|M.Sc Student||Kuperman-Benedik Hanna|
|Subject||Epitaxial Growth and Characterization of Strontium Ferrite|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Avner Rothschild|
The remarkable tunability of the SrTi1-xFexO3-d solid solution Perovskite family provides fascinating opportunities for basic science research as well as potential applications in electronic, optoelectronic, and electrochemical devices. For instance, the band gap energy can be tailored between 3.2 and 1.9 eV by changing the Fe/Ti ratio from x = 0 (SrTiO3) to 1 (SrFeO3) while the crystal structure remains nearly the same. This opens new paths to fabricate a rich platform of epitaxial heterostructures, tailor and explore their functional properties. For instance, the effects of interfacial atomic order, space charge and strain on transport properties can be systematically investigated and controlled. The enabling key to realize these exciting opportunities is growing high quality epitaxial films with precisely controlled microstructure and interfacial structure down to the atomic scale.
In this work we investigate the epitaxial growth of strontium ferrite thin films deposited by pulsed laser deposition (PLD) on SrTiO3 (STO) single crystal substrates. Prior to film growth the (001) STO substrates were treated in BHF and annealed at 950°C to produce atomically smooth TiO2-terminated terraces. The SFO films were deposited at a temperature of 700°C, oxygen pressure of 50mTorr or vacuum conditions (~10-3mTorr), and laser fluency of 1 J/cm2 using KrF excimer laser (λ=248nm). The film growth mode was monitored in-situ during deposition using reflection high energy electron diffraction (RHEED), demonstrating a strikes pattern characteristic of 2D layer-by-layer growth. The crystal structure of the SFO films and the epitaxial relationships between the SFO films and the (001) STO substrates was analyzed using high resolution x-ray diffraction (HR-XRD) and high resolution transmission electron microscopy (HR-TEM) techniques. The SFO films deposited in oxygen (50mTorr) were found to grow in the Sr8Fe8O23 orthorhombic structure with the following orientation relationships:  Sr8Fe8O23 ||  SrTiO3; (001) Sr8Fe8O23 || (001) SrTiO3 and  Sr8Fe8O23 ||  SrTiO3; (110) Sr8Fe8O23 || (001) SrTiO3. Vacuum-deposited films grew in the Sr2Fe2O5 Brownmillerite structure with the following orientation relationships:  Sr2Fe2O5 ||  SrTiO3; (101) Sr2Fe2O5 || (001) SrTiO3. The chemical composition of the films was examined by RBS and EDX, demonstrating a uniform composition with mostly 1:1 cation ratio, but in a few layers a small strontium excess was observed. HR-TEM lattice images reveal that the SFO thin films grew epitaxially on the (001) STO substrates, displaying fully coherent interfaces with the substrates. Fe-deficient films displayed strontium excess accommodated by forming SrO rich stacking faults inside the films.