M.Sc Thesis


M.Sc StudentTimna Orland-Revach
SubjectSrTiO3 Memory Diodes
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner


Abstract

SrTiO3 (STO) is an attractive model system for oxide electronic devices because it can be doped with donors (e.g., Nb) and acceptors (e.g., Fe) in order to tailor its electrical conductivity to n- or p-type, respectively. In addition, epitaxial films of undoped and doped STO can be grown on STO single crystal substrates, enabling fabrication of oxide pn homo-junctions of high crystalline quality.

We investigated pn junctions comprising STO doped with Nb and Fe. Thin films of 1site% Fe-doped STO (that is Fe/(Fe)=1%) were deposited by means of pulsed laser deposition (PLD), on 1site% Nb-doped STO single crystal (001) 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 deposition parameters of the Fe-doped STO films were tuned for optimal epitaxial growth conditions. The surface morphology of the films was examined by means of HRSEM and AFM. Films that were deposited in optimal deposition conditions displayed atomically smooth terraces similar to the substrates. The microstructure of the films was examined by means of HRXRD and HRTEM/STEM, indicating high quality films and interfaces.

I-V sweep mode measurements of these pn junctions displayed rectified and hysteretic I-V characteristics with nonzero crossing, giving rise to two distinctive open circuit voltages (OCV). This unique behavior is believed to be associated with polarization induced by slow migration of oxygen vacancies in the p-type layer. This unique behavior opens up new opportunities for non-volatile memory storage devices based on OCV writing, reading and erasing. Voltage interruption measurements were carried out to examine the stability of the OCV states over time and the repeatability over a large number of cycles, as well as to deepen the understanding of the electrical behavior.

The experimental results were analyzed by fitting to an equivalent circuit model that describes the charge transfer processes in two transmission mechanisms, ionic and electronic. Irreversible changes in the current-voltage characteristic curves and voltage interruption traces were observed after repeated tests. These changes are related to the formation of SrO islands or another derivative of this oxide on the surface of the Fe:STO epilayer as well as on the Nb:STO substrate, and the formation of large and shallow circular “craters”. These structural changes affect the conductivity and electrical properties of the device irreversibly and therefore they should be mitigated in order to maintain long-term repeatable characteristics.