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LEED observation of 4 × 1-In superstructure prepared on Si(111)-quasi-5 × 5-Cu surface
applied
surface science ELSEVIER
Applied Surface Science 113/l
14 (1997) U5-447
LEED observation of 4 X l-In superstructure prepared on Sit 111 J-quasi-5 X 5-Cu surface Yoshio Suzaki ‘, Akira Saitoh, Takeo Nakano Department
*, Shigeru Baba
of Applied Physics, Seikei UniL~ersity 3-3-I. Kichijoji-Kitu, Musashino, Tokyo 180, Japan
Abstract The (LEED) surface changes
surface structure of {Cu, In} binary system on Si( 111) has been studied with the low energy electron diffraction technique. First, the quasi-‘5 x 5’-Cu structure can be prepared with the deposition of l-2.5 ML of Cu on the clean of Si at temperatures of 300-550°C. With the additional deposition of 0.5-1.6 ML of In, the surface structure into a 4 X 1 structure at temperatures between 400 and 500°C. At 550°C however, the ‘5 X 5’-Cu structure does not
disappear, despite of the supply of In vapor. Kewords:
4 X 1 structure;
Copper; Indium; Silicon; LEED
1. Introduction The surface structure of semiconductor crystals with metal adsorbates has been a central issue of surface physics, because of its diversity which reflects the subtle difference of the physical and/or chemical nature in each atomic species. Recently, binary systems of noble metals on Sic1 11) have been attracting attentions [ 1,2], since it has been found that the existence of foreign atoms affects the surface structure and the growth mode of the film. Yuhara et al. have studied the Sic1 11) surface with six pairs of atomic species among (Au, Ag, Cu} being deposited [2]. By employing the techniques of Rutherford backscattering (RBS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED), they have observed that Cu adatoms are driven away
* Corresponding author. E-mail: [email protected]. ’ Present address R&D Center, Advantest Inc. 0169-4332/97/$17.00 Copyright PII SOl69-4332(96)00929-4
from the surface when Au or Ag is deposited, while a weaker effect is observed in the Au-Ag pair. In the present study, the effect of In atoms is examined from the observation of structure of a binary adsorbate system of In-Cu on Sic1 11) surface by means of LEED. Indium derived superstructures on Si( 111) surface have been extensively studied since the first observation of fi X fi-In/Si(ll 1) by Lander and Morrison [3]. Other superstructures such as v& X !/% [4], 4 X 1, 1 X 1 [5] and fi X 6 [6] superstructures have also been found at relatively high temperature regions from 300 to 600°C. On the other hand, the Cu/Si(l I 1) surface is known as an incommensurate structure showing a structure very close to 5 X 5 of the Sic1 11) surface, i.e., the quasi-‘5 X 5’ [7]. This structure comes to appear on Si( 11 1) at the substrate temperature of 130-600°C with the deposition of 1.3 ML of copper. Zegenhagen et al. have investigated the Cu/Si(l 11) with X-ray standing wave measurement, and sug-
0 1997 Elsevier Science B.V. All rights reserved.
446
Y. Suzaki et al./Applied
Surface Science 113 / 114 (1997) 445-447
gested that the ‘5 X 5’ structure originates from the Cu,Si compound phase [S], but the detailed atomic configuration of the surface has not yet been made clear. Furthermore, the binary system of Cu-In alloy has been studied for a variety of potential applications such as a radiation-resistive solar cell and a new soldering system in VLSI devices. It is expected that the modification of the surface structure due to additional deposition of In atoms may give us a new information about the chemical nature of the ‘5 X 5’Cu surface.
2. Experiments Experiments were carried out in a UHV system whose base pressure was less than lo-’ Pa. Substrates of a strip of 5 X 28 X 0.3 mm3 were cut out from wafers of n-type SK1 11) ( p N 1 KIcm). The substrate surface was cleaned thermally in a UHV condition by a direct resistive heating up to 1200°C. After the thermal treatment, a clear 7 X 7 LEED pattern could be observed when it was cooled to a lower temperature. Then, copper and indium were evaporated from the respective tungsten filament. The deposition rate and the total amount of the deposits were measured by a quartz crystal oscillator. The substrate temperature from 350 to 800°C was determined from the electrical resistivity of the substrate using an equation of the temperature dependence of the carrier density in the intrinsic semiconductor. The temperature was calibrated with an optical pyrometer in the range 800- 1200°C. The basic performance and the accuracy of the present experimental setup was checked by the observation of an established phase diagram of the indium-Sic 111 l-7 x 7 [5]. On the Si-7 X 7 surface, 2.5 monolayers (ML) of copper were deposited at a rate of (1.0-2.0) X lo-* ML/s at the surface temperature of 500°C and the ‘5 X 5’-Cu structure could be observed with LEED. Here 1 ML is taken as 7.8 X lOI atoms/cm2 on the Sic1 11) surface. The ‘5 X 5’-Cu surface was found stable below 550°C. After the observation of the ‘5 X 5’-Cu, the deposition of indium followed intermittently by an amount of 0.1 ML. The surface was examined with LEED after each deposition. This
experiment was repeated at temperatures of 450°C. 500°C and 550°C respectively. The deposition rate of In was kept constant at (0.8-2.1) X 10e2 ML/s. The LEED observation was carried out at temperatures lower than 150°C.
3. Results and discussion On the Sic1 11) surface with Cu adatoms, a typical LEED pattern looked like small hexagons around the 1 X 1 spots. The distance between the 1 X 1 spots and the apex of a hexagon was 0.18, which agreed well with the quasi-‘5 X 5’-Cu/Si pattern reported by Kemmann et al. [7]. When indium was deposited on the surface at 450°C and 500°C a change in the LEED pattern from ‘5 X 5’ to 4 X 1 structure could be observed at a total amount of indium from 0.5 to 1.6 ML. The turn of the appearance of LEED patterns with the deposition of In occurred almost in the same fashion at this temperature range. With the deposition of 0.2-0.3 ML of In, the apexes of a hexagon around the 1 X 1 spot were split into six spots, i.e., a clearer ‘5 X 5’ pattern was observed. As the deposition exceeded 0.3 ML, 4 X 1 pattern began to appear on this ‘5 X 5’. This 4 X 1 LEED pattern had a three-fold symmetry. It indicates that the surface is a 4 X 1 multi-domain structure. The ‘5 X 5’ pattern disappeared at the deposition of 0.5 ML of In. Only the 4 X 1 pattern could be observed up to 1.6 ML. The fractional order LEED spots of the 4 X 1 became brightest at around 1.0 ML of the indium coverage. Then the LEED spots became weaker with the increase of In coverage. The 4 X 1 spots disappeared at coverages thicker than 1.6 ML, where the 11 pattern was observed.
~~~~~~
0
0.5
1.0
1.5
Coverage (ML)
2.0
0
I .o 1.5 Coverage (ML)
0.5
Fig. 1. The phase diagram of In/%( 111) (left) and In/Cu/Si( (right) surfaces.
2.0
I 1I)
Y. Suzuki et al. /Applied Surface Science 113 / 114 ( 1997) 445-447
At 550°C the ‘5 X 5’ Cu/Si(ll 1) pattern could also be observed clearly before the indium deposition. However. its structure did not change despite that the surface was exposed to the flux of indium. The result of the observations is summarized in Fig. 1. For comparison the phase diagram of In/Si(l 1 1) [5] is illustrated on the left in Fig. 1. It should be noted that the 4 X 1 structure of In/Cu/Si(l 11) can be observed at such a high temperature as 500°C which is higher by about 50°C than the appearance upper limit in the In/Si( 111) system. The 4 X l-In structure can be maintained on the silicon surface at higher temperatures with the presence of Cu atoms. This suggests that the desorption energy of indium on the silicon surface becomes larger and the mean stay time becomes longer on the Cu modified silicon surface. It is also noted that neither 6 X fi nor fi X fi structure is observed in the In/Cu/Si(lll) system, which means that these superstructures of In are less stable than the 4 X l-In. It is interesting to compare the result of the In-Cu system on the Sic11 1) surface with those of Au-Cu and Ag-Cu systems reported by Yuhara et al. [2]. They have reported that the thermal stability of Au and Ag deposits is not affected by Cu atoms, while the Cu atoms predeposited on the Si surface were purged from the surface, i.e., traces of Cu could not be observed by any means of RBS, AES and LEED. For the case of the Ag-Cu/Si(l 11) system, Cu atoms were observed to appear again when Ag atoms disappeared from the surface at elevated temperatures. Contrary to these binary-metal systems, the interaction of In atoms with Si is not so strong as that of Au and Ag. Therefore, the Cu atoms can hinder the appearance of 6 X G-In and fi X m-In structures on the Si(ll1) surface. Furthermore, Cu atoms deposited on the Si(l11) surface are
441
not purged from the surface by the deposition of In. Copper adatoms even strengthen the bond of In atoms to the silicon surface, and the 4 X l-In structure can be observed at higher temperatures than the pure In/Si( 111) system. Although the observed 4 X 1 structure of the In/Cu/Si(l 11) system is considered to reflect the nature of In/Si(l 11) system, the difference of the 4 X 1 structure between In/Si and InCu/Si is expected to be studied in detail in order to understand the role of Cu atoms. We consider that the presence of Cu constructing a part of the 4 X 1 will possibly be evaluated in the Z-V curve of the LEED measurement. In summary, we have studied on In/Cu/Si(ll 1) system through the LEED observation at various indium thicknesses and substrate temperatures. At substrate temperatures of 450 and 5OO”C, the surface structure changes from ‘5 X 5’ to 4 X 1 with 0.5-1.6 ML of indium, while no other patterns but ‘5 X 5’ come to appear at 550°C. The interaction of In with Si atoms is found less strong compared to Au and Ag.
References 111I. Homma. Y. Tanishiro and K. Yagi, Surf. Sci. 242 (1991) 81.
121J. Yuhara, R. Ishigami and K. Morita, Surf. Sci. 326 (1995) 133.
[31 J.J. Lander and J. Morrison, Surf. Sci. 2 (1964) 553. [41 S. Ino, Surf. Sci. 82 (1979) L585. [51 M. Kawaje, S. Baba and A. Kinbara, Appl. Phys. Lett. 34 ( 1979) 748. Kraft, S.L. Sumey and F.P. Netzer, Surf. Sci. 340 (1995) 36. [71 H. Kemmann, F. Muller and H. Neddermeyer, Surf. Sci. 192 (1987) 11. [81J. Zegenhagen, E. Fontes, F. Grey and J.R. Patel, Phys. Rev. B 46 (1992) 1860.
[61J.
surface science ELSEVIER
Applied Surface Science 113/l
14 (1997) U5-447
LEED observation of 4 X l-In superstructure prepared on Sit 111 J-quasi-5 X 5-Cu surface Yoshio Suzaki ‘, Akira Saitoh, Takeo Nakano Department
*, Shigeru Baba
of Applied Physics, Seikei UniL~ersity 3-3-I. Kichijoji-Kitu, Musashino, Tokyo 180, Japan
Abstract The (LEED) surface changes
surface structure of {Cu, In} binary system on Si( 111) has been studied with the low energy electron diffraction technique. First, the quasi-‘5 x 5’-Cu structure can be prepared with the deposition of l-2.5 ML of Cu on the clean of Si at temperatures of 300-550°C. With the additional deposition of 0.5-1.6 ML of In, the surface structure into a 4 X 1 structure at temperatures between 400 and 500°C. At 550°C however, the ‘5 X 5’-Cu structure does not
disappear, despite of the supply of In vapor. Kewords:
4 X 1 structure;
Copper; Indium; Silicon; LEED
1. Introduction The surface structure of semiconductor crystals with metal adsorbates has been a central issue of surface physics, because of its diversity which reflects the subtle difference of the physical and/or chemical nature in each atomic species. Recently, binary systems of noble metals on Sic1 11) have been attracting attentions [ 1,2], since it has been found that the existence of foreign atoms affects the surface structure and the growth mode of the film. Yuhara et al. have studied the Sic1 11) surface with six pairs of atomic species among (Au, Ag, Cu} being deposited [2]. By employing the techniques of Rutherford backscattering (RBS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED), they have observed that Cu adatoms are driven away
* Corresponding author. E-mail: [email protected]. ’ Present address R&D Center, Advantest Inc. 0169-4332/97/$17.00 Copyright PII SOl69-4332(96)00929-4
from the surface when Au or Ag is deposited, while a weaker effect is observed in the Au-Ag pair. In the present study, the effect of In atoms is examined from the observation of structure of a binary adsorbate system of In-Cu on Sic1 11) surface by means of LEED. Indium derived superstructures on Si( 111) surface have been extensively studied since the first observation of fi X fi-In/Si(ll 1) by Lander and Morrison [3]. Other superstructures such as v& X !/% [4], 4 X 1, 1 X 1 [5] and fi X 6 [6] superstructures have also been found at relatively high temperature regions from 300 to 600°C. On the other hand, the Cu/Si(l I 1) surface is known as an incommensurate structure showing a structure very close to 5 X 5 of the Sic1 11) surface, i.e., the quasi-‘5 X 5’ [7]. This structure comes to appear on Si( 11 1) at the substrate temperature of 130-600°C with the deposition of 1.3 ML of copper. Zegenhagen et al. have investigated the Cu/Si(l 11) with X-ray standing wave measurement, and sug-
0 1997 Elsevier Science B.V. All rights reserved.
446
Y. Suzaki et al./Applied
Surface Science 113 / 114 (1997) 445-447
gested that the ‘5 X 5’ structure originates from the Cu,Si compound phase [S], but the detailed atomic configuration of the surface has not yet been made clear. Furthermore, the binary system of Cu-In alloy has been studied for a variety of potential applications such as a radiation-resistive solar cell and a new soldering system in VLSI devices. It is expected that the modification of the surface structure due to additional deposition of In atoms may give us a new information about the chemical nature of the ‘5 X 5’Cu surface.
2. Experiments Experiments were carried out in a UHV system whose base pressure was less than lo-’ Pa. Substrates of a strip of 5 X 28 X 0.3 mm3 were cut out from wafers of n-type SK1 11) ( p N 1 KIcm). The substrate surface was cleaned thermally in a UHV condition by a direct resistive heating up to 1200°C. After the thermal treatment, a clear 7 X 7 LEED pattern could be observed when it was cooled to a lower temperature. Then, copper and indium were evaporated from the respective tungsten filament. The deposition rate and the total amount of the deposits were measured by a quartz crystal oscillator. The substrate temperature from 350 to 800°C was determined from the electrical resistivity of the substrate using an equation of the temperature dependence of the carrier density in the intrinsic semiconductor. The temperature was calibrated with an optical pyrometer in the range 800- 1200°C. The basic performance and the accuracy of the present experimental setup was checked by the observation of an established phase diagram of the indium-Sic 111 l-7 x 7 [5]. On the Si-7 X 7 surface, 2.5 monolayers (ML) of copper were deposited at a rate of (1.0-2.0) X lo-* ML/s at the surface temperature of 500°C and the ‘5 X 5’-Cu structure could be observed with LEED. Here 1 ML is taken as 7.8 X lOI atoms/cm2 on the Sic1 11) surface. The ‘5 X 5’-Cu surface was found stable below 550°C. After the observation of the ‘5 X 5’-Cu, the deposition of indium followed intermittently by an amount of 0.1 ML. The surface was examined with LEED after each deposition. This
experiment was repeated at temperatures of 450°C. 500°C and 550°C respectively. The deposition rate of In was kept constant at (0.8-2.1) X 10e2 ML/s. The LEED observation was carried out at temperatures lower than 150°C.
3. Results and discussion On the Sic1 11) surface with Cu adatoms, a typical LEED pattern looked like small hexagons around the 1 X 1 spots. The distance between the 1 X 1 spots and the apex of a hexagon was 0.18, which agreed well with the quasi-‘5 X 5’-Cu/Si pattern reported by Kemmann et al. [7]. When indium was deposited on the surface at 450°C and 500°C a change in the LEED pattern from ‘5 X 5’ to 4 X 1 structure could be observed at a total amount of indium from 0.5 to 1.6 ML. The turn of the appearance of LEED patterns with the deposition of In occurred almost in the same fashion at this temperature range. With the deposition of 0.2-0.3 ML of In, the apexes of a hexagon around the 1 X 1 spot were split into six spots, i.e., a clearer ‘5 X 5’ pattern was observed. As the deposition exceeded 0.3 ML, 4 X 1 pattern began to appear on this ‘5 X 5’. This 4 X 1 LEED pattern had a three-fold symmetry. It indicates that the surface is a 4 X 1 multi-domain structure. The ‘5 X 5’ pattern disappeared at the deposition of 0.5 ML of In. Only the 4 X 1 pattern could be observed up to 1.6 ML. The fractional order LEED spots of the 4 X 1 became brightest at around 1.0 ML of the indium coverage. Then the LEED spots became weaker with the increase of In coverage. The 4 X 1 spots disappeared at coverages thicker than 1.6 ML, where the 11 pattern was observed.
~~~~~~
0
0.5
1.0
1.5
Coverage (ML)
2.0
0
I .o 1.5 Coverage (ML)
0.5
Fig. 1. The phase diagram of In/%( 111) (left) and In/Cu/Si( (right) surfaces.
2.0
I 1I)
Y. Suzuki et al. /Applied Surface Science 113 / 114 ( 1997) 445-447
At 550°C the ‘5 X 5’ Cu/Si(ll 1) pattern could also be observed clearly before the indium deposition. However. its structure did not change despite that the surface was exposed to the flux of indium. The result of the observations is summarized in Fig. 1. For comparison the phase diagram of In/Si(l 1 1) [5] is illustrated on the left in Fig. 1. It should be noted that the 4 X 1 structure of In/Cu/Si(l 11) can be observed at such a high temperature as 500°C which is higher by about 50°C than the appearance upper limit in the In/Si( 111) system. The 4 X l-In structure can be maintained on the silicon surface at higher temperatures with the presence of Cu atoms. This suggests that the desorption energy of indium on the silicon surface becomes larger and the mean stay time becomes longer on the Cu modified silicon surface. It is also noted that neither 6 X fi nor fi X fi structure is observed in the In/Cu/Si(lll) system, which means that these superstructures of In are less stable than the 4 X l-In. It is interesting to compare the result of the In-Cu system on the Sic11 1) surface with those of Au-Cu and Ag-Cu systems reported by Yuhara et al. [2]. They have reported that the thermal stability of Au and Ag deposits is not affected by Cu atoms, while the Cu atoms predeposited on the Si surface were purged from the surface, i.e., traces of Cu could not be observed by any means of RBS, AES and LEED. For the case of the Ag-Cu/Si(l 11) system, Cu atoms were observed to appear again when Ag atoms disappeared from the surface at elevated temperatures. Contrary to these binary-metal systems, the interaction of In atoms with Si is not so strong as that of Au and Ag. Therefore, the Cu atoms can hinder the appearance of 6 X G-In and fi X m-In structures on the Si(ll1) surface. Furthermore, Cu atoms deposited on the Si(l11) surface are
441
not purged from the surface by the deposition of In. Copper adatoms even strengthen the bond of In atoms to the silicon surface, and the 4 X l-In structure can be observed at higher temperatures than the pure In/Si( 111) system. Although the observed 4 X 1 structure of the In/Cu/Si(l 11) system is considered to reflect the nature of In/Si(l 11) system, the difference of the 4 X 1 structure between In/Si and InCu/Si is expected to be studied in detail in order to understand the role of Cu atoms. We consider that the presence of Cu constructing a part of the 4 X 1 will possibly be evaluated in the Z-V curve of the LEED measurement. In summary, we have studied on In/Cu/Si(ll 1) system through the LEED observation at various indium thicknesses and substrate temperatures. At substrate temperatures of 450 and 5OO”C, the surface structure changes from ‘5 X 5’ to 4 X 1 with 0.5-1.6 ML of indium, while no other patterns but ‘5 X 5’ come to appear at 550°C. The interaction of In with Si atoms is found less strong compared to Au and Ag.
References 111I. Homma. Y. Tanishiro and K. Yagi, Surf. Sci. 242 (1991) 81.
121J. Yuhara, R. Ishigami and K. Morita, Surf. Sci. 326 (1995) 133.
[31 J.J. Lander and J. Morrison, Surf. Sci. 2 (1964) 553. [41 S. Ino, Surf. Sci. 82 (1979) L585. [51 M. Kawaje, S. Baba and A. Kinbara, Appl. Phys. Lett. 34 ( 1979) 748. Kraft, S.L. Sumey and F.P. Netzer, Surf. Sci. 340 (1995) 36. [71 H. Kemmann, F. Muller and H. Neddermeyer, Surf. Sci. 192 (1987) 11. [81J. Zegenhagen, E. Fontes, F. Grey and J.R. Patel, Phys. Rev. B 46 (1992) 1860.
[61J.