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Scanning tunneling microscopy and spectroscopy of iron suicide epitaxially grown on Si(111)
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surface science
Surface Science 286 (1993) 203-211 No~b-Holland
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Scanning tunneling microscopy and spectroscopy of iron silicide epitaxially grown on Si( 111) Werner Raunau I, Horst Niehus, Thomas Schilling and George Comsa
Received 2 September 1992; accepted for publication 6 January 1993
Epitaxial iron silicide films have been grown on Si(lll) by solid phase epitaxy (SPE) in UHV. Structural and electronic properties have been inyestigated with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). For initial Fe deposition up to 3 A and annealing at 850 K, metallic y-FeSi, is formed. These films exhibit a perfect (2 x 2) superstructure, which is attributed to y-FeSi (111) with Si termination. SPE at higher initial iron deposition (15 .k> and annealing at 800 K results in e-FeSi showing a (6 X / 3 ) R30” superstructure. Subsequent annealing above 900 K leads to p-FeSi, formation. As determined by STS, &FeSi, films are semiconducting with ES = 0.85 eV. STM topographs show that SPE produces rough silicide surfaces with j?-FeSL#Ol) [and not fi-FeSi,(llO)] epitaxy. The atomic structure on @-FeSi, terraces is complex, consisting of many anti-phase domain boundaries and defects.
1. Introduction
During the last years there has been a considerable interest in the epitaxy of metallic and semiconducting silicides due to their possible applications in silicon technology. Among the iron silicides, FeSi, is of particular interest. According to the equilibrium Fe-Si phase diagram [l] there exist two different phases of iron disilicide: high temperature cu-FeSi, (tetragonal structure which is stable at high temperatures T > 1250 K) and &FeSi, (orthorhombic structure, stable at temperatures below 1250 K), respectively. Both phases a- as well as p-FeSi, result from distortions of the CaF,-structure. While tetragonal (YFeSi, is metallic, orthorhombic &FeSi, is semiconducting with a band gap of about 0.8 eV-0.9 eV [2-51. Its corresponding wavelength of - 1.5 pm suggests potential optical applications in glassfiber optics and might open new perspectives
* Present address: Department California, CA 92717, USA. 0039-6028/93/$06.00
of Chemistry, University of
of heterojunctions on Si, provided P-FeSi, can be grown epitaxially on Si single crystals [61. Thus, recently great effort was put on the study of growth and epitaxial relationship of /3-FeSi, on Si(ll1). Studies including diffraction methods like LEED and RHEED [7-101 showed that a complex (2 X 2) superstructure [(2 X 2) plus additional spots [ll]] as related to the Sit11 11 substrate is observed for /3-FeSi,. Recently a new, metastable FeSi, phase, yFeSi, was discovered to join the FeSi, family [I2]. Whereas Q- and P-FeSi, can be derived from the CaF, structure by distortions, y-FeSi, was proposed to have the undistorted CaF, structure, like CoSi, or Nisi,. This is surprising because cubic FeSi, does not exist in the bulk phase diagram for the Fe-Si system 111.Calculations of Christensen [2] showed that a cubic CaF, structure is not expected to be stable in the case of bulk FeSi, due to an increase of the density of states (DOS) at the Fermi energy (E,). This instability can be removed by distortions which lead to an orthorhombic unit cell and the appearance of a bandgap for /?-FeSi,. For very thin
0 1993 - Elsevier Science Publishers B.V. All rights reserved
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B! Raunau et al. / FeSi, ep~t~~~~ly grown on Sic1II)
films however, the situation may be different because of the influence of the substrate, so that cubic y-FeSi, might be stabilized on Si(lll) substrates. Similar examples of the formation of metastable structures are the growth of cr-Sn on InSb or CdTe 1131 and bee Co on GaAs [14]. Chevrier et al. [S] reported on the formation of a strained cubic phase by SPE after deposition of 2 A of iron on Si(ll1) followed by annealing at 560 K. RIIEED patterns of this thin film showed a perfect (2 x 2) periodici~ in contrast to the complex (2 x 2) RHEED pattern that is observed in the case of &FeSi,. This complex (2 x 2) pattern can be explained by the superposition of three domains of j3-FeSi,. y-FeSi, seems to be not the only metastable iron silicide phase on Si(ll1). Von Kane1 et al, 1151reported on the synthesis by molecular beam epitaxy (MBE) of FeSi, +X (0 IX I 1) with a metastable CsCl structure. Thin FeSi films ( < 20 A> were shown to evolve upon annealing below 770 K towards FeSi, with no change of the CsCl symmetry by introducing Fe vacancies in a random fashion. These FeSi,,, (0 _
2. Experimental The experiments were performed 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 Cl71 and has been modified for spectroscopical operation. Topo-
graphical images are acquired in the constant current mode. In order to obtain spectroscopical information we used a STS method, which is similar as proposed by Feenstra et al. [lS] and will be described in detail elsewhere [19]. Basically STS operates as follows: in the first step the tip is positioned in the constant current mode, then the feedback loop is opened and a voltage ramp is applied to the sample. There is roughly an exponential dependence of the tunneling probability on the tunneling voltage [18]. Thus, the tunneling current will be attenuated exponentially as the tunneling voltage approaches the zero value [201. In order to increase the sensitivity for tunneling voltages close to zero one has to amplify the tunneling current. Instead of amplifying the tunneling current electronically, which also amplifies the noise, we take advantage of the exponential dependence of the tunneling current on the tip-to-sample distance. Decreasing the tipto-sample distance linearly while the tunneling voltage ramp is applied to the sample, approximately compensates for the exponential voitage dependence of the tunneling current. The variation of the tip to sample distance is experimentally achieved by adding a V-shaped voltage ramp on the electrode of the z-piezo element of the STM so that the tip to sample distance reaches its minimum when the sample voltage goes through zero. Scanning the tip across the surface and acquiring at each (x, y) point an I/V curve gives us both, the atomically resolved topographical and spectroscopical information. The samples were cut from Si(ll1) wafers (ndoped, 30-60 Q *cm), rinsed in ethanol and cleaned in UHV by brief annealing up to 1500 K. After this pure heat treatment in UHV the STM topographs of the Si(l11) surface exhibits the well-known (7 x 7) reconstruction. Iron was evaporated by direct sublimation from a high purity iron wire heated resistively. After in situ Auger spectroscopy determination of the Fe coverage, the samples were heated for about 20 min in order to form the silicide. The samples were allowed to cool for 30 min in order to avoid thermal drift in the STM investigation. All STM measurements shown below were performed with samples at room temperature.
W. Raunau et al. / FeSi, epitaxially grown on Si(ll1)
205
3. Results and discussion Small Fe deposition in the submonolayer range and subsequent annealing at 700 K results typically in a surface as shown in fig. 1. Basically three different regions can be distinguished. (i) Patches of bare Si substrate (A, A’). Besides some areas with the normal Si(ll1) (7 x 7) reconstruction (A) also islands of Si(lll) (5 x 5) reconstruction (A’) can be found. We note, that in case of clean Si(ll1) surfaces the (5 x 5) reconstruction is found by Feenstra and Lutz [21] at cleaved Si(ll1) surfaces after annealing at 600 K. It seems to be a transition stage on the way to the more stable (7 X 7) superstructure which is obtained upon higher annealing temperatures (> 900 K). For lower annealing temperatures the (5 x 5) is supposed to be favored above the (7 x 7) reconstruction because the (5 x 5) structure exhibits the same atom density as the ideal (1 x 1) surface, whereas the (7 X 7) structure requires 4% more atoms. (ii) Disordered regions B that probably consist of a mixture of still unreacted Fe and Si. (iii) Regions C which show already a (2 x 2) structure of iron silicide. Subsequent annealing at 850 K leads to further silicide formation in re-
Fig. 1. (a) STM topograph measured after evaporating 0.2 ML Fe on Si(lll) and annealing at 700 K. Scan size 535 x535 AZ, u sample =-1.2V,Z=lnA.
Fig. 2. STM topograph showing epitaxial y-FeSi, (left) and Si substrate (right). This image has been measured after 0.2 ML Fe deposition on Si(lll) and annealing at 850 K. Scan size 220 x 220 k, U,,,,,, = - l.W, Z = 1 nA.
gions B. No longer Si(ll1) (5 X 5) but only Si(ll1) (7 x 7) areas can be found, indicating that the Si(l11) (5 X 5) areas have been transformed to the more stable Si(lll) (7 X 7) structure due to the higher annealing temperature. In fig. 2 a STM image is shown where an iron silicide island and adjacent Si(ll1) (7 x 7) substrate can be seen. In order to enhance the contrast, the image has been acquired in the “derivative mode”,in which the DC component of the measured signal is suppressed. Thus, terraces of different height appear at the same gray level in the STM image. The step edges of the iron silicide island are running parallel to the [liOl and [iOl] directions of Si. The iron silicide surface exhibits a hexagonal symmetry. The lattice constant is 7.8 & 0.2 A in contrast to 3.84 w for Si(ll1). Such a (2 x 2) structure is consistent with the (2 X 2) observed on both, y-FeSi, with CaF, structure 18,221 as well as FeSi,,, (0 I x I 1) with a defect CsCl structure [151. For the CsCl structure the distance between equivalent atomic planes is half as large as in the TaF, structure, leading to a step height of 3.1/2 A. Evaluating step heights in our (nonderivative) STM topographs results in multiples
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W. Raunau et al. / FeSi, epitaxially grown on Si(lll)
of 3.1 A which is also the Si(ll1) single-layer step height. We conclude that FeSi, with CaF, structure, i.e. y-FeSi, has been formed, lattice matched with its (111) plane parallel to the substrate. The strained cubic y-FeSi,(lll) surface is expected to exhibit a (1 X 1) superstructure referred to Si(ll1) in contrast to the measured (2 X 2) in the STM topograph in fig. 2. Similarly the (111) surface of metastable FeSi, +X(0 IX I 1) with the defect CsCl structure is also expected to show a (1 x 1) superstructure when grown on Si(lll), in contrast to the STM data from von Kane1 et al. [15]. As known for many metal-silicon compounds, Si has the tendency to segregate and to form a monolayer of pure Si on top of the silicide 123-251. We may speculate that here also Si atoms have, indeed, diffused into the outermost layer. Thus, the (2 X 2) reconstruction on the yFeSi,(lll) surface might be due to a similar effect to that on Si(ll1) (7 x 7) where Si adatoms form a local (2 X 2) arrangement in order to reduce the number of dangling bonds. Because in case of the silicide the resulting strain might not be relieved in the same manner as for clean Si(ll1) (corner holes), probably the former local (2 x 2) structure is now spread over the entire y-FeSi,(lll) surface. It has already been mentioned that cubic strained y-FeSi, is metastable and is expected to be stabilized on Si(ll1) only in the case of thin films. We obtained y-FeSi, films for initial Fe deposition up to about 3 A. Indeed, according to the Fe-Si phase diagram orthorhombic P-FeSi, is the only stable FeSi, phase at room temperature. It has been shown that p-FeSi, can epitaxially be grown on Si(ll1) matching its (101) or (110) planes parallel to the substrate [7-111. Fig. 3 shows schematically the epitaxial relationship for p-FeSi, on Si(ll1). The (110) or (101) planes of p-FeSi, are expected to be good possibilities of lattice matching to the Si(ll1) substrate. In order to produce P-FeSi, a larger amount of iron has been evaporated on the Si(ll1) substrate. A STM topograph of epitaxial p-FeS$ on Si(ll1) which stems from deposition of 15 A Fe on Si(ll1) and subsequent annealing at 950 K is shown in fig. 4a. The surface structure seems to
a.> j3-FeSi2(110)
B_FeSi2(101)
k--“-i
W
I
atiiz] 11g-Fesi[ioi] (atiizl 11g-r+.sii[tio))
Si[ilOI (Si[ilOl
II fi-FeSi&OIO] II f%-Fe.S$[oOll)
Fig. 3. (a) Schematic drawing of P-FeSi,(lOl) and pFeSi,(llO) planes in the orthorhombi$ @-FeSi, bulk unit cell. a = 9.863, b = 7.791, and c = 7.833 A 1261. (b) (101) [(llo)] epitaxy on Si(ll1): three equivalent azimuthal orientations of the rectangular unit cell are shown. its dimensions are 12.59 x 7.79 k (12.57X7.83 A’). The epitaxial relationships are Si[ilOlII /3-FeSi,[OlO] (B-FeSi,[OO11) and Si[ii2lI( &FeSi,[lOil (P-FeSiJliO]) for (101) [(llo)] epitaxy. The lattice mismatch for P-FeSi,(lOl) @FeSiz(llO)) epitaxy is about -5.3% (-5.5%) in Sini direction and + 1.4% (+ 2.0%) in Si[ilO] direction. The circles represent the silicon atoms of the Si(ll1) substrate.
be disordered. Several distinct bright lines are visible at the silicide surface which are running in nlO] direction referring to the Si substrate. As will be shown below, these bright lines represent anti-phase domain boundaries. The existence of domain boundaries is plausible considering the epitaxial relationship. According to fig. 3 the lattice mismatch of /3-FeSi, with the Si(ll1) substrate in Si(ll0) directions is small (< 2.0%),
W. Ruunau et al. f Fe%, e~i~~iu~ygrown on Si(ll1)
which might allow for growth of coincidence lattices over long distances. In the Si( 112) directions the lattice mismatch is large (> 5%). Thus, the resulting strain is relieved by the domain boundaries. As shown in fig. 3 we expect three equivalent domains because of the lower symmetry of the rectangular surface unit cell of p-FeSi,(lOl) or &FeSi,(llO) compared to the threefold symmetry of the Si(ll1) surface. However, on each individual terrace only one of the three possible domains is present. In fact, our STM data shows that on each terrace all anti-phase domain boundaries are parallel to each other. The sizes of the terraces which contain only one of the three possible domains amount to several hundred lngstriims in Si(112) directions and a few thousand angstrSms in Si(ll0) directions. Inspection of larger areas containing more terraces show anti-phase domains rotated by 60” with respect to each other. This reflects the threefold azimuthal s~rnet~ shown in fig. 3. In order to extract the periodic info~ation that is contained in the STM topograph shown in fig. 4a the image has been transformed to Fourier
207
space by means of the two dimensional Fourier transform. Although the Fourier transform shown in fig. 4b contains the same information as the STM topograph in fig. 4a, the periodicity at the surface can readily be seen. An additional feature in fig. 4b are streaks parallel to Si[ii2] which arise from the anti-phase domains separated by the boundaries visible in the STM topograph in fig. 4a. There are no streaks parallel to Si[llOl reflecting the undisturbed periodic@ in the Si[ilO] direction. We found that Fourier transformation of one single domain (area between two adjacent anti-phase domain boundaries) results in pronounced spots without any streaks. Fourier transformation of larger areas containing antiphase domain boundaries leads to spots and additional streaks in Si[ii21 direction as seen in fig. 4b. In view of the fact that there are three equivalent azimuthal orientations for p-FeSi,(lOl) [pFeSi,(llO)] epitaxy, this would result in a compIex (2 X 2) superstructure, observable by diffraction methods. As mentioned above, LEED (RHEED) studies of the Fe/Siflll) system [7-111 show, indeed, a complex (2 x 2) pattern after formation of P-FeSi,.
Fig. 4. (a) STM to~graph of P-FeSi, epitaxially grown on Si(fll) after depositing 15 A Fe on Si(ll1) and annealing at 950 K. The indicated directions are referred to the Si(ll1) substrate. Anti-phase domain boundaries of @-FeSi, along the indicated Si[ilO] direction are clearly visible. Scan size 410 x 410 k, Usampte= - 1.5 V, I = 1.5 nA. tb) Two-dimensional Fourier transform of the STM image shown in (a).
208
W. Raunau et al. / FeSi,
Next, we determined the epitaxial relationship between P-FeSi, and Si(ll1). Although it has been demonstrated that /?-FeSi, can epitaxially be grown on Si(ll1) matching with /3-FeSi,(lOl) or &FeSi,(llO) parallel to the substrate [7-111, it is hardly possible to distinguish between pFeSiJllO) and P-FeSiJlOl) epitaxy, due to the very close dimensions of their surface unit cells (cf. fig. 3b). In order to clarify the situation, the surface structure of P-FeSi, shown in fig. 4a has been analysed in more detail. According to the STM topograph in fig. 4a the P-FeSi, surface looks disordered. Transformation of the STM image in Fourier space reveals the hidden periodic information as shown in fig. 4b. In order to enhance the contrast for the periodic structure, the spots marked with a white circle in the image shown in fig. 4b have been selected and back transformed to real space, thus suppressing all the information that is not periodical. The resulting image is shown in fig. 5a. Pronounced zigzag rows are running in the Si[ii2] direction with a lateral period of 12.6 + 0.2 A along Si[?i2] and a
epitaxially grown on Si(lll)
distance between zigzag rows of 7.8 + 0.2 A. A grid with rectangular meshes is superimposed in fig. 5a in order to show the surface unit cell of the silicide. Considering the bulk structure of /3-FeSi, the observed structure can be explained by assuming (101) epitaxy. In bulk P-FeSi, there are slabs of (101) planes that only consist of silicon atoms. Fig. 5b shows a model of bulk truncated p-FeSi,(lOl) with Si termination where for the sake of simplicity only three layers of such a slab are plotted. The first and the second layer form a double layer of siliton with an intra-double layer spacing of 0.133 A. The distance of the third layer (small circles) from the surface amounts to 0.426 A. In fact, the double layer exhibits zigzag rows in P-FeSi,[lO?] direction and the surface unit mesh has the same dimensions as resulting from the structure shown in fig. 5a. This is a clear indication that /3-FeSi, grows in the (101) crystallographic modification and is terminated by a Si double layer. We have also examined /I-FeSi,(llO) crystallographic planes, but in contrast to (101) iron disilicide a similar zigzag
&F&&(101)
g-FeSiJOlOl
I
Fig. 5. (a) Periodic component of the STM image shown in fig. 4a obtained by double Fourier transformation. An enlarged area of 140 x 140 AZ is shown. Zigzag rows are running in the si[ii2] direction. The surface unit cells are shown by a net with rectangular meshes. its dimensions are 7.8 in the Si[llO] direction and 12.6 A in the Si[iiZ] direction. (b) Model of bulk truncated P-FeSi,(lOl). The size of the balls reflects its height normal to the surface. Three layers consisting of Si atoms are shown. The upper Si double layer has an intra-double layer spacing of 0.133 A. The distance of the third layer from the surface amounts to 0.426 A. Zigzag rows are running in the FeSi,[lOi] direction. The unit cell is indicated by a rectangle with dimensions 7.79 X 12.59
W Raunau et al. / FeSi,
arrangement does not occur! We therefore find for the epitaxial relationship of P-FeSi, on Si(ll1): p-FeSi,(lOl) IISi(ll1) and P-FeSi, [OlO]IlSi(llO>. In fact, on the basis of our STM data in combination with the Fourier transformations we could show that using the SPE method only p-FeSi,(lOl) and not /?-FeSi,(llO) is grown on Si(ll1). So far we have shown that for y-FeSi, and P-FeSi, there is a large difference in the atomic surface structure. Apart from these structural differences y- and P-FeSi, are also expected to display different electronic behavior. Cubic yFeSi, exhibits metallic character, while /3-FeSi, is semiconducting [2]. In order to evidence these electronic differences local scanning tunneling spectroscopy (STS) on y-FeSi, and P-FeSi, was also performed. In fig. 6a we show an Z-v curve measured by STS on the y-FeSi, surface. For this purpose the tip has been positioned above the silicide surface shown in fig. 2. We measured atomically resolved spectroscopic data and averaged the data over several unit cells in order to average out local effects. The data show an almost linear behavior with a slope of 30 nA/V between -0.1 and 10.1 V. Comparing this Z/I/ curve with the Z/V curve taken on p-FeSi, clearly demonstrates the absence of any band gap. The obvious metallic behaviour of the Z/V curve shown in fig. 6a therefore corroborates that no semiconducting P-FeSi, has been formed. Performing STS on the P-FeSi, surface shown in fig. 4a reveals a dramatic difference: fig. 6b shows the Z/I/ curve measured on P-FeSi, which has been
epitaxially grown on Si(ll1)
209
averaged over an area of about 50 A X 50 A. In contrast to the Z/V curve shown in fig. 6a a slope < 0.1 nA/V can be detected between -0.1 and +O.l V. In order to determine the size of the band gap from this Z/1/ curve we have linearly extrapolated the tunneling current to smaller tunneling voltages (see fig. 6b). As mentioned above during measurement of the Z/v curves the tip$-sample distance is decreased linearly with 2 A/V as the tunneling voltage approaches the zero value. As a consequence, high spectroscopic sensitivity at the band edges is achieved. From the Z/V curve shown in fig. 6b a band gap of 0.85 eV can be derived by the linear extrapolation of the tunneling current. This value is in good agreement with DOS calculations of Christensen [2] and experimental measurements by Bost et al. [4] and Dimitriadis et al. [5] using photothermal deflection spectroscopy. We also note that the tunneling current is not totally zero inside of the bandgap. Such a tunneling current probably arises from the large number of surface defects which introduce additional states in the bandgap region. Besides y- and P-FeSi,, another iron silicide phase, the l-FeSi monosilicide (cubic B20 structure) [16] can also be grown in an epitaxial phase. We evaporated 15 A of Fe on SXlll) and annealed the sample at 800 K. Similar to the y-FeSi, phase, the silicide shown in fig. 7 is metallic and exhibits a hexagonal structure. A closer look shows, however, substantial differences: the distance between the bright spots on the silicite terraces is no longer 7.8 + 0.2 but 6.7 k 0.2 A. Also the amount of defects has strongly in-
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-1.0
-0.5
0
0.5
1.0
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Fig. 6. Averaged I/V curves measured on (a) y-Fe& and (b) P-FeSi% D uring STS the tip has been stabilized with &,,p,e = - 3.0 V, I = 0.11 nA. The tip-to-sample distance was varied linearly with 2 A/V. In (b) the band gap Es = 0.85 eV has been determined by extrapolation of the tunneling current to smaller voltages.
210
W. Raunau et al. / Fe%, epiiaxially grown on Si(lll)
creased. In addition, the observed structure is rotated by 30” with respect to the perfect (2 x 2) structure of y-FeSi,, Such a structure can be described by a (6 x fi)R30” superstructure (7.8 %6/z = 6.75 A>. I n situ AES measurements confirm the FeSi stoichiometry. This structure is in good agreement with results from Chevrier et al. [S] and Gavriljuk, Kachanova and Lifshits [271. The epitaxial relationships of E-FeSi on SXlll) are: e-F&%(111) ]ISi(lll) and e-FeSi[liO] IISi[ii2]. Subsequent annealing of such an e-FeSi film above 900 K results in the formation of epitaxial &F&i,, fig. 8 shows a survey image of the resuiting surface. In this image the vertical roughness covers about 50 A. Typical terrace widths are of the order of several hundred ingstrijm with many step edges running parahei to the Si[ilO] direction. The STM image in fig, 4a shows a close-up of one of the terraces. The inhomogeneity of the surface shown in fig. 8 is probably due to the solid phase epitaxy. Deposition of pure metal and subsequent annealing (SPE) is often used because of its facility. On the other hand the interatomic diffusion of Si all the way through the
Fig. 8. STM tcrpograph of p-FeSi, epitaxially grown on Si(lll) after depasiting 15 k Fe on SiClll) and annealing at 950 K. Scan size 6400 X 6400 A*, Usample= + 1.8 V, I- 1 nA.
deposited iron layer is necessary for silicide formation up to the outermost layer. This large material transport probably results in many defects and a rough surface. The atomic surface structure on the terraces exhibits anti-phase domain boundaries and a great amount of surface defects as shown in fig. 4a.
4. Summary
Fig. 7. STM topograph of e-FeSi. After evaporation of 15 A Fe the film has been annealed for 20 min at 800 K. The surface shows numerous c-Fe% islands of monatomic step height. Each of the islands exhibits a (6 X fi)R30” superstructure referred to the Si(i 1I) substrate. Scan size 580 X 580
Topographical and spectroscopical features of iron silicide epitaxially grown on Si(lll) by SPE have been reported. After little Fe deposition (< 3 A) and annealing at 850 K cubic yFeSi,(f 11) has been grown epitaxialiy on Sit1 12). By means of STM and STS we showed that the y-FeSi, films exhibit a perfect (2 X 2) superstructure and are metallic. In the case of thicker films (15 &, e-F&X(111) (6 x 6)R30” is formed after annealing at 800 K. Higher annealing above 900 K transforms the E-FeSi film into p-F&i,. By application of the Fourier transform it has been shown, that p-FeSi,(lOl) grows epitaxialiy on Siflll). The local structure on the /?IFeSi,(lOl) terraces is found to consist of many
W. Raunau et al. / FeSi, epitaxially grown on Si(lll)
anti-phase domain boundaries and defects. STS performed on p-FeSi, films shows a bandgap (I& = 0.85 eV). The quality of Fe-silicide films can possibly be improved by using other techniques of silicide growth like reactive deposition epitaxy (RDE) or molecular beam epitaxy (MBE).
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[Ill Ch. Stuhlmann, J. Schmidt and H. Ibach, to be published.
[121N. Onda, J. Heinz, E. Miiller, K.A. Mader and H. von Kane], Appl. Surf. Sci. 56-58 (1992) 421. [131 R.F.C. Farrow, D.S. Robertson, G.M. Williams, A.G. Cullis, G.R. Jones, I.M. Young and P.N.J. Dennis, J. Cryst. Growth 54 (1981) 507. 1141G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051. u51 H. von KInel, K.A. Mlder, E. Miiller, N. Onda and H. Sirringhaus, Phys. Rev. B 45 (1992) 13807. M L. Pauling and A.M. Soldate, Acta Cryst. 1 (1948) 212. I171 K. Besocke, Surf. Sci. 181 (1987) 145. D81 R.M. Feenstra, J. Vat. Sci. Technol. B 7 (1989) 925. [191 W. Raunau, PhD Thesis, KFA Jiilich, Jiilich, 1992. [201We note that for small tunneling voltages the tunneling current might be attenuated into the noise level of the measured tunneling current. Thus, the I/ V curves measured at constant tip-to-sample distance would lead to the false conclusion of a too large band gap. [211R.M. Feenstra and M.A. Lutz, Surf. Sci. 243 (1991) 151. [221A.L. Vazquez de Parga, J. de la Figuera, C. Ocal and R. Miranda, Europhys. Lett. 18 (1992) 595. 1231S.A. Chambers, S.B. Anderson, H.W. Chen and J.H. Weaver, Phys. Rev. B 34 (1986) 913. [241 R. Stalder, H. Sirringhaus, N. Onda and H. von Kanel, Surf. Sci. 258 (1991) 153. [251J. Alvarez, J.J. Hinarejos, E.G. Michel, J.M. Gallego, A.L. Vazquez de Parga, J. de la Figuera, C. Ocal and R. Miranda, Appl. Phys. Lett. 59 (1991) 99. 1261Y. Dusausoy, J. Protas, R. Wandji and B. Roques, Acta Cryst. B 27 (1971) 1209. [271Y.L Gavriljuk, L.Y. Kachanova and V.G. Lifshits, Surf. Sci. Lett. 256 (1991) L589.
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surface science
Surface Science 286 (1993) 203-211 No~b-Holland
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Scanning tunneling microscopy and spectroscopy of iron silicide epitaxially grown on Si( 111) Werner Raunau I, Horst Niehus, Thomas Schilling and George Comsa
Received 2 September 1992; accepted for publication 6 January 1993
Epitaxial iron silicide films have been grown on Si(lll) by solid phase epitaxy (SPE) in UHV. Structural and electronic properties have been inyestigated with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). For initial Fe deposition up to 3 A and annealing at 850 K, metallic y-FeSi, is formed. These films exhibit a perfect (2 x 2) superstructure, which is attributed to y-FeSi (111) with Si termination. SPE at higher initial iron deposition (15 .k> and annealing at 800 K results in e-FeSi showing a (6 X / 3 ) R30” superstructure. Subsequent annealing above 900 K leads to p-FeSi, formation. As determined by STS, &FeSi, films are semiconducting with ES = 0.85 eV. STM topographs show that SPE produces rough silicide surfaces with j?-FeSL#Ol) [and not fi-FeSi,(llO)] epitaxy. The atomic structure on @-FeSi, terraces is complex, consisting of many anti-phase domain boundaries and defects.
1. Introduction
During the last years there has been a considerable interest in the epitaxy of metallic and semiconducting silicides due to their possible applications in silicon technology. Among the iron silicides, FeSi, is of particular interest. According to the equilibrium Fe-Si phase diagram [l] there exist two different phases of iron disilicide: high temperature cu-FeSi, (tetragonal structure which is stable at high temperatures T > 1250 K) and &FeSi, (orthorhombic structure, stable at temperatures below 1250 K), respectively. Both phases a- as well as p-FeSi, result from distortions of the CaF,-structure. While tetragonal (YFeSi, is metallic, orthorhombic &FeSi, is semiconducting with a band gap of about 0.8 eV-0.9 eV [2-51. Its corresponding wavelength of - 1.5 pm suggests potential optical applications in glassfiber optics and might open new perspectives
* Present address: Department California, CA 92717, USA. 0039-6028/93/$06.00
of Chemistry, University of
of heterojunctions on Si, provided P-FeSi, can be grown epitaxially on Si single crystals [61. Thus, recently great effort was put on the study of growth and epitaxial relationship of /3-FeSi, on Si(ll1). Studies including diffraction methods like LEED and RHEED [7-101 showed that a complex (2 X 2) superstructure [(2 X 2) plus additional spots [ll]] as related to the Sit11 11 substrate is observed for /3-FeSi,. Recently a new, metastable FeSi, phase, yFeSi, was discovered to join the FeSi, family [I2]. Whereas Q- and P-FeSi, can be derived from the CaF, structure by distortions, y-FeSi, was proposed to have the undistorted CaF, structure, like CoSi, or Nisi,. This is surprising because cubic FeSi, does not exist in the bulk phase diagram for the Fe-Si system 111.Calculations of Christensen [2] showed that a cubic CaF, structure is not expected to be stable in the case of bulk FeSi, due to an increase of the density of states (DOS) at the Fermi energy (E,). This instability can be removed by distortions which lead to an orthorhombic unit cell and the appearance of a bandgap for /?-FeSi,. For very thin
0 1993 - Elsevier Science Publishers B.V. All rights reserved
204
B! Raunau et al. / FeSi, ep~t~~~~ly grown on Sic1II)
films however, the situation may be different because of the influence of the substrate, so that cubic y-FeSi, might be stabilized on Si(lll) substrates. Similar examples of the formation of metastable structures are the growth of cr-Sn on InSb or CdTe 1131 and bee Co on GaAs [14]. Chevrier et al. [S] reported on the formation of a strained cubic phase by SPE after deposition of 2 A of iron on Si(ll1) followed by annealing at 560 K. RIIEED patterns of this thin film showed a perfect (2 x 2) periodici~ in contrast to the complex (2 x 2) RHEED pattern that is observed in the case of &FeSi,. This complex (2 x 2) pattern can be explained by the superposition of three domains of j3-FeSi,. y-FeSi, seems to be not the only metastable iron silicide phase on Si(ll1). Von Kane1 et al, 1151reported on the synthesis by molecular beam epitaxy (MBE) of FeSi, +X (0 IX I 1) with a metastable CsCl structure. Thin FeSi films ( < 20 A> were shown to evolve upon annealing below 770 K towards FeSi, with no change of the CsCl symmetry by introducing Fe vacancies in a random fashion. These FeSi,,, (0 _
2. Experimental The experiments were performed 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 Cl71 and has been modified for spectroscopical operation. Topo-
graphical images are acquired in the constant current mode. In order to obtain spectroscopical information we used a STS method, which is similar as proposed by Feenstra et al. [lS] and will be described in detail elsewhere [19]. Basically STS operates as follows: in the first step the tip is positioned in the constant current mode, then the feedback loop is opened and a voltage ramp is applied to the sample. There is roughly an exponential dependence of the tunneling probability on the tunneling voltage [18]. Thus, the tunneling current will be attenuated exponentially as the tunneling voltage approaches the zero value [201. In order to increase the sensitivity for tunneling voltages close to zero one has to amplify the tunneling current. Instead of amplifying the tunneling current electronically, which also amplifies the noise, we take advantage of the exponential dependence of the tunneling current on the tip-to-sample distance. Decreasing the tipto-sample distance linearly while the tunneling voltage ramp is applied to the sample, approximately compensates for the exponential voitage dependence of the tunneling current. The variation of the tip to sample distance is experimentally achieved by adding a V-shaped voltage ramp on the electrode of the z-piezo element of the STM so that the tip to sample distance reaches its minimum when the sample voltage goes through zero. Scanning the tip across the surface and acquiring at each (x, y) point an I/V curve gives us both, the atomically resolved topographical and spectroscopical information. The samples were cut from Si(ll1) wafers (ndoped, 30-60 Q *cm), rinsed in ethanol and cleaned in UHV by brief annealing up to 1500 K. After this pure heat treatment in UHV the STM topographs of the Si(l11) surface exhibits the well-known (7 x 7) reconstruction. Iron was evaporated by direct sublimation from a high purity iron wire heated resistively. After in situ Auger spectroscopy determination of the Fe coverage, the samples were heated for about 20 min in order to form the silicide. The samples were allowed to cool for 30 min in order to avoid thermal drift in the STM investigation. All STM measurements shown below were performed with samples at room temperature.
W. Raunau et al. / FeSi, epitaxially grown on Si(ll1)
205
3. Results and discussion Small Fe deposition in the submonolayer range and subsequent annealing at 700 K results typically in a surface as shown in fig. 1. Basically three different regions can be distinguished. (i) Patches of bare Si substrate (A, A’). Besides some areas with the normal Si(ll1) (7 x 7) reconstruction (A) also islands of Si(lll) (5 x 5) reconstruction (A’) can be found. We note, that in case of clean Si(ll1) surfaces the (5 x 5) reconstruction is found by Feenstra and Lutz [21] at cleaved Si(ll1) surfaces after annealing at 600 K. It seems to be a transition stage on the way to the more stable (7 X 7) superstructure which is obtained upon higher annealing temperatures (> 900 K). For lower annealing temperatures the (5 x 5) is supposed to be favored above the (7 x 7) reconstruction because the (5 x 5) structure exhibits the same atom density as the ideal (1 x 1) surface, whereas the (7 X 7) structure requires 4% more atoms. (ii) Disordered regions B that probably consist of a mixture of still unreacted Fe and Si. (iii) Regions C which show already a (2 x 2) structure of iron silicide. Subsequent annealing at 850 K leads to further silicide formation in re-
Fig. 1. (a) STM topograph measured after evaporating 0.2 ML Fe on Si(lll) and annealing at 700 K. Scan size 535 x535 AZ, u sample =-1.2V,Z=lnA.
Fig. 2. STM topograph showing epitaxial y-FeSi, (left) and Si substrate (right). This image has been measured after 0.2 ML Fe deposition on Si(lll) and annealing at 850 K. Scan size 220 x 220 k, U,,,,,, = - l.W, Z = 1 nA.
gions B. No longer Si(ll1) (5 X 5) but only Si(ll1) (7 x 7) areas can be found, indicating that the Si(l11) (5 X 5) areas have been transformed to the more stable Si(lll) (7 X 7) structure due to the higher annealing temperature. In fig. 2 a STM image is shown where an iron silicide island and adjacent Si(ll1) (7 x 7) substrate can be seen. In order to enhance the contrast, the image has been acquired in the “derivative mode”,in which the DC component of the measured signal is suppressed. Thus, terraces of different height appear at the same gray level in the STM image. The step edges of the iron silicide island are running parallel to the [liOl and [iOl] directions of Si. The iron silicide surface exhibits a hexagonal symmetry. The lattice constant is 7.8 & 0.2 A in contrast to 3.84 w for Si(ll1). Such a (2 x 2) structure is consistent with the (2 X 2) observed on both, y-FeSi, with CaF, structure 18,221 as well as FeSi,,, (0 I x I 1) with a defect CsCl structure [151. For the CsCl structure the distance between equivalent atomic planes is half as large as in the TaF, structure, leading to a step height of 3.1/2 A. Evaluating step heights in our (nonderivative) STM topographs results in multiples
206
W. Raunau et al. / FeSi, epitaxially grown on Si(lll)
of 3.1 A which is also the Si(ll1) single-layer step height. We conclude that FeSi, with CaF, structure, i.e. y-FeSi, has been formed, lattice matched with its (111) plane parallel to the substrate. The strained cubic y-FeSi,(lll) surface is expected to exhibit a (1 X 1) superstructure referred to Si(ll1) in contrast to the measured (2 X 2) in the STM topograph in fig. 2. Similarly the (111) surface of metastable FeSi, +X(0 IX I 1) with the defect CsCl structure is also expected to show a (1 x 1) superstructure when grown on Si(lll), in contrast to the STM data from von Kane1 et al. [15]. As known for many metal-silicon compounds, Si has the tendency to segregate and to form a monolayer of pure Si on top of the silicide 123-251. We may speculate that here also Si atoms have, indeed, diffused into the outermost layer. Thus, the (2 X 2) reconstruction on the yFeSi,(lll) surface might be due to a similar effect to that on Si(ll1) (7 x 7) where Si adatoms form a local (2 X 2) arrangement in order to reduce the number of dangling bonds. Because in case of the silicide the resulting strain might not be relieved in the same manner as for clean Si(ll1) (corner holes), probably the former local (2 x 2) structure is now spread over the entire y-FeSi,(lll) surface. It has already been mentioned that cubic strained y-FeSi, is metastable and is expected to be stabilized on Si(ll1) only in the case of thin films. We obtained y-FeSi, films for initial Fe deposition up to about 3 A. Indeed, according to the Fe-Si phase diagram orthorhombic P-FeSi, is the only stable FeSi, phase at room temperature. It has been shown that p-FeSi, can epitaxially be grown on Si(ll1) matching its (101) or (110) planes parallel to the substrate [7-111. Fig. 3 shows schematically the epitaxial relationship for p-FeSi, on Si(ll1). The (110) or (101) planes of p-FeSi, are expected to be good possibilities of lattice matching to the Si(ll1) substrate. In order to produce P-FeSi, a larger amount of iron has been evaporated on the Si(ll1) substrate. A STM topograph of epitaxial p-FeS$ on Si(ll1) which stems from deposition of 15 A Fe on Si(ll1) and subsequent annealing at 950 K is shown in fig. 4a. The surface structure seems to
a.> j3-FeSi2(110)
B_FeSi2(101)
k--“-i
W
I
atiiz] 11g-Fesi[ioi] (atiizl 11g-r+.sii[tio))
Si[ilOI (Si[ilOl
II fi-FeSi&OIO] II f%-Fe.S$[oOll)
Fig. 3. (a) Schematic drawing of P-FeSi,(lOl) and pFeSi,(llO) planes in the orthorhombi$ @-FeSi, bulk unit cell. a = 9.863, b = 7.791, and c = 7.833 A 1261. (b) (101) [(llo)] epitaxy on Si(ll1): three equivalent azimuthal orientations of the rectangular unit cell are shown. its dimensions are 12.59 x 7.79 k (12.57X7.83 A’). The epitaxial relationships are Si[ilOlII /3-FeSi,[OlO] (B-FeSi,[OO11) and Si[ii2lI( &FeSi,[lOil (P-FeSiJliO]) for (101) [(llo)] epitaxy. The lattice mismatch for P-FeSi,(lOl) @FeSiz(llO)) epitaxy is about -5.3% (-5.5%) in Sini direction and + 1.4% (+ 2.0%) in Si[ilO] direction. The circles represent the silicon atoms of the Si(ll1) substrate.
be disordered. Several distinct bright lines are visible at the silicide surface which are running in nlO] direction referring to the Si substrate. As will be shown below, these bright lines represent anti-phase domain boundaries. The existence of domain boundaries is plausible considering the epitaxial relationship. According to fig. 3 the lattice mismatch of /3-FeSi, with the Si(ll1) substrate in Si(ll0) directions is small (< 2.0%),
W. Ruunau et al. f Fe%, e~i~~iu~ygrown on Si(ll1)
which might allow for growth of coincidence lattices over long distances. In the Si( 112) directions the lattice mismatch is large (> 5%). Thus, the resulting strain is relieved by the domain boundaries. As shown in fig. 3 we expect three equivalent domains because of the lower symmetry of the rectangular surface unit cell of p-FeSi,(lOl) or &FeSi,(llO) compared to the threefold symmetry of the Si(ll1) surface. However, on each individual terrace only one of the three possible domains is present. In fact, our STM data shows that on each terrace all anti-phase domain boundaries are parallel to each other. The sizes of the terraces which contain only one of the three possible domains amount to several hundred lngstriims in Si(112) directions and a few thousand angstrSms in Si(ll0) directions. Inspection of larger areas containing more terraces show anti-phase domains rotated by 60” with respect to each other. This reflects the threefold azimuthal s~rnet~ shown in fig. 3. In order to extract the periodic info~ation that is contained in the STM topograph shown in fig. 4a the image has been transformed to Fourier
207
space by means of the two dimensional Fourier transform. Although the Fourier transform shown in fig. 4b contains the same information as the STM topograph in fig. 4a, the periodicity at the surface can readily be seen. An additional feature in fig. 4b are streaks parallel to Si[ii2] which arise from the anti-phase domains separated by the boundaries visible in the STM topograph in fig. 4a. There are no streaks parallel to Si[llOl reflecting the undisturbed periodic@ in the Si[ilO] direction. We found that Fourier transformation of one single domain (area between two adjacent anti-phase domain boundaries) results in pronounced spots without any streaks. Fourier transformation of larger areas containing antiphase domain boundaries leads to spots and additional streaks in Si[ii21 direction as seen in fig. 4b. In view of the fact that there are three equivalent azimuthal orientations for p-FeSi,(lOl) [pFeSi,(llO)] epitaxy, this would result in a compIex (2 X 2) superstructure, observable by diffraction methods. As mentioned above, LEED (RHEED) studies of the Fe/Siflll) system [7-111 show, indeed, a complex (2 x 2) pattern after formation of P-FeSi,.
Fig. 4. (a) STM to~graph of P-FeSi, epitaxially grown on Si(fll) after depositing 15 A Fe on Si(ll1) and annealing at 950 K. The indicated directions are referred to the Si(ll1) substrate. Anti-phase domain boundaries of @-FeSi, along the indicated Si[ilO] direction are clearly visible. Scan size 410 x 410 k, Usampte= - 1.5 V, I = 1.5 nA. tb) Two-dimensional Fourier transform of the STM image shown in (a).
208
W. Raunau et al. / FeSi,
Next, we determined the epitaxial relationship between P-FeSi, and Si(ll1). Although it has been demonstrated that /?-FeSi, can epitaxially be grown on Si(ll1) matching with /3-FeSi,(lOl) or &FeSi,(llO) parallel to the substrate [7-111, it is hardly possible to distinguish between pFeSiJllO) and P-FeSiJlOl) epitaxy, due to the very close dimensions of their surface unit cells (cf. fig. 3b). In order to clarify the situation, the surface structure of P-FeSi, shown in fig. 4a has been analysed in more detail. According to the STM topograph in fig. 4a the P-FeSi, surface looks disordered. Transformation of the STM image in Fourier space reveals the hidden periodic information as shown in fig. 4b. In order to enhance the contrast for the periodic structure, the spots marked with a white circle in the image shown in fig. 4b have been selected and back transformed to real space, thus suppressing all the information that is not periodical. The resulting image is shown in fig. 5a. Pronounced zigzag rows are running in the Si[ii2] direction with a lateral period of 12.6 + 0.2 A along Si[?i2] and a
epitaxially grown on Si(lll)
distance between zigzag rows of 7.8 + 0.2 A. A grid with rectangular meshes is superimposed in fig. 5a in order to show the surface unit cell of the silicide. Considering the bulk structure of /3-FeSi, the observed structure can be explained by assuming (101) epitaxy. In bulk P-FeSi, there are slabs of (101) planes that only consist of silicon atoms. Fig. 5b shows a model of bulk truncated p-FeSi,(lOl) with Si termination where for the sake of simplicity only three layers of such a slab are plotted. The first and the second layer form a double layer of siliton with an intra-double layer spacing of 0.133 A. The distance of the third layer (small circles) from the surface amounts to 0.426 A. In fact, the double layer exhibits zigzag rows in P-FeSi,[lO?] direction and the surface unit mesh has the same dimensions as resulting from the structure shown in fig. 5a. This is a clear indication that /3-FeSi, grows in the (101) crystallographic modification and is terminated by a Si double layer. We have also examined /I-FeSi,(llO) crystallographic planes, but in contrast to (101) iron disilicide a similar zigzag
&F&&(101)
g-FeSiJOlOl
I
Fig. 5. (a) Periodic component of the STM image shown in fig. 4a obtained by double Fourier transformation. An enlarged area of 140 x 140 AZ is shown. Zigzag rows are running in the si[ii2] direction. The surface unit cells are shown by a net with rectangular meshes. its dimensions are 7.8 in the Si[llO] direction and 12.6 A in the Si[iiZ] direction. (b) Model of bulk truncated P-FeSi,(lOl). The size of the balls reflects its height normal to the surface. Three layers consisting of Si atoms are shown. The upper Si double layer has an intra-double layer spacing of 0.133 A. The distance of the third layer from the surface amounts to 0.426 A. Zigzag rows are running in the FeSi,[lOi] direction. The unit cell is indicated by a rectangle with dimensions 7.79 X 12.59
W Raunau et al. / FeSi,
arrangement does not occur! We therefore find for the epitaxial relationship of P-FeSi, on Si(ll1): p-FeSi,(lOl) IISi(ll1) and P-FeSi, [OlO]IlSi(llO>. In fact, on the basis of our STM data in combination with the Fourier transformations we could show that using the SPE method only p-FeSi,(lOl) and not /?-FeSi,(llO) is grown on Si(ll1). So far we have shown that for y-FeSi, and P-FeSi, there is a large difference in the atomic surface structure. Apart from these structural differences y- and P-FeSi, are also expected to display different electronic behavior. Cubic yFeSi, exhibits metallic character, while /3-FeSi, is semiconducting [2]. In order to evidence these electronic differences local scanning tunneling spectroscopy (STS) on y-FeSi, and P-FeSi, was also performed. In fig. 6a we show an Z-v curve measured by STS on the y-FeSi, surface. For this purpose the tip has been positioned above the silicide surface shown in fig. 2. We measured atomically resolved spectroscopic data and averaged the data over several unit cells in order to average out local effects. The data show an almost linear behavior with a slope of 30 nA/V between -0.1 and 10.1 V. Comparing this Z/I/ curve with the Z/V curve taken on p-FeSi, clearly demonstrates the absence of any band gap. The obvious metallic behaviour of the Z/V curve shown in fig. 6a therefore corroborates that no semiconducting P-FeSi, has been formed. Performing STS on the P-FeSi, surface shown in fig. 4a reveals a dramatic difference: fig. 6b shows the Z/I/ curve measured on P-FeSi, which has been
epitaxially grown on Si(ll1)
209
averaged over an area of about 50 A X 50 A. In contrast to the Z/V curve shown in fig. 6a a slope < 0.1 nA/V can be detected between -0.1 and +O.l V. In order to determine the size of the band gap from this Z/1/ curve we have linearly extrapolated the tunneling current to smaller tunneling voltages (see fig. 6b). As mentioned above during measurement of the Z/v curves the tip$-sample distance is decreased linearly with 2 A/V as the tunneling voltage approaches the zero value. As a consequence, high spectroscopic sensitivity at the band edges is achieved. From the Z/V curve shown in fig. 6b a band gap of 0.85 eV can be derived by the linear extrapolation of the tunneling current. This value is in good agreement with DOS calculations of Christensen [2] and experimental measurements by Bost et al. [4] and Dimitriadis et al. [5] using photothermal deflection spectroscopy. We also note that the tunneling current is not totally zero inside of the bandgap. Such a tunneling current probably arises from the large number of surface defects which introduce additional states in the bandgap region. Besides y- and P-FeSi,, another iron silicide phase, the l-FeSi monosilicide (cubic B20 structure) [16] can also be grown in an epitaxial phase. We evaporated 15 A of Fe on SXlll) and annealed the sample at 800 K. Similar to the y-FeSi, phase, the silicide shown in fig. 7 is metallic and exhibits a hexagonal structure. A closer look shows, however, substantial differences: the distance between the bright spots on the silicite terraces is no longer 7.8 + 0.2 but 6.7 k 0.2 A. Also the amount of defects has strongly in-
J
-1.0
-0.5
0
0.5
1.0
M
Fig. 6. Averaged I/V curves measured on (a) y-Fe& and (b) P-FeSi% D uring STS the tip has been stabilized with &,,p,e = - 3.0 V, I = 0.11 nA. The tip-to-sample distance was varied linearly with 2 A/V. In (b) the band gap Es = 0.85 eV has been determined by extrapolation of the tunneling current to smaller voltages.
210
W. Raunau et al. / Fe%, epiiaxially grown on Si(lll)
creased. In addition, the observed structure is rotated by 30” with respect to the perfect (2 x 2) structure of y-FeSi,, Such a structure can be described by a (6 x fi)R30” superstructure (7.8 %6/z = 6.75 A>. I n situ AES measurements confirm the FeSi stoichiometry. This structure is in good agreement with results from Chevrier et al. [S] and Gavriljuk, Kachanova and Lifshits [271. The epitaxial relationships of E-FeSi on SXlll) are: e-F&%(111) ]ISi(lll) and e-FeSi[liO] IISi[ii2]. Subsequent annealing of such an e-FeSi film above 900 K results in the formation of epitaxial &F&i,, fig. 8 shows a survey image of the resuiting surface. In this image the vertical roughness covers about 50 A. Typical terrace widths are of the order of several hundred ingstrijm with many step edges running parahei to the Si[ilO] direction. The STM image in fig, 4a shows a close-up of one of the terraces. The inhomogeneity of the surface shown in fig. 8 is probably due to the solid phase epitaxy. Deposition of pure metal and subsequent annealing (SPE) is often used because of its facility. On the other hand the interatomic diffusion of Si all the way through the
Fig. 8. STM tcrpograph of p-FeSi, epitaxially grown on Si(lll) after depasiting 15 k Fe on SiClll) and annealing at 950 K. Scan size 6400 X 6400 A*, Usample= + 1.8 V, I- 1 nA.
deposited iron layer is necessary for silicide formation up to the outermost layer. This large material transport probably results in many defects and a rough surface. The atomic surface structure on the terraces exhibits anti-phase domain boundaries and a great amount of surface defects as shown in fig. 4a.
4. Summary
Fig. 7. STM topograph of e-FeSi. After evaporation of 15 A Fe the film has been annealed for 20 min at 800 K. The surface shows numerous c-Fe% islands of monatomic step height. Each of the islands exhibits a (6 X fi)R30” superstructure referred to the Si(i 1I) substrate. Scan size 580 X 580
Topographical and spectroscopical features of iron silicide epitaxially grown on Si(lll) by SPE have been reported. After little Fe deposition (< 3 A) and annealing at 850 K cubic yFeSi,(f 11) has been grown epitaxialiy on Sit1 12). By means of STM and STS we showed that the y-FeSi, films exhibit a perfect (2 X 2) superstructure and are metallic. In the case of thicker films (15 &, e-F&X(111) (6 x 6)R30” is formed after annealing at 800 K. Higher annealing above 900 K transforms the E-FeSi film into p-F&i,. By application of the Fourier transform it has been shown, that p-FeSi,(lOl) grows epitaxialiy on Siflll). The local structure on the /?IFeSi,(lOl) terraces is found to consist of many
W. Raunau et al. / FeSi, epitaxially grown on Si(lll)
anti-phase domain boundaries and defects. STS performed on p-FeSi, films shows a bandgap (I& = 0.85 eV). The quality of Fe-silicide films can possibly be improved by using other techniques of silicide growth like reactive deposition epitaxy (RDE) or molecular beam epitaxy (MBE).
References 1110. Kubaschewski, Iron-Binary Phase Diagrams (Springer, Berlin, 1982).
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[Ill Ch. Stuhlmann, J. Schmidt and H. Ibach, to be published.
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