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LEED observation of the platinum induced superstructures on Si substrates
Surface Science 100 (1980) L 4 5 7 - L 4 6 0 © North-Holland Publishing Company
SURFACE SCIENCE LETTERS LEED OBSERVATION OF THE PLATINUM INDUCED SUPERSTRUCTURES ON Si SUBSTRATES S. OKADA, Y. KISHIKAWA, K. OURA and T. HANAWA Electron Beam Laboratory, Faculty o f Engineering, Osaka University, Suita 565, Japan Received 23 May 19~0
Up to the present, various superstructures induced on Si surfaces by metals have been registered by many investigators. Very little, however, is known about the superstructures induced on the Si surfaces by metals which react with Si to form silicides, such as Ni, Pd, Pt etc., in spite of the usefulness of metal silicides in silicon devices [ 1]. For such reactive-metal/Si(111) systems, only four structures, (I × 1)Fe [2], (2 X 2)-Fe [3], (x/3 X x/3)R30°-Ta [4] and (x/19 X x/19)R + 23.4°-Ni [5,6], have been reported. As for a Pd/Si system, we had already made a detailed
LEED
~-¢7 --*
PATTERN
•
¢3
1.0 l'-"r O LIJ "I-
Pt (6Z, e.V) Si (91 ~ 92eV)
yLI.J
0.5
x5
LIJ C.9
o
O
5i(111) 0
/
0
500 ANNEALING
1000 ( * C ) TEMPERATURE
Fig. 1. Temperature range in which the (x/7 X x/7)R + 19.1 ° and (x/3 X x/3)R30 ° structures were observed and changes of the Auger peak heights of Pt (64 eV) and Si ( 9 1 - 9 2 eV) in the course of annealing, where both of the Auger peaks are normalized to their pure states. L457
L458
S. Okada et al. / P t induced superstructures on Si
O O OO
,° ,° t, 0
;l I oI I ; "l I " •;l o ; ; 0 ;•'•" b
t I
tl
Fig. 2. (a) LEED pattern of the (x/7 X x/7)R -+ 19.1 ° structure at 65 eV; (b) Schematic diagram of the LEED pattern; open circles denote the integral order spots, solid circles the fractional ones. Two reciprocal unit cells of the (x/7 × x/7)R +- 19.1° structure are shown by solid and broken lines, respectively.
LEED--AES study and found several superstructures [7]. In an extension of the previous work, we examined a Pt/Si system by LEED and AES. As a result, a Si(111) (x/7 × x/7)R -+ 19.1 ° - P t structure has been identified for the first time. All nreasurements were performed at around room temperature using a conventional three-grid LEED AES system combined with an ion gun. The base pressure of the work chamber was about 5 X 10 -~° Torr. p-Doped n-type Si wafers (5 × 25 X 0.5 mm) of the (111) and (100) orientations with bulk resistivity of 1 - 2 ~ cm were used as the substrates. Pt of 99.99% purity was evaporated with an electron gun at a pressure of the order of 10 -9 Torr. Heat-treatment of the specimen was achieved by Joule heating and its temperature was measured by a fine C A thermocouple mounted near the substrate. The error expected in such a system was corrected by comparing the thermocouple reading and the optical pyrometer reading at higher temperature. When several angstroms of platinum was deposited onto the Si(111) (7 X 7) clean surface at around room temperature, all the LEED spots were resolved into the background. At this stage, the Auger electron spectrum of platinum silicides [8] was observed. When the specimen was heat-treated in the temperature range 4 5 0 - 6 5 0 ° C , a ( x / 7 X , ~ / 7 ) R -+ 19.1 ° structure appeared and a (x/3 Xx/3)R30 ° structure was observed at higher temperature. Fig. 1 shows the temperature range in which the superstructures were observed, together with changes of the Auger peak heights of Pt (64 eV) and Si (91 -92 eV) in the course of annealing. A typical LEED pattern of the (x/7 × X/7)R -+ 19.1 ° structure is shown in fig. 2. The reciprocal lattice deduced from the LEED pattern is also given in the figure. The (x/7 × x/7)R-+ 19.1 ° structure was destroyed by slight sputtering, the Pt peak height decreased to about one third of the one before sputtering. Annealing the
S. Okada et al. / Pt induced superstructures on Si
L459
Fig. 3. LEED pattern of the (~/3 × x/3)R30° structure accompaniedwith weak facet at 45 eV.
specimen at 450-650°C regenerated the (x/7 × x/7)R +- 19.1 ° structure, though the Pt peak height was about the half of the one before sputtering. These phenomena suggest that the (x/7 × x/7)R + 19.1 ° structure may be induced by platinum atoms dispersed over several layers into the Si substrate. In fact, the change of the Pt Auger peak height from the (x/7 X X/7)R + 19.1 ° structure ranges over by a factor of ~2 as shown in fig. 1. The regeneration of superstructures in the sputtering and annealing cycle is common to the Pd/Si system. Fig. 3 shows the (x/3 X x/3)R30 ° structure. This structure was always accompanied with weak facet as well as enlarged spots, which are characteristic in the Pt/Si(111) system at higher temperature. The fact that the spot enlarging is noted for fractional order spots rather than integral ones suggests either the formation of antiphase domains or 2D crystal size effects [9], i.e. the x/3 domains are smaller than the coherence zone. The latter would effectively be promoted by the facetting. The observed facet was analyzed to belong to the (441) face of Si. The (X/3 X .X/3)R30° structure showed the same behavior as the (x/7 Xx/7)R + 19.1 ° structure in the sputtering and annealing cycle. In addition, the (x/3 × x/3)R30 ° structure persisted after annealing at 1000°C, the highest temperature in this study. On the Si(100) substrates, (2 X 1), c(2 X 4) and c(4 X 6) structures appeared in series in the annealing. Among these superstructures, the (2 X 1) and c(4 X 6) structures had also been observed in the Pd/Si(100) system. The c(4 X 6) structure persisted over 1000°C as well as the one in the Pd/Si(100). The structures observed at lower temperature may be interpreted as imperfection in a long range ordering of the c(4 X 6) structure. In summary, we have observed the platinum induced superstructures on the
L460
S. Okada et al. / Pt induced superstructures on Si
Si(111) and (100) substrates using LEED and AES. A more detailed study about the silicide formation and its thermal stability in thick Pt film is in progress.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9]
J.M. Andrews, J. Vacuum Sci. Technol. 11 (1974) 972. J.W.T. Ridgway and D. Haneman, Surface Sci. 24 (1971) 451. R.N. Thomas and M.H. Froncombe, Surface Sci. 25 (1971) 357. Y. Takeishi, 1. Sasaki and Y. loki, Surface Sci. 4 (1966) 317. A.J. van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. J.M. Charig and D.K. Skinner, Surface Sci. 19 (1970) 283. S. Okada, K. Oura, T. Hanawa and K. Satoh, Surface Sci. 97 (1980) 88. J.A. Roth and C.R. CroweU, J. Vacuum ScL Technol. 15 (1978) 1317. P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1.
SURFACE SCIENCE LETTERS LEED OBSERVATION OF THE PLATINUM INDUCED SUPERSTRUCTURES ON Si SUBSTRATES S. OKADA, Y. KISHIKAWA, K. OURA and T. HANAWA Electron Beam Laboratory, Faculty o f Engineering, Osaka University, Suita 565, Japan Received 23 May 19~0
Up to the present, various superstructures induced on Si surfaces by metals have been registered by many investigators. Very little, however, is known about the superstructures induced on the Si surfaces by metals which react with Si to form silicides, such as Ni, Pd, Pt etc., in spite of the usefulness of metal silicides in silicon devices [ 1]. For such reactive-metal/Si(111) systems, only four structures, (I × 1)Fe [2], (2 X 2)-Fe [3], (x/3 X x/3)R30°-Ta [4] and (x/19 X x/19)R + 23.4°-Ni [5,6], have been reported. As for a Pd/Si system, we had already made a detailed
LEED
~-¢7 --*
PATTERN
•
¢3
1.0 l'-"r O LIJ "I-
Pt (6Z, e.V) Si (91 ~ 92eV)
yLI.J
0.5
x5
LIJ C.9
o
O
5i(111) 0
/
0
500 ANNEALING
1000 ( * C ) TEMPERATURE
Fig. 1. Temperature range in which the (x/7 X x/7)R + 19.1 ° and (x/3 X x/3)R30 ° structures were observed and changes of the Auger peak heights of Pt (64 eV) and Si ( 9 1 - 9 2 eV) in the course of annealing, where both of the Auger peaks are normalized to their pure states. L457
L458
S. Okada et al. / P t induced superstructures on Si
O O OO
,° ,° t, 0
;l I oI I ; "l I " •;l o ; ; 0 ;•'•" b
t I
tl
Fig. 2. (a) LEED pattern of the (x/7 X x/7)R -+ 19.1 ° structure at 65 eV; (b) Schematic diagram of the LEED pattern; open circles denote the integral order spots, solid circles the fractional ones. Two reciprocal unit cells of the (x/7 × x/7)R +- 19.1° structure are shown by solid and broken lines, respectively.
LEED--AES study and found several superstructures [7]. In an extension of the previous work, we examined a Pt/Si system by LEED and AES. As a result, a Si(111) (x/7 × x/7)R -+ 19.1 ° - P t structure has been identified for the first time. All nreasurements were performed at around room temperature using a conventional three-grid LEED AES system combined with an ion gun. The base pressure of the work chamber was about 5 X 10 -~° Torr. p-Doped n-type Si wafers (5 × 25 X 0.5 mm) of the (111) and (100) orientations with bulk resistivity of 1 - 2 ~ cm were used as the substrates. Pt of 99.99% purity was evaporated with an electron gun at a pressure of the order of 10 -9 Torr. Heat-treatment of the specimen was achieved by Joule heating and its temperature was measured by a fine C A thermocouple mounted near the substrate. The error expected in such a system was corrected by comparing the thermocouple reading and the optical pyrometer reading at higher temperature. When several angstroms of platinum was deposited onto the Si(111) (7 X 7) clean surface at around room temperature, all the LEED spots were resolved into the background. At this stage, the Auger electron spectrum of platinum silicides [8] was observed. When the specimen was heat-treated in the temperature range 4 5 0 - 6 5 0 ° C , a ( x / 7 X , ~ / 7 ) R -+ 19.1 ° structure appeared and a (x/3 Xx/3)R30 ° structure was observed at higher temperature. Fig. 1 shows the temperature range in which the superstructures were observed, together with changes of the Auger peak heights of Pt (64 eV) and Si (91 -92 eV) in the course of annealing. A typical LEED pattern of the (x/7 × X/7)R -+ 19.1 ° structure is shown in fig. 2. The reciprocal lattice deduced from the LEED pattern is also given in the figure. The (x/7 × x/7)R-+ 19.1 ° structure was destroyed by slight sputtering, the Pt peak height decreased to about one third of the one before sputtering. Annealing the
S. Okada et al. / Pt induced superstructures on Si
L459
Fig. 3. LEED pattern of the (~/3 × x/3)R30° structure accompaniedwith weak facet at 45 eV.
specimen at 450-650°C regenerated the (x/7 × x/7)R +- 19.1 ° structure, though the Pt peak height was about the half of the one before sputtering. These phenomena suggest that the (x/7 × x/7)R + 19.1 ° structure may be induced by platinum atoms dispersed over several layers into the Si substrate. In fact, the change of the Pt Auger peak height from the (x/7 X X/7)R + 19.1 ° structure ranges over by a factor of ~2 as shown in fig. 1. The regeneration of superstructures in the sputtering and annealing cycle is common to the Pd/Si system. Fig. 3 shows the (x/3 X x/3)R30 ° structure. This structure was always accompanied with weak facet as well as enlarged spots, which are characteristic in the Pt/Si(111) system at higher temperature. The fact that the spot enlarging is noted for fractional order spots rather than integral ones suggests either the formation of antiphase domains or 2D crystal size effects [9], i.e. the x/3 domains are smaller than the coherence zone. The latter would effectively be promoted by the facetting. The observed facet was analyzed to belong to the (441) face of Si. The (X/3 X .X/3)R30° structure showed the same behavior as the (x/7 Xx/7)R + 19.1 ° structure in the sputtering and annealing cycle. In addition, the (x/3 × x/3)R30 ° structure persisted after annealing at 1000°C, the highest temperature in this study. On the Si(100) substrates, (2 X 1), c(2 X 4) and c(4 X 6) structures appeared in series in the annealing. Among these superstructures, the (2 X 1) and c(4 X 6) structures had also been observed in the Pd/Si(100) system. The c(4 X 6) structure persisted over 1000°C as well as the one in the Pd/Si(100). The structures observed at lower temperature may be interpreted as imperfection in a long range ordering of the c(4 X 6) structure. In summary, we have observed the platinum induced superstructures on the
L460
S. Okada et al. / Pt induced superstructures on Si
Si(111) and (100) substrates using LEED and AES. A more detailed study about the silicide formation and its thermal stability in thick Pt film is in progress.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9]
J.M. Andrews, J. Vacuum Sci. Technol. 11 (1974) 972. J.W.T. Ridgway and D. Haneman, Surface Sci. 24 (1971) 451. R.N. Thomas and M.H. Froncombe, Surface Sci. 25 (1971) 357. Y. Takeishi, 1. Sasaki and Y. loki, Surface Sci. 4 (1966) 317. A.J. van Bommel and F. Meyer, Surface Sci. 8 (1967) 467. J.M. Charig and D.K. Skinner, Surface Sci. 19 (1970) 283. S. Okada, K. Oura, T. Hanawa and K. Satoh, Surface Sci. 97 (1980) 88. J.A. Roth and C.R. CroweU, J. Vacuum ScL Technol. 15 (1978) 1317. P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1.