【Domestic Papers】Properties and fabrication of Eu:Ga₂O₃ (310) film with an ultra-long fluorescence lifetime

      Researchers from the Shandong University have published a dissertation titled “Properties and fabrication of Eu:Ga₂O₃ (310) film with an ultra-long fluorescence lifetime” in Ceramics International.

Background

      β-Ga₂O₃, as an ultra-wide bandgap semiconductor, is a core material for next-generation high-power and deep-ultraviolet optoelectronic devices. Rare-earth doping endows it with unique optical modulation capabilities, which are crucial for applications such as micro-lasers, luminescent converters, and solar-blind photodetectors. Traditional rare-earth-doped Ga₂O₃ thin films suffer from low luminescence efficiency limited by internal quantum efficiency and light extraction efficiency, and films grown on common substrates are mostly polycrystalline or of poor crystalline quality. The β-Ga₂O₃ (310) plane is an oxygen-close-packed plane with strong defect-suppression ability, and its lattice mismatch with GaN (0001) is less than 5%, suitable for heteroepitaxy. The nanoporous GaN-based DBR structure has advantages of small residual stress, high reflectivity and wide reflection band, which can significantly improve light extraction efficiency. In this work, high-quality Eu-doped β-Ga₂O₃ (310) films were prepared by PLD on NP-GaN DBR substrates for the first time, and the mechanism of structure and luminescence performance improvement was explored.

Abstract

      Generally, β-Ga₂O₃ thin films grown on GaN substrates exhibit a preferred orientation along the [-201] direction by different methods (e.g., MOCVD and PLD). In this paper, a high-quality Eu doped β-Ga₂O₃(310) film with an Eu doping concentration of 2.91 at.% was grown on a nanoporous (NP) GaN-based distributed Bragg reflector (DBR) substrate via the pulsed laser deposition (PLD) method for the first time. Compared to the reference film grown on as-grown GaN substrate, the Eu:Ga₂O₃ film on the NP-GaN DBR substrate had wider full width at half maximum (FWHM) of rocking curve in X-ray diffraction (XRD) patterns (i.e. poorer crystal quality), but exhibited a 9 fold enhancement in photoluminescence (PL) intensity and a fourfold enhancement in fluorescence lifetime (856 μs) that significantly exceeded values reported in previous works, which should be attributed to the light-coupling mechanism and suppressed non-radiative recombination processes induced by the DBR structure, respectively. This work provides an effective strategy for dramatically enhancing the luminescence properties of rare-earth-doped semiconductor films by combining with the design of DBR substrates.

Highlights

      High-quality oriented Eu:β-Ga₂O₃ (310) thin films were prepared by PLD at low temperature (550 ℃) on nanoporous GaN-based DBR substrates for the first time.

      Achieved 9-fold enhancement in PL intensity and 4-fold enhancement in fluorescence lifetime (856 μs) of Eu:Ga₂O₃ films, which is one of the longest lifetimes among reported Eu-doped systems.

      Revealed the synergistic mechanism of DBR structure: enhancing light extraction efficiency via optical coupling and prolonging fluorescence lifetime by suppressing non-radiative recombination.

      Confirmed the advantage of low lattice mismatch (<5%) between β-Ga₂O₃ (310) and GaN (0001), enabling high-quality heteroepitaxy at low temperature.

Conclusion

      High-quality Eu-doped β-Ga₂O₃(310) films on as-grown GaN and GaN-based DBR substrates via the PLD method at 550 °C were obtained. The film grown on the DBR substrate exhibits relatively good crystallinity with a narrow FWHM (XRD rocking curve) of 0.20° and an extended fluorescence lifetime of 856 μs. Compared to the reference sample, the DBR-based film presents a significant enhancement in PL intensity and lifetime. The enhancement about the PL intensity should be attributed to the multiple reflections of the excitation and emission light at the film/DBR interface and the air/film interface, which ultimately leads to constructive optical interference. For the lifetime, the enhancement should be related to the suppression of non-radiative recombination pathways. To further improve the luminescence efficiency of the thin film, the thickness of the Eu:Ga₂O₃ single-crystal film grown on DBR should satisfy the standing-wave equation, so that standing-wave resonance enhancement can occur. The low-temperature growth method paves the way for developing high-efficiency rare-earth-doped Ga₂O₃ LEDs and LDs, and future work could focus on optimizing DBR designs or extending the strategy to other rare-earth dopants in wide bandgap semiconductor optoelectronics.

Project Support

      This work was supported by the National Natural Science Foundation of China (Grant No. 61874067).