Epitaxial iron silicides on Si(001): an investigation with scanning tunneling microscopy and spectroscopy

Surface ScienceLetters 284 (1993) L375-L383 North-Holland

surface science letters

Surface Science Letters

Epitaxial iron silicides on Si(0Ol): an investigation with scanning tunneling microscopy and spectroscopy Werner Raunau, Horst Niehus and George Comsa Institut fiir Grenzfliichenforschung und Vakuumphysik, Forschungzentrum Jiilich, Postfach 1913, D-5160 Jiilich, Germany

Received 2 November 1992; accepted for publication 8 December 1992

Iron silicide films have been grown epitaxiallyon Si(001) by solid phase epitaxy (SPE) in UHV. The grown silicide films have been investigated in geometric and electronic structure with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). After deposition of small amounts of iron in the submonolayerrange and annealing at 750 K iron silicide islands are formed which may be attributed to FeSi2 with CaF2 structure (y-FeSi2). Further annealing at 900 K leads to the formation of orthorhombic/3-FeSi2 with its (100) plane parallel to the substrate, fl-FeSi2(100)has been grown for initial coverages up to about 2 ML. STS shows that /3-FeSi2 films are semiconductingwith a gap width Eg ~ 0.90 eV. Increasing the initial Fe coverage to 3 ML and annealing at 900 K changes the epitaxial relationship: not/3-FeSi2(100)but /3-FeSi2(001)is grown on the Si(001) substrate.

Among the iron silicides, the epitaxial growth of FeSi 2 has attracted much attention. In the bulk F e - S i phase diagram FeSi 2 is present in two different crystallographic phases: a-FeSi 2 (stable above 1200 K) and /3-FeSi 2 (stable below 1200 K). a-FeSi 2 displays a tetragonal structure and metallic conductivity [1,2] while/3-FeSi2 shows an orthorhombic structure and semiconducting conductivity. Dusausoy et al. [3] pointed out that in the orthorhombic unit cell two different types of silicon and iron sites, type I and type II, are present according to their different chemical surrounding. The /3-FeSi 2 primitive unit cell contains 16 Fe atoms (8 Fe(I), 8 Fe(II)) and 32 Si atoms (16 Si(I), 16 Si(II)). A band gap of Eg = 0.89 eV [4] has been inferred from optical measurements. Much effort has been focused on the achievement of epitaxial growth of fl-FeSi 2 on Si(111) [5-10]. However, despite of the fact that Si(001) bears more technical relevance compared to Si(111) only a few investigations have been conducted on the epitaxial growth of iron silicides on Si(001) substrates. Figs. la and lb show two possible matching planes for/3-FeSi2/Si(001) epitaxy. Fig. la schematically shows the epitaxial

relationships for /3-FeSi2(100) epitaxy: /3FeSi2(100) IISi(001) and /3-FeSi2[010]llSi(ll0). The /3-FeSi2(100) face is almost quadratic (7.79 A x 7.83 A) and fits well with the Si(001) substrate. The lattice mismatch amounts to + 1.4% and + 2.0%. Fig. lb shows the lattice matching for /3-FeSi2(001) epitaxy: /3-FeSi2(001)1[ Si(001) and/3-FeSi2[010] [1Si(ll0). The fl-FeSi2(.001) unit mesh is rectangular with dimensions 7.79 A x 9.86 .~ and a lattice mismatch with the Si(001) substrate of + 1.4% and + 2.7% for two silicide unit cells (cf. fig. lb). Similar relations are obtained for the /3-FeSi2(010) face which exhibits almost identical dimensions of the surface unit cell (7.83 ,~ x 9.86 ,~). As far as the lattice match is concerned /3-FeSi2(100) epitaxy is supposed to be favored. Kennou et al. [11] investigated the formation of /3-FeSi 2 by solid phase epitaxy on Si(001) and stepped Si(001) substrates with LEED. On Si(001) substrates no epitaxial growth of iron silicide has been observed in contrast to stepped Si(001) substrates where /3-FeSi 2 could be grown epitaxially, showing a p(2 x 2) L E E D pattern. Later, Geib et al. [12] succeeded in growing epitaxial /3-FeSi 2 films also on Si(001) sub-

0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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IK Raunau ~'I ~l/ / t~'pitaxial iron ~ilicides on %i(001)

strates using different techniques as molecular beam epitaxy (MBE), reactive deposition epitaxy (RDE) and solid phase epitaxy (SPE). R H E E D and TEM measurements have shown that/3-FeSi 2 grows on Si(001) matching with its (100) plane parallel to the substrate showing two types of azimuthal orientations in the epitaxial relationship. T h e epitaxial relationships arc: /3FeSi2(100)llSi(001) with the azimuthal orientations /3-FeSi2[010]llSi(ll0> (type A) or /3FeSi[010]HSi(100> (type B). Type A and type B epitaxy depend on the substrate temperature during growth: while type A epitaxy dominates for temperatures between 600 and 850 K, films with type B epitaxy have been found to occur at either lower or higher temperatures. In this Letter we report the results of an investigation on the epitaxial growth and the electronic and structural properties of iron silicide on Si(001). For surface characterization we use a combination of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in order to investigate topographic and electronic

a.)

structure in atomic resolution. The experiments were carried out in an UHV chamber equipped with AES, STM as well as facilities for sample heating and iron evaporation. The STM is of the beetle type [13] and has been modified for spectroscopic operation. All topographical images shown below are acquired in the constant current mode. In the spectroscopic mode, during scanning the feedback loop is periodically opened, a fast voltage ramp ( ~ 5 ms) is applied to the sample and the resulting tunneling current is measured. In order to enhance the tunneling current for small values of the sample voltage, the tip is linearly moved towards the sample as the magnitude of the voltage is reduced. This technique is similar to that proposed by Feenstra et al. [14] and will be described in detail elsewhere [15]. During scanning the tip across the surface, l / V - c u r v e s have been periodically acquired. Current images are obtained by plotting the current values of the l / V - c u r v e s acquired at different (x, y) location of the scanned area, for fixed bias voltages in x and y.

p-FeSi2(O01)

b.)

p-FeSi2(100)

7.79A

7.79A

I

Si[01 O]

1

•

Q J, Si[1001

•

•

si[olo]

l

"

• Si[1001

Fig. 1. (a) Schematical drawing of/3-FeS!2(100)epitaxy. The (100) face of the orthorhombic unit cell exhibits an almost quadratic size. Its dimensions are 7.79 and 7.83 A. The black points represent the silicon atoms of the silicon substrate. The epitaxial relationships are:/3-FeSi2(100)IISi(001) and/3-FeS!21010]IISi<110). (b) Schematicaldrawing of/3-FeSi2(001)epitaxy.The rectangular /3-FeSiz(001) unit mesh area amounts to 7.79 A×9.86 A. The epitaxial relationships are: /3-FeSi2(001)PlSi(001)and /3-FeSi21010]IISi(110).

W. Raunau et aL / Epitaxial iron sih'cides on Si(O01)

The samples have been cut from Si(001) wafers (n-doped, 30-60 f l . cm), rinsed in acetone and ethanol and cleaned in UHV by brief annealing up to 1500 K. After such a heat treatment the STM images exhibit the well-known (2 x 1) reconstruction. Iron has been deposited on the Si(001)-(2 x 1) surface by sublimation of a resistively heated high purity iron wire. The iron coverage was determined in situ by AES. In order to form the silicide, the samples have been annealed for about 20 min at the indicated temperatures. After cooling to room temperature the samples were transferred in situ onto the STM. All STM and STS measurements shown below have been performed with samples at room temperature. In fig. 2a STM image is shown that has been obtained after deposition of 0.5 ML Fe and subsequent annealing at 750 K. No continuous silicide film is formed. The image shows numerous silicide islands with sizes of about 150 A. The edges of the silicide islands run mainly in Si(ll0> directions. The STM image shown in fig. 3a gives a close view on the silicide island (B, C, C') and of its surroundings. In order to enhance the contrast, the image shown has been acquired in the derivative mode. Height measurements have been

L377

[nA] I0

T-FeSi2(O01 )

i

-5

-Ic

-0.3

-0.2

-0.1

0

, 0.1

, 0.2

(b) 0.3

IV]

Fig. 3. (a) STM image zoomed on one of the silicide islands shown in fig. 2. Scan size 265 /~x265 .~, Usarnple = +1.2 V. Si(001) substrate (A) and iron silicide terraces (B, C, C') are imaged in atomic resolution. Area B shows zigzag rows that are running in Si~10] direction. The silicide terraces C and C' exhibit the same surface structure. (b) I/V-curve measured on top of region C. The I./V-curve has been averaged over an area of about 20 , ~ x 2 0 A. In region B similar I/V-cures can be acquired.

Fig. 2. STM image of iron silicide islands epitaxially grown on Si(001) after deposition of 0.5 ML Fe and subsequent annealing at 750 K. Scan size 2140 ,~x 2140 .~, Usamole= + 1.2 V.

performed in the corresponding non-derivative grey scale images that have always been acquired but are not shown here. The Si(001)-(2 x 1) substrate and the silicide islands show atomic resolution. The silicon substrate exhibits many monatomic steps with 1.4 ~k step height. The inhomo-

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W. Raunau et al. / Epitaxial iron silicides on Si(O0 l )

geneities of the substrate suggests that a substantial silicon diffusion has been necessary for silicide formation. The height of the silicide island amounts to 5.4 ,~ measured from the deepest Si(001)-(2 × 1) substrate layer visible in the STM image (A) to the top of the silicide island (C). Region B shows three zigzag rows running in Si[ll0] direction. The rows are separated by 7.8 which is also the periodicity along the rows. Re~ion Cis separated from region B by a step of 1.4 A in step height. This corresponds to a monatomic step on the Si(001) substrate. The distance between the straight rows visible in region C equals the distance of the zigzag rows in region B (7.8 ,~). Along the rows no simple periodicity can be recognized in region C. The two terraces C and C' which display the same surface structure are separated by a step of 2.7 A. In order to investigate whether semiconducting /3-FeSi 2 has been formed we performed STS measurements. Fig. 3b shows an I/V-curve aver-

Fig. 4. STM image of fl-FeSi 2 on Si(001) obtained after deposition of 0.5 ML Fe on the Si(001) substrate and annealing at 900 K. Scan size 540 ,~ x 540 ,~, t/sample 1.2 V. The image has been acquired in the derivative mode for contrast enhancement. From the corresponding non-derivative grey scale image (not shown here) it can be seen that the height of the Si(001) terrace A and the silicide terrace D are at the same level. In the close vicinity of the #-FeSi 2 island there are three deeper Si(001) terraces. =

- -

aged over an area of about 20 ,~ × 20 .A in region C, clearly showing the absence of any band gap. Similar 1~V-curves are acquired in region B. We infer that no semiconducting fl-FeSi 2 has been formed yet. Such a silicide structure could be obtained for initial iron coverages up to 1 ML and annealing temperatures below 900 K. Based on our STM measurements of this metallic silicide phase, we may speculate that metallic FeSi 2 with cubic CaF 2 structure, i.e., 7-FeSi 2, has been formed. It is known that 7-FeSi2(lll) might be stabilized on Si(lll) for thin film thickness and low annealing temperatures although y-FeSi 2 is not present in the Fe/Si bulk phase diagram [10,16,17].o7-FeSi 2 exhibits a lattice constant of a 0 = 5.39 A [16,18]. Thus, assuming y-FeSi2(001) epitaxy on Si(001), the distance between equivalent atomic planes results in ao/2 ~-2.7 A. In fact, this is the step height that is measured between equivalent terraces C and C' shown in the STM image in fig. 3a. We note also that recently Alvarez et al. [19] reported on UPS spectra, obtained after depositing 2.5 ML of Fe on Si(001) and annealing at 720 K, which they suggested to be attributed to FeSi 2 with fluorite structure. This might be an additional hint that y-FeSi 2 cannot only be grown on Si(111) but also on Si(001) substrates. The STM image shown in fig. 4 has been obtained after 0.5 ML Fe deposition on the Si(001) substrate but annealing at 900 K. The STM image shows a silicide island D on the Si(001) substrate. Similar as in the STM image shown in fig. 3a this STM image has been acquired in a derivative modus in order to enhance the contrast. By evaluating the corresponding grey scale image (not shown) it can be seen that the silicide terrace D and Si(001) terrace A display the same height. Above, we already noted a substantial silicon diffusion leading to inhomogeneities of the Si(001) substrate. The amount of lower lying regions in the vicinity of the silicide island D suggests that during the silicide growth silicon was supplied not only by diffusion from below but also from the sides. Fig. 5a shows the atomic structure on top of island D. The surface unit cell is indicated by the white square. Its dimensions amount to 7.8 ,~ ×

W. Raunau et al. / Epitaxial iron silicides on Si(O01)

7.8 .~,. Edges of the unit cell are parallel to Si(ll0) directions. The pattern can be described by a c(2 x 2) superstructure. Epitaxial growth of this type of silicide films with the c(2 × 2) superstructure have been obtained in our experiments for coverages up to 2 ML Fe. In order to investigate the electronic properties of the silicide we performed scanning tunneling spectroscopy (STS) measurements. The l/V-curve shown in fig. 5b as a solid line has been averaged over an area of the unit cell in order to average out local effects within the unit cell. By extrapolating the tunneling current to smaller values of the tunneling voltage, a band gap Eg = 0.90 eV can be determined which is indicative for semiconducting /3FeSi 2. The dashed curve shown in fig. 5b shows atomary resolved spectroscopic data measured at a bright spot within the unit cell shown in the STM image in fig. 5a. In contrast to the average curve, this I/V-curve exhibits a shoulder at ~ - 0 . 8 eV. The location of the shoulder is in good agreement with density of states calculations of Christensen [18] and XPX/UPS studies by A1varez et al. [19]. LEED studies of the iron silicide formation on (stepped) Si(001) [11] show a p(2 × 2) pattern for epitaxial/3-FeSi 2 which is in contrast to the c(2 × 2) superstructure as seen in the STM topograph in fig. 5a. In order to clarify the situation we take advantage of the spectroscopic current images. Figs. 6a and 6b show current images for + 0.9 V (unoccupied states) and - 1.0 V (occupied states), respectively. As they result from plotting the current values of the l/V-curves measured at different (x, y) locations of the scanned area for fixed bias voltages in x and y, they show exactly the same area of the silicide surface without any spatial drift. The current image at positive sample bias (unoccupied states) in fig. 6a shows a quadratic unit cell as indicated by the square with dimensions of 7.8 ,~ × 7.8 .~,. There are relative bright spots (high tunneling current) at each corner of the unit cell and no spot in the center. Thus, the structure visible in fig. 6a can be described by a p(2 × 2) structure. This is, in fact, compatible with the LEED observations by Kennou et al. [11]. In addition we can deduce further structural information: at negative sample bias

L379

(occupied states) the current image in fig. 6b shows an extra spot in the center of the unit cell (square with the solid line) and thus, a c(2 x 2) structure. Such a c(2 x 2) symmetry is also observed in the topographic STM image shown in fig. 5a. Comparing the two current images, we

[nA]

4

(b)

~-FeSi2(100)

__/

2

°/

Eg ~

-2

-4

I -1.0

-0.5

I

0

0.5

] .0

Fig. 5. (a) Close-up of the silicide island D shown in fig. 3b. Scan size 88 .~ × 88 ,~, usample = + 1.2 V. The quadratic unit cell is schematically indicated. Its size is 7.8 .~ x 7.8 ,~. Edges of the unit cell are parallel to the S i ( l l 0 ) directions. (b) I/V-curves measured on top of the silicide surface shown in fig. (a). The solid curve has been averaged over a unit cell. The band gap Eg = 0.90 eV has been determined by extrapolating the tunneling current to smaller voltages. The dotted curve has been measured above one of the bright spots inside of the unit cell shown in fig. 4a. There is a shoulder at - 0 . 8 eV below Fermi level (0 V-= EF). '

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W. Raunau el al. / Epitaxial iron silicides on Si(O01)

Fig. 6. (a) Current image at + 0.9 V (empty states). Scan size 25 ,~ x 25 ,~. The unit cell is indicated by a white square (7.8 A ,~ 7.8 .~). At the corners of the unit cell there is a maximum in the tunneling current (relative bright spots). (b) Current image at - 1.0 V (filled states) that has been acquired simultaneously with the current image shown in (a), showing the same surface area. In the unit cell marked by the square with solid line, in addition to spots at the corners, an extra spot shows up in the center of the unit cell. The position of the square in (a) is marked by the dashed line in order to illustrate a lateral shift (arrow) in the maxima of the tunneling current.

also note a lateral shift in the squares drawn with the solid lines, indicating that occupied and unoccupied states are not located at the same (x, y) position inside of the surface unit cell. In order to illustrate this shift, the position of the unit cell indicated in the current image shown in fig. 6a is marked by the dashed lines. The arrow indicates the direction and the magnitude of the displacement, about 2.8 A in Si[010] direction. Next, we will discuss the topographic and spectroscopic features presented above at the model of bulk truncated fl-FeSi2(100). As already mentioned above,/3-FeSi2(100) contains two different types of silicon (Si(I), Si(II)) and iron (Fe(I), Fe(II)) sites, according to their different chemical surrounding. Ab initio calculations for fl-FeSi2 of Christensen [18] have shown that the maximum of the density of states at about -0.8 eV below Fermi energy mainly arises from Fe(I)-d states. Thus, from the observed shoulder at -0.8 eV in the I/V-curve shown in fig. 5b (dotted curve) we may conclude that the /3-FeSi2(100) surface is probably terminated with Fe(I) atoms. Indeed, it is possible to truncate bulk/3-FeSi2(100) particu-

larly with Fe(I) atoms in the topmost layer which is shown in the model in fig. 7. The epitaxial relationships are: /3-FeSi2(100)llSi(100) and /3FeSi2[010]llSi(110). In a distance of 0.85 A nor~-FeSi2(100)

~-FeSi2[O01]

~._ ~-FeSi2[01 O]

7.79 A

Fig. 7. Model of bulk truncated fl-FeSi2(i00). Only three layers have been plotted, The bulk has been truncated with iron atoms of type I (Fe(I)) in the topmost layer. In a distance of 0.85 .~ normal to the surface the second and third layer form a double layer of Si(I) and SKID with double layer spacing < 0.01 A. The surface unit cell with dimensions 7.79 ,~ x 7.83 ,~ is indicated.

W. Raunau et aL / Epitaxial iron silicides on Si(O01)

mal to the surface the second and third layer form a double layer (double layer spacing < 0.01 .~) which contains a mixture of Si(I) and Si(II) atoms. As is readily seen, with such a termination the iron atoms of the topmost layer form a c(2 x 2) arrangement. The four next (2.34 A) silicon neighbor atoms of each iron surface atom form a rectangular arrangement in the deeper double layer. This rectangular arrangement belonging to the center Fe-atom is rotated by 90° referring to the silicon neighbors of the corner Fe-atoms (fig. 7). Thus, the deeper double layer gives rise to a p(2 × 2) symmetry. The current images shown in fig. 6 are supposed to reflect the electronic influence of the deeper layers. From the current images a displacement of the tunneling current maxima for occupied and unoccupied states could be inferred. Taking advantage of the crystallographic axes shown in fig. 7 we find that the displacement occurs in/3-FeSi2[011] direction with about 2.8 ,~. This value equals about a quarter of the length of the diagonal in the (almost quadratic) /3FeSi2(100) unit cell. We note as a reminder that with a method showing the topmost layer only (STM) we would expect to see the c(2 × 2) symmetry, while with a depth averaging method (LEED) the p(2 × 2) structure should appear in the measured data. In order to produce thicker/3-FeSi 2 films the initial iron coverage has been increased up to 3 ML. After annealing the sample at 900 K, the surface exhibits a typical surface structure as shown in fig. 8a. Most prominent features on such a silicide terrace are the zigzag lines that are running in Si[ll0] direction. The areas between the zigzag lines display the same height with respect to each other and there is a phase shift between areas which are separated by one of the zigzag lines. In order to illustrate this phase shift a white line has been plotted into the STM image. Thus, the zigzag lines at the silicide surface represent anti-phase domain boundaries. A surface unit cell is indicated by a white rectangle with dimensions of 7.8 A x 10.0 .~. The edges of the unit cell are parallel to the Si directions. In fact, this surface structure is completely different from the c(2 x 2) superstructure which has been observed in the STM images (fig. 5a) in

L381

4

/

[nA]

20

13_FeSi2(O0i )

I0

0

-I0 =-'

-20 -I .o

i -.0.5

I

0

0,5

1.0

(v)

Fig. 8. (a) STM image of/3-FeSi 2 epitaxially grown on Si(001) after deposition of 3 ML Fe and annealing at 900 K. Scan size 177 ,~x 177 A, Usampte= - 1.5 V. Most striking features are the zigzag lines which are running in Si[110] direction. The white line perpendicular to the zigzags shows that there is a phase shift between adjacent areas that are separated by one of the zigzag lines (anti-phase domain boundaries). The unit cell of the area between the anti-phase domain boundaries is schematically indicated. Its size amounts to 7.8 ,~, x 10.0 ,~. (b) I/V-curve measured on top of the silicide surface shown in (a). The solid curve has been averaged over a unit cell. The dotted curve displays atomic resolved spectroscopic data above one of the bright spots within the surface unit cell shown in the STM image in (a). The drop in the tunneling current on the right results from pulling back the tip as the magnitude of the bias voltage is increased.

the case of smaller initial iron coverage. Surprisingly enough, this difference in structure appears only by variation of the initial iron coverage,

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W. Raunau et a L / Epitaxial iron silicides on Si(O01)

whereas both samples have been annealed at the same temperature (900 K). The electronic behavior of this iron silicide phase has been investigated also by scanning tunneling spectroscopy (fig. 8b). The solid curve results from measuring atomically resolved spectroscopic data and averaging the acquired 1~V-curves over the area of a unit cell. Although this 1~V-curve shows semiconducting behavior the band gap is not as well pronounced as in the I/V-curve shown in fig. 5b. At positive voltages (empty states) a broad peak is observed which seems to be composed of two peaks located at about +0.4 eV and slightly above + 0.5 eV. In order to emphasize the different behavior from the 1~V-curve shown in fig. 5b, we additionally show an l/V-curve measured above one of the bright spots in the indicated unit cell shown in fig. 8a (dotted curve). This curve displays a strong peak at + 0.4 eV which is contained in the double peak of the averaged I/Vcurve (solid curve). In addition to features visible in the average curve a small peak at -0.5 eV below Fermi energy can be observed. These peaks are probably due to surface states that are located in the band gap region. Similar surface states are not present in the case of/3-FeSi2(100) (fig. 5b). In particular we note that no shoulder at - 0 . 8 eV occurs in fig. 8b in contrast to /3FeSi2(100) epitaxy. Next, the epitaxial relationships of such /3FeSi 2 films on Si(001) have been determined. The rectangular unit mesh (7.8 A × 10.0 ~,) shown in fig. 8a does not fit with the quadratic unit mesh of /3-FeSi2(100) epitaxy (7.8 A X 7.8 .&). However, there is a good agreement in the unit cell dimensions considering the /3-FeSi2(001) (or /3-FeSi2(010)) face. Fig. lb shows a schematical drawing of /3-FeSi2(001) (or /3-FeSi2(010)) epitaxy on Si(001). The unit cell exhibits a size of 7.79 A × 9.86 ~, (7.83 A x 9.86 ,&). The lattice mismatch amounts to 1.4% (+2.0%) in Si[ll0] direction and + 2.7% in Si[ll0] direction. Assuming the/3-FeSi2(001) epitaxy, the atomic structure that is observed in the STM image shown in fig. 8a can be explained in the following way. As no shoulder at - 0 . 8 eV (Fe(I)-d states) has been observed in fig. 8b, we infer that the surface is not terminated with Fe(I) atoms. In fig. 9 we

~-FeSi2(001 )

~-FeSi2[01O]

9,86 A ~-FeSi2[100]

Fig. 9. Model of bulk truncated #-FeSi2(001). The topmost layer consists of Fe(II) atoms. The lower lying second and third layer contain Si(II) atoms. Their distance from the topmost iron layer amounts to 0.32 and 0.69 A, respectively. The /3-FeSi2(001) surface unit cell is indicated by the black rectangle.

show a model of bulk truncated fl-FeSi2(001). Three layers are plotted. Different from the Fe(I) termination proposed in the model for /3FeSi2(100) shown in fig. 7 the uppermost layer consists of Fe(II) atoms in this model for (001) epitaxy. The second and third layer consist of Si(II) atoms. Their distance from the topmost layer amounts to 0.32 and 0.69 A, respectively, The surface unit cell with dimensions 7.79 A × 9.86 ,& is indicated by a rectangle. As is clearly visible in the model the topmost layer exhibits an atomic arrangement that is observed in the STM image shown in fig. 8a. As mentioned above, /3-FeSi2(001) and /3-FeSi2(010) show very similar dimensions of their surface unit cells, We have also examined /3-FeSi2(010) crystallographic planes but different from /3-FeSi2(001), a centered unit cell similar as shown in fig. 9 does not occur. Thus, we suggest for the epitaxial relationship: /3-FeSi2(001) II Si(001) and /3-FeSi2[0t0] IISi (110). The amount of anti-phase domain boundaries present at the silicide surface is probably a consequence of the larger lattice mismatch of /3-FeSi2(001) compared to fl-FeSi2(100). Due to this lattice mismatch there is a tensile stress when /3-FeSi2(001) grows on Si(001) which is supposed to be relieved by anti-phase domain boundaries. Because of the lower twofold symmetry of /3FeSi2(001) in contrast to the fourfold symmetry o

W. Raunau et al. / Epitaxial iron silicides on Si(O01)

of the Si(001) substrate, there are two equivalent orientations of/3-FeSi2(001)/Si(001) epitaxy, azimuthally rotated by 90 °, to be expected. In fact, investigation of larger areas shows terraces with anti-phase domain boundaries which are rotated by 90 ° indicating an azimuthal rotation by 90 °. In summary, we reported on the epitaxial growth of thin silicide films on Si(001). After iron deposition in the submonolayer range silicide islands are formed that are attributed to iron disilicide with a CaF 2 structure. Higher annealing at 900 K leads to fl-FeSi 2 formation. For the growth of fl-FeSi 2 on Si(001) two different epitaxial relationships have been observed, dependent on the initial iron coverage. For initial Fe coverages below 2 ML, fl-FeSi 2 shows the epitaxial relationship: /3-FeSi2(100)llSi(001)and /3-FeSi2[010] 11Si(110). For coverages above 3 ML Fe we suggest the different epitaxial relationship: fl-FeSi 2(001) IISi(001) and fl-FeSi2[010] IIS i ( l l 0 ) .

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