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The (7 × 7) ↔ (1 × 1) phase transition on Si(111)
Surface Science 162 (1985) 163-168 North-Holland. Amsterdam
163
THE (7 × 7) ~ (I × 1) P H A S E T R A N S I T I O N ON S i ( l l i ) W. TELIEPS and E. BAUER Phvsikahsches Institut der Techntschen UnwersttiJt Clausthal, 3392 (Tausthal- Zellerfeld and SFB 126 GOttingen- Clausthal, Fed. Rep. of German)' Received 1 April 1985; accepted for publication 6 May 1985
The ( 7 x 7 ) ~ (1 x 1) phase transition of the S i ( l l l ) surface at 1100 K was studied with high lateral resolution by low energy electron reflection microscopy (LEERM) in an UHV surface electron microscope. The micrographs presented demonstrate that it is of first order. The (7 x 7) domains nucleate mainly at steps of monoatomic height but also in the terraces and grow preferentially in three high symmetry directions. Surfaces quenched from high temperatures ( > 1450 K) transform only partially to the (7 x 7) structure.
1. Introduction Twenty years ago Lander [1] mentioned for the first time the transformation of the (7 x 7) to the (1 x 1) structure on Si(ll 1). This phase transition occurs at 1100 K and is reversible. Since its discovery it was investigated several times (for a review, see [2]), but no generally accepted picture of it exists up to now. Even the type of transition - first order or continuous - is still controversial. Therefore it seemed worthwhile to look at the transition with low energy electron reflection microscopy, abbreviated LEERM [3,4]. In LEERM, surfaces are imaged by elastically reflected electrons with energies in the LEED range (0-200 V). At the same time, the LEED pattern can be obtained. The power of this combined approach - diffraction and high resolution imaging - is well known from the study of bulk properties with transmission electron microscopy and has been demonstrated for surfaces by the Tokyo group (reflection electron microscopy (REM) and R H E E D ) [5,6] some years ago in their investigations of the phase transition dealt with in this paper.
2. Experimental A schematic of the UHV surface electron microscope is shown in fig. 1. The electron beam of a field emission gun ((2) in fig. 1) is focussed into the back focal plane of the objective lens (5), an electrostatic cathode lens. The specimen 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
164
11,
IV. Teheps, E. Bauer / (7× 7) *-~ (I × 1) Phase transition on St(111)
ZI
6
15
I 16 [ ' ~
16
~'/ ~ Fig. 1. Schematic of the UHV surface micro~ope: (1) magnetic deflection field, (2) field emission electron gun. (3) quadrupoles. (4) beam forming lens, (5) cathode lens, (6) stigmator, (7) specimen. (8) screen, (9) intermediate lens, (10) proJector lens, (11 ) filter lens (121 multichannel plates, (13) -I'V camera, (16) beam alignment coils. For operation as an emission microscope a Hg lamp (14) and a conventional electron gun (15) are attached.
(7) is part of this lens. Within the cathode lens the electrons are decelerated; they reach the specimen on parallel trajectories and with an energy given by the potential difference between field emitter and specimen, typically 0 200 V. In the case of crystalline objects, the diffracted electrons form the LEED pattern in the back focal plane. The area analyzed is small (6-8/~m) compared to a conventional LEED apparatus. In addition to the diffraction pattern a real (LEERM) image of the surface is formed. The imaging column transfers either the LEED pattern or the LEERM image to the final screen (8). LEERM promises very high resolution; from calculations [3] 2 nm arc expected. So far 20 nm has been achieved. The instrument operates at 15 kV and at a pressure of 2 × 10-1° Torr. The specimen can be heated during observation by electron bombardment from the backside. The silicon surface was oriented within 0.4 ° of the (111) plane. After polishing it was oxidized in dry oxygen, etched in an H F - a l c o h o l mixture and dipped shortly in boiling HNO~. In UHV the crystal was heated in intervals for 1 h to 1450 K.
3. Results and discussion The transition temperature was measured pyrometrically as T, = 1100 K ( + 15 K). The specimen temperature relative to TOcould be controlled within 2 K. The micrographs in fig. 2 show the phase transition on wide terraces ( > 1 /~m). Only the (00) beam and its vicinity are used for imaging. The specimen
14/. Telieps. E. Bauer / ( 7 x 7) 4-. I I × I) Phase transztion on Si( l l l )
2pro
.
165
•f ~ , ~ : . . ~ :, ~.~-
2;--
.'~
•
b
•
,
- . 4 .
.
J,..
~ , _ x ~
Fig. 2. L E E R M micrographs of the (7 x 7) ~. (1 × 1) phase transition. The specimen shifted slightly during observation. Note that the uppermost step shifts considerably from (b) to (c)" in (e), it nucleates at the same position as in (a) and shifts again from (f) to (g): (h) directional dependence of domain growth. For further explanation see text• Electron energy was 3 eV. Photographs are taken from video record. The broad dark line visible on all micrographs in this article is due to a crack in one of the microchannel plates ((I 2) in fig. 1 ).
166
IV. Telieps, E. Bauer / ( 7 x 7) ~ (1 x 1) Phase transitton on S t ( I l l )
was heated to 1110 K for 1 min so that only the (1 x 1) structure appeared in the diffraction pattern. Lowering the temperature to 1094 K leads to nucleation of regions with the (7 x 7) structure on the upper side of monoatomic steps [5]. They are visible as bright areas in fig. 2a. The (7 x 7) domains spread over the terraces (fig. 2b). In the upper half of the micrograph lies a plateau, so that domains grow from both sides. Additional domains nucleate in terraces and expand. Finally most of surface is covered by (7 x 7) domains (fig. 2c). Along the bases of the steps and along the domain boundaries dark stripes persist. Between figs. 2a and 2c, 60 s elapsed. When the crystal is heated again to 1110 K, the transition reverses. The (7 x 7) domains shrink quite uniformly in width (fig. 2d). Thus, at least for these broad terrace, no pronounced dependence of the transition temperature on the place in the terrace is observed. After completion of the transition, the crystal temperature is again lowered. this time to 1088 K. Again (7 x 7) domains nucleate at the steps and in the terraces; the density of both types of domains is larger now. The domains in the terraces form preferentially in the region adjacent to the bases of the steps. The plateau, which is bounded by upper sides of steps, stays free from such domains. The directional dependence of the domain growth is obvious from all micrographs: there are three easy directions for domain growth reflecting the threefold symmetry of the crystal. This is once more shown in fig. 2h: domains which have to expand antiparailel to an easy direction grow mainly parallel to the steps. Quenching the crystal to room temperature from just above the transition temperature always yields surfaces covered completely by (7 x 7) domains. Upon quenching from higher temperatures ( > 1450 K) narrow terraces ( < 250 nm) are mainly covered by domains originating at the steps (see fig. 3). Wider terraces also show irregularly shaped domains in the terraces. The steps are decorated by narrow stripes of the (1 x 1) structure approximately 20 nm wide. Such stripes of a stress induced (1 x 1) structure were postulated earlier by Hanemann [7]. The stripes between the domains are less pronounced. Very broad terraces in the ~m range are not covered completely by (7 x 7) domains (fig. 4). The domains from the favoured nucleation sites have formed broad bands of the (7 x 7) structure along the steps. Between the steps many patches of the (7 x 7) structure are visible, but only some have expanded to large triangular domains, whose heights are oriented along the directions of easy growth. Heating the crystal to approximately 900 K makes the (7 x 7) domains grow over the whole surface; further heating to just above the transition temperature and subsequent quenching to an arbitrary temperature below the transition point always yields surfaces completely transformed to the (7 x 7) structure. These observations indicate, that the (7 x 7) domains grow slower if the
14/. Telieps, E. Bauer / ( 7 x 7) ~ ¢1 x I) Phase transttton on Si(111)
167
•" :Z1/-;
Ipm ..
,-
Fig. 3. LEERM micrograph of Si(lll spacing.
quenched from 1450 K. Electron energy is 10 eV. Small step
Fig. 4. LEERM micrograph of Si(111) quenched from 1450 K. Electron energy is 10 eV. Large step spacing. The inset shows the (slightly distorted) LEED pattern of this area at 45 eV. The LEED pattern is not filtered: the bright spot to the right is due to secondary electrons.
168
W Telteps. I-. Bauer / 17 X 7) ~ (1 × I) Phase transttton on S t ( I l l )
crystal is quenched from high temperature. A possible explanation for this is that mechanical stress in the surface persists above the transition temperature and facilitates the (7 × 7) reconstruction upon cooling. At higher temperatures this memory effect is destroyed. All the micrographs presented in this article show that the (7 × 7) and (1 × !) structures are clearly separated phases, which is typical of a first order phase transition. This result is supported by LEED investigations [2]. The Tokyo group drew the same conclusion from their first studies [5] but later favoured a continuous transition [6]. Reflection electron microscopic (REM) images [5,6] are generally strongly foreshortened in the direction of observation. This. and the reduced resolving power in this direction severely limit the method. For example the detailed shape of domains is not ascertainable in the micrographs. LEERM has a resolving power comparable to REM: the electrons are incident perpendicularly on the surface so that no foreshortening occurs.
4. Conclusion
it has been demonstrated that LEERM is an adequate method for the study of surface phase transitions. It is not possible to extract the details readily visible in a micrograph from a LEED study alone. The investigations presented are far from being complete. The kinetics of domain nucleation and growth will be studied in future work.
References [1] [21 [3] 14] [5] [6] [7]
J.J. Lander, Surfaces Sci. 1 (1964) 125. W. Witt and E. Bauer, Surface Sci., to be published. E. Bauer, Uhramicroscopy 17 t1985) 51. W. Telieps and E. Bauer, Ultramicro~opy 17 (1985) 57. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 109 (19811 353. Y. "I'anishiro. K. Takayanagi and K. Yagi, Uhramicroscopy 11 (1983)95. I). ttanemann, Phys. Rev. B25 0982) 1370.
163
THE (7 × 7) ~ (I × 1) P H A S E T R A N S I T I O N ON S i ( l l i ) W. TELIEPS and E. BAUER Phvsikahsches Institut der Techntschen UnwersttiJt Clausthal, 3392 (Tausthal- Zellerfeld and SFB 126 GOttingen- Clausthal, Fed. Rep. of German)' Received 1 April 1985; accepted for publication 6 May 1985
The ( 7 x 7 ) ~ (1 x 1) phase transition of the S i ( l l l ) surface at 1100 K was studied with high lateral resolution by low energy electron reflection microscopy (LEERM) in an UHV surface electron microscope. The micrographs presented demonstrate that it is of first order. The (7 x 7) domains nucleate mainly at steps of monoatomic height but also in the terraces and grow preferentially in three high symmetry directions. Surfaces quenched from high temperatures ( > 1450 K) transform only partially to the (7 x 7) structure.
1. Introduction Twenty years ago Lander [1] mentioned for the first time the transformation of the (7 x 7) to the (1 x 1) structure on Si(ll 1). This phase transition occurs at 1100 K and is reversible. Since its discovery it was investigated several times (for a review, see [2]), but no generally accepted picture of it exists up to now. Even the type of transition - first order or continuous - is still controversial. Therefore it seemed worthwhile to look at the transition with low energy electron reflection microscopy, abbreviated LEERM [3,4]. In LEERM, surfaces are imaged by elastically reflected electrons with energies in the LEED range (0-200 V). At the same time, the LEED pattern can be obtained. The power of this combined approach - diffraction and high resolution imaging - is well known from the study of bulk properties with transmission electron microscopy and has been demonstrated for surfaces by the Tokyo group (reflection electron microscopy (REM) and R H E E D ) [5,6] some years ago in their investigations of the phase transition dealt with in this paper.
2. Experimental A schematic of the UHV surface electron microscope is shown in fig. 1. The electron beam of a field emission gun ((2) in fig. 1) is focussed into the back focal plane of the objective lens (5), an electrostatic cathode lens. The specimen 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
164
11,
IV. Teheps, E. Bauer / (7× 7) *-~ (I × 1) Phase transition on St(111)
ZI
6
15
I 16 [ ' ~
16
~'/ ~ Fig. 1. Schematic of the UHV surface micro~ope: (1) magnetic deflection field, (2) field emission electron gun. (3) quadrupoles. (4) beam forming lens, (5) cathode lens, (6) stigmator, (7) specimen. (8) screen, (9) intermediate lens, (10) proJector lens, (11 ) filter lens (121 multichannel plates, (13) -I'V camera, (16) beam alignment coils. For operation as an emission microscope a Hg lamp (14) and a conventional electron gun (15) are attached.
(7) is part of this lens. Within the cathode lens the electrons are decelerated; they reach the specimen on parallel trajectories and with an energy given by the potential difference between field emitter and specimen, typically 0 200 V. In the case of crystalline objects, the diffracted electrons form the LEED pattern in the back focal plane. The area analyzed is small (6-8/~m) compared to a conventional LEED apparatus. In addition to the diffraction pattern a real (LEERM) image of the surface is formed. The imaging column transfers either the LEED pattern or the LEERM image to the final screen (8). LEERM promises very high resolution; from calculations [3] 2 nm arc expected. So far 20 nm has been achieved. The instrument operates at 15 kV and at a pressure of 2 × 10-1° Torr. The specimen can be heated during observation by electron bombardment from the backside. The silicon surface was oriented within 0.4 ° of the (111) plane. After polishing it was oxidized in dry oxygen, etched in an H F - a l c o h o l mixture and dipped shortly in boiling HNO~. In UHV the crystal was heated in intervals for 1 h to 1450 K.
3. Results and discussion The transition temperature was measured pyrometrically as T, = 1100 K ( + 15 K). The specimen temperature relative to TOcould be controlled within 2 K. The micrographs in fig. 2 show the phase transition on wide terraces ( > 1 /~m). Only the (00) beam and its vicinity are used for imaging. The specimen
14/. Telieps. E. Bauer / ( 7 x 7) 4-. I I × I) Phase transztion on Si( l l l )
2pro
.
165
•f ~ , ~ : . . ~ :, ~.~-
2;--
.'~
•
b
•
,
- . 4 .
.
J,..
~ , _ x ~
Fig. 2. L E E R M micrographs of the (7 x 7) ~. (1 × 1) phase transition. The specimen shifted slightly during observation. Note that the uppermost step shifts considerably from (b) to (c)" in (e), it nucleates at the same position as in (a) and shifts again from (f) to (g): (h) directional dependence of domain growth. For further explanation see text• Electron energy was 3 eV. Photographs are taken from video record. The broad dark line visible on all micrographs in this article is due to a crack in one of the microchannel plates ((I 2) in fig. 1 ).
166
IV. Telieps, E. Bauer / ( 7 x 7) ~ (1 x 1) Phase transitton on S t ( I l l )
was heated to 1110 K for 1 min so that only the (1 x 1) structure appeared in the diffraction pattern. Lowering the temperature to 1094 K leads to nucleation of regions with the (7 x 7) structure on the upper side of monoatomic steps [5]. They are visible as bright areas in fig. 2a. The (7 x 7) domains spread over the terraces (fig. 2b). In the upper half of the micrograph lies a plateau, so that domains grow from both sides. Additional domains nucleate in terraces and expand. Finally most of surface is covered by (7 x 7) domains (fig. 2c). Along the bases of the steps and along the domain boundaries dark stripes persist. Between figs. 2a and 2c, 60 s elapsed. When the crystal is heated again to 1110 K, the transition reverses. The (7 x 7) domains shrink quite uniformly in width (fig. 2d). Thus, at least for these broad terrace, no pronounced dependence of the transition temperature on the place in the terrace is observed. After completion of the transition, the crystal temperature is again lowered. this time to 1088 K. Again (7 x 7) domains nucleate at the steps and in the terraces; the density of both types of domains is larger now. The domains in the terraces form preferentially in the region adjacent to the bases of the steps. The plateau, which is bounded by upper sides of steps, stays free from such domains. The directional dependence of the domain growth is obvious from all micrographs: there are three easy directions for domain growth reflecting the threefold symmetry of the crystal. This is once more shown in fig. 2h: domains which have to expand antiparailel to an easy direction grow mainly parallel to the steps. Quenching the crystal to room temperature from just above the transition temperature always yields surfaces covered completely by (7 x 7) domains. Upon quenching from higher temperatures ( > 1450 K) narrow terraces ( < 250 nm) are mainly covered by domains originating at the steps (see fig. 3). Wider terraces also show irregularly shaped domains in the terraces. The steps are decorated by narrow stripes of the (1 x 1) structure approximately 20 nm wide. Such stripes of a stress induced (1 x 1) structure were postulated earlier by Hanemann [7]. The stripes between the domains are less pronounced. Very broad terraces in the ~m range are not covered completely by (7 x 7) domains (fig. 4). The domains from the favoured nucleation sites have formed broad bands of the (7 x 7) structure along the steps. Between the steps many patches of the (7 x 7) structure are visible, but only some have expanded to large triangular domains, whose heights are oriented along the directions of easy growth. Heating the crystal to approximately 900 K makes the (7 x 7) domains grow over the whole surface; further heating to just above the transition temperature and subsequent quenching to an arbitrary temperature below the transition point always yields surfaces completely transformed to the (7 x 7) structure. These observations indicate, that the (7 x 7) domains grow slower if the
14/. Telieps, E. Bauer / ( 7 x 7) ~ ¢1 x I) Phase transttton on Si(111)
167
•" :Z1/-;
Ipm ..
,-
Fig. 3. LEERM micrograph of Si(lll spacing.
quenched from 1450 K. Electron energy is 10 eV. Small step
Fig. 4. LEERM micrograph of Si(111) quenched from 1450 K. Electron energy is 10 eV. Large step spacing. The inset shows the (slightly distorted) LEED pattern of this area at 45 eV. The LEED pattern is not filtered: the bright spot to the right is due to secondary electrons.
168
W Telteps. I-. Bauer / 17 X 7) ~ (1 × I) Phase transttton on S t ( I l l )
crystal is quenched from high temperature. A possible explanation for this is that mechanical stress in the surface persists above the transition temperature and facilitates the (7 × 7) reconstruction upon cooling. At higher temperatures this memory effect is destroyed. All the micrographs presented in this article show that the (7 × 7) and (1 × !) structures are clearly separated phases, which is typical of a first order phase transition. This result is supported by LEED investigations [2]. The Tokyo group drew the same conclusion from their first studies [5] but later favoured a continuous transition [6]. Reflection electron microscopic (REM) images [5,6] are generally strongly foreshortened in the direction of observation. This. and the reduced resolving power in this direction severely limit the method. For example the detailed shape of domains is not ascertainable in the micrographs. LEERM has a resolving power comparable to REM: the electrons are incident perpendicularly on the surface so that no foreshortening occurs.
4. Conclusion
it has been demonstrated that LEERM is an adequate method for the study of surface phase transitions. It is not possible to extract the details readily visible in a micrograph from a LEED study alone. The investigations presented are far from being complete. The kinetics of domain nucleation and growth will be studied in future work.
References [1] [21 [3] 14] [5] [6] [7]
J.J. Lander, Surfaces Sci. 1 (1964) 125. W. Witt and E. Bauer, Surface Sci., to be published. E. Bauer, Uhramicroscopy 17 t1985) 51. W. Telieps and E. Bauer, Ultramicro~opy 17 (1985) 57. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 109 (19811 353. Y. "I'anishiro. K. Takayanagi and K. Yagi, Uhramicroscopy 11 (1983)95. I). ttanemann, Phys. Rev. B25 0982) 1370.