Ultrathin films of Rh, Ir and Pt on tungsten (110)

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ELSEVIER

Surface Science 314 (1994) 221-242

Ultrathin films of Rh, Ir and Pt on tungsten ( 110) J. Kolaczkiewicz *Ta,E. Bauer b a Instytut Fizyki Doiwiadczalnej Uniwersytetu Wroctawskiego, ul. Cybulskiego 36, 50-204 Wrociaw, Poland b Physikalisches Institut der Technixhe Universitiit Clausthal, D-3392 Clausthal-Zelle$eld, Leibnitzstrasse 4, Germany (Received 13 April 1993; accepted for publication 22 March 1994)

Abstract The initial growth of rhodium, iridium and platinum on W(110) was studied at several temperatures by low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy and work function change measurements. At room temperature Frank-van der Merwe-type growth was observed, at temperatures in excess of 600-800 K, however, Stranski-Krastanov-type growth. Several metastable structures were observed in all the adsorbates.

1. Introduction

The basic understanding of the very early stages of the growth of metal films on metal substrates has been of continuing interest for many years. The related elementary surface phenomena such as diffusion and lateral interaction between adatoms, are intensively studied. The growth processes were investigated with various surface analysis techniques, whereas the elementary phenomena have been studied mainly by field-ion microscopy (FIM). Because of experimental limitations FIM studies are confined to microscopic surfaces of the strongly bound 4d and 5d transition metals [l-5]. Little work has been reported on the mechanisms of growth of high melting point metals on macroscopic singlecrystal surfaces, apparently because of experi-

* Corresponding author.

mental difficulties. The results obtained so far on large single-crystal surfaces (Rh on Mo(ll0) [61 and Pt on W(110) [7,8]) show little correlation with those obtained by FIM. In particular, no chain structures have been observed with LEED, in contrast to the FIM studies of the same systems [9-141. Recently we have observed such structures also with LEED at temperatures up to about 450 K [15,16] in the early growth stages of Rh, Pt and Ir on large WC1101 single-crystal surfaces. These film-substrate systems are, however, not only of interest because of their tendency to chain formation at low temperatures but also from the point of view of epitaxy in general and from the point of view of bimetallic surface chemistry [17], similar to Pd on W(110) [18], Mo(ll0) [19], Nb(l10) [ZOI and Ta(ll0) [21l, and Pt on Nb(ll0) [22] which in the monolayer range differ strongly in their chemisorption behavior from bulk Pd and Pt [20-241. The atomic radii CT,,) of Pd, Pt, Rh and Ir differ less than 2% from those (r,)

0039-6028/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(94)00195-F

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J. Kdaczkiewicz, E. Bauer / Surface Science 314 (1994) 221-242

of W and MO so that these film-substrate combinations have rb/ra values intermediate between the optimum values for Nishiyama-Wassermann (NW) orientation, rJra = 0.9428, and for Kurdyumov-Sachs (KS) orientation, rb/ra = 1.0887 [25]. Pd on W(110) and Mo(ll0) has already shown complex orientation behavior [18,191 and it is interesting to see how the other three metals behave in this respect. Specific questions are: (i) at which rb/ra ratio does the transition from NW to KS orientation occur and (ii) is the formation of the close-packed (cp) monolayer preceded by a pseudomorphic (ps) monolayer, similar to other fee metal film-W,Mo(llO) substrate combinations in which the packing density of the fee (1111 plane is larger than that of the bee (110) plane such as in the case of Cu, Ni and Co layers [261. In order to answer these questions and to obtain a better understanding of the Rh, Pt and Ir on W(110) systems in general we have undertaken a multi-method study of the growth, structure and thermal stability of these systems.

2. Experimental conditions All experiments were carried out in a UHV system equipped with a cylindrical mirror analyzer (CMA) for Auger electron spectroscopy (AES), a 4-grid LEED system, a quadrupole mass analyzer for thermal desorption spectroscopy (TDS), and an electron gun for work function change measurements with the retarding potential method. The background pressure during the experiments was 4 X 10-i’ Torr. During the evaporation the pressure rose slightly but stayed below 1 x lo- ” Torr. The sample surface was oriented within +O.OY of the (1101 plane. The sample was cleaned initially by prolonged heating at 1300 K in 1 X 10v6 Torr of oxygen followed by flashing to 2400 K, later by short exposures to about 1 x lo-’ Torr oxygen at 1300 K combined with flashing up to 2400 K. The crystal was assumed to be clean when the ratio of the Auger signals from carbon and oxygen to the 163/168 eV signal from tungsten (C : W, 0 : W) was smaller than 1: 600. In the course of the measurements some residual gas adsorption occurred but at the

end of a measurement series the C : W and 0 : W ratios were never larger than 1: 200. The sample could be heated either indirectly to 1200 K or resistively to 2500 K by passing a current through it. The sample temperature was measured with a WRe20%-WReS% thermocouple which was spot-welded to the bottom of the sample. The thermocouple was calibrated with an optical pyrometer and an infrared pyrometer. No temperature gradient (along the sample) was found within the measurement accuracy of +5 K. The vapor sources consisted of fine coils of the metals of interest of 3N purity which were wound onto a tungsten wire. In the case of iridium also an Ir filament which could be resistively heated was used as a source. The adsorbate was removed by flashing to 2500 K which, however, was insufficient for Ir. Higher flashing temperatures couId not be used because of the risk of melting either the sample itself or its holder. The iridium was reduced to low levels by multiple flashing. In order to avoid the possibility of contamination by iridium, the iridium study was carried out at the end. The measurements were begun with rhodium which could be removed relatively easily. Two methods of deposition were used: (i) cumulative deposition of identical small doses at constant sample temperature and (ii) single depositions of doses of various sizes with subsequent observation of the changes of the sample as a function of temperature. In general, the AES and LEED results presented in this paper were obtained after cooling the sample to room temperature. An electron energy of 1.5 keV was used in the AES measurements; the energies in the LEED studies ranged usually from 40 to 80 eV but sometimes were also higher. The work function change was obtained either from the shift of current-voltage characteristics, whose parallelism as a function of dosis was checked, or using the constant-current method with the current fixed at one tenth of the saturation current of the clean surface. In the TDS measurements the rate of temperature increase was 3.3 or 7 K/s. TDS could not be used for iridium. During deposition, sample heating by radiation from the source could not be avoided because of the considerable power

J. Kdaczkiewicz, E. Bauer / Surface Science 314 (1994) 221-242

needed to obtain an acceptable flux density. The minimum sample temperatures during deposition were, therefore, 350 K for Rh, 400 K for Pt and predominantly 450 K, in some measurements 350 K for Ir.

3. Results and discussion 3.1. Rhodium 3.1.1. AES The deposition time dependence of the Rh 302 eV MNN and of the W 163/168 eV AES derivative peak-to-peak amplitudes (AA) is shown in Fig. 1 for various temperatures. Both W and

1

I

223

Rh AAs consist of linear segments. The crossing point of the extrapolations of the first and second segment of the 350 K deposit corresponds to a deposition time of t, = 10.0 min. The second segment ends at I, = 20.0 min. After a clear delay the AES signal rises again, initially fast, then slower. Upon deposition or annealing at elevated temperatures the Rh AES signal rises linearly up to t,. At 850 K the slope of the second segment s2 is the same as at 350 K, up to about 17 min. Upon further deposition the Rh AES signal decreases initially but rises later with the same slope as the final segment of the 350 K deposition. At the two higher temperatures s2 is much smaller and the transition between first and second segment is sharp. The W AES signal shows a

2

Fig. 1. Rh 302 eV and W 163/168 eV Auger electron signal at 350 K without (x) annealing and with annealing as a function of deposition time (bottom) and coverage (top) at T = 850 K (+ 1, T = 1100 ( . . . ), T = 1350 (0).

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J. Kdaczkiewicz, E. Bauer / Surface Science 314 (1994)

corresponding behavior. The well defined break at t, at the elevated temperatures is ascribed to the completion of the first monolayer, the transition region around t, at 350 K to a structural transition in the first monolayer (see Section 3.1.2). The delay of the increase after completion of the second layer may be due to a reorganization of the first two layers or due to initial 3D nucleation before 2D growth of the third layer starts. The decrease of the Rh AES signal at 850 K between 17 and 20 min is attributed to a 2D --) 3D transition with increasing island size, that is increasing strain energy, of the initially formed second 2D monolayer islands. The subsequent increase of the AA extrapolates to the monolayer AA with the same slope as the 3 ML signal at 350 K. This suggests that the second ML agglomerates at 850 K at about 2/3 of its full coverage into double-layer islands on top of the first ML and subsequently grows laterally as a double layer. At the two highest temperatures shown in Fig. 1 3D island growth or alloying occurs right after completion of the first monolayer and continues with further deposition. The slope ratio s2.. s1 = 0.43 at 350 K yields an ucusually small inelastic mean free path, A = 3.1 A, for the 300 eV Rh Auger electrons but a double-layer instead of a single-layer growth mode can be excluded on the basis of the results obtained with the other techniques to be reported below. Most likely, the small value of s2 : s1 is due to a change in the shape of the Auger peak similar to that observed in other systems, e.g. in the system Au/Mo(llO) 1271. If the small AA slope ratios sJsr = 0,125 and 0.037 at 1100 and 1350 K are attributed to the formation of 3D islands on top of the first ML then their thickness can be estimated from the layer attenuation coefficient IX. For flat-topped islands consisting of N monolayers sJ.sr = C CX~/N which yields N = 6 and N = 13 if (Y= 0.43 is assumed to be correct. Thus, AES shows that there is quasi-Frank-van der Merwe growth at 350 K while at higher tempera-

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tures there is alloying or Stranski-Krastanov growth with rather thick 3D crystals already in the early stages of the 3D growth. 3.1.2. LEED LEED gives more detailed information on the structure of the layer. A layer deposited at 350 K produces a streaked (3 x 1) pattern (Fig. 2a) whose intensity increases with increasing coverage to a maximum at about 0.2 ML. The (3 x 1) structure was attributed to atomic chains separated on the average by three interatomic distances of the substrate [16]. The adatom chains have the same directions and atomic spacing as the closely packed (111) directions of the substrate. Such chains are well known from FIM studies [9-141. At higher coverages the intensity of the l/3-order spots diminishes (6 = 0.25 ML) and is increasingly redistributed into streaks along the (112) directions which are initially (0 < 0.35 ML) long and sharp, later (0.35 < 8 < 0.66 ML) short, diffuse and concentrated around the integral order spots (Fig. 2b). Their intensity maxima are at a distance of l/26-1/28 [llz], from the integral order spots, apparently independent of coverage within the limits of error. Above 0.66 ML the intensity of the streaks decreases and the strongest spots of a (n X 1) structure appear (Fig. 2c) with n decreasing from 13 towards 12 at 1 ML. The satellite spots along the W[OOl] direction persist with decreasing intensity and decreasing spot distance (towards a (11 x 1) structure) up to about 3 ML until finally only one spot of every group remains which gives a hexagonal pattern with the Rh lattice parameter. Simultaneously the spot distances in the [liO] direction decrease to about 0.96 of the W spot spacing in this direction. The transition from the (12 x 1) pattern at about 1 ML to the (11 X 0.96) pattern before the disappearance of the double scattering spots corresponds to an increasing compression of the Rh layer in the W[OOl] direction and dilation in the W[liO] direction so that finally the

Fig. 2. LEED patterns of Rh on W(110) deposited at 350 K without (a, b, c) and with annealing. (a) E, = 66 eV, T= 350 K, 0 = 0.12 ML; (b) E, = 50 eV, T= 350 K, 0 = 0.42 ML; (c) E, = 49 eV, T= 350 K, 8 = 0.95 ML, (d) E, = 53 eV, T= 510 K, TV= 0.15 ML; (e) E, = 59 eV, T= 780 K, 0 = 0.54 ML, (f) E, = 49 eV, T= 1200 K, 0 = 1 ML, (g) E, = 57 eV, T= 1100 K, (J = 1.12 ML.

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hexagonal packing of the Rh(ll1) plane is reached. The mutual orientation of film and substrate is the Nishiyama-Wassermann orientation. The packing density of the monolayer (16.14 x 10’4/cm2) is about 1% larger than that of the Rh(ll1) plane in the bulk (15.97 x 1014/cm2), that of the “(11 X 0.96)” pattern is indistinguishable from it within the limits of error. Annealing at elevated substrate temperatures leads to pattern changes. The streaked (3 X 1) pattern (Fig. 2a) transforms at temperatures above 500-550 K (depending upon coverage) into a pattern with long sharp (172) streaks (Fig. 2d) similar to the one seen with increasing coverage in the 350 K layer. At higher temperatures CT> 650 K) the (1 x 1) pattern was observed. The long streaks (0 > 0.25 ML, T = 350 K) contract into elongated satellites around the integral order beams and finally sharpen into sets of satellite spots along the (112) directions (Fig. 2e). The pattern of Fig. 2b (0.35 < 8 < 0.60 ML, T = 350 K) transforms with increasing temperature above T = 650 K first into diffuse spots which resolve at T > 800 K into the sharp spots shown in Fig. 2e. The satellite spots are aligned along the (112) directions with a distance of l/30 [li2],. The (n X 1) structure at 8 > 0.6 ML (Fig. 2c) initially improves slightly with increasing temperature but transforms in the temperature range in which the (3 X 1) structure, seen at low 0, disorders into diffuse short streaks around the W spots along the (113) directions. The transformation temperature increases with coverage, contrary to the case of the (3 X 1) structure, without passing through the sharp streak stage. The transition from the diffuse satellite to the resolved satellite stage (Fig. 2f) occurs at somewhat higher temperatures (1200 K) than at low coverages (800 K). The spot distance in the (173) satellite rows is l/36 [li2],. Annealing of layers with 0 > 1 ML destroys the NW orientation and produces a pattern with sharp streaks through the integral order spots along the (li2) directions and arcs of azimuthally poorly oriented Rh crystals with KS orientation and bulk lattice parameter (Fig. 2g). The transition from the (3 x 1) pattern of Fig. 2a via the long sharp streaks of Fig. 2d to the short diffuse streak pattern of Fig. 2b, with in-

creasing coverage at 350 K or with increasing temperature at 0 < 0.35 ML, is attributed to the random filling of the empty rows of the (3 x 1) structure. This results in narrow 2D pseudomorphic (ps) crystals strongly elongated along the (111) directions which grow in width with increasing coverage or temperature. The break-up of the diffuse streak into satellite spots upon annealing is due to the increasing width of the long 2D ps crystals (“coarsening”). The distance between the satellite spots is a measure for the average width w of the long [lil]-aligned Rh crystals or the average distance of the domain walls, the spot alignment direction is perpendicular to the average direction of the crystal boundaries or domain walls. At low coverages (Fig. 2e) w = 30a,/ & = 15( 171) rows, at high coverages (Fig. 2f) w = 36a,/& 2 18 (111) rows. In unannealed films the spot spacing is larger, l/26-1/28 [1i2],, which corresponds to average crystal or domain widths of 13-14 [lil] rows. The directions of the long-crystal boundaries or domain walls are initially parallel to the W( 111) directions, that is parallel to the close-packed Rh rows ((ii2) satellites), later ((ii3) satellites) parallel to the W(li3) directions, that is inclined to the Rh rows. Upon deposition at elevated temperatures (850 and 1250 K) basically the same structures are observed as upon annealing of layers with the same coverages but with some minor differences: the spacing between the spots of the [li2] satellite rows below 19= 2/3 is l/40 [1?2], instead of i/30 [112],, the transition between (172) and (173) satellite rows is more abrupt and the NW orientation above 0 = 2/3 is not formed. Instead, at 850 K the [113] satellite spacing of l/36 [li2], is observed up to 0 = 1 where it changes within two l/10 ML doses to about l/24 [1?2],. This periodicity is attributed to the rearrangement of the domain walls which accompanies the activated incorporation of additional Rh into the first monolayer. In addition to the satellite pattern long sharp streaks through the integral order spots along the W(1?2) directions are observed over a considerable range of the second monolayer after cooling from the annealing or deposition temperature. In the 850 K deposition a slight

J. Kdaczkiewicz, E. Bauer /Surface

(3 X 1) modulation along the streaks is noticeable at the beginning of the second monolayer, similar but not as pronounced as at low coverage in the first monolayer. This indicates that the second monolayer grows at elevated temperatures initially very similar to the first monolayer with long Rh chains along the W(lil) directions which increase in width only slowly with coverage. During deposition at 1250 K, in which 3D crystals

Science 314 (1994) 221-242

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form on the monolayer, no decrease of the spacing is observed but rather a small increase to l/30 [1121,. This spacing without doubt corresponds to the saturation coverage of the Rh monolayer at 1250 K because it remains constant far beyound 8 = 1. Perfect KS orientation produces W[ li2]-oriented satellite rows but shape diffraction from long narrow [lill-oriented crystals and domain

COVERAGE 0 [ML]

Rh/W 1011) a) oT=350 K b)*T=830 K c) * T=l280 K djxannealed d

T=1550 K

measured at T-XO_t$/

L

IO

20

+J-+-

30 DEPOSITION

TIME

[min]

Fig. 3. Work function change of a W(110) surface as a function of Rh deposition time (bottom) and coverage (top) at various temperatures. Curves (a) and (d) were measured at 350 K, curves (b) and (c) at 830 and 1250 K, respectively.

.I. Kdaczkiewiez, E. Bauer / Surface Science 314 (I 994) 221-242

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increase to a saturation value. The minimum A+ value and the corresponding coverage depend on the sample temperature. At 1 ML a sharp change of slope occurs in the 350 K curve and a less well defined one at approximately the same coverage also at elevated temperatures. Below 1 ML two fundamentally different Ae(e> dependences are seen: (i> a linear one c(b) and cd)), and (ii> a strongly curved one (a). The linear dependence is attributed to the increasing coverage of the surface with large 2D islands with work function 4I < 4w(1,0j, so that 4 = 04, + (1 - 8)+wo,,,, or A4 = ($1 4 w(,,0,)8. The large linear 4 decrease of OS-O.6 eV within the monolayer of a high-4 adsorbate is understandable only if the layer is less densely packed than in the bulk or if it contains many point and line defects (islands edges) which lower the work function. In this respect, Rh is similar to Ni and Co which, however, show a much more pronounced 4 increase after the minimum because of the much larger packing density of the monolayer [26]. The 4(e) curve measured at 1280 K (c) shows a slight curvature at low coverage which is expected because at low coverages the 2D islands are surrounded by a 2D gas with

wall diffraction does too. The observation at T > 750 K of preferred crystal widths (12, 15 and 18 ]liI]w row widths) indicates that the threefold periodicity which governs the low-temperature, low-coverage structure also has influence on the growth at higher temperatures and coverages. Whether the satellite row pattern is due to pseudomorphic regions separated by domain walls or caused by a misfitting monolayer in KS orientation with misfit dislocations cannot be decided on the basis of the geometry of the diffraction pattern. This problem will be discussed later (Section 4). 3.1.3. A$ The work function changes depend very strongly on the sample temperature during deposition. Several A+(0) curves are shown in Fig. 3 in which the deposition time has already been converted into coverage using the AES data of Fig. 1. Two of the curves, (b) and (c), were obtained by measuring A4(0) at 830 and 1280 K, respectively. Curve (d) was measured after the sample had cooled. A general feature of these curves is the decrease of the work function with increasing coverage to a minimum followed by an

Rh/W

(011)

0%

--_____ 500 Fig. 4. Work function

__063”__----.o~3

__-_----

changes

550

_______ 600

(A4) with sample

__-_---gG;-

650 temperature.

700

750

TEMPERATURE

Curve parameter:

I300

[K]

coverage.

J. Kdaczkiewicz, E. Bauer / Surface Science 314 (1994)

non-negligible density. At 830 K this density is still too low to produce a noticeable effect. Individual Rh atoms have a much larger dipole moment than Rh atoms in 2D crystals and, thus, cause an additional N,,, : N,,,,,,-dependent Iowering of 4. This phenomenon was used previously to determine 2D phase diagrams [28]. The stronger and nonlinear 4 decrease during condensation at 350 K is caused by the initial formation of 1D Rh chains along the W( 171) directions in which the Rh atoms also have a larger dipole moment than in the 2D islands. The B,T range in which this occurs has already been discussed [161. The 1D + 2D transition with increasing ~9at 350 K starts already at about 2/10

/-,---_+ -+-+-_i T3

7

750-

/

650

t

1’

I

0.2

0.L

0.6 COVERAGE

Fig. 5. Dependence irreversible changes tom) and magnitude

8 [ML]

of characteristic temperatures (steps of of the work function) on coverage (botof the observed changes (top).

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ML. A segment with a considerably lower slope 0.3 < 0 < 0.6 ML - is linked to structure (1 X 1) with the short streaks, that is to the lateral growth of the chains. Above 0.6 ML 4 rises rapidly due to the transition to 2D islands with NW orientation which have a larger packing density (16.47 X 1014 atoms/cm*). The monolayer value of C#Jat 350 K is significantly higher than at the elevated temperatures shown, which is connected with the different structure of the monolayer (NW versus KS orientation or ps layer with domain walls). The 1D -+ 2D transition in the submonolayer range occurs not only with increasing coverage at 350 K but also with increasing T at fixed coverage. This is illustrated in Fig. 4 for a few coverages. The transition is connected with characteristic, gradual changes of the work function. The number of characteristic steps increases to three with increasing coverage. The temperatures at which the steps in Fig. 4 occur depend upon coverage as shown in Fig. 5. The LEED observations suggest that at low coverages the first step corresponds to the formation of narrow pseudomorphic islands. At the second step large KS layer or domain wall lattice islands develop from the pseudomorphic layer. The last step is connected with a further island size or domain wall (or misfit dislocation) distance increase. In these processes the number of edge atoms decreases which causes a corresponding work function increase. This effect is strongest for the lower coverages, because at high 8 the number of edge atoms is smaller due to the increasing contact between the islands. Above 8 = 2/3 ML the character of observed Ad(T) curves changes (Fig. 4) and instead of the third step a slight, continuous increase in 4 is revealed (by 0.05 eV). The first and second step coincide and are connected with the transition of the NW-oriented layer into a KS-oriented layer or domain wall lattice. For 0 > 1 ML, the work function decreases at T > 570 K, which is accompanied by the decay of the pattern produced by a layer of the NW orientation and is probably associated with agglomeration of the adsorbate. Above 1 ML C$ rises rapidly to a saturation value 0.115 eV above the work function of W(110) which gives ~~~~~~~~ = 5.47 eV assummg 4wo,0j

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230

= 5.35 eV. The deviation of A+(0) from linearity below 2 ML is connected with the transition region to the third monolayer seen in AES. In the layer which was annealed at 1550 K (curve Cd) in Fig. 3) there is very little (p change above 1 ML, in agreement with the conclusion from AES that after 1 ML large 3D crystals form or alloying occurs. In addition, as seen in Fig. 6, desorption sets in at 1520 K above 1 ML. The measurement at 1280 K (c) has to be interpreted similarly, with the additional feature that at the high temperature a significant number of Rh atoms is already expelled from the first monolayer so that 4 increases only little beyond the minimum. In the measurement at 830 K (b) more Rh atoms are still incorporated into the first monolayer, with a correspondingly larger 4 at 1 ML. The delayed 4 increase above 1 ML can be attributed to an initial 2D gas phase or to formation of the poorly ordered Rh chain phase with (3 x 1) periodicity which condenses at the coverage at which A+ starts to rise linearly (approximately 1.2 ML) into long narrow Rh islands (see Section 3.1.2). The much smaller C$in this coverage up to 2 ML than

at 350 K is attributed to the large number of edge atoms of these islands. The subsequent C#Jincrease is connected with the formation of flat (Ill)-oriented 3D Rh islands from this chain structure which has been deduced from the AES and LEED data. In total the A4 data are very compatible with the AES and LEED data and in part elucidate, them. 3.1.4. TDS The TDS spectra up to somewhat more than 1 ML are shown in Fig. 6. The high desorption temperatures indicate that the first Rh layer is strongly bound to the substrate. The second layer desorbs at significantly lower temperatures in a desorption peak which is seen in Fig. 6 only as a shoulder at 0 = 1.25 ML but is separated at higher coverages from the peak of the first monolayer similar to many other transition and noble metals on W(110) and Mo(ll0). The peak position of the first monolayer shifts with increasing coverage from 1840 to 1865 K at about 0.8 ML and then remains constant. This is also similar to the desorption behavior of other transition metals from

z 5

Rh/WlOll)

glO-

t-6.6K/s

$9iii

f3L

0

1-cl08

&-

F tl

2-0.20 3-0.28 4-Ok5

si6

6-0.7L5 5-0.665 7-0865 8498

i&*L:I

1500

1600

1700

18Ou

1900

2000 TEMPERATURE T[KI

Fig. 6. TDS spectra obtained with a heating rate of 6.6 K/s. The curve parameter is the coverage.

J. Koiaczkiewicz, E. Bauer /Surface

W(110) and Mo(ll0) [26,29]. An analysis of the TDS data to extract the coverage dependence of the desorption energy and of the pre-expotential factor will be made elsewhere [30]. 3.2. Platinum 3.2.1. AES The dependence of the Auger electron signals of Pt(NW 64 eV) and of W(163/168 eV) upon deposition time is shown in Figs. 7 and 8. The break points of the AA(t) curves depend upon deposition temperature. Deposition onto a sample held at 400 K resulted in two linear segments in the coverage range studied, separated by a shoulder. This shoulder is missing in the 850 K deposition curve which allows - together with the A#J and LEED data to be discussed below and with the second break of the 400 K curve - an unambiguous coverage assignment. The sharp

pt/w

11101

DEPOSITION

TIME

t [min.]

Fig. 7. Dependence of the Auger electron signal of Pt (64 eV) and W (163/168 eV), and of the work function on deposition time of platinum onto a tungsten (110) surface. Deposited and measured at 400 K.

Science 314 (1994) 221-242

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break in the 850 K Pt Auger signal at t, (Fig. 8) signals the completion of the first monolayer with NW orientation with a coverage of 13/11 ML (see Section 3.2.2), the second break in the Auger signals and the A4 maximum in Fig. 7 the completion of the double layer (1,). Accordingly, the pseudomorphic monolayer is completed at t, = 11/13t,. This is in agreement with the 400 K A4 data of Fig. 8 which show a stronger increase between 1 ps layer and 8 = 1 (marked with arrows) due to the transition from the pseudomorphic layer to the more close-packed NW layer. The shoulder in the Pt Auger signal in Fig. 7 is due to a change of the Auger shape caused by the change of the electron structure in the transition from the ps to the cp monolayer similar to that seen in Co and Ni on W(110) [31,32] and on MoUlO) [26] or in the transition from the monolayer to the double layer in the system [33]. During Au/Mo(llO) [27] and Au/W(llO) deposition at 1200 K the Pt Auger signal increases linearly until completion of the ps monolayer and is nearly constant after a short transition region which indicates either 3D growth after the ps ML or alloying. Also at 850 K the slope ratio sz/sI = 0.25 is much smaller than at 400 K (s.Jsr = 0.65) Measurements over a wider coverage range than shown in Fig. 7 give at 400 K in addition a slope ratio s3/s2 = 0.55. Thus, growth proceeds at 400 K monolayer-by-monolayer, at least for the first three monolayers, while at 850 K multilayer growth occurs beyond 1 ML. 3.2.2. LEED During Pt deposition at 400 K a pattern with sharp streaks through the integral order spots along the (112) directions and l/3-order intensity maxima, similar but not as perfect as in Rh, is observed. This (3 X 1) pattern indicates one-dimensional atomic chains along the (171) directions of tungsten at every third substrate row on the average. The maximum intensity of the 1/3order reflection from this structure occurs at 8 = 0.11 ML. Upon further deposition the l/3-order intensities decrease resulting in continuous streaks with little intensity modulation (Fig. 9a). With increasing coverage the streaks shorten and at 8 = 2/3 ML briefly point into the W(li3)

232

.i. K~czk~e~~cz,

E. Bauer / Surface Science 314 (1994]221-242

directions before the (n X 1) pattern of the NW orientation appears. This pattern persists up to 2 ML with n decreasing from 13 at 8 < 1 via 11 at 8 > 1 toward 10 for 8 > 2 ML. At the same time the spot distance along the [l?O] direction decreases to 0.98 [liOlw and the satellite reflections weaken. At the highest coverage studied (2.20 ML) only the six first-order spots from the Pt(ll1) plane are observed. The atomic density of the Pt(ll1) plane as deduced from the dimensions of its reciprocal lattice (1.19 [OOl], X 0.98 [iio],) is Npt = 1.166N,o,nt = 16.47 X lOi atoms/cm’ which is 9.2% higher than of the Pt(lll) plane in the bulk. During deposition at 850 K only short streaks in the W(li2) directions, emanating from the integraf order spots, are observed. They indicate more equiaxial 2D ps crystals than those observed at 400 K. Above 8 = 2/3 ML the streaks point in the (li3) directions and at 0 = 1 ML a (11 X 1) NW orientation appears corresponding to N,, = 0050 K> 1.18~W~~~~~. At still higher temperatures

Oli T 23

the (li2)-oriented streaks resolved into satellite spots starting at about l/3 ML. Their distance increases from about l/36 [li21, at 8 = 0.4 ML t0 i/i8 [ii21,at 8 = 2/3 ML. Shortly before 8 = 2/3 ML satellite spots around directions, spaced 3 W(lil) rows apart. Thus, Pt grows at

Pt/wlllol x 400

I<

. 850

K

COVERAGE

8

[ML]

Fig. 8. Dependence of the work function and of the Pt and W Auger electron signals of layers deposited at high temperatures and measured at room temperature ( . ’ ). T= 8.50 K (0) T= 1200 K, (+) T = 1450 K. For comparison, another 400 K cume (x) is shown too.

J. Kotaczkiewicz, E. Bauer / Surface Science 314 (1994) 221-242

233

Fig. 9. LEED patterns of Pt on W(110): (a) E, = 47 eV, T= 400 K, 0 = 0.23 ML; (b) E, = 68 eV, T= 1370 K, 0 = 0.63 ML; (c) E, = 48 eV, T = 1480 K 13= 0.29 ML; (d) E, = 48 eV, T = 1000 K, 0 = 0.29 ML.

this high temperature on the ps ML initially in a manner very similar to that seen on the bare W(110) surface. Annealing of layers deposited at 400 K leads to patterns similar to those observed after deposition at elevated temperatures. In the limited number of experiments the streaks or satellite rows in the W(li3) directions were never seen. The [li2] streaks always developed into [li2] satellite rows at a coverage-dependent transition

temperature and a coverage- and annealing-temperature-dependent spot distance. At 8 = 0.4 ML satellite spots develop already at 850 K with distances of l/18 [li2], which decrease to l/20 [li2], after annealing at 1000 K. At 0 = 0.5 ML satellites with l/22 [li2], spacing are observed after annealing at 1000 K. In general, 800 K seems to be necessary to activate the ordering processes needed for satellite formation. The satellite pattern with satellites also around the

234

.I. Kdaczkiewicz, E. Bauer /Surface

(6 x &)R30” positions at 0 = 2/3 ML become very sharp by annealing at temperatures as high as 1250-1600 K. When low coverages, e.g. 8 = 0.4 ML, are annealed that high and cooled, the satellites become very diffuse with large spacings (Fig. although at intermediate SC), e.g. l/12 [ii2],, temperatures (e.g. 1000 K) they become more sharp with small spacings (Fig. 9d) (l/20 [1?2],). This suggests that once the larger islands formed at intermediate temperature have sublimed into the 2D gas phase at high temperatures, 2D condensation upon rapid cooling results in rather small islands. LEED, thus, leads to the following picture: at lower temperatures (< 800 K), at which a NW layer develops above 13= 2/3 ML, Pt grows pseudomorphically on W(llO), with ordered vacancies at 13= 2/3 ML which produce the 2 ML. In the transition region between the ps and cp layer C$ rises more rapidly than in the second layer. The work function depends strongly on temperature, as in the case of Rh, and shows more details than the Auger data which reflect changes of the growth mechanism. At T = 850 K the changes of the work function are significantly smaller. A broad minimum occurs at about 2/33/4 ps ML. The subsequent linear increase shows

Science 314 (I 994) 221-242

c Pt/iOll)

COVERERAGE

6 [ML]

Fig. 10. Diagram of LEED structures observed after annealing. Open circles correspond to steps of irreversible work function changes. The various symbols have the following meaning: (a) (3X 11, (b) continuous streaks, (c) short streaks, (d) (n X 1) NW, (e) short streaks, (0 elongated (li2) satellites, (g) elongated (113) satellites, (h) excellent (li2) satellites, 6) “KS’, (j) “KS”, (k) (1 X 1) large spots, (I) (3 x 1) streaks.

only a minor slope change upon completion of the cp ML. The work function increases at 850 K to 5.55 eV at 0 = 1.6 ML and stays at this value up to 8 = 2 ML Co,,,). The layer deposited at 1200 K shows a more complicated coverage dependence of A+ with three local minima. The C#J change is below 0.1 eV and approximately saturates upon completion of the ps monolayer at A$ = -0.04 eV. Three local minima are also observed in layers annealed or deposited at 1450 K, two of them at the same coverages at which the minima at T = 1200 K occur, the third one close to 1 ps ML. Beyond the third minimum Ac$ rises rapidly to its saturation value + 0.025 eV.

J. Kotaczkiewicz, E. Bauer /Surface Science 314 (1994) 221-242

The slope changes of the A4(0) curves are connected with changes of diffraction patterns. The largest (negative) A#(61 slope at room temperature is connected with the (3 x 1) structure and the sharp streaks along the (172) directions. The largest (positive) slope is connected with the transition from the (1 X 1) ps to (13 X 1) cp (NW) structure. Between these two regions the transition from the quasi ID (3 x 1) structure and 2D islands of increasing size occurs which in its details is very sensitive to the deposition conditions as seen in the two A4(0) curves in Figs. 7 and 8. The Pt double layer has nearly the work function of bulk Pt (5.85 eV) and with increasing coverage by second monolayer islands this value is linearly approached (Fig. 7). At T = 850 K the AgNtl> curve is rather smooth because more equiaxial ps islands grow from the very beginning (see Section 3.22). The small initial decrease is due to the dipole moments of the island edge atoms, the increase above 3/4 ps ML is caused by their elimination. At higher temperatures the A+(e) curves are more complex, due to the more pronounced structural changes with coverage. In the 1200 K deposition the individual islands are larger and begin to reorganize into the structure which produces the fi satellite pattern and completely covers the surface at 8 = 2/3 eliminating all island edges, thus causing a work function increase towards 8 = 2/3. The subsequent decrease is attributed to the structural reorganization into ps crystals which produce the (173) satellite pattern and have island edges which are eliminated again when the coverage approaches 1 ML. The structure - Ad, correlation for the even more complex 1450 K data is indicated in Fig. 10. The differences between the A~(~~ curves at different temperatures indicate that heating causes irreversible A6, changes also in the case of platinum. The changes occur in more or less pronounced steps again (see Fig. 4 for Rh). The magnitude of the changes at the various steps shows the same trend as in the case of Rh: the changes are largest at the first step and then decrease. The largest change at a given step is coverage-dependent. For the first step it occurs at 8 = 0.11 ML, for the second step at 0.6 ML and for the third step at 0.6 ML. The temperatures of

235

the onset and completion of the changes depend both on the coverage, on the heating rate and on the heating time. A slower temperature rise or a longer heating time shifts the relevant temperatures to lower values. Heating of adlayers more than one monolayer thick decreases the work function without characteristic steps. These irreversible changes of the work function are due to transitions of the layer structure. The most significant change (first step) is related to the disappearance of the (3 x 1) structure and to the formation of sharp streaks along the (112) directions, which is attributed to the collapse of the (3 X 1) rows into narrow long 2D pseudomorphic islands along the (111) directions. Associated with this transition are also changes of the electronic structure of the layer, because the energy losses of back-scattered electrons from the surface change by about 1 eV [30]. The first step disapppears between f3= 0.45 and 8 = 0.625 because in this coverage range the long narrow ps islands are merging increasingly already during deposition so that subsequent annealing eliminates fewer and fewer island edges. The second step is connected with the transition from_ the ps islands, which produce the short W{112)-oriented streaks or satellite rows, into the structure with 6 satellites which reaches its optimum coverage at 0 = 2/3 where 4 has a local maximum at 1200 K (see Fig. 8). At 8 = 0.89 ML the second step is of a different nature: the (13 X 1) NW diffraction pattern converts at around 700 K into the [l-i31 satellite pattern (see Section 3.2.2) which indicates the transition from an incommensurate to a commensurate layer with domain walls. This layer is atomically less rough and, therefore, has a higher work function. 3.2.4. TDS TDS spectra obtained for platinum are very similar to those of rhodium (Fig. 6). The temperature at which the desorption maximum occurs is almost independent of coverage: T= 1950 K for 8 = 1, at a heating rate of 7 K/s. In the highcoverage range, 6 > 1 ML, a second peak appears at the low-temperature side of the desorption curves. It is visible initially as an inflexion in the curve and is not from a second monolayer but

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J. Kotaczkiewicz, E. Bauer / Surface Science 314 (1994) 221-242

rather from 3D crystals or from a Pt-W alloy according to the AES and A+ data (Fig. 7). 3.3. Iridium Because of its high sublimation and desorption temperatures most experiments were limited to the submonolayer range and the absolute coverage determination is rather inaccurate. The dependence of the Auger electron signals of iridium

((NW) 54 eV) and of tungsten (163/168 eV> is shown in Fig. 11. Heating the layers to temperatures above 1500 K caused changes of the ratios of the adsorbate and substrate peak heights. At 0 < 0.5 ML the ratio of the adsorbate peak to the substrate peak after annealing was lower than before, at 8 > 0.5 ML it was higher than before. This indicates a coverage-dependent redistribution of atoms, possibly accompanied by changes in electronic structure and changes in the Auger W&RAGE

0.5

0.25

B,[ML]

1.0

k/W (011)

5

Fig. Il. Auger Measurements experiments.

10

15

* annealed . annealed

at T=1650 K at T=1250 K

+deposited xdeposlted

at T=L50 K at T=350 K

20 DEPOSITION

TIME

25 [min]

electron signal of Ir(74 eV) and W(163/168 eV), and work function of Ir on W(110) as a function of deposition time. at room temperature. The tentative coverage scale on the top has been obtained by combining information from all

J. Kdaczkiewicz, E. Bauer / Surface Science 314 (1994)

signal peak shapes. TDS experiments encountered difficulties because of the risk of melting the crystal at the high temperatures needed for complete desorption and because of the strong pressure rise above 2500 K at reasonable heating rates (< 10 K/s). Therefore, the coverage could also not be calibrated by TDS. For these reasons the investigation was limited mainly to LEED and A+ studies.

221-242

237

In the submonolayer range the LEED patterns of Ir films deposited at 450 K consisted of diffuse circles around the W spots. The size of the circles decreases with increasing temperature but is independent of coverage within the limits of error. The circles are visible in the coverage range from 0.2 to 1.25 ML. They indicate small, approximately round islands whose size and average distance increase with increasing temperature. With

Fig. 12. LEED patterns of Ir on W(110): (a) E, = 50 eV, T = 720 K, 0 = 0.18 ML; (b) E, = 61 eV, T = 1150 K, 13= 0.60 ML; (c) E, = 67 eV, T = 1720 K, 0 = 0.75 ML; Cd) E, = 67 eV, T = 1450 K, 0 = 1.50 ML.

238

J. Kdaczkiewicz, E. Bauer /Surface

further increasing annealing or deposition temperature the diffuse rings transform into diffuse streaks in the (172) directions. The low-temperature (3 x 1) pattern of the chain structure seen in Rh layers could be seen only in the depositions at 350 K with the low-power evaporator and was less well pronounced in Ir layers. Upon heating the (3 x 1) pattern transforms into diffuse streaks and satellites without passing through the diffuse ring stage of the 450 K deposits (Fig. 12a). With further increasing 0 or T the diffuse satellites around the integral-order spots along the W(li2) directions contract (Fig. 12b1, weaken and become sharper, with a spot spacing of l/22 [li21,. For 8 > 2/3 ML the satellites are aligned along the (li3) directions (Fig. 12~) after high-temperature annealing very similar to Rh and Pt on W(110) (Fig. 2f). The spot disjance is, however, larger, l/20 [li2], = 0.038 A-’ which corresponds to 10 (111) rows in real space. Above 1 ML the LEED pattern of Fig. 12d is obtained after deposition or annealing at high temperatures (> 800 K). This pattern agrees with the double scattering pattern between W(110) and a Nishiyama-Wassermann-oriented Ir(ll1) crystal with bulk lattice constant (to within 0.6% which is the limit of error of the somewhat distorted LEED patterns) (Fig. 13). Of course, the double scattering spots could also be due to a misfit dislocation network. A ps layer with domain walls can be excluded, however. The results of the work function measurements as a function of deposition time at 350 and 450 K without and with annealing at 1250 and 1650 K are shown in Fig. 11 together with the AES results. At 350 K a strong initial C#Jdecrease is observed as in the Rh and Pt layers, caused by 1D chain formation. At 450 K the initial decrease is smaller because the ps 2D islands which form at this temperature have a higher work function than the 1D chains. Annealing at high temperatures causes a significant C$ increase due to the formation of large islands. The fraction of edge atoms decreases with increasing island size, e.g. from 1250 to 1650 K annealing temperature, and consequently C$increases. Upon annealing layers with fixed coverage 0 < 1 ML at increasingly higher temperatures the work function increases

Science 314 (1994) 221-242 L

Ir [Ii0 I w [OOI] /

Fig. 13. Reciprocal lattice of Idlll) on W(110). Large full circles: W; small triangles: Ir; small spots: multiple scattering or misfit vernier spots. Only the spots closest to the W spots are shown.

stepwise, similar to the increases seen in Rh and Pt. These increases are due to the structural changes mentioned earlier. They allow the determination of the temperatures at which major structural arrangements occur less tediously than with LEED. For example, the transition from the diffuse circle pattern to the streak pattern is accompanied by a work function change which occurs independent of coverage at 810 K and is about 0.05 eV from about 13= l/3 ML upwards. The transition from the diffuse streak pattern to the satellite pattern is also accompanied by a small 4 change (0.10 eV> at 950 K over a wide coverage range. The largest A+ step is, however, again that which is connected with the transition from the 1D chains to 2D islands which is very pronounced in the 350 K deposits and to a certain extent still visible in the 450 K layer. Apparently, at 450 K still a non-negligible number of 1D chains still is formed too, together with the small 2D islands. 4. Discussion 4.1. Common aspects of ultrathin Rh, Pt and Ir films on W(ll0)

The detailed results of the preceding section lead to the following general picture of the growth

J. Kotaczkiewicz,E. Bauer /Surface Science 314 (1994) 221-242

of Rh, Pt and Ir on W(110). At sufficiently low temperatures - 350 K is low enough for these materials - the atoms align initially in long 1D chains along the W(lil) directions. Because the atomic radii of film and substrate atoms differ very little these chains correspond to the densely packed (150) atomic rows in the (111) planes of the fee layer metals. (In a two- or three-dimensional layer this alignment is the KurdyumovSachs epitaxial orientation.) The rows have on the average a distance of three W(lil) rows, an ordering which is particularly well pronounced in Rh layers and which produces the (3 x l)modulated W(li2) streak LEED pattern. With increasing coverage the chains grow laterally by more or less statistical addition of rows which reduces the length of the streaks. The lateral growth of the rows is partially accompanied by the formation of diffuse satellites which are much better pronounced during and after growth at elevated temperatures and will be discussed below. At about 0 = 2/3 ML a structural rearrangement occurs in which the direction of the streaks changes from W[li2] to W[li3]. This structure is also better developed at higher temperatures and will be discussed below. Above 8 E 2/3 ML the [li3] structure is increasingly replaced by a close-packed structure with Nishiyama-wassermann orientation, at least in Rh and Pt layers, which produces the (n x 1) pattern, with the IZfold periodicity in the W[OOl] direction. Above 1 ML n decreases within 2-3 monolayers to a value compatible with a slightly compressed fee (111) plane of the metals. Simultaneously the substrate spots and double scattering - or misfit dislocation modulation - spots disappear. At the low temperatures just discussed, film growth is kinetically limited so that extreme nonequilibrium configurations such as the long (liO)-needle-like fee crystals develop initially or very small 2D crystals grow without coarsening when they join with increasing coverage. In order to approach the equilibrium structure deposition at elevated temperatures or annealing is necessary. In spite of the minor structural differences between layers deposited and those annealed at elevated temperatures - in the latter case there are more barriers which have to be overcome in

239

the approach to equilibrium - both cases will be discussed here jointly. There are four major aspects of the quasi-equilibrium case: (i) no 1D rows form but rather more equiaxial 2D crystal, (ii) the w[liZl- and [li3]-oriented streaks are replaced by satellite patterns, (iii) the transition to the NW orientation is shifted close to 1 ML or does not appear at all any more, e.g. at 1200 K in Rh layers, and above 1 ML it is replaced by a [li2] streak pattern, in the case of Rh in addition with azimuthally misoriented KS crystals; in the case of Ir the NW orientation is preserved in the limited 0 range above 1 ML which was studied. (iv) The quasi-Frank-van der Merwe-type growth is replaced by Stranski-Krastanov-type growth. Aspect (9 has been discussed in Ref. [16] and aspect (iv) is a general phenomenon observed in many misfitting ultrathin films and needs no further elaboration. Therefore, only aspects (ii) and (iii) require some discusion. As already mentioned in Section 3 the satellite pattern can have several causes: (1) shape diffraction from finitesize ps crystals, (2) diffraction from misfitting monolayer islands with KS-like orientation and (3) diffraction from a domain wall lattice of a ps structure. In cause (1) the LEED pattern is the Fourier transform of the crystal shape. In order to obtain as sharp satellites as frequently observed at least several 2D crystals must scatter in phase which is possible within the coherence limit of the LEED system used (100 A). However, these crystals must all have the same dimensions and the same distance, otherwise the satellite distances would be different and the peak smeared out. There are two more reasons which speak against cause (1): (a) The satellite spot spacing is in general independent of growth or annealing temperature; one would expect an increase in crystal size with temperature and consequently a decrease of the spot spacing. (b) The spot spacing is independent of coverage while it should decrease because of the expected increase in crystal size. In Pt layers, actually, the reverse situation has been observed: an increase of the [li2] satellite spot spacing up to 8 = 2/3 ML (Section 3.2.2). Thus, cause (1) may be discarded. The choice between causes (2) and (3) is not as straight-forward because a misfitting layer and ps

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J. Kotaczkiewicz, E. Bauer / Surface Science 314 (1994) 221-242

layer with domain walls cannot be distinguished on the basis of the geometry of the diffraction pattern if they have the same periodicity. Only an intensity analysis can do this which is not possible for such a complicated configuration at the present state of the dynamical theory. Therefore, the choice has to be based on other observations. First, however, the possibility of a domain wall lattice in a pseudomorphic layer has to be discussed. If there is only one equivalent adsorption site on the surface, e.g. the hollow site on a fee (100) surface, then a pseudomorphic, that is a (1 X 1) layer, has no domain walls. On a bee (110) surface there are two types of adsorption sites, the “lattice” site and the “surface” or “three-fold coordinated” site. The. unit mesh contains one lattice and two surface sites. The binding energy of the surface site is frequently sufficiently larger than that of the lattice site [34] so that many atoms are adsorbed in the surface site. Rh is one of them as demonstrated by FIM studies [35] and probably Pt and Ir are too. Thus, two domains of a ps layer differing in lattice site position are possible for these metals. The domain wall interpretation is favored for at least two reasons: (1) with the possible exception of Ir which has a strong tendency to NW orientation even at high temperature, the [li2] satellite pattern converts via the [li3] satellite pattern into a (1 x 1) pattern, (2) on top of this (1 X 1) structure Rh and Pt grow in a manner similar to that on the bare W(110) surface. Thus, we conclude that the satellite patterns are from large ps islands consisting of “site-exchanged” domains which are separated by W[li2] or W[li3] domain walls. Domain lattices of the second type have recently been reported for the high-coverage structures of oxygen on W(110) [36]. The observations of aspect (3) also deserve some comments. The shift of the transition from ps to cp structure to higher coverages with increasing temperature is a general feature of metal layers with atomic radii smaller than that of the substrate. This transition has been studied in detail in Ni and Co layers on Mo(ll0) [26] and has been analyzed theoretically recently [37]. The kinetically formed cp regions are metastable below the completion of the ps monolayer and

convert to the ps structure upon annealing as long as the average coverage does not exceed that of 1 ps ML. In Rh and Pt this effect is not as clearly seen as in Ni and Co layers because the packing densities of ps and cp layers differ much less. The [li2] streak pattern above 1 ML has already been discussed but the observation of misoriented KS crystals is surprising in view of the tendency to NW orientation at low temperatures. Apparently the template supplied by the NW-oriented first monolayer at low temperatures induces the NW orientation also in the subsequent layers while the pseudomorphic layer provides a surface on which the KS orientation is more favorable. A similar bimodal growth mode was found earlier in the system Pd/W(llO) [B]. Pt and Ir were not studied in the relevant B,T range so that nothing can be said about them. Ir is unusual at high temperatures. The strength of the interatomic interactions in Ir is significantly larger than that in Pt and Rh and is comparable to that in W. As a consequence, the bulk interatomic distances appear already at about 1 ML and a misfitting ML with NW orientation is stable up to high temperatures. Pt also shows a structure which was not observed in the other two metals, the satellite pattern with additional satellites centered around the ‘Yfi X fi)R30”” positions. (A similar, more diffuse pattern has been reported for Au/Mo(llO) [27].) This structure has a very narrow existence range and is also a ps layer with domain walls but with a regular distribution of Pt vacancies which form the distorted honeycomb structure of the ‘Yfi X filR30”” lattice. An analysis of this and other satellite patterns within the framework of the kinematical theory will be given within another context [38]. 4.2. Comparison with other film-substrate with comparable atomic radii ratios

systems

Film-substrate combinations in which similar structures may be expected should have rb/ra ratios above 0.97 and below 1.06, otherwise pure NW or KS orientation, respectively, would be expected. In addition, for rJra > 1 a ps structure

J. Kdaczkiewicz, E. Bauer /Surface

becomes increasingly unfavorable. These conditions suggest to consider the following combinations: Pd/W, MO; Rh, Pt, Ir/Mo, possibly also Au, Ag/W, MO (rb/ra = 1.05-1.06) and Pd, Pt/Nb, Ta (rb/ra = 0.96-0.97). The first two systems have been studied previously [l&191 but not in sufficient detail to allow a systematic comparison over a wide B,T range. Several features are similar to those described here, such as the bimodal growth of thicker films (NW or KS orientation, depending upon growth and annealing conditions) or the (3 x 1) structure above 1 ML. There are, however, also significant differences, in particular in the submonolayer range. The system Rh/W(llO) is very similar to the Rh/Mo(llO) system [6], with (112) satellites spaced l/20 [li21,,, apart at low coverages and a transition to the NW-oriented cp structure with increasing coverage. Very likely, the satellite pattern in Rh layers on Mo(ll0) has the same explanation as that on W(llO), that is in terms of a ps layer with periodic domain walls, and not in terms of a misfitting layer with KS orientation. In the cursory LEED work on Pd on Nb(llO1 [201 and Ta(ll0) 1211, and on Pt on Nb(ll0) 1221 only (1 x 1) and NW-type patterns were found similar to the systems Ni/Mo(llO) and Co/Mo(llO) 1261. This does not necessarily mean that no domain wall lattices are formed in the appropiate B,T regime because in other studies of the Pt/W(llO) systems [7,8] they have not been noticed either although clearly evident in the present study. With rb/ra = 0.963 and 0.971 for the Pd/Nb and Pt/Nb systems, values which are close to the ideal NW ratio rJr, = 0.943, it is quite possible, however, that these two systems exhibit the same scenario as the systems Ni/Mo(llO) and Co/Mo(llO) do. In contrast to these systems Au and Ag on Mo(ll0) do show satellite patterns in the submonolayer range [39] while on W(110) a satellite pattern is observed which is commensurate in the W[OOll direction and incommensurate in the W[liO] direction [40,41]. The apparent incommensurability is possibly again due to a domain wall lattice of a ps layer but with domain walls parallel to W[OOl]. In view of the similarity of W and MO these differences are surprising and indicate that details in the electronic structure

Science 314 (1994) 221-242

241

have profound influence on the structure of ultrathin films.

5. Summary and conclusions This study was conducted to elucidate the relationship between the tendency of ultrathin layers to form 1D crystals, that is cp atomic rows at low coverage and low temperatures, and the film structure at higher coverages and/or higher temperatures. It is found that these layers do not tend towards the Kurdyumov-Sachs orientation in which the close-packed rows have the same directions as in the 1D crystals, but rather towards the Nishiyama-Wassermann orientation. The KS-like diffraction patterns in the submonolayer range rather are in general caused by a periodic array of domain walls perpendicular to the W( 172) and (113) directions which separate ps domains differing in adsorption site. In a narrow region around 8 = 2/3 ML, a periodic vacancy structure can appear. The quasi-Frank-van der Merwe-type growth at low temperatures changes to a Stranski-Krastanov-type growth at higher temperatures or to alloying beyond one monolayer.

Acknowledgements The experiments were carried out with support by the Deutsche Forschungsgemeinschaft. Partial support by the State Committee of Scientific Research (KBN) Grant No. 2011199101 is also acknowledged.

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[24] P.J. Berlowitz and D.W. Goodman, Langmuir 4 (1988) 1091. [25] E. Bauer and J.H. van der Merwe, Phys. Rev. B 33 (1986) 3657. [26] M. Tikhov and E. Bauer, Surf. Sci. 232 (1990) 73, and references therein. [27] A. Pavlovska and E. Bauer, Surf. Sci. 117 (1986) 473. [28] J. Kolaczkiewicz and E. Bauer, Phys. Rev. Lett. 53 (1984) 485; Surf. Sci. 151 (1985) 333. [29] J. Kdaczkiewicz and E. Bauer, Surf. Sci. 175 (1986) 508. [30] J. Kolaczkiewicz and E. Bauer, in preparation. [31] H. Knoppe and E. Bauer, Phys. Rev. B 48 (1993) 1794. [32] C. Koziol, G. Lillienkamp and E. Bauer, Phys. Rev. B 41 (1990) 3364. [33] H. Knoppe and E. Bauer, Phys. Rev. B 48 (1993) 5621. (341 H. Gollisch, Surf. Sci. 175 (1986) 249. [3S] H. Krause, MS Thesis, Clausthal, 1981. [36] K.E. Johnson, R.J. Wilson and S. Chiang, Phys. Rev. Lett. 71 (1993) 1055. [37] J.H. van der Merwe and E. Bauer, Phys. Rev. B, to be published. [38] E. Bauer, to be published. [39] E. Bauer and H. Poppa, Thin Solid Films 121 (1984) 159. [40] E. Bauer and H. Poppa, Thin Solid Films 12 (1972) 167. [41] E. Bauer, H. Poppa, G. Todd and P.R. Davis, J. Appl. Phys. 48 (1977) 3773.