【Member Papers】Competing Self-Trapped Hole Release and Deep-Level Defect Recombination in Ga₂O₃ Photodetectors

      A paper by Professor Weifeng Yang’s team at Xiamen University, published in the journal Applied Physics Letters, is titled “Competing Self-Trapped Hole Release and Deep-Level Defect Recombination in Ga₂O₃ Photodetectors.” Using plasma-assisted molecular beam epitaxy (PAMBE), the team prepared (-201)- and (400)-oriented β-Ga₂O₃ thin films on sapphire and 4H-SiC substrates, respectively. The crystal quality and defect characteristics of the two films were systematically compared, and the temperature-dependent photoresponse performance of photodetectors based on the two films was systematically investigated. It was found that the (-201)-β-Ga₂O₃ film had a sharp interface and a lower defect density, and its photodetector exhibited nonmonotonic photocurrent behavior originating from the thermally activated release of self-trapped holes. In contrast, owing to an amorphous SiO_x interfacial layer, the (400)-β-Ga₂O₃ film had higher densities of dislocations and oxygen vacancies; deep-level defect recombination centers effectively suppressed the contribution of self-trapped holes, leading to a monotonic decrease in photocurrent and, simultaneously, a faster response speed. This work reveals a competing mechanism between self-trapped hole release and deep-level defect recombination, providing an important reference for optimizing the performance of β-Ga₂O₃ solar-blind photodetectors through substrate and orientation engineering.

Background

      Gallium oxide, as an ultra-wide-bandgap semiconductor material, exhibits a bandgap of approximately 4.5-4.9 eV, a critical breakdown field strength as high as 8 MV/cm, and a Baliga figure of merit approaching 3000, demonstrating significant application potential in high-voltage power electronics and deep-ultraviolet optoelectronic devices. However, the performance of its photodetectors is significantly affected by defects. Point defects and dislocations introduce localized energy levels within the bandgap, thereby governing carrier dynamics and temperature-dependent photoresponse behavior. Among these, the H1 level associated with self-trapped holes (STHs) can be thermally excited with an activation energy of approximately 0.2 eV, releasing mobile holes that contribute to the photocurrent, whereas deep-level defects such as E2 and H3 serve as nonradiative recombination centers and influence carrier transport behavior. Nevertheless, systematic understanding of the defect differences among differently oriented β-Ga₂O₃ films and their effects on detection performance is still lacking. Therefore, understanding the competing relationship between self-trapped hole release and deep-level defect recombination, and establishing the correlation among orientation, defects, and photoresponse, constitute an important point of departure for this study.

Abstract

      This study adopted a PAMBE strategy to achieve the epitaxial growth of (-201)-oriented and (400)-oriented β-Ga₂O₃ thin films on sapphire and 4H-SiC substrates, respectively. Through systematic characterization of the heterointerface structures, defect characteristics, and carrier dynamics, it was found that the sapphire-based film formed an atomically sharp Ga₂O₃/sapphire interface, whereas the SiC-based film possessed an amorphous SiO_x interfacial layer; the two exhibited significant differences in screw dislocation density and oxygen vacancy concentration. Experimental results showed that the (-201)-oriented film had a screw dislocation density of 4.11 × 10¹⁰ cm^-2 and a weak oxygen-vacancy EPR signal, while the (400)-oriented film had a screw dislocation density as high as 1.21 × 10¹² cm^-2 and a significantly enhanced oxygen-vacancy signal. For the corresponding devices, the photocurrent of the (-201)-oriented photodetector under 254 nm illumination varied nonmonotonically with temperature, decreasing from 93.3 μA at 25 °C to 9.7 μA at 150 °C and then recovering to 13.3 μA at 200 °C; in contrast, that of the (400)-oriented photodetector exhibited a monotonic downward trend. Furthermore, by combining temperature-dependent transient photocurrent measurements with Arrhenius analysis, the research team extracted the activation energies of the slow-response component. For the (-201)-oriented device, the activation energies in the high-temperature regime were 211 meV (rise) and 186 meV (decay), attributed to the thermal release of self-trapped holes; for the (400)-oriented device, the activation energies in the high-temperature regime were as high as 406 meV and 782 meV, revealing the dominant role of deep-level defect recombination centers in carrier behavior. These results provide an experimental basis for understanding the competing mechanism between self-trapped hole release and deep-level defect recombination in β-Ga₂O₃ photodetectors.

Highlights

      Orientation-controlled epitaxy and interface construction: Through an orientation-controlled heteroepitaxial strategy for gallium oxide, controllable growth of (-201)-oriented and (400)-oriented thin films was successfully achieved, forming two distinctly different crystalline interfaces.

      Regulation of defect configurations by orientation: The (-201)-oriented film exhibited a lower screw dislocation density and fewer oxygen vacancies, whereas the (400)-oriented film had a defect density approximately two orders of magnitude higher, revealing a strong correlation between orientation and defect configuration.

      Mechanistic competition in temperature-dependent photoresponse: As the temperature increased, the (-201)-oriented photodetector exhibited nonmonotonic photocurrent behavior due to self-trapped hole release (decreasing and then recovering by 35.4% at 200 °C), while the (400)-oriented photodetector was dominated by deep-level defect recombination and showed a continuous monotonic decrease.

      Defect-mediated fast switching characteristics: The (400)-oriented photodetector achieved a significantly faster response at 200 °C, with a rise time τ_r1 = 9 ms and a decay time τ_d1 = 139 ms, far superior to those of the (-201)-oriented device (τ_r1 = 362 ms and τ_d1 = 557 ms), which is attributed to abundant deep-level defects acting as recombination centers that greatly shorten the carrier lifetime.

Conclusion

      In this study, through an orientation-controlled epitaxial strategy, controllable growth of (-201)- and (400)-oriented β-Ga₂O₃ thin films was achieved, forming two distinctly different crystalline interfaces. Structural characterization revealed that the (-201)-oriented film had a lower screw dislocation density and fewer oxygen vacancies, whereas the defect density of the (400)-oriented film was approximately two orders of magnitude higher. This orientation-dependent defect configuration directly determined the photoresponse behavior of the devices: the (-201)-oriented photodetector exhibited nonmonotonic photocurrent due to the thermal release of self-trapped holes, recovering by 35.4% at 200 °C; in contrast, the (400)-oriented photodetector was dominated by deep-level defect recombination, and its photocurrent decreased monotonically with increasing temperature. Moreover, the abundant recombination centers significantly shortened the carrier lifetime, enabling the (400)-oriented device to achieve a faster switching speed at 200 °C, with a rise time τ_r1 = 9 ms and a decay time τ_d1 = 139 ms, far superior to those of the (-201)-oriented device (τ_r1 = 362 ms and τ_d1 = 557 ms). This study reveals the intrinsic correlation among orientation, defects, and photoresponse, elucidates the competing mechanism between self-trapped hole release and deep-level recombination, and provides an experimental basis for defect engineering and performance modulation of β-Ga₂O₃ solar-blind photodetectors.

Project Support

      This work was supported by the Shenzhen Science and Technology Program (Grant No. JCYJ20240813145617023), the National Natural Science Foundation of China (Grant No. 62171396), the Natural Science Foundation of China Young Scientists Fund (Grant No.62304187).