MA?ERIALS SCIENCE & ERGlWEERlRIG
Materials Science and Engineering A217/218 (1996) 419-423
and STM investigations of a Bi,Sr,CaCu,O, superconductor
B. Suslaa, R. Czajka”, W.S. Gordona, S. Szuba13a, J. Rauhszkiewiczb “Institute of Physics, Poznali University of Technology, ul. Piotrowo 3, 60-96.5 Poznafi, Poland bInstitrrte of Physics, Polish Academy of Sciences, Al. Lotnikdw 32146, 02-668 Warsaw, Poland
Abstract The resultsof the scanningtunneling microscopy and spectroscopy(STM/STS) as well as atomic force microscopy (AFM) measurementson the bismuth cuprate superconductorsBi,Sr,CaCu,08 (BSCCO) in the a-b plane are reported in this paper. Room-temperatureSTM/AFM has beenusedto study the BSCCO singlecrystals.Their layeredcrystalline structure allowsone to prepare them by cleaving the atomically flat surface.Experimentshave shownquite a stability of the cleavedsurfaceand it is possibleto observereproducible STM imagesof the fresh surfacein the air. The micromorphology and step heights in the c direction correspondingto multiple valuesof the unit cell have beeninvestigated.Initial testsin air have shownthat the STM was able to etch the BSCCO surface in a layer-by-layer processof subsequentremoval of non-metallic (EGO)and metallic (CuO,) planes.We have also observedlarge areaswith very stableand reproducibletunneling spectra(1-V and dI/dV) as a function of position and tip-sample spacing. Contrary to STM, AFM imaging does not modify the examined surface and reveals the surface covered with uniformly corrugated (within 1 nrn range) granules, arising due to the surfacedegradation of the BSCCO material. Keywords: AFM images; STM images; High-T, superconductor
The development of atomic resolution scanning tunneling microscopy (STM) and spectroscopy (STS) by Binnig and Rohrer [l] has opened a new era of surface science and engineering to build new structures and maybe even new materials atom-by-atom. Since the discovery of superconductivity in a LaBa-Cu-0 ceramic by Bednorz and Miiller  a large variety of copper-oxygen ceramic materials have been synthesized and investigated by different techniques. However, so far the detailed mechanism leading to high-temperature superconductivity is still unknown. Electron tunneling, has been the most popular tool to investigate the density of states (N(E)) near the Fermi level, and also to probe low lying excitations in superconductors since the experiments of Giaever . Tunneling spectroscopy has proven much more difficult to apply to high temperature superconductors than con-
’ Present address: Institute for Materials Research, Tohoku University, Katahira 2-l-1, Aoba-ku, 980 Sendai, Japan. 0921-5093/96/$15.00 0 1996- ElsevierScience S.A. All rightsreserved
ventional superconductors. This arises principally from the short coherence lengths (in-plane &, = 1.3 nm, on the order of a half of the lattice constant) in these materials, and the difficulties in preparing ideal tunneling barriers. These uncertainties about the tunneling barrier make the interpretation of the differential conductance spectra controversial in terms of intrinsic sample properties. Therefore, the structures observed in these spectra should be interpreted with caution due to the degradation of the surface after cleavage which may induce electronic modifications. 2. Experimental details
The single crystals of Bi,Sr,CaCu,Os (BSCCO) used in our experiments were prepared using methods as reported by Mitzi et al. . The magnetic susceptibility and zero resistance measurements showed that the crystals become superconducting at 90 K. The typical size of the crystals is 5 x 5 x 0.5 mm3. Samples were carefully selected for their smooth and uniform surface. Their layered crystalline structure allows one to prepare a flat surface by cleaving atomically.
et al. 1 Materials
c% sfo BiO BiO SrQ) CUO ca cue §I-0
were taken immediately after cleaving the single crystals at room temperature. The tunneling tips were electrochemically etched from 0.25 mm Et-Ir wire. The voltage applied to the tip ranged from 200 to 1000 mV and tunneling current from 0.5 to 1.0 nA. Commercially available S&N, cantilevers with integrated tips were used for AFM measurements. The maximum area available to scan was 5 x 5 urn2 and typical images consisted of 256 x 256 lines/points. The scan rate was within range of few Hz per line. The STM/AFM resolution was 0.01 nm in the z direction and 0.05 nm in the surface plane. The lateral and vertical calibration was realized by imaging the known atomic structure and height of the atomic steps of graphite and mica.
BiO 3. Results and discussion
1. The crystalline
a, = b, =, 0.54 nm.
Room-temperature STM/AFM (OMICRON) has been used to study the topography and spectroscopic properties of the crystals. The STM images in constant current mode and AFM images in constant force mode
The crystalline structure of BSCCO is face-centered orthorhombic, with the lattice parameters: c, = 3.09 nm, and a, = b, = 0.54 nm, as presented in Fig. 1. Experiments have shown quite a stability of the cleaved surface and possibility to observe reproducible AFM images of the fresh surface in the air. Fig. 2 presents an AFM image of a 500 x 500 nm2 area with 1 nm corrugation. We have not observed any changes on the examined BSCCO surface with changing the AFM experimental parameters, e.g. force set-point. However, initial tests in air have shown that the STM was able to
1.048 0.983 0.927 0.852 0.766 0.722 0.655 0.590 0. m 0.459 0.393 0.328 a 262 0.197 0.131 0.066 0. OQO
Fig. 2. AFM
image of 500 x 500 nm2 Bi,Sr,CaCu,O,
z scale is 1.1 nm.
et al. / Materials
Fig. 3. STM top view image of a Bi,Sr,CaCu,O, siingle crystal 200 x 200 nm* surface. The hole on the left side was produced during beforehand STM scanning. The cross-line section of the obtained fault line is shown at the bottom part.
“etch” the 3SCCO surface in a layer-by-layer process of subsequent removal of non-metallic (BiO) and metallic (CuO,) planes. This process is controlled by the voltage bias and takes place above a 0.5 V bias threshold, and may be explained in terms of field evaporation due to the presence -of a high electric field of lo9 V m - ‘. This corresponds to a removal of a half sublattice cell or less. Fig. 3 shows the STM top view image of a 200 x 200 nm2 surface. The hole on the left side was produced during beforehand STM scanning. The cross-line section of the obtained fault line is shown at the bottom part of the Fig. 3 and it reveals the steps of sub-lattice and lattice constants according to the crystal structure presented in Fig. 1. Beginning from the left side of the cross-line section (Fig. 3) one can observe steps of l/3, 2/3, 1, and two times l/2 of the c,, lattice constant, for example.
We have also performed scanning tunneling spectroscopy (STS) measurements of the local I-V and dl/dY vs. V characteristics. We have found that the I-V curves are generally of three types depending on the tip-sample distance and exhibited spatial variation (Fig. 4(a)). They exhibit features from fully metallic (A) to semiconductor type (B). These might be related to the inhomogeneity of the BSCCO ab plane, the cha’rging effect and the coupling strength between the tip and the ab plane. Some of these STS spectra show the opening of the semiconducting gap. The I-V characteristics show a flat region around zero sample bias voltage. They are asymmetric-the conductance is higher for negative tip polarity. The dI/dV versus V curves recorded at 300 K are shown in Fig. 4(b), and they reveal a characteristic semiconductor gap of - 1.0 eV. We have also observed non-zero conductance at near-zero bias, when the tip-surface distance,was de-
B. &da et al. / Materials Science and Engineering A21 71218 (i996) 419-423
Fig. 4. (a) Tunneling current vs. voltage curves for a Pt-Ir tip and l$Sr,CaCu,O, sample taken at different spots of the STM image. Fully metallic (A) and semiconducting (B) characteristics are seen. (b) Tunneling conductance vs. voltage for BiO layer. The semiconducting gap is about 1 eV at 300 K. creasing
(due to increasing the tunnel current for a given bias polarization). In this case the near-zero-bias conductance is due to tunneling into a deeper layer of CuO, through the BiO top layer. The average distance between CuO, and BiO layers is only 0.45 nm which is also important to mention .
a nanometer-scale in this high-ir, material, The similar processes were not observed during AFM imaging. STS spectra on the cleaved surface of BSCCO show both the metallic and semiconducting features. We could estimate the width of a semiconducting gap to be of 1.0 eV at room temperature for the BiO monolayer.
The cleaved surface of a BSCCO single crystal has been studied using room-temperature STM/AFM. Using STM, we were able to modify the BSCCO surface in the air. It is possible to prepare atomically flat and clean surfaces, as well as to create the new structures in
This work was supported by the Poznan University of Technology under the project DS 62-112/4. The UHV STM/AFM (OMICRON) purchase was sponsored by The Foundation for Polish Science (Contract “SEZAM” No 33/94).
B. Susia et al. / Materials
References [l] G. Binnig and H. Rohrer, IBM J. Res. Deu., 30 (1986) 335. !2] J.G. Bednorz and K.A. Miiller, 2. Phys., B64 (1986) 189.
 I. Giaever, Plqx Rev. Lett., 5 (1960) 147.  D.B. Mitzi et al., Phys. Rev., B41 (1990) 6564.  B. Susla, R. Czajka and S. Szuba, &fol. Pkys. Rep., (1995), in press.