Ab initio study of the BaTiO3/Ge interface

We present a comprehensive first-principles study of BaTiO3 ultrathin films epitaxially grown on Ge(001). We recently reported on the experimental realization of this system and analyzed the 2 x 1 structural distortions in the BTO thin film which may give rise to technologically relevant functional properties [D. P. Kumah et al., Phys. Rev. Lett. 116, 106101 (2016)]. In this work, we describe the structural and electronic properties of the experimentally observed interface configuration, as well as a distinct metastable interface configuration with a higher out-of-plane polarization. We show that these two distinct interface structures can be made energetically degenerate by choosing a top electrode with an appropriate work function, thus enabling, in principle, an epitaxial ferroelectric thin film oxide. We analyze the interface chemistry and electronic structure and show that in the two polarization states the bands align differently, indicating a strong ferroelectric field effect. We also show that, surprisingly, in the intrinsic limit for the semiconductor, switching the oxide polarization state can cause the dominant charge carrier to switch between electrons and holes. The coupling of ferroelectric switching in the oxide with charge carrier type modulation in the semiconductor may have novel technological applications.