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Strontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity.
We have investigated a point defect, common to all SiC substrates, that is thought to be a broken carbon bond. Electron paramagnetic resonance spectroscopy performed in combination with three different etching methods using p-type, n-type, and semi-insulating substrates demonstrate that the center lies near the surface of a wafer. The results suggest that on the order of 1013 cm-2 defects are removed within the first micron of the surface of a wafer.
Chromium impurities in SrTiO3 grown by Verneuil and Float-zone methods were investigated using photo-electron paramagnetic resonance spectroscopy. The samples are the substrates typically used for deposition of multifunctional and ferromagnetic films. A maximum optical cross section for Cr3+ of 2.6×10−18 cm2 is obtained from the time-dependent data, and steady state measurements suggest the presence of a defect level 2 eV from one of the band edges. The cross section is similar to that obtained from optical absorption studies of Cr3+ in semiconductors. The results produced here should be useful for those trying to interpret photoluminescence or similar optical characterization data.
Strontium Titanate (STO) substrates were studied by electron paramagnetic resonance (EPR) spectroscopy to assess possible changes incurred by deposition of multiferroic thin films. To this effect, STO was vacuum annealed at pressures of 10−5 Torr for one hour at temperatures in the range of 200 – 500 °C. EPR spectra, measured before and after each anneal, revealed changes in the amount of three different defects, Cr3+, Fe3+ and an iron-oxygen vacancy complex, Fe3+Vo. The latter was used to monitor the diffusion of oxygen. EPR analysis showed that Fe3+Vo increases from its as-grown value, suggesting that a charged oxygen species is mobile in the substrate under film deposition conditions. Coupled with a subsequent O2 anneal showing minimal change in the Fe3+Vo signal, the data indicate a loss of oxygen from the sample during vacuum annealing. As charged oxygen vacancies may affect the substrate as well as the substrate/ thin film interface, these results are important for understanding the behavior of multiferroic devices built on STO substrates.
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