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Step dynamics on highly oriented and vicinal surfaces of Si(111)
surface science ELSEVIER
Surface Science 331-333 (1995) 1408-1413
Step dynamics on highly oriented and vicinal surfaces of Si(111) G. Wilhelmi a, T. Kampschulte a, H. Neddermeyer b,, a InstitutfiJr Experimentalphysik der Ruhr-Universitiit Bochum, D-44780 Bochum, Germany b Fachbereich Physik, Martin-Luther-Universit~it Halle-Wittenberg, D-06099 Halle / Saale, Germany
Received 4 August 1994; accepted for publication 2 December 1994
Abstract
By using high-temperature scanning tunneling microscopy we have measured step fluctuations on highly oriented and vicinal surfaces of Si(lll) in the temperature range up to the (7 x 7)/(1 X 1) phase transition temperature (1100 K). On mono-layer steps these fluctuations mostly occur in a width corresponding to a 7 X 7 unit cell half. By an Arrhenius plot of the fluctuation rate an activation energy for 7 x 7 kink diffusion of 1.8 +__0.3 eV was estimated. On vicinal surfaces and just below the phase transition temperature fluctuations of entire 7 X 7 domains are observed. Keywords: Scanning tunneling microscopy; Silicon; Surface thermodynamics
1. Introduction
On vicinal surfaces of S i ( l l l ) the (7 x 7 ) / ( 1 X 1) phase transition and the step structure are intimately related to each other. Above the phase transition temperature (Tc = 1100 K [1]) the steps are equally spaced due to their repulsive interaction [2]. Below Tc a delicate balance between formation of 7 × 7 reconstructed (111) terraces (leading to an energy gain) and partial approach of steps to step bunches (which costs energy) is realized [2]. In situ studies of such problems at high temperatures have mostly been performed by using low-energy electron diffraction (LEED) [1], low-energy electron diffraction microscopy (LEEM) [3] and reflection electron microscopy [4,5]. Since recently, these effects can also be investigated by using of high-temperature scanning tunneling microscopy
* Corresponding author.
(HTSTM), which has been developed by a number of groups [6-12]. The application of HTSTM measurements to such problems represents an essential progress in this field since the individual high-temperature processes can be analyzed with atomic resolution. It has to be mentioned in this context that the atomic fluctuations at high temperatures usually are much faster than the scanning speed of the STM. This means that one has to consider that the HTSTM image normally does not correspond to a " s n a p s h o t " of the system but rather provides information on the time-averaged z position of the tip for each pixel. Since for a conventional identification of some individual atomic feature (for example, one adatom of the 7 × 7 structure under optimum resolution, and at least one 7 × 7 unit cell half in case of worse resolution) several scanning lines are necessary, the time scale for the atomic events which can be resolved directly is not much shorter than 0.01-0.1 s. In the present work we describe results on stepped S i ( l l l ) which have been obtained by a newly devel-
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00131-X
G. Wilhelmi et aI./Surface Science 331-333 (1995) 1408-1413
oped H T S T M equipment. Two aspects will be considered here. Firstly, we follow the behavior o f individual kinks on mono- and double-layer steps, respectively. These kinks normally are observed with a width which corresponds to the extension of one 7 X 7 unit cell half. By measuring in the temperature range around 1000 K w e determined the kink fluctuation as a function of temperature. F r o m these measurements an Arrhenius plot can be constructed and the activation energy for diffusion of the 7 X 7 kink be evaluated. Secondly, on a vicinal surface o f S i ( l l l ) we have studied the formation and growth of 7 X 7 reconstructed parts directly b e l o w Tc. In this case we have observed fluctuations in the size of the 7 X 7 domains which have not been reported before.
1409
2. Experimental For the measurements we have developed a HTSTM, which has been used so far up to sample temperatures of 1100 K. The coarse approach between sample and tip is accomplished b y means of a mechanical system, where the single-tube scanner is mounted on precision guide rails and shifted by a lever system. The thermal stability is fairly good, although from time to time unwanted sudden changes in the z position probably due to thermal relaxation in the entire set-up gives rise to lateral and vertical shifts in the z position. The sample m a y be heated indirectly b y radiation or by a direct current. The latter has been used for the S i ( l l l ) samples, which
Fig. 1. CCT from Si(lll) at a sample temperature T= 875 K. The image was measured at a sample bias voltage U= -2.4 V (sample negative), a tunneling current I = 0.2 nA on an area of 45 X 50 nm2.
1410
G. Wilhelmi et al. / Surface Science 331-333 (1995) 1408-1413
a
b
c
Fig. 2. Steps on Si(111) measured in a time of 2 s/frame at a temperature of (a) 960 K (24 X 24 nm2), (b) 1005 K (18 X 18 nm 2) and (c) 1115 K (11 X 11 nm2).
have a size of approximately 2 x 6 m m 2. The measurements reported here have been obtained on highly oriented wafers and others with a misorientation of around 4 ° . After thermal equilibrium has been reached (which is usually accomplished by continuous mild heating) the thermal drift is rather small. A t the highest temperature and for t h e stepped Si(111) samples the thermal fluctuation o f the step edges are considerably larger than the drift as m a y be concluded from the stable location of 7 X 7 reconstruction. Examples will be given later. The temperature has been determined by means of an optical pyrometer which has been controlled once by a thermocouple. The absolute accuracy of the temperature values is better than 20 K. Sample treatment consisted in degassing and flashing off the native oxide film. Most of the S T M measurements have been performed by using a fast mode where a data acquisition rate of up to 100 k H z / p i x e l can be reached. Normally, the system h a s been used with moderate speed (a few s / i m a g e ) to improve the signal to noise ratio of the constant current topographies (CCTs).
are needed to find instabilities on a step. Thermal excitations may be observed in form of kinks which move along the step edge. In case of Si(111) these kinks are not monatomic but rather correspond to 7 x 7 unit cell halves (we call them 7 X 7 kinks). Surprisingly, when we examined the behavior of 7 X 7 kinks on mono-layer steps (in case of Si(111) they actually correspond to a Si double-layer), we found that double-layer steps were the first step arrangements to become instable. It has to be emphasized that at room temperature double-layer steps are not known. In a sequence of fast measurements we found that such double-layer steps decompose during 100
%
3. Results and discussion While for metal surfaces thermal excitation of steps are already observed at room and slightly elevated temperatures [6,12] (they may be identified by a " f r i z z y " appearance o f the step edge), for Si(111) temperatures in the order of at least 800 K
1 . . . . 0,850
I . . . . 0,900
I . . . . 0,950
I ' 1,000
'
1 / T . IO00K
Fig. 3. Arrhenius plot of the 7 × 7 kink fluctuations.
G. Wilhelmi et al. / S u r f a c e Science 331-333 (1995) 1408-1413
cooling into mono-layer steps. The width of the newly formed terrace between these two mono-layer steps mostly approaches multiples of 7 X 7 unit cell halves. The stability of such a double-layer step is also connected to the step orientation. In Fig. 1 an example is given. Here a double-layer step is running from the top to the bottom of the image. The upper part of the step is oriented parallel to the corner holes of the 7 X 7 unit cells ([110] direction), while the lower part approximately follows the long diagonal. It is obvious from the image that the upper part of the step is more stable than the lower one. The "frizziness" on the upper part has an amplitude corresponding to the size of an adatom of the 7 X 7 structure and on the lower part the amplitude may reach the width of a unit cell half. This means that in case of a [110] step adatom diffusion along the step edge (including attachment and detachment to a 7 x 7 unit cell) is the first process which may be excited
1411
thermally. For a more instable step configuration away from [110] these processes and in addition formation of complete unit cell halves (giving rise to 7 X 7 kinks) have to be taken into account. Increasing the temperature to 1000 K the monolayer steps become instable, too. In Fig. 2 three CCTs are reproduced which have been acquired at temperatures of 960 K (a), 1005 K (b) and 1115 K (c). On Fig. 2a two 7 X 7 kinks are recognized which exhibit a small "frizziness" due to adatom diffusion. For 1005 K (Fig. 2b) 7 X 7 kink diffusion is found. The image shows two steps where the upper one shows a kink (right-hand side) and in addition two positions where the step has retracted to the left-hand side (referred to the horizontal scanning direction) by a 7 x 7 unit cell half for one or two scanning lines. These step "excursions" may be understood by a forwards or backwards diffusion of a 7 X 7 kink along the step edge which is followed by a subse-
Fig. 4. CCT from Si(111) at T = 1100 K, measured at U = - 1.8 V, I = 1.5 nA on an area of 50 X 55 nmz.
1412
G. Wilhelmi et al. / Surface Science 331-333 (1995) 1408-1413
quent movement of the kink in the opposite direction. Increasing the temperature gives rise to a drastic increase of these step excursions (Fig. 2c). It is apparent from the latter image that the time-averaged step position remains constant. W e should mention that within the accuracy of the temperature measurement the temperature is still somewhat below Tc, since 7 × 7 unit cells are still visible on the right-hand side. It is tempting to analyze the step excursions due to diffusion and fluctuation of 7 X 7 kinks as a function of temperature. If w e assume that the number of observed excursions (backwards or forwards) per s ( n / s ) is proportional to the diffusion coefficient, which should follow the Boltzmann factor, we m a y estimate the activation energy E D for 7 X 7 kink diffusion by an Arrhenius plot (Fig. 3). The
data points follow reasonably well a straight line from which an activation energy of 1.8 __+0.3 eV is determined. One difficulty in this kind of analysis is that only those excursions are considered which happen to occur between subsequent scanning lines. If we assume that the number of o b s e r v e d excursions is proportional to their actual number and this fraction is independent of the magnitude of that number, the data analysis still provides the correct value for E D, since a constant factor does not change the slope of the Arrhenius plot. Williams et al. [15] determined the kink energy of a 7 × 7 reconstructed monoatomic step by fitting an experimental phase diagram of vicinal S i ( l l l ) . They found a value of 1.5 eV which is comparable to our activation energy. Since many individual atomic processes contribute to a 7 X 7 kink diffusion it is difficult to decide, at present,
Fig. 5. Three subsequent CCTs of Si(lll) at T = 1025 K, measured at U = -3.3 V, I = 1 nA on an area of 15 X 35 nm2. The measurement time was 7 s per frame.
G. Wilhelmi et aL /Surface Science 331-333 (1995) 1408-1413
whether such an agreement is reasonable or somewhat fortuitous. In case of a vicinal surface the behavior of steps directly below the phase transition temperature Tc = 1100 K is particularly interesting. For vicinal surfaces of S i ( l l l ) the formation of narrow 7 × 7 reconstructed domains along the [110] directions just below Tc has already been described in the H T S T M work of Miki et al. [14]. In the neighboring area of the 7 X 7 domain, which should correspond to the equally spaced (1 X 1) parts of S i ( l l l ) , atomic features could hardly be resolved in Ref. [14]. In contrast to the latter work, our results clearly indicate the presence of stepped structures on both sides of the 7 X 7 domain (Fig. 4). Step excursions by one 7 X 7 kink can also be identified on the right-hand side of the 7 X 7 domain. The width of the terraces in the stepped parts is slightly larger that that of a 7 X 7 unit cell. Otherwise one could speculate whether or not these narrow terraces have already developed 7 X 7 unit cells. As a final result we show in Fig. 5 three subsequent images of the same part of the surface, where local fluctuations of the size of 7 × 7 domains are recognized. The measured temperature is already distinctly below Tc. That drift effects are negligible may be inferred from the shape of the 7 X 7 unit cells, which in case of noticeable drift should be distorted correspondingly. This means that we have indeed observed that the size of the domain shows a fluctuation on a time scale of 7 s which in this case was the measurement time for one frame. In summary, we have developed equipment for HTSTM, where S i ( l l l ) can be measured up to temperatures of 1100 K. By using a fast data acquisition system and due to the rigid construction of the microscope the images were measured in a time scale down to 2 s. From an Arrhenius plot of the step fluctuations up to 1100 K an activation energy for
1413
7 × 7 kink diffusion of 1.8 ___0.3 eV was evaluated. Just below the (7 × 7 ) / ( 1 × 1) phase transition temperature we observed growth and fluctuations of 7 X 7 domains.
Acknowledgements This work has been supported by the Volkswagenstiftung and by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg " D y n a m i s c h e Prozesse an Festkfrperoberfliichen".
References [1] P.A. Bennett and M.W. Webb, Surf. Sci. 104 (1981) 74. [2] R.J. Phaneuf, E.D. Williams and N.C. Bartelt, Phys. Rev. B 38 (1988) 1984. [3] W. Telieps and E. Bauer, Surf. Sci. 162 (1985) 163. [4] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surf. Sci. 109 (1981) 353. [5] A.V. Latyshev, A.L. Aseev, A.B. Krasilnikovand S.L Stenin, Surf. Sci. 227 (1990) 24. [6] L. Kuipers,M.S. Hoogemanand J.W.M. Frenken,Phys. Rev. Lett. 71 (1993) 3517. [7] R.M. Feenstra,A.J. Slavin, G.A. Held and M.A. Lutz, Phys. Rev. Lett. 66 (1991) 3257. [8] H. Tokumotoand M. Iwatsuld,Jpn. J. Appl. Phys. 32 (1993) 1368. [9] H.J.W. Zandvliet, E.B. Elswijk and E.J. van Loenen, Surf. Sci. 272 (1992) 264. [10] B.S. Swartzentruber, Y.-W. Mo, R. Kariotis, M.G. Lagally and M.B. Webb, Phys. Rev. Lett. 65 (1990) 1913. [11] R.J. Behm, private communication. [12] M. Giesen-Seibert,R. Jentjens, M. Poensgen and H. Ibach, Phys. Rev. Lett. 71 (1993) 3521. [13] R. Kliese, B. R6ttger, D. Badt and H. Neddermeyer,Ultramicroscopy 42-44 (1992) 824. [14] K. Miki, Y. Morita, H. Tokumoto, T. Sato, M. Iwatsuki, M. Suzuki and T. Fukuda, Ultramicroscopy42-44 (1992) 851. [15] E.D. Williams, R.J. Phaneuf, J. Wie, N.C. Bartelt and T.L. Einstein, Surf. Sci. 310 (1994) 451.
Surface Science 331-333 (1995) 1408-1413
Step dynamics on highly oriented and vicinal surfaces of Si(111) G. Wilhelmi a, T. Kampschulte a, H. Neddermeyer b,, a InstitutfiJr Experimentalphysik der Ruhr-Universitiit Bochum, D-44780 Bochum, Germany b Fachbereich Physik, Martin-Luther-Universit~it Halle-Wittenberg, D-06099 Halle / Saale, Germany
Received 4 August 1994; accepted for publication 2 December 1994
Abstract
By using high-temperature scanning tunneling microscopy we have measured step fluctuations on highly oriented and vicinal surfaces of Si(lll) in the temperature range up to the (7 x 7)/(1 X 1) phase transition temperature (1100 K). On mono-layer steps these fluctuations mostly occur in a width corresponding to a 7 X 7 unit cell half. By an Arrhenius plot of the fluctuation rate an activation energy for 7 x 7 kink diffusion of 1.8 +__0.3 eV was estimated. On vicinal surfaces and just below the phase transition temperature fluctuations of entire 7 X 7 domains are observed. Keywords: Scanning tunneling microscopy; Silicon; Surface thermodynamics
1. Introduction
On vicinal surfaces of S i ( l l l ) the (7 x 7 ) / ( 1 X 1) phase transition and the step structure are intimately related to each other. Above the phase transition temperature (Tc = 1100 K [1]) the steps are equally spaced due to their repulsive interaction [2]. Below Tc a delicate balance between formation of 7 × 7 reconstructed (111) terraces (leading to an energy gain) and partial approach of steps to step bunches (which costs energy) is realized [2]. In situ studies of such problems at high temperatures have mostly been performed by using low-energy electron diffraction (LEED) [1], low-energy electron diffraction microscopy (LEEM) [3] and reflection electron microscopy [4,5]. Since recently, these effects can also be investigated by using of high-temperature scanning tunneling microscopy
* Corresponding author.
(HTSTM), which has been developed by a number of groups [6-12]. The application of HTSTM measurements to such problems represents an essential progress in this field since the individual high-temperature processes can be analyzed with atomic resolution. It has to be mentioned in this context that the atomic fluctuations at high temperatures usually are much faster than the scanning speed of the STM. This means that one has to consider that the HTSTM image normally does not correspond to a " s n a p s h o t " of the system but rather provides information on the time-averaged z position of the tip for each pixel. Since for a conventional identification of some individual atomic feature (for example, one adatom of the 7 × 7 structure under optimum resolution, and at least one 7 × 7 unit cell half in case of worse resolution) several scanning lines are necessary, the time scale for the atomic events which can be resolved directly is not much shorter than 0.01-0.1 s. In the present work we describe results on stepped S i ( l l l ) which have been obtained by a newly devel-
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00131-X
G. Wilhelmi et aI./Surface Science 331-333 (1995) 1408-1413
oped H T S T M equipment. Two aspects will be considered here. Firstly, we follow the behavior o f individual kinks on mono- and double-layer steps, respectively. These kinks normally are observed with a width which corresponds to the extension of one 7 X 7 unit cell half. By measuring in the temperature range around 1000 K w e determined the kink fluctuation as a function of temperature. F r o m these measurements an Arrhenius plot can be constructed and the activation energy for diffusion of the 7 X 7 kink be evaluated. Secondly, on a vicinal surface o f S i ( l l l ) we have studied the formation and growth of 7 X 7 reconstructed parts directly b e l o w Tc. In this case we have observed fluctuations in the size of the 7 X 7 domains which have not been reported before.
1409
2. Experimental For the measurements we have developed a HTSTM, which has been used so far up to sample temperatures of 1100 K. The coarse approach between sample and tip is accomplished b y means of a mechanical system, where the single-tube scanner is mounted on precision guide rails and shifted by a lever system. The thermal stability is fairly good, although from time to time unwanted sudden changes in the z position probably due to thermal relaxation in the entire set-up gives rise to lateral and vertical shifts in the z position. The sample m a y be heated indirectly b y radiation or by a direct current. The latter has been used for the S i ( l l l ) samples, which
Fig. 1. CCT from Si(lll) at a sample temperature T= 875 K. The image was measured at a sample bias voltage U= -2.4 V (sample negative), a tunneling current I = 0.2 nA on an area of 45 X 50 nm2.
1410
G. Wilhelmi et al. / Surface Science 331-333 (1995) 1408-1413
a
b
c
Fig. 2. Steps on Si(111) measured in a time of 2 s/frame at a temperature of (a) 960 K (24 X 24 nm2), (b) 1005 K (18 X 18 nm 2) and (c) 1115 K (11 X 11 nm2).
have a size of approximately 2 x 6 m m 2. The measurements reported here have been obtained on highly oriented wafers and others with a misorientation of around 4 ° . After thermal equilibrium has been reached (which is usually accomplished by continuous mild heating) the thermal drift is rather small. A t the highest temperature and for t h e stepped Si(111) samples the thermal fluctuation o f the step edges are considerably larger than the drift as m a y be concluded from the stable location of 7 X 7 reconstruction. Examples will be given later. The temperature has been determined by means of an optical pyrometer which has been controlled once by a thermocouple. The absolute accuracy of the temperature values is better than 20 K. Sample treatment consisted in degassing and flashing off the native oxide film. Most of the S T M measurements have been performed by using a fast mode where a data acquisition rate of up to 100 k H z / p i x e l can be reached. Normally, the system h a s been used with moderate speed (a few s / i m a g e ) to improve the signal to noise ratio of the constant current topographies (CCTs).
are needed to find instabilities on a step. Thermal excitations may be observed in form of kinks which move along the step edge. In case of Si(111) these kinks are not monatomic but rather correspond to 7 x 7 unit cell halves (we call them 7 X 7 kinks). Surprisingly, when we examined the behavior of 7 X 7 kinks on mono-layer steps (in case of Si(111) they actually correspond to a Si double-layer), we found that double-layer steps were the first step arrangements to become instable. It has to be emphasized that at room temperature double-layer steps are not known. In a sequence of fast measurements we found that such double-layer steps decompose during 100
%
3. Results and discussion While for metal surfaces thermal excitation of steps are already observed at room and slightly elevated temperatures [6,12] (they may be identified by a " f r i z z y " appearance o f the step edge), for Si(111) temperatures in the order of at least 800 K
1 . . . . 0,850
I . . . . 0,900
I . . . . 0,950
I ' 1,000
'
1 / T . IO00K
Fig. 3. Arrhenius plot of the 7 × 7 kink fluctuations.
G. Wilhelmi et al. / S u r f a c e Science 331-333 (1995) 1408-1413
cooling into mono-layer steps. The width of the newly formed terrace between these two mono-layer steps mostly approaches multiples of 7 X 7 unit cell halves. The stability of such a double-layer step is also connected to the step orientation. In Fig. 1 an example is given. Here a double-layer step is running from the top to the bottom of the image. The upper part of the step is oriented parallel to the corner holes of the 7 X 7 unit cells ([110] direction), while the lower part approximately follows the long diagonal. It is obvious from the image that the upper part of the step is more stable than the lower one. The "frizziness" on the upper part has an amplitude corresponding to the size of an adatom of the 7 X 7 structure and on the lower part the amplitude may reach the width of a unit cell half. This means that in case of a [110] step adatom diffusion along the step edge (including attachment and detachment to a 7 x 7 unit cell) is the first process which may be excited
1411
thermally. For a more instable step configuration away from [110] these processes and in addition formation of complete unit cell halves (giving rise to 7 X 7 kinks) have to be taken into account. Increasing the temperature to 1000 K the monolayer steps become instable, too. In Fig. 2 three CCTs are reproduced which have been acquired at temperatures of 960 K (a), 1005 K (b) and 1115 K (c). On Fig. 2a two 7 X 7 kinks are recognized which exhibit a small "frizziness" due to adatom diffusion. For 1005 K (Fig. 2b) 7 X 7 kink diffusion is found. The image shows two steps where the upper one shows a kink (right-hand side) and in addition two positions where the step has retracted to the left-hand side (referred to the horizontal scanning direction) by a 7 x 7 unit cell half for one or two scanning lines. These step "excursions" may be understood by a forwards or backwards diffusion of a 7 X 7 kink along the step edge which is followed by a subse-
Fig. 4. CCT from Si(111) at T = 1100 K, measured at U = - 1.8 V, I = 1.5 nA on an area of 50 X 55 nmz.
1412
G. Wilhelmi et al. / Surface Science 331-333 (1995) 1408-1413
quent movement of the kink in the opposite direction. Increasing the temperature gives rise to a drastic increase of these step excursions (Fig. 2c). It is apparent from the latter image that the time-averaged step position remains constant. W e should mention that within the accuracy of the temperature measurement the temperature is still somewhat below Tc, since 7 × 7 unit cells are still visible on the right-hand side. It is tempting to analyze the step excursions due to diffusion and fluctuation of 7 X 7 kinks as a function of temperature. If w e assume that the number of observed excursions (backwards or forwards) per s ( n / s ) is proportional to the diffusion coefficient, which should follow the Boltzmann factor, we m a y estimate the activation energy E D for 7 X 7 kink diffusion by an Arrhenius plot (Fig. 3). The
data points follow reasonably well a straight line from which an activation energy of 1.8 __+0.3 eV is determined. One difficulty in this kind of analysis is that only those excursions are considered which happen to occur between subsequent scanning lines. If we assume that the number of o b s e r v e d excursions is proportional to their actual number and this fraction is independent of the magnitude of that number, the data analysis still provides the correct value for E D, since a constant factor does not change the slope of the Arrhenius plot. Williams et al. [15] determined the kink energy of a 7 × 7 reconstructed monoatomic step by fitting an experimental phase diagram of vicinal S i ( l l l ) . They found a value of 1.5 eV which is comparable to our activation energy. Since many individual atomic processes contribute to a 7 X 7 kink diffusion it is difficult to decide, at present,
Fig. 5. Three subsequent CCTs of Si(lll) at T = 1025 K, measured at U = -3.3 V, I = 1 nA on an area of 15 X 35 nm2. The measurement time was 7 s per frame.
G. Wilhelmi et aL /Surface Science 331-333 (1995) 1408-1413
whether such an agreement is reasonable or somewhat fortuitous. In case of a vicinal surface the behavior of steps directly below the phase transition temperature Tc = 1100 K is particularly interesting. For vicinal surfaces of S i ( l l l ) the formation of narrow 7 × 7 reconstructed domains along the [110] directions just below Tc has already been described in the H T S T M work of Miki et al. [14]. In the neighboring area of the 7 X 7 domain, which should correspond to the equally spaced (1 X 1) parts of S i ( l l l ) , atomic features could hardly be resolved in Ref. [14]. In contrast to the latter work, our results clearly indicate the presence of stepped structures on both sides of the 7 X 7 domain (Fig. 4). Step excursions by one 7 X 7 kink can also be identified on the right-hand side of the 7 X 7 domain. The width of the terraces in the stepped parts is slightly larger that that of a 7 X 7 unit cell. Otherwise one could speculate whether or not these narrow terraces have already developed 7 X 7 unit cells. As a final result we show in Fig. 5 three subsequent images of the same part of the surface, where local fluctuations of the size of 7 × 7 domains are recognized. The measured temperature is already distinctly below Tc. That drift effects are negligible may be inferred from the shape of the 7 X 7 unit cells, which in case of noticeable drift should be distorted correspondingly. This means that we have indeed observed that the size of the domain shows a fluctuation on a time scale of 7 s which in this case was the measurement time for one frame. In summary, we have developed equipment for HTSTM, where S i ( l l l ) can be measured up to temperatures of 1100 K. By using a fast data acquisition system and due to the rigid construction of the microscope the images were measured in a time scale down to 2 s. From an Arrhenius plot of the step fluctuations up to 1100 K an activation energy for
1413
7 × 7 kink diffusion of 1.8 ___0.3 eV was evaluated. Just below the (7 × 7 ) / ( 1 × 1) phase transition temperature we observed growth and fluctuations of 7 X 7 domains.
Acknowledgements This work has been supported by the Volkswagenstiftung and by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg " D y n a m i s c h e Prozesse an Festkfrperoberfliichen".
References [1] P.A. Bennett and M.W. Webb, Surf. Sci. 104 (1981) 74. [2] R.J. Phaneuf, E.D. Williams and N.C. Bartelt, Phys. Rev. B 38 (1988) 1984. [3] W. Telieps and E. Bauer, Surf. Sci. 162 (1985) 163. [4] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surf. Sci. 109 (1981) 353. [5] A.V. Latyshev, A.L. Aseev, A.B. Krasilnikovand S.L Stenin, Surf. Sci. 227 (1990) 24. [6] L. Kuipers,M.S. Hoogemanand J.W.M. Frenken,Phys. Rev. Lett. 71 (1993) 3517. [7] R.M. Feenstra,A.J. Slavin, G.A. Held and M.A. Lutz, Phys. Rev. Lett. 66 (1991) 3257. [8] H. Tokumotoand M. Iwatsuld,Jpn. J. Appl. Phys. 32 (1993) 1368. [9] H.J.W. Zandvliet, E.B. Elswijk and E.J. van Loenen, Surf. Sci. 272 (1992) 264. [10] B.S. Swartzentruber, Y.-W. Mo, R. Kariotis, M.G. Lagally and M.B. Webb, Phys. Rev. Lett. 65 (1990) 1913. [11] R.J. Behm, private communication. [12] M. Giesen-Seibert,R. Jentjens, M. Poensgen and H. Ibach, Phys. Rev. Lett. 71 (1993) 3521. [13] R. Kliese, B. R6ttger, D. Badt and H. Neddermeyer,Ultramicroscopy 42-44 (1992) 824. [14] K. Miki, Y. Morita, H. Tokumoto, T. Sato, M. Iwatsuki, M. Suzuki and T. Fukuda, Ultramicroscopy42-44 (1992) 851. [15] E.D. Williams, R.J. Phaneuf, J. Wie, N.C. Bartelt and T.L. Einstein, Surf. Sci. 310 (1994) 451.