Microstructure investigations of the epitaxial growth of ultrathin Bi 2Sr 2CaCu 2O x superconducting films on (001) SrTiO 3 substrates

Abstract

The microstructure of epitaxial, c-axis oriented Bi 2Sr 2CaCu 2O x superconducting thin films with a mean thickness of 3 to 100 nm on (001) SrTiO 3 substrates was studied by plan-view and cross-section transmission electron microscopy (TEM) and atomic force microscopy (AFM). An inductively measured transition temperature could be observed down to 12.5-nm thick films. In plan-view TEM samples, islands were observed, which began to coalesce at a thickness of ≈ 12 nm forming 90° twin domains. By analysis of the Moiré patterns we noted an increasing relaxation of the in-plane lattice parameters of the Bi 2Sr 2CaCu 2O x phase with increasing film thickness. No Bi 2Sr 2CaCu 2O x phase was seen in cross-section TEM samples up to a film thickness of 12.5 nm inclusive but two unidentified phases with a fringe spacing in [001] direction of the SrTiO 3 of ≈3.3 A ̊ and ≈8 A ̊ , respectively. The first one occurred often in discrete film thicknesses of 4, 8 and 12 nm, the other one did not. We suggest, that a precursor mechanism for the growth of the ultrathin Bi 2Sr 2CaCu 2O x films exists. AFM investigations showed a three stage growth mechanism: 1. appearance of small circular nuclei with a tendency to coalesce, 2. islands form a facet-like film surface and 3. smooth surface. An high density of small holes clustering round needlelike outgrowths was seen in 12.5-nm and thicker films.