M.Sc Thesis


M.Sc StudentRoman Rechter
SubjectGrowth and Characterization of the SrTiO3 -Based Thin
Films
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Zolotoyabko Emil
Professor Rothschild Avner
Full Thesis textFull thesis text - English Version


Abstract

SrTiO3 (STO)-based thin films attract growing attention of research groups worldwide due to their tunable multi-functional properties. Epitaxial thin films and heterostructures, comprising STO and its doped-derivatives, provide a rich platform for exploring and tailoring their functional properties. In particular, the effects of interfacial phenomena and strain on transport properties can be systematically investigated and controlled. The enabling key to these investigations is the growth of high quality epitaxial films with precisely controlled microstructure and interfacial structure down to the atomic scale.

In this work we investigate the possibility to epitaxially grow Fe- or Nb-doped STO thin films (10 - 150 nm thick) by Pulsed Laser Deposition (PLD). The films were deposited on (001)-oriented single crystal STO substrates that had been treated in BHF solution and annealed at 9500C in order to form atomically smooth TiO2-terminated terraces that enable 2D epitaxial film growth. The films were deposited at a substrate temperature of 7000C under oxygen pressure of 50 mTorr or in high vacuum (0.003 mTorr) conditions, using KrF excimer laser (λ = 248 nm) at a fluence of 1 J/cm2. The film surfaces displayed smooth terraces similar to the substrate topography features, as was revealed by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (HRSEM). The terrace morphology indicates a 2D-mode of the film growth. The chemical composition of the grown films was measured by Rutherford backscattering (RBS) and energy dispersive spectrometry (EDS) in SEM, demonstrating a uniform composition close to the expected stoichiometry.

The crystal structure, epitaxial orientation relations, degree of strain and in some cases d-spacing fluctuations, and interface roughness of the grown films were analyzed by means of high resolution X-ray diffraction (HRXRD). The degree of strain was found to vary with film thickness and composition. The Nb-doped or Fe-doped films were found to be coherent to the substrate and fully strained up to a thickness of 150 or 50 nm, respectively. In Fe-doped films, thicker than 50 nm, strain relaxation by crack propagation was observed. It was also found that strontium ferrite (SrFeO3-x, x ≤ 0.5) films can be grown epitaxially on STO and their crystal structure can be modified from cubic (perovskite) to orthorhombic (brownmillerite) by growing the films in oxygen flow or in high vacuum.

This work demonstrates that high quality epitaxial films of Nb- or Fe-doped STO can be grown by PLD, enabling further investigations of their functional properties towards possible device applications.