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Initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces
Surface Science 492 (2001) 34±40
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Initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces Kang-Ho Park *, Jeong Sook Ha, Wan Soo Yun Basic Research Laboratory, Electronics and Telecommunications Research Institute, Yusong P.O. Box 106, Taejon 305-600, South Korea Received 2 January 2001; accepted for publication 15 May 2001
Abstract The initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces were investigated with a scanning tunneling microscopy/spectroscopy. Due to the saturation of Si dangling bonds by Sb adatoms, nucleation sites for Fe growth are highly reduced on Sb-terminated Si(1 0 0) surfaces. We ®nd that the sizes of islands grown on Sb-terminated surfaces are larger but the island densities are smaller than those grown on clean surfaces. Through the local I±V measurements on Fe islands, it was found that the conduction properties gradually changed from semiconducting to metallic as Fe coverage increased. It was explained in terms of the coalescence of Fe clusters and the formation of bulk metallic Fe ®lms. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface structure, morphology, roughness, and topography; Iron; Silicon; Antimony
1. Introduction The initial stage of metal thin ®lm growth on the Si surfaces has been studied extensively. In particular the growth of Ag ®lms on Si surfaces, which is a standard example of a metal-semiconductor interface with no intermixing, has been thoroughly investigated [1,2]. It was reported that the growth mode was drastically modi®ed with the help of surfactant layer such as Sb or H element [3±6] and the elaborate manipulation of Ag clusters with a scanning tunneling microscopy (STM) tip was possible due to the altered surface properties [7,8]. * Corresponding author. Tel.: +82-428606028; fax: +82428606836. E-mail address: [email protected] (K.-H. Park).
However, the initial stage of metal silicide formation on the passivated Si surfaces and the eects of surfactant or interfactant adsorbates on those strongly reactive systems have been rarely explored [9]. Recently, strong attention is paid to the growth of Fe thin ®lm on the passivated semiconductor surfaces because the growth of Fe thin ®lms without silicide formation is a challenging goal for the realization of Si based spin electronics [10,11]. We have done STM/STS measurements on the initial stages of the Fe growth on clean and the Sbterminated Si(1 0 0) surfaces. STM images showed that three dimensional (3-D) Fe islands are formed on Sb-terminated surfaces without introduction of a complete wetting layer at the initial stage. Island densities are smaller than those grown on clean Si
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 3 8 2 - 6
K.-H. Park et al. / Surface Science 492 (2001) 34±40
surfaces, whereas the sizes are larger. It is attributed to the elimination of nucleation sites by the passivation of Si dangling bonds. Actually in the early stages of Fe growth on clean Si(1 0 0) surfaces, the concentration of nucleation sites is dense and continuous ®lms evolve via the increase and merging of each islands with further growth. Similarly to the growth of Ag ®lms on Sb-terminated Si surfaces [5,6], it was found that no wetting layer was formed in the growth of Fe ®lms because of the existence of an Sb passivation layer. However, the detailed features are dierent from the Ag growths. Ultrasmall clusters are formed at the early stages and their shapes are not regular in contrast to the Ag clusters in Ag/Sb/Si system, which indicates the reactive interaction between Fe and Si and the lower mobility of Fe on Si surfaces. Through the local I±V measurements on Fe islands on clean and Sb-terminated Si(1 0 0) surfaces, it was found that the conduction properties of Fe islands gradually changed from semiconducting to metallic with no band gap as Fe coverage increased. The onset thickness of metallic conduction behavior on clean and the Sb-terminated surface is This behavior is explained in terms of the about 6 A. gradual contact between adjacent Fe clusters and the formation of electrical conduction channels be tween the connected clusters at the coverage of 6 A. 2. Experiment In order to obtain a clean Si surface, p-type Si(1 0 0) with a resistivity of 1 X cm was repeatedly ¯ashed at 1200°C under the ultra-high vacuum (UHV) condition of 1 10 10 Torr. As a passivation adsorbate, Sb was deposited on the clean Si(1 0 0) surface at 375°C up to 1±2 monolayers (MLs) coverage, and then the sample was annealed for several minutes at 550°C in order to form a well-ordered Sb-terminated Si(1 0 0) surface. As a result, not only the dangling bonds of Si are fully passivated by a dimerized Sb adlayer [6], but also the outermost Si layer showed bulk-like properties via the structural change from a 2 1 dimer reconstruction to bulk-like 1 1 [12]. Fe was deposited on the Sb-terminated Si(1 0 0) sample at a deposition rate of 0.2±0.4 A/min at
35
room temperature. Fe was thermally evaporated from a hot tungsten ®lament at 5 10 10 Torr, and the coverage was monitored with a quartz crystal oscillator (INFICON). The sample was held at room temperature during evaporation. The thickness of the Fe ®lms was nominally estimated assuming perfect sticking of Fe on the surface. We ignored the apparent broadening of Fe islands by a tip convolution eect when measuring the sizes of Fe islands, hence the actual lateral sizes might be slightly smaller than the measured ones. The local tunneling spectra were measured under a z toward the sample after displacement of 1 A disconnecting the feedback loop at selected spots. The sweep time for single I±V measurement was set to be about 1 s in order to raise S=N ratio. The total resolution of the tunneling spectrum was set at 74 meV after smoothing. All STM/STS measurements were performed under the pressure of 1 10 10 Torr at room temperature. Tip cleaning process is very important for the reliable STM spectroscopy data. W tip was sharpened by the electrochemical etching and then transferred to the vacuum chamber. The contaminant of tip edge was removed by the fast scanning at a high bias voltage of 5±7 V. The cleanliness and sharpness of the tip were con®rmed by the atomic resolution image and by spectroscopy of clean Si surfaces. Occasionally we cleaned the tip edge by the irradiation of the high energy electron beam from hot tungsten ®laments, but it is not an necessary procedure for atomic resolution image and spectroscopy. 3. Results and discussion 3.1. Formation of Fe silicide on clean Si(1 0 0) surfaces The initial stages of Fe silicide formation on clean Si(1 0 0)-2 1 surfaces are shown in Fig. 1. Very small sized clusters appear on the surface The sizes of the with a ®lm thickness of 0.5 A. in diameter and 0.2±2.0 silicide clusters are 8±18 A in height at the Fe coverage of 0.5 A. It should A be noted that the nucleated silicide clusters are uniformly and regularly distributed on the surface.
36
K.-H. Park et al. / Surface Science 492 (2001) 34±40
(sample bias voltage VS Fig. 1. STM images of Fe ®lms grown on Si(1 0 0)-2 1 surfaces at the thicknesses of (a) 0.5 A (VS 2:5 V), (c) 2.0 A (VS 2:5 V), and (d) 6.0 A (VS 2:5 V), scan area 300 300 A 2 . 1.0 A
The linearly aligned clusters matched with the underlying 2 1 dimer rows. The spacing between the rows of aligned clusters is estimated to be two times of surface lattice unit length, 2a0 ( 7.68 A). Fe seems to be reactively bonded with the underlying Si atoms similarly to the previous reports of Miranda group [13±17]. We could not identify the chemical elements of the clusters simply by STM images. In the Fe growth on Si(1 1 1), it was analyzed that outer surface is composed of Si atoms because of the intermixing reaction between Fe and Si [17]. As reported in many reports, various Fe-silicide complexes are formed at the submonolayer coverages depending on the substrates at room temperature [13±17]. For instance, they are assigned as FeSi on a Si(1 1 1) surface [17], but Fe3 Si on a Si(1 0 0) surface by the comparison of density of states from UPS measurements [15,16].
2:0 V), (b)
As more Fe is deposited, the sizes of silicide clusters increase, and they coalesce into the larger ones resulting in the formation of elongated silicide islands as shown in Fig. 1(d). The size distribution ®lm is 7±25 (14±32) A in diameter of 1.0 (2.0) A and 0.5±3.7 (0.9±3.0) A in height, respectively. We measured the local I±V characteristics of various Fe ®lms with a variation of measurement positions. The local I±V characteristics measured on Fe clusters changed with the coverage. Fig. 2 shows the typical I±V characteristics of Fe clusters The forbidden on the ®lms of 0.5, 2.0, and 6.0 A. gap of 1 eV still existed on Fe clusters of the ®lm despite of the band gap narrowing, and 0.5 A whereas it comit decrease to 0.5 eV at 2.0 A ®lm. In the previous pletely disappeared at a 6.0 A UPS measurements, there was noticeable density of states at the Fermi level indicating the appearance
K.-H. Park et al. / Surface Science 492 (2001) 34±40
37
3.2. Growth of Fe ®lm on Sb-terminated Si(1 0 0) surface
Fig. 2. The local I±V characteristics on clusters at Fe ®lms on clean Si(1 0 0) surfaces with a variation of the thickness. The dot (dotted line), 2.0 and solid lines denote the data for 0.5 A (thin line), 6.0 A (thick line), respectively. A
of metallic silicide phase even at 1 ML (0.83 A) [15,16]. But it was reported that the line shape of bulk metallic Fe valence band is only obtained at the coverage of 5 ML [15]. Even if the metallic phase is locally formed on the Si surface, semiconducting gap feature in the I±V measurement is still revealed because of the local Schottky barrier formation. The change of conductivity from semiconducting to metallic in I±V measurement is considered to result from the onset of surface conduction channels by the contact between the adjacent Fe islands similarly to Ag growths. I±V characteristics are considered to reveal metallic properties when clusters begin to merge at a thick According to the UPS analysis of ness of 6.0 A. Miranda group [15], it is considered that continuous bulk metallic Fe ®lm is formed near 5 ML (4.2 and our metallic I±V characteristics appear as a A) result of it.
When Fe was deposited on a Sb-terminated Si(1 0 0) surface, small islands were formed at the early stages without a Fe wetting layer. Fig. 3(a) shows nucleated islands on the Sb-terminated The sizes Si(1 0 0) surface at a thickness of 0.5 A. of islands ranged from 11 to 24 A in diameter and in height. These islands were from 0.8 to 2.5 A separated from each other. The irregularly shaped islands as well as round ones were distributed on the Sb-terminated Si(1 0 0) surface. The islands reside on the sites between the rows of voids and rectangular pits similarly to the Ag growth on Sbterminated Si(1 0 0) surfaces as shown in Fig. 3(a) [6]. It should be noted that the islands are not formed right on the Sb dimer voids or within the dark pits. The underlying Sb-terminated Si(1 0 0) surface composed of linear Sb dimer row `A', dimer voids `B', and dark rectangular pits `C', which are observed even at the higher coverage as shown in the Fig. 3(b). Preferential growth of the islands on the topmost layer is commonly observed in Ag and Fe growth on Sb-terminated Si(1 0 0). As the Fe thickness increases, the lateral and vertical sizes of clusters become larger as shown in Fig. 3(b) and the sizes of islands are (c). At a thickness of 1.0 A, in diameter and 0.9±4.2 A in height. At 13±28 A they became 14±40 A in a thickness of 2.0 A, in height, respectively. The diameter and 1.1±4.2 A sizes of clusters grown on Sb-terminated Si(1 0 0) were much larger than those grown on the clean Si(1 0 0) at similar Fe thickness. The morphological evolution of Fe ®lms is similar to that observed in the Ag growth on Sb-terminated Si(1 0 0) surfaces [6] at room temperature except for the small cluster sizes and the irregular shapes. In the initial stage, the ®lm consists of isolated and round clusters formed on the Sb overlayer. As more metal is deposited, the size and density grow. At a later stage with a Fe thickness metal clusters coalesce into elonhigher than 6 A, gated ones, implying the contact between metal clusters as shown in Fig. 3(d). Since the chemical elements cannot be identi®ed by STM image, the composition of clusters cannot be de®nitely addressed. In the Fe growth on the Si(1 1 1) and
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K.-H. Park et al. / Surface Science 492 (2001) 34±40
(sample bias voltage VS Fig. 3. STM images of Fe ®lms grown on Sb-terminated Si(1 0 0) surfaces at Fe thicknesses of (a) 0.5 A (VS 2:5 V), (c) 2.0 A (VS 2:5 V), and (d) 6.0 A (VS 2:5 V), scan area 300 300 A 2 . 3:0 V), (b) 1.0 A
Si(1 0 0) surfaces, the strong intermixing reaction of Fe and Si occurs resulting in the formation of various silicide layer with a variation of the thickness and temperature [13,15±17]. From the morphological evolution, we ®nd that the nucleation sites are greatly reduced due to the Sb-passivation and the intermixing reaction between Fe and Si should be suppressed due to the existence of Sb layer. In order to estimate the bonding strength between the cluster and substrate, we performed desorption experiment of Fe clusters on Sb-terminated surfaces by STM manipulation technique [7,8]. However, we could not detach them from the surface by application of moderate ®eld or by the formation of point contact. This implies that the bonding strength is larger than in the Ag/Sb/Si case. In Fig. 4, changes of the average island diameter and the density on clean and Sb-terminated
Si(1 0 0) surfaces are recorded with a variation of Fe thickness. The size is larger but the density is smaller on the Sb-terminated surface. It should be noted that the island density of Fe ®lm on a clean Si(1 0 0) surface decreases as coverage increases. This feature is strikingly dierent from the growth on Sb-terminated Si(1 0 0) surfaces where the island density increases with increasing thickness. Such behavior stems from the surface passivation by Sb elements. The local I±V characteristics are very similar to the case of Fe growth on clean Si(1 0 0) surfaces. Fig. 5(a) shows typical I±V characteristics of Fe The clusters on the ®lms of 0.5, 4.0, and 6.0 A. forbidden gap of 1 eV still existed on Fe clusters ®lm, and it decreased to 0.5 eV at of the 0.5 A whereas it completely disappeared at a 6.0 4.0 A ®lm similarly to the case of Fe ®lms on clean A
K.-H. Park et al. / Surface Science 492 (2001) 34±40
Fig. 4. The average island diameter and island density of Fe clusters as a function of Fe ®lm thickness on clean and Sbterminated Si(1 0 0) surfaces.
Si(1 0 0) surfaces. The overall characters are nearly the same in most of the Fe clusters within a ®lm except for the slight change (0.1 eV) of gap size. The change of conductivity from semiconducting to metallic is also considered to result from the onset of surface conduction channels by contact between adjacent Fe clusters at a thickness of 6.0 The underlying Sb is not considered to make a A. signi®cant eect on the local I±V characteristics, since we could not ®nd the apparent dierence of I±V characteristics between clean and Sb-terminated surfaces. 4. Summary We have found that nanometer sized Fe clusters were formed in the initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces. The Sbterminated Si(1 0 0) surfaces are found to be more resistive to the silicide formation than the clean Si(1 0 0) surface since the nucleation sites are reduced due to the saturation of reactive Si dangling
39
Fig. 5. The local I±V characteristics on clusters at Fe ®lms with a variation of thickness. The dot and solid lines denote the data (dotted line), 4.0 A (thin line), 6.0 A (thick line), refor 0.5 A spectively.
bonds by Sb. However, it is necessary to analyze the surface/interface of Fe silicide ®lms in order to get the detailed compositional information. The conduction properties of Fe clusters gradually changed from semiconducting to metallic with no band gap as Fe coverage increased. Acknowledgements This work has been supported by the Ministry of Informations and Communications and Ministry of Commerce, Industry and Energy, Korea. References [1] G. Le Lay, Surf. Sci. 132 (1983) 169. [2] E.-J. van Loenen, M. Iwami, R.M. Tromp, J.F. van der Veen, Surf. Sci. 137 (1984) 1.
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[3] K. Sumitomo, T. Kobayashi, F. Shoji, K. Oura, I. Katayama, Phys. Rev. Lett. 66 (1991) 1193. [4] M. Naitoh, F. Shoji, K. Oura, Surf. Sci. 242 (1991) 152. [5] K.-H. Park, J.S. Ha, S.-J. Park, E.-H. Lee, Surf. Sci. 380 (1997) 258. [6] K.-H. Park, J.S. Ha, W.S. Yun, E.-H. Lee, Surf. Sci. 415 (1998) 320. [7] K.-H. Park, J.S. Ha, W.S. Yun, E.-H. Lee, J. Vac. Sci. Technol. A 17 (1999) 1441. [8] K.-H. Park, J.S. Ha, W.S. Yun, Y.-J. Ko, Jpn. J. Appl. Phys. 39 (2000) 4629. [9] M. Copel, R.M. Tromp, Appl. Phys. Lett. 65 (1994) 3102. [10] P. Ma, G.W. Anderson, P.R. Norton, Surf. Sci. 420 (1999) 134.
[11] F. Zavaliche, W. Wulfhekel, H. Xu, J. Kirschner, J. Appl. Phys. 88 (2000) 5289. [12] D.H. Rich, T. Miller, G.E. Franklin, T.-C. Chiang, Phys. Rev. B 39 (1989) 1438. [13] J.M. Gallego, R. Miranda, J. Appl. Phys. 69 (1991) 1377. [14] A.L. Vazquez de Parga, J. de la Figuera, C. Ocal, R. Miranda, Europhys. Lett. 18 (1992) 595. [15] J. Alvarez, J.J. Hinarejos, E.G. Michel, G.R. Castro, R. Miranda, Phys. Rev. B 45 (1992) 14042. [16] J.M. Gallego, J.M. Garcõa, J. Alvarez, R. Miranda, Phys. Rev. B 46 (1992) 13339. [17] J. Alvarez, A.L. Vazquez de Parga, J.J. Hinarejos, J. de la Figuera, E.G. Michel, C. Ocal, R. Miranda, Phys. Rev. B 47 (1993) 16048.
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Initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces Kang-Ho Park *, Jeong Sook Ha, Wan Soo Yun Basic Research Laboratory, Electronics and Telecommunications Research Institute, Yusong P.O. Box 106, Taejon 305-600, South Korea Received 2 January 2001; accepted for publication 15 May 2001
Abstract The initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces were investigated with a scanning tunneling microscopy/spectroscopy. Due to the saturation of Si dangling bonds by Sb adatoms, nucleation sites for Fe growth are highly reduced on Sb-terminated Si(1 0 0) surfaces. We ®nd that the sizes of islands grown on Sb-terminated surfaces are larger but the island densities are smaller than those grown on clean surfaces. Through the local I±V measurements on Fe islands, it was found that the conduction properties gradually changed from semiconducting to metallic as Fe coverage increased. It was explained in terms of the coalescence of Fe clusters and the formation of bulk metallic Fe ®lms. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Surface structure, morphology, roughness, and topography; Iron; Silicon; Antimony
1. Introduction The initial stage of metal thin ®lm growth on the Si surfaces has been studied extensively. In particular the growth of Ag ®lms on Si surfaces, which is a standard example of a metal-semiconductor interface with no intermixing, has been thoroughly investigated [1,2]. It was reported that the growth mode was drastically modi®ed with the help of surfactant layer such as Sb or H element [3±6] and the elaborate manipulation of Ag clusters with a scanning tunneling microscopy (STM) tip was possible due to the altered surface properties [7,8]. * Corresponding author. Tel.: +82-428606028; fax: +82428606836. E-mail address: [email protected] (K.-H. Park).
However, the initial stage of metal silicide formation on the passivated Si surfaces and the eects of surfactant or interfactant adsorbates on those strongly reactive systems have been rarely explored [9]. Recently, strong attention is paid to the growth of Fe thin ®lm on the passivated semiconductor surfaces because the growth of Fe thin ®lms without silicide formation is a challenging goal for the realization of Si based spin electronics [10,11]. We have done STM/STS measurements on the initial stages of the Fe growth on clean and the Sbterminated Si(1 0 0) surfaces. STM images showed that three dimensional (3-D) Fe islands are formed on Sb-terminated surfaces without introduction of a complete wetting layer at the initial stage. Island densities are smaller than those grown on clean Si
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 3 8 2 - 6
K.-H. Park et al. / Surface Science 492 (2001) 34±40
surfaces, whereas the sizes are larger. It is attributed to the elimination of nucleation sites by the passivation of Si dangling bonds. Actually in the early stages of Fe growth on clean Si(1 0 0) surfaces, the concentration of nucleation sites is dense and continuous ®lms evolve via the increase and merging of each islands with further growth. Similarly to the growth of Ag ®lms on Sb-terminated Si surfaces [5,6], it was found that no wetting layer was formed in the growth of Fe ®lms because of the existence of an Sb passivation layer. However, the detailed features are dierent from the Ag growths. Ultrasmall clusters are formed at the early stages and their shapes are not regular in contrast to the Ag clusters in Ag/Sb/Si system, which indicates the reactive interaction between Fe and Si and the lower mobility of Fe on Si surfaces. Through the local I±V measurements on Fe islands on clean and Sb-terminated Si(1 0 0) surfaces, it was found that the conduction properties of Fe islands gradually changed from semiconducting to metallic with no band gap as Fe coverage increased. The onset thickness of metallic conduction behavior on clean and the Sb-terminated surface is This behavior is explained in terms of the about 6 A. gradual contact between adjacent Fe clusters and the formation of electrical conduction channels be tween the connected clusters at the coverage of 6 A. 2. Experiment In order to obtain a clean Si surface, p-type Si(1 0 0) with a resistivity of 1 X cm was repeatedly ¯ashed at 1200°C under the ultra-high vacuum (UHV) condition of 1 10 10 Torr. As a passivation adsorbate, Sb was deposited on the clean Si(1 0 0) surface at 375°C up to 1±2 monolayers (MLs) coverage, and then the sample was annealed for several minutes at 550°C in order to form a well-ordered Sb-terminated Si(1 0 0) surface. As a result, not only the dangling bonds of Si are fully passivated by a dimerized Sb adlayer [6], but also the outermost Si layer showed bulk-like properties via the structural change from a 2 1 dimer reconstruction to bulk-like 1 1 [12]. Fe was deposited on the Sb-terminated Si(1 0 0) sample at a deposition rate of 0.2±0.4 A/min at
35
room temperature. Fe was thermally evaporated from a hot tungsten ®lament at 5 10 10 Torr, and the coverage was monitored with a quartz crystal oscillator (INFICON). The sample was held at room temperature during evaporation. The thickness of the Fe ®lms was nominally estimated assuming perfect sticking of Fe on the surface. We ignored the apparent broadening of Fe islands by a tip convolution eect when measuring the sizes of Fe islands, hence the actual lateral sizes might be slightly smaller than the measured ones. The local tunneling spectra were measured under a z toward the sample after displacement of 1 A disconnecting the feedback loop at selected spots. The sweep time for single I±V measurement was set to be about 1 s in order to raise S=N ratio. The total resolution of the tunneling spectrum was set at 74 meV after smoothing. All STM/STS measurements were performed under the pressure of 1 10 10 Torr at room temperature. Tip cleaning process is very important for the reliable STM spectroscopy data. W tip was sharpened by the electrochemical etching and then transferred to the vacuum chamber. The contaminant of tip edge was removed by the fast scanning at a high bias voltage of 5±7 V. The cleanliness and sharpness of the tip were con®rmed by the atomic resolution image and by spectroscopy of clean Si surfaces. Occasionally we cleaned the tip edge by the irradiation of the high energy electron beam from hot tungsten ®laments, but it is not an necessary procedure for atomic resolution image and spectroscopy. 3. Results and discussion 3.1. Formation of Fe silicide on clean Si(1 0 0) surfaces The initial stages of Fe silicide formation on clean Si(1 0 0)-2 1 surfaces are shown in Fig. 1. Very small sized clusters appear on the surface The sizes of the with a ®lm thickness of 0.5 A. in diameter and 0.2±2.0 silicide clusters are 8±18 A in height at the Fe coverage of 0.5 A. It should A be noted that the nucleated silicide clusters are uniformly and regularly distributed on the surface.
36
K.-H. Park et al. / Surface Science 492 (2001) 34±40
(sample bias voltage VS Fig. 1. STM images of Fe ®lms grown on Si(1 0 0)-2 1 surfaces at the thicknesses of (a) 0.5 A (VS 2:5 V), (c) 2.0 A (VS 2:5 V), and (d) 6.0 A (VS 2:5 V), scan area 300 300 A 2 . 1.0 A
The linearly aligned clusters matched with the underlying 2 1 dimer rows. The spacing between the rows of aligned clusters is estimated to be two times of surface lattice unit length, 2a0 ( 7.68 A). Fe seems to be reactively bonded with the underlying Si atoms similarly to the previous reports of Miranda group [13±17]. We could not identify the chemical elements of the clusters simply by STM images. In the Fe growth on Si(1 1 1), it was analyzed that outer surface is composed of Si atoms because of the intermixing reaction between Fe and Si [17]. As reported in many reports, various Fe-silicide complexes are formed at the submonolayer coverages depending on the substrates at room temperature [13±17]. For instance, they are assigned as FeSi on a Si(1 1 1) surface [17], but Fe3 Si on a Si(1 0 0) surface by the comparison of density of states from UPS measurements [15,16].
2:0 V), (b)
As more Fe is deposited, the sizes of silicide clusters increase, and they coalesce into the larger ones resulting in the formation of elongated silicide islands as shown in Fig. 1(d). The size distribution ®lm is 7±25 (14±32) A in diameter of 1.0 (2.0) A and 0.5±3.7 (0.9±3.0) A in height, respectively. We measured the local I±V characteristics of various Fe ®lms with a variation of measurement positions. The local I±V characteristics measured on Fe clusters changed with the coverage. Fig. 2 shows the typical I±V characteristics of Fe clusters The forbidden on the ®lms of 0.5, 2.0, and 6.0 A. gap of 1 eV still existed on Fe clusters of the ®lm despite of the band gap narrowing, and 0.5 A whereas it comit decrease to 0.5 eV at 2.0 A ®lm. In the previous pletely disappeared at a 6.0 A UPS measurements, there was noticeable density of states at the Fermi level indicating the appearance
K.-H. Park et al. / Surface Science 492 (2001) 34±40
37
3.2. Growth of Fe ®lm on Sb-terminated Si(1 0 0) surface
Fig. 2. The local I±V characteristics on clusters at Fe ®lms on clean Si(1 0 0) surfaces with a variation of the thickness. The dot (dotted line), 2.0 and solid lines denote the data for 0.5 A (thin line), 6.0 A (thick line), respectively. A
of metallic silicide phase even at 1 ML (0.83 A) [15,16]. But it was reported that the line shape of bulk metallic Fe valence band is only obtained at the coverage of 5 ML [15]. Even if the metallic phase is locally formed on the Si surface, semiconducting gap feature in the I±V measurement is still revealed because of the local Schottky barrier formation. The change of conductivity from semiconducting to metallic in I±V measurement is considered to result from the onset of surface conduction channels by the contact between the adjacent Fe islands similarly to Ag growths. I±V characteristics are considered to reveal metallic properties when clusters begin to merge at a thick According to the UPS analysis of ness of 6.0 A. Miranda group [15], it is considered that continuous bulk metallic Fe ®lm is formed near 5 ML (4.2 and our metallic I±V characteristics appear as a A) result of it.
When Fe was deposited on a Sb-terminated Si(1 0 0) surface, small islands were formed at the early stages without a Fe wetting layer. Fig. 3(a) shows nucleated islands on the Sb-terminated The sizes Si(1 0 0) surface at a thickness of 0.5 A. of islands ranged from 11 to 24 A in diameter and in height. These islands were from 0.8 to 2.5 A separated from each other. The irregularly shaped islands as well as round ones were distributed on the Sb-terminated Si(1 0 0) surface. The islands reside on the sites between the rows of voids and rectangular pits similarly to the Ag growth on Sbterminated Si(1 0 0) surfaces as shown in Fig. 3(a) [6]. It should be noted that the islands are not formed right on the Sb dimer voids or within the dark pits. The underlying Sb-terminated Si(1 0 0) surface composed of linear Sb dimer row `A', dimer voids `B', and dark rectangular pits `C', which are observed even at the higher coverage as shown in the Fig. 3(b). Preferential growth of the islands on the topmost layer is commonly observed in Ag and Fe growth on Sb-terminated Si(1 0 0). As the Fe thickness increases, the lateral and vertical sizes of clusters become larger as shown in Fig. 3(b) and the sizes of islands are (c). At a thickness of 1.0 A, in diameter and 0.9±4.2 A in height. At 13±28 A they became 14±40 A in a thickness of 2.0 A, in height, respectively. The diameter and 1.1±4.2 A sizes of clusters grown on Sb-terminated Si(1 0 0) were much larger than those grown on the clean Si(1 0 0) at similar Fe thickness. The morphological evolution of Fe ®lms is similar to that observed in the Ag growth on Sb-terminated Si(1 0 0) surfaces [6] at room temperature except for the small cluster sizes and the irregular shapes. In the initial stage, the ®lm consists of isolated and round clusters formed on the Sb overlayer. As more metal is deposited, the size and density grow. At a later stage with a Fe thickness metal clusters coalesce into elonhigher than 6 A, gated ones, implying the contact between metal clusters as shown in Fig. 3(d). Since the chemical elements cannot be identi®ed by STM image, the composition of clusters cannot be de®nitely addressed. In the Fe growth on the Si(1 1 1) and
38
K.-H. Park et al. / Surface Science 492 (2001) 34±40
(sample bias voltage VS Fig. 3. STM images of Fe ®lms grown on Sb-terminated Si(1 0 0) surfaces at Fe thicknesses of (a) 0.5 A (VS 2:5 V), (c) 2.0 A (VS 2:5 V), and (d) 6.0 A (VS 2:5 V), scan area 300 300 A 2 . 3:0 V), (b) 1.0 A
Si(1 0 0) surfaces, the strong intermixing reaction of Fe and Si occurs resulting in the formation of various silicide layer with a variation of the thickness and temperature [13,15±17]. From the morphological evolution, we ®nd that the nucleation sites are greatly reduced due to the Sb-passivation and the intermixing reaction between Fe and Si should be suppressed due to the existence of Sb layer. In order to estimate the bonding strength between the cluster and substrate, we performed desorption experiment of Fe clusters on Sb-terminated surfaces by STM manipulation technique [7,8]. However, we could not detach them from the surface by application of moderate ®eld or by the formation of point contact. This implies that the bonding strength is larger than in the Ag/Sb/Si case. In Fig. 4, changes of the average island diameter and the density on clean and Sb-terminated
Si(1 0 0) surfaces are recorded with a variation of Fe thickness. The size is larger but the density is smaller on the Sb-terminated surface. It should be noted that the island density of Fe ®lm on a clean Si(1 0 0) surface decreases as coverage increases. This feature is strikingly dierent from the growth on Sb-terminated Si(1 0 0) surfaces where the island density increases with increasing thickness. Such behavior stems from the surface passivation by Sb elements. The local I±V characteristics are very similar to the case of Fe growth on clean Si(1 0 0) surfaces. Fig. 5(a) shows typical I±V characteristics of Fe The clusters on the ®lms of 0.5, 4.0, and 6.0 A. forbidden gap of 1 eV still existed on Fe clusters ®lm, and it decreased to 0.5 eV at of the 0.5 A whereas it completely disappeared at a 6.0 4.0 A ®lm similarly to the case of Fe ®lms on clean A
K.-H. Park et al. / Surface Science 492 (2001) 34±40
Fig. 4. The average island diameter and island density of Fe clusters as a function of Fe ®lm thickness on clean and Sbterminated Si(1 0 0) surfaces.
Si(1 0 0) surfaces. The overall characters are nearly the same in most of the Fe clusters within a ®lm except for the slight change (0.1 eV) of gap size. The change of conductivity from semiconducting to metallic is also considered to result from the onset of surface conduction channels by contact between adjacent Fe clusters at a thickness of 6.0 The underlying Sb is not considered to make a A. signi®cant eect on the local I±V characteristics, since we could not ®nd the apparent dierence of I±V characteristics between clean and Sb-terminated surfaces. 4. Summary We have found that nanometer sized Fe clusters were formed in the initial stages of Fe growth on clean and Sb-terminated Si(1 0 0) surfaces. The Sbterminated Si(1 0 0) surfaces are found to be more resistive to the silicide formation than the clean Si(1 0 0) surface since the nucleation sites are reduced due to the saturation of reactive Si dangling
39
Fig. 5. The local I±V characteristics on clusters at Fe ®lms with a variation of thickness. The dot and solid lines denote the data (dotted line), 4.0 A (thin line), 6.0 A (thick line), refor 0.5 A spectively.
bonds by Sb. However, it is necessary to analyze the surface/interface of Fe silicide ®lms in order to get the detailed compositional information. The conduction properties of Fe clusters gradually changed from semiconducting to metallic with no band gap as Fe coverage increased. Acknowledgements This work has been supported by the Ministry of Informations and Communications and Ministry of Commerce, Industry and Energy, Korea. References [1] G. Le Lay, Surf. Sci. 132 (1983) 169. [2] E.-J. van Loenen, M. Iwami, R.M. Tromp, J.F. van der Veen, Surf. Sci. 137 (1984) 1.
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[3] K. Sumitomo, T. Kobayashi, F. Shoji, K. Oura, I. Katayama, Phys. Rev. Lett. 66 (1991) 1193. [4] M. Naitoh, F. Shoji, K. Oura, Surf. Sci. 242 (1991) 152. [5] K.-H. Park, J.S. Ha, S.-J. Park, E.-H. Lee, Surf. Sci. 380 (1997) 258. [6] K.-H. Park, J.S. Ha, W.S. Yun, E.-H. Lee, Surf. Sci. 415 (1998) 320. [7] K.-H. Park, J.S. Ha, W.S. Yun, E.-H. Lee, J. Vac. Sci. Technol. A 17 (1999) 1441. [8] K.-H. Park, J.S. Ha, W.S. Yun, Y.-J. Ko, Jpn. J. Appl. Phys. 39 (2000) 4629. [9] M. Copel, R.M. Tromp, Appl. Phys. Lett. 65 (1994) 3102. [10] P. Ma, G.W. Anderson, P.R. Norton, Surf. Sci. 420 (1999) 134.
[11] F. Zavaliche, W. Wulfhekel, H. Xu, J. Kirschner, J. Appl. Phys. 88 (2000) 5289. [12] D.H. Rich, T. Miller, G.E. Franklin, T.-C. Chiang, Phys. Rev. B 39 (1989) 1438. [13] J.M. Gallego, R. Miranda, J. Appl. Phys. 69 (1991) 1377. [14] A.L. Vazquez de Parga, J. de la Figuera, C. Ocal, R. Miranda, Europhys. Lett. 18 (1992) 595. [15] J. Alvarez, J.J. Hinarejos, E.G. Michel, G.R. Castro, R. Miranda, Phys. Rev. B 45 (1992) 14042. [16] J.M. Gallego, J.M. Garcõa, J. Alvarez, R. Miranda, Phys. Rev. B 46 (1992) 13339. [17] J. Alvarez, A.L. Vazquez de Parga, J.J. Hinarejos, J. de la Figuera, E.G. Michel, C. Ocal, R. Miranda, Phys. Rev. B 47 (1993) 16048.