Controlling of lattice strains for crack-free and strong ferroelectric barium titanate films by post-thermal treatment

In this study, we experimentally demonstrate fabrication of ultra-smooth and crystalline barium titanate (BTO) films on magnesium oxide (MgO) substrates by engineering lattice strain and crystal structure via thermal treatment. We observe that oxygen-depleted deposition allows growth of highly strained BTO films on MgO substrates with crack-free surface. In addition, post-thermal treatment relaxes strain, resulting in an enhancement of ferroelectricity. Surface roughening of the BTO films caused by recrystallization during post-thermal treatment is controlled by chemical–mechanical polishing (CMP) to retain their initial ultra-smooth surfaces. From Raman spectroscopy, reciprocal space map (RSM), and capacitance–voltage (C–V) curve measurements, we confirm that the ferroelectricity of BTO films strongly depend on the relaxation of lattice strain and the phase transition from a-axis to c-axis oriented crystal structure.