ELSEVIER Physica C 249(1995)151-156

PHYSICA

Cleaved surface structure of BizSr2CuO6+ by scanning tunneling microscopy

Akira Inoue a,1, Masao Nakao a,1, Ryouzo Yoshizaki b a SANYO Tsukuba Research Center, 2-1 Koyadai, Tsukuba, Ibaraki 305, Japan

b Institute of Applied Physics, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, lbaraki 305, Japan

Received 3 April 1995

Abstract

We have carried out scanning tunneling microscopic (STM) observations of the surface Bi-O plane of Bi2Sr2CuO6÷ 8 (Bi2201). Single-crystal samples were cleaved in situ for observing under ultrahigh-vacuum conditions at room temperature. The atomic-resolution STM images dearly show a modulated lattice structure formed by bright dots which is characteristic of the surface Bi-O plane. In contrast to our previous report on the surface structure of Bi2Sr:CaCu20 s + ~, no "missing atom row" was found. The topographic structure of the out-of-plane modulation is sinusoidal. The images of Bi2201 are independent of bias polarity. We have also observed steps at the surface. The typical step is about 12 A, which is half the lattice parameter of the c direction. The existence of steps lower than 12 ,~ indicates that cleavage does not always occur between the double Bi-O plane, but sometimes occurs between other planes.

1. Introduction

The crystal structure of Bi2Sr2CaCu2Os+ ~ (Bi2212) has incommensurate modulation which is formed by the in-plane and the out-of-plane periodic lattice displacements [1-8]. The period was reported to be about 4.7 times the fundamental unit. The structural and electronic properties of the surface B i - O monoplane have also been studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) [3-8]. The STM studies revealed the existence of a special feature of the surface called the "miss ing atom row" [3]. It was reported that atoms are not missing, but shrinking downward at "miss ing atom rows" [5,7,8]. Thus we call these

1 Corresponding author. Present address: SANYO Microelec- tronics Research Center, 2-1 Koyadai, Tsukuba, Ibaraki 305, Japan.

rows "shrinking atom rows" in this paper. The modulated structure of Bi2Sr2CuO6+ z (Bi2201) was also found and widely studied [9-21]. The electron diffraction (ED) and transmission electron mi- croscopy (TEM) studies reported that the period of bulk averaged modulation is intrinsically incommen- surate, depending on the composition ratio of Bi to Sr [10]. The structural and electronic properties of the surface B i - O monoplane were first reported by Ikeda et al. [12]. They observed the atomic-resolu- tion STM image and reported the semiconductive nature of the surface B i - O monolayer. Up to now, there have been a few STM and STS analyses on Bi2201. More STM and STS studies are expected to reveal the surface structure of Bi2201.

Previously, we reported STM images of Bi2212 with atomic resolution and proposed a new model of the surface B i - O plane structure [7]. Applying the same procedure to a high-quality Bi2201 single crys-

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152 A. Inoue et al. /Physica C 249 (1995) 151-156

tal, we succeeded in obtaining STM images of Bi2201 with atomic resolution and observing the step struc- ture on the cleaved surface. In this paper, we present the atomic-resolution STM images as well as an STM image around steps of the Bi2201 cleaved surface. The surface modulated structure is discussed in comparison to that of Bi2212.

2. Experiment a

B D

0.18 ...... n r n

i Single crystals of Bi2201 were prepared by a flux

method. The temperature dependence of resistivity for the as-grown sample is semiconductive. No su- perconductive transition was observed above 4.2 K. The results of electron probe microanalysis show that the compositional ratio of B i :S r :Cu for this crystal is 2.16:1.84: 1.00, suggesting that Bi atoms may partially substitute Sr sites. The lattice parame- ters of the fundamental unit cell are determined from X-ray diffraction and precession studies to be about a = b = 5.38 .~ and c = 26.8 A. The period of modu- lation along the b axis is about 5.0 times b. This modulation period is consistent with the ED data reported by Hiroi et al. [10].

STM measurements were made with a commer- cial instrument (Park Scientific Instruments, model STM-SU2) under ultrahigh-vacuum (UHV ~ 3 × 10 -s Pa) conditions at room temperature in the constant-current mode. The tunneling tips that we used were made of electrochemically etched tungsten wire. As-grown samples were used for measure- ments. Prior to the STM observation, the single- crystal samples were cleaved in situ at a Bi -O plane.

3. Results and discussion

The top view of an atomic-resolution STM image is shown in Fig. 1. This image is obtained after noise reduction by low-pass filtering. The tip bias relative to the sample is - 1 .28 V and the tunneling current is 0.67 nA. The topographic height is indicated by the gray scale. The size of the area in this image is about 13.9 × 13.9 nm 2. The mean values of dis- tances between neighboring bright dots along a and b axes are 6.8 and 4.9 A, respectively. Since one

10nm b

Fig. 1. Top view (13.9×13.9 nm 2) of an STM image at the cleaved Bi2Sr2CuO6÷ ~ surface taken at a tip-to-sample bias of - 1.28 V and a tunneling current of 0.67 nA. This image is taken in a constant-current mode. The topographic height is indicated by the gray scale. Image was obtained after noise reduction by low-pass filtering.

bright dot is observed for each fundamental unit, as in the case of STM images of Bi2212, these values of distance indicate the lattice parameters of the fundamental unit cell. The topographic structure (out-of-plane lattice displacements) of the modula- tion at the surface is clearly seen as contrast in the brightness of the dots. The period of the modulation is about 5.2 times b. Figs. 2(a) and 2(b) show the atomic corrugations of the two inequivalent neigh- boring atomic rows along the b axis, which are marked as A - B and C - D in Fig. 1, respectively. The amplitude of the out-of-plane modulation is about 1 A. Fig. 3 is another STM image of Bi2201 at a different area on the same cleaved surface. The tip bias relative to the sample is - 1 . 2 7 V and the tunneling current is 0.56 nA. The size of the area in this image is about 13.9 × 13.9 nm 2. Also in this image, one bright dot is observed for each fundamen- tal unit. The lattice parameters estimated from this image are about a = 7 . 1 A and b = 6 . 2 ,~. The period of the modulation is about 4.2 times b. Figs. 4(a) and 4(b) show the atomic corrugations of the two inequivalent neighboring atomic rows, which are marked as A - B and C - D in Fig. 3, respectively. The amplitude of the out-of-plane modulation is about 0.7 A. Although the out-of-plane modulation is clearly seen, the in-plane lattice displacements of

A. lnoue et al . /Physica C 249 (1995) 151-156 153

(a)

1.0

0.5

0.0

-0.5

-1.0 0 50 100 150

(a) 1.0

, ~ 0.5

..~ 0.0

"~ -0.5

-I .o 0 50 100 150

(b) 1.0

*~ 0.5

0.0

-0.5

-1.0

c D

0 50 100 150

Distance (,~ )

Fig. 2. Atomic corrugations along the b axis of (a) A - B and (b) C - D shown in Fig. 1. Every fifth peak is marked with an arrow. The period of modulation is longer than 5 units of b.

modulation are too small to observe in our STM images.

Figs. 5(a) and 5(b) show STM images simultane-

~D

- - 0.13 i!ii+ ..... n r n

i b

Fig. 3. Top view (13.9×13.9 nm 2) of an STM image at a different area on the same cleaved surface as in Fig. 1. This image is taken at a tip-to-sample bias of - 1 . 2 7 V and a tunneling current of 0.56 nA in a constant-current mode. The topographic height is indicated by the gray scale. Image was obtained after noise reduction by low-pass filtering.

(b) 1.0

0.5

..~ o . o

-0.5

-1.0 50 100

Distance (A )

150

Fig. 4. Atomic corrugations along the b axis of (a) A - B and (b) C - D shown in Fig. 3. Every fifth peak is marked with an arrow. The period of modulation is shorter than 5 units of b.

ously taken at the tip biases of 1.25 and - 1 . 2 8 V, respectively. The tunneling current is 0.52 nA. Fig. 5 shows the in-phase atomic corrugations between op- posite bias polarities, and the modulated structure is also in phase. Here we note that in-phase corrugation means that both the empty and the occupied states near the Fermi level at the surface are derived from the same atom (or the same position). This result is quite similar to the case of Bi2212 [5], so that the STM images of both Bi2212 and Bi2201 are inde- pendent of the tip-to-sample bias polarity. Ikeda et al. reported that the STS spectrum of Bi2201 is similar to that of Bi2212 [12]. Their result strongly suggests that the local electronic structures at the surface B i -O plane of Bi2201 are equivalent to those of Bi2212. Thus, it is expected that the bright dots of the STM image correspond to Bi atoms. The STM images also represent the real surface topo- graphic structure of Bi2201, as in the case of Bi2212 [3,6].

Because of the effect of drift, in Figs. 1 and 3, the bright dots do not form the square lattice and the

154 A. Inoue et al . /Physica C 249 (1995) 151-156

0.1 m n m

t inm

(a) b (b)

Fig. 5. STM images (6.95 × 6.95 nm 2) simultaneously taken at tip biases of (a) 1.25 and (b) - 1.28 V. Tunneling current is 0.52 nA. These images show in-phase atomic corrugations. The crosshairs are located at identical locations in both images.

lattice parameters are different from X-ray data. It is difficult to determine the distance between atoms accurately from STM images. However, the resolu- tion of our STM images is high enough to observe the atomic images and allow discussion of the sur- face structure. The period of modulation can be estimated from the fundamental unit. In Figs. 2(a), 2(b), 4(a) and 4(b), we labeled every fifth peak with an arrow. Although the bulk averaged modulation

period is about 5.0 times b, it is clear that the modulation period at the local area observed is longer than 5 units in Fig. 2 and is shorter than 5 units in Fig. 4. The period of modulation is varying locally. According to the result of ED [10], the period of modulation becomes short, as the ratio of Bi to Sr increases. Therefore, the local composition ratio of Bi to Sr in the area observed in Fig. 1 may be less than that in the area observed in Fig. 3. The bulk

(a) (b)

I

2 n:

40 nm

40 nm

30 30

2O 20

10 10

0 0

•

] i

! .....

40 30 20 10 0 0 10 20 30 40 nm nlTl

Fig. 6. (a) Perspective view (48.7 x 48.7 nm z) of an STM image around some steps on the same cleaved surface as in Figs. 1 and 3. Four terraces are observed and labeled a to d. The modulated structure can be clearly seen as a stripe pattern at terraces a and d. (b) Top view of the curvature image calculated from (a). The modulated structure is clearly seen as a stripe pattern.

A. Inoue et al. /Physica C 249 (1995) 151-156 155

period 5b, which is the result obtained from X-ray studies, is the average of locally varying incommen- surate modulation. On nanometer-scale order, the period of the modulation is intrinsically incommen- surate.

In contrast to the STM images of Bi2212 [3,5-8], there is no indication of a "shrinking atom row" in our atomic-resolution STM images of Bi2201. Some- times, we observed dark rows which resemble "shrinking atom rows". When the dark rows were observed in images, the resolution of the images was not at the atomic level. Atomic-resolution images show the surface structure more accurately. Our STM images indicate that there is no "shrinking atom row" in the cleaved surface of Bi2201. The dark rows may be due to some fluctuation of the z-piezoscanner.

We reported that the modulated structure at the surface B i -O plane of Bi2212 is formed due to the ordering of two kinds of atoms, the lower-position atoms and the higher-position ones, where the modu- lation period is commensurate (5 times b) [7]. In the case of Bi2201, however, atoms cannot be divided into these two kinds. As seen in Figs. 2 and 4, the height of the peaks changes sinusoidally, and the period of the modulation is not commensurate. Un- like Bi2212, the surface B i -O plane of Bi2201 has an incommensurate sinusoidal wave structure with- out the "shrinking atom row", although both cleaved surfaces have a common Bi -O plane which has the same electronic structure.

At the cleaved surface we observed some steps. In Fig. 6(a), we show an oblique STM view around steps. In this image, there are four terraces. We labeled these terraces a to d. At terraces a and d, the modulated structure can be clearly seen as a stripe pattern. In Figs. 7(a) and 7(b), we show the cross- sectional structures along A - B and C - D in Fig. 6(a), respectively. In Fig. 7, the modulated structure can be seen at terraces a and d as a corrugated structure. Because Bi2201 crystals are most likely to cleave along a B i -O plane, the step he!ght at the cleaved surface is thought to be about 12 A, which is half the length of lattice parameter c. As shown in Fig. 7, the step heights from d to a, d to b, and d to c

o

are about 12.0, 7.3, and 4.3 A, respectively. There- fore, it can be thought that terraces a and d are B i -O planes, but the terraces b and c are not. According to

(a)

ca

20

10

-10

A

0 1 ~ 2 ~ 3 ~ 400 5 ~

(b) 20

10

.¢~

-10

.C

. . . . . . . . . . . . . . . . . . . . . . d - . .

0 1 O0 200 300 400 500

Distance (A )

Fig. 7. The cross-sectional structures along (a) A - B and (b) C - D shown in Fig. 6(a). The modulated structure can be seen at the terraces a and d as a corrugated structure. The step heights from d to a, d to b, and d to c are about 12.0, 7.3, and 4.3 ,~, respectively.

the X-ray studies, the distance from the lower Bi -O plane to the C u - O p l a n e and to the Sr -O plane are about 7.7 and 5.5 A, respectively [11]. This suggests that terraces b and c are Cu-O and Sr -O plane, respectively. Cleavage also occurs between Sr-O and Cu-O planes. In Fig. 6(b), we show a curvature image of Fig. 6(a). Phase shift of the modulation is observed between terraces a and d. This is consistent with ED data [9,10].

Although the STS spectrum of the surface Bi -O plane of Bi2201 resembles that of Bi2212 [12], the modulated structure of the surface Bi-O plane shows some differences between Bi2201 and Bi2212. "Shrinking atom rows" are found for the surface of Bi2212, but no evidence for the surface of Bi2201. The modulated structure of Bi2201 is an incommen- surate sinusoidal wave, while that of Bi2212 is a commensurate rectangular wave. The step structure of the cleaved surface suggests that cleavage also occurs between Sr -O and Cu-O planes. Phase shift of the modulation was also observed by STM.

156 A. lnoue et aL /Physica C 249 (1995) 151-156

4. Conclusions

We have obtained STM images of a cleaved surface of Bi2201. Atomic-resolution STM images show various periods of the local modulated struc- ture. There was no indication of a "shrinking atom row" on the surface Bi -O plane. Voltage-dependent measurements show that the corrugations in the im- ages taken at opposite bias polarities are in phase. The step structure of STM images show that some

o

steps have lower step height than 12 A, which is half the length of lattice parameter c.

References

[1] Y. Gao, P. Lee, P. Coppens, M.A. Subramanian and A.W. Sleight, Science 241 (1988) 954.

[2] For example, Y. Matsui, H. Maeda, Y. Tanaka and S. Horiuchi, Jpn. J. Appl. Phys. 27 (1988) L361.

[3] M.D. Kirk, J. Nogami, A.A. Baski, D.B. Mitzi, A. Kapitul- nik, T.H. Geballe and C.F. Quate, Science 242 (1988) 1673.

[4] C.K. Shih, R.M. Feenstra, J.R. Kirtley and G.V. Chan- drashekar, Phys. Rev. B 40 (1989) 2682.

[5] C.K. Shih, R.M. Feenstra and G.V. Chandrashekar, Phys. Rev. B 43 (1991) 7913.

[6] K. Ikeda, K. Takamuku, R. Itti and N. Koshizuka, J. Vac. Sci. Technol. B 10 (1992) 2311.

[7] A. Inoue, H. Mukaida, M. Nakao and R. Yoshizaki, Physica C 233 (1994) 49.

[8] Wu Ting, R. Itti, Y. Ishimaru, G. Gu, Y. Enomoto, N. Koshizuka and S. Tanaka, J. Mater. Res. 10 (1995) 817.

[9] Y. Matsui, S. Takekawa, S. Horiuchi and A. Umesono, Jpn. J. Appl. Phys. 27 (1988) L1873.

[10] Z. Hiroi, Y. Ikeda, M. Takano and Y. Bando, J. Mater. Res. 6 (1991) 435.

[11] H. Sawa, H. Fujiki, K. Tomimoto and J. Akimitsu, Jpn. J. Appl. Phys. 27 (1988) L830.

[12] K. Ikeda, I. Tomeno, K. Takamuku, K. Yamaguchi, R. Itti and N. Koshizuka, Physica C 191 (1992) 505.