In-Gap States in Wide-Band-Gap SrTiO3 Analyzed by Cathodoluminescence

Abstract

The cathodoluminescence spectra between 300 and 900 nm were investigated for undoped and Nb2O5‐doped SrTiO3 sintered both in air and under low oxygen partial pressure ( ). Two broadened emission peaks observed experimentally were deconvoluted into five luminescences in the visible range of 2.9 eV (427 nm) to 2.4 eV (516 nm), and in the infrared range of 1.57 eV (790 nm), 1.55 eV (800 nm), and 1.44 (861 nm); each of which was assigned to interband transitions between in‐gap energy states correspondingly by considering the intrinsic and extrinsic defect reactions. It is suggested that the 2.4 eV emission originated from charge transfer between Ti4+ and O2− by a mechanism of charge‐transfer vibronic excitons. The 2.9 eV luminescence was due to electrons trapped in the donor states of extrinsic NbTi• generated by donor doping, and of extrinsic VO•• produced by low‐ sintering, and of the Schottky intrinsic VO•• in undoped samples deexciting directly to the valence band. The triplet of (1.57+1.44) and 1.55 eV was from the emissions between the Schottky intrinsic defects in SrTiO3 and in second‐phase TiO2, respectively.

A Comparison of Different A-, A-B-, and B-Site Incorporated in (Ba0.5Sr0.5)TiO3 on Photocatalytic Application

“…Two oxygen vacancy sites are generally exhibited at the positions below conduction band approximately 0.3 eV and 3 meV, respectively [37]. At 2.33 eV, this peak is attributed to the charge-transfer vibronic excitons (CTVE) of Ti-O-Ti bonds [39,40]. This result agreed well with energy migration arising from [V O ] in bulk SrTiO 3 that occurs below the conduction band ca.…”

Recent Advances on Carrier and Exciton Self-Trapping in Strontium Titanate: Understanding the Luminescence Emissions

“…Depending on sample, excitation conditions, and measuring temperature, several overlapped Gaussian emissions centered at 2.0 eV, 2.5 eV, and 2.8 eV are generally distinguished [32]. Remarkably, the (red) 2.0 eV band has only been clearly observed for heavily strained and amorphous samples suggesting that it is closely related to structural disorder [31,42,43]. Another band at 3.2 eV has been identified at low temperatures (below 100 K) but it has not been much investigated.…”

“…More relevant for our review are the studies based on the monitoring of oxygen vacancy defects and Ti 3+ states at the surfaces and its impact on the luminescence behavior [20,21]. However, it is important to mention that the luminescence spectra and behavior observed under different excitation sources from UV light [10,17,[22][23][24][25][26][27][28][29][30] to X-rays [16], electrons [31], and ion beams [5,32], having very different penetration depths, show a similar pattern apparently associated to a bulk response. In spite of these considerations, we believe that surface luminescence is beyond the scope of our review and deserves independent attention.…”

Local structural changes due to the electric field‐induced migration of oxygen vacancies at Fe‐doped SrTiO3 interfaces

“…In the bulk of the crystal, translational symmetry allows electronic energy bands to form . Impurities existing in the crystal lattice, such as ionic or structural defects, can break symmetries and alter local electronic band structure by creating electronic defect states . These defect states result in the stabilization of electronic energy levels within the bandgap and, depending on the nature of the defect, may act as electron donors or acceptors.…”

“…Three primary photoemission peaks are expected to arise during radiative recombination from the Fe:STO crystal surface for emission wavelengths between 335 nm to 640 nm. The first is the bandgap energy (~2.9‐3.0 e V), the second is an oxygen vacancy donor state (~2.8‐2.9 e V), and the third is a shallow acceptor state (~2.8‐2.9 e V), as shown in Figure . Due to their very similar emission energies, the oxygen vacancy donor and shallow acceptor state emissions cannot be fully separated within fluorescence spectra and must be carefully analyzed according to the characteristics of the interface being probed.…”