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Exchange-Polarisation in Perovskite-Wurtzite heterostructures

Research goals

Coupling between the spontaneous polarization in wurtzite structures and the ferroelectric polarization in perovskite structures is the subject of this research field. Of particular interest is whether the ferroelectric phase transition can be influenced by the spontaneous electric polarization in perovskite-wurtzite heterostructures. The concept is to bring bound ionic charges at the interface of wurtzite structure materials in contact with switchable ionic charges at the interface of ferroelectric perovskite materials. The amount of the charge can be varied by the thickness of adjacent layers. If the polarization axies are aligned properly, the ferroelectric polarization can be switched upon application of external electric fields. In this concept, the wurtzite material also acts as a transparent conducting electrode, which allows electro-optical measurement of the polarization reversal within the perovskite layers.

polarization coupling
Fig. 1.

State of the art

Bariumtitanate (BTO) is attractive for capacitive, piezoelectric, pyroelectric and electro-optic (EO) device applications. Successful growth of high-quality BTO and ZnO layers by pulsed laser deposition (PLD) was reported previously. The electrical conductivity of the ZnO layers can be controlled over many orders of magnitude. ZnO possesses permanent spontaneous polarization.

GaN BTO crystal structure Fig. 2.


Calculations predict values similar to GaN and AlN. Spontaneous polarization values of BTO thin films are within the same range. Whereas the wurtzite polarization cannot be reversed, external electric fields can switch the polarization direction in the perovskite structure.


ZnO-BTO-ZnO layer structures are grown by PLD on (0001) sapphire (10mm x 10mm, Fig. 3.)

TEM picture
Fig. 3

X-ray diffraction, transmission electron microscopy (TEM), selected area diffraction (SAD), Raman scattering, infrared ellipsometry (Fig. 4) and electrical measurements are performed for analysis of structural, optical and electrical properties. TEM and SAD results are shown in Fig.3.

IRSE and Raman data
Fig. 4.

The ZnO films adopt (0001) orientation and possess high structural quality. The BTO layer (approx. 300 nm thick) reveals (111)-oriented domains, and is electrically resistive. Raman scattering reveals phonon mode properties of ferroelectric BTO. Electrical contacts were processed onto both ZnO layers. Masks are used during growth.
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