Preparation of [Ba 2 CuO 2 (CO 3 ) ]m[ACuO 2 ]n(A=Sr,Ca) Films by MBE Technique
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Preparation of [Ba2CuO2(CO3) ]m[ACuO2]n(A=Sr,Ca) Films by MBE Technique Yutaka Adachi, Yoshio Matsui, Isao Sakaguchi, Hajime Haneda and Koichiro Takahashi Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, JAPAN ABSTRACT [Ba2CuO2(CO3)]m[ACuO2]n(A=Sr) superlattices containing oxycarbonate blocks as charge reservoir have been prepared on SrTiO3 using the molecular beam epitaxy technique. First, thin films of the oxycarbonate cuprate Ba2CuO2(CO3) have been prepared on SrTiO3(001) using NO2 gas as an oxidant and CO2 gas. The films have been grown at 500℃. At higher substrate temperature or at lower CO2 pressure Ba2CuO3 was formed instead of Ba2CuO2(CO3), and the films becomes amorphous at lower temperature. X-ray diffraction and reflection high-energy electron diffraction observations indicated that (BaxSr1-x)2CuO2(CO3) grew along the [001] crystal orientation on SrTiO3(001) with the following epitaxial relationship: Ba2CuO2(CO3)[100]//SrTiO3[110] and Ba2CuO2(CO3)[110]//SrTiO3 [100]. Depth profile of secondary ion mass spectrometry signals indicated the incorporation of carbon into the films. Secondly, the oxycarbonate cuprates and infinite layers have been alternately stacked. It was confirmed that Ba2CuO2(CO3)was inserted between several unit cells of SrCuO2. Electrical measurements show the as grown films to have a semiconducting behavior. INTRODUCTION Since the discovery of cuprate superconductors containing a metal carbonate in the blocking layer [1], several series of oxycarbonte cuprates have been confirmed by high-pressure method. One of them, (Cu,C)Ba2Can-1CunOy has attracted considerable attention because of the high critical temperature, high critical current densities and high irreversibility fields [2]-[4]. The structure of these oxycarbonates consists of infinite layer structure interconnected through layers of carbonate groups. However these compounds can be synthesized only by high-pressure synthesis techniques, therefore these materials can be obtained in the form of polycrystals. On the other hand, it is known that the thin-film processing make it possible to obtain metastable phases synthesized at high pressure. So far, few studies have been done on cuprate oxycarbonate thin films. Thin films of the Ba-Ca-Cu-C-O system have been grown by sputtering [5] and pulse laser deposition [6] and superconductivity was confirmed. But the resistance curves of the films show a broad transition, because different members having variable numbers of CuO2 layer usually coexist. E5.24.1
To obtain single-phase thin films, precise stacking of CuO2 layer blocks and oxycarbonate blocks along the c direction is required. One of the most promising ways to form an artificial lattice will be molecular beam epitaxy (MBE) method. MBE has the advantage of in situ reflection high-energy electron diffraction (RHEED) observations, which can used to provide feedback. In this study we report on our efforts to prepare superlattices containing oxycarbonate blocks as charge
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