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Study of ultrathin Fe films on Pd{111)}, Ag{111} and A1{111}
Surface Science 280 (1993) 289-297 North-Holland
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Study of ultrathin Fe films on Pd{ ill)}, Ag( 111) and Al{ 111) A.M. Begley a, D. Tim a*t,F. Jona a and P.M. Marcus b aDepartment ofMaterials Science and Engineering, State University of New York, Stony Brook, NY I1 794-2275, USA b IBM Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA
Received 6 Juty 1992; accepted for publication 15 September 1992
The growth of epitaxial Fe films of 1 to 10 layer equivalents (LIE) on Pd{lllJ, Al(111) and Ag{lll) has been studied with quantitative low-energy electron diffraction and Auger electron spectroscopy. On Pd(lll), both at room temperature and at 2oo”C, the growth starts with pseudomorphic layers which may involve intermixing of Fe and Pd. At both temperatures, when the coverage reaches 6 LE, the Fe films develop into large bee (IlO} domains related to the substrate by the Kurdjumov-Sachs orientation. On Ag{lllf Fe grows initially in a very similar manner to Fe on ~{~l~, i.e., by way of small islands of bee Fe. These islands then grow into bee Fe{1101 domains in the Nishiyama-Wasserman orientation. On Alflll) Fe behaves differentiy, despite the fact that the lattice mismatch for Fe on Al{lll) is nearly identical to that for Fe on Ag(lll}: for coverages of less than 1 LE the surface region becomes completely disordered and no LEED pattern is visible.
1. Int~uction Although the study of magnetic ultrathin films on metal surfaces is long-established and widely reported, not much is well-established about the growth mode and the ~stalIograph~c structure of the films, and the results are often controversial. Both the magnetic and the structural properties of films of a few atomic layers can turn out to be very sensitive to strain, substrate interaction and substrate temperature [I]. The resulting problems of fiIm reproducibiii~ between laboratories have led to a lack of general agreement over growth modes for several epitaxial systems. This situation is unfortunate, because without a detailed understanding and characterization of film growth and structure the interpretation of results from ma~etic measurements must always remain open to question. Claims about growth modes (layer-by-layer, island growth or combinations thereof) and about crystallographic structure of ultrathin films are t Di Tian was tragically kilied in a car accident on February 9, 1992.
usually made on the basis three types of measurements: (1) qualitative reflection high-energy electron diffraction (RHEED) observations (in particular, the presence or absence of intensity oscillations); (2) qualitative low-energy electron diffraction (LEED) observations (the presence or absence of 1 X 1 patterns); (3) qualitative Auger electron spectroscopy (AES) observations (presence or absence of “breaks” in the plot of Auger intensities versus deposition time). These observations may in some cases allow the identification of the growth mode, but such an identification should be done with extreme care because, e.g., ABS breaks can be affected by electronescape depth, diffraction effects and band structure [2] whereas RHEED oscillations are merely a result of periodically repeated surface conditions, not necessarily flat surfaces, in the course of film growth [3]. In any case, the conclusions reached from AES breaks or RHEED oscillations or qualitative LEED about the growth mode of an ultrathin film can and should be tested quantitatively with appropriate su~a~-sensitive techniques such as surface X-ray diffraction, medium-energy ion
0039-6028/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
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A.M. Begley et al. / Ultrathin Fefilms on Pd{lll},
scattering or quantitative LEED intensity analysis. Indeed, if the growth is layer-by-layer, for example, any one of these techniques should be able to confirm it and in favorable cases should be able to determine both the inter- and the intra-layer atomic distances involved. If the growth is by islands the determination of interatomic distances may not be possible in the early stages, but the absence of layer-by-layer growth should be recognizable in any case. Quantitative LEED procedures have been recently used in several epitaxial studies [4] and have in general produced results in disagreement with the conclusions reached by means of AES breaks, RHEED oscillations or qualitative LEED observations. It is possible of course that the disagreement may not be ascribable to the fact that different techniques were employed, but rather to differences in experimental procedures, such as, e.g., substrate preparation, surface morphology, materials purity, etc. The only way to know for sure is to carry out all experiments (film growth, magnetic measurements and quantitative crystallographic determinations) on the same sample, in the same experimental chamber and at the same time. Unfortunately, such an undertaking has not been made to date, but is desirable. We present here some results of epitaxial studies done with quantitative LEED methods, which, again, are at least partially in disagreement with the conclusions reached in other laboratories. The epitaxial systems considered here are Fe/Pd{lll), Fe/Ag{lll) and Fe/Al{lll). Both the Fe/Pd{lll) and the Fe/Ag{lll) are thin-film systems on which magnetic measurements were made and interpreted without the benefit of a more extensive understanding of the growth as provided by quantitative structure determinations. The Fe/Pd(lll) system was investigated by Binns et al. [5] in the ultrathin range (O-2 layers). These authors used AES data and qualitative LEED observations to infer that the growth occurred in the “simultaneous multilayer” mode (a mode which involves the development of the nth layer before the (n - list layer is complete) and claimed to be able to determine the coverage by iron to within + 0.05 monolayers. They concluded
Ag{lII}
andAl{lll}
that the growth of Fe on Pd(ll1) “is perfectly epitaxial for two atomic layers, with relaxation beginning in the third layer” [5]. The growth of Fe on Ag(ll1) was investigated by three different groups with somewhat contrasting results. Olsen and Snyman [6] determined by electron microscopy that the growth “initiates with the formation of distinct three-dimensional nuclei”. Gutierrez et al. [7] recognized, by using RHEED, that bee FelllO) domains grow epitaxially in three related Nishiyama-Wasserman (NW) orientations, but found that at 180°C two of these orientations are suppressed and, as a result, Fe{ 110) grows epitaxially at that temperature with only one orientation. Qiu et al. [S], more recently, studied the magnetic properties of l-3 layersthick Fe films on Ag{ 111) which they grew with the procedures described by Gutierrez et al. [7], and concluded, from AES breaks, RHEED and qualitative LEED observations, that the growth mode is layer-by-layer, and that a two-layer Fe{llO) film is “single crystal . . . with only atomic-scale roughness” [8]. Our own results of quantitative LEED studies are described below in the following order: we give experimental details in section 2; we present the results in section 3 under separate headings for Fe/Pd{lll), Fe/Ag(lll) and Fe/Al{lll); and we discuss the conclusions in section 4.
2. Experiments The experiments were done in a chamber with a base pressure of 1 X 10-l’ Torr. The sample surfaces were cleaned by standard sputtering-annealing cycles. Sputtering was done with 400 eV argon ions at a typical pressure of 5 x lo-’ Torr for several hours, while the subsequent anneal was done typically for about 1 h at temperatures of 800°C (Pd), 600°C (Ag) and 450°C (Al). Small amounts of sulfur were removed from the surface of the Pd sample by sputtering the sample at 750°C for several hours. All the resulting surfaces were free of S, 0 and C as checked by AES via a double-pass cylindrical-mirror analyzer, and all produced sharp 1 x 1 LEED patterns with low background. The ItI’) spectra from the clean
A.M. Begfey et al. / Uitr~t~~ Fe films on Pdflll),
surfaces were in good agreement with comparable data that are available in the literature [9,10]. The iron source was a 99.999 at% pure wire in a tungsten spiral, heated resistively. Since this evaporation technique is known often to produce Fe films which are contaminated by carbon and oxygen, great care was taken to outgas the source extensively and to test that the deposited films were free from contamination. The depoosition rates varied in the range from 0.3 to 1 A/min. The film thicknesses were determined from AES by using the peak-to-peak heights of the Fe (47 and 651 eV) and the Pd (330 eV), Ag (356 eV), and Al (68 and 1396 eV) AES signals. This standard procedure has been well-documented before [ll]. The mean-free paths used in the calibration were taken from Seah et al. [12]: 13.8 A (Fe 651 eV), 4.35 A (Fe 47 eV), 9.8 A (Pd 330 eV), 10.2 A (Ag 356 eV), 4.76 A (Al 68 eV) and 20.2 A (Al 1396 eV). The thicknesses thus obtained are quoted in the text, according to our standard procedure, in units o$ the hyer e~~~u~Ze~~(LE). The inversion from A to LE was done using the bulk substrate i$erlayer spacings, i.e., 2.25 ,A for Pd{lll}, 2.36 A for Ag{lll}, and 2.34 A for Al{lll}. LEED I(Y) curves were collected with a video-LEED system [131 directly after each deposition had been made and calibrated by AES.
3. Experimental results
The LEED pattern remained 3-fold s~met~c 1 x 1 as that of Pd{ll l} until the coverage reached about 6 LE. The background increased steadily with coverage after 0.9 LE, and between the thicknesses of 2 and 5 LE the beam sizes varied with energy, an indication of the presence of steps. The 10’) spectra began to show small but detectable changes at 0.9 LE, then stayed stable until about 3 LE. At the coverage of 6 LE, the 3-fold 1 X 1 pattern changed to one characteristic of bee (110) domains on fee Illl} in the Kurdjumov-Sachs (KS) orientation [14,153. This pattern takes the form of a 6-fold complex pattern in which the 10 beams of the original 1 X 1 pattern
Agflll]
.
a~Al~l~1~
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l
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.
.
l
.
: ,
. *
291
./
.
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.
.
.
:\
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.
~~
(a)
(4
Fig. 1. Schematic LEED patterns from bee Fe(llO} domains (full circles) on fee (ill} (open circles) in: (a) the KS orientation; (b) the NW orientation.
are substituted by a cluster of five beams, three inside and two outside (see fig. la and ref. [16]). This KS pattern remained unchanged up to 10 LE, the thickest film studied. ItI/) spectra were measured on a 10 LE film with a larger scan window covering all five spots. (The resolution of our data-~llection system did not allow us to measure I(V) curves of the five indi~dual beams in the cluster.) The resulting spectra are shown in fig. 2 together with equivalent spectra from Fe films grown on fee {ill] or hcp (0001) faces of other substrates on which the KS growth mode has been observed. Also shown in fig. 2 is the I@‘) curve obtained by adding 10 and 11 spectra from a clean Fe{ 1lo] surface [ 171. The equality of all these curves is offered here as proof of the presence of bee Fe{llO] domains rotationally related to one another by the symmetry of the substrate net. Upon deposition onto the substrate at 200°C the LEED patterns were of slightly better quality than their room-temperature counterparts in terms of background, beam intensities and spot sizes, but the geometry remained 3-fold symmetric 1 x 1, as in the room-temperature case, while the background increased with coverage between 0.9 and 6 LE. With increasing coverage the Z(I/) spectra showed changes, with respect to those from clean Pd{lll], between 0.8 and 1.3 LE, and then they stayed similar up to about 3 LE, but they were visibly different from their room-temperature counterparts. Above 6 LE the patterns again assumed the complex form due to the KS
A.M. Begley et al. / Ultrathin Fefilms on Pd{lll),
292
orientation, with a quality similar to patterns from the substrate at room temperature. 3.2. Fe on Ag{lll) The sharp 3-fold symmetric 1 X 1 LEED pattern from clean Ag(lll} remained unchanged for Fe coverages up to 1 LE, then became increasingly less sharp and with steadily increasing background, but was still visible at about 8 LE. From coverages of about 3 LE onward, weak “clouding” outside each beam developed into the diffuse intensity patches characteristic of the presence of rotationally related domains of bee Fe{1101 in the Nishiyama-Wassermann (NW) orientation [15,18]. Such a pattern is 6-fold symmetric and exhibits clusters of 3 beams (from the 3 possible domains) in the vicinity of the original lo-type beams of the 1 X 1 structure (see fig. lb and ref. [19]), although in the present case the pattern was too diffuse to resolve each of the 3 individual beams in each cluster. At a coverage of about 9 LE the original lo-type beams were no longer visible - only very diffuse intensities due to the bee Fe{1101 domains remained. Deposition on the substrate at 200°C produced no visible differences in the LEED patterns compared to roomtemperature deposition.
Ag{lll}
and Al(111)
3.3. Fe on Al{lll}
At a coverage of less than 1 LE the sharp 3-fold symmetric 1 x 1 LEED pattern from the clean Al{1 11) surface had completely disappeared - only a diffuse uniform background remained. From a coverage of about 3 LE onwards very diffuse, very broad (- 2 cm diameter) “spots” could just be resolved from the non-uniform, high background. This pattern improved slightly in intensity and sharpness (to - 1.5 cm diameter) for coverages from 10 to 20 LE such that its symmetry could be perceived to be 6-fold. An Z(V) curve of these broad spots is shown in fig. 2 (top>. No changes in this behavior occurred when depositing onto a substrate at 200°C.
4. Analysis 4.1. Fe on Pd{lll}
- room-temperature deposition
The fact that the LEED I(V) curves did not change from zero coverage up to a coverage of about 0.9 LE indicates that in this range the diffracted intensities stemmed only or predominantly from the clean substrate. Also, the increase in background was not significant, indicat-
bee Fe]llOt on
0
40
A
80
1.20
160
Energy
200
240
280
320
360
(eV)
Fig. 2. Each of the top 5 curves represents the integrated intensity of the cluster of LEED spots that appear in the vicinity of the lo-type substrate spots when thicker (S-20 LE) films of Fe are grown on Al{llll, Ag{lllJ, Pd(lll), Cu(ll1) [19] and RutOOOl) [16] surfaces. The bottom curve was obtained by adding together the 10 and the 11 I(V) spectra measured on bulk Fe(ll0) by Shih et al. [lo].
A.M. Begleyet al. / UltrathinFefilms on Pd{lll}, Ag{lll)
ing that the deposited Fe was probably not disordered, but its amount was too small to be detectable in the Z(Y) spectra. From a coverage of 0.9 to 3 LE the spectra were stable and different in some detail from those of the clean substrate. In trying to identify the atomic arrangement in this coverage range we tested over 40 models, all pseudomo~hic with the substrate surface. These models involved either 1 or 2 Fe layers, or a thick (semi-infinite) film of Fe, or mixed layers of Fe and Pd [20], and we varied both the overlayer-substrate distance and the interlayer spacings. The fit to experiment was evaluated both visually and by reliabili~ factors (Rw, E211,f-a [22] and R, [23]). Although none of the tested models provided a convincing fit to experiment,two were somewhat better than the others and produced the lowest R-factor values, namely: (A) a monolayer of pseudomo~hic Fe 2.1 A above the Pd substrate CR,, = 0.25, r zT = 0.16, R, = 0.48) and (3) a single 70%-30% mixed layer of Fe and Pd separated from Pd by a distance of 2.1 A (RvHT = 0.27, rzl = 0.26, R, = 0.35). Fig. 3 depicts experimental Z(V) spectra compared to the theoretical spectra from the two models. The fit is not wholly satisfactory for either model, perhaps because the actual coverage was not precisely one layer and/or the long-range order was not perfect. The most appropriate conclusion at this point is that the first layer was very pr~~~iy pseudomorphic Fe, although some intermixing of Fe and Pd cannot be ruled out. The invariance of the spectra in the coverage range between 0.9 and 3 LE (see section 3.1) suggests that the excess Fe beyond 1 LE was either disordered or in the form of small islands - calculations show that flat m&i-layer films of Fe would change the Z(Y) spectra dramatically. The former hypothesis is less likely than the latter, because the observed increase in background was only moderate and the beam intensities were not visibly weakened. The formation of small islands is more probable: if the islands were less than 80 or 100 A across they would contribute little to the Z(V) spectra and cause only a moderate increase in background intensity of the LEED pattern. Thus, the Z(V) spectra would still be characteristic of the underlying 1 LE film. This
andAl{lll)
---------Theory A -Experiment ...... Theory 6 III,,, 0
40
I,, 80
120
180
293
o2
,‘-\
,q _ : :
--a
I II ;. I . I I I ..I..-.\._......_ 200
240
280
320
360
EnefgyteVl Fig. 3. Z(Y) spectra measured from a Fe film of nominal 0.9 LE thickness grown on Pd(lllJ at room temperature (solid curves). Theory A (dashed curves) refers to calculations for a pseudomorphic film of pure Fe at a distance of 2.1 w from the first Pd layer. Theory B (dotted curves) refers to calculations for a pseudomorphic mixed Ityer of 70% Fe and 30% Pd also at a distance of 2.1 A from the first Pd layer.
hypothesis also agrees with the observation of beam-spot size variation with energy - a characteristic indication of the existence of steps which in this case would be associated with islands. The islands are believed to consist of ordered Fe atoms in the stable bee structure, as demonstrated by the fact that when they grow larger, at a nominal film thickness of 6 LE, they produce the LEED pattern characteristic of bee Fe{llO} domains in the KS orientation (see section 3.1). 4.2. Fe on Pdflll)
- 200°C deposition
The intensity data from the 0.8 LE and the 1.2 LE films on the hot substrate were fitted with all of the various models described above as well as
with a model involving two mixed layers of Fe and Pd [20]. Moderate agreement was obtained between the 0.8 LE data and a model of 2 identical layers of a 50%-500/n random alloy on top of Pd(lll1, as can be seen in fig. 4, Theo interlayer distance of the mixed layer is 2.10 A while the distance between the mixed layer and the Pd substrate distance is 2.17 A. The corresponding R-factors 0.50, tZI = 0.37, R, = 0.55. However, are Rm= the 1.2 LE film was found to be better fit by a model in which the first layer is pure Fe, whereas the second layer is a 50%-50% mixture. This fit can be judged by ~mparing 109 spectrain fig. 5. The first interlayer spacing is 2.10 A, the distance between the second layer and the Pd substrate is 2.14 A. The R-factors are RVHT = 0.33, rz = 0.20, R, = 0.49.
0
40
a5
720
160
200
240
285
320
360
EnergyieVl Fig. 5. I@‘) spectra measured from a Fe film of nominal 1.2 LE thickness grown on Pdjltl} at 200°C (solid WV& The theoretical curves (dotted) were calculated for a model involving a pseudomorphic layer of Fe above a mixed tayer of SO% Fe and 50% Pd on top of the Pdjlll) substrate.
For both these models the R-factor values are larger than desirable for a reliable structure determination: indeed, they reflect oniy moderate agreement between theory and experiment. We found that large variations in the composition of the random alloy model did not cause large changes in the I(V) spectra. The two models described above are just somewhat better than ail other tested, and a conclusion that they suggest is that at 2OOYZthere was ~r~~~~~~ interdiffusion of Fe and Pd at the deposition rates used in these experiments.
i
3 i J: E ;
0
40
80
1.70
160
2rm
240
280
320
360
EnergyW~
Fig. 4. I(V) spectra measured from a Fe film of nominal 0.X LE thickness grown on Pd{lllJ at 200°C (solid curves). The theoretical curves (dotted) were calculated for a model involving two pseudomorphic mixed layers of 50% Fe and 50% Pd on top of the Pdfl II} substrate.
4.3, Fe rm Ag{llf] The experiment showed that while the LEED pattern remained 1 x 1 with increasing deposition of Fe, the 1(V) spectra remained identical to
A.M. Begley et al. / Ultrathin Fefilms on Pd{lll},
those from the clean Ag{lll} surface for coverages of up to 4 LE. Intensity calculations for 1, 2 and 3 layers of flat pseudomorphic Fe on Ag{lll] show that the Z(V) spectra would change considerably. Hence, we must conclude that the deposited Fe atoms did not form a uniform flat layer, but rather conglomerated into islands which were initially too small to contribute to the Z(V) spectra and left therefore most of the clean substrate surface exposed. Prolonged deposition caused these islands to grow both in height and in width, thereby covering more and more of the substrate and weakening the LEED pattern. A similar behavior was observed in the case of Fe on Ag{OOl} [24]. The initial island growth is in contrast to the conclusions reached by Qiu et al. [81. At higher coverages both the geometry of the LEED pattern and the Z(V) spectra (fig. 2) show that the film consisted of domains of bee Fe(llO] rotationally related to one another by the 6-fold symmetry of the substrate surface net in the NW orientation. This orientation is expected to occur for bee (110) overlayers on fee (111) substrates [15] when the lengths of one of the two diagonals of the respective surface meshes are almost equal. In the present case, the thort diagonal of the Ag{lll} unit mesh is 2.89 A long whereas that of the oblique bee Fe(llO] unit mesh is 2.87 A long - a misfit of only 0.8%. 4.4. Fe on Al{lll} No intensity analysis was attempted in this case. At low coverages, the LEED pattern was wholly obliterated. At high coverages (15-20 LE) the pattern suggested, and the Z(V) curve shown in fig. 2 confirmed, the presence of small islands of very poorly ordered bee Fe(llO].
5. Discussion
and conclusion
5.1. Fe on Pd{lll}
With the caueut that the results of our LEED intensity analysis are not wholly unambiguous, we can conclude that in the earliest stages of growth
Ag{lll}
andAl{lll}
295
the Fe overlayer is probably pseudomorphic with the Pd{lll) substrate. The first layer may consist of pure Fe or may be partially intermixed with Pd. For prolonged deposition on a room-temperature substrate we found no evidence of a second or third complete layer of Fe. This finding is reasonable since pseudomorphic multilayer growth is quite improbable based upon the lattice mismatches - an fee Fe{lll} plane (a = 2.539 A) is stretched 8.3% iaorder to fit onto the Pd(lll} surface (a 3 2.751 A) while a bee Fe(ll1) plane (a = 4.053 A) would be compressed 32%. Rather the Z(V) spectra were indicative of the growth of small islands on top of the first (pseudomorphic) layer, and these islands eventually developed into larger domains of bee Fe(ll0) in three KS orientations rotationally related to one another by the symmetry of the substrate net. The KS orientation is based on the matching of inter-row distances in film and substrate [15]: such inter-row distances are 2.382 A on Pd{lll) and 2.340 A on bee Fe(llO], so that the corresponding misfit for the KS orientation is 1.8%. With the substrate at higher temperatures (200°C) partial intermixing of Fe and Pd may have occurred, which made it possible for a second pseudomorphic layer to grow, but thicker films still consisted of bee Fe{llO} domains in the KS orientation. These results are only partially in agreement with the conclusions reached by Binns et al. [5]. These authors interpreted their AES data to indicate growth, at room temperature, of simultaneous multilayer islands, growth which was allegedly “perfectly epitaxial for two atomic layers, with relaxation beginning in the third layer”. This conclusion cannot be justified by our LEED experiments. However, growth at 180°C was interpreted by Binns et al. to produce “two atomic layers of an fee lattice with a lattice constant identical to that of palladium”, the LEED pattern degrading thereafter. This conclusion would fit our present results if the two atomic layers were mixed Fe-Pd layers. It is possible, of course, that the growth may have been different in the two experiments (ours and Binns et al.), owing perhaps to different preparation of the substrate surface or other ex-
296
A.M. Be&y et al. / Ultrathin Fefilms on Pd{lll),
perimental procedures. However, as mentioned in the introduction, conclusions reached only on the basis of AES data and qualitative LEED observations are in general not as convincing for characterization of film growth as Z(I’) spectra. We conclude that, if our analysis of the LEED intensities is correct, Fe on Pd{ 111) grows in the Stranski-Krastanov mode and is in this respect similar (with the exception of the possibility of alloy formation) to the growth of Fe on Ru(00011
M 5.2. Fe on Ag{lll) Our experiments indicate that Fe grows on Ag( 111) in a similar manner to that found for Ag(OO1) [24]. This growth involves the initial formation of small islands which do not contribute to the LEED pattern, but merely reduce the signal from exposed areas of the clean substrate - the LEED ICI/) spectra do not change, except for the background, with respect to those from the substrate up to a coverage of about 5 LE, whereupon the reflections due to the Fe(ll0) domains become detectable. These conclusions are consistent with the results of early electron-microscopy investigations by Olsen and Snyman [6]. These authors found that iron deposited on Ag(ll1) platelets specially prepared for epitaxial studies grew by formation of distinct three-dimensional islands, the islands being elongated in the (110) directions of the substrate net. Fe films with average thickness of 20 A were still discontinuous and the thickness of individual islands was at least twice the value of the average thickness. At temperatures lower than 500°C growth was found to start with small nuclei that coalesced at greater thickness. Gutierrez et al. [7] examined the growth of multilayers of Fe(ll0) on Ag( 111) by molecular beam epitaxy. The Ag(ll1) substrates were prepared by depositing Ag (1500 A> on Fe-free synthetic mica, and Fe films of thicknesses varying between 1.3 and 8 layers were then alternated with Ag films of thickness varying between 15 and 30 layers. No evidence for RHEED oscillations was found and the Fe(ll0) films grew, at temperatures lower than 180°C in the three NW orien-
Ag{lll}
andAl{lll}
tations consistent with the 3-fold symmetry of the substrate net. At temperatures higher than 180°C two of these orientations were suppressed and the Fe film grew epitaxially with only one orientation. This particular result, which was attributed to the effect of steps in the original mica substrate, and which apparently inspired the preparation procedures adopted by Qiu et al. [8] (see below), could not be confirmed in our own experiments. In our case deposition on the substrate at 200°C produced the same sequence of LEED patterns and I@‘) curves as observed for deposition on the room-temperature substrate. Qiu et al. [8] also started from a thick Ag( 111) film grown on mica, and then grew an Fe film l-3 layers thick at 450 K, but interpreted their AES data to indicate layer-by-layer growth, and their RHEED and LEED patterns to indicate formation of a single-crystal Fe(ll0) film “with only atomic scale roughness” (81. We note, however, that the LEED pattern depicted in fig. Id of ref. [81 confirms qualitatively our own observations, namely, that it is a 1 x 1 LEED pattern of an fee (111) substrate. The corresponding Z(V) curves, not measured by Qiu et al., in our experiment prove that the pattern stemmed from Ag(ll1). We note also that a single crystal Fe(ll0) film in the NW orientation on Ag(ll1) would give a LEED pattern in which both “Ag spots” (i.e., spots produced by the Ag substrate) and “Fe spots” (i.e., spots produced by the Fe film) could be observed. The separation of the “Ag spot” from the “Fe spot” along a line from the center of the pattern would be about 20% of the Ag reciprocal net vector - a difference that is clearly detectable in our multi-domain LEED pattern. However, the only spots visible in fig. Id of ref. [Sl are, juding from the symmetry of the pattern, the “Ag spots”. 5.3. Fe on Al{lll} The experiment indicates that thicker Fe films (thicker than about 5 LE) on Al(111) may have been partially ordered in the NW orientation, but the disorder dominated and quantitative measurements were not possible. Very low coverages completely obliterated the LEED pattern. The
result that submonolayer amounts of Fe destroy the long-range order in the whole surface selvedge of Ah1111 down to the fifth or sixth layer (as required in order to obliterate the LEED pattern), is remarkable in view of the fact that the unit-mesh edge of AI{lll) (2.863 & is very close to that of Ag{lll} (2.889 A>. In fact, the misfit for growth of bee Fe{llO} on AlIlll} in the NW orientation is only O.l%, which is even less than for AgUll) (see above). We speculate that the Fe atoms diffuse into the AI very rapidly and form an amorphous alloy layer on the surface of the substrate.
This work was sponsored in part by the National Science Foundation and by the Department of Energy with Grants DMR-8921123 and DE-FG02-86ER45239, respectively.
References [l] F.J. Himpsel, Phys. Rev. Lett. 67 (1991) 2363, and references therein. [2J M. Tikhov and E. Bauer, Surf. Sd. 232 (19QO)73. [3] J.W. Evans, D.E. Sanders, P.A. Tbiel and A.J. DePristo, Phys. Rev. B 41 (1990) 5410. [4] F. Jona and P.M. Marcus, in: Surface Physics and Related Topics, Eds. Fu-Jia Yang, Guang-Jiong Ni, Xun Wang, Kai-Ming Zhang and Dong Lu (World Scientific, Singapore, 1991) p. 213.
Ed C. Binns, C. Norris, G.P. Williams, M.G. Barthes and H.A. Padmore, Phys. Rev. B 34 (1986) 8221.
I61G.H. Olsen and H.C. Snyman, Acta Metall. 21 (1973) 769. 171 C.J. Gutierrez, S.H. Mayer and J.C. Walker, J. Magn. Magn. Mater. 80 (1989) 299. Is1 Z.Q. Qiu, J. Pearson and S.D. Bader, Phys. Rev. Lett. 67 (1991) 1646; Phys. Rev. B 45 (1992) 7211. 191 H. Ohtani, M.A. Van Hove and G.A. Somorjai, Surf. Sci. 187 (1987) 372. ml F. Jona, D. Sondericker and P.M. Marcus, J. Phys. C 73 (1980) L155. ml See, e.g., J. Quinn, Y.S. Li, H. Li, D.Tian, F. Jona and P.M. Marcus, Phys. Rev. B 43 (1991) 3959. WI M.P. Seah and W.A. Dench, Surf. Interface Anal. 1 (1979) 2. 1131 F. Jona, J.A. Strozier, Jr. and P.M. Marcus, in: The Structure of Surfaces, Eds. M.A. Van Hove and S.Y. Tong (Springer, Berlin, 1985) p. 92. D41 G. Kurdjum~ and G. Sachs, Z. Phys. 64 (19301325. WI J.H. van der Merwe, Philos. Mag. A 45 (1982) 127, 145, 159; Appt. Surf. Sci. 22/23 (1985) 545. Ml D. Tian, H. Li, F. Jona and P.M. Marcus, Solid State CInmnun. 80 (1991) 783. t171 H.D. Shih, F. Jona, U. Bardi and P.M. Marcus, J. Phys. C 13 (1980) 3801. f181 Z. Nishiyama, Sci. Rep. Tohoku Univ. 23 (1934) 638. 1191D. Tian, F. Jona and P.M. Marcus, Phys. Rev. B 45 (1992) 11216. m In the calculations of models with mixed layers of Fe and Pd the scattering amplitudes of Fe and Pd were replaced by suitable averages of these same scattering amplitudes, see F. Jona, KG. Legg, H.D. Shih, D.W. Jepsen and P.M. Marcus, Phys. Rev. Lett. 40 (1978) 1466. ml M.A. Van Hove, S.Y. Tong and M.H. Elconin, Surf. Sci. 64 (1977) 85. WI E. Zanazzi and F. Jona, Surf. Sci. 62 (1977) 61. D31 J.B. Pendry, J. Phys. C 13 (1980) 937. L241H. Li, Y.S. Li, J. Quinn, D. Tian, J. Sokolov, F. Jona and P.M. Marcus, Phys. Rev. B 42 (lQQOI9195.
kurface
scienck
Study of ultrathin Fe films on Pd{ ill)}, Ag( 111) and Al{ 111) A.M. Begley a, D. Tim a*t,F. Jona a and P.M. Marcus b aDepartment ofMaterials Science and Engineering, State University of New York, Stony Brook, NY I1 794-2275, USA b IBM Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA
Received 6 Juty 1992; accepted for publication 15 September 1992
The growth of epitaxial Fe films of 1 to 10 layer equivalents (LIE) on Pd{lllJ, Al(111) and Ag{lll) has been studied with quantitative low-energy electron diffraction and Auger electron spectroscopy. On Pd(lll), both at room temperature and at 2oo”C, the growth starts with pseudomorphic layers which may involve intermixing of Fe and Pd. At both temperatures, when the coverage reaches 6 LE, the Fe films develop into large bee (IlO} domains related to the substrate by the Kurdjumov-Sachs orientation. On Ag{lllf Fe grows initially in a very similar manner to Fe on ~{~l~, i.e., by way of small islands of bee Fe. These islands then grow into bee Fe{1101 domains in the Nishiyama-Wasserman orientation. On Alflll) Fe behaves differentiy, despite the fact that the lattice mismatch for Fe on Al{lll) is nearly identical to that for Fe on Ag(lll}: for coverages of less than 1 LE the surface region becomes completely disordered and no LEED pattern is visible.
1. Int~uction Although the study of magnetic ultrathin films on metal surfaces is long-established and widely reported, not much is well-established about the growth mode and the ~stalIograph~c structure of the films, and the results are often controversial. Both the magnetic and the structural properties of films of a few atomic layers can turn out to be very sensitive to strain, substrate interaction and substrate temperature [I]. The resulting problems of fiIm reproducibiii~ between laboratories have led to a lack of general agreement over growth modes for several epitaxial systems. This situation is unfortunate, because without a detailed understanding and characterization of film growth and structure the interpretation of results from ma~etic measurements must always remain open to question. Claims about growth modes (layer-by-layer, island growth or combinations thereof) and about crystallographic structure of ultrathin films are t Di Tian was tragically kilied in a car accident on February 9, 1992.
usually made on the basis three types of measurements: (1) qualitative reflection high-energy electron diffraction (RHEED) observations (in particular, the presence or absence of intensity oscillations); (2) qualitative low-energy electron diffraction (LEED) observations (the presence or absence of 1 X 1 patterns); (3) qualitative Auger electron spectroscopy (AES) observations (presence or absence of “breaks” in the plot of Auger intensities versus deposition time). These observations may in some cases allow the identification of the growth mode, but such an identification should be done with extreme care because, e.g., ABS breaks can be affected by electronescape depth, diffraction effects and band structure [2] whereas RHEED oscillations are merely a result of periodically repeated surface conditions, not necessarily flat surfaces, in the course of film growth [3]. In any case, the conclusions reached from AES breaks or RHEED oscillations or qualitative LEED about the growth mode of an ultrathin film can and should be tested quantitatively with appropriate su~a~-sensitive techniques such as surface X-ray diffraction, medium-energy ion
0039-6028/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
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A.M. Begley et al. / Ultrathin Fefilms on Pd{lll},
scattering or quantitative LEED intensity analysis. Indeed, if the growth is layer-by-layer, for example, any one of these techniques should be able to confirm it and in favorable cases should be able to determine both the inter- and the intra-layer atomic distances involved. If the growth is by islands the determination of interatomic distances may not be possible in the early stages, but the absence of layer-by-layer growth should be recognizable in any case. Quantitative LEED procedures have been recently used in several epitaxial studies [4] and have in general produced results in disagreement with the conclusions reached by means of AES breaks, RHEED oscillations or qualitative LEED observations. It is possible of course that the disagreement may not be ascribable to the fact that different techniques were employed, but rather to differences in experimental procedures, such as, e.g., substrate preparation, surface morphology, materials purity, etc. The only way to know for sure is to carry out all experiments (film growth, magnetic measurements and quantitative crystallographic determinations) on the same sample, in the same experimental chamber and at the same time. Unfortunately, such an undertaking has not been made to date, but is desirable. We present here some results of epitaxial studies done with quantitative LEED methods, which, again, are at least partially in disagreement with the conclusions reached in other laboratories. The epitaxial systems considered here are Fe/Pd{lll), Fe/Ag{lll) and Fe/Al{lll). Both the Fe/Pd{lll) and the Fe/Ag{lll) are thin-film systems on which magnetic measurements were made and interpreted without the benefit of a more extensive understanding of the growth as provided by quantitative structure determinations. The Fe/Pd(lll) system was investigated by Binns et al. [5] in the ultrathin range (O-2 layers). These authors used AES data and qualitative LEED observations to infer that the growth occurred in the “simultaneous multilayer” mode (a mode which involves the development of the nth layer before the (n - list layer is complete) and claimed to be able to determine the coverage by iron to within + 0.05 monolayers. They concluded
Ag{lII}
andAl{lll}
that the growth of Fe on Pd(ll1) “is perfectly epitaxial for two atomic layers, with relaxation beginning in the third layer” [5]. The growth of Fe on Ag(ll1) was investigated by three different groups with somewhat contrasting results. Olsen and Snyman [6] determined by electron microscopy that the growth “initiates with the formation of distinct three-dimensional nuclei”. Gutierrez et al. [7] recognized, by using RHEED, that bee FelllO) domains grow epitaxially in three related Nishiyama-Wasserman (NW) orientations, but found that at 180°C two of these orientations are suppressed and, as a result, Fe{ 110) grows epitaxially at that temperature with only one orientation. Qiu et al. [S], more recently, studied the magnetic properties of l-3 layersthick Fe films on Ag{ 111) which they grew with the procedures described by Gutierrez et al. [7], and concluded, from AES breaks, RHEED and qualitative LEED observations, that the growth mode is layer-by-layer, and that a two-layer Fe{llO) film is “single crystal . . . with only atomic-scale roughness” [8]. Our own results of quantitative LEED studies are described below in the following order: we give experimental details in section 2; we present the results in section 3 under separate headings for Fe/Pd{lll), Fe/Ag(lll) and Fe/Al{lll); and we discuss the conclusions in section 4.
2. Experiments The experiments were done in a chamber with a base pressure of 1 X 10-l’ Torr. The sample surfaces were cleaned by standard sputtering-annealing cycles. Sputtering was done with 400 eV argon ions at a typical pressure of 5 x lo-’ Torr for several hours, while the subsequent anneal was done typically for about 1 h at temperatures of 800°C (Pd), 600°C (Ag) and 450°C (Al). Small amounts of sulfur were removed from the surface of the Pd sample by sputtering the sample at 750°C for several hours. All the resulting surfaces were free of S, 0 and C as checked by AES via a double-pass cylindrical-mirror analyzer, and all produced sharp 1 x 1 LEED patterns with low background. The ItI’) spectra from the clean
A.M. Begfey et al. / Uitr~t~~ Fe films on Pdflll),
surfaces were in good agreement with comparable data that are available in the literature [9,10]. The iron source was a 99.999 at% pure wire in a tungsten spiral, heated resistively. Since this evaporation technique is known often to produce Fe films which are contaminated by carbon and oxygen, great care was taken to outgas the source extensively and to test that the deposited films were free from contamination. The depoosition rates varied in the range from 0.3 to 1 A/min. The film thicknesses were determined from AES by using the peak-to-peak heights of the Fe (47 and 651 eV) and the Pd (330 eV), Ag (356 eV), and Al (68 and 1396 eV) AES signals. This standard procedure has been well-documented before [ll]. The mean-free paths used in the calibration were taken from Seah et al. [12]: 13.8 A (Fe 651 eV), 4.35 A (Fe 47 eV), 9.8 A (Pd 330 eV), 10.2 A (Ag 356 eV), 4.76 A (Al 68 eV) and 20.2 A (Al 1396 eV). The thicknesses thus obtained are quoted in the text, according to our standard procedure, in units o$ the hyer e~~~u~Ze~~(LE). The inversion from A to LE was done using the bulk substrate i$erlayer spacings, i.e., 2.25 ,A for Pd{lll}, 2.36 A for Ag{lll}, and 2.34 A for Al{lll}. LEED I(Y) curves were collected with a video-LEED system [131 directly after each deposition had been made and calibrated by AES.
3. Experimental results
The LEED pattern remained 3-fold s~met~c 1 x 1 as that of Pd{ll l} until the coverage reached about 6 LE. The background increased steadily with coverage after 0.9 LE, and between the thicknesses of 2 and 5 LE the beam sizes varied with energy, an indication of the presence of steps. The 10’) spectra began to show small but detectable changes at 0.9 LE, then stayed stable until about 3 LE. At the coverage of 6 LE, the 3-fold 1 X 1 pattern changed to one characteristic of bee (110) domains on fee Illl} in the Kurdjumov-Sachs (KS) orientation [14,153. This pattern takes the form of a 6-fold complex pattern in which the 10 beams of the original 1 X 1 pattern
Agflll]
.
a~Al~l~1~
. .
l
.*
l
.
.
l
.
: ,
. *
291
./
.
. .
.
.
.
:\
’ . .
.
~~
(a)
(4
Fig. 1. Schematic LEED patterns from bee Fe(llO} domains (full circles) on fee (ill} (open circles) in: (a) the KS orientation; (b) the NW orientation.
are substituted by a cluster of five beams, three inside and two outside (see fig. la and ref. [16]). This KS pattern remained unchanged up to 10 LE, the thickest film studied. ItI/) spectra were measured on a 10 LE film with a larger scan window covering all five spots. (The resolution of our data-~llection system did not allow us to measure I(V) curves of the five indi~dual beams in the cluster.) The resulting spectra are shown in fig. 2 together with equivalent spectra from Fe films grown on fee {ill] or hcp (0001) faces of other substrates on which the KS growth mode has been observed. Also shown in fig. 2 is the I@‘) curve obtained by adding 10 and 11 spectra from a clean Fe{ 1lo] surface [ 171. The equality of all these curves is offered here as proof of the presence of bee Fe{llO] domains rotationally related to one another by the symmetry of the substrate net. Upon deposition onto the substrate at 200°C the LEED patterns were of slightly better quality than their room-temperature counterparts in terms of background, beam intensities and spot sizes, but the geometry remained 3-fold symmetric 1 x 1, as in the room-temperature case, while the background increased with coverage between 0.9 and 6 LE. With increasing coverage the Z(I/) spectra showed changes, with respect to those from clean Pd{lll], between 0.8 and 1.3 LE, and then they stayed similar up to about 3 LE, but they were visibly different from their room-temperature counterparts. Above 6 LE the patterns again assumed the complex form due to the KS
A.M. Begley et al. / Ultrathin Fefilms on Pd{lll),
292
orientation, with a quality similar to patterns from the substrate at room temperature. 3.2. Fe on Ag{lll) The sharp 3-fold symmetric 1 X 1 LEED pattern from clean Ag(lll} remained unchanged for Fe coverages up to 1 LE, then became increasingly less sharp and with steadily increasing background, but was still visible at about 8 LE. From coverages of about 3 LE onward, weak “clouding” outside each beam developed into the diffuse intensity patches characteristic of the presence of rotationally related domains of bee Fe{1101 in the Nishiyama-Wassermann (NW) orientation [15,18]. Such a pattern is 6-fold symmetric and exhibits clusters of 3 beams (from the 3 possible domains) in the vicinity of the original lo-type beams of the 1 X 1 structure (see fig. lb and ref. [19]), although in the present case the pattern was too diffuse to resolve each of the 3 individual beams in each cluster. At a coverage of about 9 LE the original lo-type beams were no longer visible - only very diffuse intensities due to the bee Fe{1101 domains remained. Deposition on the substrate at 200°C produced no visible differences in the LEED patterns compared to roomtemperature deposition.
Ag{lll}
and Al(111)
3.3. Fe on Al{lll}
At a coverage of less than 1 LE the sharp 3-fold symmetric 1 x 1 LEED pattern from the clean Al{1 11) surface had completely disappeared - only a diffuse uniform background remained. From a coverage of about 3 LE onwards very diffuse, very broad (- 2 cm diameter) “spots” could just be resolved from the non-uniform, high background. This pattern improved slightly in intensity and sharpness (to - 1.5 cm diameter) for coverages from 10 to 20 LE such that its symmetry could be perceived to be 6-fold. An Z(V) curve of these broad spots is shown in fig. 2 (top>. No changes in this behavior occurred when depositing onto a substrate at 200°C.
4. Analysis 4.1. Fe on Pd{lll}
- room-temperature deposition
The fact that the LEED I(V) curves did not change from zero coverage up to a coverage of about 0.9 LE indicates that in this range the diffracted intensities stemmed only or predominantly from the clean substrate. Also, the increase in background was not significant, indicat-
bee Fe]llOt on
0
40
A
80
1.20
160
Energy
200
240
280
320
360
(eV)
Fig. 2. Each of the top 5 curves represents the integrated intensity of the cluster of LEED spots that appear in the vicinity of the lo-type substrate spots when thicker (S-20 LE) films of Fe are grown on Al{llll, Ag{lllJ, Pd(lll), Cu(ll1) [19] and RutOOOl) [16] surfaces. The bottom curve was obtained by adding together the 10 and the 11 I(V) spectra measured on bulk Fe(ll0) by Shih et al. [lo].
A.M. Begleyet al. / UltrathinFefilms on Pd{lll}, Ag{lll)
ing that the deposited Fe was probably not disordered, but its amount was too small to be detectable in the Z(Y) spectra. From a coverage of 0.9 to 3 LE the spectra were stable and different in some detail from those of the clean substrate. In trying to identify the atomic arrangement in this coverage range we tested over 40 models, all pseudomo~hic with the substrate surface. These models involved either 1 or 2 Fe layers, or a thick (semi-infinite) film of Fe, or mixed layers of Fe and Pd [20], and we varied both the overlayer-substrate distance and the interlayer spacings. The fit to experiment was evaluated both visually and by reliabili~ factors (Rw, E211,f-a [22] and R, [23]). Although none of the tested models provided a convincing fit to experiment,two were somewhat better than the others and produced the lowest R-factor values, namely: (A) a monolayer of pseudomo~hic Fe 2.1 A above the Pd substrate CR,, = 0.25, r zT = 0.16, R, = 0.48) and (3) a single 70%-30% mixed layer of Fe and Pd separated from Pd by a distance of 2.1 A (RvHT = 0.27, rzl = 0.26, R, = 0.35). Fig. 3 depicts experimental Z(V) spectra compared to the theoretical spectra from the two models. The fit is not wholly satisfactory for either model, perhaps because the actual coverage was not precisely one layer and/or the long-range order was not perfect. The most appropriate conclusion at this point is that the first layer was very pr~~~iy pseudomorphic Fe, although some intermixing of Fe and Pd cannot be ruled out. The invariance of the spectra in the coverage range between 0.9 and 3 LE (see section 3.1) suggests that the excess Fe beyond 1 LE was either disordered or in the form of small islands - calculations show that flat m&i-layer films of Fe would change the Z(Y) spectra dramatically. The former hypothesis is less likely than the latter, because the observed increase in background was only moderate and the beam intensities were not visibly weakened. The formation of small islands is more probable: if the islands were less than 80 or 100 A across they would contribute little to the Z(V) spectra and cause only a moderate increase in background intensity of the LEED pattern. Thus, the Z(V) spectra would still be characteristic of the underlying 1 LE film. This
andAl{lll)
---------Theory A -Experiment ...... Theory 6 III,,, 0
40
I,, 80
120
180
293
o2
,‘-\
,q _ : :
--a
I II ;. I . I I I ..I..-.\._......_ 200
240
280
320
360
EnefgyteVl Fig. 3. Z(Y) spectra measured from a Fe film of nominal 0.9 LE thickness grown on Pd(lllJ at room temperature (solid curves). Theory A (dashed curves) refers to calculations for a pseudomorphic film of pure Fe at a distance of 2.1 w from the first Pd layer. Theory B (dotted curves) refers to calculations for a pseudomorphic mixed Ityer of 70% Fe and 30% Pd also at a distance of 2.1 A from the first Pd layer.
hypothesis also agrees with the observation of beam-spot size variation with energy - a characteristic indication of the existence of steps which in this case would be associated with islands. The islands are believed to consist of ordered Fe atoms in the stable bee structure, as demonstrated by the fact that when they grow larger, at a nominal film thickness of 6 LE, they produce the LEED pattern characteristic of bee Fe{llO} domains in the KS orientation (see section 3.1). 4.2. Fe on Pdflll)
- 200°C deposition
The intensity data from the 0.8 LE and the 1.2 LE films on the hot substrate were fitted with all of the various models described above as well as
with a model involving two mixed layers of Fe and Pd [20]. Moderate agreement was obtained between the 0.8 LE data and a model of 2 identical layers of a 50%-500/n random alloy on top of Pd(lll1, as can be seen in fig. 4, Theo interlayer distance of the mixed layer is 2.10 A while the distance between the mixed layer and the Pd substrate distance is 2.17 A. The corresponding R-factors 0.50, tZI = 0.37, R, = 0.55. However, are Rm= the 1.2 LE film was found to be better fit by a model in which the first layer is pure Fe, whereas the second layer is a 50%-50% mixture. This fit can be judged by ~mparing 109 spectrain fig. 5. The first interlayer spacing is 2.10 A, the distance between the second layer and the Pd substrate is 2.14 A. The R-factors are RVHT = 0.33, rz = 0.20, R, = 0.49.
0
40
a5
720
160
200
240
285
320
360
EnergyieVl Fig. 5. I@‘) spectra measured from a Fe film of nominal 1.2 LE thickness grown on Pdjltl} at 200°C (solid WV& The theoretical curves (dotted) were calculated for a model involving a pseudomorphic layer of Fe above a mixed tayer of SO% Fe and 50% Pd on top of the Pdjlll) substrate.
For both these models the R-factor values are larger than desirable for a reliable structure determination: indeed, they reflect oniy moderate agreement between theory and experiment. We found that large variations in the composition of the random alloy model did not cause large changes in the I(V) spectra. The two models described above are just somewhat better than ail other tested, and a conclusion that they suggest is that at 2OOYZthere was ~r~~~~~~ interdiffusion of Fe and Pd at the deposition rates used in these experiments.
i
3 i J: E ;
0
40
80
1.70
160
2rm
240
280
320
360
EnergyW~
Fig. 4. I(V) spectra measured from a Fe film of nominal 0.X LE thickness grown on Pd{lllJ at 200°C (solid curves). The theoretical curves (dotted) were calculated for a model involving two pseudomorphic mixed layers of 50% Fe and 50% Pd on top of the Pdfl II} substrate.
4.3, Fe rm Ag{llf] The experiment showed that while the LEED pattern remained 1 x 1 with increasing deposition of Fe, the 1(V) spectra remained identical to
A.M. Begley et al. / Ultrathin Fefilms on Pd{lll},
those from the clean Ag{lll} surface for coverages of up to 4 LE. Intensity calculations for 1, 2 and 3 layers of flat pseudomorphic Fe on Ag{lll] show that the Z(V) spectra would change considerably. Hence, we must conclude that the deposited Fe atoms did not form a uniform flat layer, but rather conglomerated into islands which were initially too small to contribute to the Z(V) spectra and left therefore most of the clean substrate surface exposed. Prolonged deposition caused these islands to grow both in height and in width, thereby covering more and more of the substrate and weakening the LEED pattern. A similar behavior was observed in the case of Fe on Ag{OOl} [24]. The initial island growth is in contrast to the conclusions reached by Qiu et al. [81. At higher coverages both the geometry of the LEED pattern and the Z(V) spectra (fig. 2) show that the film consisted of domains of bee Fe(llO] rotationally related to one another by the 6-fold symmetry of the substrate surface net in the NW orientation. This orientation is expected to occur for bee (110) overlayers on fee (111) substrates [15] when the lengths of one of the two diagonals of the respective surface meshes are almost equal. In the present case, the thort diagonal of the Ag{lll} unit mesh is 2.89 A long whereas that of the oblique bee Fe(llO] unit mesh is 2.87 A long - a misfit of only 0.8%. 4.4. Fe on Al{lll} No intensity analysis was attempted in this case. At low coverages, the LEED pattern was wholly obliterated. At high coverages (15-20 LE) the pattern suggested, and the Z(V) curve shown in fig. 2 confirmed, the presence of small islands of very poorly ordered bee Fe(llO].
5. Discussion
and conclusion
5.1. Fe on Pd{lll}
With the caueut that the results of our LEED intensity analysis are not wholly unambiguous, we can conclude that in the earliest stages of growth
Ag{lll}
andAl{lll}
295
the Fe overlayer is probably pseudomorphic with the Pd{lll) substrate. The first layer may consist of pure Fe or may be partially intermixed with Pd. For prolonged deposition on a room-temperature substrate we found no evidence of a second or third complete layer of Fe. This finding is reasonable since pseudomorphic multilayer growth is quite improbable based upon the lattice mismatches - an fee Fe{lll} plane (a = 2.539 A) is stretched 8.3% iaorder to fit onto the Pd(lll} surface (a 3 2.751 A) while a bee Fe(ll1) plane (a = 4.053 A) would be compressed 32%. Rather the Z(V) spectra were indicative of the growth of small islands on top of the first (pseudomorphic) layer, and these islands eventually developed into larger domains of bee Fe(ll0) in three KS orientations rotationally related to one another by the symmetry of the substrate net. The KS orientation is based on the matching of inter-row distances in film and substrate [15]: such inter-row distances are 2.382 A on Pd{lll) and 2.340 A on bee Fe(llO], so that the corresponding misfit for the KS orientation is 1.8%. With the substrate at higher temperatures (200°C) partial intermixing of Fe and Pd may have occurred, which made it possible for a second pseudomorphic layer to grow, but thicker films still consisted of bee Fe{llO} domains in the KS orientation. These results are only partially in agreement with the conclusions reached by Binns et al. [5]. These authors interpreted their AES data to indicate growth, at room temperature, of simultaneous multilayer islands, growth which was allegedly “perfectly epitaxial for two atomic layers, with relaxation beginning in the third layer”. This conclusion cannot be justified by our LEED experiments. However, growth at 180°C was interpreted by Binns et al. to produce “two atomic layers of an fee lattice with a lattice constant identical to that of palladium”, the LEED pattern degrading thereafter. This conclusion would fit our present results if the two atomic layers were mixed Fe-Pd layers. It is possible, of course, that the growth may have been different in the two experiments (ours and Binns et al.), owing perhaps to different preparation of the substrate surface or other ex-
296
A.M. Be&y et al. / Ultrathin Fefilms on Pd{lll),
perimental procedures. However, as mentioned in the introduction, conclusions reached only on the basis of AES data and qualitative LEED observations are in general not as convincing for characterization of film growth as Z(I’) spectra. We conclude that, if our analysis of the LEED intensities is correct, Fe on Pd{ 111) grows in the Stranski-Krastanov mode and is in this respect similar (with the exception of the possibility of alloy formation) to the growth of Fe on Ru(00011
M 5.2. Fe on Ag{lll) Our experiments indicate that Fe grows on Ag( 111) in a similar manner to that found for Ag(OO1) [24]. This growth involves the initial formation of small islands which do not contribute to the LEED pattern, but merely reduce the signal from exposed areas of the clean substrate - the LEED ICI/) spectra do not change, except for the background, with respect to those from the substrate up to a coverage of about 5 LE, whereupon the reflections due to the Fe(ll0) domains become detectable. These conclusions are consistent with the results of early electron-microscopy investigations by Olsen and Snyman [6]. These authors found that iron deposited on Ag(ll1) platelets specially prepared for epitaxial studies grew by formation of distinct three-dimensional islands, the islands being elongated in the (110) directions of the substrate net. Fe films with average thickness of 20 A were still discontinuous and the thickness of individual islands was at least twice the value of the average thickness. At temperatures lower than 500°C growth was found to start with small nuclei that coalesced at greater thickness. Gutierrez et al. [7] examined the growth of multilayers of Fe(ll0) on Ag( 111) by molecular beam epitaxy. The Ag(ll1) substrates were prepared by depositing Ag (1500 A> on Fe-free synthetic mica, and Fe films of thicknesses varying between 1.3 and 8 layers were then alternated with Ag films of thickness varying between 15 and 30 layers. No evidence for RHEED oscillations was found and the Fe(ll0) films grew, at temperatures lower than 180°C in the three NW orien-
Ag{lll}
andAl{lll}
tations consistent with the 3-fold symmetry of the substrate net. At temperatures higher than 180°C two of these orientations were suppressed and the Fe film grew epitaxially with only one orientation. This particular result, which was attributed to the effect of steps in the original mica substrate, and which apparently inspired the preparation procedures adopted by Qiu et al. [8] (see below), could not be confirmed in our own experiments. In our case deposition on the substrate at 200°C produced the same sequence of LEED patterns and I@‘) curves as observed for deposition on the room-temperature substrate. Qiu et al. [8] also started from a thick Ag( 111) film grown on mica, and then grew an Fe film l-3 layers thick at 450 K, but interpreted their AES data to indicate layer-by-layer growth, and their RHEED and LEED patterns to indicate formation of a single-crystal Fe(ll0) film “with only atomic scale roughness” (81. We note, however, that the LEED pattern depicted in fig. Id of ref. [81 confirms qualitatively our own observations, namely, that it is a 1 x 1 LEED pattern of an fee (111) substrate. The corresponding Z(V) curves, not measured by Qiu et al., in our experiment prove that the pattern stemmed from Ag(ll1). We note also that a single crystal Fe(ll0) film in the NW orientation on Ag(ll1) would give a LEED pattern in which both “Ag spots” (i.e., spots produced by the Ag substrate) and “Fe spots” (i.e., spots produced by the Fe film) could be observed. The separation of the “Ag spot” from the “Fe spot” along a line from the center of the pattern would be about 20% of the Ag reciprocal net vector - a difference that is clearly detectable in our multi-domain LEED pattern. However, the only spots visible in fig. Id of ref. [Sl are, juding from the symmetry of the pattern, the “Ag spots”. 5.3. Fe on Al{lll} The experiment indicates that thicker Fe films (thicker than about 5 LE) on Al(111) may have been partially ordered in the NW orientation, but the disorder dominated and quantitative measurements were not possible. Very low coverages completely obliterated the LEED pattern. The
result that submonolayer amounts of Fe destroy the long-range order in the whole surface selvedge of Ah1111 down to the fifth or sixth layer (as required in order to obliterate the LEED pattern), is remarkable in view of the fact that the unit-mesh edge of AI{lll) (2.863 & is very close to that of Ag{lll} (2.889 A>. In fact, the misfit for growth of bee Fe{llO} on AlIlll} in the NW orientation is only O.l%, which is even less than for AgUll) (see above). We speculate that the Fe atoms diffuse into the AI very rapidly and form an amorphous alloy layer on the surface of the substrate.
This work was sponsored in part by the National Science Foundation and by the Department of Energy with Grants DMR-8921123 and DE-FG02-86ER45239, respectively.
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