Defect-induced photoluminescence of strontium titanate and its modulation by electrostatic gating

Abstract

The photoluminescence (PL) spectra of Ar + -ion irradiated single crystals of SrTiO3 (STO) excited by 325 nm line of a He-Cd laser are compared with those of pristine crystals, epitaxial films and amorphous layers of STO at several temperatures down to 20 K. The 550 eV Ar + -beam irradiation activates distinctly visible three PL peaks; blue (∼430 nm), green (∼550 nm), and infra-red (∼820 nm) at room temperature making the photoluminescence multi-colored. The abrupt changes in PL properties below ≈100 K are discussed in relation with the antiferrodistortive structural phase transition in SrTiO3 from cubic-to-tetragonal symmetry which makes it a direct bandgap semiconductor. The photoluminescence spectra are also tuned by electrostatic gate field in a field-effect transistor geometry. At 20 K, we observed a maximum increase of ∼20% in PL intensity under back gating of SrTiO3.

Correlation between Cr3+ Luminescence and Oxygen Vacancy Disorder in Strontium Titanate under MeV Ion Irradiation

“…This is the fluence scale for the lattice damage caused by the irradiation, in which the 1.55 eV emission of Cr experiences a rapid decay (Figure ). On the other hand, the 2.0 eV band, sometimes observed as an unresolved shoulder to the 2.5 eV band in heavily disordered/strained or amorphous STO, − shows a quite different behavior (Figure b). It grows slowly from the start of irradiation and, at variance with the other bands, appears clearly related to lattice damage.…”

“…In addition to the Cr 3+ emissions, other spectral bands at 2.8 , 2.5, and 2.0 eV are observed, as illustrated in Figure 5, in accordance with previous reports. [3][4][5][6]9,10,[34][35][36][37][38][39]53,54 These bands have been attributed to different decay channels for the electronic excitations, 39 and they will serve as a reference to discuss the effect of irradiation on the chromium luminescence. The kinetic behavior of all these bands is quite different, as illustrated in Figure 6.…”

“…Moreover, it manifests intense photocatalytic activity for hydrogen generation from water decomposition . This latter property is closely related to doping with transition metal impurities, such as chromium, as well as to thermal treatments and ion irradiation that introduce oxygen vacancies. − In particular, chromium doping appears to play a key role in all these processes and allows for a reversible insulator-to-conductor transition or resistive memory effect . − It is generally accepted that the Cr dopant ions in STO substitute for Ti 4+ and occupy an octahedral site as Cr 4+ but shift to Cr 3+ under electronic excitation. The possibility of other valence states, such as Cr 6+ , as well as the occupancy of Sr sites has been also discussed, − mostly in connection with the photocatalytic response of STO, since this activity is very dependent on the charge state of chromium. − One should remark that the charge state is strongly influenced by the location of the Fermi level in the STO crystal, which can be modified through impurity doping, thermochemical treatments, and irradiation.…”

Intrinsically Activated SrTiO3: Photocatalytic H2 Evolution from Neutral Aqueous Methanol Solution in the Absence of Any Noble Metal Cocatalyst

“…The occurrence of visible PL from STO has particularly been observed for defective STO and has been ascribed to a superposition of emission from trapped excitons and various defect-related emission bands. 62,63 This PL from the hydrogenated crystal is surface-related because it is strongly dependent on the ambient atmosphere (Figure S12). 64,65 In our case, considering the onset and maximum of the PL, the involved defect states show an emission peak at an energy of ∼2.88 eVthese states thus lie around 0.3 eV below the conduction band of STO.…”

Revealing the competing contributions of charge carriers, excitons, and defects to the non-equilibrium optical properties of ZnO

“…Nevertheless, their triumph was hampered by the persistent lack of understanding of the detailed origin of the high conductivity in these materials, partially due to the complex involvement of different types of defects. [1] Beyond their direct impact on carrier density and mobility, defects also affect the optical and opto-electronic properties of TCOs, directly reflected in the absorption and emission characteristics [2,3,4,5,6] or more subtly manifested in the lifetimes and non-equilibrium dynamics of optical excitations as, for example, in TiO2 [7,8] and ZnO [9] . With its 3.4 eV direct band gap, bulk exciton binding energy of 60 meV, native n-type doping and high conductivity, ZnO has the potential to be an excellent TCO for optoelectronic applications in the visible and UV photon energy range and for light harvesting applications.…”