Materials and magnetic studies of cobalt-doped anatase titanium dioxide and perovskite strontium titanate as potential dilute magnetic semiconductors
Deposition of cobalt-doped anatase TiO2 and cobalt-doped perovskite SrTiO3 (STO) utilizing molecular beam epitaxy (MBE) was investigated in an effort to achieve a dilute magnetic semiconductor (DMS) system with desirable materials and magnetic properties for application in future Si-based spintronic devices. A prospective DMS material must have a uniform dopant distribution within the semiconductor lattice without the presence of secondary phases, and the dopant must ferromagnetically order such that the semiconductor carriers are spin-polarized. Compatibility with Si requires the DMS material to be deposited under mildly oxidizing conditions to protect the Si interface from oxidation. Epitaxial anatase TiO2 can be realized on Si utilizing an epitaxial SrTiO3 buffer layer. Despite the use of the oxygen plasma during Co:TiO2 deposition, which was confirmed to result in significant oxidation of the Si interface, the charge state of Co in the resultant film was found to be a mixture of Co(0), Co(II), and Co(III). Although no secondary phases were observed, faceted Co-enriched surface particles were present on all films. The observed room temperature ferromagnetism of the films could not be conclusively attributed to a true Co-doped anatase DMS phase. The inability to fully oxidize Co to Co(II) under strongly oxidizing conditions and incorporate it uniforrmly into the anatase lattice, preclude the use of Co-doped anatase TiO2 as a DMS material in future spintronic devices based on Si technology. Alternatively, Co-doped perovskite STO was explored. To evaluate the materials and magnetic properties of Co:STO as a potential DMS material, Co:STO films were deposited on STO(001) substrates. Smooth, epitaxial films free of surface particles or clusters were obtained; further evidence showed Co substituted as Co(II) for Ti in the STO lattice. Films doped with <5% Co were ferromagnetic at room temperature. F-center mediated exchange was proposed as the mechanism of ferromagnetic ordering in these insulating films (p > 5 kOcm). For doping concentrations >5%, no ferromagnetic hysteresis was observed. From these results, Co:STO is a prime candidate as a ferromagnetic semiconductor for integration into future Si-based spintronic devices.
- Chemical engineering