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An STM study of Fe3O4(100) grown by molecular beam epitaxy
surface science ELSEVIER
Surface Science 373 (1997) 85-94
An STM study of F e 3 0 4 ( 1 0 0 )
grown by molecular beam epitaxy
J.M. Gaines a,., P.J.H. B l o e m e n b,1 J.T. K o h l h e p p b, C.W.T. B u l l e - L i e u w m a a, R.M. W o l f a, A. Reinders a, R.M. Jungblut a, P.A.A. van der Heijden b, J.T.W.M. van Eemeren a, J. aan de Stegge a, W.J.M. de Jonge b a Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands b Department of Physics, Eindhoven University of Technology (EU T), 5600 MB Eindhoven, The Netherlands Received 23 June 1996; accepted for publication 6 September 1996
Abstract
STM imaging of MBE-grown pseudomorphic (100) FeaO4 surfaces reveals terrace widths that are typically a few hundred ~ngstrrms broad, and can be as broad as 1000 ~,. These terraces are separated by steps that are 1/4 of the spinel lattice constant high, corresponding to the distance (2.1 ~,) between planes of oxygen (or equivalent iron) atoms. The images show that the p( 1 x 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [ 110l direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90 ° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 ,~ spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 ,~ step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred hngstr6ms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide, Magnesium oxides, Magnetic surfaces, Molecular beam epitaxy, Scanning tunneling microscopy, Single crystal surfaces, Surface defects
1. Introduction Magnetic oxidic multilayers composed of Fe304 (spinel structure) combined with MgO, CoO, or NiO (rocksalt structure) are being used to study topics such as magnetic coupling across nonmagnetic barriers [ 1], coupling between ferromagnetic * Corresponding author. E-mail: [email protected] 1 Present address: Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands.
and antiferromagnetic layers (exchange biasing) [2], magnetic anisotropy [1,3] and electrical transport [4]. These properties are of importance in the development of devices such as magnetic field sensors. The lattice mismatch between the four compounds ranges from 0.3 % for MgO/Fe304 (using half the Fe304 spinel lattice constant) to 1.9% for NiO/CoO. The lattice constants of these cubic oxides are primarily determined by the fcc oxygen sublattice rather than by the small (relative to oxygen) metal ions. Several groups have shown that these materials can be grown by molecular
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0039-6028(96)01145-4
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J.M. Gaines et aL / Surface Science 373 (1997) 85-94
beam epitaxy (MBE) [5-8]. Observations of RHEED oscillations during oxide growth [7-9] demonstrate that layer-by-layer epitaxial growth can be performed. Crystalline quality has been evaluated, yielding high-resolution X-ray rocking curve widths as low as 65 arcsec for a CoO/Fe304 multilayer grown on MgO [6], and interface roughnesses that are typically about 3 ~, for Fe304/CoO multilayers [10]. High-resolution transmission electron microscopy [6] shows sharp interfaces in CoO/Fe304 multilayers, with roughnesses of 1-2 atomic layers, despite the presence of misfit dislocations. The magnetic and transport properties depend strongly on the quality of the interfaces between the various layers. A recent example of a relation between structure and magnetic properties was found for the CoO/Fe304 (100) system where a correlation was observed between the structural perfection as inferred from reflection high energy electron diffraction (RHEED) and the magnitude of the exchange biasing strength [ 11 ]. In this article, we present results of an STM investigation of the surface of an MBE-grown (100) Fe304 layer. STM images showed the p(1 × 1) reconstruction, and revealed the presence of stacking faults, presumably propagating from the interface with the higher symmetry MgO substrate to the surface. STM observations of bulk crystal (100) surfaces [12], bulk crystal (110) surfaces [13] and epitaxial (110) surfaces [14] of Fe304 have been reported. Crucial to our observations were a rapid transfer of the sample from the growth system to the STM system and the in-situ surface cleaning treatment done before the STM measurements.
2. Experimental The samples discussed in this paper are single 400-500 A thick fe304 layers grown by molecular beam epitaxy (MBE) on MgO(100) substrates. Before deposition the MgO substrates were annealed for 20 min at 550°C at an oxygen pressure of 2.8 x 10 -5 mbar. The Fe304 layers were deposited by e-gun evaporation from an iron target. Oxygen was supplied through a ring shaped doser located close to the substrate holder. During
growth, a substrate temperature of 225°C was used and the oxygen pressure was maintained at 2.8 × 10 -5 mbar [6]. Information on the structural quality of the epitaxial layer was found from high-resolution X-ray diffraction (HRXRD) experiments done to determine the in- and out-of-plane lattice constants of a 475 A Fe30 4 film grown on MgO(100). The HRXRD measurements were performed using a Ge four-crystal monochromator with Cu K~I radiation. A two-dimensional X-ray diffraction map of reciprocal space near the asymmetric (622) reflection ( ~ = 62.579 °, 20 = 74.679 °) is obtained by carrying out several ~0/20 scans for different values of o~. A contour plot of the intensity as a function of the in-plane (k,) and the out-of-plane (k±) reciprocal wave vectors (Fig. 1) shows both the substrate and epilayer peaks. The strong, sharp peak (with a weaker satellite, indicating that the substrate is not a perfect single crystal) originates from the MgO(100) substrate. The weaker elongated peak (top of Fig. 1) originates from the Fe304 layer. The peak is weak and elongated because the layer is thin. k, is, within the experimental accuracy, equal to k, of the MgO. Thus, the in-plane lattice con-
0.554. Fe304
0.552-
,#
I~..IL 0.550-
0.548. 0.252
0.2Bo
0.2~8
~°MgO
0.2~
0.2~
"1 Fig. 1. Two-dimensionalhigh-resolutionX-ray diffractionplot of a 475 .ApseudomorphicFe304 layer,grown on a MgO(100) substrate. Along the x-axis,ktl is plotted (a, = V~2/2kN with 2 = 1.5418 ~i) and k. is given along the y-axis (at=32/2ki). The position of the intense substrate peak (622) is adjusted to the bulk MgO position (a=4.212 A).
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
stants of substrate and epilayer are equal, and the Fe304 layer grows coherently (pseudomorphically) on the MgO(100) substrate. From k±, the perpendicular lattice constant is calculated to be 4.182 + 0.006 A, which was confirmed by a 0-20 scan. This lattice constant, compared to the bulk value of 4.198 A (half of the spinel cubic lattice constant [15]), demonstrates the tetragonal distortion occurring during pseudomorphic growth. Using a MgO lattice constant of 4.212 [15], Poisson's ratio is found to be 0.37. Poisson's ratio, calculated using bulk elastic constants [16] is 0.357 and agrees well with the measurement. The STM experiments were performed in the constant current mode in an UHV chamber with a background pressure of 1 × 10 -1° mbar. The tungsten tips were sharpened by ex-situ electrochemical etching in NaOH. To collect the images, the sample was biased at - 1 . 4 to - 2 . 4 V with respect to the tip. Tunneling currents of 0.25-0.3 nA were used. We use the assumption [12] that the STM images only the iron atoms, and not the oxygen atoms, which are presumed to have all occupied orbitals far below the Fermi level. All images presented here are raw data, with only a background correction applied. The STM was not attached to the MBE system. Thus, the F e 3 0 4 ( 1 0 0 ) sample had to be exposed to air for about 3 min during the transfer from one system to the other. LEED images, taken after the transfer (Fig. 2a) contained a high background with weak, but nevertheless sharp spots, indicating that the Fe304 surface was well ordered but was contaminated during transfer. The surface was then cleaned, first by sputtering with 500 eV Ar ÷ ions at room temperature for 15 min. This was followed by a temperature ramp of 20 min to 600°C during which the sputtering was continued until a temperature of 500°C was exceeded. The ramp was followed by a 30 min anneal at 600°C after which the sample was cooled down to room temperature. The sample was not exposed to oxygen during or after the sputtering and annealing stages. LEED patterns taken after the surface treatment (Figs. 2c and 2d) show lower background intensity and more intense diffraction spots, indicating a clean, well-ordered surface with large terraces. The clean
87
(a)
(c)
@
Fig. 2. LEED patterns observed ((a),(b)) immediately after transfer of the Fe304 sample into the STM chamber, and ((c),(d)) following the sputter/anneal surface treatment. The LEED images in (a) and (c) were taken at about 60 V. The images in (b) and (d) were taken at about 150 V.
sample yielded LEED patterns at energies as low as 15 V. Following the sputter-anneal treatment, X-ray photo-electron spectroscopy (XPS) was used to examine the chemical composition of the surface: only iron and oxygen signals were observed. The contamination level is thus below the detection limit of XPS (less than about 1% of a monolayer). The surface treatment was done at temperatures considerably higher than the MBE growth temperature. Thus, there is some question as to whether the images presented here represent the actual surface condition during growth. Experiments in a combined STM/MBE system would be necessary to resolve this issue.
3. Results and discussion A large area 1700 × 1700 ,&image (Fig. 3a) shows atomically-flat terraces. Steps on the surface are aligned primarily along [ 110] directions. The terrace widths are typically several hundred ~ngstr6ms wide, but in some directions on some terraces can be broader than 1000 ,~. A line profile through the image (Fig. 3b) reveals that the steps
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J.M. Gaines et al. / Surface Science 373 (1997) 85-94
J 0.4 0.0
,
, 50
'
1 O0 nm
'
150
Fig. 3. Largearea (170 x 170 nm2) STM image (a) of an Fe304 surface, revealing large terrace widths with steps that are integral multiples of 2 A high. The steps run primarily along [110] directions. The sample was biased at - 2 V with respect to the tip, and the tunneling current was 0.28 hA. The line profile (b) was taken from the image (a) as indicated by the white line. are integral multiples of about 2 ,~ high, corresponding to 1/4 of the spinel lattice constant, which is the distance between two adjacent planes of oxygen or equivalent iron atoms (Fig. 4a). The octahedrally-coordinated iron and oxygen atoms lie nearly in the same (100) plane and are separated from the nearest plane of tetrahedrally coordinated iron atoms by 1/8 of the spinel lattice constant. The rms roughness of the surface calculated from this image is 4 A which agrees well with interface roughnesses (3,~) for Fe304/MgO multilayers, determined by low-angle X-ray diffraction [ 10].
The Fe304 surface, imaged with unit cell resolution (Fig. 5), displays a regular network of features (bright features in Fig. 5) elongated along [110] directions. These features are aligned in rows along a [-110] direction. The surface unit cell is a simple square cell (Fig. 6), with a lattice constant equal to the bulk lattice constant of spinel Fe304 (8.4 ,~). Thus, the surface has reconstructed in a p(1 x l) unit cell, eliminating the c(1 x 1) structure (based on the bulk spinel lattice) expected for an unreconstructed bulk spinel surface. LEED patterns of the surface (Fig. 2) also show a simple square structure. These observations are consistent with earlier R H E E D and L E E D [,7,12] observations showing that the Fe30 4 surface reconstructs in a simple square unit cell. The step separating the two major terraces (Fig. 5) is approximately 2 ,~ high, corresponding to the distance between two adjacent planes of either O or equivalent Fe atoms. The orientation of the unit cells on one plane, however, is rotated by 90 ° with respect to the other plane. The octahedral iron atoms on the surface are each missing only one bond with oxygen atoms nearly perpendicular to the surface, and tetrahedral iron atoms on the surface are missing two bonds. Thus, the surface reconstruction is likely to be dominated by the tetrahedrally coordinated Fe atoms. The dangling bonds of the tetrahedral iron atoms all lie in the same [110] direction (Fig. 4b), thus an asymmetry in the reconstruction (with respect to the two [110] directions lying in the surface) is also expected. At step edges, the image is noticeably brighter (Fig. 5). This may be caused either by an actual height difference resulting from an additional stepedge reconstruction, or by a difference in charge at the step edges. Such a contrast difference caused by a localized accumulation of charge was observed on GaAs surfaces at kinks in the surface reconstruction [ 17]. The step-edge effect is also observable in the line profile of the large area scan (Fig. 3b), where an apparent spike in the surface height is observed at most step edges. Two distinct surface reconstructions are observed with atomic resolution STM (Fig. 6). The terrace in the top left region of Fig. 6 shows the same structure observed in larger area scans
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
89
J
(a)
0
0
%
0
%
k
(b)
Fig. 4. Schematic structure of a unit cell of (a) bulk, and (b) the unreconstructed surface of FeaO4. In (a), the iron atoms are dark, with octahedral iron atoms located at corners of the small cubes (edge length 2.1 A), and tetrahedral atoms located at corners of the large cubes (edge length 4.2 ,~). The box defines the entire unit cell, but only the atoms in the front half of the cell are shown. In (b), the oxygen atoms are shown larger than the iron atoms. The dangling bonds of the tetrahedral iron atoms are indicated. The octahedral iron atoms each have a dangling bond nearly perpendicular to the plane of the page. The four oxygen atoms farthest from the tetrahedral iron atoms each have a dangling bond nearly perpendicular to the surface. (Fig. 5). T h e r e s o l u t i o n o n this terrace, u n d e r t h e c o n d i t i o n s u s e d t o c o l l e c t t h e i m a g e o f Fig. 6 is insufficient to s e p a r a t e t h e i n d i v i d u a l a t o m s . A
diffuse c l u s t e r o f a t o m s is s i t u a t e d o n t h e surface in e a c h p(1 x 1) u n i t cell. T h e s p a c i n g b e t w e e n t h e p l a n e s o f o c t a h e d r a l a n d t e t r a h e d r a l Fe a t o m s in
J.M. Gaines et aL / Surface Science 373 (1997) 85-94
90
0.6
(b) 0.4
E ¢0.2
0-00
2'0
10
30
nm
Fig. 5. A 28 x 28 nm 2 STM image (a) with intermediate resolution, revealing the unit cells on the Fe304 surface. The surface is reconstructed with p(1 x 1) square unit cells rather than the c(1 x 1) unit cell expected for an unreconstructed surface. The sample was biased at - 2 . 3 V with respect to the tip, and the tunneling current was 0.24 nA. The line profile (b) was measured along the black line indicated in (a).
bulk is only 1 A, and it is likely to be less on the surface because of charging effects. Thus, these clusters may consist of both tetrahedral and octahedral iron atoms. Imaging with different voltages yields better resolution of the dusters, but also causes the form of the atom cluster in the unit cell to change. Thus, a complicated surface atomic arrangement and/or density of states makes the interpretation of the atomic-resolution images from this terrace difficult. Further experiments are necessary to clarify the details of this surface. Regions of the surface appear devoid of the
Fig. 6. A 7 x 7 nm 2 STM image of two terraces, showing two different reconstructions. Individual atoms, separated by about 3 A are visible on the lower terrace. On the upper terrace, a cluster of atoms appears in each unit cell. The clusters are aligned along a [110] direction. The sample was biased at - 2 . 3 V with respect to the tip, and the tunneling current was 0.31 nA. The terraces are separated by a step of about 1 ,~.
p(1 × 1) reconstruction (Fig. 5). Atomic-resolution imaging of these regions reveals a second surface reconstruction, as shown by the terrace in the lower right region of Fig. 6. The features of this surface are arranged in what appears to be a c(1/2× 1/2) pattern, based on the spinel lattice constant. (Note that no extra spots are expected in the LEED pattern from these regions.) We stress that no filtering of the image was done to enhance the contrast of these features. The nearest-neighbor separation between atomic features is about 3 A. The step height from the p(1 × 1) surface to the c(1/2 × 1/2) surface is approximately 1 A which is close to the distance between the tetrahedral iron plane and the oxygen plane immediately below. Two conceivable explanations for this fcc closepacked surface are: (1) the STM is imaging oxygen atoms or (2) the STM is imaging iron atoms that
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
are in a rocksalt FeO-like surface structure. Although the unreconstructed spinel oxygen atoms are arranged in nearly a c(1/2 x 1/2) pattern, with oxygen atoms slightly displaced (approximately 0.04.~) from the precise c(1/2x 1/2) positions, option (1) is thought to be unlikely because the filled oxygen 2p orbitals are expected to be far below the Fermi energy and thus inaccessible to STM investigation [12]. Electron counting arguments [18] support option (2), the FeO surface. Surfaces reconstruct to leave dangling bonds on the electronegative element filled and dangling bonds on the electropositive element empty [18]. Thus, all oxygen atoms are expected to have filled dangling bonds, and all iron atoms empty dangling bonds. If the iron atoms at the spinel surface have the same valency that they would have in the bulk, then octahedral sites are filled by an equal mixture of Fe 3÷ and Fe 2 ÷ ions and tetrahedral sites are filled by Fe 3÷ ions. If the spinel structure is cleaved between a plane of oxygen/octahedral iron and a plane of tetrahedral iron atoms, and no reconstruction occurs (Fig. 4a), then there are 4 octahedral iron atoms per unit cell that are each missing one of the oxygen atoms required to complete their octahedral coordination. These four iron atoms have an average of 2.5 valence electrons and, in the bulk, each would contribute 5/12 of an electron to each of its six coordinating oxygen atoms. Thus, at the surface, these four atoms contribute a total of 5/3 excess electrons per unit cell. Each bulk oxygen atom has three bonds to octahedrallycoordinated iron atoms and one bond to a tetrahedrally-coordinated iron atom. In the topmost oxygen plane, (Fig. 4a), there are 4 oxygen atoms that are each missing one tetrahedral bond to iron. The missing charge per oxygen atom is 3/4 electron for a total of 3 electrons per unit cell. There are four additional oxygen atoms that each have one unsatisfied octahedral bond, because of the missing octahedral iron atoms above them. These oxygen atoms thus need 4 × 5/12=5/3 electrons per unit cell to completely fill their 2p shells. Thus, there is a total of 3 missing electrons per unit cell. The electron counting arguments can be satisfied either by adding more atoms to the unit cell (by filling half of the tetrahedral Fe 3 + sites, for instance), or
91
by a reconstruction allowing sharing of charge. However, in the close packed fcc regions (Fig. 6), the STM reveals a high-symmetry surface with no reconstruction. If the STM w a s imaging the oxygen atoms in the close-packed region, and not the iron atoms, then it also reveals no difference between the atoms that could correspond to the difference in charge for the two types of surface oxygen (those missing a tetrahedral and those missing an octahedral bond with iron). Thus, we conclude that this surface is not the oxygen plane of the spinel structure. Option (2), the FeO-like surface, readily satisfies electron counting arguments. FeO crystallizes in the rocksalt structure with a lattice constant of 4.31 ~,. In comparison to the spinel structure, all of the bulk octahedral sites are filled with Fe 2÷ ions (only half of these sites are filled in spinel), and all of the bulk tetrahedral sites are empty. The Fe 2÷ ions in a (100) surface of FeO are each missing one octahedral bond to oxygen, and thus each has an extra charge of 1/3 electron. Similarly, the 0 2 - ions, which are equal in number to the Fe 2÷ ions, are each missing one octahedral bond to iron, and thus each is missing 1/3 electron. Therefore, a simple transfer of the excess electrons from the iron atoms to the oxygen atoms satisfies electron counting arguments. There is also no electric dipole moment normal to an FeO surface. Thus, no reconstruction of the FeO surface is expected. Note that, depending on the structure of the interface between the underlying Fe304 and the FeO surface layer, there may still be a surface dipole present at FeO-like regions in our samples. An FeO-like surface could possibly have been induced by the high temperature sputter/anneal procedure used to prepare the surface. Perhaps the surface of the F e 3 0 4 in these regions lost enough oxygen and has a sufficient excess of Fe to fill all of the surface (and perhaps some below-surface) octahedral sites. The transition from spinel to rocksalt occurs below the surface, and is therefore inaccessible to STM investigation. The surface observed in the STM is then best described, based on the FeO lattice constant, as an unreconstructed c(1 × 1) rocksalt structure. If the FeO structure was not induced by the surface treatment, but is commonly present at the surface during growth,
J.M. Gaines et aL / Surface Science 373 (1997) 85-94
92
then its presence at interfaces in heterostructures could play a role in forming the magnetically "dead" regions observed at interfaces in such heterostructures [19]. In-situ growth/STM measurements could possibly clarify this issue. We have also observed grown-in defects in the spinel structure (Fig. 7). The lower central portion of Fig. 7 shows a region where the atomic clusters are arranged at 90 ° with respect to the atomic clusters in the upper part of the image. However, there is no height difference between these two regions; only a disordered band separates them.
0.3
0.2
O'CO
(b)
10
2'0 nm
Fig. 7. A 33 x 33 n m 2 STM image (a) of a region of the p(1 x 1) surface showing two domains rotated by 90 ° with respect to each other. The line profile (b) shows that the two domains are at the same height on the surface. The domains are separated by a disordered region indicating the presence of a stacking fault. The sample was biased at - 2 . 7 V with respect to the tip, and the tunneling current was 0.32 nA.
Thus the unit cells in the two regions are oriented at 90 ° with respect to each other and must be separated by a stacking fault. The stacking fault extends along a [100] direction. A similar defect can be seen in Fig. 5a. The two regions at the ends of the line are at the same height (Fig. 5b), but again the unit cells are in domains rotated by 90 ° with respect to each other. There is a dark, disordered band, lying approximately perpendicular to the line, between the domain at the top left and the step leading to the higher terrace in the center of the line profile. The orientation of the unit cells is identical in the two domains separated by the dark band. However, the orientation rotates by 90 ° where the line profile traverses the step at the lower right. Thus the darker region along the edge of the upper step must also be a stacking fault. The few unit cells visible on the highest terrace along the topmost edge of the figure appear to be oriented at 90 ° with respect to the unit cells on the same terrace, but located more centrally in the image. Apparently, the stacking fault also traverses this highest terrace, along the top edge of the figure, making a 45 ° turn from a generally [110] direction to a [ 100] direction near the double step in the top central region of the image. The growth of lower-symmetry spinel on highsymmetry MgO substrates is expected to produce stacking faults, since the Fe304 unit cells can be accommodated in several equivalent ways on the rocksalt substrate. During nucleation of Fe304 on the MgO surface, islands may form on the same terrace separated by an odd number of unit cells of the rocksalt structure. Then, the two islands are separated by a distance that is a non-integral number of spinel lattice constants. The islands may also nucleate on the same terrace, but rotated by 90 °, or they may nucleate on different terraces, in such a way that the spinel unit cells in the two islands are separated by a non-integral number of unit cell lengths. Later in growth, these cases lead to a stacking fault at the boundary where two such "out-of-phase" islands merge. It is difficult to estimate the density of these defects from the STM data, but the presence of single domain regions many unit cells broad (Figs. 5, 7 and 6), indicate that they can be at least a few hundred ~ngstr6ms apart. This implies that the surface mobility of the
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
iron atoms during the initiation of growth on the M g O substrate is high enough that the island sizes during nucleation are also at least a few hundred ~ngstr6ms apart. Stacking faults have been observed to annihilate each other in the first 1000 ~, of ZnSe growth, leading to fewer defects present at the surface [20]. However, the annihilation of such stacking faults must be accompanied by relaxation of strain and the accompanying formation of misfit dislocations. Relaxation does not occur with Fe304, for the thicknesses that we have used. Comparison of our results to those of Tarrach, et al. [12], for the (100) surface of bulk Fe304, show closest agreement for what they refer to as phase C, a surface that was prepared also by a sputter/annealing treatment. Fig. 3. is comparable to Fig. 8 from Ref. [12], showing islands with similar width, step height and step edge orientations. The clusters of atoms in the individual unit cells (Fig. 5a) may be the same as those in Fig. 9 of Ref. [12]. However, the features appear more circular and diffuse in Ref. [12]. We have seen no features comparable to phases A and B from Ref. [ 12]. Also, no FeO-like surface was reported in Ref. [12], which m a y have been present, but unobservable because of the lower resolution in their images. Alternatively, this surface m a y be unique to heteroepitaxial films. The presence of stacking faults appears to be unique to the heteroepitaxial films, as expected from the origin of the faults discussed above.
4. Conclusions STM imaging of an MBE-grown (100) Fe304 surface reveals flat terraces that are hundreds of gmgstr6ms broad, separated by steps that are integral multiples of the distance between planes of oxygen (or equivalent iron) atoms. Two surface reconstructions are present. A spinel p(1 x 1) surface reconstruction covers most of the surface. In this reconstruction, a cluster of atoms is seen in each unit cell. The clusters are elongated in a [ 110] direction, causing the c(1 x l) unreconstructed spinel surface to reconstruct in a p(1 x 1) surface. The second reconstruction is a c(1 x 1) unreconstructed FeO-type surface, which forms on top
93
of the spinel epilayer. This surface satisfies electron counting requirements. Regions containing stacking fault defects were imaged. These defects are thought to originate from the coalescence of spinel islands that nucleated with incompatible orientations on the highersymmetry rocksalt substrate and later grew together. The width of single domain regions can be at least a few hundred ~ngstr6ms. Thus, the surface mobility at the beginning of growth is inferred to be high enough to allow iron atoms to diffuse by at least the island width, before bonding to the surface.
Acknowledgements The research of P.J.H. Bloemen has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences. The authors gratefully acknowledge discussions with E.J. van Loenen and N. Kramer.
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J.M. Gaines et aL / Surface Science 373 (1997) 85-94 Jungblut, P.J. van der Zaag and W.J.M. de Jonge, Mater. Res. Soc. Symp. Proc. 401, to be published. P.J. van der Zaag et al., to be published. G. Tarrach, D. Bilrgler, T. Schaub, R. Wiesendanger and H.-J. GQnterodt, Surf. Sci. 285 (1993) 1. R. Jansen, B.J. Nelissen, D.L. Abraham, H. van Kempen and V.A.M. Brabers, IEEE Trans. Magn. 30 (1994) 4506; R. Jansen, V.A.M. Brabers and H. van Kempen, Surf. Sci. 328 (1995) 237. R. Jansen, H. van Kempen and R.M. Wolf, J. Vac. Sci. Technol. B, to be published. Powder Diffraction File: Inorganic Phases, Ed. W.F.
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McClung (International Center for Diffraction Data, Swarthmore, PA, 1990). V.A.M. Brabers, private communication (Cl1=2.70, C12 = 1.50 GPa). M.D. Pashley, K.W. Haberern and R.M. Feenstra, J. Vac. Sci. Technol. B 10 (1992) 1874. M.D. Pashley, Phys. Rev. B 40 (1989) 10481. S.S.P. Parkin, R. Sigsbee, R. Felici and G.P. Felcher, Appl. Phys. Lett. 48 (1986) 604. J. Petruzzello, B. Greenberg, D. Cammack and R. Dalby, J. Appl. Phys. 63 (1988) 2299.
Surface Science 373 (1997) 85-94
An STM study of F e 3 0 4 ( 1 0 0 )
grown by molecular beam epitaxy
J.M. Gaines a,., P.J.H. B l o e m e n b,1 J.T. K o h l h e p p b, C.W.T. B u l l e - L i e u w m a a, R.M. W o l f a, A. Reinders a, R.M. Jungblut a, P.A.A. van der Heijden b, J.T.W.M. van Eemeren a, J. aan de Stegge a, W.J.M. de Jonge b a Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands b Department of Physics, Eindhoven University of Technology (EU T), 5600 MB Eindhoven, The Netherlands Received 23 June 1996; accepted for publication 6 September 1996
Abstract
STM imaging of MBE-grown pseudomorphic (100) FeaO4 surfaces reveals terrace widths that are typically a few hundred ~ngstrrms broad, and can be as broad as 1000 ~,. These terraces are separated by steps that are 1/4 of the spinel lattice constant high, corresponding to the distance (2.1 ~,) between planes of oxygen (or equivalent iron) atoms. The images show that the p( 1 x 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [ 110l direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90 ° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 ,~ spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 ,~ step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred hngstr6ms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide, Magnesium oxides, Magnetic surfaces, Molecular beam epitaxy, Scanning tunneling microscopy, Single crystal surfaces, Surface defects
1. Introduction Magnetic oxidic multilayers composed of Fe304 (spinel structure) combined with MgO, CoO, or NiO (rocksalt structure) are being used to study topics such as magnetic coupling across nonmagnetic barriers [ 1], coupling between ferromagnetic * Corresponding author. E-mail: [email protected] 1 Present address: Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands.
and antiferromagnetic layers (exchange biasing) [2], magnetic anisotropy [1,3] and electrical transport [4]. These properties are of importance in the development of devices such as magnetic field sensors. The lattice mismatch between the four compounds ranges from 0.3 % for MgO/Fe304 (using half the Fe304 spinel lattice constant) to 1.9% for NiO/CoO. The lattice constants of these cubic oxides are primarily determined by the fcc oxygen sublattice rather than by the small (relative to oxygen) metal ions. Several groups have shown that these materials can be grown by molecular
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PH S0039-6028(96)01145-4
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J.M. Gaines et aL / Surface Science 373 (1997) 85-94
beam epitaxy (MBE) [5-8]. Observations of RHEED oscillations during oxide growth [7-9] demonstrate that layer-by-layer epitaxial growth can be performed. Crystalline quality has been evaluated, yielding high-resolution X-ray rocking curve widths as low as 65 arcsec for a CoO/Fe304 multilayer grown on MgO [6], and interface roughnesses that are typically about 3 ~, for Fe304/CoO multilayers [10]. High-resolution transmission electron microscopy [6] shows sharp interfaces in CoO/Fe304 multilayers, with roughnesses of 1-2 atomic layers, despite the presence of misfit dislocations. The magnetic and transport properties depend strongly on the quality of the interfaces between the various layers. A recent example of a relation between structure and magnetic properties was found for the CoO/Fe304 (100) system where a correlation was observed between the structural perfection as inferred from reflection high energy electron diffraction (RHEED) and the magnitude of the exchange biasing strength [ 11 ]. In this article, we present results of an STM investigation of the surface of an MBE-grown (100) Fe304 layer. STM images showed the p(1 × 1) reconstruction, and revealed the presence of stacking faults, presumably propagating from the interface with the higher symmetry MgO substrate to the surface. STM observations of bulk crystal (100) surfaces [12], bulk crystal (110) surfaces [13] and epitaxial (110) surfaces [14] of Fe304 have been reported. Crucial to our observations were a rapid transfer of the sample from the growth system to the STM system and the in-situ surface cleaning treatment done before the STM measurements.
2. Experimental The samples discussed in this paper are single 400-500 A thick fe304 layers grown by molecular beam epitaxy (MBE) on MgO(100) substrates. Before deposition the MgO substrates were annealed for 20 min at 550°C at an oxygen pressure of 2.8 x 10 -5 mbar. The Fe304 layers were deposited by e-gun evaporation from an iron target. Oxygen was supplied through a ring shaped doser located close to the substrate holder. During
growth, a substrate temperature of 225°C was used and the oxygen pressure was maintained at 2.8 × 10 -5 mbar [6]. Information on the structural quality of the epitaxial layer was found from high-resolution X-ray diffraction (HRXRD) experiments done to determine the in- and out-of-plane lattice constants of a 475 A Fe30 4 film grown on MgO(100). The HRXRD measurements were performed using a Ge four-crystal monochromator with Cu K~I radiation. A two-dimensional X-ray diffraction map of reciprocal space near the asymmetric (622) reflection ( ~ = 62.579 °, 20 = 74.679 °) is obtained by carrying out several ~0/20 scans for different values of o~. A contour plot of the intensity as a function of the in-plane (k,) and the out-of-plane (k±) reciprocal wave vectors (Fig. 1) shows both the substrate and epilayer peaks. The strong, sharp peak (with a weaker satellite, indicating that the substrate is not a perfect single crystal) originates from the MgO(100) substrate. The weaker elongated peak (top of Fig. 1) originates from the Fe304 layer. The peak is weak and elongated because the layer is thin. k, is, within the experimental accuracy, equal to k, of the MgO. Thus, the in-plane lattice con-
0.554. Fe304
0.552-
,#
I~..IL 0.550-
0.548. 0.252
0.2Bo
0.2~8
~°MgO
0.2~
0.2~
"1 Fig. 1. Two-dimensionalhigh-resolutionX-ray diffractionplot of a 475 .ApseudomorphicFe304 layer,grown on a MgO(100) substrate. Along the x-axis,ktl is plotted (a, = V~2/2kN with 2 = 1.5418 ~i) and k. is given along the y-axis (at=32/2ki). The position of the intense substrate peak (622) is adjusted to the bulk MgO position (a=4.212 A).
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
stants of substrate and epilayer are equal, and the Fe304 layer grows coherently (pseudomorphically) on the MgO(100) substrate. From k±, the perpendicular lattice constant is calculated to be 4.182 + 0.006 A, which was confirmed by a 0-20 scan. This lattice constant, compared to the bulk value of 4.198 A (half of the spinel cubic lattice constant [15]), demonstrates the tetragonal distortion occurring during pseudomorphic growth. Using a MgO lattice constant of 4.212 [15], Poisson's ratio is found to be 0.37. Poisson's ratio, calculated using bulk elastic constants [16] is 0.357 and agrees well with the measurement. The STM experiments were performed in the constant current mode in an UHV chamber with a background pressure of 1 × 10 -1° mbar. The tungsten tips were sharpened by ex-situ electrochemical etching in NaOH. To collect the images, the sample was biased at - 1 . 4 to - 2 . 4 V with respect to the tip. Tunneling currents of 0.25-0.3 nA were used. We use the assumption [12] that the STM images only the iron atoms, and not the oxygen atoms, which are presumed to have all occupied orbitals far below the Fermi level. All images presented here are raw data, with only a background correction applied. The STM was not attached to the MBE system. Thus, the F e 3 0 4 ( 1 0 0 ) sample had to be exposed to air for about 3 min during the transfer from one system to the other. LEED images, taken after the transfer (Fig. 2a) contained a high background with weak, but nevertheless sharp spots, indicating that the Fe304 surface was well ordered but was contaminated during transfer. The surface was then cleaned, first by sputtering with 500 eV Ar ÷ ions at room temperature for 15 min. This was followed by a temperature ramp of 20 min to 600°C during which the sputtering was continued until a temperature of 500°C was exceeded. The ramp was followed by a 30 min anneal at 600°C after which the sample was cooled down to room temperature. The sample was not exposed to oxygen during or after the sputtering and annealing stages. LEED patterns taken after the surface treatment (Figs. 2c and 2d) show lower background intensity and more intense diffraction spots, indicating a clean, well-ordered surface with large terraces. The clean
87
(a)
(c)
@
Fig. 2. LEED patterns observed ((a),(b)) immediately after transfer of the Fe304 sample into the STM chamber, and ((c),(d)) following the sputter/anneal surface treatment. The LEED images in (a) and (c) were taken at about 60 V. The images in (b) and (d) were taken at about 150 V.
sample yielded LEED patterns at energies as low as 15 V. Following the sputter-anneal treatment, X-ray photo-electron spectroscopy (XPS) was used to examine the chemical composition of the surface: only iron and oxygen signals were observed. The contamination level is thus below the detection limit of XPS (less than about 1% of a monolayer). The surface treatment was done at temperatures considerably higher than the MBE growth temperature. Thus, there is some question as to whether the images presented here represent the actual surface condition during growth. Experiments in a combined STM/MBE system would be necessary to resolve this issue.
3. Results and discussion A large area 1700 × 1700 ,&image (Fig. 3a) shows atomically-flat terraces. Steps on the surface are aligned primarily along [ 110] directions. The terrace widths are typically several hundred ~ngstr6ms wide, but in some directions on some terraces can be broader than 1000 ,~. A line profile through the image (Fig. 3b) reveals that the steps
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J 0.4 0.0
,
, 50
'
1 O0 nm
'
150
Fig. 3. Largearea (170 x 170 nm2) STM image (a) of an Fe304 surface, revealing large terrace widths with steps that are integral multiples of 2 A high. The steps run primarily along [110] directions. The sample was biased at - 2 V with respect to the tip, and the tunneling current was 0.28 hA. The line profile (b) was taken from the image (a) as indicated by the white line. are integral multiples of about 2 ,~ high, corresponding to 1/4 of the spinel lattice constant, which is the distance between two adjacent planes of oxygen or equivalent iron atoms (Fig. 4a). The octahedrally-coordinated iron and oxygen atoms lie nearly in the same (100) plane and are separated from the nearest plane of tetrahedrally coordinated iron atoms by 1/8 of the spinel lattice constant. The rms roughness of the surface calculated from this image is 4 A which agrees well with interface roughnesses (3,~) for Fe304/MgO multilayers, determined by low-angle X-ray diffraction [ 10].
The Fe304 surface, imaged with unit cell resolution (Fig. 5), displays a regular network of features (bright features in Fig. 5) elongated along [110] directions. These features are aligned in rows along a [-110] direction. The surface unit cell is a simple square cell (Fig. 6), with a lattice constant equal to the bulk lattice constant of spinel Fe304 (8.4 ,~). Thus, the surface has reconstructed in a p(1 x l) unit cell, eliminating the c(1 x 1) structure (based on the bulk spinel lattice) expected for an unreconstructed bulk spinel surface. LEED patterns of the surface (Fig. 2) also show a simple square structure. These observations are consistent with earlier R H E E D and L E E D [,7,12] observations showing that the Fe30 4 surface reconstructs in a simple square unit cell. The step separating the two major terraces (Fig. 5) is approximately 2 ,~ high, corresponding to the distance between two adjacent planes of either O or equivalent Fe atoms. The orientation of the unit cells on one plane, however, is rotated by 90 ° with respect to the other plane. The octahedral iron atoms on the surface are each missing only one bond with oxygen atoms nearly perpendicular to the surface, and tetrahedral iron atoms on the surface are missing two bonds. Thus, the surface reconstruction is likely to be dominated by the tetrahedrally coordinated Fe atoms. The dangling bonds of the tetrahedral iron atoms all lie in the same [110] direction (Fig. 4b), thus an asymmetry in the reconstruction (with respect to the two [110] directions lying in the surface) is also expected. At step edges, the image is noticeably brighter (Fig. 5). This may be caused either by an actual height difference resulting from an additional stepedge reconstruction, or by a difference in charge at the step edges. Such a contrast difference caused by a localized accumulation of charge was observed on GaAs surfaces at kinks in the surface reconstruction [ 17]. The step-edge effect is also observable in the line profile of the large area scan (Fig. 3b), where an apparent spike in the surface height is observed at most step edges. Two distinct surface reconstructions are observed with atomic resolution STM (Fig. 6). The terrace in the top left region of Fig. 6 shows the same structure observed in larger area scans
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
89
J
(a)
0
0
%
0
%
k
(b)
Fig. 4. Schematic structure of a unit cell of (a) bulk, and (b) the unreconstructed surface of FeaO4. In (a), the iron atoms are dark, with octahedral iron atoms located at corners of the small cubes (edge length 2.1 A), and tetrahedral atoms located at corners of the large cubes (edge length 4.2 ,~). The box defines the entire unit cell, but only the atoms in the front half of the cell are shown. In (b), the oxygen atoms are shown larger than the iron atoms. The dangling bonds of the tetrahedral iron atoms are indicated. The octahedral iron atoms each have a dangling bond nearly perpendicular to the plane of the page. The four oxygen atoms farthest from the tetrahedral iron atoms each have a dangling bond nearly perpendicular to the surface. (Fig. 5). T h e r e s o l u t i o n o n this terrace, u n d e r t h e c o n d i t i o n s u s e d t o c o l l e c t t h e i m a g e o f Fig. 6 is insufficient to s e p a r a t e t h e i n d i v i d u a l a t o m s . A
diffuse c l u s t e r o f a t o m s is s i t u a t e d o n t h e surface in e a c h p(1 x 1) u n i t cell. T h e s p a c i n g b e t w e e n t h e p l a n e s o f o c t a h e d r a l a n d t e t r a h e d r a l Fe a t o m s in
J.M. Gaines et aL / Surface Science 373 (1997) 85-94
90
0.6
(b) 0.4
E ¢0.2
0-00
2'0
10
30
nm
Fig. 5. A 28 x 28 nm 2 STM image (a) with intermediate resolution, revealing the unit cells on the Fe304 surface. The surface is reconstructed with p(1 x 1) square unit cells rather than the c(1 x 1) unit cell expected for an unreconstructed surface. The sample was biased at - 2 . 3 V with respect to the tip, and the tunneling current was 0.24 nA. The line profile (b) was measured along the black line indicated in (a).
bulk is only 1 A, and it is likely to be less on the surface because of charging effects. Thus, these clusters may consist of both tetrahedral and octahedral iron atoms. Imaging with different voltages yields better resolution of the dusters, but also causes the form of the atom cluster in the unit cell to change. Thus, a complicated surface atomic arrangement and/or density of states makes the interpretation of the atomic-resolution images from this terrace difficult. Further experiments are necessary to clarify the details of this surface. Regions of the surface appear devoid of the
Fig. 6. A 7 x 7 nm 2 STM image of two terraces, showing two different reconstructions. Individual atoms, separated by about 3 A are visible on the lower terrace. On the upper terrace, a cluster of atoms appears in each unit cell. The clusters are aligned along a [110] direction. The sample was biased at - 2 . 3 V with respect to the tip, and the tunneling current was 0.31 nA. The terraces are separated by a step of about 1 ,~.
p(1 × 1) reconstruction (Fig. 5). Atomic-resolution imaging of these regions reveals a second surface reconstruction, as shown by the terrace in the lower right region of Fig. 6. The features of this surface are arranged in what appears to be a c(1/2× 1/2) pattern, based on the spinel lattice constant. (Note that no extra spots are expected in the LEED pattern from these regions.) We stress that no filtering of the image was done to enhance the contrast of these features. The nearest-neighbor separation between atomic features is about 3 A. The step height from the p(1 × 1) surface to the c(1/2 × 1/2) surface is approximately 1 A which is close to the distance between the tetrahedral iron plane and the oxygen plane immediately below. Two conceivable explanations for this fcc closepacked surface are: (1) the STM is imaging oxygen atoms or (2) the STM is imaging iron atoms that
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
are in a rocksalt FeO-like surface structure. Although the unreconstructed spinel oxygen atoms are arranged in nearly a c(1/2 x 1/2) pattern, with oxygen atoms slightly displaced (approximately 0.04.~) from the precise c(1/2x 1/2) positions, option (1) is thought to be unlikely because the filled oxygen 2p orbitals are expected to be far below the Fermi energy and thus inaccessible to STM investigation [12]. Electron counting arguments [18] support option (2), the FeO surface. Surfaces reconstruct to leave dangling bonds on the electronegative element filled and dangling bonds on the electropositive element empty [18]. Thus, all oxygen atoms are expected to have filled dangling bonds, and all iron atoms empty dangling bonds. If the iron atoms at the spinel surface have the same valency that they would have in the bulk, then octahedral sites are filled by an equal mixture of Fe 3÷ and Fe 2 ÷ ions and tetrahedral sites are filled by Fe 3÷ ions. If the spinel structure is cleaved between a plane of oxygen/octahedral iron and a plane of tetrahedral iron atoms, and no reconstruction occurs (Fig. 4a), then there are 4 octahedral iron atoms per unit cell that are each missing one of the oxygen atoms required to complete their octahedral coordination. These four iron atoms have an average of 2.5 valence electrons and, in the bulk, each would contribute 5/12 of an electron to each of its six coordinating oxygen atoms. Thus, at the surface, these four atoms contribute a total of 5/3 excess electrons per unit cell. Each bulk oxygen atom has three bonds to octahedrallycoordinated iron atoms and one bond to a tetrahedrally-coordinated iron atom. In the topmost oxygen plane, (Fig. 4a), there are 4 oxygen atoms that are each missing one tetrahedral bond to iron. The missing charge per oxygen atom is 3/4 electron for a total of 3 electrons per unit cell. There are four additional oxygen atoms that each have one unsatisfied octahedral bond, because of the missing octahedral iron atoms above them. These oxygen atoms thus need 4 × 5/12=5/3 electrons per unit cell to completely fill their 2p shells. Thus, there is a total of 3 missing electrons per unit cell. The electron counting arguments can be satisfied either by adding more atoms to the unit cell (by filling half of the tetrahedral Fe 3 + sites, for instance), or
91
by a reconstruction allowing sharing of charge. However, in the close packed fcc regions (Fig. 6), the STM reveals a high-symmetry surface with no reconstruction. If the STM w a s imaging the oxygen atoms in the close-packed region, and not the iron atoms, then it also reveals no difference between the atoms that could correspond to the difference in charge for the two types of surface oxygen (those missing a tetrahedral and those missing an octahedral bond with iron). Thus, we conclude that this surface is not the oxygen plane of the spinel structure. Option (2), the FeO-like surface, readily satisfies electron counting arguments. FeO crystallizes in the rocksalt structure with a lattice constant of 4.31 ~,. In comparison to the spinel structure, all of the bulk octahedral sites are filled with Fe 2÷ ions (only half of these sites are filled in spinel), and all of the bulk tetrahedral sites are empty. The Fe 2÷ ions in a (100) surface of FeO are each missing one octahedral bond to oxygen, and thus each has an extra charge of 1/3 electron. Similarly, the 0 2 - ions, which are equal in number to the Fe 2÷ ions, are each missing one octahedral bond to iron, and thus each is missing 1/3 electron. Therefore, a simple transfer of the excess electrons from the iron atoms to the oxygen atoms satisfies electron counting arguments. There is also no electric dipole moment normal to an FeO surface. Thus, no reconstruction of the FeO surface is expected. Note that, depending on the structure of the interface between the underlying Fe304 and the FeO surface layer, there may still be a surface dipole present at FeO-like regions in our samples. An FeO-like surface could possibly have been induced by the high temperature sputter/anneal procedure used to prepare the surface. Perhaps the surface of the F e 3 0 4 in these regions lost enough oxygen and has a sufficient excess of Fe to fill all of the surface (and perhaps some below-surface) octahedral sites. The transition from spinel to rocksalt occurs below the surface, and is therefore inaccessible to STM investigation. The surface observed in the STM is then best described, based on the FeO lattice constant, as an unreconstructed c(1 × 1) rocksalt structure. If the FeO structure was not induced by the surface treatment, but is commonly present at the surface during growth,
J.M. Gaines et aL / Surface Science 373 (1997) 85-94
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then its presence at interfaces in heterostructures could play a role in forming the magnetically "dead" regions observed at interfaces in such heterostructures [19]. In-situ growth/STM measurements could possibly clarify this issue. We have also observed grown-in defects in the spinel structure (Fig. 7). The lower central portion of Fig. 7 shows a region where the atomic clusters are arranged at 90 ° with respect to the atomic clusters in the upper part of the image. However, there is no height difference between these two regions; only a disordered band separates them.
0.3
0.2
O'CO
(b)
10
2'0 nm
Fig. 7. A 33 x 33 n m 2 STM image (a) of a region of the p(1 x 1) surface showing two domains rotated by 90 ° with respect to each other. The line profile (b) shows that the two domains are at the same height on the surface. The domains are separated by a disordered region indicating the presence of a stacking fault. The sample was biased at - 2 . 7 V with respect to the tip, and the tunneling current was 0.32 nA.
Thus the unit cells in the two regions are oriented at 90 ° with respect to each other and must be separated by a stacking fault. The stacking fault extends along a [100] direction. A similar defect can be seen in Fig. 5a. The two regions at the ends of the line are at the same height (Fig. 5b), but again the unit cells are in domains rotated by 90 ° with respect to each other. There is a dark, disordered band, lying approximately perpendicular to the line, between the domain at the top left and the step leading to the higher terrace in the center of the line profile. The orientation of the unit cells is identical in the two domains separated by the dark band. However, the orientation rotates by 90 ° where the line profile traverses the step at the lower right. Thus the darker region along the edge of the upper step must also be a stacking fault. The few unit cells visible on the highest terrace along the topmost edge of the figure appear to be oriented at 90 ° with respect to the unit cells on the same terrace, but located more centrally in the image. Apparently, the stacking fault also traverses this highest terrace, along the top edge of the figure, making a 45 ° turn from a generally [110] direction to a [ 100] direction near the double step in the top central region of the image. The growth of lower-symmetry spinel on highsymmetry MgO substrates is expected to produce stacking faults, since the Fe304 unit cells can be accommodated in several equivalent ways on the rocksalt substrate. During nucleation of Fe304 on the MgO surface, islands may form on the same terrace separated by an odd number of unit cells of the rocksalt structure. Then, the two islands are separated by a distance that is a non-integral number of spinel lattice constants. The islands may also nucleate on the same terrace, but rotated by 90 °, or they may nucleate on different terraces, in such a way that the spinel unit cells in the two islands are separated by a non-integral number of unit cell lengths. Later in growth, these cases lead to a stacking fault at the boundary where two such "out-of-phase" islands merge. It is difficult to estimate the density of these defects from the STM data, but the presence of single domain regions many unit cells broad (Figs. 5, 7 and 6), indicate that they can be at least a few hundred ~ngstr6ms apart. This implies that the surface mobility of the
J.M. Gaines et al. / Surface Science 373 (1997) 85-94
iron atoms during the initiation of growth on the M g O substrate is high enough that the island sizes during nucleation are also at least a few hundred ~ngstr6ms apart. Stacking faults have been observed to annihilate each other in the first 1000 ~, of ZnSe growth, leading to fewer defects present at the surface [20]. However, the annihilation of such stacking faults must be accompanied by relaxation of strain and the accompanying formation of misfit dislocations. Relaxation does not occur with Fe304, for the thicknesses that we have used. Comparison of our results to those of Tarrach, et al. [12], for the (100) surface of bulk Fe304, show closest agreement for what they refer to as phase C, a surface that was prepared also by a sputter/annealing treatment. Fig. 3. is comparable to Fig. 8 from Ref. [12], showing islands with similar width, step height and step edge orientations. The clusters of atoms in the individual unit cells (Fig. 5a) may be the same as those in Fig. 9 of Ref. [12]. However, the features appear more circular and diffuse in Ref. [12]. We have seen no features comparable to phases A and B from Ref. [ 12]. Also, no FeO-like surface was reported in Ref. [12], which m a y have been present, but unobservable because of the lower resolution in their images. Alternatively, this surface m a y be unique to heteroepitaxial films. The presence of stacking faults appears to be unique to the heteroepitaxial films, as expected from the origin of the faults discussed above.
4. Conclusions STM imaging of an MBE-grown (100) Fe304 surface reveals flat terraces that are hundreds of gmgstr6ms broad, separated by steps that are integral multiples of the distance between planes of oxygen (or equivalent iron) atoms. Two surface reconstructions are present. A spinel p(1 x 1) surface reconstruction covers most of the surface. In this reconstruction, a cluster of atoms is seen in each unit cell. The clusters are elongated in a [ 110] direction, causing the c(1 x l) unreconstructed spinel surface to reconstruct in a p(1 x 1) surface. The second reconstruction is a c(1 x 1) unreconstructed FeO-type surface, which forms on top
93
of the spinel epilayer. This surface satisfies electron counting requirements. Regions containing stacking fault defects were imaged. These defects are thought to originate from the coalescence of spinel islands that nucleated with incompatible orientations on the highersymmetry rocksalt substrate and later grew together. The width of single domain regions can be at least a few hundred ~ngstr6ms. Thus, the surface mobility at the beginning of growth is inferred to be high enough to allow iron atoms to diffuse by at least the island width, before bonding to the surface.
Acknowledgements The research of P.J.H. Bloemen has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences. The authors gratefully acknowledge discussions with E.J. van Loenen and N. Kramer.
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McClung (International Center for Diffraction Data, Swarthmore, PA, 1990). V.A.M. Brabers, private communication (Cl1=2.70, C12 = 1.50 GPa). M.D. Pashley, K.W. Haberern and R.M. Feenstra, J. Vac. Sci. Technol. B 10 (1992) 1874. M.D. Pashley, Phys. Rev. B 40 (1989) 10481. S.S.P. Parkin, R. Sigsbee, R. Felici and G.P. Felcher, Appl. Phys. Lett. 48 (1986) 604. J. Petruzzello, B. Greenberg, D. Cammack and R. Dalby, J. Appl. Phys. 63 (1988) 2299.