Ph.D Thesis


Ph.D StudentMiri Markovich
SubjectGrowth, Characterization and Investigation of the Transport
Properties of SrTio3 Epilayers and pn
Junctions
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner


Abstract

In this research we investigated oxide pn junctions comprising STO doped with Nb or Fe (0.1 - 0.5at%) and hetero-structures comprising STO and SrTi1-xFexO3 (0.2 ? x ?0.75). Thin films of these compositions were deposited by means of pulsed lased deposition (PLD) on un-doped or Nb-doped STO (Nb:STO) single crystal substrates, aiming at growing coherent epilayers free of extended defects such as grain boundaries, cracks and dislocations that could falter the electrical properties of the junction. The depositions were carried out at a substrate temperature of ~700°C, in vacuum (0.003mTorr) or oxygen atmospheres (50 - 150mTorr). The microstructure of the films was characterized by means of RHEED, HRXRD, RSM, HRTEM/STEM, HRSEM and AFM, and their chemical composition was examined by EDS/WDS and ToF-SIMS. We found that in order to achieve precise stoichiometry transfer from target to film the substrates should be placed 1 cm or more above the tip of the plume. Otherwise, non-stoichiometric cation ratio was observed, leading to lattice expansion of the films.

Nb:STO and SrTi0.8Fe0.2O3 films exhibited Stranski-Krastanov growth, resulting in small angle grain boundaries. Films thicker than few hundred nm displayed mosaicity and eventually dendritic structure. Quasi-periodic oscillations of the majority and minority cations were observed across the thickness of the Nb:STO films. SrTi0.25Fe0.75O3 epilayers grew coherently to the substrate, displaying partial ordering of the B-site cations. Cracks were observed in films thicker than 70nm, leading to local strain relaxation close to the crack but no detachment farther than few nm from the crack.

In most films, the critical thickness for strain release was substantially higher than the value predicted by classical models of strain release by misfit and threading dislocations. This discrepancy suggests that different strain release mechanisms took place in our films rather than by dislocations. These mechanisms include the formation of small angle grain boundaries and eventually crack when the strain energy exceeded the threshold for crack formation.

PN junctions comprising Fe:STO epilayers on Nb:STO substrates or Fe- and Nb-doped STO epilayers on un-doped STO substrates displayed rectified and hysteretic I-V characteristics with nonzero crossing, giving rise to two distinctive open circuit voltages. This unique behaviour is believed to be associated with polarization induced by slow migration of oxygen vacancies in the p-type layer, in qualitative agreement with numerical simulations of the defect distribution across the junction. This unique behaviour opens up new opportunities for non-volatile memory storage devices.