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PHYSICA m ELSEVIER Physica C 282-287 (1997) 629-630

Morphology of Bi2201 Thin FilIns Fabricated on the Thennal and Chemical Treated M gO(100)

Farid Ahmed, Yang Qi, Masafumi Kawakami, Kazuo Sakai, and Ryozo Aoki a

aDept. of Electrical Eng., Osaka Univ., 2-1 Yamadaoka, Suita-shi 565, Osaka, .Japan

The Bi2201 thin films have been fabricat,cd by t,he ion beam sp"tt,ering method on the various thermal anncaled and chemically ct,ched M gO( 100) substrat.es. Thc surface morphology before and an.er deposi­t,ion arc examined by AFM and IUIEED. Although surface morphology of MgO(IOO) is significantly improved, t.hat of the Bi2201 thin film fabricated is det.eriorated.

The lattice matching is an important fac­tor for the thin film fabrication, therefore the substrates with the lattice constants as large as those of the fabricating films are se­lected. Nevertheless, the single crystal M gO is widely used as a substrate for fabricating high-7~ superconducting thin films due to its low dielectric constant and loss etc. in spite of the large lattice mismatches by ca.10%.

It is required to construct the atomically flat surface on these substrates for obtaining the high quality superconducting thin films. Y BCO thin film quality on the treated M gO substrate was reported to be improved in con­trast to that of the as-received one[l], while the effect of the treatment for the M gO(100) substrate on the superconducting BSCCO thin film quality is still in dark.

An ion beam sputtering system (I B S) was employed for the Bi2201 thin film fab­rication.[2] The various thermal annealing and/or chemically treatment was carried out on the MgO(lOO) substrates polished me­chanical1y.[3] M gO homoepitaxial deposition on the as-received spedmcJl was also per­formed. The surface morphology both be­fore and after the Bi2201 deposition were examined by means of atomic force mi­croscopy (AFM) and reflection high en­ergy electron diffraction (RR EED). Crys-

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tal structures and compositional analyses of the thin films fabricated were inspected by X -ray diffractometer(X RD) and energy dis­persive X -ray spectrometer (EDX), respec­tively.

The as-received M gO( 100) surface exhib­ited the obscure step lines running along < OlD > and < 001 > azimuths with small undulations by AF M observation and the broad RII EED streak pattern.[4] After the three repetitions of the lOOO°C annealing for three hours with intermediate etching and a 13.50°C final annealing for 2 hours were carried ol1t, the atomically flat wide ter­races separated by steps running only along < 011 > azimuth were observed besides a little amount of Ca segregation as shown in Fig.1. Fig.2 shows the su rface morphology of the Bi2201 thin film fabricated under same condition on the treated substrate. This im­age shows a rough surface morphology with a large number of outgrowths, and could not be improved or be even worse than those de­posited the a.s-received substrate in spite of the surface improvement of the M.qO(100) substra.te. Namely, as increasing iteration of the thermal annealing with chemical etching, the Bi2201 thin film quality was deteriorated despite the M gO surface morphology was im­proved.

630 F. Ahmed e/ aU Physica C 282-287 (1997) 629-630

Fig. 3 shows the RH E E D pattern of MgO(100) after MgO homoepitaxy. This sharp and clear RH EED pattern with the Kikuchi lines shows the construction of very flat surface on MgO(100) surface compared to those produced by the other treatments. This result is also supported by AF M obser­vation. The Bi2201 thin film quality on the MgO deposited, however, are also worse than those on the as-received substrate.

We cannot understood the cause of the inconsistency between the cleanness of M gO(100) and the Bi2201 thin film qual­ity. The high-Tc superconductor can be fab­ricated regardless of the lattice mismatch by ca.l0% between MgO(lOO) and Bi2201. The relaxation of this stress energy come from this difference must be essential to be capable of fabricating the high-Tc superconductors. The huge stress energy would be relaxed by a larger unit cells rather than within several unit cells. Some high-order superstructures on the MgO(100) substrates have already been confirmed according to different ther­mal and/or etching treatments by RH EED analyses. The surface improvement may im­ply the exhaustion of the high-order super­structures on the substrate surface. It may be unable to construct the high quality Bi2201 thin film on the treated M gO(100) substrate.

REFERENCES

1. S.King, L.Coccia, and I.W.Boyed, Appl. Surf. Sci., 86, (1995) 13.

2. S.Migita, K.Sakai, R.Ota, Z.Mori, and R.Aoki, Thin Solid Films, 281-282, (1996) 50.

3. F.Ahmed, K. Sakai, R.Ota, R. Aoki, N. Ikemiya, and S. Rara, J.Low temp. Phys., 105, (1996) 134.

4. R.Ota, K.Sakai, R. Aoki, N. Ikemiya, and S. Rara, Surj.Sci., 357-358,(1996) 15.

Figure 1. AF M image of the M gO(100) sur­face after finally annealed at 1350°C for 2 hours.

Figure 2. Surface Image of the Bi2201 thin film fabricated on the treated substrate.

Figure 3. RH EED pattern of the MgO(100) surface after M gO homoepitaxy.