【International Papers】Heteroepitaxial growth of Eu-doped Ga₂O₃ films on flexible substrates

      Researchers from Saga University, University of Bristol have published a dissertation titled "Heteroepitaxial growth of Eu-doped Ga₂O₃ films on flexible substrates" in Japanese Journal of Applied Physics.

Background

      Rare-earth-doped wide-bandgap oxides are of great value for flexible displays and wearable optoelectronic devices. Eu³⁺ ions can produce characteristic sharp red emission and serve as ideal activators for red-light-emitting functional layers. β-Ga₂O₃ features a wide bandgap and favorable thermochemical stability, making it an excellent host material for rare-earth-ion-doped luminescence. At present, most Eu-doped Ga₂O₃ luminescent films are fabricated on rigid substrates such as sapphire and silicon, which cannot adapt to curved and deformable device applications. Polymer-based flexible substrates have low thermal budgets and cannot support direct high-temperature crystallization of oxide films. The transfer-after-growth method on rigid substrates tends to introduce cracks and interfacial damage, which is unfavorable for large-area fabrication. Hastelloy metallic tapes possess high thermal stability and mechanical flexibility, while bare tapes lack oriented crystalline templates, requiring multi-layer buffers for epitaxy of oriented oxide films. For flexible luminescent devices, controllable fabrication strategies for Eu-doped β-Ga₂O₃ films that simultaneously achieve good crystallinity, strain-tunable luminescence and reliable cyclic-bending performance remain to be explored.

Abstract

      Flexible luminescent oxides require mechanically compliant substrates that also provide crystalline templates for ordered film growth. Eu-doped Ga₂O₃ films were deposited on flexible MgO(100)/amorphous Al₂O₃-buffered Hastelloy by oxygen-plasma-assisted pulsed laser deposition. Growth at 500 °C under 300 W plasma produced the most ordered film, exhibiting a β-Ga₂O₃(400) reflection, a rocking-curve full width at half maximum of 9.3°, and fourfold in-plane ordering with rotational domains. This film also showed the strongest Eu³⁺ red emission, dominated by the ⁵D₀ → ⁷F₂ transition at 613–614 nm. Under outward bending, the characteristic emission profile was retained, while the integrated ⁵D₀ → ⁷F₂ intensity increased by up to 82.3% at a nominal strain of approximately 0.42%. The Raman mode near 201 cm⁻¹ shifted down to 196.7 cm⁻¹ at 6 mm and recovered to 200.0 cm⁻¹ after release. After 10³ bending cycles, the integrated emission retained 99.6% of its initial value, demonstrating strain-tunable luminescence and cyclic-bending stability.

Highlights

      Realize heteroepitaxy of Eu-doped β-Ga₂O₃films on flexible buffered Hastelloy tape by oxygen-plasma-assisted pulsed laser deposition.

      Optimized sample shows strong Eu³⁺red luminescence originating from ⁵D₀ → ⁷F₂ transition and four-fold in-plane rotational domain structure.

      Demonstrate strain-enhanced Eu³⁺luminescence under tensile outward bending with maximum intensity increment of 82.3 %.

      The film maintains 99.6 % luminescence intensity after 10³ bending cycles, realizing strain-tunable luminescence with good cyclic mechanical stability.

Conclusion

      Eu-doped β-Ga₂O₃ films were integrated with flexible MgO(100)/amorphous Al₂O₃-buffered Hastelloy by oxygen-plasma-assisted PLD. β-Ga₂O₃ crystallization began at 300 °C, while growth at 500 °C under 300 W plasma produced the most ordered film. This film exhibited preferential β-Ga₂O₃(100) orientation, a β-Ga₂O₃ (400) rocking-curve FWHM of 9.3°, fourfold in-plane alignment with rotational domains, and the strongest Eu³⁺ ⁵D₀→⁷F₂ emission among the investigated samples. Under outward bending, the Eu³⁺ emission profile and peak positions were retained, while the integrated ⁵D₀→⁷F₂ intensity increased by 82.3% at a radius of curvature of 6 mm, corresponding to a nominal strain of approximately 0.42%. The accompanying Raman redshift confirmed a bending-induced lattice response that largely recovered after release, although a residual shift remained. After 103 bending cycles, the integrated emission retained 99.6% of its initial value, and the Raman mode remained resolved without cumulative linewidth broadening. The buffered-Hastelloy architecture therefore enables preferentially oriented Eu-doped β-Ga₂O₃ films with strain-tunable red emission and stable optical and vibrational responses under repeated bending.