VOLUME 87, NUMBER 24

PHYSICAL REVIEW LETTERS

10 DECEMBER 2001

Direct Observation of Surface Alloying and Interface Roughening: Growth of Au on Fe(001)
M. M. J. Bischoff, T. Yamada, A. J. Quinn,* R. G. P. van der Kraan,† and H. van Kempen‡
Research Institute for Materials, University of Nijmegen, 6525 ED Nijmegen, The Netherlands(Received 11 July 2001; published 21 November 2001)
Scanning tunneling microscopy studies on the growth of Au on Fe(001) are reported. A surface
alloy is observed for submonolayer deposition (,0.5 monolayer) at temperatures higher than 370 K.
This surface-confined alloy demixes when it is covered with Au and in combination with imperfectlayer-by-layer growth a rough interface consisting of Au islands in and Fe islands on the original Fe(001)substrate is created. A real-space high resolution study of this buried interface is possible because of thelarge difference in interlayer spacing between bcc Fe(001) and fcc Au(001).
DOI: 10.1103/PhysRevLett.87.246102

PACS numbers: 68.35.Ct, 68.35.Bs, 68.37.Ef, 68.55.Jk

Interfaces play a key role in the transport and magnetic
properties of multilayer systems which are of technologicaland fundamental interest. Effects such as giant magnetoresistance [1], tunnel magnetoresistance [2], and oscillatoryexchange coupling [3] display extreme sensitivity to interfacial conditions. Intermixing and interface roughening areimportant processes which strongly influence the interface
quality.
During the last decade, the scanning tunneling microscope (STM) has proven to be a very useful tool in studying surface alloying. Chemical contrast has often beenachieved either with special tip conditions (see, e.g., [4]and references therein) or by means of characteristic features in scanning tunneling spectroscopy (STS) [5]. However, reports on imaging of structures buried under metallic
overlayers (i.e., a buried interface) are rare [6–8].
The large difference in interlayer spacing between bcc
Fe(001) and fcc Au(001) (0.144 and 0.204 nm, respectively) makes this system ideal for a high resolution STM
study of the subsurface interface. Their close lateral match(only 0.6% difference) is very favorable and, consequently,
growth of Fe on Au(001) has been studied extensively (e.g.,[9 –11]). Despite the unfavorable surface energy relation(gAu ø gFe 兲 layer-by-layer growth was often found andseems promoted by a surfactant Au monolayer floating on
top of the Fe layer. Nevertheless, Blum et al. recently
concluded that the growth of a flat and defect-free bcc-Fe
film on Au(001) remains a challenge [10]. In contrast, thegrowth of Au on Fe(001) is not as widely studied. RHEED
studies by Unguris et al. indicated that at 373 K the growthof Au on Fe(001) is well ordered and layer by layer [12].
However, the exchange coupling strength for a Fe兾Au兾Fe
trilayer was found to be much lower than predicted by theory, in spite of the high quality of the Fe(001) whisker
substrates used [3]. Recently, Opitz et al. postulated thatthe origin of this discrepancy is due to impurity scattering(i.e., intermixing of Au and Fe) at the interface rather thanmesoscopic roughness [13].
In this Letter we show that in spite of the immiscibilityof Au in bulk Fe a surface alloy is found for submonolayer deposition of Au on Fe(001) at temperatures higher
than 370 K. For coverages larger than 0.5 monolayer thesurface-confined alloy demixes and in combination withnonideal layer-by-layer growth behavior, a rough interface
is obtained which comprises Au islands in and Fe islandson the original atomically flat Fe(001) substrate layer. A
real-space high resolution STM study on this buried interface is presented.
The STM experiments were performed in ultrahigh vacuum using an Omicron UHV STM-1. Electrochemicallyetched W tips were cleaned and sharpened in situ by Ne
self-sputtering [14]. A single crystal Fe(001) whisker
was used as the substrate which was cleaned by cycles
of Ar ion sputtering around 1000 K followed by radiative
cooling to room temperature, which was sufficient to repair any surface damage (terraces .200 nm wide can beobserved in STM). The concentration of surface impurities (mainly oxygen) was mostly below 1% as estimatedfrom Auger electron spectroscopy and atomically resolved
STM images. Gold (99.99% pure) was evaporated (rate of
0.2 monolayer兾min as determined by a quartz crystal microbalance) from a Knudsen cell inside a liquid nitrogencryoshroud to keep the pressure below 3 3 10210 mbar
during growth. One monolayer (ML) is defined as theamount of atoms in one Fe(001) layer. The sample temperature was measured by a thermocouple mounted closeto (but not in direct contact with) the sample making thetemperature measurements quite inaccurate 共630 K兲.
An STM image of the Fe(001) surface after depositionof 0.20 ML Au at 400 K is shown in Fig. 1(a) and reveals islands, depressions, and protrusions. The terrace
depressions (marked D; 30 pm deep and 7% concentration) are also visible on the uncovered Fe(001) surface andare related to oxygen contaminants. [Note that the contamination in this image is slightly higher compared to
the following images, which was related to an impropercleaning of the Fe(001) whisker.] The small protrusions(marked P; 10% concentration) are visible only after Au
deposition at temperatures higher than 400 K with increasing concentrations for increasing deposition temperatures.
Because of their spherical symmetry (0.4 nm wide), lowapparent height [10–20 pm; see Fig. 1(a) inset] and extreme sensitivity of their appearance on the tip conditions
these features are identified as single substitutional Au

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FIG. 1. (a) STM image of 0.20 ML Au deposited on Fe(001)
at 400 K (10 3 10 nm2 , Vs 苷 20.77 V, I 苷 0.23 nA). In addition to the depressions (e.g., arrow marked D, protrusions(P) are observed on the terrace, while on the islands only
depressions (I) are observed. The inset shows a cross section along the dashed black line over protrusion P. (b) STM
image of 0.44 ML Au deposited at 500 K (14.6 3 14.6 nm2 ,Vs 苷 20.07 V, I 苷 0.52 nA). The terrace and the island (bottom) have the same composition for growth at this temperature. The close-packed directions of the Fe(001) lattice havebeen copied from Fig. 2. The arrows marked 1 and 2 indicate
a local c共2 3 2兲 ordered area and a bright line parallel to the[010] direction, respectively. In both images the grey scale used
is stepped: the black-white range corresponds to 0.05 nm on
the terrace and on the island surface. The black (white) island
perimeters are grey scale artifacts. The heights of the islands
equal the Fe(001) step height (0.14 nm) within the accuracy ofthe measurement. All images presented in this Letter are raw
data and have not been corrected for drift which explains thenonorthogonal appearance of the lattice directions.

atoms in the Fe(001) surface layer. Analogously, the weak
depressions visible on the islands (marked I, 10 –20 pm
deep) are interpreted as place exchanged Fe atoms whichhave formed islands with Au adatoms. This interpretation
is in agreement with the fact that 40% of the surface of
the islands is covered with depressions. Since the islands
cover 0.20 ML of the Fe(001) substrate (which equals theamount of Au deposited and shows that the number of Au
atoms attached to step edges can be neglected for large
terrace whisker substrates), it follows that 0.08 ML Fe canbe found in the islands. Consequently, 0.08 ML Au is
substituted in the Fe(001) terrace which perfectly agrees
with the concentration observed. This balance and results
obtained at slightly different growth temperatures and coverages also show that the Au atoms stay in the surface layer.
Upon deposition of 0.44 ML Au at 500 K a homogeneous surface layer is formed [Fig. 1(b)]. Protrusions are
now visible on both the terrace and the island layers. Although the protrusions order locally in a c共2 3 2兲 structure(arrow 1) or even in clusters along close-packed directions(arrow 2), long-range order was not observed. Postannealing this sample to 650 K did not generate long-range order.
STS shows that the Fe(001) surface state (see, e.g., [15])has disappeared on this surface; instead a peak is observed
around 0.6–0.7 eV above the Fermi level (see Bischoff[21]) on both terrace and islands giving another indication
for the equal character of terrace and islands.
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FIG. 2. (a) STM image of 0.96 ML Au deposited on Fe(001)
at 500 K (100 3 100 nm2 , Vs 苷 1.0 V, I 苷 0.07 nA). The
growth mode is not perfectly layer by layer at this temperature:
0.15 ML of the original substrate (marked 0) is still uncovered,
while already 0.11 ML of the first deposited layer (marked 1)
is covered by islands (marked 2). All step heights in this image
are 0.14 nm. (b) Higher resolution image of the same sample as(a). (20 3 20 nm2 , Vs 苷 20.58 V, I 苷 0.25 nA). A steppedgrey scale has been used (numbers in image refer to level). On
the p共1 3 1兲 lattice, asymmetric protrusions (marked A) andcircular depressions (marked B) are observed which are ascribedto intermixed Fe atoms and contaminants, respectively. The inset
shows the boxed area. The depression is centered at a fourfold
hollow site, while the asymmetric feature comprises two unit
cell depressions separated by a protrusion at a bridge site. These(highly tip dependent) appearances of the Fe atoms are ascribed
to nontrivial tip-sample interactions leading to chemical contrast.

The formation of a surface alloy for Au on Fe(001)might be surprising considering the fact that Au is immiscible in bulk Fe [16]. However, Tersoff showed that
surface-confined mixing may generally arise in systems
dominated by atomic size mismatch [17] which can explain the present observations (Au atom is 16% larger thanthe Fe atom).
Figure 2(a) shows the Fe(001) surface after depositionof 0.96 ML Au at 500 K. The growth is certainly not perfectly layer by layer: 0.11 ML of the first layer (marked 1)
has been covered with islands (marked 2), while 0.15 MLof the original substrate (marked 0) is still uncovered. An
atomically resolved STM image is shown in Fig. 2(b). In
addition to the p共1 3 1兲 lattice, asymmetric bright features (marked A; 20 pm high) and spherical depressions(marked B; 80 pm deep) are visible. The latter ones havea much lower concentration (1.5% and 0.3% on level 0
and level 1, respectively) and are visible under various tip
conditions. Consequently, they are ascribed to impurities.
On the contrary, the appearance of the asymmetric featuresstrongly depends on the tip conditions. These features are
attributed to intermixed Fe atoms. Their concentration is
6% (2%) on levels 0 and 1 (2). Therefore, adding 0.52 ML
Au to the mixed surface layer shown in Fig. 1(b) whichcontained ⬃50% Fe results in a surface layer which contains only 6% Fe. Apparently, the Fe concentration in thesubstrate layer decreased.
To clarify the observations a simple model is sketchedin Fig. 3. For simplicity, it is assumed that the surface
alloy saturates at exactly 0.5 ML Au [Fig. 3(a)]. The
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FIG. 3. Schematic model for the alloying-dealloying mechanism leading to interface roughness. (a) Deposition of 0.5 ML
Au at 500 K leads to a homogeneous surface alloy. (b) Areasof the alloy which are covered by Au become pure Fe. This
Fe is taken out of the alloyed layer which therefore becomes
Fe depleted. I, II, and III refer to the explanation in the text.
(c) At deposition of ⬃1 ML all Fe has diffused out of the alloy
which has now become a pure Au layer. Since growth of thesecond Au layer has started before completing the first one, arough interface is created. (d) Situation after 3.5 ML Au deposition. Fe, Au, and reconstructed surface layers are 0.14, 0.20,
and 0.26 nm high, respectively. The reconstructed Au layer contains 20% more Au atoms than the unreconstructed Au layers[18]. The rough interface, the different interlayer spacings ofFe and Au, and the surface reconstruction for Au films thicker
than three layers can explain all the levels (numbered) observedin Fig. 4(b).

morphology after deposition of an additional 0.46 ML isshown in Fig. 3(b). According to the experimental results,
0.15 ML of the substrate is uncovered at this stage, while
0.11 ML of the first layer is covered by islands. Therefore, 0.35 ML of the additionally deposited Au covered
the alloyed substrate layer. The AuFe alloy is assumed
to be surface confined and covering it with Au causes it
to demix and to form a pure Fe layer. The Au segregation is probably driven by the large difference in surface energy between Au and Fe and is also reported forstudies on Fe兾Au(001); see, e.g., [10]. The Fe concentration in the Fe-depleted surface alloy can be calculated
if it is realized that 0.46 ML Fe is needed to fill the Aucovered areas denoted I, II, and III in Fig. 3(b). If the Fe inlevel 2 is neglected (only 0.02 3 0.11 苷 0.002 ML), theFe concentration in the uncovered alloyed layer reduces to
共0.5 0.46兲兾共1 0.11兲 3 100% 苷 4.5% which is in good
agreement with the experimental results [19].
Coverage of more Au will completely deplete the alloy from Fe: at a coverage of ⬃1 ML [20] the alloy becomes a pure Au layer and Au islands embedded in theoriginal Fe(001) substrate and Fe islands covering the substrate have been created [Fig. 3(c)]. The model shows that
this roughened interface is purely a result of the imperfect
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FIG. 4. STM images of 3.5 ML Au grown on Fe(001) at
500 K. (a) Large scale image (100 3 100 nm2 , Vs 苷 21.0 V,
I 苷 0.05 nA). The surface is rough and partially reconstructed. The circular shapes (see, e.g., two arrows) of
the reconstructed layer are obvious. The dotted white arrow indicates the lowest level (marked 0; h01 苷 0.14 nm).
(b) Small scale image (50 3 50 nm2 , Vs 苷 1.0 V, I 苷
0.07 nA). On the lowest unreconstructed level (marked 1),
reconstructed (2) and unreconstructed (3) islands are visible.
On the reconstructed layer (4), ridges (5) are visible. Small
unreconstructed patches (6) which are connected to the reconstructed layer can be observed on level 3. The black circles in(a) show that the ridges are connected to reconstructed patcheson the lower terrace. (c) Line profile along the white dotted linein (b). The different height levels indicated in (a), (b), and (c)can be explained by the interface roughness model of Fig. 3.

layer-by-layer growth which in turn may be a result of thealloying. If at a deposition of 1 ML the first adlayer had
been fully completed without second layer islands, afterdemixing the first monolayer would be a perfectly flat Au
layer. Apparently, the second layer Au islands trap the
Fe atoms underneath preventing them from filling the substrate voids.
In contrast to growth at lower temperatures for which
perfect layer-by-layer growth is found [21], Fig. 4(a)shows that after deposition of 3.5 ML Au at 500 K arough surface is obtained. Although various levels canbe observed, remarkably all step heights are less thanthe Au interlayer spacing (0.20 nm). The observationof the characteristic modulation of the quasihexagonalreconstruction on parts of the surface [Fig. 4(b)] implies
that these areas are covered by at least 4 ML Au [12].
This reconstruction appears as ridges weakly meandering
along the close-packed lattice directions with a periodicityof 1.4 nm [i.e., 共5 3 n兲; see, e.g., [18] ] and a corrugationof 0.04 nm. The line profile of Fig. 4(c) shows that
relative to the lowest unreconstructed level in Fig. 4(b)(marked 1) the reconstructed islands (2) are 0.12 nm
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unreconstructed islands (3) are 0.14 nm high (h13 ). The
reconstructed layer (4) is 0.12 nm high compared to level
3 (h34 ). The height of the ridges (5) on the reconstructed
layer (4) is only 0.06 nm relative to level 4 (h45 ). Finally,small features (6) which are 0.06 nm high relative to level
3 are observed (h36 ). Although the corrugation of thereconstruction and consequently h12 and h34 depend on
the tunneling conditions [22], such a dependence was notfound for h13 , h45 , and h36 .
The reconstructed ridges (5) are connected to the reconstructed islands on the lower terrace (2) forming ringlike patterns as can be seen in Fig. 4(a) (e.g., in areas
within black circles). Therefore, it seems likely that thesefeatures with heights smaller than the Fe(001) interlayer
distance have the same origin. Comparison of Fig. 4(a)with Fig. 2(a) shows that the ringlike patterns resemble
the shape of the uncovered substrate patches. Following
the model depicted in Fig. 3 describing the results for thesubmonolayer growth, these patches will turn into Au islands embedded in the Fe substrate upon further deposition. The situation after deposition of 3.5 ML has beensketched in Fig. 3(d). The interface roughness and the surface reconstruction for Au films thicker than three layers
can perfectly explain the observed levels. Level 1 consistsof three Au layers on top of the original Fe(001) surface.
Levels 2 and 5 [and level 0 in Fig. 4(a)] arise from Au islands embedded in the substrate layer [four and five (three)
Au layers high, respectively]. Level 3 can be explained bya buried Fe island covered by three Au layers. Level 4 consists of four Au layers (top one reconstructed) on top of theoriginal substrate. Finally, level 6 might be explained byassuming that very small patches of four layer thick Au
films are not reconstructed.
From Fig. 3(d) it is clear that h25 is the Au step height(0.14 1 0.06 苷 0.20 nm), while h14 is the height of thereconstructed Au layer (0.12 1 0.14 苷 0.26 nm). The latter depends on tunneling conditions [22] which can be explained by electronic effects [9,21]. However, the height
differences between unreconstructed layers (i.e., h13 andh36 ) and two reconstructed layers (i.e., h45 ) do not depend
on tunneling conditions which justifies an interpretation ofthese height differences as geometrical ones caused by interface roughness.
In summary, it was shown that for Au coverages lessthan 0.5 ML deposited at temperatures above 400 K on
Fe(001) a surface alloy is formed. This alloy is stable onlyat the surface and demixes when it is covered by additional
Au. In combination with imperfect layer-by-layer growth
conditions, this demixing leads to a rough interface for
growth at 500 K. The large difference in interlayer spacing

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between Fe and Au made it possible to study the buriedinterface with STM. These results show that the qualityof the interface can be controlled and can be studied withhigh resolution in real space.
This work was supported by the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (NWO). T. Y. acknowledges
Gakushuin University for a traveling grant. We gratefully thank B. Heinrich (Simon Fraser University, Burnaby, Canada) for supplying the Fe whiskers.

*Present address: National Microelectronics Research Centre, University College, Cork, Ireland.
†
Present address: Philips Semiconductors, Gerstweg 2, 6534
AE Nijmegen, The Netherlands.
‡
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[19] The uncertainties in the experimentally obtained concentrations and coverages are estimated to be 61%.
[20] The exact coverage depends on the details of the processes
involved.
[21] R. G. P. van der Kraan, Ph.D. thesis, University of Nijmegen, 1995; M. M. J. Bischoff et al. (to be published).
[22] For example, at Vs 苷 20.5 V and I 苷 0.5 nA the “hex”
corrugation reduced to ⬃0.01 nm and consequently h12
共h14 兲 to 0.09 nm (0.23 nm).

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