【Member Papers】MOCVD-grown α-Ga₂O₃ with increased growth rate and improved crystal quality for deep UV optoelectronics application

      Researchers from the Northeast Normal University have published a dissertation titled "MOCVD-grown α-Ga₂O₃ with increased growth rate and improved crystal quality for deep UV optoelectronics application" in Applied Physics Reviews.

Background

      Ultra-wide bandgap (UWBG) semiconductor materials are indispensable for solar-blind ultraviolet (UV) photodetection applications. Although the monoclinic β-Ga₂O₃ phase is the most stable and extensively studied, the corundum-structured α-Ga₂O₃ offers certain advantages. Its wider bandgap enables more precise cutoff characteristics for deep-UV light. α-Ga₂O₃ shares the same crystal structure (corundum) as the commonly used sapphire (Al₂O₃) substrates, making high-quality heteroepitaxial growth theoretically easier and offering better compatibility with existing GaN-based optoelectronic processes. However, α-Ga₂O₃ is a metastable phase. Under conventional epitaxial growth conditions, it readily transforms into the β phase at high temperatures. To maintain the α phase, growth is typically conducted at relatively low temperatures. Low-temperature growth, however, leads to incomplete precursor decomposition, resulting in extremely low growth rates and poor crystal quality. Slow growth rates not only increase production costs but also limit the development of thick-film devices, while poor crystal quality directly causes slow detector response and high dark current. This study proposes a novel growth strategy aimed at simultaneously overcoming the trade-offs among phase stability, growth rate, and crystal quality.

Abstract

      α-Ga₂O₃ films with a more than threefold increase in growth rate were epitaxially grown on m-Al₂O₃ at 530 °C by employing a 700 °C high-temperature buffer layer (HTBL) in metal–organic chemical vapor deposition growth. The underlying mechanism of the enhanced growth was understood experimentally by the surface analysis and theoretically by a model based on Gibbs–Thomson equation and first-principles calculations. With HTBL, the crystal quality of the α-Ga₂O₃ was also significantly improved, as evidenced by the decreased x-ray diffraction rocking curve linewidth of the (30-30) peak. The linewidth is down to 0.387°, which is significantly narrower than 0.592° from sample without HTBL. The surface morphology of HTBL features larger, more sparsely distributed islands, resulting in a lower chemical potential on the surface, which promotes the adsorption of Ga₂O₃-clusters by reducing the desorption and thus increases the growth. x-ray photoelectron spectroscopy showed more oxygen vacancies (V_o) on the HTBL surface. First-principles calculations indicated that V_o in the (30-30)-surface reduced the adsorption energy from −5.48 to −6.61 eV, thereby promoting cluster adsorption. A solar-blind ultraviolet photodetector based-on α-Ga₂O₃ exhibited an ultra-low dark current of 0.1 pA at 10 V, a high light-to-dark current ratio of 10⁶, a responsivity of 0.54 A/W, and a detectivity of 5.5 × 10¹² Jones.

Conclusion

      In summary, pure phase α-Ga₂O₃ thin films were successfully epitaxially grown on m-plane sapphire substrates via the MOCVD method. By introducing a HTBL, the crystalline quality of the α-Ga₂O₃ films was significantly improved, as evidenced by a reduction in the full width at half maximum (FWHM) of the (30-30) XRD rocking curve from 0.592° to 0.417°. Simultaneously, the film growth rate increased markedly from 1.56 ± 0.05 nm/min (mean ± standard deviation) to 5.38 ± 0.10  nm/min (mean ± standard deviation), demonstrating the effectiveness of the buffer layer strategy in enhancing both film quality and deposition efficiency. Further optimization of the growth parameters led to an even narrower FWHM of 0.387°, indicating superior crystallinity. Based on this high-quality film, a metal – semiconductor–metal (MSM) structured solar-blind ultraviolet photodetector was fabricated using thermally evaporated interdigital Al electrodes. The resulting device exhibited excellent performance, including an ultra-low dark current of 0.1 pA under a 10 V bias, a high photo-to-dark current ratio of 10⁶ , a peak responsivity of 0.54 A/W, and a detectivity of 5.5 × 10¹² Jones. Notably, under low-light conditions (0.52 μW/cm²), the photodetector maintained a responsivity of 16 mA/W and a detectivity of 1.62 × 10¹¹ Jones, highlighting its potential for high-sensitivity DUV detection in low-intensity environments. These results demonstrate that the combined use of HTBL and optimized growth conditions offers a viable pathway toward the scalable fabrication of high-performance α-Ga₂O₃-based solar-blind photodetectors.