【Member Papers】Bond-angle modulation in nucleation layer overcomes lattice-mismatch limits in Ga₂O₃ heteroepitaxy

      Researchers from the Xiamen University have published a dissertation titled "Bond-angle modulation in nucleation layer overcomes lattice-mismatch limits in Ga₂O₃ heteroepitaxy" in Materials Futures.

Background

      β-Ga₂O₃ is an ideal material for high-power electronic devices and deep-ultraviolet optoelectronic applications due to its ultra-wide bandgap (~4.9 eV), high breakdown electric field (8 MV/cm), and excellent Baliga's figure of merit. However, the high cost and poor thermal properties of bulk single-crystal substrates make heteroepitaxial growth a critical pathway for its application. During heteroepitaxy, lattice mismatch and interfacial stress lead to high defect densities, severely limiting device performance. Traditional buffer layers like GaN can partially mitigate the mismatch, but issues like residual strain and interface disorder persist. Recently, gallium oxynitride (GaON) has been preliminarily explored as a nucleation layer, but its atomic-scale structure, nucleation mechanism, and influence on band structure and carrier dynamics remain unclear.

Abstract

      This study demonstrates a multilayer transition strategy for growing high-quality β-Ga₂O₃ thin films on sapphire substrates via plasma-assisted molecular beam epitaxy (PAMBE). The structure incorporates an AlN/GaN buffer layer and a GaON nucleation layer formed by in-situ oxygen plasma treatment. First-principles calculations reveal the mechanism of GaON as an effective nucleation layer: its N-Ga-O bond angle (90.27°) closely matches the average bond angle (90.4°) of GaO₆ octahedra in β-Ga₂O₃, and its effective plane energy (0.109 eV/Ų) is significantly lower than that of sapphire (0.218 eV/Ų), promoting heterogeneous nucleation.

      Experimental results show the GaON layer is approximately 5 nm thick with an oxygen gradient, transitioning from GaO₀.₁₇N₀.₈₃ at the outer region to GaN deeper within. HRTEM confirms this layer is atomically ordered, contrasting with the amorphous interface in samples without GaON. XRD analysis shows that introducing the GaON layer reduces the rocking curve FWHM of the β-Ga₂O₃ film from 7063 to 4341 arcsec, decreasing screw dislocation density by about 62%. XPS and EPR analyses further confirm that the GaON layer effectively suppresses oxygen vacancy defects.

      At the device level, metal-semiconductor-metal photodetectors (MSM-PDs) based on this structure exhibit excellent performance under 254 nm illumination: an ultra-low dark current of 13 pA, a photo-to-dark current ratio of 6.29×10⁶, a high responsivity of 3821.6 A/W, a high specific detectivity of 3.3×10¹⁶ Jones, and fast response times (rise/decay: 30/20 ms). Band structure analysis reveals that the GaON layer creates a smooth band transition from GaN to β-Ga₂O₃, reducing the valence band offset from 1.7 eV to 1.52 eV and the conduction band offset from 0.12 eV to 0.02 eV, significantly enhancing carrier extraction efficiency.

Highlights

      Proposes and validates a bond-angle-matched GaON nucleation layer design strategy, overcoming traditional lattice-matching limitations.

      First to combine DFT calculations with experiments to reveal the oxygen-gradient structure of GaON and its role in mitigating lattice strain.

      The GaON layer not only improves crystal quality but also achieves a graded energy band, significantly optimizing carrier transport.

      The fabricated MSM ultraviolet photodetectors achieve world-leading performance in key metrics like responsivity, detectivity, and response speed.

Conclusion

      This research successfully achieved high-quality heteroepitaxial growth of β-Ga₂O₃ thin films on sapphire substrates by introducing a GaON nucleation layer within an AlN/GaN multilayer buffer structure. The GaON layer, with its bond-angle compatibility with β-Ga₂O₃ and low effective plane energy, effectively mitigates stress and defects caused by lattice mismatch. Band analysis further indicates that the GaON layer enables a smooth band transition from GaN to β-Ga₂O₃, improving carrier separation efficiency. The resulting ultraviolet photodetectors demonstrate excellent performance across multiple metrics, validating the universality and application potential of this strategy for heteroepitaxy of wide-bandgap semiconductors.

Project Support

      This work was supported the National Natural Science Foundation of China (Grant No. 62171396), the Shenzhen Science and Technology Program (Grant No. JCYJ20240813145617023), the Central University Basic Research Fund of China under (Grant No. 20720230040), Fujian Minjiang Distinguished Scholar Program, and Xiamen Double Hundred-Talent Program.

DOI：

10.1088/2752-5724/ae4e4c