EPITAXIALY STRAINED STRONTIUM TITANATE
Open Access
- Author:
- Biegalski, Michael David
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 01, 2006
- Committee Members:
- Susan E Trolier Mckinstry, Committee Chair/Co-Chair
Darrell G Schlom, Committee Chair/Co-Chair
Long Qing Chen, Committee Member
Venkatraman Gopalan, Committee Member
Qiming Zhang, Committee Member - Keywords:
- Thin Film
Strontium Titante
Strain
Ferroelectric
Relaxor
Epitaxy - Abstract:
- Although SrTiO<SUB>3</SUB> is normally not ferroelectric at any temperature, predictions that predate this thesis based on thermodynamic analysis concluded that a biaxial tensile strain of order 1% would shift the paraelectric-to-ferroelectric transition temperature (<I>T</I><SUB>c</SUB>) of SrTiO<SUB>3</SUB> to the vicinity of room temperature. In practice, uniformly straining SrTiO<SUB>3</SUB> or related perovskite ferroelectrics to such strain levels is challenging and hitherto unheard of. Nonetheless, using epitaxy and the misfit strain imposed by an underlying substrate, I have strained SrTiO<SUB>3</SUB> thin films to percent levels far beyond where they would crack in bulk. Epitaxial ferroelectric films are often grown to thicknesses greatly exceeding their critical values, resulting in undesirable relaxation toward a zero-strain state by the introduction of dislocations. Dislocation densities of ~10<SUP>11</SUP> cm<SUP>-2</SUP> are common in epitaxial ferroelectric films grown on lattice-mismatched substrates, and the resulting inhomogeneous strain smears out the ferroelectric phase transition. My approach to achieving the desired high strain levels in SrTiO<SUB>3</SUB> films to assess strain predictions made use of new substrates (DyScO<SUB>3</SUB> and GdScO<SUB>3</SUB>) that enabled the growth of uniformly strained SrTiO<SUB>3</SUB> films below, or at least far closer to, the critical thickness for relaxation. The resulting strained SrTiO<SUB>3</SUB> films have better structural perfection (narrower rocking curve widths) than the best bulk SrTiO<SUB>3</SUB> single crystals. These films have the narrowest rocking curves ever reported for any heteroepeitaxial oxide thin film (6.5 arc sec). Modeling of ferroelectrics under these strain levels predicts dramatic shifts in the transition temperature and enhancement of the polarization. Indeed, in our strained SrTiO<SUB>3</SUB>, a ferroelectric state was induced with a Tc near room temperature. These films also exhibit a peak dielectric constant near room temperature of ~20,000, comparable to that seen at very low temperatures (~4 K) in bulk SrTiO<SUB>3</SUB>. Unexpectedly, the strained SrTiO<SUB>3</SUB> films exhibit a frequency dependence of their dielectric constant consistent with relaxor ferroelectricity. Due to the anisotropy in the in-plane strain, the polarization develops at different temperatures along two orthogonal in-plane directions indicating an anisotropy in the dielectric properties due to the orhorhombicity of the substrate. These results, illustrate that in thin films strain is a viable alternative to the traditional method of chemical substitutions for shifting <I>T</I><SUB>c</SUB> by large amounts.