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).


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.

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Fig. 6.

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