The initial stages of deposition of Au and Ag on Cu(001) studied by low-energy ion scattering

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Surface Science 287/288 (1993) 974-978 North-Holland

surface science

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The initial stages of deposition of Au and Ag on Cu(OO1) studied by low-energy ion scattering S.

Nakanishi, K. Kawamoto and IS. Umezawa

Department

of Physics, University of Osaka Pr~f’rcture, Gnknen-Cho,

Sakai 593, Osaka, Japan

Received 4 September 1992; accepted for publication 17 November 1992

Using impact collision ion scattering spectroscopy. we have studied the initial stage of the deposition of Au and Ag onto the CufOO11surface. Our analysis provides evidencefor surface alloying by the substitution of Cu with Au atoms in the surface, even at very low coverages at room temperature. However, the diffusion of Au atoms into the Cu substrate was not observed at coverages below 0.25 ML. The growth of an hexagonal ~(2 X 14)-Au layer on top of the first ~(2 X 2) structure was found to promote the diffusion of Au atoms into the Cu substrate, destroying the ordered arrangement of the ~(2 x 2) structure. Ag was confirmed to form a simple over-layer up to 1 ML without any diffusion of Ag atoms into the substrate. The growth mode of the Ag deposition was identified to be of the Stranski-Krastanov type.

1. Introduction

Studies of the initial stage of metal thin~film deposition onto metal substrates are of great importance for understanding the mechanism of heteroepitaxy and the formation of superlattices in metal-metal systems. Among others, the Au/Cu(OOl) [l-41 and Ag/Cu(OOl) [1,.5,61 systems are of great interest due to the formation of the hexagonal overlayer structure which differs from the substrate in 2D-crystallographic syrnmetry. Also, these two systems are extremely different in their miscibility. Although much work has been reported on these systems, there are few works on real-space analysis of the overlayer structure or of the interdiffusion process. Impact collision ion scattering spectrometty (KISS) is a powerful tool for such real space analysis. We report detailed properties of the initial growth of Au and Ag on the Cu(OO1) surface using ICISS.

2. Experimental The experiments were carried out in an ultrahigh vacuum chamber (1O-8 Pal equipped with 0039-602S/93/$06.~0

standard LEED optics, an Auger electron spectrometer (CMA) and coaxial ICISS. The ICISS equipment is of the time-of-flight version with a flight length of about 40 cm, capable of detecting both ions and neutrals scattered from the sample surface with a scattering angle of 180”. Pulsed 3 keV Ne’ ion beams were used as an incidence probe. A disk-shaped Cu(OO1) sample of large area (10 mm diameter) was cut out of a singlecrystal rod (99.99% pure) and the orientation of the surface was confirmed by Laue back-scattering to be less than 1” with respect to the (001) crystallographic plane. A mirror finish of the sample surface was obtained by a combination of mechanical polishing with fine alumina powder and a chemical etch in acid solution. An atomicaIly clean surface was prepared by conventional Ar (or Ne) ion sputtering followed by repeated cycles of sample heating and cooling. Prior to the measurements it was verified that the LEED pattern was (1 x 1) and that the surface was free of impurities as judged by AES. Pure Au and hg (99.999% for both Au and Ag) were vapor-deposited onto the Cu(OO1) surface by using a miniceramic effusion cell surrounded by a W coil filament. The rate of deposition was typically

@ 1993 - EIsevier Science Publishers B.V. Ah rights reserved

S. Nakanishi et al. /Au

about 0.1 ML/min, as measured oscillator film-thickness monitor.

and Ag deposition on Cu(OO1)

975

by a quartz Ne+(3keV) -> Cu(OO1) azimuth [OIOI

3. Results and discussion 3.1. Au / Cu(OO1) We first examined the change of the LEED pattern during depositions of Au at room temperature in order to compare it with previous reports [1,3]. At the initial stage of deposition below 0.3 ML, a c(2 X 2) pattern with broad (l/2, l/2) spots was observed and, at around 0.5 ML, the pattern became a clear ~$2 x 2) pattern with faint streaks along the [llO] directions. At coverages beyond 0.5 ML, two c(2 x 14) patterns originating from two equivalent orientations of the hexagonal overlayer of Au developed. During this stage, the (l/2, l/2) spots for the c(2 x 2) pattern gradually broadened and faded out with increasing coverage. Eventually, the c(2 x 14) structure without any trace of the (l/2, l/2) spots was completed at around 1.2 ML. These results are in good agreement with those of previous reports [1,3]. On the basis of the structural change in the LEED pattern, we first investigated the adsorption site of Au on the Cu(OO1) surface at the very initial stage of deposition. Fig. 1 shows for a coverage less than 0.1 ML a polar-angle scan of the ICISS signal, which is the integrated value of the quasi-single scattering peak in the time-offlight spectrum (not shown). The data was taken along the [OlO] azimuth at every 2” of the polar angle. According to the basic concept of ICI!%, structural information is derived, as a first approximation, from the position of the focusing peak in polar scan, resulting from sequential scattering by an atom pair; the incident projectile is deflected and focused by the first shadowing atom and successively back-scattered by the second target atom almost by a head-on collision. The position of the second atom relative to the first one is determined using the shape of the shadow cone for the first atom. In this way, the peak “a” appearing at070” was assigned to scattering from Cu-Cu (3.6 A) pairs in the surface. The shoulder “b” at 63” for the Cu signal and the peak “c” at

a b

-60

-40

-20

0

20

40

60

80

J

POLAR ANGLE (deg.) Fig. 1. Polar angle scan along the [OlO] azimuth for a nearly clean Cu(OO1)surface ( < 0.1 ML Au coverage), taken at room temperature. Open and closed circles correspond to Au and Cu signals, respectively. Arrows “a”, “b” and “c” show the focusing peaks due to scattering by Cu-Cu, Au-Cu and Cu-Au atom pairs in the surface, respectively.

74" for

Au were assigned to scattering from AuCu and Cu-Au pairs, respectively, where a slight deviation of the Au atoms out of the Cu surface plane by 0.18 _t 0.06 A is taken into considerations. The peak positions obtained from both experiment and calculation are summarized in table 1 together with other results to be discussed later. The above results suggest that a substitution of Cu with Au atoms takes place in the surface, resulting in a dilute surface alloy layer. Table 1 Experimental values of the polar angles at which peaks occur in the KISS signal (accuracy f 1”) and calculated values estimated using a Thomas-Fermi-Molibre potential Peak

: C

d e x F

Fig. 1

Fig. 2

Fig. 5

Exp.

Calc.

Exp.

Calc.

Exp.

Calc.

70” 63” 74” -

70” 65” 73” _ _ -

69” 63” 74” 69” _ -

70 65” 73” 68”

73” 35” 1” -12” - 61” -35” 59”

74” 36” 2 -11” - 59” - 36” 59”

-

-

S. Nakanishi et al. /Au

976

This means that Au and Cu atoms are essentially coplanar, apart from a small distortion of Au atoms out of the plane by an amount of 0.18 F 0.06 A (to be compared with the value of 0.1 .& found in a LEED analysis by Wang et al. [7]). The ICISS polar scan measured at a coverage of 0.25 ML, is shown in fig. 2. Peaks ‘“a”, “‘b” and “c” correspond to those in fig. 1. The appearance of additional peak “d” at 69” may correspond to scatt$ng from Au-Au pairs with a distance of 3.6 A along the [OlO] direction and corresponds we11 with the appearance of the ~$2 x 2) LEED pattern. This indicates the formation of a two-dimensional ordered alloy. Figs. 1 and 2 also show that bulk alloying is absent, because the polar scan does not exhibit any of the peaks that are expected for a three-dimensional Au-Cu alloy; bulk alloying would have resulted in two characteristic peaks at +20”, as seen for the Cu signal in fig. 2 or for the Au signal in refs. [9,10] in the case of a Au&u alloy. The strong surface segregation of Au in Cu [lo] must be the principal reason for the fact that Au does not diffuse into the substrate at low coverages. On the other hand, bulk alloying was clearly observed at cover-

I

Net (3keV)

-> Au(0.25ML)/Cu(OOl)

I I

azimuth [OIOI

and Ag deposition on Cu(OO1)

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-20 0 20 40 60 -& POLAR ANGLE (deg.) Fig. 3. Evidence of diffusion of Au into the Cu substrate. Peaks appearing at *20” indicate scattering from Au atoms intermixed with the substrate fee lattice. The surface peaks at 60-W originate from the hexagonal c(2 X 14) overlayer.

ages beyond 0.5 ML where the ~(2 X 141 or hexagonal overlayer structure was found to develop in coexistence with the ct2 x 21 structure. Fig. 3 shows the polar angle scan for the Au signal measured at about 1 ML coverage. In this case, the presence of the characteristic peaks at - 20” is obvious. We conclude that Au has diffused into the substrate. The diffusion is considered to extend to the third layer of the substrate fee lattice, because similar characteristic peaks have been observed to come from the second or third layer Au atoms in the case of Au,Cu(OOll [9,101. 3.2. Ag / Cu ~00~~

-60

-40

-20

0

POLAR ANGLE (deg.) Fig. 2. Polar angle scan along the [OlO] azimuth after deposition of 0.25 ML of Au at room temperature. Notations “a”, “b” and “c” correspond to those in fig, 1. Peak “d” for the Au signal comes from the scattering by Au-Au pairs in the surface.

The change of the LEED pattern with deposition of Ag was very simple: two-fold c(2 X 10) patterns originating from two hexagonal overlayers, orientations with respect to the substrate lattice, were observed at about one monolayer Ag deposition. Further depositions of Ag resulted in simple two-fold hexagonal patterns due to the growth of two equivalent AgUll) epitaxial layers; the extra spots characteristic of the d2 x 101 pattern faded out with increasing coverage. These

S. Nakanishi et al. /Au and Ag deposition on Cu(Wl)

results agree with previous works [1,5]. A marked difference between the Ag/Cu(OOl) and the Au/Cu(OOl) systems is in the diffusion of the overlayer atoms into the Cu substrate [S]. In the case of Ag/Cu(OOl), no diffusion of Ag into the Cu substrate was observed, at least not till full monolayer coverage. Direct evidence is given in fig. 4 which shows a polar scan of the Ag signal after 1 ML deposition at 150°C. Obviously, the polar scan does not exhibit any focusing peaks, especially at N 20”. The peak at about 70” comes from the scattering by Ag-Ag pairs on the surface. We therefore conclude that there is no diffusion of Ag into the Cu substrate. This result significantly differs from that described for the case of 1 ML deposition of Au on Cu@Ol) and may be explained by the presence of a miscibility gap in the phase diagram for Ag-Cu alloy systems. After deposition of 2 ML at room temperature a complicated structure was observed in the polar

i hlet(3keV) -> Ag(lML)/Cu(OOll

977

Net (3keV) -> Ag(2ML)/Cu(OOl)

- at R.T.

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I

I

I

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I

I

I

I

-60 -40 -20 0 20 40 60 POLAR ANGLE (deg.)

I

XI 80

Fig. 5. Polar angle scan taken after deposition of 2 ML of Ag at room temperature. The azimuth is shown in the upper right by the long arrow together with a sketch of a hexagonal overlayer formed on top of the Ctr(OO1)surface. The origin of the peaks denoted by “a’‘-“e” is illustrated in the upper left in a side view of the stack of the hexagonal layers, see also table 1. Other peaks “E”, I%” and peaks at f50” come from other equivalent hexagonal layers rotated by 180“ and 90” with respect to the first one, respectively.

at 150'C

a

azimuth [OlOl

I

b b

a

CU

1,~__J -60 -40 -20 0 20 40 60 POLAR ANGLE (deg.)

80

Fig. 4. Polar angle scan along the [OlO] azimuth measured after deposition of 1 ML of Ag at 150°C; the polar scan is essentially the same as that measured after deposition at room temperature. The inset shows a top view of the hexagonal overlayer formed on top of the substrate square net; arrows indicate trajectories of incident ions along the [OlO] azimuth giving rise to the peak “a” and “b” in the polar angle plot. The dotted line is a schematic outline of the shadow cone.

scan, see fig. 5. Most peaks can be related to the epitaxial growth of Ag(ll1) layers, see table 1 and the inset of fig. 5. However, the growth mode cannot be strictly layer by layer. Fig. 6 shows the coverage dependence of the Ag signal taken at two different polar angles, 0” and 7”. It should be noted that the first, second and third layers are detectable at a polar angle of 0” but only the first two layers at 7”, because the third layer atoms are shadowed by the first layer atoms. The first linear increase of both signals up to 1 ML corresponds to simple overlayer growth. However, the second linear increase up to about 1.5 ML followed by its split at higher coverages indicates a partial occupation of the second layer and the successive nucleation of the third and further layers; the stepwise increase of the slope at each break point is explained by the increase of the scattering yield due to focusing of the projectile on the second and third layer atoms. From these considerations

r

i Ne'(3keV) -> Ag/Cu(OOl)

P OS0

at R.T. azimuth IO101

we conclude that Ag on Cu(OO1) grows in the Stranski-Krastanov mode. It may be that ideal layer-by-layer growth is precluded because the second layer stacking involves structural transformation from a slightly strained hexagonal c(2 x 10) structure to less strained Ag(ll1) layers.

References

_t

30

Fig. 6. Variation of the intensity of the Ag signal as a function of deposition time at two different polar angles. The first break in the linear slope corresponds to the completion of the first hexagonal overlayer ( - 1 ML).

[t] P.W. Palmberg and T.N. Rhodin, J. Chem. Phys. 49 (1968) 134. (21 Y. Fijinaga, Surf. Sci. 86 (1979) 581. [3] G.W. Graham, Surf. Sci. 184 (1987) 137. 141 D.D. Chambliss and S. Chiang, Surf. Sci. Lett. 264 (1992) L187. [51 J.G. Tobin, S.W. Robey, L.E. Klebanoff and D.A. Shirly, Phys. Rev. B 35 (1987) 9056. [6] J.G. Tobin, S.W. Robey and D.A. Shirly, Phys. Rev. B 33 (1986) 2270. [7) Z.Q. Wang Y.S. Li, C.K.C. Lok, J. Quinn, F. Jona and P.M. Marcus, Solid State Commun. 62 (1987) 181. 181 G. Gladyszewski and Z. Mitura, Surf. Sci. 231 (1990) 90. 191 K. Kawamoto and S. Nakanishi, J. Vat. Sot. Jpn. 34 (1991) 35 [in Japanese]. I101 S. Nakanishi, K. Kawamoto, N. Fukuoka and K. Umezawa, Surf. Sci. 261 (1992) 342.