Thin Films and Superlattices in the Bi-Fe-O System Prepared by ALD

Bismuth ferrite (BiFeO₃) is one of the most technologically promising multiferroics with a simple perovskite structure and high ferroelectric and antiferromagnetic ordering temperatures. High-quality heteroepitaxial growth of BiFeO₃ thin films has been demonstrated mainly by PLD, MBE, RF sputtering and metal-organic chemical vapor deposition (MOCVD). While the first three techniques listed above require ultrahigh vacuum and thus are rather expensive, particularly when used on an industrial scale, MOCVD may serve as an appropriate deposition method for the device fabrication. However, as nanoelectronic device sizes shrink, the demand for a high-precision deposition of thin layers on high-aspect-ratio structures grows stronger. ALD simply does not encounter this obstacle and can produce atomically smooth coverage on structures with extremely high aspect ratio. We will discuss the growth of BiFeO₃ thin films and (Bi-O)_x(Fe-O)_y superlattices by ALD and the influence of the growth parameters on the phase composition of the resulting films. In short, the preparation of the films involved two stages: the deposition of the amorphous films and superlattices on Si wafers, SrTiO₃(001), Nb:SrTiO₃(001) at 250 C and a subsequent annealing of as-deposited films at various temperatures. Using XRD analysis and TEM imaging the annealed BiFeO₃ films were demonstrated to be highly crystalline; piezoresponse force microscopy showed an expected response of the ferroelectric phase. The ALD growth of BiFeO₃ will provide a new way of convenient, technologically-oriented and commercially realizable deposition of a promising multiferroic for nanoelectronic devices.

This work was supported by the NSF DMR under 1124696 and by the ARO under W911.