【Member Papers】Low-Temperature Synthesis of Highly Preferentially Oriented ε-Ga₂O₃ Films for Solar-Blind Detector Application

      Researchers from the Xi’an Jiaotong University have published a dissertation titled "Low-Temperature Synthesis of Highly Preferentially Oriented ε-Ga₂O₃ Films for Solar-Blind Detector Application" in Nanomaterials.

Project Support

      This research was funded by the National Natural Science Foundation of China (Grants number 51702253, 51332003), Natural Science Foundation of Shaanxi Province (No. 2017JQ5095), and the “111 Project” of China (Grant number B14040). Z.-G.Y. acknowledges the support from the Natural Sciences and Engineering Research Council of Canada (NSERC, DG, RGPIN-2013-04416).

Background

      The realization of high-performance power electronic devices in such applications as electric vehicle charging, grid-scale energy storage, and improvements in high-speed trains and other systems depends heavily on wide-bandgap semiconductor materials. Owing to its high breakdown field, ultra-high Baliga’s figure of merit (BFOM), wide bandgap, excellent thermal stability, and chemical inertness, Ga₂O₃ has emerged as one of the most promising materials beyond the third-generation semiconductors of silicon carbide (SiC) and gallium nitride (GaN). It has attracted significant attention in the fields of power devices and solar-blind ultraviolet detection. To date, at least five crystalline polymorphs of gallium oxide have been reported: β, α, δ, γ, and ε (also referred to as κ-phase). Among them, the extensively studied β-phase is the only thermodynamically stable form. At high temperatures, all the other metastable phases transform irreversibly into the β-phase. Their stability follows the order: β < ε(κ) < α < γ < δ. Notably, ε-Ga₂O₃ exhibits an orthorhombic, pseudo-hexagonal structure with rotational nanodomains of gallium oxide (also defined as κ-Ga₂O₃).

Abstract

      As one of the polymorphs of the gallium oxide family, ε gallium oxide (ε-Ga₂O₃) demonstrates promising potential in high-power electronic devices and solar-blind photodetection applications. However, the synthesis of pure-phase ε-Ga₂O₃ remains challenging through low-energy consumption methods, due to its metastable phase of gallium oxide. In this study, we have fabricated pure-phase and highly oriented ε-Ga₂O₃ thin films on c-plane sapphire substrates via thermal atomic layer deposition (ALD) at a low temperature of 400 °C, utilizing low-reactive trimethylgallium (TMG) as the gallium precursor and ozone (O₃) as the oxygen source. X-ray diffraction (XRD) results revealed that the in situ-grown ε-Ga₂O₃ films exhibit a preferred orientation parallel to the (002) crystallographic plane, and the pure ε phase remains stable following a post-annealing up to 800 °C, but it completely transforms into β-Ga₂O₃ once the thermal treatment temperature reaches 900 °C. Notably, post-annealing at 800 °C significantly enhanced the crystalline quality of ε-Ga₂O₃. To evaluate the optoelectronic characteristics, metal–semiconductor–metal (MSM)-structured solar-blind photodetectors were fabricated using the ε-Ga₂O₃ films. The devices have an extremely low dark current (<1 pA), a high photo-to-dark current ratio (>10⁶), a maximum responsivity (>1 A/W), and the optoelectronic properties maintained stability under varying illumination intensities. This work provides valuable insights into the low-temperature synthesis of high-quality ε-Ga₂O₃ films and the development of ε-Ga₂O₃-based solar-blind photodetectors for practical applications.

Conclusions

      Ultra-thin, pure-phase ε-Ga₂O₃ films with a preferred orientation were deposited in situ on sapphire substrates by thermal ALD at a relatively low temperature of 400 °C. Post-annealing studies show that the metastable ε-Ga₂O₃ phase could be stabilized upon post-annealing at temperatures up to 800 °C, and it transforms into the more stable β-Ga₂O₃ after annealing at 900 °C. The surface roughness of the ɛ-Ga₂O₃ does not change much after annealing, and the roughness slightly increases after the transformation into β-Ga₂O₃. The dark current of the solar-blind photodetector prepared based on ɛ-Ga₂O₃ is lower than 1 pA, and the PDCR is greater than 10⁶, which increases with the increase in light intensity. At the same time, the detector responsivity is almost independent of light intensity, which demonstrates a good day-blind detection performance and light intensity stability.