H2 bubbles from collecting on the etching surface. On the atomic scale, the etched surface was dominated by a “missing row” motif that consisted of alternating rows of unstrained and highly strained silicon dihydrides. These two dihydride structures had characteristic vibrational resonances (two each) that were assigned on the basis of polarized infrared absorption spectroscopy. Although the etched surface was nearly flat, the vibrational resonances were significantly heterogeneously broadened. This broadening was attributed to point defects which give rise to local variations in interadsorbate strain. Although the etched surface had approximately one-half as many strained sites as the (predicted) bulk-terminated H/Si(100) morphology, microfaceted surfaces would have much lower strain. Acknowledgment. This work was supported by the National Science Foundation (NSF) under Award No. CHE-0515436 and the Cornell Center for Nanoscale Systems under Award No. EEC-0646547 and made use of the Cornell Center for Materials Research Shared Experimental Facilities which are supported through the NSF MRSEC Program (Grant DMR-0520404). References and Notes (1) Ohmi, T.; Kotani, K.; Teramoto, A.; Miyashita, M. IEEE Electron DeVice Lett. 1991, 12, 652–654. (2) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656–658. (3) Allongue, P. Phys. ReV. Lett. 1996, 77, 1986–1989. (4) Garcia, S. P.; Bao, H.; Hines, M. A. Phys. ReV. Lett. 2004, 93, 166102.
Clark et al. (5) Chabal, Y. J.; Higashi, G. S.; Small, R. J. Handbook of Semiconductor Wafer Cleaning Technology; William Andrew, Inc.: Norwich, NY, 2008; pp 515-606. (6) Dumas, P.; Chabal, Y. J.; Jakob, P. Surf. Sci. 1992, 269/270, 867– 878. (7) Gra¨f, D.; Bauer-Mayer, S.; Schnegg, A. J. Vac. Sci. Technol., A 1993, 11, 940–944. (8) Bjorkman, C. H.; Fukuda, M.; Yamazaki, T.; Miyazaki, S.; Hirose, M. Jpn. J. Appl. Phys. 1995, 34, 722–726. (9) Niwano, M.; Takeda, Y.; Ishibashi, Y.; Kurita, K.; Miyamoto, N. J. Appl. Phys. 1992, 71, 5646–5649. (10) Nakamura, M.; Song, M.-B.; Ito, M. Electrochim. Acta 1996, 41, 681–686. (11) Le Thanh, V.; Bouchier, D.; Hincelin, G. J. Appl. Phys. 2000, 87, 3700–3706. (12) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. Phys. ReV. Lett. 1990, 65, 504–507. (13) Ciraci, S.; Batra, I. P. Surf. Sci. 1986, 178, 80–89. (14) Northrup, J. E. Phys. ReV. B 1991, 44, 1419–1422. (15) Endo, K.; Arima, K.; Hirose, K.; Kataoka, t.; Mori, Y. J. Appl. Phys. 2002, 91 (7), 4065–4072. (16) Freking, U.; Kru¨ger, P.; Mazur, A.; Pollmann, J. Phys. ReV. B 2004, 69, 035315. (17) Arima, K.; Katoh, J.; Horie, S.; Endo, K.; Ono, T.; Sugawa, S.; Akahori, H.; Teramoto, A.; Ohmi, T. J. Appl. Phys. 2005, 98, 103525. (18) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897–2909. (19) Faggin, M. F.; Hines, M. A. ReV. Sci. Instrum. 2004, 75, 4547– 4553. (20) Boland, J. J. Phys. ReV. Lett. 1991, 67, 1539–1542. (21) Chabal, Y. J.; Raghavachari, K. Phys. ReV. Lett. 1984, 53, 282– 285. (22) Clark, I. T.; Aldinger, B. S.; Gupta, A.; Hines, M. A. J. Chem. Phys. 2008, 128, 144711. (23) Wierenga, P. E.; Kubby, J. A.; Griffith, J. E. Phys. ReV. Lett. 1987, 59, 2169–2172. (24) Poon, T. W.; Yip, S.; Ho, P. S.; Abraham, F. F. Phys. ReV. B 1992, 45, 3521–3531. (25) Chabal, Y. J.; Raghavachari, K. Phys. ReV. Lett. 1985, 54, 1055– 1058. (26) Weldon, M. K.; Queeney, K. T.; Gurevich, A. B.; Stefanov, B. B.; Chabal, Y. J.; Raghavachari, K. J. Chem. Phys. 2000, 113 (6), 2440–2446. (27) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Burrows, V. A. J. Vac. Sci. Technol. A 1989, 7, 2104–2109. (28) Dumas, P.; Chabal, Y. J.; Higashi, G. S. Phys. ReV. Lett. 1990, 65, 1124–1127. (29) Hines, M. A. Annu. ReV. Phys. Chem. 2003, 54, 29–56.
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