Molecular beam epitaxy of dysprosium barium cuprous oxides using molecular oxygen
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Molecular beam epitaxy of dysprosium barium cuprous oxides using molecular oxygen E. S. Hellman, E. H. Hartford, and E. A. Fitzgerald AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974 (Received 8 February 1991; accepted 21 November 1991)
Epitaxy of cupric oxides, such as the high temperature superconductor YBa 2 Cu 3 0 7 , using "vacuum" techniques requires either activated forms of oxygen, such as atomic oxygen, oxygen plasma, or ozone, or a relatively high pressure of molecular oxygen. In contrast, cuprous oxides (those with formal valence of copper less than +2) can be grown epitaxially in molecular oxygen at pressures below 1CT4 Torr. We have explored this regime of epitaxial growth because of the possibility of forming DyBa 2 Cu 3 0 7 through low temperature ex situ oxidation of DyBa 2 Cu 3 0 6 . We find that the dominant phases growing epitaxially on MgO are CuDyO2, Cu 2 O, CuBa 2 O 2 , DyBa 2 Cu 3 0 6 , and the barium-rich perovskite solid solutions. Sticking coefficients of barium and dysprosium depend on substrate temperature and flux composition for substrate temperatures between 550° and 700 °C. We have obtained superconducting films by annealing Dy-rich, Cu-deficient films in oxygen at 400 °C. The nonstoichiometry (with respect to DyBa 2 Cu 3 0 6 ) appears to stabilize "DyBa 2 Cu 3 0 6 ," at low oxygen pressures. We also discuss the use of copper in effusion cells.
I. INTRODUCTION Since the discovery of the high Tc superconductors, a great many techniques have been used to synthesize these materials in the form of epitaxial thin films. One class of techniques that has been successful involves the evaporation of the constituent metals in vacuum onto a heated substrate in a flux of an oxidizing species to obtain an epitaxial oxide film. Such techniques include those known as molecular beam epitaxy (MBE) and reactive evaporation. For the most widely studied material, YBa 2 Cu 3 0 7 _ x , commonly known as "1-2-3" (we will use this term to refer also to the isostructural phases with rare earths replacing the yttrium), it is generally advantageous to raise the oxygen pressure during the cooldown following epitaxial growth, to obtain in situ superconducting films. This is because superconducting YBa 2 Cu 3 0 7 _ x (x < 0.4) is impossible to grow by conventional vacuum evaporation techniques. However, semiconducting YBa 2 Cu 3 0 7 x (x > 0.5) can be grown by vacuum evaporation, and it is readily transformed into the 90 K superconductor on cooling. Hammond and Bormann1 have correlated the optimal conditions for growth of YBa 2 Cu 3 0 7 _ x for a wide variety of techniques with the thermodynamic stability range of YBa 2 Cu 3 0 7 _ x as determined by solid state electrolytic measurements. They argue that optimal epitaxial growth is obtained near the high temperature, low oxygen pressure stability limit of YBa 2 Cu 3 0 7 _ x , i.e., in conditions where x ~ 1 in equilibrium. One implication of the "Hammond-Bormann Line", taken at face value, is that for a given substrate temperature, opti
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