Observation of the Early Stages of B-Axis Oriented Prba 2 Cu 3 O 7-x Thin Film Growth by AFM
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INTRODUCTION The a-axis oriented YBa 2Cu 3OT 7 x (YBCO) thin films are important to fabricate the planar Josephson junctions because of the longer coherence length along the a-axis or b-axis. Among the a-axis oriented YBCO films grown by various methods[l-8], in-plane aligned a-axis oriented YBCO thin films on LaSrGaO 4 (LSGO) substrate with PrBa 2Cu 3OT 7 x (PBCO) template have the highest zero resistance temperature(Tc,zero)[ 6 - 91. The a-axis YBCO films with a Tczero of 83 K have been reported by Inam et al.[5] and these films are formed by inserting an a-axis oriented PBCO thin film as a template layer between the SrTiO 3 (100) substrate and the YBCO film. Their low Tczero was due to the existence of these 90' grain boundaries compared with c-axis YBCO films. Recently, in-plane alignment of the a-axis YBCO films has been realized by using LSGO (100) substrates with PBCO templates, so that the a-axis oriented YBCO films having Tc,zero of 87 - 90.5 K were obtained. Since much of a-axis oriented microstructure is determined by the initial formation of the film on the substrate, it is valuable to investigate the early regime of deposition to better understand nucleation and subsequent growth of the film and the creation of microstructural features. Investigation of the surface morphology with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have revealed a high density of growth spirals on c-axis oriented YBCO thin films[10, 11]. But the STM and AFM studies for a-axis oriented YBCO and PBCO films are very limited. Therefore we have focussed our efforts on the growth mode of PBCO films on LSGO (100) substrate. In this study we report the observation of the growth of b-axis oriented PBCO thin films on LSGO (100) substrates, the in-plane aligned a-axis oriented YBCO thin films on LSGO (100) substrates with b-axis oriented PBCO templates, and the c-axis oriented YBCO thin films on SrTiO 3(100) substrates. EXPERIMENTAL PROCEDURE PBCO thin films with various thicknesses, ranging from 0.34 nm up to 100 nm, were 433 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
prepared by pulsed laser deposition (PLD). Hot-pressed stoichiometric PBCO targets were mounted on a rotating, water-cooled 4-pole target holders. LSGO (100) substrates were bonded onto the heating block using silver paste. The substrates, held at 4 cm from the targets, were pre-annealed at 850 'C under 10-6 Torr to remove surface defects including contaminants and scratches created during mechanical polishing. After in-situ pre-annealing, the PBCO films were deposited at 630 'C using a 308 nm XeC1 excimer laser operated at repetition rate of 1 Hz and produced 14±4 ns pulses with energy density of 1 J/cm 2 . All depositions were carried out in a dynamic oxygen atmosphere of 100 mTorr. The thickness of the films was controlled by scaling the number of laser pulses. Deposition rate of the PLD conditions was 0. 34 nm per single laser pulse. In-plane aligned a-axis oriented YBCO thin films were grown by two steps PLD proces
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