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Triangular facets on the GaAs(110) surface observed by scanning tunneling microscopy
Surface Science 217 (1989) 289-297 North-Holland, Amsterdam
289
TRIANGULAR FACETS ON THE GaAs(ll0) SURFACE OBSERVED BY SCANNING TUNNELING MICROSCOPY R. MILLER,
R. COENEN,
B. KOSLOWSKI
Sektion Physik, Universitiit Miinchen, Schellingstrasse
and M. RAUSCHER
4, 8000 Munich 40, Fed. Rep. of Germany
Received 23 November 1988; accepted for publication 13 February 1989
The cleaved (110) surface of p-doped GaAs has been investigated by STM. Facets with a height of single atomic layers were found to extend over large areas of a few thousand A*. The steps occur along the [OOl], the [112] and in a few cases the [114] direction, leading to a repetitive pattern of triangular structures. While the steps along the [112] and [114] direction can be formed by every multiple of a single atomic layer, only multiples of two layers are found for the [OOl] direction.
1. Introduction The surface of GaAs cleaved in the (110) plane is generally assumed to form atomically flat terraces of large dimensions. This has different practical applications, e.g. the cleaved (110) faces form the laser mirrors of the GaAslaser diodes. From indirect measurements by LEED [1,2] and AES [3] it has been concluded that different step heights of one and two atomic layers are present on cleaved GaAs. Images taken by REM [4] revealed facets of different forms, however the exact height of the steps could not be resolved. Feenstra and Fein [5] investigated the (110) surface on the atomic scale by means of STM revealing a row structure formed by chains of alternating Ga and As atoms. The STM was shown to be capable of selectively imaging of either of the atomic species [6]. By tunneling spectroscopy the influence of different overlayers on the electronic states at the surface could be analyzed [7-91. In this paper we present the first investigated of facets on the (110) surface of GaAs performed by STM. The steps are found to follow exclusively three directions which can easily be identified from the crystallographic structure. For one direction the step height is given only by even multiples of atomic layers. 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
290
R. Mijller et al. / Triangular facets on GaAs(Il0)
observed by STM
2. Experimental The experiments were performed in a UHV apparatus at a pressure of about 3 x lOPi mbar. The STM described in detail elsewhere [lo] uses a stack of metal plates with Viton spacers for vibrational damping very similar to the “pocket size STM” proposed by Gerber et al. [ll]. The coarse positioning is performed by piezo-driven micrometer screws allowing a three-dimensional adjustment of the tip relative to the sample. A “single tube scanner” [12] is used for the fine positioning of the tip. The motion of the tube scanner has been measured optically with a quadrant detector before the instrument was assembled. Furthermore the instrument was calibrated in x-y direction by measurements on graphite, the z-direction was calibrated relative to x and y by measurements on inclined surfaces. The tip has been cleaned prior to the measurement by field emission at a few PA for about half an hour. The sample has been prepared from a wafer of p-doped (Zn) material with a doping concentration of n = 1 X lo’* cmv3. The rectangular shaped piece of GaAs (8 x 6 mm) was scratched to predetermine where the sample will cleave. It has been mounted such that about 3 mm in length are held tightly by the sample holder, to which it has been contacted by an indium-tin alloy, providing a low contact resistance. It has been cleaved in UHV by applying a
I (nA)
l1
-3
-2
2
3
-1 -1 --
-2 --3 '$.V
4 Fig.
1. I-V
characteristics ((d ln Z)/dV)
(au.)
h
measured on the p-doped GaAs sample: (a) Z versus V versus V, corresponding to the density of states.
V; (b)
R Miiller et al. / Triangular facets on GaAs(Il0)
observed by STM
291
force in the [OOi] direction at the overhanging end. The remaining piece has a size of 4-6 mm.’ In order to check the preparation of the sample and the tip, the band structure at the surface has been analyzed by tunneling spectroscopy. To measure the 1-v characteristics the feedback control for the z-position of tunneling tip is stopped at a given z-value for a time of about 0.1 s. During that interval the tunneling voltage undergoes a linear ramp from + 2.5 to - 2.5 V sample voltage. The tunneling current 1 as a function of Y is recorded by a computer. Fig. 1 shows the I-f/ characteristic obtained for the p-doped sample averaged over hundred cycles. At positive sample bias one can see the contribution of electrons tunneling from the tip into the conduction band of GaAs, conversely, at negative sample bias the contribution of electrons tunneling out of the valence band to the tip is observed. To visualize the band structure we have calculated the quantity ((d ln f)/dV)V= (dl/dY)(V/I) which has been shown [13] to correspond to the local density of states. Fig. lb gives ((d ln f)/dV)V calculated numerically from I. The band gap of 1.43 eV is clearly visible. The results agree well with the data given by Feenstra et al. [61.
3. Results Fig. 2 shows a linescan of the cleaved (170) plane of GaAs recorded by an x-y recorder. The tunneling current was 300 pA at a positive sample bias of 2.5 V. The size of the scan area is 840 X 840 A2. The z-axis is labeled in units of 2 A, which means that the z-axis is enhanced by a factor of 4 relative to the x- and y-scaling. The sample is oriented with the [OOl] direction along the y-axis. As can be recognized from the figure all steps have heights which are multiples of 2 A. This value corresponds well to the spacing of atomic layers of GaAs in the (110) orientation, which is d = (1/2&)a = 1.99 A for a lattice constant of a = 5.65 A. The step lines are found along three different directions, named “ a,” “b” and “c”. The steps in fig. 2 are labelled by two indices, the first for the direction, the second for the height in units of atomic layers. In order to get a better representation of the geometry of those structures, data were acquired by a computer providing a top view of the investigated areas. Figs. 3a and 3b show a ,‘op view and the corresponding linescan of a large scan area of 3000 x 3000 A. Due to the enlarged scan area the pattern of facets appears denser than in the previous figure. Again most of the steps are 2 or 4 A high, a few having a height of 6 or 8 A. The x and y directions have the same orientation as in the previous figure. In the topview one can see that within the experimental error the direction labeled “a” runs along the [OOlJ direction. The second major step line labeled
292
R. Miller et al. / Triangular facets on GaAs(I IO) observed by STM
a2
cl
b’2
a2
cl
c2
cl
bi
Fig. 2. STM image of the cleaved (110) surface of GaAs recorded by an x-y recorder. The size of the scan area is 840 X 840 R. The tunneling current is 300 pA at a positive sample bias of 2.5 V. Three different directions of step lines labeled “ a”, “ b”, and “c” can be recognized. The height of the steps is given in number of monolayers by the second index.
“c” forms an angle of about (Y= 33O to the [OOl] direction. The step line “b”, which is less frequent, appears at an angle of p = 18” relative to [OOl]. The two angles almost exactly fulfill the relation tan /3= : tan (Y. By comparing two subsequent scans the effect of thermal drift can be estimated The drift is below 20 A for the time which is needed to acquire the data for the whole image. The drift vector is mainly directed along the x-axis. Due to the large scan range of 3000 x 3000 A this leads to a maximum error for the measured angles of less then lo and is neglected. A schematic representation of the atomic structure of the top layers of the GaAs(li0) surface is shown in fig. 4a. It is characterized by zig-zag-chains of alternating Ga and As atoms running along the [llO] direction. They are perpendicularly intersected by the [OOl] direction forming the major step line indicated by the full line. This is expected since the face exposed by steps in the [OOl] direction is the (110) plane which is a macroscopic cleavage plane. The second orientation of step lines is the [112] direction represented by the dashed line. It connects neighbouring chains with a difference of one pair of Ga and As atoms. The angle between the [OOl] and the [112] direction is (Y= 35’ given by tan (Y= 1/2fi. This agrees fairly well with the observed angle of the facet structure. The difference lies within the experimental error due to the relative calibration of the piezo scanner for the x and y direction.
R. MNer et al. / Triangular facets on GaAs(Il0) observed by STM
293
a
Fig. 3. STM image of a large scan area of the cleaved (li0) surface of GaAs recorded by a computerized data acquisition system. The size of the scan area is 3000 X 3000 A*. The tunneling current is 300 pA at a positive sample bias of 2.5 V: (a) linescan representation; (b) topview representation.
294
Triangular facets on GaAs(Il0)
observed by STh4
a Ga-Atas
As-Atcau
2nd layer 1st
layer
0th
layer
@
b Fig. 4. Schematic representation of the atomic arrangement forming a triangular facet on the (ITO) surface of GaAs. (a) Topview representation: (0) Ga-atom top layer; (0) As-atom top layer; (s) Ga-atom second layer; (0) As-atom second layer. (b) Pseudo-three-dimensional representation.
The face exposed by the [112] step line lies in the (117) plane which is known to be As-terminated [14]. Therefore, it is assumed that the step line is formed by As atoms which is also suggested by the better packing compared to the Ga-terminated arrangement indicated by the dotted line.
R. Miller et al. / Triangular facets on GaAs(II0)
1
2
3
observed by STM
295
step height
Fig. 5. Histogram of the step heights of the area shown in fig. 3 sorted according to the direction of the steps.
The step line at an angle p with tan p = 3 tan (Yis readily identified by the dashed line in the [114] direction. It connects the next neighbouring zig-zag lines with a difference of one pair of Ga and As atoms. The theoretical value for the angle is p T 19.5 ‘, the observed one is about 18”. The corresponding face lies in the (221) plane. From the figure it is evident that this step line is less favoured since only every second row fits exactly in the geometry. This is also reflected in irregularities in the step line which can be clearly seen in fig. 2. From our measurement it cannot be concluded whether this face is terminated by only one atomic species or, as suggested in fig. 4, altematingly by both As and Ga atoms. Considering the height of the steps in figs. 2 and 3, a correlation between the step height and the direction of the steps is clearly recognized. To point this out, fig. 5 shows histograms of the step heights for the three directions evaluated from fig. 3. For the [112] and the [114] direction every multiple of a single atomic step is possible with a decreasing probability for increasing height. However, for the [OOl] direction only steps which are multiples of two monolayers are found, more than 90% being steps with a height of two layers. This can be explained by a rearrangement of the otherwise unsaturated dangling bonds. If there was a step of one monolayer along the [OOl] direction, there would be one dangling bond at the end of every zig-zag chain of Ga and As atoms, as can be seen in fig. 4a. If the step consists of two layers the
296
R Mijller et al. / Triangular facets on GaAs(ll0)
observed by STM
dangling bonds of the top layer can form saturated pairs with those of the next lower. For every atom at the end of a chain in the top layer there is an atom of the corresponding other species at the end of a chain in the lower layer, which is at the normal binding distance. The experimental results strongly suggest this pairwise arrangement. To visualize the structure of a facet with a height of two atomic layers formed by steps in the [OOl] and the [112] directions, fig. 4b shows a three-dimensional representation. The structure has been simplified so far that all bond lengths and angles are identical to those of the bulk structure of GaAs. The rotation of the outermost Ga-As bonds is neglected. As one can see the (110) face exposed by the step in the [OOl] direction is terminated by pairs of Ga and As atoms.
4. Discussion Our measurements reveal that Jhe (li0) surface of cleaved GaAs, although it is essentially flat with a few A height difference over a range of several thousands of A, can exhibit very characteristic step lines. Similar structures were previously observed with REM by Hsu et al. [4], however their identification of the orientation of the step lines appears to be incorrect. In contrast to their measurement no evidence for dislocations was found on our sample. The two major orientations for step lines are the [OOl] and the [112] direction, running parallel in the (ii0) and the (lli) planes respectively. With a lower probability also steps along the [114] direction have been observed. By comparison to the macroscopic situation and because of the better packing of the atoms at the step it is concluded that the steps in the [112] direction are probably As-terminated. From indirect measurements by LEED [1,2] and AES [3] it was deduced that monoatomic as well as diatomic steps are to be found on a cleaved (110) surface of GaAs. By our investigation the step height can be correlated to the direction of the steps. While the steps along [112] and [114] are 1, 2 and 3 monolayers high, only steps with a height of 2 and in a few cases 4 have been found for the [OOl] direction. This can be explained by a pairwise arrangement of the otherwise dangling bonds at the step. This is only possible for multiples of two monolayers.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft for generous support, and we gratefully acknowledge helpful discussions with F. Giessibl and T. HZnsch.
R Miiller et al. / Triangular facets on G&s{1 10) observed by STM
297
The authors would like to thank Dr. Loehnert and Mrs. Ludwig (Wacker Chemitronic) and Dr. J. Eckstein (Varian) for providing the GaAs material.
References [l] [2] [3] [4] [5] (61 [7] [8] [9] [lo] [ll] [12] [13] [14]
M. Henzler, Surface Sci. 22 (1970) 12. D.W. Welkie and M.G. LagaBy, J. Vacuum Sci. Technol. 16 (1979) 784. W. Rat&e, Y.R. Xing and G.D. Shen, Surface Sci. 120 (1982) 67. T. Hsu, S. Iijima and J.M. Cowley, Surface Sci. 137 (1984) 551. R.M. Feenstra and A.P. Fein, Phys. Rev. B 32 (1985) 1394. R.M. Feenstra, J.A. Stroscio, J. Tersoff and A.P. Fein, Phys. Rev. Letters 58 (1987) 1192. J.A. Stroscio, R.M. Feenstra and A.P. Fein, Phys. Rev. Letters 58 (1987) 1668. R.M. Feenstra and P. M&rtensson, Phys. Rev. Letters 61 (1988) 447. P. M%rtensson and R.M. Feenstra, to be published. R. Moller, B. Koslowski and M. Rauscher, to be published. Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer, Rev. Sci. Instr. 57 (1986) 221. G. Binnig and D.P.E. Smith, Rev. Sci. Instr. 57 (1986) 1688. N.D. Lang, Phys. Rev. B 43 (1986) 5947. R.D. Bringans and R.Z. Bachrach, in: The Structure of Surfaces, Eds. M.A. Van Hove and S.Y. Tong (Springer, Berlin, 1985).
289
TRIANGULAR FACETS ON THE GaAs(ll0) SURFACE OBSERVED BY SCANNING TUNNELING MICROSCOPY R. MILLER,
R. COENEN,
B. KOSLOWSKI
Sektion Physik, Universitiit Miinchen, Schellingstrasse
and M. RAUSCHER
4, 8000 Munich 40, Fed. Rep. of Germany
Received 23 November 1988; accepted for publication 13 February 1989
The cleaved (110) surface of p-doped GaAs has been investigated by STM. Facets with a height of single atomic layers were found to extend over large areas of a few thousand A*. The steps occur along the [OOl], the [112] and in a few cases the [114] direction, leading to a repetitive pattern of triangular structures. While the steps along the [112] and [114] direction can be formed by every multiple of a single atomic layer, only multiples of two layers are found for the [OOl] direction.
1. Introduction The surface of GaAs cleaved in the (110) plane is generally assumed to form atomically flat terraces of large dimensions. This has different practical applications, e.g. the cleaved (110) faces form the laser mirrors of the GaAslaser diodes. From indirect measurements by LEED [1,2] and AES [3] it has been concluded that different step heights of one and two atomic layers are present on cleaved GaAs. Images taken by REM [4] revealed facets of different forms, however the exact height of the steps could not be resolved. Feenstra and Fein [5] investigated the (110) surface on the atomic scale by means of STM revealing a row structure formed by chains of alternating Ga and As atoms. The STM was shown to be capable of selectively imaging of either of the atomic species [6]. By tunneling spectroscopy the influence of different overlayers on the electronic states at the surface could be analyzed [7-91. In this paper we present the first investigated of facets on the (110) surface of GaAs performed by STM. The steps are found to follow exclusively three directions which can easily be identified from the crystallographic structure. For one direction the step height is given only by even multiples of atomic layers. 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
290
R. Mijller et al. / Triangular facets on GaAs(Il0)
observed by STM
2. Experimental The experiments were performed in a UHV apparatus at a pressure of about 3 x lOPi mbar. The STM described in detail elsewhere [lo] uses a stack of metal plates with Viton spacers for vibrational damping very similar to the “pocket size STM” proposed by Gerber et al. [ll]. The coarse positioning is performed by piezo-driven micrometer screws allowing a three-dimensional adjustment of the tip relative to the sample. A “single tube scanner” [12] is used for the fine positioning of the tip. The motion of the tube scanner has been measured optically with a quadrant detector before the instrument was assembled. Furthermore the instrument was calibrated in x-y direction by measurements on graphite, the z-direction was calibrated relative to x and y by measurements on inclined surfaces. The tip has been cleaned prior to the measurement by field emission at a few PA for about half an hour. The sample has been prepared from a wafer of p-doped (Zn) material with a doping concentration of n = 1 X lo’* cmv3. The rectangular shaped piece of GaAs (8 x 6 mm) was scratched to predetermine where the sample will cleave. It has been mounted such that about 3 mm in length are held tightly by the sample holder, to which it has been contacted by an indium-tin alloy, providing a low contact resistance. It has been cleaved in UHV by applying a
I (nA)
l1
-3
-2
2
3
-1 -1 --
-2 --3 '$.V
4 Fig.
1. I-V
characteristics ((d ln Z)/dV)
(au.)
h
measured on the p-doped GaAs sample: (a) Z versus V versus V, corresponding to the density of states.
V; (b)
R Miiller et al. / Triangular facets on GaAs(Il0)
observed by STM
291
force in the [OOi] direction at the overhanging end. The remaining piece has a size of 4-6 mm.’ In order to check the preparation of the sample and the tip, the band structure at the surface has been analyzed by tunneling spectroscopy. To measure the 1-v characteristics the feedback control for the z-position of tunneling tip is stopped at a given z-value for a time of about 0.1 s. During that interval the tunneling voltage undergoes a linear ramp from + 2.5 to - 2.5 V sample voltage. The tunneling current 1 as a function of Y is recorded by a computer. Fig. 1 shows the I-f/ characteristic obtained for the p-doped sample averaged over hundred cycles. At positive sample bias one can see the contribution of electrons tunneling from the tip into the conduction band of GaAs, conversely, at negative sample bias the contribution of electrons tunneling out of the valence band to the tip is observed. To visualize the band structure we have calculated the quantity ((d ln f)/dV)V= (dl/dY)(V/I) which has been shown [13] to correspond to the local density of states. Fig. lb gives ((d ln f)/dV)V calculated numerically from I. The band gap of 1.43 eV is clearly visible. The results agree well with the data given by Feenstra et al. [61.
3. Results Fig. 2 shows a linescan of the cleaved (170) plane of GaAs recorded by an x-y recorder. The tunneling current was 300 pA at a positive sample bias of 2.5 V. The size of the scan area is 840 X 840 A2. The z-axis is labeled in units of 2 A, which means that the z-axis is enhanced by a factor of 4 relative to the x- and y-scaling. The sample is oriented with the [OOl] direction along the y-axis. As can be recognized from the figure all steps have heights which are multiples of 2 A. This value corresponds well to the spacing of atomic layers of GaAs in the (110) orientation, which is d = (1/2&)a = 1.99 A for a lattice constant of a = 5.65 A. The step lines are found along three different directions, named “ a,” “b” and “c”. The steps in fig. 2 are labelled by two indices, the first for the direction, the second for the height in units of atomic layers. In order to get a better representation of the geometry of those structures, data were acquired by a computer providing a top view of the investigated areas. Figs. 3a and 3b show a ,‘op view and the corresponding linescan of a large scan area of 3000 x 3000 A. Due to the enlarged scan area the pattern of facets appears denser than in the previous figure. Again most of the steps are 2 or 4 A high, a few having a height of 6 or 8 A. The x and y directions have the same orientation as in the previous figure. In the topview one can see that within the experimental error the direction labeled “a” runs along the [OOlJ direction. The second major step line labeled
292
R. Miller et al. / Triangular facets on GaAs(I IO) observed by STM
a2
cl
b’2
a2
cl
c2
cl
bi
Fig. 2. STM image of the cleaved (110) surface of GaAs recorded by an x-y recorder. The size of the scan area is 840 X 840 R. The tunneling current is 300 pA at a positive sample bias of 2.5 V. Three different directions of step lines labeled “ a”, “ b”, and “c” can be recognized. The height of the steps is given in number of monolayers by the second index.
“c” forms an angle of about (Y= 33O to the [OOl] direction. The step line “b”, which is less frequent, appears at an angle of p = 18” relative to [OOl]. The two angles almost exactly fulfill the relation tan /3= : tan (Y. By comparing two subsequent scans the effect of thermal drift can be estimated The drift is below 20 A for the time which is needed to acquire the data for the whole image. The drift vector is mainly directed along the x-axis. Due to the large scan range of 3000 x 3000 A this leads to a maximum error for the measured angles of less then lo and is neglected. A schematic representation of the atomic structure of the top layers of the GaAs(li0) surface is shown in fig. 4a. It is characterized by zig-zag-chains of alternating Ga and As atoms running along the [llO] direction. They are perpendicularly intersected by the [OOl] direction forming the major step line indicated by the full line. This is expected since the face exposed by steps in the [OOl] direction is the (110) plane which is a macroscopic cleavage plane. The second orientation of step lines is the [112] direction represented by the dashed line. It connects neighbouring chains with a difference of one pair of Ga and As atoms. The angle between the [OOl] and the [112] direction is (Y= 35’ given by tan (Y= 1/2fi. This agrees fairly well with the observed angle of the facet structure. The difference lies within the experimental error due to the relative calibration of the piezo scanner for the x and y direction.
R. MNer et al. / Triangular facets on GaAs(Il0) observed by STM
293
a
Fig. 3. STM image of a large scan area of the cleaved (li0) surface of GaAs recorded by a computerized data acquisition system. The size of the scan area is 3000 X 3000 A*. The tunneling current is 300 pA at a positive sample bias of 2.5 V: (a) linescan representation; (b) topview representation.
294
Triangular facets on GaAs(Il0)
observed by STh4
a Ga-Atas
As-Atcau
2nd layer 1st
layer
0th
layer
@
b Fig. 4. Schematic representation of the atomic arrangement forming a triangular facet on the (ITO) surface of GaAs. (a) Topview representation: (0) Ga-atom top layer; (0) As-atom top layer; (s) Ga-atom second layer; (0) As-atom second layer. (b) Pseudo-three-dimensional representation.
The face exposed by the [112] step line lies in the (117) plane which is known to be As-terminated [14]. Therefore, it is assumed that the step line is formed by As atoms which is also suggested by the better packing compared to the Ga-terminated arrangement indicated by the dotted line.
R. Miller et al. / Triangular facets on GaAs(II0)
1
2
3
observed by STM
295
step height
Fig. 5. Histogram of the step heights of the area shown in fig. 3 sorted according to the direction of the steps.
The step line at an angle p with tan p = 3 tan (Yis readily identified by the dashed line in the [114] direction. It connects the next neighbouring zig-zag lines with a difference of one pair of Ga and As atoms. The theoretical value for the angle is p T 19.5 ‘, the observed one is about 18”. The corresponding face lies in the (221) plane. From the figure it is evident that this step line is less favoured since only every second row fits exactly in the geometry. This is also reflected in irregularities in the step line which can be clearly seen in fig. 2. From our measurement it cannot be concluded whether this face is terminated by only one atomic species or, as suggested in fig. 4, altematingly by both As and Ga atoms. Considering the height of the steps in figs. 2 and 3, a correlation between the step height and the direction of the steps is clearly recognized. To point this out, fig. 5 shows histograms of the step heights for the three directions evaluated from fig. 3. For the [112] and the [114] direction every multiple of a single atomic step is possible with a decreasing probability for increasing height. However, for the [OOl] direction only steps which are multiples of two monolayers are found, more than 90% being steps with a height of two layers. This can be explained by a rearrangement of the otherwise unsaturated dangling bonds. If there was a step of one monolayer along the [OOl] direction, there would be one dangling bond at the end of every zig-zag chain of Ga and As atoms, as can be seen in fig. 4a. If the step consists of two layers the
296
R Mijller et al. / Triangular facets on GaAs(ll0)
observed by STM
dangling bonds of the top layer can form saturated pairs with those of the next lower. For every atom at the end of a chain in the top layer there is an atom of the corresponding other species at the end of a chain in the lower layer, which is at the normal binding distance. The experimental results strongly suggest this pairwise arrangement. To visualize the structure of a facet with a height of two atomic layers formed by steps in the [OOl] and the [112] directions, fig. 4b shows a three-dimensional representation. The structure has been simplified so far that all bond lengths and angles are identical to those of the bulk structure of GaAs. The rotation of the outermost Ga-As bonds is neglected. As one can see the (110) face exposed by the step in the [OOl] direction is terminated by pairs of Ga and As atoms.
4. Discussion Our measurements reveal that Jhe (li0) surface of cleaved GaAs, although it is essentially flat with a few A height difference over a range of several thousands of A, can exhibit very characteristic step lines. Similar structures were previously observed with REM by Hsu et al. [4], however their identification of the orientation of the step lines appears to be incorrect. In contrast to their measurement no evidence for dislocations was found on our sample. The two major orientations for step lines are the [OOl] and the [112] direction, running parallel in the (ii0) and the (lli) planes respectively. With a lower probability also steps along the [114] direction have been observed. By comparison to the macroscopic situation and because of the better packing of the atoms at the step it is concluded that the steps in the [112] direction are probably As-terminated. From indirect measurements by LEED [1,2] and AES [3] it was deduced that monoatomic as well as diatomic steps are to be found on a cleaved (110) surface of GaAs. By our investigation the step height can be correlated to the direction of the steps. While the steps along [112] and [114] are 1, 2 and 3 monolayers high, only steps with a height of 2 and in a few cases 4 have been found for the [OOl] direction. This can be explained by a pairwise arrangement of the otherwise dangling bonds at the step. This is only possible for multiples of two monolayers.
Acknowledgements We thank the Deutsche Forschungsgemeinschaft for generous support, and we gratefully acknowledge helpful discussions with F. Giessibl and T. HZnsch.
R Miiller et al. / Triangular facets on G&s{1 10) observed by STM
297
The authors would like to thank Dr. Loehnert and Mrs. Ludwig (Wacker Chemitronic) and Dr. J. Eckstein (Varian) for providing the GaAs material.
References [l] [2] [3] [4] [5] (61 [7] [8] [9] [lo] [ll] [12] [13] [14]
M. Henzler, Surface Sci. 22 (1970) 12. D.W. Welkie and M.G. LagaBy, J. Vacuum Sci. Technol. 16 (1979) 784. W. Rat&e, Y.R. Xing and G.D. Shen, Surface Sci. 120 (1982) 67. T. Hsu, S. Iijima and J.M. Cowley, Surface Sci. 137 (1984) 551. R.M. Feenstra and A.P. Fein, Phys. Rev. B 32 (1985) 1394. R.M. Feenstra, J.A. Stroscio, J. Tersoff and A.P. Fein, Phys. Rev. Letters 58 (1987) 1192. J.A. Stroscio, R.M. Feenstra and A.P. Fein, Phys. Rev. Letters 58 (1987) 1668. R.M. Feenstra and P. M&rtensson, Phys. Rev. Letters 61 (1988) 447. P. M%rtensson and R.M. Feenstra, to be published. R. Moller, B. Koslowski and M. Rauscher, to be published. Ch. Gerber, G. Binnig, H. Fuchs, 0. Marti and H. Rohrer, Rev. Sci. Instr. 57 (1986) 221. G. Binnig and D.P.E. Smith, Rev. Sci. Instr. 57 (1986) 1688. N.D. Lang, Phys. Rev. B 43 (1986) 5947. R.D. Bringans and R.Z. Bachrach, in: The Structure of Surfaces, Eds. M.A. Van Hove and S.Y. Tong (Springer, Berlin, 1985).