Functional oxide films with engineered ferroelectric properties
- PDF / 2,153,598 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 69 Downloads / 211 Views
Functional oxide films with engineered ferroelectric properties R. Wördenweber, T. Ehlig, J. Schubert, R. Kutzner, and E. Hollmann Peter Grünberg Institute (PGI) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT The ferroelectric properties of anisotropically strained SrTiO3 films are analyzed by detailed measurements of the complex dielectric constant as function of temperature, frequency, bias voltage and electric field direction. The strain induces a relaxor-ferroelectric phase that persists up to room temperature. However, transition temperature and ferroelectric properties strongly depend on the orientation of the electric field and therefore on the amount of structural strain in the given electric field direction. Frequency and time dependent relaxation experiments reveal the presence and properties of polar nanoregions with randomly distributed directions of dipole moments in the film. INTRODUCTION Pseudomorph strained-layer epitaxial growth has quickly turned from a mere curiosity into a major technology for advanced semiconductor devices and structures for solid state research. It has been demonstrated that mechanical strain can have dramatic effects on the electromagnetic properties of thin films. Significant effects have not only been observed in semiconductor films for which the strain affects key material properties including the energy gap and effective mass - and thus the mobility - of the charge carriers. Prominent examples are also the strain-induced enhancement of magnetization observed in ultrathin spinel ferrite films [1] and the strain induced enhancement of the ferroelectricity transition temperature in perovskite ferroelectric films [2]. For heteroepitaxially grown thin films, mechanical stress is naturally induced by the underlying substrate. The stress is caused by the lattice mismatch and the differences in thermal expansion coefficients between the substrate and the film. It naturally leads to strain in the layer that results in structural modifications of the film ranging from (i) the development of defects [3], (ii) structural transitions [4], or (iii) modification of the lattice constants [9-11]. Even more complex is the combination of substrate and film with different crystallographic symmetries. In this case the structural mismatch will be different in different crystallographic directions. As a result, not only the structural but also the electromechanical properties of the layer might be different in different crystallographic directions. An ideal candidate to study this effect is provided by the growth of a nominally cubic ferroelectric perovskite (e.g. SrTiO3) on a noncubic substrate. In this paper, the ferroelectric properties of anisotropically strained SrTiO3 on DyScO3 substrates are analyzed and discussed. The dependence of the complex dielectric constant on temperature, frequency, bias voltage and electric field direction provides detailed information about the underlying ferroelectricity and its modification du
Data Loading...
