Electronic Structure and Interface Formation during Nickel Deposition

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J. Phys. Chem. B 2000, 104, 9647-9652

9647

Electronic Structure and Interface Formation during Nickel Deposition on Polycrystalline Aluminum C. Palacio* and A. Arranz Departamento de Fı´sica Aplicada, Facultad de Ciencias, C-XII, UniVersidad Auto´ noma de Madrid, Cantoblanco, 28049-Madrid, Spain ReceiVed: March 23, 2000; In Final Form: July 31, 2000

The deposition of nickel on polycrystalline aluminum substrates has been studied at room temperature by X-ray photoelectron spectroscopy (XPS), angle-resolved X-ray photoelectron spectroscopy (ARXPS), and valence band X-ray photoelectron spectroscopy (VBXPS). The growth of the nickel on the aluminum surface has been found to occur in two stages: Formation of NiAlx (x ≈ 0.45) islands 10 ML thick up to a coverage θNiAlx ) 0.7, followed by the formation of metallic nickel islands with constant thickness of 8 ML that grow over the intermetallic islands previously formed. The simultaneous lateral growth of both kinds of islands is also observed during the second stage. During the first stage, the VBXPS spectra suggest a net charge transfer from nickel to aluminum with the sequential formation of NiAl3, Ni2Al3, NiAl, and Ni3Al phases.

Introduction The characterization of nickel ultrathin films deposited on different supports has attracted considerable attention in recent years, as a consequence of the magnetic and catalytic properties of these systems.1-10 The morphology and composition at the Ni/support interface have been found to be determinants of the electronic structure and therefore of the physical and chemical properties of these films.1 The Ni-Al alloys are widely used for applications as hightemperature materials7-18 and as metallization layers and diffusion barriers in microelectronics.19,20 In these applications, the optimal performance of the material is strongly correlated with the properties of the Ni-Al interface. Therefore, the study of the Ni-Al interface is important in order to understand and control the properties of the Ni-Al alloys. In a previous work,21 we studied the formation of the Ni-Al interface during the deposition of nickel on polycrystalline aluminum substrates at room temperature using Auger electron spectroscopy combined with factor analysis (AES-FA) and electron energy loss spectroscopy (EELS). The formation of the interface was found to occur in two stages. A first stage was characterized by the growth of a nickel aluminide compound (NiAlx), followed by the formation of metallic nickel. Likewise, several workers have also studied Ni deposition on different monocrystalline Al substrates and polycrystalline evaporated ultrathin Al films.7-10 Ruckman et al.9 have found the formation of a NiAl3 intermetallic compound at room temperature, during the first stages of Ni deposition on polycrystalline evaporated Al thin films. Shutthanandan et al.,7 using high-energy ion scattering (HEIS) and X-ray photoelectron spectroscopy (XPS), found the initial formation of a NiAl phase followed by a Ni3Al phase, suggesting the formation of thick Ni-Al islands. Hanamoto et al.10 have also proposed the formation of Ni islands during Ni deposition on ultrathin evaporated Al films. However, neither they nor Lu et al.8 found the alloy formation at the interface during Ni deposition on Al. Therefore, published results * Corresponding author: fax ++34 91 3974949; e-mail carlos.palacio@ uam.e.

show a considerable amount of disagreement related to the morphology and composition at the Ni-Al interface. In addition and surprisingly enough, no attempt at using angle-resolved X-ray photoelectron spectroscopy (ARXPS) to carry out a quantitative approach to the study of the Ni-Al interface during Ni deposition on Al substrates has been described to our knowledge. The aim of this work is to use XPS, ARXPS, and valence band X-ray photoelectron spectroscopy (VBXPS) to characterize the growth mode and electronic structure of thin Ni films deposited on polycrystalline aluminum substrates at room temperature. Experimental Section The experiments were performed in an ultrahigh vacuum (UHV) system at a base pressure better than 8 × 10-10 torr. High-purity polycrystalline aluminum substrates (nominal composition 99.994% Al, 0.004% Si, 0.001% Fe, and 0.001% Cu) manufactured by Toyo Aluminum Co., Japan, were used throughout this work. They were degreased by successive boiling in carbon tetrachloride, acetone, and ethanol. Then the sample was introduced into the UHV chamber. The aluminum substrates were sputter-cleaned in situ with 3 keV Ar+ until no impurities were detected by AES. Nickel was deposited by sublimation of a directly heated filament of 99.98% purity onto the Al substrates at room temperature. The XPS spectra were measured on a hemispherical analyzer (SPECS EA-10 Plus). The pass energy was 15 eV, giving a constant resolution of 0.9 eV. The Ag 3d5/2 line was used to calibrate the sample binding energies. A twin anode (Mg and Al) X-ray source was operated at a constant power of 300 W, with Mg KR (1253.4 eV) radiation. The sample was placed in a sample stage with four degrees of freedom in such a way that the takeoff angle can be varied between 0 and 70° in order to perform angle-resolved measurements.

10.1021/jp001097e CCC: $19.00 © 2000 American Chemical Society Published on Web 09/21/2000

9648 J. Phys. Chem. B, Vol. 104, No. 41, 2000

Palacio and Arranz TABLE 1: Parameters of the Synthetic Bands Used for Deconvolution of XPS Spectra of Figure 1a band Ni0 3p3/2 3p1/2 satellite NiAl 3p3/2 3p1/2 satellite Al0 AlNi Al(2p) satellites

Figure 1. XPS spectra measured during nickel deposition on aluminum. (a) Al 2p and Ni 3p bands; (b) Ni 2p band.

Results Figure 1 shows (a) the Al 2p and Ni 3p and (b) Ni 2p XPS spectra measured for a takeoff angle φ ) 0° and for different nickel deposition times, after background subtraction based on the Shirley method.22 The spectrum labeled as Al0 is representative of the clean sputtered Al substrate. The X-ray source is not monochromatic, and therefore the KR3 and KR4 satellites are also observed on the low binding energy side of Figure 1a. As the Ni dose increases, an attenuation and shift to lower binding energies of the Al 2p band are observed. In addition to that, the Al 2p band becomes more symmetrical. The spectrum labeled as Ni0 in Figure 1a is representative of a clean sputtered nickel substrate. It is characterized by a broad band centered at ∼65.7 eV. Both, the Al 2p band and Ni 3p band overlap in this region. As the deposition proceeds, the Ni 3p band increases and slowly shifts to reach the binding energy of the metallic Ni 3p band. To determine the bands associated with the different Al and Ni species, a peak deconvolution with synthetic spectra and a least-squares optimization has been carried out. For the metallic Al 2p band the optimal synthetic peak is an asymmetrical Gaussian-Lorentzian function (GL) centered at 72.7 eV and 1 eV fwhm. To reproduce the Al 2p band for the deposited substrates, it is necessary to introduce a new band (AlNi) on the low binding energy side of the Al 2p band, which can be associated with the formation of an intermetallic compound NiAlx, in good agreement with our previous results.21 For this aluminum species the optimal synthetic peak is a symmetrical GL function centered at 72.3 eV and 1 eV fwhm. For the KR3 and KR4 satellites, two symmetrical GL functions centered at

Eo (eV)

2p 2p KR3 KR4

fwhm (eV)

65.7 67.4 69.8

2.3 2.3 3.5

66.7 ( 0.05 68.4 ( 0.05 69.4 ( 0.10 72.7 72.3 64.3 62.5

2.3 2.3 2.7 1 1 1.1 1.1

64.3 and 62.5 eV, respectively, and with 1.1 eV fwhm have been used. On the other hand, for the metallic nickel Ni0, the optimal synthetic band is the sum of three symmetrical GL functions. Two of them correspond to the Ni 3p3/2,1/2 doublet, and the third one corresponds to the characteristic satellite also observed in other nickel bands.23 To reproduce the Ni 3p band during the first stages of deposition, it is necessary to introduce a new band, NiAl, on the low binding energy side, associated with the formation of the nickel aluminide. For this additional nickel state, the optimal synthetic band was assumed to be composed of three symmetrical GL functions, two of them for the NiAl 3p doublet and the other one for the satellite. Table 1 shows the parameters defining the synthetic bands Al0, AlNi, Ni0, NiAl, and Al satellites. The AlNi 2p and NiAl 3p binding energies (E0), the Ni 3p spin-orbit splitting (sos), and the Ni 3p3/2-satellite separation are in good agreement with the values observed in the literature for several Ni-Al bulk compounds.16,24-26 The spectrum labeled as Ni0 in Figure 1b is characterized by the Ni 2p3/2,1/2 doublet with peaks at 852.4 and 869.7 eV respectively. The associated satellites corresponding to the final state c-13d94s2 at 858.3 and 874.2 eV, respectively, are also observed.23,26 The splitting between the main line and the satellite is in good agreement with the value measured by Hillebrecht et al.26 for metallic nickel. As the deposition proceeds, an increase of the Ni 2p band asymmetry and a shift of ∼1 eV to lower binding energies of this band are observed. In addition, the shape of the satellites changes, and the splitting between the Ni 2p3/2 band and the associated satellite decreases from 8.2 eV, in the first stages of deposition, to 5.8 eV in clean Ni. According to our previous AES-FA results on the Ni/Al system,21 for Ni deposition times below 9 min, there is not metallic nickel (Ni0) on the surface and therefore the measured Ni 2p band should be attributed to the intermetallic compound NiAlx. Figure 2 shows the evolution of Al 2p and Ni 2p XPS signals, for a takeoff angle φ ) 0°, as a function of Ni deposition time. The intensities, I (peak area), are normalized to the corresponding sensitivity factors, SAl ) 0.11 and SNi ) 5.4.27 The standards used for the deconvolution of the Ni 2p spectra were the spectrum labeled as Ni0 in Figure 1b and that measured for a deposition time of 9 min (NiAlx). The application of factor analysis (FA)28-30 to the spectra of Figure 1, panels a and b, for t g 9 min, gives three and two principal factors, respectively. The application of the target testing transformation,28,29 using as target spectra the synthetic bands Al0, Ni0, and AlNi + NiAl for Al 2p and Ni 3p, and the standard spectra Ni0 and NiAlx for Ni 2p, gives results analogous to those of Figure 2. In addition, a good agreement between the target spectra and their reproduction after target testing transformation is obtained. This supports

Nickel Deposition on Polycrystalline Aluminum

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9649

Figure 2. XPS intensities for the different Al and Ni chemical species as a function of deposition time.

Figure 4. Variation of NiAl/Al0 XPS peak area ratio as a function of the takeoff angle for different deposition times up to 9 min. Filled circles represent experimental data. The different curves were calculated from the models explained in the text.

Figure 3. Valence band XPS spectra measured during nickel deposition on aluminum.

the validity of the chosen target spectra. As can be observed in Figure 2, the AlNi and NiAl 2p intensities reach a maximum at 9 min. Above 9 min, the evolution of the Ni0 2p band exhibits a sigmoidal shape and continuously increases whereas the Al0 signal continuously decreases. This suggests that the reaction of the Ni with the Al surface occurs in two stages: formation of an intermetallic compound NiAlx up to ∼9 min of nickel deposition, followed by the formation of metallic nickel islands on top of the intermetallic compound. The calculated nickel atomic concentration at the end of the first stage (t ) 9 min) is found to be around 69%, i.e., NiAlx with x ≈ 0.45, in good agreement with the average composition obtained previously from AES results,21 which according to the phase diagram of the Ni-Al system31 could be attributed to a mixture of Ni3Al and NiAl phases. Figure 3 shows the evolution of the VBXPS spectra, for a takeoff angle φ ) 70°, as a function of Ni deposition time. Metallic nickel (Ni0) shows a narrow VB mainly composed of 3d states with its maximum at ∼0.5 eV below the Fermi level (EF), and a characteristic associated satellite at ∼6 eV below EF.23,32 Considering the high inelastic mean free path of the VBXPS photoelectrons, a part of the signal of the VB spectrum should be attributed to the Al substrate (Al0). However, for the Ni/Al interface, the influence of the Al substrate in the VB spectra of Figure 3 can be neglected due to the high density of states (DOS) of the Ni deposited layer. For small Ni deposition times, the VB has its maximum at ∼2.2 eV and no satellite is observed. As the deposition time increases, this maximum shifts to lower binding energies, and an increase of the BV asymmetry and the appearance of the nickel satellite are observed. More detailed information on the structure of the deposited film can be obtained from ARXPS. Figure 4 shows the NiAl/ Al0 peak area ratio as a function of the takeoff angle for different

Figure 5. Variation of Ni0/Al0 XPS area ratio as a function of the takeoff angle for different deposition times during the second stage of growth. Filled circles represent experimental data. The continuous lines were calculated from the model explained in the text.

deposition times up to 9 min. Figure 5 shows the Ni0/Al0 peak area ratio as a function of the takeoff angle for deposition times above 9 min. Results of Figures 4 and 5 are consistent with the two stage model for nickel growth proposed above. As will be discussed later, the first stage is characterized by the formation of NiAlx islands of ∼10 ML constant thickness (1 ML ) 2.2 Å) up to a coverage θNiAlx ≈ 0.7, and the second stage is characterized by the formation of Ni0 islands that grow over the NiAlx previously formed. A schematic diagram of the model proposed for the Ni-Al interface formation is given in Figure 6. Discussion Several groups have studied the Ni growth on different Al substrates in recent years. Ruckman et al.,9 using AES and UPS, have found the formation of a NiAl3 intermetallic compound at room temperature, during the first stages of nickel deposition on polycrystalline evaporated Al thin films. Shutthanandan et al.,7 using HEIS and XPS, found the formation of a NiAl phase up to 2.3 ML and a Ni3Al phase between 2.3 and 8.1 ML, and they suggest the formation of thick Ni-Al islands on the Al-

9650 J. Phys. Chem. B, Vol. 104, No. 41, 2000

Figure 6. Schematic model of the Ni-Al interface formation during Ni deposition on polycrystalline Al substrates. (a) First stage of growth; (b) second stage of growth. The first stage is characterized by the formation of NiAlx islands of 10 ML thickness up to a coverage θNiAlx ≈ 0.7. The second stage is characterized by the formation and growth of pure nickel islands over the NiAlx islands previously formed.

(110) surface during the first stages of Ni deposition. Hanamoto et al.10 have also proposed the formation of Ni islands during Ni deposition on ultrathin evaporated Al films. However, neither they nor Lu et al.8 found the alloy formation at the interface during Ni deposition on Al. In present work, the results of Figures 1-3 show the formation of a Ni-Al compound during the first stages of Ni deposition. Furthermore, the observed changes in the Al 2p, Ni 3p, Ni 2p, and VB spectra of Figures 1 and 3 should be attributed to a charge-transfer process from nickel to aluminum during the formation of the NiAlx intermetallic compound, in good agreement with the changes observed in the Al LVV, Ni LVV, and Ni MVV Auger transitions.21 The XPS results of Figure 1b show marked changes in the Ni 2p band as the Ni deposition proceeds. For Ni deposition times below 9 min, the Ni 2p3/2 core level exhibit a chemical shift, in relation to the metallic Ni 2p3/2 core level, that amounts to ∼1 eV. This shift as well as the shape and position of the associated Ni 2p3/2 satellite suggest the formation of the NiAl3 phase during the first stages of deposition, in agreement with the results of Hillebrecht et al.26 for bulk Ni-Al alloys. As the nickel dose increases, the shift of the Ni 2p3/2 band decreases, indicating a nickel enrichment of the formed intermetallic compound. According to Hillebrecht et al.,26 these changes should be attributed to the sequential formation of NiAl3, Ni2Al3, NiAl, and Ni3Al phases. As will be discussed later, this sequence is followed by metallic nickel growth on the surface. However, it is necessary to point out that there is not a general agreement on the alloy phases formed during Ni deposition on Al. Ruckman et al.9 reported the growth of NiAl3 during the formation of the Ni/Al interface at room temperature, in good agreement with other studies carried out at high temperature.33-35 Shutthanandan et al.7 have observed the formation of NiAl followed by Ni3Al during the first stages of Ni deposition over Al(110), but they did not observe the formation of NiAl3. Additional information on the electronic structure of the interface can be obtained by analyzing VB spectra. As it can be observed in Figure 3, the VB shifts to lower binding energies with increasing deposition time up to 9 min, indicating the nickel enrichment of the alloy in agreement with Fuggle et al.32 According to theoretical calculations,24,32 the opposite shift of the Ni 3d band to higher binding energies (BE) is a consequence of the filling of the Ni 3d band as the Al content increases, and

Palacio and Arranz it is related to the decrease of the density of states at EF. The filling of the Ni 3d band could be interpreted to imply that electrons are transferred from Al to Ni. However, the observed shifts of the Ni 2p and Ni 3p bands to higher binding energies, and that of the Al 2p band to lower binding energies, are consistent with a net charge transfer from nickel to aluminum.24 For lower Ni doses, the maximum of the VB is located at ∼2.2 eV, which should be attributed to the initial formation of NiAl3.32 With increasing Ni deposition time up to 9 min, the VB broadens and shifts to ∼1.4 eV. The broadening and the shift are consistent with the formation of the phases Ni2Al3, NiAl, and Ni3Al during the first stage of Ni deposition.32 Above 9 min, the VB broadens considerably and shifts to ∼0.5 eV as a consequence of the metallic nickel formation during the second stage of growth. As the deposition proceeds, a satellite shoulder at ∼6 eV, attributed to d-like final states, becomes noticeable. As can be observed in Figures 1b and 3, the asymmetry of the Ni 2p band and VBXPS bands increases as the deposition proceeds, as a consequence of the increase of the DOS at EF.36 Likewise, the intensity and the relative position of the satellites associated with the Ni 2p band depend on the 3d unoccupied states above EF,26 in such a way that the filling of the Ni 3d band as the Al content increases, goes along with changes in the intensity and position of the Ni 2p satellites. A further attempt was made to characterize the composition and structure of the interface by using the ARXPS intensity data of Figures 4 and 5, which correspond to the first and second stage of nickel growth on the Al surface, respectively. To establish the correct growth mechanism during the first stage, the experimental data of Figure 4 were compared to three plausible models previously developed to study Fe growth on Al:37 (i) uniform NiAlx layer growth of thickness dNiAlx on the metallic Al substrate, (ii) NiAlx island growth of constant thickness DNiAlx and coverage θNiAlx, and (iii) NiAlx monolayer growth followed by NiAlx island growth (mixed model). Since the equations of the models were developed elsewhere,37 they will not be repeated here. In these models, the I(NiNiAlx)/I(Al0) XPS peak area ratio depends on dNiAlx, DNiAlx, θNiAlx, and C ) I∞(NiNiAlx)/I∞(Al0) which are used as fitting parameters to fit the experimental data to the equations of the model. To determine these parameters, the following values were assumed for the attenuation lengths (AL) of Ni 2p and Al 2p photoelectrons in NiAlx: λNiNiAlx ) 9 Å and λAlNiAlx ) 15.5 Å.38 For the uniform layer model the only free parameter is dNiAlx. For the other models, the only free parameter was θNiAlx. DNiAlx and C were calculated by a trial-and-error procedure in such a way that the total root-mean-square deviation (rms) for all curves of Figure 4 is a minimum. The calculated values were DNiAlx ) 10 ML and DNiAlx ) 9 ML for the island growth model and for the mixed model, respectively, and C ≈ 24. This last value is of the same order of magnitude as the value C ≈ 18, obtained from

C)

I∞(NiNiAlx) I∞(Al0)

)

NNiNiAlxλNiNiAlxσNi(2p) NAlAlλAlAlσAl(2p)

(1)

assuming λAlAl ) 18 Å,38 NNiNiAlx ) 0.095 mol/cm3, NAlAl ) 0.1 mol/cm3, and σNi(2p)/σAl(2p) ≈ 37.39 Here N stand for bulk atomic densities and σ stand for photoionization cross sections. Such a type of deviations is expected, owing not only to the lack of knowledge of the surface atomic density for the intermetallic compound but also to the influence of matrix effects on the photoionization cross-section calculations which can modify the tabulated values.39,40 It should be indicated that

Nickel Deposition on Polycrystalline Aluminum

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9651

TABLE 2: Best-Fit Parameters for the First Stage of Ni Growth onto Al t (min)

uniform model dNiAlx (Å)

1 2 3 4 5.5 7 8 9

0.7 ( 0.1 1.5 ( 0.1 2.1 ( 0.2 3.5 ( 0.3 4.1 ( 0.4 5.8 ( 0.5 7.1 ( 0.6 8.0 ( 0.7

TABLE 3: Best-Fit Parameters for the Second Stage of Ni Growth onto Al

mixed model DNiAlx ≈ 9 ML θNiAlx

island model DNiAlx ≈ 10 ML θNiAlx

0.19 ( 0.01 0.25 ( 0.01 0.42 ( 0.02 0.51 ( 0.03 0.58 ( 0.03

0.12 ( 0.01 0.22 ( 0.01 0.28 ( 0.01 0.41 ( 0.01 0.46 ( 0.01 0.58 ( 0.01 0.64 ( 0.01 0.70 ( 0.01

despite the variation of the NiAlx composition during the first stage of Ni deposition, C was kept constant for all curves of Figure 4, in order to simplify the proposed models. The best fit is represented by the solid lines (island growth), the dashed lines (mixed model), and the dotted lines (uniform layer growth) in Figure 4. The parameters leading to the best fit of the curves are shown in Table 2. Results of Figure 4 clearly show that the appropriate model to characterize the first stage of nickel growth on aluminum is that of island growth. According to this model, if the sticking coefficient is assumed to be equal to unity for t ) 0, the calculated nickel deposition rate is about ∼1.5 × 1015 atoms‚cm-2‚min-1. It is interesting to indicate that according to Monte Carlo simulations and HEIS measurements of Shutthanandan et al.7 the formation mechanism of NiAlx should be that of Ni atoms diffusing into the Al surface and displacing the Al atoms from the initial positions. For the second stage of the kinetics of growth (t > 9min), XPS results of Figure 2 are consistent with the formation and growth of pure nickel islands. The model, which describes the lateral growth of nickel islands on an Al substrate covered with NiAlx islands, is given by 0

I(Ni ) I

(Al0)

[ ( )] [ ( ) (

) kθNi 1 - exp θNi) exp

-DNi / (1 - θNiAlx) + (θNiAlx λNiNi cos φ

) )]

-DNiAlx -DNiAlx + θ exp Ni λAlNiAlx cos φ λAlNiAlx cos φ exp

(

-DNi λAlNi cos φ

(2)

I∞(Ni0) I∞(Al0)

)

NNiNiλNiNiσNi(2p) NAlAlλAlAlσAl(2p)

t (min)

θNiAlx

θNi

11 12 15

0.75 ( 0.05 0.82 ( 0.01 0.85 ( 0.01

0.29 ( 0.04 0.34 ( 0.01 0.56 ( 0.02

and (ii) θNi e θNiAlx which limits the nickel nucleation sites to NiAlx islands. As can be observed in Figure 5, the fit is very good. Hanamoto et al.10 have also found the formation of Ni islands during Ni deposition on ultrathin evaporated Al films. However, they did not detect the alloy formation at the interface. Although Shutthanandan et al.7 have suggested the formation of thick islands of NiAl and Ni3Al alloys on the Al surface during Ni deposition on Al(110), no model for island growth of the intermetallic compounds was proposed by these authors. In this work, a model for Ni growth on polycrystalline Al substrates, which involves the formation of NiAlx islands during the first stages of Ni deposition, is proposed. Conclusions The formation of the Ni-Al interface during Ni deposition on polycrystalline Al substrates has been studied at room temperature by means of XPS, ARXPS, and VBXPS. With increasing nickel dose two stages can be distinguished. The first stage is characterized by the formation of NiAlx (x ≈ 0.45) islands 10 ML thick that reach a maximum coverage θNiAlx ) 0.7 before the formation of metallic nickel on the surface. The second stage is characterized by the formation of metallic nickel islands 8 ML thick, which grow over the NiAlx islands previously formed. The lateral growth of both kinds of islands has been observed during this stage. The formation of an intermetallic compound during the first stage involves a net charge-transfer process from nickel to aluminum, as well as the sequential formation of NiAl3, Ni2Al3, NiAl, and Ni3Al phases. Likewise, a decrease in the DOS near EF, a loss in the asymmetry of the Ni 2p and VB bands, and changes in the intensity and relative position of the Ni 2p satellites, in comparison with the metallic nickel spectra, are observed for the intermetallic compound. Acknowledgment. We thank D. Dı´az for technical assistance. This work was financially supported by the Spanish Comisio´n Interministerial de Ciencia y Tecnologı´a (Project MAT94-0662).

where K can be approximated by eq 3.

K)

island model DNi ) 8 ML

(3)

Here λNiNi and λAlNi are the ALs in pure nickel of Ni 2p and Al 2p photoelectrons respectively, and NNiNi is the nickel atomic density of pure nickel. Equation 2 has been used to fit the experimental data of Figure 5. To have only two free parameters, θNi and θNiAlx, the parameters DNi and K were calculated by the trial and error procedure described above. The calculated values were DNi ) 8 ML (1 ML ) 2.2 Å) and K ≈ 27. This last parameter is in good agreement with the value obtained from eq 3 assuming λNiNi ) 8.6 Å, λAlNi ) 14.7 Å, and NNiNi ) 0.15 mol/cm3. The best fit is represented by the solid lines in Figure 5 and the parameters leading to the best fit of the curves are shown in Table 3. It is necessary to point out that two constraints were imposed in order to improve the fitting: (i) θNiAlx g 0.7 on account of the coverage reached at the end of the first stage

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