Oxygen vacancy induced phase and conductivity transition of epitaxial BaTiO_3-δfilms directly grown on Ge (001) without surface passivation

The heterogeneous epitaxial system of BaTiO3/Ge (BTO/Ge) is of great interest for both fundamental research and device applications, thanks to its quasi-lattice-matching feature and the integration of functional oxides on semiconductors. Currently, the heteroepitaxial growth of crystalline BTO films on Ge includes the utilization of ultrahigh vacuum tools and complex surface passivation pre-treatment as well as careful control of oxygen partial pressure during the growth. Meanwhile, oxygen vacancies in oxides strongly impact their structural and electrical properties. Here, we report a facile method to directly grow single crystalline BTO films on Ge using pulsed laser deposition. The strict control of oxygen partial pressure ensures a sharp interface with an atom-to-atom registry and also leads to the oxygen-deficient characteristics of BTO. The epitaxial relationship of BTO and Ge is [110] BTO (001)//[100] Ge (001). Detailed crystallographic studies on BTO films with different thicknesses show that, for the films with a thickness less than 20 nm, BTO shows a mixture of tetragonal and cubic phases due to the oxygen vacancies and the strain from the Ge substrate and the cubic phase eventually dominates as the film thickness increases. Such oxygen-deficient BTO films reveal conducting characteristics rather than dielectric properties. The oxygen vacancies can be partly "cured"after a low temperature annealing process. These results not only demonstrate the possibility to directly grow single crystalline oxides on semiconductors without surface passivation but also highlight the importance of oxygen vacancies and lattice strain on the crystallographic and electrical properties of BTO films.