Molecular Beam Epitaxy of Layered Bi-Sr-Ca-Cu-O Compounds
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MOLECULAR BEAM EPITAXY OF LAYERED Bi-Sr-Ca-Cu-O COMPOUNDS D.G. SCHLOMtt, J.N. ECKSTEIN*, I. BOZOVIC*, A.F. MARSHALL§, J.T. SIZEMOREt, Z.J. CHENt, K.E. VON DESSONNECK*, J.S. HARRIS, JR.t, AND J.C. BRAVMANt t Department of Electrical Engineering, Stanford University, Stanford, CA *
94305
Varian Research Center, Palo Alto, CA 94303
: Department of Materials Science & Engineering, Stanford University, Stanford, CA 94305 § Center for Materials Research, Stanford University, Stanford, CA 94305 ABSTRACT The in situ epitaxial growth of Bi-Sr-Ca-Cu-O films by molecular beam epitaxy (MBE) is reported. The suitability of ozone to the MBE growth of cuprate superconductors is discussed. Molecular beams of the constituents were periodically shuttered to grow various Bi2Sr2Can-lCunOx phases, including 2201, 2212, 2223, 2245, and layered mixtures of these phases. Using these techniques a superconducting film with Tnset near 100 K and Tc (p--0) of 81 K was achieved under entirely MBE conditions (Pchamnber< 2xl0-4 Torr during growth and cooling). The films are smooth on an atomic scale. The results demonstrate the ability of shuttered MBE growth to selectively grow Bi2Sr2Can-lCunOx phases. INTRODUCTION The syntactic intergrowths of Bi2Sr2Can-lCunO2n+4 phases or T12Ba2Can_ 1CunO2n+4 phases commonly observed in bulk samples makes one wonder if this layering can be controlled on a layer by layer basis. Such layering might not only encompass the bismuth and thallium families of superconducting compounds, but also mixtures of any perovskite-related layers having compatible lattice geometries. If available, such a growth technique would be useful for device fabrication, would offer an unparalleled technique to fabricate metastable superlattice mixtures to test high-Tc theories, and may allow the growth of higher temperature superconducting compounds once the mechanism of high-Tc superconductivity is understood. It is with these goals in mind that we have undertaken a systematic study of the growth of oxide compounds by molecular beam epitaxy (MBE). The growth of fully oxidized compounds by MBE involves significant challenges compared to the conventional use of MBE in the growth of non-oxide semiconductors. Clearly to realize the described layered metastable structures, a low temperature in situ growth method is required. Further an activated species of oxygen is necessary to bring the constituent elements to the proper oxidation state Mat. Res. Soc. Symp. Proc. Vol. 169. (c1990 Materials Research Society
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while maintaining the long mean free path necessary for MBE. Finally, composition control is crucial in order to repeatedly lay down "monolayers" of the desired composition. This paper describes our progress related to the in situ growth of layered Bi2Sr2Can-lCunOx compounds by MBE. The Bi-Sr-Ca-Cu-O films were grown in a custom-built MBE machine, specifically designed for the growth of oxide superconductors. Four conventional resistively heated MBE furnaces were used to provide fluxes of the constituent elements. Four independe
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