An ultrahigh vacuum scanning tunneling microscope for the investigation
of clean surfaces
R. Wiesendanger, D. Buergler, G. Tarrach, D. Anselmetti, H. R. Hidber,
and H.-J. GOntherodt
University ofBasel, Department ofPhysics, Klingelbergstrasse 82. CH-4056 Basel, Switzerland
(Received 10 July 1989; accepted 1 August 1989)
An ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been built into a VG
surface analysis system consisting of a VG Escalab and three additional UHV chambers. Samples
and tips can be prepared in UHV. Surface analysis can be performed by low-energy electron
diffraction (LEEO), x-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy
(UPS), and scanning electron/Auger microscopy (SEM/SAM). The STM design is based on an
electromagnetically driven approach system. Easy exchange of samples is possible since the
scanner is incorporated in a scan head which can be flipped backwards. The performance of the
STM was tested by resolving the surface structure of graphite and Si ( III ) 7 X 7. However, the
main application will be the investigation of well characterized metal surfaces. We have studied
the Au( 111) surface where series of mono atomic steps could be imaged. In addition, we resolved
the atomic surface structure. The conditions for atomic resolution imaging on close-packed metal
surfaces are discussed. For comparison, we have also studied polycrystalline Au films vacuum
deposited onto various substrates. For the first time, atomic resolution could also be obtained on
these polycrystalline films. Finally, we present preliminary results of STM studies on a gallium
single crystal.
I. INTRODUCTION
Scanning tunneling microscopy (STM) has helped to solve
many problems in surface science during the past few years.
However, some limitations of the STM technique have become obvious, such as the missing information about the
structural and chemical state of the tipl and the lack of
chemical sensitiveness except from a few examples including
the atom-selective imaging of the GaAs( 110) surface. 2
Therefore, it is highly desirable to have a combination of
STM with other surface analytical techniques 3 allowing the
detailed characterization of both tip and sample surface and
giving additional complementary information about the surface atomic and electronic structure. We have realized an
ultrahigh vacuum (UHV) system where novel scanning
techniques such as STM-Iater on also atomic force microscopy (AFM)-are combined with "conventional" surface
analytical techniques such as low-energy electron diffraction
(LEEO), x-ray photoelectron spectroscopy (XPS), UVphotoelectron spectroscopy (UPS), and scanning electron/Auger microscopy (SEM/SAM). The main application will be the investigation of well characterized metal
surfaces. Besides studies of single crystal surfaces, we want
to focus on polycrystalline, nanocrystalline, quasicrystalline, and amorphous materials continuing our earlier STM
investigations in this field.4-6
II. GENERAL CONCEPT AND STM DESIGN
The UHV system consists of an analysis chamber of a VG
Escalab and three additional vacuum chambers: a preparation, a transfer, and a separate chamber. Samples and tips
can be introduced into the system through either of two Fast
Entry Air Locks connected to the preparation chamber with
facilities for resistive and electron beam heating, ion etching,
339
J. Vac. Sci. Technol. A 8 (1), Jan/Feb 1990
evaporation, etc., and the transfer chamber with LEEO optics and further sample preparation facilities. A STM has
been built into the separate chamber which will also take up
a UHV compatible AFM in the near future. The analysis
chamber equipped with XPS, UPS, SEM/SAM, and preparation facilities such as ion etching and resistive heating will
additionally allow to build in a combined STM/AFM with
the sample located in the center of the chamber. The base
pressure is 3 X 10- 11 mbar for the preparation, transfer, and
analysis chambers and 1 X 10- II mbar for the separate
chamber. The whole UHV system rests on sixteen air pads
for vibration isolation.
The design of the STM in the separate chamber is based on
an electromagnetically driven approach system. Easy exchange of the samples mounted on sample holders is possible
since the scanner (either tripod or tubes with sensitivities
between 0.2 and 15 nm/V) is incorporated in a scan head
which can be flipped backwards. 7 In Fig. 1(a) we present a
schematic drawing of the STM and in Fig. 1(b) the corresponding cross section.
III. S1M PERFORMANCE
To test the performance of the STM and to calibrate the
scanners, we have first studied the highly oriented pyrolytic
graphite (HOPG) and the Si (111 ) 7 X 7 surface. Atomic
resolution images on both surfaces yield an accurate calibration of the lateral length scale whereas the calibration of the
vertical length scale perpendicular to the sample surface was
achieved by imaging mono- and biatomic steps on the
Si( 111)7 X 7 and Au( 111) surface.
The graphite surface can be prepared by just cleaving the
sample in air. Additional preparation such as heating the
sample under vacuum 8 did not lead to significant differences
0734-2101/90/010339-06$01.00
© 1990 American Vacuum Society
339
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43
340
Wiesendanger et al.: UHV STM
340
(al
(blL-_ _ _ _ _ _ _ _ _ _ _ _ _- '
FIG. \. (a) Schematic drawing of the STM. (b) Cross section of the STM.
An electromagnetically driven wedge transfonns its horizontal movement
into the vertical movement of a sample stage which carries the clamped
sample holder with the horizontally mounted sample. The scanhead rests
on three ballbearings and can be flipped backwards giving easy access to the
sample and tip.
in the quality of atomic resolution STM images. A phosphorus doped, 1 n cm Si ( III ) wafer was used for the studies
of the Si ( III ) 7 X 7 surface. After a chemical cleaning procedure 9 the samples were introduced into the UHV system and
thoroughly outgassed. A maximum annealing temperature
of 960°C and a cooling rate of 15 K/min was found to be
appropriate for obtaining sharp LEED patterns characteristic of the well-reconstructed Si ( III ) 7 X 7 surface. Traces of
residual oxygen, carbon, or other impurities could not be
detected by Auger electron spectroscopy (AES). In Fig.
2(a) we present an example of an atomic resolution STM
image showing a 16 X 18 nm 2 surface area on SiC 111)7 X 7.
Besides the 7 X 7 reconstruction, which has already been described and discussed extensively, 10-13 various kinds of defects appear such as missing adatoms and "cloudy" regions
which may be due to impurity sites (e.g., dopant sites) or
adsorbates. In Fig. 2(b) a unit cell located in the defect free
region is shown. The asymmetry between the two halves of
the unit cell can clearly be seen and was explained within the
dimer ada tom stacking fault (DAS) model ofTakayanagi et
al. 14 : the unfaulted half of the unit cell gives rise to an in-
(bl
FIG. 2. (a) STM constant current image of a 16X 18 nm 2 surface area on
Si ( 111 ) 7 X 7 obtained with a sample bias voltage U = + 2.0 V and a tunneling current I = I nA. (b) Enlarged part of the STM image presented in
(a) showing the asymmetry of the two halves of the surface unit cell. The
height difference between the corner holes and the adatoms is 0.18 nm.
creased tunneling current compared to the faulted half of the
unit cell. Therefore, the adatoms of the unfaulted half appear
brighter.
IV. STM INVESTIGATIONS OF SINGLE CRYSTAL
AND POLYCRYSTALLINE METAL SURFACES
After calibrating and testing the performance of the instrument, we have focused on the investigation of metal sur-
J. Vac. Sci. Technol. A, Vol. 8, No.1, Jan/Feb 1990
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43
341
Wiesendanger et al.: UHV STM
341
(8)
FIG. 3. STM overview image of a 170 X 130 nm 2 surface area on Au ( III )
showing terraces up to 30 nm width and separated by steps of various
heights (U = + 500 mV, I = I nA, scan speed 3s/scan line).
faces. First, we want to describe experimental results obtained on a Au( 111) single crystal surface before a
comparison with STM results on polycrystalline Au films
will be made.
The mechanically and electrolytically polished Au( 111)
single crystal surface was prepared in situ by several cycles of
Ar+ ion etching and annealing. Preparation was controlled
by AES, XPS, and LEED. Diffuse 1 X 1 LEED patterns
were found to be correlated with STM images showing relatively rough surface regions on a large scale. In Fig. 3 we
present a STM overview image of a 170 X 130 nm 2 area on
the Au( 111) surface. Several terraces up to 30 nm wide and
separated by steps of various heights can be seen. After
zooming into smaller regions, the terraces separated by series of monoatomic steps (height of 0.234 nm) could be studied in more detail as shown in the perspective view of Fig.
4(a) which represents a 35 X 26 nm 2 surface area on
Au( 111). The STM line scan image [Fig. 4(b) j of the series
of mono atomic steps demonstrates the remarkable good signal to noise ratio of the raw data. Furthermore, a corrugation with an amplitude of about 0.02 nm can clearly be identified on the terraces in the line scan representation. This
corrugation is due to a reconstruction of the Au( 111) surface. STM studies of this reconstruction have recently been
reported 15.16 and will not further be discussed here. We now
want to focus on the conditions leading to atomic resolution
images on the close-packed Au ( 111 ) surface which has first
been reported some time ago as an unexpected experimental
result. 17 In Fig. 5 (a) we present a perspective view of a constant current STM image of a 2 X 2 nm 2 surface area on
Au( 111) showing stripes with a corrugation amplitude of
about 0.02 nm which result from the surface reconstruction
mentioned above. An additional corrugation due to the
atomic surface structure is totally absent in this image indi-
FIG. 4. (a) Perspective view of a STM image showing series of monoatomic
steps on a 35X26 nm 2 surface area of Au( III). (U = + 0.1 V, 1= 1 nA.)
(b) STM line scan image (24 X 18 nm') of monoatomic steps on Au ( III )
demonstrating the good signal-to-noise ratio of the raw data. The vertical
scale is greatly expanded.
cating that this corrugation is well below 0.01 nm, which
would agree with results obtained from He scattering experiments. 18 In Fig. 5 (b) a surface area of the same size imaged
with similar tunneling parameters is shown. An additional
corrugation due to the atomic surface structure can now be
clearly observed-perhaps even better in the line scan representation of Fig. 5 (c). Since the tunneling parameters were
similar and a drastic change in the shape of the tip is not
expected when the tunneling junction is not greatly disturbed [e.g., by an increase of the sample bias voltage or by
voltage pulses as reported earlier for obtaining atomic resolution on epitaxially grown Au films on mica l7 .19 or on an
AI(lll) single crystal 20 j we conclude that small changes
such as a rearrangement of the cluster in front of the tip or
even only a change of the single atom being closest to the
sample surface, which may result from an atom transfer
from the sample to the tip or an adsorption of an atom out of
the residual gas, can change the STM resolution considerably. Apart from the structural state of the tip,21 electronic
structure effects may also play an essential role for determining the lateral resolution.
For comparison, we have also studied Au films vacuum
deposited onto various substrates. Epitaxially grown Au
J. Vac. Sci. Techno!. A, Vo!. 8, No.1, Jan/Feb 1990
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43
342
Wiesendanger et al.: UHV STM
(a)
342
(c)
FIG. 5. (a) Perspective view ofa constant current STM imageofa 2X 2 nm 2
surface area on Au( III) showing stripes which are due to a reconstruction
of the Au( III) surface. An additional corrugation due to the atomic surface structure is totally absent. (U = + 0.03 V,/ = InA.) (b) Surface area
of the same size as that shown in (a). The additional corrugation due to the
atomic surface structure can now clearly be observed, although the image
was obtained with similar tunneling parameters (U = + 0.05 V, 1 = I
nA). (c) Line scan representation of the STM image presented in (b).
(b)
films on mica as well as polycrystalline Au films on HOPG
have been prepared in situ. Since many STM results on epitaxially grown Au films on mica have already been reported,
we want to concentrate on the STM studies of the polycrystalline Au films showing for the first time that atomic resolution can also be obtained on polycrystalline metal surfaces.
In Fig. 6(a) we present a perspective view of a 64 X 74 nm 2
surface area showing the morphology of the polycrystalline
Au film on a large scale. Grains with diameters between 10
and 50 nm can typically be observed. After zooming into a
tiny 2 X 2 nm" surface area on top of a single grain, we were
able to resolve the atomic structure in the constant current
mode of operation as shown in Fig. 6(b). The hexagonal
symmetry of this atomic resolution image indicates that the
tip was scanning over a ( 111) oriented facet of the grain. The
corrugation was determined to be about 0.02 nm.
Atomic resolution studies could not only be performed on
polycrystalline Au films but more recently also on polycrystalline Fe samples.
Finally, we want to describe preliminary STM studies on a
gallium single crystal. Interest in these studies arises because
gallium with its melting temperature of about 30·C is an
ideal sample for investigating the solid-liquid phase transition directly in real space by STM. Additionally, changes in
the elastic properties at this phase transition can be probed
by STM or more reliably by AFM. Here, we report UHVSTM measurements performed in the solid state at room
temperature after cleaving the crystal in air. In Fig. 7(a) we
present a perspective view of a 170X 100 nm 2 surface area
showing three different crystal faces. Two of them are sepa-
J. Vac. Sci. Technol. A, Vol. 8, No.1, Jan/Feb 1990
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43
343
343
Wiesendanger et al.: UHV STM
(.)
(b)
(.)
(b)
FIG. 6. (a) Perspective view of a 64 X 74 nm' surface area showing the
morphology of the polycrystalline Au film vacuum deposited onto a HOPG
substrate. (U = + 0.2 V, I = I nA.) (b) Perspective view of a constant
current STM image showing the atomic surface structure on a 2 X 2 nm'
surface area on top of a single grain. (U = + 0.05 V, I = InA.)
FIG. 7. (a) Perspective view of a 170X 100 nm' surface area on a cleaved
gallium single crystal showing three different crystal faces. (U = + 0.2 V,
I = I !lA.) (b) Atomic-scale STM image located on the crystal plane in the
front part of the image shown in (a). The corrugation in this constant
current image is about 0.05 nm. (U = + 0.2 V, I = InA.)
rated from the third by a step of approximately 50 nm height.
On the crystal plane in the front part of the image a fine
structure is already visible. After zooming into this part of
the surface we were able to get atomic-scale STM images
such as that presented in Fig. 7 (b). A remarkable high corrugation of about 0.05 nm was measured. Future experiments on in situ cleaved single crystals have to clarify
whether this corrugation is mainly due to the atomic surface
structure of gallium, to the elastic response of the sample
surface near the melting point, or due to a contamination
layer which was present at the surface because the gallium
single crystal was not yet prepared in situ.
V.SUMMARY
An ultrahigh vacuum system has been realized where novel scanning techniques capable of providing information
about surfaces on an atomic scale such as STM-later on
J. Vac. Sci. Techno!. A, Vol. 8, No.1, Jan/Feb 1990
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43
344
Wiesendanger et al.: UHV STM
AFM-are combined with several other surface analytical
techniques which yield information averaged over macroscopic surface areas. A STM has already been built into this
UHV system which allows atomic resolution studies of
layered materials, semiconductors, and metals. Besides
atomic resolution investigations of single crystal surfaces,
we showed for the first time that atomic resolution can also
be obtained on polycrystalline metal surfaces. This experimental result is important for instance in view of chemical
reaction studies where differences in chemical reactivity of
the grains and at grain boundaries are expected.
ACKNOWLEDGMENTS
We would like to thank Dr. A. W. Moore (Union Carbide) for providing HOPG, Wacker Chemitronic for providing the Si wafers and Alusuisse for providing Ga single
crystals. We also thank M. Baur, H. Breitenstein, P. Cattin,
D. Michel, P. Reimann, W. Roth, R. Schnyder, A. Tonin,
and P. Wunderli for technical help and T. Richmond for
proofreading the manuscript. Financial support from the
Swiss National Science Foundation and the Kommission
zur Forderung der wissenschaftlichen Forschung is gratefully acknowledged.
'J. E. Demuth, U. Koehler, and R. J. Hamers, J. Microsc. 152,299 (1989).
2R. M. Feenstra, J. A. Stroscio, J. Tersotf, and A. P. Fein, Phys. Rev. Lett.
58,1192 (1987).
3S. Chiang, R. J. Wilson, Ch. Gerber, and V. M. Hallmark, I. Vac. Sci.
Techno!. A 6,386 (1988).
4B. W. Corb, M. Ringger, H.-I. Giintherodt, and F. E. Pinkerton, App!.
Phys. Lett. 50, 353 (1987).
'R. Wiesendanger, M. Ringger, L. Rosenthaler, H. R. Hidber, P. Oelhafen,
344
H. Rudin, and H.-I. Giintherodt, Surf. Sci. 181,46 (1987).
OR. Wiesendanger, L. Rosenthaler, H. R. Hidber, L. Eng, U. Staufer, H.-I.
Giintherodt, M. Diiggelin, and R. Guggenheim, I. Vac. Sci. Techno!. A 6,
529 (1988).
7The basic STM design has already been described earlier. M. Ringger, R.
Wiesendanger, L. Eng, U. Staufer, H. Rudin, and H.-I. Giintherodt, in
Proceedings of the 1st International Conference of Scanning Tunneling
Microscopy, 1986, Santiago de Compostela, Spain (unpublished). It has
now been further improved regarding stability and flexibility.
8J. I. Metois, I. C. Heyraud, and Y. Takeda, Thin Solid Films 51, \05
( 1978).
°A. Ishizaka, K. Nagakawa, and Y. Shiraki, in Proceedings of the 2nd
International Symposium on Molecular Beam Epitaxy and Related Clean
Surface Techniques, Tokyo, 1982, p. 183.
lOG. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett. 50,120
(1983).
"R. S. Becker, J. A. Golovchenko, D. R. Hamann, and B. S. Swartzentruber, Phys. Rev. Lett. 55, 2032 (1985).
I2R. I. Hamers, R. M. Tromp, and J. E. Demuth, Phys. Rev. Lett. 56,1972
(1986).
13Th. Berghaus, A. Brodde, H. Neddermeyer, and St. Tosch, Surf. Sci. 193,
235 (1988).
14K. Takayanagi, Y. Tanishiro, M. Takahashi, and S. Takahashi, J. Vac.
Sci. Techno I. A 3,1502 (1985).
"Ch. Woll, S. Chiang, R. J. Wilson, and P. H. Lippel, Phys. Rev. B 39,7988
(1989).
lOR. I. Behm, presented at International School of the NATO Advanced
Study Institute, Erice, Italy, April 17-29, 1989.
"Y. M. Hallmark, S. Chiang, J. F. Rabolt, J. D. Swalen, and R. J. Wilson,
Phys. Rev. Lett. 59, 2879 (1987).
"u. Harten, A. M. Lahee, I. P. Toennies, and Ch. Woll, Phys. Rev. Lett. 54,
2619 (1985).
'OR. Emch, I. Nogami, M. M. Dovek, C. A. Lang, and C. F. Quate, J. App!.
Phys. 65, 79 (1989).
20J. Wintterlin, I. Wiechers, H. Brune, T. Gritsch, H. Hofer, and R. J.
Behm, Phys. Rev. Lett. 62, 59 (1989).
21y' Kuk and P. I. Silverman, Appl. Phys. Lett. 48, 1597 (1986).
J. Vac. Sci. Techno!. A, Vol. 8, No.1, Jan/Feb 1990
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.70.28.248 On: Wed, 06 Nov 2013 09:59:43