Natural strong pinning sites in laser-ablated thin films

Abstract

At low temperatures dislocations are the dominant flux-pinning centers in thin films of (Formula presented) deposited on (100) (Formula presented) substrates [B. Dam et al., Nature (London) 399, 439 (1999)]. Using a wet-chemical etching technique in combination with atomic force microscopy, both the length and the lateral dislocation distribution are determined in laser ablated (Formula presented) films. We find that (i) dislocations are induced in the first stages of film growth, i.e., close to the substrate-film interface, and persist all the way up to the film surface parallel to the c axis, resulting in a uniform length distribution, and (ii) the radial dislocation distribution function exhibits a universal behavior: it approaches zero at small distances, indicating short-range ordering of the defects. This self-organization of the growth-induced correlated disorder makes epitaxial films completely different from single crystals with randomly distributed columnar defects created by means of heavy-ion irradiation. Since the substrate temperature can be used to tune the dislocation density (Formula presented) over almost two orders of magnitude (∼1-100/μ(Formula presented)), the mechanism by which dislocations are induced is closely related to the (Formula presented) nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations. © 2000 The American Physical Society.