【Domestic Papers】Influence of the number of growth cycles on the solar-blind photoresponse of amorphous and crystalline Ga₂O₃ films by atomic layer deposition

      Researchers from the Xi'an University of Posts & Telecommunications have published a dissertation titled "Influence of the number of growth cycles on the solar-blind photoresponse of amorphous and crystalline Ga₂O₃ films by atomic layer deposition" in Materials Science and Engineering: B.

Project Support

      This work was supported by the National Natural Science Foundation of China (Grant No. 62304178) and Natural Science Basic Research Program of Shaanxi Province (Grant No. 2022JQ-684).

Background

      Solar-blind photodetectors, which are specifically sensitive to the 200–280 nm ultraviolet band, have attracted significant interest for applications including missile plume detection, corona discharge monitoring, and flame sensing, owing to their inherent solar blindness and reliable operation under strong background illumination. Among wide-bandgap semiconductors, gallium oxide (Ga₂O₃) has emerged as a highly promising material for solar-blind photodetectors, thanks to its tunable ultra-wide bandgap (4.4–5.3 eV) [4], which aligns naturally with the solar-blind window without requiring complex bandgap engineering. It also possesses advantages such as high-temperature resistance, UV radiation resistance, high voltage resistance, and good chemical stability, making it gradually become a research hotspot in the field of solar-blind UV detection.

      In this work, amorphous and crystalline Ga₂O₃ films of varying thicknesses were deposited on sapphire substrates via atomic layer deposition to fabricate metal-semiconductor-metal (MSM) solar-blind photodetectors. A key finding is the observed saturation of photocurrent/responsivity in amorphous Ga₂O₃ PDs beyond a critical thickness, in stark contrast to the monotonic increase exhibited in crystalline Ga₂O₃ within the studied range. Also, our work provides a direct parallel comparison of amorphous and crystalline Ga₂O₃ PDs with the same thickness. These findings provide valuable guidance for engineering Ga₂O₃-based photodetectors, highlighting the significant role of material phases and film thickness in influencing device performance.

Abstract

      This study investigates the photodetection performance of amorphous Ga₂O₃ and crystalline films grown by atomic layer deposition with varying growth cycles (1000–7000 cycles). The fundamental difference in structural order – long-range in crystalline versus short-range in amorphous - governs their distinct optoelectronic behaviors. For amorphous Ga₂O₃ devices, photocurrent/responsivity nealrly saturates beyond a certain thickness. This limitation comes from low carrier mobility and short diffusion length due to abundant trap states. In contrast, crystalline Ga₂O₃ photodetectors show a monotonic increase in responsivity with film thickness, benefiting from its long-range order which contributes to a longer carrier diffusion length and superior charge collection efficiency. Consequently, for thinner films, the high photoconductive gain from carrier trapping in amorphous films leads to higher responsivity for amorphous Ga₂O₃ photodetecors. However, for thicker films, the photoresponsivity and response speed of crystalline Ga₂O₃ photodetectors are both superior to those of amorphous Ga₂O₃ photodetectors. These advantages can be attributed to the greater absorption depth and longer carrier diffusion length. This work establishes how the material phase (amorphous vs. crystalline) and film thickness jointly influence the performance of Ga₂O₃ photodetectors. The obtained results provide valuable insights for the design of Ga₂O₃ photodetectors.

Highlights

      ● Saturation of the photoresponsivity in amorphous Ga₂O₃ is observed for the first time as the film thickness increases.

      ● The photoresponsivity of crystalline Ga₂O₃ PDs increases monotonically as a function of the number of growth cycles.

      ● A parallel comparison of amorphous and crystalline Ga₂O₃ PDs is provided across a wide range of ALD cycles.

Conclusions

      In conclusion, this work systematically elucidates the distinct thickness-dependent photoresponse of amorphous and crystalline Ga₂O₃ photodetectors prepared by atomic layer deposition. The primary finding is a fundamental divergence in behavior: the photocurrent/responsivity of amorphous Ga₂O₃ saturates beyond a critical thickness (∼130 nm), whereas the photocurrent/responsivity of crystalline Ga₂O₃ PDs increases monotonically across the studied range (up to ∼300 nm). This contrast is governed by the inherent structural disorder of the amorphous phase. The low mobility and short carrier diffusion length in a-Ga₂O₃, induced by abundant trap states limits the charge collection efficiency by the electrodes, leading to the saturation of the photocurrent/responsivity. Conversely, the long-range order in crystalline Ga₂O₃ facilitates a higher mobility, a longer carrier diffusion length and superior charge collection efficiency, enabling sustained performance improvement with increasing thickness. This study also provides a parallel comparison of amorphous and crystalline Ga₂O₃ PDs with the same film thickness. When the film thickness is relatively thin, due to the significant difference of absorption coefficients and photoconductive gain between amorphous and crystalline state, the photoresponsivity after 500 ◦C annealing is greater than that after 900 ◦C annealing. However, when the film is relatively thick, the photoresponsivity and response speed of crystalline Ga₂O₃ photodetector are both superior to those of amorphous Ga₂O₃ photodetector. These insights deliver a crucial design rule: amorphous Ga₂O₃ is suited for cost-effective, low-speed applications in thin-film form, while crystalline Ga₂O₃ is optimal for high-responsivity and high-speed devices, underscoring the critical role of material phase in device engineering.