Thin films of multiferroic BiCoO3 by ALD

Abstract

This work presents the first reported thin film deposition of α-Bi2O3 by ALD (Atomic Layer Deposition). In addition, thin films of Co3O4 and CoO are for the first time reported deposited by ALD from the respective novel precursor combinations Co(thd)3/O3 and Co(thd)2/H2O (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate). Finally, BiPh3 (Ph = phenyl), Bi(t-OBu)3 (t-OBu = tert-butoxide) and Bi(thd)3 were all investigated for potential use as bismuth precursors in the ALD process.

Thin films of Co3O4 deposited from the novel precursor combination Co(thd)3/O3 was investigated and a comparison to the already well investigated precursor combination Co(thd)2/O3 is given.

It was found that BiPh3 can be used as a bismuth precursor for deposition in the Bi-Co-O system by ALD, although the concentration of bismuth, with respect to the metal content, was limited to a maximum of 22.8 at.%. Bi(t-OBu)3 was synthesized and investigated for the potential use as an ALD precursor, however, it was found that this compound has too low thermal stability to be applicable in the ALD processes.

Bi(thd)3 was synthesized and investigated for use as precursor in the ALD process. It was found that uniform films of α-Bi2O3 could be deposited from the Bi(thd)3/H2O precursor combination, however, in-situ QCM measurements indicated a non-ideal ALD growth behavior. It is suggested that a surface controlled reaction occurs between Bi(thd)3∙n(H2O) and its own crystal water. In addition, a suggestion for a new and yet unreported phase of Bi(thd)3 is given.

Deposition and investigation of thin films in the Bi-Co-O system are also presented. However, an etching process was observed between the Bi(thd)3 precursors and the Co3O4 surface. A possible mechanism is presented. Thin films of composition near 50:50 at.% of Bi and Co has been obtained. Mild heat-treatment under oxygen atmosphere resulted in the formation of multiple phases such as Co3O4 and a sillenite phase, with the proposed composition Bi3.43Co0.57O5.90.