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Surface step and defect structure of Cr(001) studied by scanning tunneling microscopy
Surface Science 235 (1990) 1-4 North-Holland
Surface step and defect structure scanning tunneling microscopy R. Wiesendanger Department
Received
and H.-J. Glintherodt of Bawl, Klingelbergstrmse
of Physics, University
23 March
1990; accepted
for publication
of Cr(OO1) studied
82, CH-4056
by
Basel, Switzerland
30 April 1990
The real-space surface structure of a Cr(OO1) single crystal has been examined by scanning tunneling microscopy (STM). Terraces separated predominantly by monatomic steps have been found consistent with a recent microscopic model of this surface by Bltigel et al. [Phys. Rev. B 39 (1989) 13921 which has been developed on the basis of self-consistent total-energy calculations. We have also obtained clear images of screw dislocations on the Cr(OO1) surface by using STM.
1. Introduction
2. Experiment
Scanning tunneling microscopy (STM) has already proven to be an invaluable surface analytical technique because it allows one to study the real-space surface structure at the atomic level. Microscopic models of surfaces based either on theoretical studies or on experimental results obtained by techniques which provide information averaged over macroscopic surface areas, can relatively easily be proved or refuted by STM. We have used STM to investigate the real-space surface structure of a Cr(OO1) single crystal. This work was motivated by a recent publication by Bliigel et al. [l] offering a microscopic model of the Cr(OO1) surface. It was shown on the basis of self-consistent total-energy calculations that topological antiferromagnetism between ferromagnetic terraces separated by single steps is the energetically most favourable structure of this surface. Our STM data are consistent with this microscopic model of the Cr(OO1) surface with terraces separated predominantly by monatomic steps. Besides the atomic step and terrace structure, we have also studied other defect structures on the Cr(OO1) surface such as screw dislocations.
The STM experiments were performed in a multichamber UHV system (NANOLAB) with several surface preparation and analysis facilities [2]. The calibration of the STM scanning unit was performed laterally by imaging the Si(111)7 X 7 [3] and Si(OO1)2 x 1 [4] surface structures and perpendicular to the surface by imaging monatomic steps on the Si(OO1)2 X 1 surface. On this surface, a clear distinction between monatomic and biatomic steps can be made, since monatomic steps separate terraces with the dimer rows oriented at 90 o relative to each other whereas the same orientation of the dimer rows is observed on terraces separated by biatomic steps. We used electrochemically etched tungsten tips for our STM study of the Cr(OO1) surface. The mechanically and electrolytically polished Cr(OO1) surface was prepared in-situ over a time period of several months by cycles of Ar+ ion etching and annealing. The pressure remained in the lo-” mbar range even at the highest annealing temperature of 900” C. The chemical state of the surface was monitored by X-ray photoelectron spectroscopy (XPS) whereas the surface order was controlled by LEED.
0039-6028/90/$03.50
0 1990 - Elsevier
Science Publishers
B.V. (North-Holland)
2
R. Wiesendanger,
H. J. Giiniherodt / Surface step and defect structure of Cr(OOl) studied by STM
A p(1 X 1) LEED pattern was obtained characteristic for a clean Cr(OO1) surface [5]. Only small traces of oxygen and nitrogen could be detected by XPS. (Particularly the nitrogen impurity level made the long, continuous sample preparation necessary.) The STM experiments were performed at 1 X lop” mbar. The STM data presented in this work represent raw data and have been acquired at a constant tunneling current of I = 1 nA and a sample bias voltage between U = +50 mV and U = + 100 mV. A scan speed of typically l-3 s per scan line was used.
3. STM results on Cr(OO1) In fig. 1 we present a topographic STM image giving an overview of the morphology of the Cr(OO1) surface after the preparation procedure described above. Terraces of different size are the most prominent surface structures appearing in the STM image of fig. 1. These terraces are predominantly separated by monatomic steps as deduced from single line scans crossing the steps.
Fig. 1. Top view image (128 nm)* of the Cr(OO1) surface showing terraces of different size separated by monatomic steps. In this top-view representation, the topographic height is translated into a grey scale. Therefore, high lying terraces appear brighter, whereas low lying terraces appear darker. A screw dislocation located in the right hand part of the image is marked by an arrow.
The experimentally determined monatomic step height is (0.149 + 0.008) nm in good agreement with half of the cubic unit cell height of 0.144 nm. The rounded shape of the terraces seen in fig. 1 is striking. However, the detailed shape of the terraces as well as their size were found to depend on the annealing conditions of the Cr(OO1) sample. On the other hand, the preferred occurrence of monatomic steps on the Cr(OO1) surface was found to be independent of the annealing conditions and was observed after at least twenty different ion etching and annealing cycles. Apart from the atomic step and terrace structure of the Cr(OO1) surface, other surface structures appear in the STM image of fig. 1. Firstly, several spots of increased topographic height distributed over the imaged surface region are visible. These spots are attributed to impurity or contamination sites because their frequency of occurrence could be correlated with the amount of residual surface impurities as detected by XPS. From the chemical analysis by XPS, these residual surface impurities can almost exclusively be attributed to nitrogen and oxygen. It is evident from the STM image shown in fig. 1 that these residual impurity or contamination sites are not correlated with the surface step structure. However, most parts of the Cr(OO1) surface appear atomically clean as further verified by the measured reasonable high tunneling barrier heights of 3-5 eV deduced from local log I versus s characteristics where s denotes the distance between the probing tip and the sample surface. Additionally, we observed screw dislocations and other types of dislocations on the Cr(OO1) surface. One screw dislocation can be found in the right hand part of the STM image shown in fig. 1. After zooming into this surface region, the STM image presented in figs. 2a and 2b was obtained. It can now clearly be seen that two monatomic surface steps originate from this screw dislocation. The spiral appearance of these steps is also nicely visible. Another screw dislocation led to the surface morphology observed in the upper right hand part of the STM image presented in fig. 3. In this surface region, a curved terrace is visible which changes its height by two monatomic steps. In the upper part, this terrace is one atomic layer below
R. Wi~sg~dan~e~, H.-J. Giintherodt / Surface sfep and defect sflucwe
of Cr(OOlf studied by STM
3
Fig. 2. (a) Top-view image (80 nm)* of the screw dislocation (marked by an arrow) from which two monatomic steps originate. The surface now appears under the illumination by an artificial light source giving an impression of the surface morphology similar to electron microscope images. (b) Same topographic image as in (a), but now represented in a perspective view.
the neighbouring terrace on the left hand side whereas the curved terrace becomes one atomic layer higher compared to the neighbouring terrace
towards the right hand part of the imaged surface region.
4. Discussion
Fig. 3. (a) Top-view image of another surface region (SO nm)’ showing a curved terrace caused by a screw dislocation. This terrace in the upper right hand part of the image (marked by an arrow) changes its height by two monato~~ steps as can directly be seen by comparison with the topographic height of the neighbouring terrace. (The topographic height is translated into a grey scale as in fig. 1.)
The real-space observation of the surface step and terrace structure by STM is consistent with the microscopic model of the Cr(OO1) surface by Bltigel et al. (11. The topological antiferromagnetism between ferromagnetic terraces separated by single steps is not only the energetically most favourable structure of the Cr(OO1) surface but is also Further substantiated by the real-space surface structure as determined by STM. Since the terraces on the Cr(OO1) surface, which are separated by monatomic steps, are alternately magnetized in opposite directions [l], the Cr(OO1) surface provides an interesting magnetic test structure for surface sensitive magnetic probes with a high spatial resolution. In particular, the Cr(OO1) surface can be used as a test structure for spin-polarized tunneling experiments with the STM, as will be described elsewhere 161. It should also be noted that the preferred occurrence of single atomic steps on the Cr(OO1) surface is in contrast to the STM observation of
4
R. Wiesendanger,
H. ;I. Giintherodt / Surface step and deject structure of Cr(OOI) studied by STM
double atomic steps on the surface of the ordered alloy NiAl(lll) which has recently been reported [7]. Thus, the surface step structure is found to depend on the constituents of the single crystal and the crystal face under investigation. We have also demonstrated that surface defects arising from dislocations can well be characterized by STM. The STM technique complements more established techniques such as transmission electron microscopy for the characterization of crystal defects. Most importantly, the samples have not to be thinned for STM investigations. Therefore, the influence of thinning on the defect structures of the crystal including their strain fields can be avoided. It will be the aim of future STM investigations of single crystal metal surfaces to study defect structures arising from dislocations also with in-plane atomic resolution.
5. Summary In summary, we have used STM to support a microscopic model of the Cr(OO1) surface, which was favoured on the basis of self-consistent totalenergy calculations. The surface atomic step and terrace structure as well as surface defects arising from dislocations could be characterized in realspace by STM. Besides the recently reported STM study of the NiAl(111) surface, the present work gives another example for the usefulness of the determination of the surface step structure by STM in order to discriminate between different microscopic models.
Acknowledgements
We would like to thank Professor G. Guntherodt for bringing ref. [l] to our attention and Professor A. Hubert for electropolishing the Cr(OO1) single crystal. We also thank D. Btirgler and G. Tarrach for their assistance as well as H. Breitenstien, H.R. Hidber, P. Reimann, R. Schnyder and A. Tonin for their technical help. Financial support from the Swiss National Science Foundation and the Kommission zur Fijrderung der wissenschaftlichen Forschung is gratefully acknowledged.
References [l] S. Bliigel, D. Pescia and P.H. Dederichs, Phys. Rev. B 39 (1989) 1392. [2] R. Wiesendanger, G. Tarrach, D. Btirgler, T. Jung, L. Eng and H.-J. Guntherodt, Proc. IVC-ll/ICSS-7, Cologne, WGermany, Vacuum, in press; R. Wiesendanger, D. Bbrgler, G. Tarrach, D. Anselmetti, H.R. Hidber and H.-J. Guntherodt, Proc. STM’89, Oarai, Japan, J. Vat. Sci. Technol. A 8 (1990) 339. [3] R. Wiesendanger, G. Tarrach, D. Btirgler and H.-J. Giintherodt, Europhys. Lett. 12 (1990) 57. [4] R. Wiesendanger, D. Btirgler, G. Tarrach and H.-J. Gtintherodt, Surf. Sci. 232 (1990) 1. [5] L.E. Klebanoff, R.H. Victoria, L.M. Fahcov and D.A. Shirley, Phys. Rev. B 32 (1985) 1997. [6] R. Wiesendanger, H.-J. Gtintherodt, G. Gtintherodt, R.J. Gambino and R. Ruf, 2. Phys. B, in press. [7] H. Niehus, W. Raunau, K. Besocke, R. Spitz1 and G. Comsa, Surf. Sci. Lett. 225 (1990) L8.
Surface step and defect structure scanning tunneling microscopy R. Wiesendanger Department
Received
and H.-J. Glintherodt of Bawl, Klingelbergstrmse
of Physics, University
23 March
1990; accepted
for publication
of Cr(OO1) studied
82, CH-4056
by
Basel, Switzerland
30 April 1990
The real-space surface structure of a Cr(OO1) single crystal has been examined by scanning tunneling microscopy (STM). Terraces separated predominantly by monatomic steps have been found consistent with a recent microscopic model of this surface by Bltigel et al. [Phys. Rev. B 39 (1989) 13921 which has been developed on the basis of self-consistent total-energy calculations. We have also obtained clear images of screw dislocations on the Cr(OO1) surface by using STM.
1. Introduction
2. Experiment
Scanning tunneling microscopy (STM) has already proven to be an invaluable surface analytical technique because it allows one to study the real-space surface structure at the atomic level. Microscopic models of surfaces based either on theoretical studies or on experimental results obtained by techniques which provide information averaged over macroscopic surface areas, can relatively easily be proved or refuted by STM. We have used STM to investigate the real-space surface structure of a Cr(OO1) single crystal. This work was motivated by a recent publication by Bliigel et al. [l] offering a microscopic model of the Cr(OO1) surface. It was shown on the basis of self-consistent total-energy calculations that topological antiferromagnetism between ferromagnetic terraces separated by single steps is the energetically most favourable structure of this surface. Our STM data are consistent with this microscopic model of the Cr(OO1) surface with terraces separated predominantly by monatomic steps. Besides the atomic step and terrace structure, we have also studied other defect structures on the Cr(OO1) surface such as screw dislocations.
The STM experiments were performed in a multichamber UHV system (NANOLAB) with several surface preparation and analysis facilities [2]. The calibration of the STM scanning unit was performed laterally by imaging the Si(111)7 X 7 [3] and Si(OO1)2 x 1 [4] surface structures and perpendicular to the surface by imaging monatomic steps on the Si(OO1)2 X 1 surface. On this surface, a clear distinction between monatomic and biatomic steps can be made, since monatomic steps separate terraces with the dimer rows oriented at 90 o relative to each other whereas the same orientation of the dimer rows is observed on terraces separated by biatomic steps. We used electrochemically etched tungsten tips for our STM study of the Cr(OO1) surface. The mechanically and electrolytically polished Cr(OO1) surface was prepared in-situ over a time period of several months by cycles of Ar+ ion etching and annealing. The pressure remained in the lo-” mbar range even at the highest annealing temperature of 900” C. The chemical state of the surface was monitored by X-ray photoelectron spectroscopy (XPS) whereas the surface order was controlled by LEED.
0039-6028/90/$03.50
0 1990 - Elsevier
Science Publishers
B.V. (North-Holland)
2
R. Wiesendanger,
H. J. Giiniherodt / Surface step and defect structure of Cr(OOl) studied by STM
A p(1 X 1) LEED pattern was obtained characteristic for a clean Cr(OO1) surface [5]. Only small traces of oxygen and nitrogen could be detected by XPS. (Particularly the nitrogen impurity level made the long, continuous sample preparation necessary.) The STM experiments were performed at 1 X lop” mbar. The STM data presented in this work represent raw data and have been acquired at a constant tunneling current of I = 1 nA and a sample bias voltage between U = +50 mV and U = + 100 mV. A scan speed of typically l-3 s per scan line was used.
3. STM results on Cr(OO1) In fig. 1 we present a topographic STM image giving an overview of the morphology of the Cr(OO1) surface after the preparation procedure described above. Terraces of different size are the most prominent surface structures appearing in the STM image of fig. 1. These terraces are predominantly separated by monatomic steps as deduced from single line scans crossing the steps.
Fig. 1. Top view image (128 nm)* of the Cr(OO1) surface showing terraces of different size separated by monatomic steps. In this top-view representation, the topographic height is translated into a grey scale. Therefore, high lying terraces appear brighter, whereas low lying terraces appear darker. A screw dislocation located in the right hand part of the image is marked by an arrow.
The experimentally determined monatomic step height is (0.149 + 0.008) nm in good agreement with half of the cubic unit cell height of 0.144 nm. The rounded shape of the terraces seen in fig. 1 is striking. However, the detailed shape of the terraces as well as their size were found to depend on the annealing conditions of the Cr(OO1) sample. On the other hand, the preferred occurrence of monatomic steps on the Cr(OO1) surface was found to be independent of the annealing conditions and was observed after at least twenty different ion etching and annealing cycles. Apart from the atomic step and terrace structure of the Cr(OO1) surface, other surface structures appear in the STM image of fig. 1. Firstly, several spots of increased topographic height distributed over the imaged surface region are visible. These spots are attributed to impurity or contamination sites because their frequency of occurrence could be correlated with the amount of residual surface impurities as detected by XPS. From the chemical analysis by XPS, these residual surface impurities can almost exclusively be attributed to nitrogen and oxygen. It is evident from the STM image shown in fig. 1 that these residual impurity or contamination sites are not correlated with the surface step structure. However, most parts of the Cr(OO1) surface appear atomically clean as further verified by the measured reasonable high tunneling barrier heights of 3-5 eV deduced from local log I versus s characteristics where s denotes the distance between the probing tip and the sample surface. Additionally, we observed screw dislocations and other types of dislocations on the Cr(OO1) surface. One screw dislocation can be found in the right hand part of the STM image shown in fig. 1. After zooming into this surface region, the STM image presented in figs. 2a and 2b was obtained. It can now clearly be seen that two monatomic surface steps originate from this screw dislocation. The spiral appearance of these steps is also nicely visible. Another screw dislocation led to the surface morphology observed in the upper right hand part of the STM image presented in fig. 3. In this surface region, a curved terrace is visible which changes its height by two monatomic steps. In the upper part, this terrace is one atomic layer below
R. Wi~sg~dan~e~, H.-J. Giintherodt / Surface sfep and defect sflucwe
of Cr(OOlf studied by STM
3
Fig. 2. (a) Top-view image (80 nm)* of the screw dislocation (marked by an arrow) from which two monatomic steps originate. The surface now appears under the illumination by an artificial light source giving an impression of the surface morphology similar to electron microscope images. (b) Same topographic image as in (a), but now represented in a perspective view.
the neighbouring terrace on the left hand side whereas the curved terrace becomes one atomic layer higher compared to the neighbouring terrace
towards the right hand part of the imaged surface region.
4. Discussion
Fig. 3. (a) Top-view image of another surface region (SO nm)’ showing a curved terrace caused by a screw dislocation. This terrace in the upper right hand part of the image (marked by an arrow) changes its height by two monato~~ steps as can directly be seen by comparison with the topographic height of the neighbouring terrace. (The topographic height is translated into a grey scale as in fig. 1.)
The real-space observation of the surface step and terrace structure by STM is consistent with the microscopic model of the Cr(OO1) surface by Bltigel et al. (11. The topological antiferromagnetism between ferromagnetic terraces separated by single steps is not only the energetically most favourable structure of the Cr(OO1) surface but is also Further substantiated by the real-space surface structure as determined by STM. Since the terraces on the Cr(OO1) surface, which are separated by monatomic steps, are alternately magnetized in opposite directions [l], the Cr(OO1) surface provides an interesting magnetic test structure for surface sensitive magnetic probes with a high spatial resolution. In particular, the Cr(OO1) surface can be used as a test structure for spin-polarized tunneling experiments with the STM, as will be described elsewhere 161. It should also be noted that the preferred occurrence of single atomic steps on the Cr(OO1) surface is in contrast to the STM observation of
4
R. Wiesendanger,
H. ;I. Giintherodt / Surface step and deject structure of Cr(OOI) studied by STM
double atomic steps on the surface of the ordered alloy NiAl(lll) which has recently been reported [7]. Thus, the surface step structure is found to depend on the constituents of the single crystal and the crystal face under investigation. We have also demonstrated that surface defects arising from dislocations can well be characterized by STM. The STM technique complements more established techniques such as transmission electron microscopy for the characterization of crystal defects. Most importantly, the samples have not to be thinned for STM investigations. Therefore, the influence of thinning on the defect structures of the crystal including their strain fields can be avoided. It will be the aim of future STM investigations of single crystal metal surfaces to study defect structures arising from dislocations also with in-plane atomic resolution.
5. Summary In summary, we have used STM to support a microscopic model of the Cr(OO1) surface, which was favoured on the basis of self-consistent totalenergy calculations. The surface atomic step and terrace structure as well as surface defects arising from dislocations could be characterized in realspace by STM. Besides the recently reported STM study of the NiAl(111) surface, the present work gives another example for the usefulness of the determination of the surface step structure by STM in order to discriminate between different microscopic models.
Acknowledgements
We would like to thank Professor G. Guntherodt for bringing ref. [l] to our attention and Professor A. Hubert for electropolishing the Cr(OO1) single crystal. We also thank D. Btirgler and G. Tarrach for their assistance as well as H. Breitenstien, H.R. Hidber, P. Reimann, R. Schnyder and A. Tonin for their technical help. Financial support from the Swiss National Science Foundation and the Kommission zur Fijrderung der wissenschaftlichen Forschung is gratefully acknowledged.
References [l] S. Bliigel, D. Pescia and P.H. Dederichs, Phys. Rev. B 39 (1989) 1392. [2] R. Wiesendanger, G. Tarrach, D. Btirgler, T. Jung, L. Eng and H.-J. Guntherodt, Proc. IVC-ll/ICSS-7, Cologne, WGermany, Vacuum, in press; R. Wiesendanger, D. Bbrgler, G. Tarrach, D. Anselmetti, H.R. Hidber and H.-J. Guntherodt, Proc. STM’89, Oarai, Japan, J. Vat. Sci. Technol. A 8 (1990) 339. [3] R. Wiesendanger, G. Tarrach, D. Btirgler and H.-J. Giintherodt, Europhys. Lett. 12 (1990) 57. [4] R. Wiesendanger, D. Btirgler, G. Tarrach and H.-J. Gtintherodt, Surf. Sci. 232 (1990) 1. [5] L.E. Klebanoff, R.H. Victoria, L.M. Fahcov and D.A. Shirley, Phys. Rev. B 32 (1985) 1997. [6] R. Wiesendanger, H.-J. Gtintherodt, G. Gtintherodt, R.J. Gambino and R. Ruf, 2. Phys. B, in press. [7] H. Niehus, W. Raunau, K. Besocke, R. Spitz1 and G. Comsa, Surf. Sci. Lett. 225 (1990) L8.