Recent progress on BEEM

Recent progress on BEEM

Ultramicroscopy 73 (1998) 175—183 Recent progress on BEEM Hans von Ka¨nel*, Thomas Meyer Laboratorium fu( r Festko( rperphysik, ETH Zu( rich, CH-8093...

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Ultramicroscopy 73 (1998) 175—183

Recent progress on BEEM Hans von Ka¨nel*, Thomas Meyer Laboratorium fu( r Festko( rperphysik, ETH Zu( rich, CH-8093 Zu( rich, Switzerland Received 7 July 1997; received in revised form 11 July 1997

Abstract The application of ballistic-electron-emission microscopy (BEEM) and spectroscopy (BEES) to epitaxial CoSi /Si 2 interfaces is reviewed. Dislocations in 2—3 nm thick CoSi (0 0 1)/Si(0 0 1) films with Burgers vectors b"a/4S1 1 1T are 2 shown to lower the barrier height measured at 77 K by almost 0.1 eV. This corresponds to a decrease of the Schottky barrier height at the metallurgical interface from the value of U "0.74$0.03 eV, characteristic for defect-free regions, B to near zero within the range of their strain field of a few nanometers. By contrast, the dislocations of type b"a/6S1 1 21 T present at CoSi /Si(1 1 1) interfaces do not affect the barrier height. Films of CoSi /Si(1 1 1) are more suitable for 2 2 studying interfacial scattering by BEEM because of their simpler surface structure and because of band structure effects. Here individual point defects can be resolved. Their spatial distribution indicates diffusion along the interface during film growth. By analysing the shape of BEES spectra obtained on isolated point defects, and by making use of the projected band structure predicting the BEES current to set in &0.2 eV above the Schottky barrier, it may be concluded that the defects are located at the interface. ( 1998 Elsevier Science B.V. All rights reserved.

1. Introduction Nearly a decade has passed since the invention of ballistic-electron-emission microscopy (BEEM) by Kaiser and Bell [1]. BEEM offers a way to apply the spatial resolution capabilities of the scanning tunnelling microscope (STM) to the study of buried interfaces. In principle, the method is very simple: a tunnelling current is established between the tip of the STM and a sample, consisting of a metallic film on a semiconducting substrate. In addition to

* Corresponding author. Tel.: #41 1 633 2261; fax: #41 1 633 1072.

the tip height recorded as a function of its lateral position, giving rise to the usual STM topography image, the current entering the semiconducting substrate is measured by means of an electrical contact attached to the substrate. In general, this collector current, also called BEEM current, is only a small fraction of the tunnelling current injected by the STM tip. It is due to hot carriers with energies sufficient to cross all potential barriers on their way to the collecting electrode. At sufficiently small tunnelling bias, only ballistic carriers are able to enter the collector, i.e. those carriers which have not been subject to any scattering event. In addition to the energy, the momentum component parallel to an interface is ususally conserved to varying degrees. In fact, in the simplest theories capable to

0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 5 2 - 6

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quantitatively describe the BEEM current as a function of the tunnelling bias, perfect conservation of the parallel momentum has been assumed [2,3]. Energy and parallel momentum conservation have the effect that the BEEM current, at a given tunnelling bias, is not only sensitive to lateral variations of the potential barriers, but also to any scattering processes taking place between the point of injection at the surface and the collector. The simplest structures which can be studied by BEEM are those consisting just of a metallic film deposited on the semiconducting collector. The height of the Schottky barrier formed at the metal/semiconductor interface is then one of the quantities of interest, determining the potential barrier to be surmounted by the hot charge carriers. The barrier height, U , can be obtained with B very high precision by measuring the collector current, I , as a function of the tunnelling bias » , and # 5 by fitting the resulting spectra to a power law of the form I (» )"R(e» !U )a. (1) # 5 5 B The exponent a has a value of 2 when quantum mechanical reflection at the interface is neglected [2], or 5 if it is taken into account within an effec2 tive mass approximation [3]. While the value of U is of interest in itself, its variation on a lateral B scale is even more important. Schottky barrier height inhomogeneities are responsible for many anomalies in the behaviour of macroscopic Schottky diodes [4]. Schottky barrier height fluctuations on a nanometer scale have been observed by several groups [5—7]. In this review, we shall focus on recent results obtained at epitaxial CoSi /Si 2 interfaces, where the nature of the defects giving rise to the barrier variations is known for the most part [7]. It should be pointed out, however, that more complicated heterostructures have been studied successfully by BEEM, such as p—n diodes [8], semiconductor heterointerfaces [9—11], resonant tunnelling structures [12], a well as metal—insulator—semiconductor (MIS) [13] and metal—oxide— semiconductor heterostructures [14]. Many more references can be found in a number of excellent review articles on BEEM which have appeared recently [15—17].

2. Experimental procedure The details of the growth procedures used here for the formation of epitaxial CoSi /Si(1 1 1) and 2 CoSi /Si(0 0 1) interfaces have been described in 2 Refs. [18] and [19], respectively. The only one of importance for high-resolution BEEM measurements is the need to use very thin films, i.e. films with thicknesses of the order of 2—3 nm, and ohmic collector contacts. The silicide films were deposited through a shadow mask, defining the active area of the Schottky diode of approx. 0.5 cm2. In order to ensure ohmic collector contacts, 10—20 nm thick CoSi films were deposited on the entire back side 2 of the degenerately doped 3 in Si substrates. On the front side, diode behaviour was achieved by growing a &300 nm thick undoped Si buffer layer by MBE before the deposition of the thin silicide. Since CoSi surfaces oxidize easily, the BEEM 2 measurements were carried out in the same UHV environment as the sample preparation. A further complication arises from the rather low barrier heights of CoSi /Si interfaces (see below), making it 2 necessary to perform all BEEM experiments at low temperatures in order to improve the signal-tonoise ratio [15,16]. For this reason a low-temperature STM was designed for BEEM operation at 77 K. The microscope is a “Besocke walker” [20] standing on top of the 3 in wafer [21].

3. Results 3.1. Variations of the Schottky barrier height at interfacial defects Epitaxial CoSi /Si interfaces have been exten2 sively studied for the past 15 years. For the present discussion the defect structure obtained by transmission electron microscopy (TEM) is of greatest relevance. For both substrate orientations interfacial steps were shown to be coupled to dislocations relieving part of the misfit strain of !1.2%, even when the films are thin enough to be coherently strained to the Si lattice constant in the (hypothetical) absence of steps. The most prominent dislocations found at CoSi /Si(1 1 1) interfaces 2 have Burgers vectors b"a/6 S1 1 21 T lying in the

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interfacial plane [22]. They can be seen in topographic STM images because their associated strain field leads to a corrugation of the surface [23]. As a result of the lattice relaxation induced by these dislocations the surface of CoSi /Si(1 1 1) 2 films is unreconstructed, whereas films strained to the Si lattice exhibit a 2]1 reconstruction [24]. The absence of surface reconstructions is desirable for high-resolution BEEM studies, since strong variations of the BEEM current with the periodicity of surface reconstructions have been observed, which may easily mask features stemming from the interface [25]. The partial dislocations mentioned above do not have any measurable influence on the Schottky barrier height, which is hence homogeneous at CoSi /Si(1 1 1) interfaces with a value of 2 U "0.66$0.03 eV at 77 K [26]. This is only true, B however, as long as the CoSi assumes its usual 2 CaF structure characterized by Si atoms arranged 2 on a simple cubic lattice, with the Co atoms occupying the centre of every other Si cube. Instead of being arranged in this regular way, the Co atoms can occupy the Si cubes in a random fashion, giving rise to a defect-CsCl structure with half the cation sites remaining empty. This defect structure seems to be characteristic of an intermediate phase forming during the synthesis of fluorite-like CoSi 2 [27,28]. When present, grains with the defect-CsCl structure do in fact lead to a substantial lowering of the Schottky barrier height to U "0.4—0.5 eV B [29]. Since the grain size can be as large as several tens of nanometers, the classical parallel conduction model can be used to analyse the I—» curves of the corresponding macroscopic diodes [30], yielding good agreement with the BEEM experiments [29]. Perhaps more interesting, from the point of view of barrier height variations, are CoSi /Si(0 0 1) 2 interfaces. In contrast to CoSi /Si(1 1 1), where a 2 single epitaxial orientation can easily be achieved, it is hard to grow truly single-crystalline films for this substrate orientation. Thus, apart from perfectly aligned CoSi (0 0 1)/Si(0 0 1) with 2 [1 0 0] E[1 0 0] , very often misoriented C0S*2 S* CoSi (0 1 1)/Si(0 0 1) grains can also be found, 2 covering an areal fraction depending on the exact deposition procedure [31]. The presence of CoSi 2 (0 1 1) grains has been seen to lower the barrier

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height from U "0.74$0.04 eV, typical for deB fect-free CoSi (0 0 1)/Si(0 0 1) interfaces, to 2 U "0.58$0.04 eV [32]. Since the grains are B again large enough to apply the parallel conduction model, we shall not discuss this simple case any further here. Instead, we shall focus attention to pure CoSi (0 0 1)/Si(0 0 1) interfaces. Two different 2 interfacial structures were found to coexist at these interfaces, one of them being 2]1 reconstructed and the other one unreconstructed [33,34]. The boundaries between 2]1 and 1]2 reconstructed domains are separated by monolayer steps (where 1 monolayer"a /4) giving rise to dislocations S* with Burgers vectors b"a/4S1 1 1T. Dislocations of the same type, coupled to interfacial steps, were found between pairs of unreconstructed domains and at boundaries between reconstructed and unreconstructed domains [35]. In contrast to CoSi /Si(1 1 1), the surfaces of 2 CoSi /Si(0 0 1) films are always reconstructed inde2 pendent of their state of strain. Well-annealed films are usually 3J2]J2R45° reconstructed with characteristic double rows of Si atoms running along the S1 0 0T directions [19]. As mentioned above, the surface reconstructions lead to a substantial modulation of the BEEM current even in regions of defect-free interfaces [25]. In order to eliminate this surface-induced BEEM contrast, we have tried to simplify the surface structure of CoSi . 2 One possibility is to terminate the surface by one or two monolayers (MLs) of Fe, followed by &2 ML of Si and annealing to typically 500°C [29]. This does not result in the desired unreconstructed surface, but at least in a reduction of the surface unit cell. Fig. 1a shows a topographic STM image of such a surface. It is homogeneously J2]J2R45° reconstructed. The only prominent features present in the STM image are two small Si islands, and a domain boundary, appearing as a faint protruding line on the right-hand side terrace in Fig. 1a. The most interesting object is a “soft” surface step where the surface height rises by one ML on a lateral scale of about 3—4 nm (see also Fig. 3). Such steps are due to interfacial dislocations having a component of the Burgers vector normal to the surface, i.e. the dislocations of the type b"a/4S1 1 1T mentioned above. Fig. 1b shows a BEEM image acquired simultaneously with the topography image

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Fig. 1. (a) Topographic STM image of a 2.4 nm thick CoSi /n2 Si(1 0 0) film capped with approximately 0.7 nm of FeSi . x A “soft” surface step is visible in the left part of the image (b) Corresponding BEEM image, obtained for a tunnelling current of I "3 nA and a tunnelling voltage of » "!1.7 V. In (a) the 5 5 gray scale ranges from 0 to 0.33 nm. In (b) it varies within *I "152 pA. #

of Fig. 1a. First of all it has to be emphasized that the BEEM current is surprisingly nonuniform, despite of the fact that we are dealing with an epitaxial interface. In order to figure out the reason for this nonuniformity, local spectroscopy is needed. Here we discuss only those features which can definitely be attributed to crystallographically identified defects, i.e. the dislocation at which the BEEM current may be seen to be larger than on the terraces to the left and to the right (Fig. 1b). Fig. 2 shows two spectra, one of which was acquired right on top of a dislocation and the other one in a defect-free area. Here the square root of the collector current is plotted as a function of the tip bias, assuming a quadratic dependence according to Eq. (1). From the x-axis intercepts the barrier heights can be seen to be U "0.74 eV and U$*4-"0.66 eV in the deB B fect-free region and on top of the dislocation, respectively. We therefore have to conclude that the Schottky barrier height is significantly depressed at dislocations with Burgers vectors b"a/4S1 1 1T, and that this is the reason for the enhanced BEEM current in Fig. 1b. The spatial extent of the lowbarrier height region can be obtained from crosssections through the BEEM image (Fig. 1b), taken in a direction perpendicular to the soft surface step in Fig. 1a. A representative example can be seen in Fig. 3, in which cross-sections through both the topography and the BEEM image are shown. The

Fig. 2. BEES spectra taken on top of the dislocation of Fig. 1 (filled circles) and in the defect-free area (open circles). The tunnelling current was set to I "3 nA. 5

Fig. 3. Cross-sections through the topography image of Fig. 1a (filled circles) and through the BEEM image of Fig. 1b (empty circles), respectively.

width of the soft step visible in the topography cross-section gives an indication of the extent of the elastic strain field around the core of the dislocation. The profile can be fitted very well to a Fermi function, the derivative of which has a full-width at half-maximum (FWHM) of 6.3 nm. The FWHM of the Lorentzian fitted to the BEEM profile (Fig. 3) is given by 4.3 nm. It can thus be seen that the extent of the region with lowered barrier height coincides roughly with the extent of the strain field. Since the width of the low-barrier region is so small, the onset of the BEEM current obtained from Fig. 2 does not

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reflect the true value of the Schottky barrier height at the metallurgical interface. This is the result of screening effects, leading to a smoother variation of the potential inside the semiconductor, compared to the abruptly changing Schottky barrier height right at the interface [4]. The parallel conduction model mentioned above [30] is no longer applicable for the description of the interfacial transport in such a case. The value of the Schottky barrier height lowering at the dislocation can be estimated, however, by means of a model described in Ref. [4]. Within this model a measured barrier height lowering dU "U !U$*4- of &0.1 eV indicates that the B B B Fermi level is pinned to the Si conduction band minimum at the core of the dislocation [7,36]. This would correspond to a perfectly ohmic contact if the low barrier height region extended across the whole interface! A close inspection of Fig. 1 reveals that the BEEM current is also enhanced at other features which are not correlated with surface steps. Local spectroscopy performed in such regions have shown that there the barrier height is lowered too, by the same amount as at the dislocations. The nature of the defects responsible for the low barrier height in these regions could not, however, be established to date [7]. 3.2. Hot carrier scattering at interfacial defects As pointed out above, the Schottky barrier height is constant at CoSi /Si(1 1 1) interfaces as 2 long as the silicide crystallizes exclusively in its bulk stable fluorite form. The CoSi /Si(1 1 1) interface is, 2 however, ideally suited to study scattering effects for the following reasons: f The surfaces of relaxed films are unreconstructed and hence do not lead to any contrast in BEEM images. f The Si conduction band minima (CBM) do not project onto the centre of the interface Brillouin zone. In a semiclassical picture the injected tunnelling distribution is strongly forward focussed, leading to a small BEEM current across the epitaxial interfaces, unless hot carriers are provided with the necessary parallel momentum in a scattering event.

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f Calculations of the band structure show that within a range of 0.2 eV above the Schottky barrier CoSi does not have any states that over2 lap in energy and parallel momentum with the Si conduction band states [37]. As a result of this lack of matching states no BEEM current is expected at a perfect interface up to 0.2 eV above the Schottky barrier. This means that particularly in the energy range between U and B U #0.2 eV the BEEM measurements should be B very sensitive to interface scattering. The extent to which the BEEM current is sensitive to scattering processes can be envisioned from Fig. 4. Here the BEEM images and corresponding topography images, obtained on a 2.8 nm thick film are shown for two tip biases, » "!1.2 eV and 5 !1.6 eV. Since the displayed surface area does not contain any steps, the contrast in the topography images of Fig. 4a and Fig. 4c is dominated by the surface protrusions due to the strain fields of interfacial dislocations [23]. The latter are visible also in the BEEM images of Fig. 4b and Fig. 4d. In addition, the BEEM current exhibits planar contrast variations which change with the tip bias. They are caused by quantum interference effects in the thin metal film. Quantum interference peaks have also been seen in the local density of states measured by scanning tunnelling spectroscopy, up to a tip bias of typically !2 V [38]. Since the energy and momentum distribution of the injected electrons depends on the local band structure, the quantum interference contrast appears also in BEEM, between film areas differing in thickness. For the flat surface of Fig. 4 the film thickness changes only at dislocations, which, as explained above, are always associated with interfacial steps [35]. The most interesting features in Fig. 4b and Fig. 4d are the bright speckles, distributed over the whole area. They are due to hot electron scattering at individual point defects [39]. The half-widths of these features are typically of the order of 1 nm, indicating that the scattering centres responsible for their appearance are indeed of atomic dimensions. The question then arises where exactly the scattering centres giving rise to the enhanced collector current are located. It is easy to show that most of the defects must be below the CoSi 2

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Fig. 4. (a) STM topography image of a 2.8 nm thick CoSi /n-Si(1 1 1) film, obtained for » "!1.2 V and I "10 nA. Dislocations are 2 5 5 labelled by D. In the upper part of the image the surface is 2]1 reconstructed. (b) Simultaneously acquired BEEM image. The arrow points towards an interfacial point defect PD. (c) and (d) show the topography and BEEM image, respectively, measured during the back scan at a tip bias of » "!1.6 V. The ranges of the gray scales are 0.19 nm in the topography images of (a) and (c), and *I "270 pA 5 # and *I "480 pA in the BEEM images of (b) and (d), respectively. #

surface, since otherwise they ought to be visible in atomically resolved STM images. In order to locate them within the silicide film, we shall have to take recourse to the band structure calculation, however. As mentioned before, ballistic-electron-emission spectroscopy (BEES) yields a barrier height of U "0.66$0.03 eV for CoSi /Si(1 1 1) at 77 K B 2 [26]. This is the value obtained by averaging many spectra over an area bounded by dislocations (see Fig. 4). It can only be explained by the presence of interface scattering in view of the lack of matching states in the projected band structures of CoSi and 2 Si (see above). Since individual scattering centres can be resolved in BEEM images such as those in Fig. 4, it becomes possible to acquire BEES spectra in truly defect-free areas, and to compare them with spectra obtained right on top of a point defect. In Fig. 5 two such spectra are shown,

together with their first derivatives. They are averages over 14 spectra recorded at a spacing of 0.05 nm. Also shown are the results of fits to the sum of two quadratic terms, each with the form of Eq. (1), but with different prefactors and threshold values I (» )"R (e» !U )2#R (e» !U )2. (2) # 5 1 5 B1 2 5 B2 Most important is the presence of two thresholds in both kinds of spectra. The lower threshold, U "0.69$0.03 eV, coincides with the value obB1 tained before for the Schottky barrier. The fact that it is present even in the defect-free area suggests that parallel momentum conservation must be violated even there, by some other mechanism such as interface phonon scattering [7]. The second threshold, U "0.83$0.02 eV, agrees well with B2 the value expected from theory for the delayed onset [37].

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Fig. 5. Average of 14 BEES spectra, and corresponding first derivative spectra, taken on top of an interfacial point defect (diamonds) and in a defect-free area (circles). The spectra were measured with a tunnelling current of I "20 nA, and are nor5 malized to I "1 nA. Fits to Eq. (2) are indicated by the solid 5 lines.

From the results presented so far we cannot yet deduce the location of the point defects, since the two thresholds are found in defect-free regions as well. It seems likely, however, that, below the onset predicted by theory, an interfacial point defect

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should lead to an increase of the BEES current to a value above the one observed at the perfect interface. In order to verify this expectation, use was made of the possibility to fit single spectra in view of the excellent signal-to-noise ratio. For this purpose a 5 nm wide region containing a single point defect was chosen. Fig. 6a shows the featureless topographic image of this region. In Fig. 6b the corresponding BEEM image is displayed for » "!1.7 V and I "20 nA. The elongated shape 5 5 of the point defect is due to the drift of the microscope. The image was generated from a total of 100]9 spectra, recorded at a nominal spacing of 0.05 nm both along and perpendicular to the scan direction. Each single spectrum was then fitted to Eq. (2), by varying only the prefactors R and 1 R while keeping the barriers fixed at U "0.69 eV 2 B1 and U "0.83 eV, respectively. In Fig. 6c and B2 Fig. 6d the scale factors R and R are plotted, 2 1 respectively. The most important finding is that the contrast due to the point defect appears only in the R image of Fig. 6d, whereas the R image of 1 2 Fig. 6c is featureless. This is exactly what should be expected if the point defect was located at the interface. As explained above, it is only under this condition that scattering at the point defect can increase the BEES current below the delayed onset.

Fig. 6. (a) Topographic STM image and corresponding BEEM image (b) obtained for » "!1.7 V and I "20 nA. A total of 100]9 5 5 BEES spectra was recorded at a nominal spacing of 0.05 nm. The elongated shape of the point defect is due to microscope drift during the measurement. Each spectrum was fitted to Eq. (2), by keeping the barriers fixed at U "0.69 eV and U "0.83 eV. The scale factor B1 B2 R deduced from the fit is shown in (c). Similarly, the scale factor R is displayed in (d). Gray scale ranges were 0.1 nm, *I "200 pA, 2 1 # R "28—84 pA/(eV)2, and R "14—42 pA/(eV)2 in (a), (b), (c) and (d), respectively. 2 1

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Finally, we should say a word about the nature of the interfacial point defects. So far a spectroscopic identification of these objects has not been possible. A close inspection of the BEEM images does, however, enable us to make an educated guess at the very least. A statistical evaluation of point defect density n(r) shows that this quantity is not constant along the interface. Instead, a clear decrease of n(r) is observed, as a dislocation is approached [39]. The dislocations themselves do not represent homogeneous linear scattering centres, as originally believed [26]. Most of their scattering efficiency stems rather from point defects trapped within their strain fields, as can clearly be seen in the high-resolution image of Fig. 7. This figure shows an area containing a single dislocation (dashed line in the topography image of Fig. 7a). The point defects appear in the BEEM image of Fig. 7b like beads on a string. Whereas far away from dislocations the silicide film is under

Fig. 7. (a) STM topography image and (b) corresponding BEEM image of a 2.8 nm CoSi /n-Si(1 1 1) film, recorded with 2 » "!1.2 V and I "20 nA. Interfacial point defects have been 5 5 trapped within the dislocation D. Empty and occupied sites in the dislocation are designated by E and O, respectively. The range of gray scales is 0.2 nm in (a) and *I "260 pA in (b). #

tensile strain, because of the negative misfit, the dislocations represent regions of locally compressive strain. This allows us to interpret the observations in the following way: The point defects, which we tentatively identify with Co vacancies, diffuse along the interface during the annealing procedure and are preferentially trapped within the regions of compressive strain, i.e. at the dislocations. In view of the defected CsCl structure of CoSi which nu2 cleates from the as deposited amorphous phase, it appears indeed probable that some Co vacancies might survive the annealing process leading to fluorite CoSi . 2 4. Conclusions The results of BEEM and BEES experiments performed on epitaxial CoSi (0 0 1)/Si(0 0 1) and 2 CoSi /Si(1 1 1) interfaces have been described. The 2 former interfaces offer the possibility to correlate variations of the Schottky barrier height U , obB served by BEES, with defects characterized by TEM. One important example is dislocations with Burgers vectors b"a/4S1 1 1T, in the strain field of which the measured barrier height is lowered by almost 0.1 eV from the value U "0.74 eV measB ured at the perfect interface. When screening of the potential variations inside the semiconductor is taken into account, the measured barrier lowering corresponds to a Schottky barrier height of almost zero at the metallurgical interface. In the case of CoSi /Si(1 1 1) interfaces the Schottky barrier is 2 unaffected by interfacial defects. As a result of this, variations of the BEEM current are due only to scattering of hot carriers. Most interesting is scattering by single-point defects. They can be shown to be located at the interface by taking into account the properties of the projected band structures of Si and CoSi . This offers the possibility to apply 2 BEEM to study the diffusion of point defects along the epitaxial interfaces.

Acknowledgements Financial support by the Swiss National Science Foundation is gratefully acknowledged.

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