Microstructural properties of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Cu}}_n{\mathrm{O}}_y$ multilayers grown by molecular beam epitaxy

The microstructure of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Cu}}_n{\mathrm{O}}_y$ multilayers grown by molecular beam epitaxy on atomically flat SrTi${\mathrm{O}}_3$ substrates has been studied by reflection high-energy electron diffraction (RHEED), atomic force microscopy, and x-ray-diffraction (XRD) techniques. The overall RHEED data, collected in situ at different $\frac{{\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Cu}{\mathrm{O}}_y}{{\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_y}$ (2201/2212) multilayer growth stages, demonstrated a two-dimensional growth and rather a high quality of the interfaces. Following the evolution of RHEED patterns, some evidence of an increase in surface roughness after several multilayer periods, was detected. A one-dimensional x-ray-diffraction model was applied for a quantitative analysis of growth defects in the multilayers. The substitutional disorder in the lattice and stacking faults in the molecular layers were determined by an iterative comparison of simulated x-ray-diffraction spectra with the experimental XRD data. The observed changes in the $c$-axis lattice parameter of 2201 molecular layers were interpreted as being caused by ionic substitutions of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{Ca}}^{2+}$ in the lattice and governed by the growth interdiffusion. The fitting procedure also revealed that two types of growth disorder were present in the layers: (1) stacking faults randomly distributed within the layers and (2) stacking faults localized at the interfaces. The two types of growth defect are expected to influence the superconducting properties differently and this has to be considered before the transport properties of superconducting multilayers are studied.