Oxygen vacancy induced phase and conductivity transition of epitaxial BaTiO3−δ films directly grown on Ge (001) without surface passivation

Abstract

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.

Highly heterogeneous epitaxy of flexoelectric BaTiO3-δ membrane on Ge

“…These results indicate that the epitaxial BTO 3-δ grown on graphene/Ge (011) has a cubic phase due to the 35 induced by the oxygen-poor growth conditions required by the interface engineering. Moreover, different from the BTO/Ge heteroepitaxy 22 , 33 , 36 , the BTO 3-δ lattice was already partially relaxed at the very beginning of the growth, due to the weakening of the clamping effect of Ge substrates by the graphene monolayer. As the film thickness increases, BTO 3-δ relaxes more and becomes fully relaxed in the 90 nm-thick film, while the film becomes less deficiency in oxygen due to a higher oxygen partial pressure during the growth, and thus the lattice volume decreases 37 .…”

“…S10 in SI. Moreover, interestingly, it was reported that BTO films grown on bare Ge (001) substrates using interface engineering were single crystalline 33 whereas BTO films grown on the graphene/SiO 2 /Si template were similarly textured with (001)-orientation, which reveals that graphene layer hinders the epitaxy of BTO on Ge (001) but induces the textured structure of BTO 34 .

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“…This evidently indicates the important passivation role of graphene, which protects Ge (011) surface from oxidizing at the beginning of BTO growth at a relatively high temperature. It is interesting to point out that the BTO directly grown on Ge (001) substrate, however, shows an epitaxial feature 33 . This can be possibly attributed to a more active feature of Ge (011) compared to Ge (001), and further confirms the passivation role of graphene.…”

Dissociation mechanism of lithium salt by BaTiO3 with spontaneous polarization

“…In X-ray photo-electron spectroscopy (XPS), the O 1s peak can be fitted as lattice-O and absorbed-O. 27 The results indicate that the ratio of T-BTO 3− x lattice-O has reduced compared to T-BTO (Fig. S4a, ESI†).…”