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Metal induced step arrangement on Si(1 1 1) surface observed by LEED
Applied Surface Science 169±170 (2001) 88±92
Metal induced step arrangement on Si(1 1 1) surface observed by LEED T. Urano*, K. Watanabe, S. Hongo Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501 Japan Received 2 August 1999; accepted 22 October 1999
Abstract Adsorbed structures of Ba on Si(1 1 1) surfaces have been observed by LEED. In elevating the annealing temperature after p p a few monolayers (MLs) deposition at room temperature ( 3 2 3)R308, 2 8 and 3 1 patterns were observed as reported by Weitering [H.H. Weitering, Surf. Sci. 355 (1996) L271]. Additionally a one-dimensional 10 1 pattern was observed in the temperature range between the appearance of two-dimensional 2 8 and 3 1 patterns. This pattern is considered to be produced by a combination of the periodical terrace with a width of 10 unit lengths separating each step and a 5 1 reconstruction on the terrace. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Si(1 1 1); Ba; Low energy electron diffraction (LEED); Step arrangement; Reconstruction
1. Introduction A study of metal±semiconductor interfaces is important for a basic knowledge of semiconductor devices. Superstructures, silicide formation, ®lm growth and diffusion have been investigated for various metals on semiconductor surfaces. Alkaline-earth metals as well as alkali metals deposited on a semiconductor surface cause a drastic decrease of the work function. Weitering [1] observed 2 8, 5 1 and 3 1 reconstructions on the Ba/Si(1 at p1 1) surface p submonolayer coverage, and the ( 3 2 3)R308 reconstruction by the silicide formation of BaSi2 near sample edge during the isothermal desorption at high temperatures of 1115 and 1225 K. In this study, a one-dimensional 10 1 structure was observed to occur in the annealing temperature
* Corresponding author. Fax: 81-78-803-6079. E-mail address: [email protected] (T. Urano).
range between which two-dimensional 2 8 and 3 1 reconstructions appeared after a few MLs Ba deposition at room temperature (RT). 2. Experimental The experiments were carried out in an ultra high vacuum chamber equipped with a four-grid AESLEED optics (OCI Vacuum Micro-engineering, BLD-800). The base pressure of the chamber was 1 10ÿ8 Pa. LEED patterns were taken with a highly sensitive charged couple device (CCD) (Hamamatsu Photonics Co., C5985) and stored on a personal computer. The Si(1 1 1) wafer (P-type, B-doped, 10±20 O cm) substrate was cut into a strip of 5 mm width and 15 mm length and was supported by a pair of tantalum foils. The silicon was cleaned by ¯ashing at 12008C for 15 s followed by annealing at 8008C for several minutes. A clear 7 7 LEED pattern was observed
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 4 5 - 0
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
and no trace of contamination was detected by AES. The substrate temperature was estimated by the extrapolation to lower temperatures of the direct current passing through the sample. This was previously calibrated at higher temperatures by the relation between the direct current passing through the sample and the indicated temperatures of the optical pyrometer. Ba atoms were deposited from a commercial dispenser (SAES Getters Co.). LEED observations were carried out for the sample at RT. 3. Experimental results and discussions Ba atoms were deposited on the Si(1 1 1) substrate at RT. A changing curve of Ba Auger signal (NOO, 56 eV) as a function of deposition time showed the layer-by-layer fashion up to at least two MLs. The coverage was normalized with respect to the Auger intensity ratio of Ba(NOO)/Si(LVV, 92 eV) at the ®rst break point in this changing curve. Fig. 1 shows the changing curve of the ratio of Ba(NOO) to Si(LVV, 92 eV) Auger intensity as a function of annealing temperature after 3 MLs deposition at RT. Annealing time is 1 min or 15 s at every stage of temperatures lower than or higher than 10008C, respectively. There is no change of Auger intensity ratio up to 3008C and in this temperature range only a few faint integer spots were seen in the LEED pattern. At the temperature of 4008C thepAuger p intensity ratio decreased suddenly and a ( 3 2 3)R308
Fig. 1. Changing curve of Auger peak ratio of Ba(NOO, 56 eV) to Si(LVV, 92 eV) as a function of annealing temperature after deposition of 3 ML at room temperature.
89
LEED pattern of three orientational domains was observed as shown in Fig. 2(b). At 4008C no Ba atom desorption occurs because on the Si(0 0 1) surface extra Ba atoms more than 1 ML are desorbed around 8008C at once and after that the remaining 1 ML atoms are gradually desorbed up to 12008C as observed by p thermal p desorption spectroscopy (TDS) [2]. This ( 3 2 3)R308 pattern is considered to be caused by a silicide formation of BaSi2 on the Si(1 1 1) substrate, because the silicide peak at 2.7± 2.9 eV has been observed in this temperature range by p p UPS in our laboratory [3]. The ( 3 2 3)R308 structure can be shown to be a pseudo (0 0 1) plane of orthorhombic BaSi2 which has lattice parameters of Ê [4]. The lengths of rectangular 6.80 and 11.58 A Ê is the unit length lattice of H3a and 3a (a 3:84 A Ê , respectively. of Si(1 1 1) plane) are 6.65 and 11.51 A The magnitudes of lattice mismatch are only 2.2 and 0.6%, respectively. At temperatures higher than 7008C, 1/8 order spots began to appear in the LEED pattern around the integer and half-order beams and at 8008C the 2 8 pattern is completed as shown in Fig. 2(c). At this stage, the coverage is about 0.8 ML. At temperatures higher than 8008C, a one-dimensional fashion of LEED pattern began to appear in which spots became brighter in one direction (Fig. 2(d)). In this direction, 1/10 order spots appeared, while in the other two directions, 1/8 order spots still remained. At about 9508C a single domain 10 1 pattern is shown in Fig. 2(e). This pattern is shown at a different energy in Fig. 3(a). There are faint spots between bright 1/5 order spots. In this temperature region, a 5 1 reconstruction was observed in [1]. Therefore, it is considered that this pattern is produced by a combination of the periodical terrace having the width of 10 times that of the unit length of Si(1 1 1)1 1 surface and a 5 1 reconstruction on the terrace, as indicated in Fig. 3(b) schematically. The domains of 5 1 reconstruction of the other two orientations are suppressed. At this stage the coverage seems to be about 0.4 ML. It is well known that the terrace width on the vicinal Si(1 1 1) surface changes by direct current passing through the sample [5±7]. Step bunching or uniform distribution of single height steps depends on whether the current direction is in the step-down or step-up direction and on the annealing temperature. When the
90
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
Fig. 2. Typical LEED patterns in the annealing process after 3 ML deposition at RT. The electron energy is 80 eV. (a) Si(1 1 1) 7 7 clean surface; (b) (H32H3)R308 at 6008C; (c) 2 8 at 8008C; (d) one-dimension-like 2 8 at 8508C; (e) one-dimensional 10 1 at 9508C; (f) one-dimension-like 3 1 at 10508C; (g) 3 1 at 11008C; and (h) 3 1 at 11508C; one domain is missing.
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
Fig. 3. (a) The 10 1 structure with a different energy of 50 eV; (b) a schematic model of step arrangement and 1 5 reconstruction on the terrace.
current direction is reversed, the dependence of the step structure on the temperature is also reversed. To make sure that there is no current effect the same experiments with the reverse current passing through the substrate were carried out and the result was similar. Furthermore, in this study the steps are parallel to the heating current which ¯ows in the vertical direction with respect to the LEED pattern. Then, a new substrate was cut out in the perpendicular direction from the same Si(1 1 1) wafer as was used before, and the one-dimensional fashion is con®rmed in the direction perpendicular to the former result mentioned
91
above. Therefore, the one-dimensionality observed in this study is not considered to be a current effect. We have observed metal induced faceting on the Ba/Si(0 0 1) surface by annealing at temperatures around 7008C [8]. Metal adsorption induced step ordering on Au/Si(1 1 1) [9] and Au/Si(0 0 1) [10], step bunching on Cu/Si(1 1 1) [11], and faceting on Au/Si [12,13] have been also observed. In these studies, it is considered that the Si±Si bond is weakened by electrons transfered from the metal atom, allowing the Si atom to move easily to a more stable position. In this study, the same mechanism may apply to form a periodical step distribution. The driving force is the surface reconstruction produced on the terrace. Therefore, this one-dimensionality is a metal induced and reconstruction induced effect. At the temperatures higher than 10008C the twodimensional fashion returned and a 3 1 pattern of three domains appeared as shown in Fig. 2(g). However, in this stage the coverage seems to be 0.15 ML which is far less than 1/3 ML. Therefore, the 3 1 might be produced on only a part of the surface. Moreover, at the temperatures around 11508C just before complete desorption of Ba, the 3 1 pattern is still observed as shown in Fig. 2(h). But one of three domains of 3 1 reconstruction has disappeared. This feature of one-dimensionality is opposite to the 10 1 structure mentioned above. It might be that the threefold periodicity perpendicular to the step edge direction is suppressed by the obstruction of the steps at higher temperatures. After the appearance of the one-dimensional 10 1 structure, one-dimensional 9 1-like pattern based on 3 1 structure was sometimes observed. Then, the terrace width seems to be ¯exible in coincidence with the reconstruction on the terrace. Therefore, further investigation is necessary as to whether the disappearance of one domain of 3 1 structure is caused by a regular step width or an irregular step width. 4. Summary Adsorbed structures of Ba on Si(1 1 1) surfaces have been investigated by LEED. In elevating the annealing temperature after p a fewp MLs deposition at room temperature ( 3 2 3)R308, 2 8, 10 1 and 3 1 patterns were observed. The
92
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
10 1 structure indicates a clear one-dimensional fashion. This pattern can be considered as a combination of 5 1 reconstruction on the terrace and a regular terrace width of 10-times the unit length of the Si(1 1 1) 1 1 surface. This is caused by a metal induced and reconstruction induced step arrangement. According to this model the terrace width is about Ê and the miscut angle of this substrate seems to be 33 A 5.48. STM observations would con®rm this. Acknowledgements The Authors would like to thank Dr. S.P. Tear, University of York, UK, for his helpful discussions. References [1] H.H. Weitering, Surf. Sci. 355 (1996) L271.
[2] S. Hongo, K. Ojima, S. Taniguchi, T. Urano, T. Kanaji, Appl. Surf. Sci. 82/83 (1994) 537. [3] Y. Takeda, T. Ohtani, K. Ojima, T. Urano, S. Hongo, J. Electron. Spectr. Rel. Phen. 88/91 (1998) 619. [4] F.A. Shunk, Constitution of Binary Alloys (Suppl. 2), McGraw-Hill, New York, 1986. [5] A.V. Latyshev, A.L. Aseev, A.B. Krasilnikov, S.I. Stenin, Phys. Stat. Solid. A113 (1989) 421. [6] Y. Homma, R.J. McClelland, H. Hibino, Jpn. J. Appl. Phys. 29 (1990) L2254. [7] Y.-N. Yang, E.S. Fu, E.D. Williams, Surf. Sci. 356 (1996) 101. [8] T. Urano, Y. Takeda, T. Ohtani, K. Tamiya, S. Hongo, Jpn. J. Appl. Phys. 37 (1998) 617. [9] M. Jalochowski, M. Strozak, R. Zdyb, Surf. Sci. 375 (1997) 203. [10] T. Shimakura, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 407 (1998) L657. [11] Y. Takahashi, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 433±435 (1999) 512. [12] H. Minoda, K. Yagi, Phys. Rev. B59 (1999) 2363. [13] H. Minoda, T. Shimaoka, K. Yagi, F.-J. Meyer zu Heringdorf, M. Horn von Hoegen, Surf. Sci. 432 (1999) 69.
Metal induced step arrangement on Si(1 1 1) surface observed by LEED T. Urano*, K. Watanabe, S. Hongo Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501 Japan Received 2 August 1999; accepted 22 October 1999
Abstract Adsorbed structures of Ba on Si(1 1 1) surfaces have been observed by LEED. In elevating the annealing temperature after p p a few monolayers (MLs) deposition at room temperature ( 3 2 3)R308, 2 8 and 3 1 patterns were observed as reported by Weitering [H.H. Weitering, Surf. Sci. 355 (1996) L271]. Additionally a one-dimensional 10 1 pattern was observed in the temperature range between the appearance of two-dimensional 2 8 and 3 1 patterns. This pattern is considered to be produced by a combination of the periodical terrace with a width of 10 unit lengths separating each step and a 5 1 reconstruction on the terrace. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Si(1 1 1); Ba; Low energy electron diffraction (LEED); Step arrangement; Reconstruction
1. Introduction A study of metal±semiconductor interfaces is important for a basic knowledge of semiconductor devices. Superstructures, silicide formation, ®lm growth and diffusion have been investigated for various metals on semiconductor surfaces. Alkaline-earth metals as well as alkali metals deposited on a semiconductor surface cause a drastic decrease of the work function. Weitering [1] observed 2 8, 5 1 and 3 1 reconstructions on the Ba/Si(1 at p1 1) surface p submonolayer coverage, and the ( 3 2 3)R308 reconstruction by the silicide formation of BaSi2 near sample edge during the isothermal desorption at high temperatures of 1115 and 1225 K. In this study, a one-dimensional 10 1 structure was observed to occur in the annealing temperature
* Corresponding author. Fax: 81-78-803-6079. E-mail address: [email protected] (T. Urano).
range between which two-dimensional 2 8 and 3 1 reconstructions appeared after a few MLs Ba deposition at room temperature (RT). 2. Experimental The experiments were carried out in an ultra high vacuum chamber equipped with a four-grid AESLEED optics (OCI Vacuum Micro-engineering, BLD-800). The base pressure of the chamber was 1 10ÿ8 Pa. LEED patterns were taken with a highly sensitive charged couple device (CCD) (Hamamatsu Photonics Co., C5985) and stored on a personal computer. The Si(1 1 1) wafer (P-type, B-doped, 10±20 O cm) substrate was cut into a strip of 5 mm width and 15 mm length and was supported by a pair of tantalum foils. The silicon was cleaned by ¯ashing at 12008C for 15 s followed by annealing at 8008C for several minutes. A clear 7 7 LEED pattern was observed
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 4 5 - 0
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
and no trace of contamination was detected by AES. The substrate temperature was estimated by the extrapolation to lower temperatures of the direct current passing through the sample. This was previously calibrated at higher temperatures by the relation between the direct current passing through the sample and the indicated temperatures of the optical pyrometer. Ba atoms were deposited from a commercial dispenser (SAES Getters Co.). LEED observations were carried out for the sample at RT. 3. Experimental results and discussions Ba atoms were deposited on the Si(1 1 1) substrate at RT. A changing curve of Ba Auger signal (NOO, 56 eV) as a function of deposition time showed the layer-by-layer fashion up to at least two MLs. The coverage was normalized with respect to the Auger intensity ratio of Ba(NOO)/Si(LVV, 92 eV) at the ®rst break point in this changing curve. Fig. 1 shows the changing curve of the ratio of Ba(NOO) to Si(LVV, 92 eV) Auger intensity as a function of annealing temperature after 3 MLs deposition at RT. Annealing time is 1 min or 15 s at every stage of temperatures lower than or higher than 10008C, respectively. There is no change of Auger intensity ratio up to 3008C and in this temperature range only a few faint integer spots were seen in the LEED pattern. At the temperature of 4008C thepAuger p intensity ratio decreased suddenly and a ( 3 2 3)R308
Fig. 1. Changing curve of Auger peak ratio of Ba(NOO, 56 eV) to Si(LVV, 92 eV) as a function of annealing temperature after deposition of 3 ML at room temperature.
89
LEED pattern of three orientational domains was observed as shown in Fig. 2(b). At 4008C no Ba atom desorption occurs because on the Si(0 0 1) surface extra Ba atoms more than 1 ML are desorbed around 8008C at once and after that the remaining 1 ML atoms are gradually desorbed up to 12008C as observed by p thermal p desorption spectroscopy (TDS) [2]. This ( 3 2 3)R308 pattern is considered to be caused by a silicide formation of BaSi2 on the Si(1 1 1) substrate, because the silicide peak at 2.7± 2.9 eV has been observed in this temperature range by p p UPS in our laboratory [3]. The ( 3 2 3)R308 structure can be shown to be a pseudo (0 0 1) plane of orthorhombic BaSi2 which has lattice parameters of Ê [4]. The lengths of rectangular 6.80 and 11.58 A Ê is the unit length lattice of H3a and 3a (a 3:84 A Ê , respectively. of Si(1 1 1) plane) are 6.65 and 11.51 A The magnitudes of lattice mismatch are only 2.2 and 0.6%, respectively. At temperatures higher than 7008C, 1/8 order spots began to appear in the LEED pattern around the integer and half-order beams and at 8008C the 2 8 pattern is completed as shown in Fig. 2(c). At this stage, the coverage is about 0.8 ML. At temperatures higher than 8008C, a one-dimensional fashion of LEED pattern began to appear in which spots became brighter in one direction (Fig. 2(d)). In this direction, 1/10 order spots appeared, while in the other two directions, 1/8 order spots still remained. At about 9508C a single domain 10 1 pattern is shown in Fig. 2(e). This pattern is shown at a different energy in Fig. 3(a). There are faint spots between bright 1/5 order spots. In this temperature region, a 5 1 reconstruction was observed in [1]. Therefore, it is considered that this pattern is produced by a combination of the periodical terrace having the width of 10 times that of the unit length of Si(1 1 1)1 1 surface and a 5 1 reconstruction on the terrace, as indicated in Fig. 3(b) schematically. The domains of 5 1 reconstruction of the other two orientations are suppressed. At this stage the coverage seems to be about 0.4 ML. It is well known that the terrace width on the vicinal Si(1 1 1) surface changes by direct current passing through the sample [5±7]. Step bunching or uniform distribution of single height steps depends on whether the current direction is in the step-down or step-up direction and on the annealing temperature. When the
90
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
Fig. 2. Typical LEED patterns in the annealing process after 3 ML deposition at RT. The electron energy is 80 eV. (a) Si(1 1 1) 7 7 clean surface; (b) (H32H3)R308 at 6008C; (c) 2 8 at 8008C; (d) one-dimension-like 2 8 at 8508C; (e) one-dimensional 10 1 at 9508C; (f) one-dimension-like 3 1 at 10508C; (g) 3 1 at 11008C; and (h) 3 1 at 11508C; one domain is missing.
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
Fig. 3. (a) The 10 1 structure with a different energy of 50 eV; (b) a schematic model of step arrangement and 1 5 reconstruction on the terrace.
current direction is reversed, the dependence of the step structure on the temperature is also reversed. To make sure that there is no current effect the same experiments with the reverse current passing through the substrate were carried out and the result was similar. Furthermore, in this study the steps are parallel to the heating current which ¯ows in the vertical direction with respect to the LEED pattern. Then, a new substrate was cut out in the perpendicular direction from the same Si(1 1 1) wafer as was used before, and the one-dimensional fashion is con®rmed in the direction perpendicular to the former result mentioned
91
above. Therefore, the one-dimensionality observed in this study is not considered to be a current effect. We have observed metal induced faceting on the Ba/Si(0 0 1) surface by annealing at temperatures around 7008C [8]. Metal adsorption induced step ordering on Au/Si(1 1 1) [9] and Au/Si(0 0 1) [10], step bunching on Cu/Si(1 1 1) [11], and faceting on Au/Si [12,13] have been also observed. In these studies, it is considered that the Si±Si bond is weakened by electrons transfered from the metal atom, allowing the Si atom to move easily to a more stable position. In this study, the same mechanism may apply to form a periodical step distribution. The driving force is the surface reconstruction produced on the terrace. Therefore, this one-dimensionality is a metal induced and reconstruction induced effect. At the temperatures higher than 10008C the twodimensional fashion returned and a 3 1 pattern of three domains appeared as shown in Fig. 2(g). However, in this stage the coverage seems to be 0.15 ML which is far less than 1/3 ML. Therefore, the 3 1 might be produced on only a part of the surface. Moreover, at the temperatures around 11508C just before complete desorption of Ba, the 3 1 pattern is still observed as shown in Fig. 2(h). But one of three domains of 3 1 reconstruction has disappeared. This feature of one-dimensionality is opposite to the 10 1 structure mentioned above. It might be that the threefold periodicity perpendicular to the step edge direction is suppressed by the obstruction of the steps at higher temperatures. After the appearance of the one-dimensional 10 1 structure, one-dimensional 9 1-like pattern based on 3 1 structure was sometimes observed. Then, the terrace width seems to be ¯exible in coincidence with the reconstruction on the terrace. Therefore, further investigation is necessary as to whether the disappearance of one domain of 3 1 structure is caused by a regular step width or an irregular step width. 4. Summary Adsorbed structures of Ba on Si(1 1 1) surfaces have been investigated by LEED. In elevating the annealing temperature after p a fewp MLs deposition at room temperature ( 3 2 3)R308, 2 8, 10 1 and 3 1 patterns were observed. The
92
T. Urano et al. / Applied Surface Science 169±170 (2001) 88±92
10 1 structure indicates a clear one-dimensional fashion. This pattern can be considered as a combination of 5 1 reconstruction on the terrace and a regular terrace width of 10-times the unit length of the Si(1 1 1) 1 1 surface. This is caused by a metal induced and reconstruction induced step arrangement. According to this model the terrace width is about Ê and the miscut angle of this substrate seems to be 33 A 5.48. STM observations would con®rm this. Acknowledgements The Authors would like to thank Dr. S.P. Tear, University of York, UK, for his helpful discussions. References [1] H.H. Weitering, Surf. Sci. 355 (1996) L271.
[2] S. Hongo, K. Ojima, S. Taniguchi, T. Urano, T. Kanaji, Appl. Surf. Sci. 82/83 (1994) 537. [3] Y. Takeda, T. Ohtani, K. Ojima, T. Urano, S. Hongo, J. Electron. Spectr. Rel. Phen. 88/91 (1998) 619. [4] F.A. Shunk, Constitution of Binary Alloys (Suppl. 2), McGraw-Hill, New York, 1986. [5] A.V. Latyshev, A.L. Aseev, A.B. Krasilnikov, S.I. Stenin, Phys. Stat. Solid. A113 (1989) 421. [6] Y. Homma, R.J. McClelland, H. Hibino, Jpn. J. Appl. Phys. 29 (1990) L2254. [7] Y.-N. Yang, E.S. Fu, E.D. Williams, Surf. Sci. 356 (1996) 101. [8] T. Urano, Y. Takeda, T. Ohtani, K. Tamiya, S. Hongo, Jpn. J. Appl. Phys. 37 (1998) 617. [9] M. Jalochowski, M. Strozak, R. Zdyb, Surf. Sci. 375 (1997) 203. [10] T. Shimakura, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 407 (1998) L657. [11] Y. Takahashi, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 433±435 (1999) 512. [12] H. Minoda, K. Yagi, Phys. Rev. B59 (1999) 2363. [13] H. Minoda, T. Shimaoka, K. Yagi, F.-J. Meyer zu Heringdorf, M. Horn von Hoegen, Surf. Sci. 432 (1999) 69.