Nanoscale physics

Polar phonon behaviour in polycrystalline SrTiO3 thin films: 
the role of the substrate
Tkach O.1, Okhay O.1, Nuzhnyy D.2 Petzelt J.2, Vilarinho P.M.1
1CICECO – Aveiro Institute of Materials, Dept. of Materials and Ceramic Eng., University of Aveiro. Campus de Santiago, 3810-193, Aveiro,Portugal. 
E-mail: atkach@ua.pt 
2Institute of Physics of the Czech Academy of Sciences. Na Slovance 2, 182 21 Prague 8, Czech Republic.

Undoped SrTiO3 (ST) is a quantum paraelectric, where zero-point fluctuations preclude the condensation of the polar lattice soft mode, saturating the dielectric permittivity at low temperatures T < 4 K [1]. ST is also characterized by very low dielectric losses, hence ST based thin films are excellent candidates for such high quality factor applications as tunable devices.
In ST films with nanoscale thickness a ferroelectric anomaly accompanied by soft mode condensation has been induced at room temperature due to in-plane stresses from the DyScO3 substrates and according to the lattice mismatch theoretical predictions [2,3]. However, optimizing performance using low cost scalable processing and substrates is critical if functional oxide thin films are to achieve commercialisation. Here, we present a comprehensive investigation of the role low cost Al2O3 and MgO substrates on the structure, microstructure and the polar phonon behaviour of ST thin films, deposited by sol gel processing.
We verify that ST films deposited on Al2O3 substrates, being under a tensile stress due to thermal expansion mismatch, present the soft mode behaviour similar to that of ST single crystals, deviating just below 150 K. In contrast, ST films deposited on MgO substrates, being under compressive stress, have higher soft mode frequency from 10 to 300 K. It is thus proved that for polycrystalline ST films the intrinsic in-plane relative permittivity may be tuned through an induced stress state due to thermal expansion mismatch.

1.	Müller K. A., Burkard H. SrTiO3: An intrinsic quantum paraelectric below 4 K // Phys Rev B.-1979.-19.-3593-3602.
2.	Haeni J. H., et al. Room-temperature ferroelectricity in strained SrTiO3 // Nature.-2004.-430.-758-761.
3.	Nuzhnyy, D. et al. Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions // Appl. Phys. Lett. 2009.-95.-232902.