Abstract
Results recently reported on the effect of thermochemical treatments on the (He-Cd) laser-excited emission spectra of strontium titanate (STO) are re-analyzed here and compared with results obtained under ion-beam irradiation. Contributing bands centered at 2.4 eV and 2.8 eV, which appear under laser excitation, present intensities dependent upon previous thermal treatments in oxidizing (O2) or reducing atmosphere (H2). As a key result, the emission band centered at 2.8 eV is clearly enhanced in samples exposed to a reducing atmosphere. From a comparison with the ionoluminescence data, it is concluded that the laser-excited experiments can be rationalized within a framework developed from ion-beam excitation studies. In particular, the band at 2.8 eV, sometimes attributed to oxygen vacancies, behaves as expected for optical transitions from conduction-band (CB) states to the ground state level of the self-trapped exciton center. The band at 2.0 eV reported in ion-beam irradiated STO, and attributed to oxygen vacancies, is not observed in laser-excited crystals. As a consequence of our analysis, a consistent scheme of electronic energy levels and optical transitions can now be reliably offered for strontium titanate.Graphical abstract
Highlights
Some interesting results on the effect of thermal treatments in different atmospheres on the emission spectra of strontium titanate (STO) have been reported for a broad range of temperatures from 10 K to room temperature (RT) [11]
More recent results suggest that the blue emission at 2.8 eV is not consistent with a previous assignment to oxygen vacancies [31], and it has been very recently associated with optical transitions from conduction band (CB) states to the ground state of the self-trapped exciton (STE) [15]
It is shown that the effect of thermal treatments on the emission bands centered at 2.4 eV and 2.8 eV is consistent with the model derived from the ion-beam experiments [12,13,14,15,16], some unsettled questions may still remain
Summary
Some interesting results on the effect of thermal treatments in different atmospheres on the emission spectra of STO have been reported for a broad range of temperatures from 10 K to RT [11]. The broad green emission band centered at 2.5 eV, accompanied by strong lattice relaxation, is associated with a triplet-singlet optical transition of a self-trapped exciton (STE) inside a regular octahedron [24,25,26,27] It has been observed under a large variety of excitation methods including visible light, X-rays, and ion beams [25, 26, 28, 29], and the mean lifetime exhibits a long decay time of milliseconds in accordance with the spin-forbidden character of the transition [25, 27, 30]. It is shown that the effect of thermal treatments on the emission bands centered at 2.4 eV and 2.8 eV is consistent with the model derived from the ion-beam experiments [12,13,14,15,16], some unsettled questions may still remain
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