Eu3+-activated CaGa2O4 wide band gap (WBG) material for solar blind UV conversion: fluorescence and photo-conductivity performance

Abstract

Herein, we have probed a wide band gap material, CaGa₂O₄ co-doped with Eu³⁺/Na⁺, which is fairly transparent (≥95%) in the visible region and efficiently absorbs solar blind UV radiation. Importantly, the absorbed UV energy can be realized both in terms of fluorescence and photoconductivity as well. X-ray diffraction studies confirmed that CaGa₂O₄ exhibits two orthorhombic polymorphs: CaO·Ga₂O₃ I (major) and CaO·Ga₂O₃II (minor). Vibrational spectroscopy provided the first quantitative insight into the phonon frequency of the material. Upon UV illumination the host itself gives UV-blue and red emissions due to band to band and trap level transitions, respectively. (no additional dopant or activator ion were present). A substantial overlap between the emission spectrum of the host and the excitation spectrum of Eu³⁺; decrease in emission intensity and decay time (8.82 μs from 9.2 μs) of the host emission, after Eu³⁺ doping; and evolution of a short rise-time (∼10 μs) in decay curve of Eu³⁺ revealed that the host efficiently transfers its energy to the Eu³⁺ ions. Co-doping of Na⁺ further enhances the emission intensity (∼7 times), decay time and rise-time of the Eu³⁺ emission. Likewise the intrinsic behavior of Ga₂O₃ this material also shows significant transient photoconductivity under UV illumination, photocurrent is of the order of μA, but with slow time constant ∼40 min. Thus, the excellent optical properties prove to be superior for photoconductivity for UV-to-visible converter applications. Such WBG optical materials can be coupled to silicon-based commercial detectors for their wide use in areas ranging from invisible flame sensing to UV astronomy.