Mat. Res. Bull., Vol. 28, pp. 271-277, 1993. Printed in the USA. 0025-5408/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd. INTERLOCKED GRAIN GROWTH OF YBCO FILM ON MAGNESIUM OXIDE AS OBSERVED BY SCANNING TUNNELING MICROSCOPY. M.V.H.Rao, S.K. Mishra*, L.C. Pathak*, B.K. Mathur, D. Bhattacharya* and K.L.Chopra. Department of Physics, Materials Science Center Indian Institute of Technology, Kharagpur -721 302. India (Received October 6, 1992; Communicated by C.N.R. Rao) ABSTRACT: Thin films of YBa2Cu307 on MgO deposited by RF magnetron sputtering to study their superconducting behavior, were examined by a Scanning Tunneling Microscope. The films were deposited at room temperature, sintered exsitu at 850°C, 900°C, 930°C,950°C and 980°C for 30 minutes each and cooled at rate of 100°C per hour in oxygen ambient. Topographic images taken in air show that the films which were initially amorphous in nature, gradually transform to a layered grain structure with the increase of sintering temperature. Films sintered at temperatures above 900°C are c-axis oriented and have a coarse grain structure. These grains are connected mutually by a well defined interlock, formed between the layers of neighboring grains. The mechanism of interlocking of layers from adjacent grains and its possible influence on the superconducting properties has been discussed. MATERIALS INDEX: Yttrium, Barium, Cuprates, Superconductors, Thin films. Introduction The microstructure of thin films of high temperature superconductors (I-ITS) controls their electrical properties. The final microstructure of the film is dependent on the nucleation and early stages of growth and post deposition processing. The substrate is expected to exert an influence on growth. Due to characteristic transport properties of YBa2Cu307 (YBCO) film on MgO and the fact that highly oriented films can be grown on this substrate, even though there is a large lattice mismatch, it is of interest to study the growth process of YBCO on MgO. These films deposited at room temperature are amorphous and insulating. To make them superconducting, it is required to do post deposition annealing at high temperatures for grain growth and crystallization to occur and slow cooling in oxygen atmosphere for its incorporation into the lattice. It is important to understand the mechanism of grain growth of these films and the factors influencing it, so that the annealing parameters can be optimized to get high quality superconducting films. Here in this report we present our Scanning Tunneling Microscopy (STM) study on 271