XPS study on the Bi-2212 single crystals grown by a modified vertical Bridgman method

XPS study on the Bi-2212 single crystals grown by a modified vertical Bridgman method

Physica C 392–396 (2003) 153–156 www.elsevier.com/locate/physc XPS study on the Bi-2212 single crystals grown by a modified vertical Bridgman method H...

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Physica C 392–396 (2003) 153–156 www.elsevier.com/locate/physc

XPS study on the Bi-2212 single crystals grown by a modified vertical Bridgman method H. Tanaka a

a,*

, S. Kishida a, H. Yoshikawa b, A. Kimura b, A. Tanaka c, S. Fukushima b

Department of Electrical and Electronic Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552, Japan b Harima Office, Advanced Materials Laboratory, National Institute for Materials Science, Kohto 1-1, Mikazuki, Sayo, Hyogo 679-5143, Japan c ULVAC-PHI, Inc. 370 Enzo, Chigasaki, Kanagawa 253-0084, Japan Received 13 November 2002; accepted 21 February 2003

Abstract We carried out surface cleaning of Bi2 Sr2 CaCu2 Oy (Bi-2212) single crystals grown by a modified vertical Bridgman method (modified VB method), and investigated the chemical state with X-ray photoelectron spectroscopy. From the results, we found that carbon impurities on the surface of the single crystals were mostly removed by cleaving them in the high vacuum (7.3  106 Pa). The O-1s peak from the cleaved surface of the single crystal was very sharp and its FWHM value was about 2.0 eV. Moreover, we knew that the crystals did not include any carbon impurities, and that they had few mutual substitutions between Ca and Sr. Therefore, the single crystals grown by a modified VB method is expected to have a high quality. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.72.H; 81.10.Fq; 81.65.Cf; 79.60.)i Keywords: Bi2 Sr2 CaCu2 Oy ; Single crystal; Modified vertical Bridgman method; X-ray photoelectron spectroscopy; Surface cleaning

1. Introduction Since high-Tc superconductors (HTSC) were discovered [1–5], many researcher have made enormous effort to understand their superconducting mechanism. In order to clarify the mechanism of HTSC, it is very important to investigate the accurate chemical bond nature and changes

*

Corresponding author. Tel./fax: +81-857-31-5244. E-mail address: [email protected] (H. Tanaka).

of valence (hereafter, accurate chemical state) in HTSC. For the accurate study, the very clean surfaces of the high quality HTSC single crystals are necessary because the impurity adsorbed on the surface can affect the results of the measurements which mainly observe the surface chemical state and it can lead to misunderstanding of intrinsic properties. Since the Bi2 Sr2 Can1 Cun Oy (Bi-system) single crystals with a layered structure are easily cleaved, their surface seems to be easy for cleaning. Recently, it was reported that the large Bi-2212 single crystals with high crystallinity were obtained by a modified vertical Bridgman method (modified

0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)00853-0

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VB method) [6,7]. However, the optimum surface cleaning method and the accurate chemical states of the single crystals have not yet been clarified. In this study, we carried out surface cleaning of Bi-2212 single crystals grown by a modified VB method, and investigated the surfaces with X-ray photoelectron spectroscopy (XPS).

2. Experimental In this study, we used the Bi-2212 single crystals grown by a modified VB method. The growth conditions for the crystal growth were following; temperature gradient: 3.3 °C/mm, temperature for stating crystal growth: 1000 °C, rotation speed of crucible: 25 rpm. The Tc the FWHM value of (0 0 1 0) XRD peak and the DM value at 60 K in magnetization-magnetic field characteristic of the single crystals were 82 K, 0.05° and 2 emu/g, respectively. In order to obtain the clean surface of the single crystals, we cleaved them in the various atmospheres; air, N2 gas and high vacuum (7.3  106 Pa). The XPS measurements were carried out by ULVAC-PHI DAPHNIA, which was furnished in SPring-8 BL15XU(NIMS), with an energy resolution of less than 0.1 eV at pass energy of 23.5 eV. The base pressure and the X-ray source for the measurements were less than 3  107 Pa and MgKa (1253.6 eV), respectively. Moreover, we etched the cleaved surfaces by Ar ion and observed the C-1s XPS intensity to check whether the crystals include the carbon impurities or not.

3. Results and discussion Fig. 1 shows the O-1s spectra from the surfaces of Bi-2212 single crystals cleaved in various atmospheres. As shown in the figure, the very sharp O-1s peaks around 529 eV were observed from the single crystals cleaved in the high vacuum (7.3  106 Pa). The O-1s spectrum has few peaks due to impurity oxygen at the side of higher binding energy ðEb Þ. The FWHM value of the sharp O-1s peak at 529 eV was about 2.0 eV, which is the smallest in the value reported on Bi-2212 so far.

Fig. 1. O-1s spectra of as-grown single crystals cleaved in (a) air, (b) N2 gas and (c) 7.3  106 Pa.

The sharp O-1s peak indicates that the impurities rarely exist on the surface of the sample, and the surface is very clean enough to enable to discuss the accurate chemical state. We can consider that the O-1s peak just came from the constituent elements of the Bi-2212 single crystals. On the other hand, the O-1s spectrum from the surface cleaved in N2 flowing gas was slightly broader. This broadening would be caused by the overlapping of another peak due to the impurity oxygen which adsorbed on the crystal surface. Moreover, the peaks due to the adsorption of hydroxides were also observed from the surface of the crystals cleaved in air. Fig. 2 shows the C-1s spectra observed from the surfaces of Bi-2212 single crystals cleaved in various atmospheres. In addition, the intensity ratio of C-1s (around 285 eV) to Sr-3p1=2 was shown in the Table 1. The peaks at 285 and 288 eV were observed in the C-1s spectra from the surface cleaved in air. The intensity of the 285 eV peak was very large. The intensities of the peaks at 285 and 288 eV drastically decreased by cleaving the single crystals in the high vacuum (7.3  106 Pa). As the carbonic compounds were used for the starting materials of crystal growth, the single crystals are likely to include the carbon impurities. Therefore, we etched the cleaved surfaces by Ar ion and observed the intensity changes of the C-1s peak at 285 eV to tell whether the carbon impurities exist in the surface or do in the bulk. Fig. 3 shows the

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Fig. 2. C-1s spectra of as-grown single crystals cleaved in (a) air, (b) N2 gas and (c) 7.3  106 Pa.

Table 1 Intensity ratio of C-1s to Sr-3p1=2 in the single crystals cleaved in air, N2 gas and high vacuum (7.3  106 ) Pa Atmosphere for cleavage

IC-1s /ISr-3p

Air N2 gas High vacuum (7.3  106 Pa)

1.4 1.1 0.3

155

contamination by etching because of mixing effect which the etched materials are buried into the inside of the sample in etching. Considering that the remaining peak is very small, the peak seems to just come from the surface contamination buried into the inside of the sample. Furthermore, we could not find any carbon contents in EPMA analysis. From the result, we found that the observed C-1s came from the impurity carbon which adsorbed on the crystal surface, and that the crystals did not include any carbon impurities. Therefore, the carbon impurities would be completely removed by cleaving the single crystals in the higher vacuum or by heating in the vacuum [8]. Fig. 4 shows the Ca-2p spectra observed from the surface of the single crystal cleaved in the 7.3  106 Pa. In general, Ca and Sr are known to be easily substituted for each other. This substitution commonly happens in Bi-2212 superconductors and affects to the superconducting properties. It was reported that Ca-2p and Sr-3d XPS peaks due to the substitution were observed at the side of higher Eb and that the spectra of Ca-2p and Sr-3d became broad [9,10]. The inset represents a reference data [9]. However, as shown in Fig. 4, the Ca2p peaks at 345 and 348 eV were observed and very sharp. This indicated that the peaks due to the substitution of Sr for Ca site were weak. Therefore, Ca and Sr of the single crystal are thought to occupy their respective original sites. Form the results, we suggest that the Bi-2212 single crystals grown by a modified VB method

Fig. 3. C-1s spectra of as-grown single crystal cleaved in N2 gas: (a) before and (b) after Ar ion etching.

C-1s spectra of the single crystal cleaved in N2 gas: (a) before and (b) after Ar ion etching. The result indicated that the intensity of the C-1s peak became almost zero after the surface of the single crystal was etched well. Though there is a small remaining peak at 285 eV after etching, it is generally difficult to completely eliminate the surface

Fig. 4. Ca-2p spectrum of as-grown single crystal cleaved in 7.3  106 Pa. Inset represents a reference data [9].

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have good quality and are suitable for the accurate study of chemical states in HTSC.

by a modified VB method are expected to have a high quality.

4. Conclusions

References

In this study, we carried out surface cleaning of Bi-2212 single crystals grown by a modified VB method, and investigated their chemical state with XPS. From the results, we found that the carbon impurities on the surface of the single crystals were mostly removed by cleaving them in the high vacuum (7.3  106 Pa). The O-1s peak from the cleaved surface of the single crystal was very sharp and there were no peaks due to impurity oxygen at the higher Eb . The FWHM value of the sharp O-1s peak was about 2.0 eV. Moreover, we found that the observed C-1s came from the impurity carbon which adsorbed on the surface of single crystals, and that the crystals did not include any carbon impurities. In addition, we knew that there were few mutual substitutions between Ca and Sr in the single crystals. Therefore, the single crystals grown

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