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LEEM observation of formation of Cu nano-islands on Si(111) surface by hydrogen termination
Surface Science 493 (2001) 381±388
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LEEM observation of formation of Cu nano-islands on Si(1 1 1) surface by hydrogen termination T. Yasue a,*, T. Koshikawa a, M. Jalochowski b, E. Bauer c a
Fundamental Electronics Research Institute and Academic Frontier Promotion Center, Osaka Electro-Communication University, 18-8 Hatsu-cho, Neyagawa, Osaka 572-8530, Japan b Institute of Physics, University of Marie Curie-Sklodowska, pl M. Curie-Sklodowskiej 1, PL 20-031 Lublin, Poland c Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA Received 21 September 2000; accepted for publication 15 February 2001
Abstract The growth of Cu on Si(1 1 1) surface with and without hydrogen termination was studied with low energy electron microscopy. On the clean surface the two-dimensional ``5 5'' incommensurate layer is ®rst formed followed by the formation of three-dimensional (3D) islands. On the hydrogen-terminated surface the formation of the ``5 5'' structure is suppressed and nano-scale 3D islands decorate the steps and domain boundaries of the d7 7 structure. Many LEED spots from the nano-islands move with electron energy, which indicates that the islands are faceted. From the analysis of the LEED pattern it is suggested that the nano-islands are the (1 1 1)-oriented b-phase Cu±Si alloy and are terminated by (1 1 1), {5 5 4} and {15 16 13} faces. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low-energy electron microscopy (LEEM); Low energy electron diraction (LEED); Growth; Copper; Silicon; Semiconducting ®lms
1. Introduction Semiconductor surfaces have many dangling bonds. Since dangling bonds are quite active, semiconductor surfaces reconstruct to reduce the number of dangling bonds. For example, the number of dangling bonds is 19 on the well-known Si(1 1 1) 7 7 reconstructed surface, which is much smaller than that of the unreconstructed ideal Si(1 1 1) surface (49 dangling bonds per 7 7 unit cell). However, the chemical reactivity of such a
*
Corresponding author. Fax: +81-72-825-4590. E-mail address: [email protected] (T. Yasue).
surface is still high because of the remaining dangling bonds. Hydrogen is the simplest element that can passivate the dangling bonds on the surface as well as in the bulk. The crystal growth processes can be modi®ed by using hydrogen termination as ®rst demonstrated by Oura's group [1]. However, the growth behavior on the hydrogen-terminated surface varies from system to system. Observation with the microscopic techniques such as low energy electron microscopy (LEEM) [2] gives direct information on the growth processes. Compared with REM, LEEM has the advantage of distortion-free imaging, so that it is easier to understand the growth processes. In addition, LEED provides structural information. Although STM is also a powerful tool, it is better
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T. Yasue et al. / Surface Science 493 (2001) 381±388
suited for the investigation of processes on the atomic level, such as nucleation etc. In the present study, we used LEEM to observe the growth of Cu on the hydrogen-terminated Si(1 1 1) surface. The growth of Cu on the clean Si(1 1 1) 7 7 surface has been studied previously with various surface analysis techniques [3±12]. At elevated temperatures between 130 and 600°C, the ``5 5'' structure is formed. The ``5 5'' structure is a complicated incommensurate structure, but is a stable phase. It has been reported that triangular and elongated islands are formed on the ``5 5'' structure [3,12]. The suppression of the formation of the ``5 5'' structure that might lead to island formation directly on the substrate is one ®rst step to modify the growth process. On an ideal hydrogen-terminated surface, there are no dangling bonds. Therefore it is expected that the reaction of Cu with Si should be strongly suppressed and surface migration might be enhanced. Then threedimensional (3D) island growth could take place directly. In the present paper, we will discuss the modi®cation of the growth process of Cu on the hydrogen-terminated surface [13,14]. The structure of the islands formed on the hydrogen-terminated surface as derived from their LEED patterns will be also discussed.
2. Experimental The specimen used was B-doped p-type Si(1 1 1) resistance > 7000 X cm whose miscut was 0:08°. The sample was ¯ashed by passing direct current through it. After ¯ashing, the clear contrast of monoatomic steps on the surface was observed in LEEM, and sharp 7 7 spots could be seen in the LEED pattern. Hydrogen gas whose partial pressure was 6:7 10 5 Pa was dissociated on a hot W ®lament placed at about 75 mm from the specimen. We have observed with MEIS that when the temperature of the W ®lament is about 1800°C, a small amount of W sublimes and contaminates the substrate. Therefore the temperature of the W ®lament was kept at about 1500°C in order to avoid the sublimation of W. The exposure time was 90
min. During exposure the Si(1 1 1) surface was kept at the same temperature as during Cu deposition in order to ensure the thermal equilibrium adsorption state of atomic hydrogen at that temperature. After exposure to atomic hydrogen, the d7 7 LEED pattern was observed for all temperatures used. Cu was evaporated from a BN crucible, and the deposition rate was about 0.55 ML/min as estimated from the completion of the Cu/Si(1 1 1) ``5 5'' structure at 1.3 ML [3,4]. The substrate temperature was estimated from the resistance. However, once Cu was deposited onto the surface, the resistance of the specimen may change because Cu can easily diuse into the bulk. Therefore the estimated temperature may include errors of several tens of °C. The growth processes were observed with a compact LEEM. The compact LEEM is a bolt-on type instrument and the designed resolution is about 15 nm. In the present work, we used a nonoptimal contrast aperture of 20 lm in diameter to obtain enough intensity, so that the resolution is several tens of nm. The details of the compact LEEM have been described elsewhere [15]. All LEEM images shown in the present paper are bright ®eld images obtained with the (0 0) beam. During LEEM observation of the hydrogen-terminated surface there is the possibility of electronstimulated desorption of hydrogen. The threshold energy of desorption of hydrogen ions is reported to be about 26 eV [16] which is much higher than the electron energy used in the LEEM studies. The ion fraction of the desorbed species, however, is generally quite small. In order to check for desorption of neutral hydrogen a hydrogen-terminated surface was exposed to a 7 eV electron beam for about 12 h at room temperature. The intensity distributions of the fractional order spots of the d7 7 structure before and after irradiation did not dier. Since a drift of the sample during irradiation could not be excluded, we observed the LEED patterns at several points of the sample. All LEED patterns were essentially the same. The LEEM image after irradiation was also the same as that before irradiation. Therefore, we conclude that there is no signi®cant electron-stimulated desorption during observation of the growth processes.
T. Yasue et al. / Surface Science 493 (2001) 381±388
3. Results and discussion The eect of hydrogen termination can be seen by a comparison of the island growth process on the clean and hydrogen-terminated surface. On the clean surface, islands form after the completion of the ``5 5'' structure whose coverage is about 1.3 ML [3,4,12]. Fig. 1 shows the LEEM images of islands formed on the clean Si(1 1 1) 7 7 surface at (a) about 280°C, (b) about 380°C and (c) about 600°C at a Cu coverage of 5.5 ML. In images triangular and elongated 3D islands are seen. The edges of the islands are along the h 1 1 0i crystallographic orientations of the Si(1 1 1) surface. Such islands have been observed previously by SEM [3]
Fig. 1. LEEM images of islands formed on the clean Si(1 1 1) surface at (a) about 280°C, (b) about 380°C and (c) about 600°C. The coverage of Cu is 5.5 ML. Electron energy used is (a,b) 4.5 eV and (c) 4.1 eV.
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and LEEM/PEEM [12]. Another type of island with a ¯at top face was also observed in the present study [14] as seen, for example, at the rightupper side in Fig. 1(b). With increasing substrate temperature, the size of the islands increases and their number density decreases. This is attributed to the temperature dependence of the nucleation rate and of the surface migration length. The contrast of islands is not uniform. Some triangular islands have stripes of dark and bright contrast along their edges while the central part of island is bright (see Fig. 1(c)), but only around 4 eV. The bright contrast can be explained by the shape of island. If an island has a ¯at top face parallel to the substrate, the (0 0) beam passes through the contrast aperture and the face appears bright whenever the (0 0) beam has high intensity. If the top face is tilted against the substrate then it will appear bright only at energies at which a non-(0 0) beam diracted from it passes through the aperture. The elongated islands were dark at all energies studied, so that they have de®nitely tilted faces as previously reported [12]. Fig. 2 shows the LEEM images of nano-islands formed on the hydrogen-terminated Si(1 1 1) d7 7 surface at (a) about 350°C, (b) about 380°C and (c) about 450°C. The coverage of Cu is 4.4 ML for (a) and (c), and 5.5 ML for (b). As seen in Fig. 2(a) and (b), the island formation is quite dierent from that on the clean surface. Nano-islands form preferentially along step edges and domain boundaries of the original hydrogen-terminated surface. In addition nano-islands grow on the ¯at terraces between the step edges and the domain boundaries. The number density of nano-islands on the terrace decreases with increasing substrate temperature because the nucleation rate decreases and the surface migration length increases with increasing substrate temperature. In the corresponding LEED patterns, the fractional order spots due to the hydrogen-terminated d7 7 structure were still observed. Therefore the region between the islands is still covered by hydrogen. The deposited Cu atoms migrate on the hydrogenterminated surface and nucleate the islands. The migration length on the hydrogen-terminated surface is sucient to reach the step edges and domain boundaries, so that the steps and domain
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Fig. 2. LEEM images of islands formed on the hydrogen-terminated Si(1 1 1) surface at (a) about 350°C, (b) about 380°C and (c) about 450°C. The coverage of Cu is (a, c) 4.4 ML, and (b) 5.5 ML. Electron energy is (a) 4.2 eV, (b) 4.5 eV and (c) 7.5 eV.
boundaries are preferentially decorated. When the terrace width is large, the nucleation on the terrace also takes place. The formation of islands at temperatures above about 400°C, however, is similar to that on the clean surface. In Fig. 2(c) triangular and elongated islands can be seen. The corresponding LEED pattern showed the ``5 5'' structure. The desorption temperature of hydrogen from Si is about 550°C [17]. Although slow desorption of hydrogen takes place at around
400°C, a large amount of hydrogen atoms should still cover the substrate surface during deposition of Cu [18]. Since the reactivity of Cu with Si is quite high, an exchange between hydrogen and Cu may occur above about 400°C for energetic reasons. It is also possible that other eects, for example the current passing through the substrate for heating and/or the electric ®eld, may assist the exchange of Cu with hydrogen. In order to determine the structure of the nanoislands on the hydrogen-terminated surface, the LEED pattern was observed as a function of coverage up to 66 ML. At low coverages clear spots due to the d7 7 structure were observed together with weak spots from the nano-islands. With increasing coverage the intensity of the d7 7 spots decreased while that from the nanoislands increased. No additional spots appeared with increasing coverage. This indicates that the structure of the nano-islands does not change in the coverage region examined. Fig. 3 shows the LEED patterns observed at a coverage of about 33 ML, in which the spots from the nano-islands can be clearly seen, on the hydrogen-terminated surface taken at (a) 3 eV, (b) 6 eV and (c) 9 eV. The substrate temperature is about 350°C. The LEEM image showed that the many nano-islands formed on the surface, however the substrate was still observed between nano-islands. The 1=7 order spots of the d7 7 structure indicated by arrows in Fig. 3(a) stay at the same position in Fig. 3(b) and (c). Spots from the nano-islands on the hydrogenterminated surface, which are visible at the energies used in Fig. 3, however, move with changing electron energy. The positions of the diraction spots observed at several electron energies are shown in Fig. 4. The large open circles indicate the positions of the fractional spots of the d7 7 structure. The small circles represent the diraction spots from the nano-islands on the hydrogenterminated surface. The electron energy increases from white to black shading. The LEED pattern shows threefold symmetry and there are two kinds of spots. One is moving along a line connecting the fundamental spots of the substrate, that is along the substrate h1 1 2i directions, and the other deviates from these directions by about 19°. The large solid circles mark spots also from the nano-
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Fig. 4. Schematic drawing of the LEED patterns observed at several electron energies. Open circles show the spots of the hydrogen-terminated Si(1 1 1) d7 7 surface. The small circles and the solid circles indicate the positions of the diraction spots from nano-islands. The electron energy increases from white (3 eV) to black (10.5 eV) shading in 1.5 eV steps.
Fig. 3. LEED patterns after deposition of 33 ML Cu on the hydrogen-terminated Si(1 1 1) surface at about 350°C. Electron energy is (a) 3 eV, (b) 6 eV and (c) 9 eV. Arrows in (a) indicate the 1=7 order spots of the hydrogen-terminated Si(1 1 1) d7 7 surface.
islands, however these spots do not move with electron energy. In a LEEM, a bias voltage of 5000 V is applied to the optics, so that the direction of the diracted beam in the LEEM is dierent from that on the sample. The observed angle h of the diracted beam from an inclined face is related to the angle
h0 of the diracted beam on the sample by 1=2 sin h sin h0 V0 =5000 , where h0 and h are measured from the normal of the substrate and V0 is the energy of electrons on the sample. The angle h0 of the diracted beam on the sample can be obtained by the Ewald construction. Since sin h0 is 1=2 for the diraction from the proportional to V0 surface parallel to the substrate, the diraction spots do not change their positions even when the electron energy on the sample is changed. For a face that is not parallel to the substrate, however, the observed angle h changes with changing the electron energy. The movement of the spots shown in Fig. 4 shows that the nano-islands have inclined faces with dierent orientations, and there are three equivalent orientations rotated by 120° with respect to each other. In Fig. 4 the spots from the nano-islands (large solid circles) which does not move with the electron energy can be observed. This indicates that there is a ¯at face parallel to the substrate on top of the islands. These spots are observed just inside of the ®rst order spots of the substrate at a reciprocal lattice distance from the (0 0) spot of about 1 . This distance is incompatible with Cu 0:285 A
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so that it has to be assigned to a Cu±Si alloy. The phase diagram of the Cu±Si system is quite complicated with several alloys [19,20]. The phase which ®ts best to the solid circles is the bodycentered cubic b-phase alloy with a lattice constant of 0.285 nm at 85.1 at.% Cu1 [19]. The reciprocal agrees within the lattice distance of 0:285 A limits of error with that expected for the (1 0) spots of a (1 1 1)-oriented b-phase crystal (the reciprocal 1 ). The b-phase is a 3:2 lattice distance is 0:286 A electron compound, in which the composition of Cu is 82.8±85.8 at.% [19,20]. The fact that the bphase forms in the bulk at high temperatures around 800°C does not speak against this interpretation because in epitaxy frequently non-equilibrium structures are formed. For the moving spots the observed angle h for the (0 0) beam from the inclined face can be represented as a function of the inclination angle u that is de®ned as the angle of the normal directions between the inclined face and the substrate. The observed angle h can be deduced from Fig. 4 with reference to those of the spots originated from the substrate. Fig. 5 shows the relationship between
Fig. 5. The relationship between the observed angle h of the (0 0) beam from the inclined face and the inclination angle u. Circles show the measured angle h in the LEED patterns.
the angle h of the (0 0) beam and the inclination angle u. Solid circles show the measured angles for the (0 0) spots moving along h1 1 2i direction and open circles those for the spots moving 19° o from these directions. The thick lines show the errors of the measured angles. From this ®gure we can estimate the inclination angles u are about 7:5° and 5° for two kinds of spots. The inclination angle of 7:5° is close to that of the [4 4 3] direction. Hereafter we call the facet normal to the [4 4 3] direction as the [4 4 3]-facet and the other facet with the same manner. The [15 16 13]-facet u 4:9° or [14 15 12]-facet u 5:2° is plausible for 19° o-directions. The observed angles for the (1 0) and (1 0) spots can be calculated as a function of the reciprocal lattice spacing, the inclination angle u of the facet and the electron energy on the sample. If the combination of the reciprocal lattice spacing and the inclination angle was not properly given, the observed angles are not reproduced. In order to obtain the best ®t of the observed angle for all electron energies, the inclination angle of around 1 are 5:5° and the lattice spacing of about 0:124 A required. This inclination angle agrees with that of the [5 5 4]-facet that is 5:8°. The inclination angle of the facet along h1 1 2i direction obtained in Fig. 5 (about 7:5°) in which the (0 0) beam is used is dierent from that obtained by using (1 0) and (1 0) spots (about 5:5°). The former is close to the angle of the [4 4 3]-facet, while the latter corresponds to the [5 5 4]-facet. Now we consider the structure of the island. Although we cannot exclude the other facets with higher inclination angle that could not be observed in the LEED patterns, the island is composed by a (1 1 1) face, three [4 4 3]- or [5 5 4]-facets and six [15 16 13]- or [14 15 12]-facets. Fig. 6 shows the structure model of an island. In this model the [4 4 3]- or [5 5 4]-facet has to connect to the [15 16 13]- or [14 15 12]-facet. The atomic arrangement should be considered in order to see such connection in details. The [4 4 3]- or [5 5 4]-facet of the b-phase alloy consists of the (1 1 1) terraces and steps along the [1 1 0] direction. The reciprocal lattice spacing depends on the terrace width (the step height) of the facet. In order to consider the reciprocal lattice spacing we have to specify
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Fig. 6. The schema of the shape of an island. The island is composed by the top (1 1 1) face, three [5 5 4]- or [4 4 3]-facets and six [15 16 13]- or [14 15 12]-facets. Note that the vertical scale in the side view is magni®ed.
the face 1index. The obtained lattice spacing of from the movement of the (1 0) and ( 1 0) 0:124 A spots is in good agreement with that of the (515 4) . In face along the [1 1 2] direction that is 0:123 A the LEED patterns another two moving spots, which are not shown in Fig. 4, were observed on each line connecting the large solid circles, that are the integer spots of the (1 1 1) face of the b-phase alloy. Although it is quite dicult to determine the exact positions of these spots because the o-normal incidence was used to observe the spots, these are close to those of the reciprocal lattice points of the (5 5 4) face. Therefore we consider the (5 5 4) face ®rst. Lower part of Fig. 7 shows the top view of the atomic arrangement of the (5 5 4) face. Solid lines indicate the step edges. The step down direction is the 1 1 2 direction of the nano-island and the steps run along the [ 1 1 0] direction. Here note that it is not clear that the 1 1 2 direction of the nano-island is parallel or anti-parallel to that of the substrate because the substrate orientation cannot be deduced from the LEED pattern. Steps running along [ 1 1 0] direction appear on many surfaces of a bcc crystal. The 19° rotated facet can be drawn to ®t to the (5 5 4) face as shown in the
Fig. 7. The top view of the atomic arrangement of the (1 1 1)oriented b-phase Cu±Si compound. The solid lines indicate the step edges toward the 1 1 2 direction. The step distance is 0.814 nm that corresponds to the (5 5 4) face. The step con®guration for the (15 16 13) face is also indicated in the upper part of the ®gure.
upper part of Fig. 7. In this case the appeared face is the (15 16 13) face. Although the inclination angle of the (14 15 12) face is also close to that estimated in Fig. 5, the (14 15 12) face does not ®t to the step arrangement of the (5 5 4) face. In the case of the [4 4 3]-facet the reciprocal lat 1 cannot be explained and tice spacing of 0:124 A both [15 16 13]- and [14 15 12]-facets do not ®t to the atomic arrangement of the [4 4 3]-facet. As discussed above, the (5 5 4) face can explain the reciprocal lattice spacing estimated from the LEED pattern and the (15 16 13) face ®ts well to the (5 5 4) face. However the observed angle of the (0 0) spot from the nano-islands is dierent from that expected from the (5 5 4) face in the LEED pattern. Instead that corresponding to the [4 4 3]facet is observed. The discrepancy is not resolved
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at present. The three equivalent atomic arrangements shown in Fig. 7 rotated by 120° with respect to each other have to be connected to construct the island. Then two or three atomic layer high steps can be easily introduced. For example, when a two atomic layer high step is introduced with every two steps, the [4 4 3]-facet appears. The another possibility to explain the discrepancy might be the vertical expansion of the atomic spacing. In order to get the inclination angle of about 7:5°, the is required. vertical expansion of about 0:25 A This expansion, however, is about 30% of the atomic spacing of the b-phase alloy and it seems too large. The further experiment such as the direct observation of the shape of the islands, the step arrangement etc., e.g. by STM, is required in order to clear the problem. 4. Conclusion The growth process of Cu on the Si(1 1 1) surface can be modi®ed by hydrogen termination of the surface dangling bonds. On the clean surface islands grow on the 2D ``5 5'' layer, while on the hydrogen-terminated surface 3D nano-islands form directly. The main role of the hydrogen termination is the suppression of the formation of the ``5 5'' structure that results in an enhancement of the surface migration length. Thus the nanoislands can decorate steps and domain boundaries. The nano-islands are the b-phase alloy and have the facet structure consisted of {5 5 4} and {15 16 13} faces. The inclination angle of {5 5 4} face, however, is not consistent with that obtained by the analysis of the movement of the LEED spots. The further study will be required in order to discuss the details. Acknowledgements This work was supported by a Grant-in-Aid for Creative Basic Research (09NP1201), a Grant-in-
Aid for Scienti®c Research (10044184) from the Ministry of Education, Science and Culture. This work was also supported by the Murata Science Foundation. The authors are grateful to Dr. H. Minoda, Tokyo Institute of Technology, for his invaluable discussions to analyze the LEED patterns. References [1] K. Oura, V.G. Lifshits, A.A. Saranin, A.V. Zotov, M. Katayama, Surf. Sci. Rep. 35 (1999) 1 and references therein. [2] E. Bauer, Rep. Prog. Phys. 57 (1994) 895. [3] E. Daugy, P. Mathiez, F. Salvan, J.M. Layet, Surf. Sci. 154 (1985) 267. [4] H. Kemmann, F. M uller, H. Neddermeyer, Surf. Sci. 192 (1987) 11. [5] J. Zegenhagen, E. Fontes, F. Grey, J.R. Patel, Phys. Rev. B 46 (1992) 1860. [6] K. Mortensen, Phys. Rev. 66 (1991) 461. [7] T. Koshikawa, T. Yasue, H. Tanaka, I. Sumita, Y. Kido, Surf. Sci. 331±333 (1995) 506. [8] T. Koshikawa, T. Yasue, H. Tanaka, I. Sumita, Y. Kido, Nucl. Instrum. Methods B 99 (1995) 495. [9] K. Yamashita, T. Yasue, T. Koshikawa, A. Ikeda, Y. Kido, Nucl. Instrum. Methods B 136±138 (1998) 1086. [10] T. Kawasaki, T. An, H. Ito, T. Ichinokawa, Surf. Sci. 487 (2001) 39. [11] K. Takayanagi, Y. Tanishiro, T. Ishitsuka, K. Akiyama, Appl. Surf. Sci. 41/42 (1989) 337. [12] M. Mundschau, E. Bauer, W. Telieps, J. Appl. Phys. 65 (1989) 4747. [13] T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer, Surf. Rev. Lett. 7 (2000) 595. [14] T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer, Surf. Sci. 480 (2001) 118. [15] P. Adamec, E. Bauer, B. Lencova, Rev. Sci. Instrum. 69 (1998) 3538. [16] T. Yasue, A. Ichimiya, S. Ohtani, Vacuum 41 (1990) 561. [17] G. Schulze, M. Henzler, Surf. Sci. 124 (1983) 336. [18] K. Murano, K. Ueda, Surf. Sci. 357±358 (1996) 910. [19] M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 629. [20] T.B. Massalski, Binary Alloy Phase Diagrams, second ed., ASM International, Ohio, 1990, p. 1477.
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LEEM observation of formation of Cu nano-islands on Si(1 1 1) surface by hydrogen termination T. Yasue a,*, T. Koshikawa a, M. Jalochowski b, E. Bauer c a
Fundamental Electronics Research Institute and Academic Frontier Promotion Center, Osaka Electro-Communication University, 18-8 Hatsu-cho, Neyagawa, Osaka 572-8530, Japan b Institute of Physics, University of Marie Curie-Sklodowska, pl M. Curie-Sklodowskiej 1, PL 20-031 Lublin, Poland c Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA Received 21 September 2000; accepted for publication 15 February 2001
Abstract The growth of Cu on Si(1 1 1) surface with and without hydrogen termination was studied with low energy electron microscopy. On the clean surface the two-dimensional ``5 5'' incommensurate layer is ®rst formed followed by the formation of three-dimensional (3D) islands. On the hydrogen-terminated surface the formation of the ``5 5'' structure is suppressed and nano-scale 3D islands decorate the steps and domain boundaries of the d7 7 structure. Many LEED spots from the nano-islands move with electron energy, which indicates that the islands are faceted. From the analysis of the LEED pattern it is suggested that the nano-islands are the (1 1 1)-oriented b-phase Cu±Si alloy and are terminated by (1 1 1), {5 5 4} and {15 16 13} faces. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Low-energy electron microscopy (LEEM); Low energy electron diraction (LEED); Growth; Copper; Silicon; Semiconducting ®lms
1. Introduction Semiconductor surfaces have many dangling bonds. Since dangling bonds are quite active, semiconductor surfaces reconstruct to reduce the number of dangling bonds. For example, the number of dangling bonds is 19 on the well-known Si(1 1 1) 7 7 reconstructed surface, which is much smaller than that of the unreconstructed ideal Si(1 1 1) surface (49 dangling bonds per 7 7 unit cell). However, the chemical reactivity of such a
*
Corresponding author. Fax: +81-72-825-4590. E-mail address: [email protected] (T. Yasue).
surface is still high because of the remaining dangling bonds. Hydrogen is the simplest element that can passivate the dangling bonds on the surface as well as in the bulk. The crystal growth processes can be modi®ed by using hydrogen termination as ®rst demonstrated by Oura's group [1]. However, the growth behavior on the hydrogen-terminated surface varies from system to system. Observation with the microscopic techniques such as low energy electron microscopy (LEEM) [2] gives direct information on the growth processes. Compared with REM, LEEM has the advantage of distortion-free imaging, so that it is easier to understand the growth processes. In addition, LEED provides structural information. Although STM is also a powerful tool, it is better
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 2 4 3 - 2
382
T. Yasue et al. / Surface Science 493 (2001) 381±388
suited for the investigation of processes on the atomic level, such as nucleation etc. In the present study, we used LEEM to observe the growth of Cu on the hydrogen-terminated Si(1 1 1) surface. The growth of Cu on the clean Si(1 1 1) 7 7 surface has been studied previously with various surface analysis techniques [3±12]. At elevated temperatures between 130 and 600°C, the ``5 5'' structure is formed. The ``5 5'' structure is a complicated incommensurate structure, but is a stable phase. It has been reported that triangular and elongated islands are formed on the ``5 5'' structure [3,12]. The suppression of the formation of the ``5 5'' structure that might lead to island formation directly on the substrate is one ®rst step to modify the growth process. On an ideal hydrogen-terminated surface, there are no dangling bonds. Therefore it is expected that the reaction of Cu with Si should be strongly suppressed and surface migration might be enhanced. Then threedimensional (3D) island growth could take place directly. In the present paper, we will discuss the modi®cation of the growth process of Cu on the hydrogen-terminated surface [13,14]. The structure of the islands formed on the hydrogen-terminated surface as derived from their LEED patterns will be also discussed.
2. Experimental The specimen used was B-doped p-type Si(1 1 1) resistance > 7000 X cm whose miscut was 0:08°. The sample was ¯ashed by passing direct current through it. After ¯ashing, the clear contrast of monoatomic steps on the surface was observed in LEEM, and sharp 7 7 spots could be seen in the LEED pattern. Hydrogen gas whose partial pressure was 6:7 10 5 Pa was dissociated on a hot W ®lament placed at about 75 mm from the specimen. We have observed with MEIS that when the temperature of the W ®lament is about 1800°C, a small amount of W sublimes and contaminates the substrate. Therefore the temperature of the W ®lament was kept at about 1500°C in order to avoid the sublimation of W. The exposure time was 90
min. During exposure the Si(1 1 1) surface was kept at the same temperature as during Cu deposition in order to ensure the thermal equilibrium adsorption state of atomic hydrogen at that temperature. After exposure to atomic hydrogen, the d7 7 LEED pattern was observed for all temperatures used. Cu was evaporated from a BN crucible, and the deposition rate was about 0.55 ML/min as estimated from the completion of the Cu/Si(1 1 1) ``5 5'' structure at 1.3 ML [3,4]. The substrate temperature was estimated from the resistance. However, once Cu was deposited onto the surface, the resistance of the specimen may change because Cu can easily diuse into the bulk. Therefore the estimated temperature may include errors of several tens of °C. The growth processes were observed with a compact LEEM. The compact LEEM is a bolt-on type instrument and the designed resolution is about 15 nm. In the present work, we used a nonoptimal contrast aperture of 20 lm in diameter to obtain enough intensity, so that the resolution is several tens of nm. The details of the compact LEEM have been described elsewhere [15]. All LEEM images shown in the present paper are bright ®eld images obtained with the (0 0) beam. During LEEM observation of the hydrogen-terminated surface there is the possibility of electronstimulated desorption of hydrogen. The threshold energy of desorption of hydrogen ions is reported to be about 26 eV [16] which is much higher than the electron energy used in the LEEM studies. The ion fraction of the desorbed species, however, is generally quite small. In order to check for desorption of neutral hydrogen a hydrogen-terminated surface was exposed to a 7 eV electron beam for about 12 h at room temperature. The intensity distributions of the fractional order spots of the d7 7 structure before and after irradiation did not dier. Since a drift of the sample during irradiation could not be excluded, we observed the LEED patterns at several points of the sample. All LEED patterns were essentially the same. The LEEM image after irradiation was also the same as that before irradiation. Therefore, we conclude that there is no signi®cant electron-stimulated desorption during observation of the growth processes.
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3. Results and discussion The eect of hydrogen termination can be seen by a comparison of the island growth process on the clean and hydrogen-terminated surface. On the clean surface, islands form after the completion of the ``5 5'' structure whose coverage is about 1.3 ML [3,4,12]. Fig. 1 shows the LEEM images of islands formed on the clean Si(1 1 1) 7 7 surface at (a) about 280°C, (b) about 380°C and (c) about 600°C at a Cu coverage of 5.5 ML. In images triangular and elongated 3D islands are seen. The edges of the islands are along the h 1 1 0i crystallographic orientations of the Si(1 1 1) surface. Such islands have been observed previously by SEM [3]
Fig. 1. LEEM images of islands formed on the clean Si(1 1 1) surface at (a) about 280°C, (b) about 380°C and (c) about 600°C. The coverage of Cu is 5.5 ML. Electron energy used is (a,b) 4.5 eV and (c) 4.1 eV.
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and LEEM/PEEM [12]. Another type of island with a ¯at top face was also observed in the present study [14] as seen, for example, at the rightupper side in Fig. 1(b). With increasing substrate temperature, the size of the islands increases and their number density decreases. This is attributed to the temperature dependence of the nucleation rate and of the surface migration length. The contrast of islands is not uniform. Some triangular islands have stripes of dark and bright contrast along their edges while the central part of island is bright (see Fig. 1(c)), but only around 4 eV. The bright contrast can be explained by the shape of island. If an island has a ¯at top face parallel to the substrate, the (0 0) beam passes through the contrast aperture and the face appears bright whenever the (0 0) beam has high intensity. If the top face is tilted against the substrate then it will appear bright only at energies at which a non-(0 0) beam diracted from it passes through the aperture. The elongated islands were dark at all energies studied, so that they have de®nitely tilted faces as previously reported [12]. Fig. 2 shows the LEEM images of nano-islands formed on the hydrogen-terminated Si(1 1 1) d7 7 surface at (a) about 350°C, (b) about 380°C and (c) about 450°C. The coverage of Cu is 4.4 ML for (a) and (c), and 5.5 ML for (b). As seen in Fig. 2(a) and (b), the island formation is quite dierent from that on the clean surface. Nano-islands form preferentially along step edges and domain boundaries of the original hydrogen-terminated surface. In addition nano-islands grow on the ¯at terraces between the step edges and the domain boundaries. The number density of nano-islands on the terrace decreases with increasing substrate temperature because the nucleation rate decreases and the surface migration length increases with increasing substrate temperature. In the corresponding LEED patterns, the fractional order spots due to the hydrogen-terminated d7 7 structure were still observed. Therefore the region between the islands is still covered by hydrogen. The deposited Cu atoms migrate on the hydrogenterminated surface and nucleate the islands. The migration length on the hydrogen-terminated surface is sucient to reach the step edges and domain boundaries, so that the steps and domain
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Fig. 2. LEEM images of islands formed on the hydrogen-terminated Si(1 1 1) surface at (a) about 350°C, (b) about 380°C and (c) about 450°C. The coverage of Cu is (a, c) 4.4 ML, and (b) 5.5 ML. Electron energy is (a) 4.2 eV, (b) 4.5 eV and (c) 7.5 eV.
boundaries are preferentially decorated. When the terrace width is large, the nucleation on the terrace also takes place. The formation of islands at temperatures above about 400°C, however, is similar to that on the clean surface. In Fig. 2(c) triangular and elongated islands can be seen. The corresponding LEED pattern showed the ``5 5'' structure. The desorption temperature of hydrogen from Si is about 550°C [17]. Although slow desorption of hydrogen takes place at around
400°C, a large amount of hydrogen atoms should still cover the substrate surface during deposition of Cu [18]. Since the reactivity of Cu with Si is quite high, an exchange between hydrogen and Cu may occur above about 400°C for energetic reasons. It is also possible that other eects, for example the current passing through the substrate for heating and/or the electric ®eld, may assist the exchange of Cu with hydrogen. In order to determine the structure of the nanoislands on the hydrogen-terminated surface, the LEED pattern was observed as a function of coverage up to 66 ML. At low coverages clear spots due to the d7 7 structure were observed together with weak spots from the nano-islands. With increasing coverage the intensity of the d7 7 spots decreased while that from the nanoislands increased. No additional spots appeared with increasing coverage. This indicates that the structure of the nano-islands does not change in the coverage region examined. Fig. 3 shows the LEED patterns observed at a coverage of about 33 ML, in which the spots from the nano-islands can be clearly seen, on the hydrogen-terminated surface taken at (a) 3 eV, (b) 6 eV and (c) 9 eV. The substrate temperature is about 350°C. The LEEM image showed that the many nano-islands formed on the surface, however the substrate was still observed between nano-islands. The 1=7 order spots of the d7 7 structure indicated by arrows in Fig. 3(a) stay at the same position in Fig. 3(b) and (c). Spots from the nano-islands on the hydrogenterminated surface, which are visible at the energies used in Fig. 3, however, move with changing electron energy. The positions of the diraction spots observed at several electron energies are shown in Fig. 4. The large open circles indicate the positions of the fractional spots of the d7 7 structure. The small circles represent the diraction spots from the nano-islands on the hydrogenterminated surface. The electron energy increases from white to black shading. The LEED pattern shows threefold symmetry and there are two kinds of spots. One is moving along a line connecting the fundamental spots of the substrate, that is along the substrate h1 1 2i directions, and the other deviates from these directions by about 19°. The large solid circles mark spots also from the nano-
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Fig. 4. Schematic drawing of the LEED patterns observed at several electron energies. Open circles show the spots of the hydrogen-terminated Si(1 1 1) d7 7 surface. The small circles and the solid circles indicate the positions of the diraction spots from nano-islands. The electron energy increases from white (3 eV) to black (10.5 eV) shading in 1.5 eV steps.
Fig. 3. LEED patterns after deposition of 33 ML Cu on the hydrogen-terminated Si(1 1 1) surface at about 350°C. Electron energy is (a) 3 eV, (b) 6 eV and (c) 9 eV. Arrows in (a) indicate the 1=7 order spots of the hydrogen-terminated Si(1 1 1) d7 7 surface.
islands, however these spots do not move with electron energy. In a LEEM, a bias voltage of 5000 V is applied to the optics, so that the direction of the diracted beam in the LEEM is dierent from that on the sample. The observed angle h of the diracted beam from an inclined face is related to the angle
h0 of the diracted beam on the sample by 1=2 sin h sin h0 V0 =5000 , where h0 and h are measured from the normal of the substrate and V0 is the energy of electrons on the sample. The angle h0 of the diracted beam on the sample can be obtained by the Ewald construction. Since sin h0 is 1=2 for the diraction from the proportional to V0 surface parallel to the substrate, the diraction spots do not change their positions even when the electron energy on the sample is changed. For a face that is not parallel to the substrate, however, the observed angle h changes with changing the electron energy. The movement of the spots shown in Fig. 4 shows that the nano-islands have inclined faces with dierent orientations, and there are three equivalent orientations rotated by 120° with respect to each other. In Fig. 4 the spots from the nano-islands (large solid circles) which does not move with the electron energy can be observed. This indicates that there is a ¯at face parallel to the substrate on top of the islands. These spots are observed just inside of the ®rst order spots of the substrate at a reciprocal lattice distance from the (0 0) spot of about 1 . This distance is incompatible with Cu 0:285 A
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so that it has to be assigned to a Cu±Si alloy. The phase diagram of the Cu±Si system is quite complicated with several alloys [19,20]. The phase which ®ts best to the solid circles is the bodycentered cubic b-phase alloy with a lattice constant of 0.285 nm at 85.1 at.% Cu1 [19]. The reciprocal agrees within the lattice distance of 0:285 A limits of error with that expected for the (1 0) spots of a (1 1 1)-oriented b-phase crystal (the reciprocal 1 ). The b-phase is a 3:2 lattice distance is 0:286 A electron compound, in which the composition of Cu is 82.8±85.8 at.% [19,20]. The fact that the bphase forms in the bulk at high temperatures around 800°C does not speak against this interpretation because in epitaxy frequently non-equilibrium structures are formed. For the moving spots the observed angle h for the (0 0) beam from the inclined face can be represented as a function of the inclination angle u that is de®ned as the angle of the normal directions between the inclined face and the substrate. The observed angle h can be deduced from Fig. 4 with reference to those of the spots originated from the substrate. Fig. 5 shows the relationship between
Fig. 5. The relationship between the observed angle h of the (0 0) beam from the inclined face and the inclination angle u. Circles show the measured angle h in the LEED patterns.
the angle h of the (0 0) beam and the inclination angle u. Solid circles show the measured angles for the (0 0) spots moving along h1 1 2i direction and open circles those for the spots moving 19° o from these directions. The thick lines show the errors of the measured angles. From this ®gure we can estimate the inclination angles u are about 7:5° and 5° for two kinds of spots. The inclination angle of 7:5° is close to that of the [4 4 3] direction. Hereafter we call the facet normal to the [4 4 3] direction as the [4 4 3]-facet and the other facet with the same manner. The [15 16 13]-facet u 4:9° or [14 15 12]-facet u 5:2° is plausible for 19° o-directions. The observed angles for the (1 0) and (1 0) spots can be calculated as a function of the reciprocal lattice spacing, the inclination angle u of the facet and the electron energy on the sample. If the combination of the reciprocal lattice spacing and the inclination angle was not properly given, the observed angles are not reproduced. In order to obtain the best ®t of the observed angle for all electron energies, the inclination angle of around 1 are 5:5° and the lattice spacing of about 0:124 A required. This inclination angle agrees with that of the [5 5 4]-facet that is 5:8°. The inclination angle of the facet along h1 1 2i direction obtained in Fig. 5 (about 7:5°) in which the (0 0) beam is used is dierent from that obtained by using (1 0) and (1 0) spots (about 5:5°). The former is close to the angle of the [4 4 3]-facet, while the latter corresponds to the [5 5 4]-facet. Now we consider the structure of the island. Although we cannot exclude the other facets with higher inclination angle that could not be observed in the LEED patterns, the island is composed by a (1 1 1) face, three [4 4 3]- or [5 5 4]-facets and six [15 16 13]- or [14 15 12]-facets. Fig. 6 shows the structure model of an island. In this model the [4 4 3]- or [5 5 4]-facet has to connect to the [15 16 13]- or [14 15 12]-facet. The atomic arrangement should be considered in order to see such connection in details. The [4 4 3]- or [5 5 4]-facet of the b-phase alloy consists of the (1 1 1) terraces and steps along the [1 1 0] direction. The reciprocal lattice spacing depends on the terrace width (the step height) of the facet. In order to consider the reciprocal lattice spacing we have to specify
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Fig. 6. The schema of the shape of an island. The island is composed by the top (1 1 1) face, three [5 5 4]- or [4 4 3]-facets and six [15 16 13]- or [14 15 12]-facets. Note that the vertical scale in the side view is magni®ed.
the face 1index. The obtained lattice spacing of from the movement of the (1 0) and ( 1 0) 0:124 A spots is in good agreement with that of the (515 4) . In face along the [1 1 2] direction that is 0:123 A the LEED patterns another two moving spots, which are not shown in Fig. 4, were observed on each line connecting the large solid circles, that are the integer spots of the (1 1 1) face of the b-phase alloy. Although it is quite dicult to determine the exact positions of these spots because the o-normal incidence was used to observe the spots, these are close to those of the reciprocal lattice points of the (5 5 4) face. Therefore we consider the (5 5 4) face ®rst. Lower part of Fig. 7 shows the top view of the atomic arrangement of the (5 5 4) face. Solid lines indicate the step edges. The step down direction is the 1 1 2 direction of the nano-island and the steps run along the [ 1 1 0] direction. Here note that it is not clear that the 1 1 2 direction of the nano-island is parallel or anti-parallel to that of the substrate because the substrate orientation cannot be deduced from the LEED pattern. Steps running along [ 1 1 0] direction appear on many surfaces of a bcc crystal. The 19° rotated facet can be drawn to ®t to the (5 5 4) face as shown in the
Fig. 7. The top view of the atomic arrangement of the (1 1 1)oriented b-phase Cu±Si compound. The solid lines indicate the step edges toward the 1 1 2 direction. The step distance is 0.814 nm that corresponds to the (5 5 4) face. The step con®guration for the (15 16 13) face is also indicated in the upper part of the ®gure.
upper part of Fig. 7. In this case the appeared face is the (15 16 13) face. Although the inclination angle of the (14 15 12) face is also close to that estimated in Fig. 5, the (14 15 12) face does not ®t to the step arrangement of the (5 5 4) face. In the case of the [4 4 3]-facet the reciprocal lat 1 cannot be explained and tice spacing of 0:124 A both [15 16 13]- and [14 15 12]-facets do not ®t to the atomic arrangement of the [4 4 3]-facet. As discussed above, the (5 5 4) face can explain the reciprocal lattice spacing estimated from the LEED pattern and the (15 16 13) face ®ts well to the (5 5 4) face. However the observed angle of the (0 0) spot from the nano-islands is dierent from that expected from the (5 5 4) face in the LEED pattern. Instead that corresponding to the [4 4 3]facet is observed. The discrepancy is not resolved
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at present. The three equivalent atomic arrangements shown in Fig. 7 rotated by 120° with respect to each other have to be connected to construct the island. Then two or three atomic layer high steps can be easily introduced. For example, when a two atomic layer high step is introduced with every two steps, the [4 4 3]-facet appears. The another possibility to explain the discrepancy might be the vertical expansion of the atomic spacing. In order to get the inclination angle of about 7:5°, the is required. vertical expansion of about 0:25 A This expansion, however, is about 30% of the atomic spacing of the b-phase alloy and it seems too large. The further experiment such as the direct observation of the shape of the islands, the step arrangement etc., e.g. by STM, is required in order to clear the problem. 4. Conclusion The growth process of Cu on the Si(1 1 1) surface can be modi®ed by hydrogen termination of the surface dangling bonds. On the clean surface islands grow on the 2D ``5 5'' layer, while on the hydrogen-terminated surface 3D nano-islands form directly. The main role of the hydrogen termination is the suppression of the formation of the ``5 5'' structure that results in an enhancement of the surface migration length. Thus the nanoislands can decorate steps and domain boundaries. The nano-islands are the b-phase alloy and have the facet structure consisted of {5 5 4} and {15 16 13} faces. The inclination angle of {5 5 4} face, however, is not consistent with that obtained by the analysis of the movement of the LEED spots. The further study will be required in order to discuss the details. Acknowledgements This work was supported by a Grant-in-Aid for Creative Basic Research (09NP1201), a Grant-in-
Aid for Scienti®c Research (10044184) from the Ministry of Education, Science and Culture. This work was also supported by the Murata Science Foundation. The authors are grateful to Dr. H. Minoda, Tokyo Institute of Technology, for his invaluable discussions to analyze the LEED patterns. References [1] K. Oura, V.G. Lifshits, A.A. Saranin, A.V. Zotov, M. Katayama, Surf. Sci. Rep. 35 (1999) 1 and references therein. [2] E. Bauer, Rep. Prog. Phys. 57 (1994) 895. [3] E. Daugy, P. Mathiez, F. Salvan, J.M. Layet, Surf. Sci. 154 (1985) 267. [4] H. Kemmann, F. M uller, H. Neddermeyer, Surf. Sci. 192 (1987) 11. [5] J. Zegenhagen, E. Fontes, F. Grey, J.R. Patel, Phys. Rev. B 46 (1992) 1860. [6] K. Mortensen, Phys. Rev. 66 (1991) 461. [7] T. Koshikawa, T. Yasue, H. Tanaka, I. Sumita, Y. Kido, Surf. Sci. 331±333 (1995) 506. [8] T. Koshikawa, T. Yasue, H. Tanaka, I. Sumita, Y. Kido, Nucl. Instrum. Methods B 99 (1995) 495. [9] K. Yamashita, T. Yasue, T. Koshikawa, A. Ikeda, Y. Kido, Nucl. Instrum. Methods B 136±138 (1998) 1086. [10] T. Kawasaki, T. An, H. Ito, T. Ichinokawa, Surf. Sci. 487 (2001) 39. [11] K. Takayanagi, Y. Tanishiro, T. Ishitsuka, K. Akiyama, Appl. Surf. Sci. 41/42 (1989) 337. [12] M. Mundschau, E. Bauer, W. Telieps, J. Appl. Phys. 65 (1989) 4747. [13] T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer, Surf. Rev. Lett. 7 (2000) 595. [14] T. Yasue, T. Koshikawa, M. Jalochowski, E. Bauer, Surf. Sci. 480 (2001) 118. [15] P. Adamec, E. Bauer, B. Lencova, Rev. Sci. Instrum. 69 (1998) 3538. [16] T. Yasue, A. Ichimiya, S. Ohtani, Vacuum 41 (1990) 561. [17] G. Schulze, M. Henzler, Surf. Sci. 124 (1983) 336. [18] K. Murano, K. Ueda, Surf. Sci. 357±358 (1996) 910. [19] M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, p. 629. [20] T.B. Massalski, Binary Alloy Phase Diagrams, second ed., ASM International, Ohio, 1990, p. 1477.