【Member Papers】Epitaxial Growth of Degenerately Doped Ga₂O₃ Films on GaN (0001) as a Deep Ultraviolet Transparent Electrode for Opto-Electronics Applications

      Researchers from the Xiamen University have published a dissertation titled "Epitaxial Growth of Degenerately Doped Ga₂O₃ Films on GaN (0001) as a Deep Ultraviolet Transparent Electrode for Opto-Electronics Applications" in Advanced Optical Materials.

Project Support

      This work was supported by the Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20240813145501003), the National Natural Science Foundation of China (Grant No. 22275154), and the Science & Technology Commission of Shanghai Municipality (Grant No. 25JD1404600).

Background

      Deep ultraviolet (DUV, 200–300 nm) transparent conducting oxides (TCOs) have garnered significant research interest as transparent electrodes for DUV optoelectronic devices, such as laser diodes (LDs), light-emitting diodes (LEDs), and solar-blind photodetectors (SBPDs). DUV LEDs play a crucial role in various applications such as sterilization, water purification, lithography, biological detection, and optical communication systems. The development of GaN-based DUV LEDs in recent years has further advanced the LED industry into the DUV spectral region. However, the external quantum efficiency (EQE) of these devices is quite low (only ≈1%), in part because of the non-availability of a deep UV transparent electrode. Conventional TCO materials such as Sn-doped In₂O₃ (ITO), F-doped SnO₂ (FTO), and Al-doped ZnO (AZO) are opaque in the deep UV region because of their small E_g of less than 3.5 eV. Hence, the development of new TCOs with high DUV transparency and conductivity is imperative for advancing DUV optoelectronics devices.

Abstract

      GaN-based deep ultraviolet (DUV) optoelectronic devices have garnered considerable attention for applications in sterilization, biological detection, and optical communications. However, the performance of current DUV optoelectronic devices is limited by the insufficient DUV transparency of conventional electrodes. In this work, the epitaxial growth of degenerately Si-doped Ga₂O₃ films on GaN as a promising DUV transparent electrode is reported. The 0.5% Si doped Ga₂O₃ (n⁺-Ga₂O₃) films exhibit DUV transparency exceeding 85% in the spectral range from 280 to 400 nm wavelength. Such a high DUV transparency is attributed to the ultrawide bandgap of ≈5.0 eV of the n⁺-Ga₂O₃ film induced by the Burstein–Moss effect due to degenerate doping. Moreover, the n⁺-Ga₂O₃ film exhibits a very low specific contact resistance of 1.96 × 10⁻⁴ Ω cm² to GaN. High-resolution X-ray photoemission spectroscopic (XPS) study reveals that n⁺-Ga₂O₃ forms a type-II staggered band alignment with GaN with a low interface barrier of 0.15 eV and a narrow band bending thickness of a few nm. The small barrier, together with the degenerately doped Ga₂O₃ film, enables excellent electrical contact at the n⁺-Ga₂O₃/GaN interface and low contact resistance. This work demonstrates n⁺-Ga₂O₃ as a promising alternative for DUV transparent electrode for GaN-based DUV devices.

Conclusion

      In this work, we report the epitaxial growth of n⁺-Ga₂O₃ thin films on (0001)-oriented GaN as a new DUV transparent electrode. Our n⁺-Ga₂O₃ films exhibit exceptional optical performance, with > 85% transparency in the 280–400 nm wavelength range, attributed to the ultra-wide bandgap (≈5.0 eV) of n⁺-Ga₂O₃, which is further widened by the Burstein-Moss effect. Moreover, these films achieve a low specific contact resistance of 1.96 × 10⁻⁴ Ω cm² to GaN. Detailed high-resolution XPS quantitative analysis at the interface reveals that the low specific contact resistance at the n⁺-Ga₂O₃/GaN interface can be attributed to the favorable interfacial energetics with a low interface barrier of 0.15 eV and a narrow band bending thickness of a few nm. The low barrier and the narrow band bending enable efficient carrier transport via tunneling, facilitating the establishment of good Ohmic contact. The excellent ohmic contact and high DUV transmittance demonstrate n⁺-Ga₂O₃ as a promising alternative for DUV transparent electrode for GaN-based DUV devices. However, the resistivity of the n⁺-Ga₂O₃ thin films in this work (≈0.17 Ω cm) is still higher than that of traditional TCO materials (10⁻⁴–10⁻³ Ω cm). To meet the electrode requirements for practical applications, it has to sacrifice optical transparency and appropriately increase the electrode thickness. Such a trade-off strategy based on performance compromise is hard to fully exploit the application potential of this DUV transparent electrode. Future work will focus on improving its carrier mobility and doping efficiency to further optimize the comprehensive performance.