Optical properties of BaTiO3/ZnO heterostructures under the effect of an applied bias
Introduction
Recently the coupling between the spontaneous ZnO polarization and the switchable ferroelectric polarization of BaTiO3(BTO) has been demonstrated to cause remanent effects in ZnO/BTO MISFET structures [1]. Also a model for the description of the interaction has been proposed [2] and was applied to ZnO/BTO/ZnO heterostructures [3], [4]. These works focused on the electrical properties of BTO/ZnO heterostructures. However such polarization coupling and more over the remanent effect from creating a ferroelectric polarization can also be exploited for electro-optical applications. By using doped ZnO as electrode material instead of metals the whole heterostructure becomes transparent making it even more appealing for optical applications. Electro-optical effects have been observed in ZnO/BTO/ZnO heterostructures by applying an external bias [5], [6]. But up to now no studies of the optical properties of such structures with regard to a remanent effect are available. This remanent effect is caused by the formation of an electric polarization in the BTO layer. A key part for the investigation of a heterostructure is the knowledge of the individual components. Recently model dielectric functions for ZnO and BTO have been developed and are now available [7]. Here we report electro-optical measurements of BTO/ZnO heterostructures and the analysis of the optical properties with respect to an applied bias.
Section snippets
Experimental
All samples have been grown by pulsed laser deposition (PLD) with a KrF excimer laser at a pulse energy of 600 mJ on a-sapphire substrate. The growth was carried out at a substrate temperature of 740 °C and an oxygen partial pressure of 2 ⋅ 10− 3 mbar. The bottom layer of the heterostructure consists of Al doped ZnO (ZnO:Al) to provide an ohmic back electrode [8]. A 300 pulse ZnO:Al nucleation layer was deposited first at a repetition rate of 1 Hz. Subsequently the repetition rate was set to 15 Hz for
Structural properties
Fig. 1 shows a 2θ − ω XRD-scan of the heterostructures. It is clearly visible, that both the ZnO and BTO are single phase with a (0001) and (001) orientation respectively. The BTO peaks show a slight deviation from the single crystal values to lower angles. This indicates a higher c-axis lattice constant for the BTO thin films (∼ 4.1 Å) compared to the single crystal (4.03 Å) and is likely due to the incorporation of oxygen vacancies which are known to cause lattice deformation in (PLD grown) BTO [9]
Optical properties
To evaluate the effect of an applied bias and, more importantly, the effect of the formation of a polarization in the BTO, first the optical properties of the heterostructure need to be measured in the as grown state before any bias is applied. This state of the heterostructure will be referred to as the virgin state. Subsequently different bias conditions were applied, namely + 10 V, 0 V, −10 V and again 0 V. At each voltage the optical constants were measured. Experimental spectra of the
Conclusion
We have shown measurements of the optical properties of BTO/ZnO heterostructures and the effect of an applied electric bias. A linear electro-optical effect caused a change in the band gap thus altering the optical properties of the heterostructure. Furthermore heterostructures containing a BTO layer show a remanent change in the band gap which was attributed to the formation of a ferroelectric polarization in the BTO. The strength of the remanent effect in the current heterostructures could be
Acknowledgment
The authors acknowledge the support of Deutsche Forschungsgemeinschaft in the framework of Sonderforschungsbereich SFB762 “Functionality of oxidic interfaces”. Tammo Böntgen also acknowledges financial support by the graduate school BuildMoNa.
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