Exchange-Polarisation in Perovskite-Wurtzite heterostructures |
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.
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. 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.
Fig. 2.Experiment
ZnO-BTO-ZnO layer structures are grown by PLD on (0001) sapphire (10mm x 10mm, 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. 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.
Electro-optic measurements
A voltage U is applied between the two ZnO layers, and a 512-wavelength (1700 - 370 nm) ellipsometer (J.A. Woollam Co.) is used for measurements of the electro-optic (EO) effect in the BTO layer. The voltage is varied between U = -100 ... +100V. Changes in the ellipsometric parameters Delta and Psi are due to electric field induced index-of-refraction changes parallel to the growth surface within the BTO layer (Fig. 5). Spectrally-averaged index differences versus voltage U are depicted in Fig. 5. A linear current-voltage curve is obtained, with maximum current of approx. 40 mA at U = 100 V. To begin with, the index change is linear with U until approx. U = 50 V, according to the linear EO-effect (virgin curve). For higher voltages the index change is proportional to the square of U, indicative for paraelectric BTO (quadratic EO-effect). The index change remains constant upon voltage reversal until U = -50 V, indicative for reverse transition into ferroelectric BTO. Electro-optic Raman scattering measurements show disappearance of the ferroelectric phonon modes (MOdes at 308 and 580 cm-1, Fig. 6). The cause of the paraelectric phase transition is the parasitic leakage current, which increases the sample temperature. Preliminary temperature dependent dielectric measurements reveal this phase transition at T = 111°C. In this structure, the voltage required for reversal of the BTO domains is above U = 50 V, since only the upper branch of the ferroelectric BTO hysteresis loop can be seen. Further experiments are under way.
Collaborators: MPI for Microstructure Physics Halle (PD. Dr. D. Hesse), Semiconductor Physics Group, University Leipzig (Prof. Dr. Marius Grundmann, Dr. Michael Lorentz)
Funding: Deutsche Forschungsgemeinschaft (SCHUH 1338/4-1,2) within Forschergruppe "Oxidic interfaces" FOR 404
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