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STM-imaging of a SrTiO3(100) surface with atomic-scale resolution
Surface Science Letters 278 (19921 Li53-L158 North-Holland
surface science letters
Surface Science Letters
ST&f-imaging of a SrTiO,( 100) surface with atomic-scale Takuya Matsumoto, The Institute
of Scientific
resolution
Hiroyuki Tanaka, Tomoji Kawai and Shichio Kawai
and Industrial Research, Osaka University, Mihogaoka, lbaraki, Osaka 567, Japan
Received 74 June 1992; accepted for publication 25 August 1992
We have obtained atomic-scale resolution STM images of a SrTiO,(lOO) surface annealed in UHV at 1200°C for the first time. The 2 X 2 surface superstructure indicating oxygen vacancy ordering in the TiO, topmost layer has been observed. The STM image corresponds to the surface orbital induced by a Ti-oxygen vacancy complex.
The surface of metal oxides with a perovskite structure (ABO,, where A is a group I or II ion and B is a transition-metal) plays an important role in surface photocatalysis 111 and as a substrate for thin films f2). The surface defect of these materials is especially important as a center of chemical reaction [3] and for the nucleation of crystal growth. The surface of strontium titanate (SrTiO,) is interesting from the viewpoint of not only a typical perovskite but also a typical dielectric material. For these reasons, there have been many surface science studies of SrTiO,. In low energy electron diffraction (LEED), the 2 X 2 superstructure after annealing the crystal in ultra high vacuum KJHV> has been reported. Photoelectron spectroscopic (PES) study has proposed the superstructure model made of ordered surface defects, where the defects are q-Ti3+-0 (Cl: oxygen vacancy) complexes forming a state’ inside the band gap [4,51. Moreover, the surface electronic states related with the oxygen vacancy have been calculated [6,7]. These experimental and theoretical results are concerned with a TiO, surface layer. However, a different surface termination with either SrO or TiO, at the top layer of the SrTiO, surface is possible. In fact, both terminations have been suggested by PES and LEED study [5,8]. In order the characterize such a sur-
face, scanning tunneling microscope @TM) is one of the most powerful tools. However, a metal oxide STM-image with atomic-scale resolution is scarcely reported except for cuprate superconductor and TiO, so far. In this Letter, we report the first successful atomic-scale resolution STM image of the SrTiO, surface. We will show high resolution images of a 2 x 2 superstructure on the annealed surface and also show the specific position of particles on the surface which are assigned to be Sr atoms. The STM imaging was performed with a UNISOKU USM301 (Osaka, Japan). The experiments were conducted in a UHV chamber with a base pressure of 8 X 10-i’ Torr. Both mechanically formed Pt-Ir and AC etched W tips were used. We calibrated the XYZ-scale by imaging a well-known Si(lll)-7 x 7 surface. The STM chamber was equipped with a preparation chamber for sample heating and various surface treatments. Polished and (100) oriented plate-shaped crystals of SrTiO, and Nb doped SrTiO, (0.01%) were purchased from Earth-Jewelry Co. (Osaka, Japan). The SrTiO, crystal was clamped on a Si heater mounted on a hoider made of Ta and MO. The sample was pre-heated for degassing in the preparation chamber and annealed at high tem-
0039~6028/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
perature was measured with an optical pyrometer. The chamber pressure during annealing did not exceed 1 x fOP Torr, After the anneaIing, the sample was transferred to the STM head. It was possible to obtain STM images with atomicresolution within 3 h even after the annealing. Nb doped SrTiU, has enough conductivity for STM measurements with good signal-to-noise ratio. Typical STM images of the Nb-doped SrTiO,UOO) surface after the anneafing in UHV at various temperatures are shown in fig. 1. The surface morphology of the annealed surface at 800°C as shown in fig. la is irregular and has no flat regions. This ~o~bolo~ does nut change even after a Ior~g time annealing of 10 h, Annealing at 11OU”Cyields a more flat surface, shown in fig. lb, than that of fig. la. Surface steps are faintly observed, while the surface itself is not atomically flat. An atomically flat surface is obrained by a~~ea~~ng at 3200°C. A dear step structure and surface boundary are shown in fig. Ic_ The number of the small particles adsorbed on the flat region decreases with increasing the annealing time. However, annealing for long time, beyond Ih, or at higher temperature, beyond 125O”C, yields a degraded surface which cannot be observed by STM. These changes in STM image with annealing temperature suggest that the as-polished surface of SrTiO, is strongly damaged and this damaged layer can be removed and/or rearranged by annealing in UHV at fZttO”C for a few minutes, The images of nondoped SrTiU, can also be observed after the annealing in vacuum above 1000°C. The conductivity of this crystal arises from oxygen defect by the annealing because the Ti’+-O-vacancy cumplex induces a defect state around the Fermi surface inside the band gap without the change of the main electronic strup ture [43. The surface images of both nondoped and Nb-doped SrTiO, annealed at high temperature are completely identical. Any influence of ~b-do~~n~ was nor observed by STM imaging. A current image presenting a surface step is shown in fig. 2a. Fig. 2b shows a cross section taken by the constant-current line scan indicated in fig, 2a. The atomically flat terraces and clear
Boundary Fig. 2, Current images of a 50 nm by 50 nm region of the ~~~~~~~~~~ surface taken with rr tip bias of 2.0 V and a tunneling current of 200 pA. The sample surface annealed in UFIV at (a): @WC, (b): 11WC, and (c)z 1200°C. The sensitivity of(b) and (c) is 1.5 and 3.8 times higher than that of (a).
steps are observed. SrTiO, has a perovskite structure and a different surface termination with either SrO or TiO, at the top layer is possible.
T. ~ats~moto et al. / STM-imaging of a S~Ti~~~~~~~surface with atomic-scale resolution
The observed height of the step is 0.42 nm, in agreement with the unit cell dimension for bulk SrTiO, of 0.39 nm. Sub-unit cell steps or multiple unit cell steps are not observed in any similar image. However, the surface image of terrace (A> is the same as that of terrace (B). Accordingly, the top layer of the surface is always of one kind, either SrO or TiO,. It will be discussed later which layer is the topmost on the surface. An atomic-scale image of the SrTiO, surface is shown in fig. 3. The bright spots are ordered as square lattices. The sub-atomic corrugation is approximately 0.02 nm, similar to what has been observed on metal surfaces. The surface can be divided into three domains (a)-(c) by surface boundaries. The lattice of domain (a) slides against that of domain cc> by half a period of the lattice as illustrated in the expanded image. Another type boundary is observed between domain (a) and (b). The axis of the square lattice in domain (a) is rotated with respect to domain (b). Similar rotation between surface layer and bulk crystal is also found in fig. 4 and in any other sample. All the axes of these square lattice rotate
by 26 f 2 degrees with respect to the axis of the bulk crystal. In addition, surface undulation which may be caused by the rotation is observed as indicated by arrows in fig. 3. The direction of this undulation is diagonal to the square lattice in the corresponding region of the surface. To examine the size of the square lattice, fast-scan current images have been observed as shown in fig. 5a. The observed lattice of 0.8 nm x 0.8 nm corresponds to two times the lattice constant of the SrTiO, (0.39 nm> bulk crystal. This 2 X 2 surface has already been reported by LEED study and characterized as a Ti-rich surface including a lot of oxygen vacancies. Moreover, we have recently obtained the evidence of a TiO, top layer from the study of SrO and TiO, monolayer growth [PI. Considering these reports and unit cell steps mentioned before, it is suggested that the top layer of SrTiO, surface annealed in UHV is TiO, showing 2 x 2 superstructure due to oxygen defect ordering or oxygen-defect driven surface reconstruction. Tsukada et al. has calculated the surface electronic structure of SrTiO, with oxygen defect by
l
0
fjJ+
Sr2’
OCP-
Fig. 2. (a) A current image of a 50 nm by 50 nm region of the SrTiO,(lOO) surface taken with a tip bias of 1.0 V and a tunneling current of 500 pA. (bf A cross section of the surface indicated in (a), illustrating the single unit cell step height of 0.42 nm. (c): An illustration of the perovskite structure of SrTiO,.
Fig. 3. An atomic-scafe resofucion current-image of 40 nm by 40 nm segion of the SrTiO,surface taken with a tip bias of 0.6 V and a tunneling current of 4CMpA. The axes of the square lattice rotate by 26 I 2” with respect to the axis of the bulk crystal. The rotational boundary between domain (a) and (b) is observed. The extended image of a edge dislocation between domain
the DV-Xtu cluster method. ~rig~~a~i~~ SrTiO, has no Ievel near the Fermi Ievet. By annealing in vacuum, a surface defect state induced by oxygen
vacancies appears near the Fermi surface. In the STM ~~as~~~~~nt, the tunneling current flows from this swface defect state to the tip. There-
coo13 Fig. 4. An atomic-scale resolution current-image of a 40 nm by 40 nm region of the SrTi0,(100) surface taken with a tip bias of 3.0 V and a tunneling current of 500 pA. Many particles are observed on the surface. At the. fiat region, the particles are located at the lattice points.
0.78 nm 1-t
2
0.8 nm
0.8 nm Fig. 5. ia> A fast-scan current image of a 5 nm by 5 nm region of the SrTi~~~~~~ surface taken with a tip bias of 0.5 V and a tunneling current of 200 PA. A square Lattice of 0.8 nm by 0.8 nm corresponding to the lattice dimension of the bulk SrTiOz crystal. (b) A passible model of SrTiO&Otl) surface with 2 X 2 ordered oxygen vacancy and Sr atom on the surface.
state. The highest
occupied
moI&l~ai orbital
~~~~~r~~ induced by the oxygen defect shows a broad maximum around the Ti atom and swells toward the vacuum side 171.Consequently, the bright spot of the STM image may correspond to the periodicity of the q-Ti3’-0 complex as shown in fig. 5b. Many atomic-size particles are also observed on the surface as shown in figs. 3 and 4. The annealing time of the surface shown in fig. 3 is longer than that in fig. 4. At the flat region the ~~rn~~~ of particks decreases by Xonger anneding as shown in figs. 3 and 4. However, long time a~~~ali~g cannot remove the particles at the surface boundary. These particles are strongly stabilized at the disordered boundary. These particles are not due to contamination from background gases, because the number of particles does not change even after two days, In the flat region, these particles are of atomic size and are located on a specific position which is the lattice point of the 2 x 2 su~~~truct~lre. Many previous reports have suggested that the top SUP face is a TiUz fayer as ment~o~ed above. Wowever, the Sr signaf of the photoemission has not completely disappeared on these surfaces ISI. We propose the picture that the well-ordered ffat surface is TiO, as the top layer and the adsorbed particles on this TiO, layer are Sr-atoms or SrO,
In Sudan, we haye obtained atomic-scale resolution STM images of the ~rT~~~~~~~ surface for the first time. The 2 x 2 surface superstructure indicating oxygen vacancy ordering in the TiO, layer has been observed. The STM image corresponds to the surface orbital induced by a Ti-oxygen vacancy complex. Such a direct observation of oxygen vacancy in real-space is important to reveal the mechanism of surface reaction and the initial stage of crystal growth on SrTiU,.
[I] J.G. Mvlavroides,Sk Kafalas and D.F. Kolisar, Appl. Phys. Lett. 28 (lQ76) 241. [2] P, Chaudhari, R.W. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino. Phys. Rev. Lett* 58 (1987) 2687, 1331S. Ferrer and G.A. Somorjai, Surf. Sci. 94 (1980) 41 I [4] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Fhys. Rev. B 17 C2978)4%x
f71 M. Tsukada, I-i. Adachi and C. Satoko, Prog. Surf. Sci. 14 WS3) 113. [8] N. Bickel, Ci. Schmidt, K. Heinz and K. h?ullcr, S+ WC. Sci. Technol. 41 (1990) 46. [9] M. Ran& T. Kawai and S. Kawai, Appl. Phys. Lett. 31 (1992) L.331”
surface science letters
Surface Science Letters
ST&f-imaging of a SrTiO,( 100) surface with atomic-scale Takuya Matsumoto, The Institute
of Scientific
resolution
Hiroyuki Tanaka, Tomoji Kawai and Shichio Kawai
and Industrial Research, Osaka University, Mihogaoka, lbaraki, Osaka 567, Japan
Received 74 June 1992; accepted for publication 25 August 1992
We have obtained atomic-scale resolution STM images of a SrTiO,(lOO) surface annealed in UHV at 1200°C for the first time. The 2 X 2 surface superstructure indicating oxygen vacancy ordering in the TiO, topmost layer has been observed. The STM image corresponds to the surface orbital induced by a Ti-oxygen vacancy complex.
The surface of metal oxides with a perovskite structure (ABO,, where A is a group I or II ion and B is a transition-metal) plays an important role in surface photocatalysis 111 and as a substrate for thin films f2). The surface defect of these materials is especially important as a center of chemical reaction [3] and for the nucleation of crystal growth. The surface of strontium titanate (SrTiO,) is interesting from the viewpoint of not only a typical perovskite but also a typical dielectric material. For these reasons, there have been many surface science studies of SrTiO,. In low energy electron diffraction (LEED), the 2 X 2 superstructure after annealing the crystal in ultra high vacuum KJHV> has been reported. Photoelectron spectroscopic (PES) study has proposed the superstructure model made of ordered surface defects, where the defects are q-Ti3+-0 (Cl: oxygen vacancy) complexes forming a state’ inside the band gap [4,51. Moreover, the surface electronic states related with the oxygen vacancy have been calculated [6,7]. These experimental and theoretical results are concerned with a TiO, surface layer. However, a different surface termination with either SrO or TiO, at the top layer of the SrTiO, surface is possible. In fact, both terminations have been suggested by PES and LEED study [5,8]. In order the characterize such a sur-
face, scanning tunneling microscope @TM) is one of the most powerful tools. However, a metal oxide STM-image with atomic-scale resolution is scarcely reported except for cuprate superconductor and TiO, so far. In this Letter, we report the first successful atomic-scale resolution STM image of the SrTiO, surface. We will show high resolution images of a 2 x 2 superstructure on the annealed surface and also show the specific position of particles on the surface which are assigned to be Sr atoms. The STM imaging was performed with a UNISOKU USM301 (Osaka, Japan). The experiments were conducted in a UHV chamber with a base pressure of 8 X 10-i’ Torr. Both mechanically formed Pt-Ir and AC etched W tips were used. We calibrated the XYZ-scale by imaging a well-known Si(lll)-7 x 7 surface. The STM chamber was equipped with a preparation chamber for sample heating and various surface treatments. Polished and (100) oriented plate-shaped crystals of SrTiO, and Nb doped SrTiO, (0.01%) were purchased from Earth-Jewelry Co. (Osaka, Japan). The SrTiO, crystal was clamped on a Si heater mounted on a hoider made of Ta and MO. The sample was pre-heated for degassing in the preparation chamber and annealed at high tem-
0039~6028/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
perature was measured with an optical pyrometer. The chamber pressure during annealing did not exceed 1 x fOP Torr, After the anneaIing, the sample was transferred to the STM head. It was possible to obtain STM images with atomicresolution within 3 h even after the annealing. Nb doped SrTiU, has enough conductivity for STM measurements with good signal-to-noise ratio. Typical STM images of the Nb-doped SrTiO,UOO) surface after the anneafing in UHV at various temperatures are shown in fig. 1. The surface morphology of the annealed surface at 800°C as shown in fig. la is irregular and has no flat regions. This ~o~bolo~ does nut change even after a Ior~g time annealing of 10 h, Annealing at 11OU”Cyields a more flat surface, shown in fig. lb, than that of fig. la. Surface steps are faintly observed, while the surface itself is not atomically flat. An atomically flat surface is obrained by a~~ea~~ng at 3200°C. A dear step structure and surface boundary are shown in fig. Ic_ The number of the small particles adsorbed on the flat region decreases with increasing the annealing time. However, annealing for long time, beyond Ih, or at higher temperature, beyond 125O”C, yields a degraded surface which cannot be observed by STM. These changes in STM image with annealing temperature suggest that the as-polished surface of SrTiO, is strongly damaged and this damaged layer can be removed and/or rearranged by annealing in UHV at fZttO”C for a few minutes, The images of nondoped SrTiU, can also be observed after the annealing in vacuum above 1000°C. The conductivity of this crystal arises from oxygen defect by the annealing because the Ti’+-O-vacancy cumplex induces a defect state around the Fermi surface inside the band gap without the change of the main electronic strup ture [43. The surface images of both nondoped and Nb-doped SrTiO, annealed at high temperature are completely identical. Any influence of ~b-do~~n~ was nor observed by STM imaging. A current image presenting a surface step is shown in fig. 2a. Fig. 2b shows a cross section taken by the constant-current line scan indicated in fig, 2a. The atomically flat terraces and clear
Boundary Fig. 2, Current images of a 50 nm by 50 nm region of the ~~~~~~~~~~ surface taken with rr tip bias of 2.0 V and a tunneling current of 200 pA. The sample surface annealed in UFIV at (a): @WC, (b): 11WC, and (c)z 1200°C. The sensitivity of(b) and (c) is 1.5 and 3.8 times higher than that of (a).
steps are observed. SrTiO, has a perovskite structure and a different surface termination with either SrO or TiO, at the top layer is possible.
T. ~ats~moto et al. / STM-imaging of a S~Ti~~~~~~~surface with atomic-scale resolution
The observed height of the step is 0.42 nm, in agreement with the unit cell dimension for bulk SrTiO, of 0.39 nm. Sub-unit cell steps or multiple unit cell steps are not observed in any similar image. However, the surface image of terrace (A> is the same as that of terrace (B). Accordingly, the top layer of the surface is always of one kind, either SrO or TiO,. It will be discussed later which layer is the topmost on the surface. An atomic-scale image of the SrTiO, surface is shown in fig. 3. The bright spots are ordered as square lattices. The sub-atomic corrugation is approximately 0.02 nm, similar to what has been observed on metal surfaces. The surface can be divided into three domains (a)-(c) by surface boundaries. The lattice of domain (a) slides against that of domain cc> by half a period of the lattice as illustrated in the expanded image. Another type boundary is observed between domain (a) and (b). The axis of the square lattice in domain (a) is rotated with respect to domain (b). Similar rotation between surface layer and bulk crystal is also found in fig. 4 and in any other sample. All the axes of these square lattice rotate
by 26 f 2 degrees with respect to the axis of the bulk crystal. In addition, surface undulation which may be caused by the rotation is observed as indicated by arrows in fig. 3. The direction of this undulation is diagonal to the square lattice in the corresponding region of the surface. To examine the size of the square lattice, fast-scan current images have been observed as shown in fig. 5a. The observed lattice of 0.8 nm x 0.8 nm corresponds to two times the lattice constant of the SrTiO, (0.39 nm> bulk crystal. This 2 X 2 surface has already been reported by LEED study and characterized as a Ti-rich surface including a lot of oxygen vacancies. Moreover, we have recently obtained the evidence of a TiO, top layer from the study of SrO and TiO, monolayer growth [PI. Considering these reports and unit cell steps mentioned before, it is suggested that the top layer of SrTiO, surface annealed in UHV is TiO, showing 2 x 2 superstructure due to oxygen defect ordering or oxygen-defect driven surface reconstruction. Tsukada et al. has calculated the surface electronic structure of SrTiO, with oxygen defect by
l
0
fjJ+
Sr2’
OCP-
Fig. 2. (a) A current image of a 50 nm by 50 nm region of the SrTiO,(lOO) surface taken with a tip bias of 1.0 V and a tunneling current of 500 pA. (bf A cross section of the surface indicated in (a), illustrating the single unit cell step height of 0.42 nm. (c): An illustration of the perovskite structure of SrTiO,.
Fig. 3. An atomic-scafe resofucion current-image of 40 nm by 40 nm segion of the SrTiO,
the DV-Xtu cluster method. ~rig~~a~i~~ SrTiO, has no Ievel near the Fermi Ievet. By annealing in vacuum, a surface defect state induced by oxygen
vacancies appears near the Fermi surface. In the STM ~~as~~~~~nt, the tunneling current flows from this swface defect state to the tip. There-
coo13 Fig. 4. An atomic-scale resolution current-image of a 40 nm by 40 nm region of the SrTi0,(100) surface taken with a tip bias of 3.0 V and a tunneling current of 500 pA. Many particles are observed on the surface. At the. fiat region, the particles are located at the lattice points.
0.78 nm 1-t
2
0.8 nm
0.8 nm Fig. 5. ia> A fast-scan current image of a 5 nm by 5 nm region of the SrTi~~~~~~ surface taken with a tip bias of 0.5 V and a tunneling current of 200 PA. A square Lattice of 0.8 nm by 0.8 nm corresponding to the lattice dimension of the bulk SrTiOz crystal. (b) A passible model of SrTiO&Otl) surface with 2 X 2 ordered oxygen vacancy and Sr atom on the surface.
state. The highest
occupied
moI&l~ai orbital
~~~~~r~~ induced by the oxygen defect shows a broad maximum around the Ti atom and swells toward the vacuum side 171.Consequently, the bright spot of the STM image may correspond to the periodicity of the q-Ti3’-0 complex as shown in fig. 5b. Many atomic-size particles are also observed on the surface as shown in figs. 3 and 4. The annealing time of the surface shown in fig. 3 is longer than that in fig. 4. At the flat region the ~~rn~~~ of particks decreases by Xonger anneding as shown in figs. 3 and 4. However, long time a~~~ali~g cannot remove the particles at the surface boundary. These particles are strongly stabilized at the disordered boundary. These particles are not due to contamination from background gases, because the number of particles does not change even after two days, In the flat region, these particles are of atomic size and are located on a specific position which is the lattice point of the 2 x 2 su~~~truct~lre. Many previous reports have suggested that the top SUP face is a TiUz fayer as ment~o~ed above. Wowever, the Sr signaf of the photoemission has not completely disappeared on these surfaces ISI. We propose the picture that the well-ordered ffat surface is TiO, as the top layer and the adsorbed particles on this TiO, layer are Sr-atoms or SrO,
In Sudan, we haye obtained atomic-scale resolution STM images of the ~rT~~~~~~~ surface for the first time. The 2 x 2 surface superstructure indicating oxygen vacancy ordering in the TiO, layer has been observed. The STM image corresponds to the surface orbital induced by a Ti-oxygen vacancy complex. Such a direct observation of oxygen vacancy in real-space is important to reveal the mechanism of surface reaction and the initial stage of crystal growth on SrTiU,.
[I] J.G. Mvlavroides,Sk Kafalas and D.F. Kolisar, Appl. Phys. Lett. 28 (lQ76) 241. [2] P, Chaudhari, R.W. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino. Phys. Rev. Lett* 58 (1987) 2687, 1331S. Ferrer and G.A. Somorjai, Surf. Sci. 94 (1980) 41 I [4] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Fhys. Rev. B 17 C2978)4%x
f71 M. Tsukada, I-i. Adachi and C. Satoko, Prog. Surf. Sci. 14 WS3) 113. [8] N. Bickel, Ci. Schmidt, K. Heinz and K. h?ullcr, S+ WC. Sci. Technol. 41 (1990) 46. [9] M. Ran& T. Kawai and S. Kawai, Appl. Phys. Lett. 31 (1992) L.331”