Surface Science Letters 247 (199t) L215 L220 L215

North-Holhmd

Surface Sc i ence Let t e r s

Oxygen induced surface segregation of Cu on the Au0.7Cu0.3(100) surface

S. Nakan i sh i , N. Fukuoka , K. K a w a m o t o , K. U m e z a w a , Y. T e r a o k a and K. N a k a h i g a s h i

Department of Materials Science, University of Osaka Prefecture, Mozu-Umemachi, Sakai 591, Japan

Received 1 August 1990: accepted for publication 7 January 1991

The oxygen induced surface segregation of Cu on the Au0.vCu03(100) surface was investigated by means of LEED and AES techniques. The dissociative adsorption of O 2 did not take place on this clean surface for a long time exposure at least up to 104 L,

and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive

oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the

clean surface, the segregation of Au was clearly observed and the surface concentration of Au was estimated to be about 86%, greater

than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was

observed. But. above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 × 4) LEED pattern

was observed in a wide range of oxygen coverage. The maximum intensity of the (2 × 4) was observed at about 0.45 ML.

1. Introduction

The surface segregation of one constituent in a binary alloy system is a common phenomenon and has been extensively studied by many authors. As is well known, there is a tendency for the segregation of the constituent with a lower heat of vaporization, a lower surface energy or a smaller cohesive energy. On the other hand, the chemisorption of foreign atoms onto the alloy surface frequently changes the equilibrium con- centration at the surface resulting in a chemisorp- tion-induced surface segregation; in some cases, it results in an alternation of the segregation ele- ment. The binary alloy system of C u - A u is one of the typical targets for a study of the segregation phenomena and widely investigated from experi- mental [1-5] and theoretical [6] standpoints. How- ever, the chemisorption induced effects on this alloy surface have hardly been studied so far, especially in the Au rich regions such as the pre- sent Au0vCu0.3, in spite of the importance for the systematic understandings of the segregation phe- nomena. In the case of the Au-rich sample, the surface top layer is thought to be occupied almost

completely by Au atoms due to a surface segrega- tion of Au and, therefore, the oxygen chemisorp- tion onto the surface is extremely depressed in contrast to the case of Cu-rich sample [7]. How- ever, this does not mean the absence of the chem- isorption-induced segregation: if we use the ap- propriate way to apply oxygen atoms forcibly onto the surface, we can reveal the chemisorp- tion-induced segregation on this alloy surface.

In the present work, we investigated the oxygen chemisorption-induced surface segregation of Cu on the (100) surface of Au,~.TCuo. 3 single crystal alloy, using LEED (low energy electron diffrac- tion), AES (Auger electron spectroscopy) tech- niques.

2. Experimental

Experiments were performed using a standard L E E D / A E S system with hemispherical 4-grid electron optics. The base pressure of the working space was 2 × 10 8 Pa in opt imum condition after a 15 h bake at 250°C. The AES measurements were performed at primary electron energy of 2

0039-6028/91/$03.50 '~' 1991 Elsevier Science Publishers B.V. (North-Holland)

L 2 1 6 S. Nakanishi et al. / Oxygen induced surface segregation of Cu on the A Uo. 7Cuo. f1100) surJ~ce

kV, the current of 5 ~A and the modulation of 1.5 to 5 Wrm s. The AU0.TCU0. 3 single crystal rod with a size of about 6 mm diameter × 20 mm length was grown by the Bridgman method using high quality source materials of Au(99.999%) and Cu(99.999%). The crystallization and the orientation of the pel- let type sample (5 mm diameter × 1 mm thick) with the (100) surface cut from the single crystal rod were tested by the X-ray diffraction method. Some deviation (1-2 ° ) of the orientation from the ideal (100) surface was recognized. The clean surfaces were prepared by the conventional Ar ion sputter-annealing procedure in vacuum. As the clean surface was very stable for the oxygen gas exposure, the surface oxygen coverage was con- trolled by the following procedures: (1) For the compulsive oxygen introduction onto the chemically stable clean surface, 1-2 ML Cu was deposited onto the clean surface prior to the oxygen adsorption. (2) The Cu coated surface was fully exposed to oxygen gas and saturated with oxygen atoms. (3) In order to obtain the desired oxygen cover-

age, the sample was heated up to an appropriate temperature leading to the reduction of the excess portion of oxygen due to the desorption from the surface. (4) To achieve the thermal equilibrium concentra- tion, the sample was slowly cooled to r~×~m tem- perature. According to Canning [8], atomic oxygen created on the plat inum hot filament can chemicsorb even on the pure gold surface. In the present work. however, this method was not employed, because of its inefficiency in our system.

3. Results and discussion

The sputter-annealed clean surface of At|t~.~ Cu0.3(100 ) showed a p(1 × 1) LEED pattern. No marked impurity was observed in AES measure- ments as shown in fig. 1. The clean surface was very stable against a long time exposure of resid- ual gases in a UHV chamber. For the estimation of the relative sensitivity factor for Cu(60 eV) and

W -tJ

L~

Z

r -

c

¢_

J

i ~(70) (60)

^. ^"

n t t , i i l n j l n l a n u ~ l l l n n l l n ~ J l ~ n I f l l l n n l i , J l f n a l l n t l n j 100 200 300 400 500

Electron Energy (eV) Fig. 1. Auger s p u t u m from the clean surface of the Auo.TCuo.3(100 ).

S. Nakanishi et al. / Oxygen induced surface segregation of Cu on the A Uo. 7Cuo._~( I O0) surface L217

Au(70 eV) Auger signal, 1 ML copper was de- posited on the clean surface where the calibration of the deposition rate was monitored by a quartz oscillator. The monolayer-over-growth of the de- position time and the first break point of the linear slope agreed well with the expected value derived from the deposition rate. Fig. 2 shows the Auger spectrum at a monolayer coverage of Cu deposition. From fig. 2, the relative sensitivity S of the Auger signal height of Cu with respect to that of Au was estimated to be about 1.4 by using the following relation,

S = (1~., - l c u ) / ( I A u -- IAu ), (1)

where 1Ao and lco, IAo and I~. u represent Auger signal intensities of Au and Cu obtained from initial clean surface and from the Cu coated surface, respectively. It was assumed reasonably that the mean escape depths of the Auger elec- trons both for the Au(70 eV) and the Cu(60 eV) are the same in the alloy crystal. Using the relative

sensitivity S = 1.4, the mean concentration of Cu [Cu] near the surface was calculated by

[Cu] = I c u / ( SIAu + Icu ). (2)

Fig. 3 shows the variations of Auger signal intensities for the two cases as a function of the heat treatment temperature. In the first case repre- sented by curves (a) and (b) corresponding to the Au and Cu Auger signal, respectively, about 2 ML Cu was initially deposited onto the clean surface. The sample was heated and held at the relevant temperature for about 30 min and almost the same time was spend to cool the sample down to room temperature where the measurements were made. In the second case indicated by the curves (c), (d) and (e) for the Au, Cu and oxygen Auger signal, respectively, the Cu coated surface was exposed to the oxygen gas until its saturation coverage was reached and then the measurements were made with the same procedure as in the first case. From the behavior of curves (a) and (b) in

I.a.J

I , I

Z

> . .

03

C

:D .,,::E

[A]

~ AuoaCuo.3 CLEAN

/1 ///

/ V Cu ( 1 HL ) DEPO.

Cu Au

40 50 60 70 80 90 Elect ron Energy (eV)

[B]

Cu

40 50 6o 70 80 9o Elect ron Energy (eV)

Fig. 2. Auger spectra [A] and syntheses [B] of the spectra: Syntheses were performed by using two reference spectra for Au and Cu with help of the least square method.

1+218 S. Nakanishi et aL / Oxygen reduced surJace segregation oj ('u on the A u.. 7('uo.+(lO0; Yurtac~

the first case, we can state that, for heat t r ea tment t empera tu res above 300 o C, the excess Cu migra ted into the bulk crysta l far enough to reach the

t he r m a l e q u i l i b r i u m c o n c e n t r a t i o n near the surface, because bo th signals for Au (denoted by (a)) and Cu (by (b)) reach the values co r respond- ing to that f rom the clean surface. In the second case, however, the A u (c) and Cu (d) Auger signals g radua l ly change with increas ing heat t r ea tment t empera tu re and, even above 300 ° C, the in tensi ty of the Cu signal was still kep t at a high level away from that for the clean surface. Obviously , this indica tes the presence of the oxygen chemisorp- l ion- induced surface segregat ion of Cu. The near ly para l le l re la t ionship be tween the Auger intensi t ies of Cu (d) and oxygen (e) in the t empera tu re range f rom 3 0 0 ° C to 6 0 0 ° C means that the surface segregat ion is app rox ima te ly p ropo r t i ona l temper- a tures b e y o n d 600 o C, Auger intensi t ies for A u (c) and Cu (d) a lmos t reach to that for the clean surface, in spi te of the existence of the oxygen signal (e) and its change. This suggests the insensi-

bi l i ty of the induced segregauon of ( u under low oxygen coverages.

In o rde r to conf i rm that oxygen a toms arc loca ted in the al loy surface, we also measure the var ia t ion of the Auger signal tntensit3 ~t~ a func- t ion of the inc ident angle Idef ined a~ the angle with the surface) of the p r imary e lect ron beam. The results are shown in fig. 4. where the da ta were taken after a mild hea t ing ~ > 20() ~' ( '~ o f the oxidized Cu coated sample. Accord ing to our pre- vious repor t [9], the present results s t rongly sug- gest that the oxygen a toms are present at the topmos t layer or very close to the surface, because the Auger signal becomes more surface-sensi t ive with a decreas ing incident angle. In tact. if we assume oxygen a toms to be loca ted at the surface. the ca lcu la ted curve for the oxygen Auger inten-.

sity agreed well with the exper imenta l result as shown by the do t t ed line in fig. 4+ where the ca lcula t ion was made by using our previous for- mula [9] with r easonab le parameters .

L E E D observa t ions were also carr ied out in the

!

1.01 o Au F- - (2X4)+C(2X2) - i O..+~O'~j- O -~_ }- , ...... o / (a) / -:

¢.- i s

i ~ / to) .. c r- . i 0 . .0

"..T. k O .............. O - ~ J "

z ~ 0 .5 ~ ~ ........... m A .................. ~.,.__. ..:

~-- i m "- m ............... ..~.__... ~ (d)

rv-" "x,, .4

< ................................. Cu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X ~ ~ A.--. '11 , '-- c1 c a n s u r £ a c e 0 \ I

"- \M.

O i , , , , , , , , , , , , " " ~ _ _ _ _ _ J 0 1 [}0 200 300 400 500 600 700

TEMPERATURE ( ' C) Fig. 3. Variations of the Auger peak intensities as a function of the heat treatment temperature. In the case of curves (a) and (bL about 2 ML Cu was initially deposited on to the clean surface at room temperature. For curves (c), (d) and (eL oxygen was fully

adsorbed after the 2 ML Cu deposition at room temperature.

S. Nakanishi et al. / Oxygen induced surface segregation of Cu on the A u o 7Cu( ~(100) surface L 2 1 9

E

_d .(._ o~ ¢-

°_

>- I---

Z LIA I - - Z

r r " I.J_l (._0

<

1.0

0.5

\

a

~-. ~"~o "~'-...? ~"~---------~o A~ -

[ I -0- . . . . .

0 I I I 0 I IO 210 3'0

INCIDENT ANGLE ~ (deg.)

Fig. 4. Va r i a t i ons of A u g e r in tens i ty as a func t ion of the inc iden t angle a (in deg) of the p r i m a r y e lec t ron b e a m .

course of the heat treatment. As indicated in fig. 5, (2 × 4) LEED patterns were mainly observed in a wide range of oxygen coverages. The maximum intensity of the (2 × 4) pattern was obtained at about 400°C, where the oxygen coverage was about 0.45 ML. Taking into account the weakness of the scattering factor for oxygen atoms alone, the diffraction of the (2 × 4) pattern suggests the

Fig. 5. L E E D pa t t e rn of the p(2 × 4 ) s ingle d o m a i n s t ruc tu re .

ordering of Au and Cu atoms in the surface layer, because of the fairly strong intensity of the ob- served extra spots. However, detailed structure analysis will be needed for tile definite conclusion.

Fig. 6 shows the concentration change of Cu and Au with respect to the oxygen coverage, where the oxygen coverage was estimated by using the reference signal for the monolayer coverage ob- tained by in situ measurements on the p(1 x 1)-O structure on Fe(100) surface which is well estab- lished [10,11]. However, the difference of back scattering factors between Fe and the present al- loy were not taken into account. Therefore, some errors are included in the oxygen coverage scale. The concentration for Cu and Au was derived from the formula (2) described above. From this figure, we can conclude that: (1) Au atoms segre- gate to the surface in the case of the clean surface: the surface concen~ation of Au is about 86%, greater than the bulk concentration of 70%. (2) At low oxygen coverages below 0.16 ML, the ad- sorbed oxygen does not change the segregation phenomena, or the oxygen induced surface segre- gation of Cu is negligibly small. (3) At high cover-

L220 S. Nakanishi et a L / Oxygen induced surface segregation of Cu on the Auo. 7CUo.3(100) sur]~lce

100

70

z 60

t - -

~- 50 Z

z ° 40

9 0 -

80

<

30 <

20;

10

0

-4

O~ 0 .

0 --.,_.

"-o-...o

Au Au ....... .0_. ..... -4

BULK CONCENTRATION

/ -Cu

. . . . . . . . . I

0.1

A ~ / / / I / A . ....

CLI .... . / ~ / . . / a

A - - ~ .... i L . . . . . . . . . . . . (2x4) +c (2x21 ......... d

i

0.2 0,3 0,4 0,5 OXYGEN COVERAGE {0)

Fig. 6. Surface concentration changes of [Au] and [Cu] as a function of oxygen coverage.

ages beyond 0.2 ML, oxygen atoms induce Cu atoms to segregate to the surface almost propor- t ional ly to the coverage. The first result indicates

the strong tendency of the segregation of Au atoms and agrees with the segregation behavior observed at the Cu 3Au(100) surface [3]. The second and the third results may be explained in terms of the

energy compet i t ion between the decrease in surface energy due to the oxygen chemisorpt ion and the increasde by the exchange of Au and Cu atoms°

Teraoka [12] has recently treated the detailed

chemisorpt ion effect on the segregation phenom- ena on the fcc b inary alloy crystal surface (100)

within a framework of the Bragg-Wi l l i ams ap-

proximat ion and obta ined very similar results m a

quali tat ive m a n n e r to the present experimental results shown in fig. 6.

R e f e r e n c e s

[1] R.A. van Santen, L.H. Toneman and R. Bouwman, Surf. Sci. 47 (1975) 64.

[2] J.M. McDavid and S.C. Fain, Jr, Surf. Sci. 52 (1975) lt5]. [3] T.M. Buck and G.H. Wheatley and I.. Marchut, Phys.

Rev. Let. 51 (1983) 43. [4] R.-S. Li, T. Koshikawa and K. Got(), Surf, Sci. 129 (1983)

192. [5] M.J. Sparnaay and G.E. Thomas, Surf. Sci. 135 (1983)

184. [6] J.M. Sanchez and J.L. Moran-Lopez, Surf. Sc:i. 157 (198511

L297. [7] G.W. Graham, Surf. Sci. 137 (1984) 1,79. [8] N.D.S. Canning, D. Outka and R.J. Madix, Surf. Sci 141

(1984) 240. [9] S. Nakanishi, N. Fukuoka, K. Nakahigashi, M. Kogacht,

H. Sasakura, S. Minamigawa and A. Yanase, Jpn. J. App]. Phys. 28 (1989) L71.

[10] T. Horiguchi and S. Nakanishi, Jpn. J. Appt. Phys. SuppL 2, part 2 (1974) 89.

[11] K.O. Legg, F.P. Jona, D.W~ Jepsen and P.M. Marcus. J Phys. C8 (1975) L492.

[12] Y. Teraoka, Surf. Sci., submitted