Microstructure and growth mechanisms of YBa 2 Cu 3 O x films prepared by rf thermal plasma evaporation
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We prepared YBa 2 Cu 3 0^ films on (100) MgO and (100) SrTiO3 substrates by rf thermal plasma evaporation, and investigated microstructure and growth mechanisms of these films by observation of the surfaces using an AFM technique. As a result, 2D nucleation and further coalescence between vicinal grains were observed in the initial stage of growth. In the films prepared for a deposition time of 3 min, the different complex growth modes, including spiral growth, "birth and spread" growth, and 2D growth, were observed, which might be due to high growth rate over 55 nm/min of this process. I. INTRODUCTION The rf thermal plasma evaporation technique is a vapor phase process using thermal O 2 plasma with an extremely high temperature and active O2 atmosphere.1"3 As reported in previous papers, 4 " 6 as-grown films prepared by rf thermal plasma evaporation have the advantages of a high deposition rate, a large area deposition, and a high critical current density (Jc) with an order of the magnitude of 105 A/cm 2 (77 K, 0T). Accordingly, this technique is considered to be one of the practical processes for synthesizing superconducting tape,7 wire, and magnetic shielding for engineering power applications. However, growth mechanisms of films prepared by this process have not been clearly understood. Since microstructure of YBa 2 Cu 3 0^ films often influences their superconducting properties, particularly Jc values and the magnetic field dependence, investigation of microstructure and growth mechanisms is important to improve the film qualities prepared by this process for engineering application. Recently, atomic force microscopy (AFM) and scanning tunneling microscopy (STM) imaging were developed to study the microstructure of the superconducting films. These techniques could be used to investigate nucleation and growth mechanisms in the vapor-phase process8"12; for example, the spirally expanding step edge on the YBa 2 Cu 3 0^ films prepared by sputtering could be observed.8 In this paper, we exhibited morphologies and structure of typical YBa 2 Cu 3 0^ films prepared on (100)MgO and (100)SrTiO3 and a deposition rate of 55 nm/min for deposition times from 1 s to 3 min by rf thermal plasma evaporation, and further investigated microstructure and growth mechanisms of these films by observation of the surfaces using an AFM technique. II. EXPERIMENTAL PROCEDURES The schematic diagram of the experimental setup is shown in Fig. 1. Major experimental conditions are J. Mater. Res., Vol. 10, No. 2, Feb 1995 http://journals.cambridge.org
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listed in Table I. The 4 MHz rf A r - O 2 plasma was operated at 43 kW and 200 Torr as the input power of the plate power level and the total pressure, respectively. Ar/O 2 flowing gas ratio was 7/50. The Y 12 Ba 2 Cu 3 O^ powder4'13 with an average particle size of approximately 3 fjiva was fed axially into the plasma with Ar carrier gas. After the growth, the sample was quickly cooled for approximately 10 min to room temperature under 200 Torr O 2 atmosphere. As a subst
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