Analysis of Growing Films of Complex Oxides by RHEED
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Why Atomic-Layer Engineering of Oxides? Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) is the technique we have been developing and applying to synthesis of complex oxides over the last six years.3,4 Several other groups—mostly in Japan, and a few in Europe—are also pursuing closely related programs.5,6 So far, ALL-MBE has provided us with heterostructures that have atomically abrupt interfaces, tunnel junctions that contain barriers as thin as a few angstroms, and novel metastable materials, as well as a variety of multilayers and superlattices. The atomic-layer engineering capability has also generated some unique samples for the study of basic physics issues such as electron localization, band line-up, and the nature of transport across the interfaces. It has also enabled fabrication of unique HTS Josephson sandwich junctions and their vertical stacks. Several recent MRS Bulletin articles offer further information on HTS applications of ALL-MBE3,4 and on crystal engineering of oxides in general.7 Conceivable utilization for oxide ALL-MBE is even broader. It could be
employed to manufacture a variety of devices, ranging from traditional electronic and optoelectronic devices to more recent innovations, such as nonvolatile ferroelectric memory elements, and still others that are under development like single-electron transistors (SETs) or superconducting field-effect transistors (SuFETs). Overall, it seems safe to predict that oxide MBE will play an important role, at least at the research-tool level, in the development of the ultrafast nanoelectronics of the future.
Problems and Difficulties Throughout this article, we frequently mention complex oxides, referring to ternary or higher oxides, including superconductors such as YBa 2 Cu 3 O 7 or Bi2Sr2CaCu2O8, ferroelectrics such as (Pb,La)(Zr,Ti)O3, and ferromagnets such as La0.7Cao.3Mn03. Compared to Si or GaAs, these materials are more complex in several respects. To begin with, one has to deal with rich phase diagrams in this case. As an example, Table I is a partial list of phases that appear in the Bi-Sr-Ca-Cu-O phase diagram (i.e., phases that can be obtained by mixing and firing Bi2O3, SrO, CaO, and CuO). Notice the occurrence of polytypes such as 2201, 2212, and 2223. We will mention these phases frequently in this article, since we have studied them most extensively. An attempt to grow 2212, for example, may easily provide a mixture of two or more phases from the list in Table I. In general, the result will depend on the initial stoichiometry of the mixture as well as on the thermodynamic conditions such as the temperature T and the gas pressure p. To complicate things further, 2212 is merely a shorthand notation; the stoichiometry of single crystals grown from t h e m e l t m a y a c t u a l l y be Bi2]Sric)5Ca().95Cu2O8+x. Occurrence of a
Table I: A partial list of phases that appear in the Bi-Sr-Ca-Cu-0 phase diagram.8 Bi 2 O 3 , SrO, CaO, CuO; SrBi2O4, Sr 2 Bi 2 0 5 , Sr 3 Bi 2 0 6 , Sr 6 Bi 2 0 9 , SrsBi^O,; Ca 5 Bi 1 4 0 2 6 , Ca 4 Bi 6 0
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