Stacking of Ag layers on Pt(111)

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surface science

ELSEVIER

Surface Science 331-333 (1995) 948-951

Stacking of Ag layers on Pt( 111 ) G. R a n g e l o v a, T h . F a u s t e r a,.,1, U . S t r t i b e r b, J. K t i p p e r s b a Sektion Physik, Universitiit Mfinchen, Schellingstrasse 4, D-80799 Miinchen, Germany b Experimentalphysik VI, UniversitEt Bayreuth, D-95440 Bayreuth, Germany

Received 1 August 1994; accepted for publication 6 December 1994

Abstract

Ultrathin Ag films grown epitaxially on Pt(111) were studied by photoelectron forward scattering using synchrotron radiation. The complete angular distributions of the Ag 3d electrons at ,~ 500 eV kinetic energy were recorded in a cone with 88 ° opening angle using a two-dimensional display-type analyzer. The images reveal that the second Ag layer grows at room temperature predominantly in a hcp stacking sequence with respect to the pseudomorphic first Ag layer. After annealing to 750 K most of the second layer Ag atoms assume fcc sites. Further Ag layers continue to grow in a fcc stacking sequence relative to the first two Ag layers. Annealing does not change the stacking of thicker films. Thick fcc Ag films can be grown in a twin orientation with respect to the Pt( 111 ) substrate at room temperature or in the substrate orientation if the second Ag layer is annealed. Keywords: Low index single crystal surfaces; Photoelectron diffraction; Platinum; Silver; Single crystal epitaxy; Surface structure; X-ray

photoelectron spectroscopy

1. Introduction

In a previous study we have reported results on the growth and structure o f thin Ag films on P d ( l l l ) [ 1 ]. At room temperature Ag grows epitaxially on P d ( l l l ) in a layer-by-layer mode as confirmed by a two-photon photoemission investigation [2]. The first Ag layer grows pseudomorphically, followed by a stacking fault between the first and the second layer related to the compensation of the lattice mismatch. Thick Ag films show a fcc crystal structure with the Ag lattice constant, are almost as well ordered as the P d ( l l l ) substrate, and grow in (111) twin orientation with respect to the substrate. Films grown at ele* Corresponding author. 1Present address: Max-Planck-Insfitutf'tir Plasmaphysik, Boltzmannstrasse, D-80799 Mtinchen, Germany.

vated temperatures do not show the stacking fault, but an alloying between A g and Pd occurs above 500 K [3,4]. In this work we investigate the growth o f Ag on Pt( 111 ). Platinum and palladium have an almost identical lattice constant which is ,-~ 5% smaller than the one of Ag. Previous studies [ 5 - 8 ] find a growth behavior very similar to P d ( 1 1 1 ) . The main difference is that Ag and Pt do not intermix at high temperatures, except for the monolayer case [9,10]. A scanning tunneling microscopy (STM) study [6] investigated the strain relief in the second Ag layer on P t ( l l l ) and found different domain structures depending on temperature. They assigned independent of temperature the majority domains to Ag islands continuing the regular fcc stacking sequence. The similarities to the Ag on Pd(111) system motivated us to study the stacking

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G. Rangelov et al./Surface Science 331-333 (1995) 948-951

of Ag on Pt(111) in detail. In both cases the second Ag layer grows on a 5% compressed Ag layer and an analogous behavior should be expected. The main experimental technique used was photoelectron forward scattering which is particulary suitable to yield direct information on the growth and structure of thin films on single-crystal metal surfaces [ 11-14]. For the study of complex growth behavior it is important to sample the complete angular distribution of the photoelectrons at high kinetic energies [ 15,16] which can be most easily done using a twodimensional display-type electron spectrometer [ 17]. These experiments were complemented by angleintegrated X-ray photoelectron spectroscopy (XPS), thermal desorption spectrometry (TDS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED).

2. Experiment Sample preparation, TDS, AES, and LEED measurements were performed in a separate ultra-high vacuum chamber which is connected to the chamber of the display-type analyzer [ 17] by a transfer system. The Pt( 111 ) crystal was cleaned following standard procedures [8]. The surface cleanliness and crystallographic order were verified with AES, TDS, and LEED. Silver was evaporated from a resistively-heated tungsten basket at a rate of 0.01 monolayers (ML) per second onto the substrates at room temperature ( < 50°C) and at a pressure of < 2 x 10 -1° mbar. Coverages are given in monolayer equivalents and were determined with an uncertainty of 10% by means of a quartz microbalance. The coverage calibration was checked by thermal desorption spectrometry of Ag [8]. For the forward-scattering experiments synchrotron radiation with an energy of ,-~ 900 eV from the HETGM-1 beam line at the BESSY storage ring was employed. The angular distribution patterns of the photoelectrons were recorded by a two-dimensional displaytype electron spectrometer [ 17] which registers the complete angular distribution in an acceptance cone of 88 ° . The data processing and the normalization with respect to the spatial analyzer efficiency has been described previously [ 1,15,18].

949

2 N

.,= 1

It

N ~ x Ag

÷

at + ~

Ag / (Ag + Pt) /

o 0.8

/

-

/

-

-

500 1 554 eV /

~ 0.6 i'

N o.4 °N ~

calculation: layer-by-layer growlh )L= 9 . 1 A

~ 0.2 / ¢ o 0.0-" 0

i

i

I

I

i

i

I

l

I

I

I

I

I

I

10 Ag coverage (A)

I

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20

Fig. 1. Bottom: Variation of the XPS intensity ratio A g / ( A g + P t ) (open squares) as a function of Ag coverage compared to a model calculation for layer-by-layer growth (solid line) with the mean free path h = 9.1 /~. The experimental data were normalized relative to the clean surface and a thick film. The kinetic energy of the Pt 4d and Ag 3d electrons was 554 and 500 eV, respectively. Top: Normalized inelastic XPS intensity for Ag ( + ) and Pt ( × ) as a function of coverage.

3. X-ray photoelectron spectroscopy To study the growth mode of thin films we present first the results of the XPS measurements. In these experiments we determine the relative intensities and inelastic contributions by a line fitting procedure using the spectra from the clean Pt( 111 ) surface and from an 8 ML thick Ag film on Pt( 111 ) as reference (compare to Refs. [ 15,18] ). The kinetic energy of the electrons from the Ag 3d and Pt 4d core levels was 500 and 554 eV, respectively. The film signal normalized to the sum of film and substrate signal is shown at the bottom of Fig. 1 for Ag on Pt(111). The data for samples heated to 750 K are also included in Fig. 1 and show only minor changes in the A g / P t ratios. This confirms that no Ag is lost due to diffusion, into deeper layers or evaporation and only small changes o f the surface morphology like smoothening of the films occur during the anneal [8]. The results agree well with data for Ag on Pd(111) and a calculation for layer-by-layer growth with a mean free path of 9.1

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G. Rangelov et al./Surface Science 33i-333 (1995) 948-951

[ 1,18]. The data are also compatible with a StranskiKrastanov growth mode for higher coverages at room temperature [ 8 ],

pattern reverses and the triangle exhibits the same orientation as the Pt(111) substrate (lower right corner in Fig. 2). This illustrates clearly that the stacking sequence changes from hcp at RT to fcc after annealing. A closer inspection of the patterns for 2 ML of Ag reveals that there is in both cases some intensity corresponding t o the opposite orientation of the main stacking. The intensity ratio is in qualitative agreement with the STM observations [6]. For 3 and 8 ML of Ag forward-scattering directions from deeper layers appear and the emission patterns show the stacking fault which is found to be stable upon annealing. For 8 ML the pictures look very similar to the pattern observed for Pt(111) except for the different stacking sequence. We conclude that thick Ag films grow in a well-oriented fcc (111) structure. The growth of thicker Ag films without stacking fault can be achieved in the following way: Two layers of Ag are deposited at room temperature and annealed shortly to obtain the regular fcc-stacking sequence (Fig. 2 top right). Further Ag layers then continue to grow in fcc orientation. This method is illustrated in Fig. 2 (center right) for a film with a total coverage of 3 ML which could otherwise only be grown with a stacking-fault orientation. The fcc-oriented overlayers could presumably be grown also by deposition at elevated temperatures.

4. Angular distributions of photoelectrons

5. Discussion

The angular distributions of the photoelectrons are shown as gray-scale pictures in Fig. 2 in a slightly distorted polar diagram with an opening angle of N 88 ° and the surface normal approximately in the center [ 1,15,18 ]. In order to show the structures as clearly as possible, the lowest (highest) intensity of each pattern is shown in the picture as black (white). For 0.8 ML Ag no forward scattering is seen in agreement with the layer-by-layer growth. All other patterns show a threefold axis around the surface normal and are slightly distorted by the ellipsoidal mirror of the analyzer [ 1,17]. The s polarization of the synchrotron radiation reduces the symmetry to a reflection along a vertical line [ 15]. For 2 ML of Ag deposited at room temperature (RT) the three main forward-scattering directions at 35 ° polar angle form a triangle pointing downward. After a short anneal to 750 K the emission

Our findings about the growth and structure of Ag films on P t ( l l l ) agree well with previous work on this system [5,6,8]. The only discrepancy to the STM work [6] is the identification of the majority domains in the second monolayer deposited at room temperature as hcp sites by the forward-scattering experiments. The STM study assumed from the appearance of these domains at downward steps that these domains would be fcc stacked. The room temperature structure converts after annealing into a structure with a majority of fcc domains. This proves that the room temperature phase with its majority of unfavorable hcp sites is metastable and its growth is determined by kinetics. A plausible mechanism would be that the growth starts at the downward steps in a fcc sequence. After a certain width the lattice mismatch demands the occupation of hcp sites. This can be avoided by the

0.8 ML Ag RT

2.0 ML Ag RT

2.0 ML Ag 750 K

3.0 ML Ag RT

3.0 ML Ag 750 K

2 + 1 ML Ag

8.0 ML Ag RT

8.0 ML Ag 750 K

Pt(111)

Fig. 2. Angular distribution patterns fog various coveragesof Ag on Pt( 111) as deposited and heated to the indicated temperatures.

G. Rangelov et al./Surface Science 331-333 (1995) 948-951

growth o f finger-like p r o t r u s i o n s p e r p e n d i c u l a r to the [ 1 i 0 ] steps often f o u n d in S T M pictures [ 5 ] . Further growth fills in the space b e t w e e n the fingers. These atoms have to o c c u p y the unfavorable hcp sites. This scenario w o u l d explain the orientation o f the d o m a i n walls a l o n g the [ 112] direction and the hcp atoms at the step edges o f the second m o n o l a y e r [ 6 ] . A n n e a l i n g to 750 K is necessary to convert into a stable c o n f i g u r a t i o n w h i c h involves a large scale rearrangem e n t o f the surface. This c a n n o t be achieved d u r i n g the adsorption and diffusion o f single a t o m s at r o o m temperature.

Acknowledgements W e a c k n o w l e d g e s t i m u l a t i n g discussions with K. Kern. This w o r k was supported in part by the B u n d e s m i n i s t e r ftir F o r s c h u n g u n d Technologie ' B M F T 05 5 W M A B B 2 ) .

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[3] R. Fischer, Th. Fauster and W. Steinmann, Phys. Rev. B 48 (1993) 15496. [4] R. Fischer and Th. Fauster, in: Physics and Chemistry of Alloy Surfaces, F_As. K. Wandelt and K. Heinz (World Scientific, Singapore, 1995). [5] H. Rtider, H. Brnne, J.-P. Bucher and K. Kern, Surf. sci. 298 (1993) 121. [6] H. Brnne, H. Rfder, C. Boragno and K. Kern, Phys. Rev. B 49 (1994) 2997. [7] H. R6der, E. Hahn, H. Brune, J.-R Bucher and K. Kern, Nature 366 (1993) 14t. [8] T. H~tel, U. Striiber and J. Kiippers, Thin Solid Films 229 (1993) 163. [9] H. R/3der, R. Schuster, H. Brnne and K. Kern, Phys. Rev. Lett. 71 (1993) 2086. [10] U. Strfiber and J. Kiippers, Surf. Sci. 294 (1993) L924. [ 11] W.E Egelhoff, Jr., CRC Crit. Rev. Solid State Mater. Sci. 16 (1990) 213. [ 12] C.S. Fadley, in: Synchrotron Radiation Research: Advances in Surface Science, Eds. R.Z. Bachrach (Plenum, New York, 1992) ch. 11. [13] S.A. Chambers, Adv. Phys. 40 (1991) 357. [14] J. Osterwalder, Arab. J. Sci. Eng. 15 (1990) 273. [15] Th~ Fauster, G. Rangelov, J. Stober and B. Eisenhut, Phys. Rev. B 48 (1993) 11361. [ 16] J. Osterwalder, T. Greber, A. Stuck and L. Schlapbach, Phys. Rev. B 44 (1991) 13764. [17] D. Rieger, R.D. Schnell, W. Steinmann and V. Saile, Nucl. Instrum. Methods 208 (1983) 777. [18] B. Eisenhut, J. Stober, G. Rangelov and Th. Fauster, Phys. Rev. B 49 (1994) 14676.