Transmission electron study of heteroepitaxial growth in the BiSrCaCuO system
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Transmission electron study of heteroepitaxial growth in the BiSrCaCuO system A. Chaiken, M. A. Wall, and R. H. Howell Materials Science and Technology Division, Lawrence Livermore National Laboratory, Livermore, California 94550
I. Bozovic, J. N. Eckstein, and G. F. Virshup E. L. Ginzton Research Laboratory, Varian Associates, Inc., Palo Alto, California 94304-1025 (Received 22 January 1996; accepted 5 March 1996)
Films of Bi2 Sr2 CaCu2 O8 and Bi2 Sr2 CuO6 have been grown using Atomic-Layerby-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely related metastable compounds like Bi2 Sr2 Ca7 Cu8 O20 (2278) and rare-earth-doped compounds like Bi2 Sr2 Dyx Ca12x Cu2 O8 (Dy : 2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi2 Sr2 CaCu2 O8 .
I. INTRODUCTION
Bi2 Sr2 CaCu2 O8 , called 2212, is the prototypical compound of a class of layered copper-oxide superconductors which have been studied extensively due to their high superconducting critical temperatures, Tc . Other known compounds in the BiSrCaCuO family, with the general formula Bi2 Sr2 Can21Cun O2n14, include Bi2 Sr2 CuO6 , called 2201, and Bi2 Sr2 Ca2 Cu3 O10 , called 2223. Like other cuprate superconductors, 2212 is a highly anisotropic layered compound, with lattice ˚ and c 30.66 A. ˚ 1,2 Despite parameters a 3.818 A the complexity of the large unit cell, layer-by-layer growth of films has been achieved, as has been demonstrated by the observation of oscillations in the intensity of reflection high-energy electron diffraction (RHEED) features during deposition.3 Many cubic materials, notably GaAs, Si, and various transition metals, can also be grown in a layer-by-layer fashion, so in itself the occurrence of RHEED oscillations is not remarkable. The unusual aspect of the 2212 oscillations is that each molecular unit is composed of 14 layers so that the cyclical growth pattern involves the formation of ordered layers within the unit cell, as well as the accumulation of completed unit cells. Putting 2212 films in a multilayer along with other BiSrCaCuO compounds adds a further level of complexity. Because of the three different levels of ordering and the intrinsically strong anisotropy, the dynamics of growth in a multilayer made up of 2212 and its analogs can therefore be expected to be quite different from the case of semiconductor or metallic multilayers. Cross-sectional TEM is a useful tool for studying the microstructure of multilayers and for classifying
stacking faults in layered structures. Here TEM images have been used to study the morphology of single films and heterostructur
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