Solar-blind (200 to 280 nm) deep-ultraviolet (DUV) photodetectors offer marked advantages such as low background noise, all-weather operation, and a high signal-to-noise ratio [1,2]. In recent years, DUV photodetectors have attracted increasing attention due to their vast application potential in areas such as missile tracking, flame detection, and ozone-hole monitoring [3]. Compared to traditional Si-based solar-blind DUV photodetectors with vacuum multiplier tube structure, those based on wide-bandgap (E_g) semiconductors offer notable advantages, including simpler structures, smaller sizes, higher detection sensitivity, low operation voltages, and ease of integration [4]. Extensive research has been conducted on wide-bandgap semiconductors, including GaN (E_g = 3.4 eV), 4H-SiC (E_g = 3.2 eV), AlGaN (E_g = 3.4 to 6.2 eV), ZnMgO (E_g = 3.4 to 7.8 eV), diamond (E_g = 5.5 eV), boron nitride (BN) (E_g = 6.4 eV), and Ga₂O₃ (E_g = 4.6 to 4.9 eV) [5,6]. However, the bandgaps of GaN and 4H-SiC are too narrow to effectively cover the solar-blind spectral range, and the complex growth techniques required for AlGaN and ZnMgO limit their use for DUV detection. Furthermore, the maturity of BN and diamond materials is relatively low, and the high cost of diamond poses challenges for its widespread application in DUV photodetectors. Among these wide-bandgap semiconductors, Ga₂O₃ stands out for solar-blind DUV photodetectors due to its intrinsic solar blindness, high thermal stability, high absorption coefficient, excellent radiation hardness, large breakdown field, and availability of high-quality crystal [7,8]. Moreover, the high-quality Ga₂O₃ material has been utilized to fabricate various device structures for solar-blind DUV photodetectors, including planar metal–semiconductor–metal (MSM) types, Schottky diodes, p–n diodes, and van der Waals heterostructure devices [9].

Recent efforts have focused on enhancing the photoelectric performance of Ga₂O₃ photodetectors, particularly targeting key parameters such as responsivity (R) and response time [10]. For instance, Shen et al. developed an MSM-structured Ga₂O₃ solar-blind DUV photodetector by optimizing the annealing process. By thermally annealing the material to repair surface defects and suppress carrier trapping, the device achieved a fast response time of 41 ms but a low photoresponsivity of 0.01 A/W [11]. Additionally, Tang and colleagues [12] fabricated a Ga₂O₃ solar-blind DUV heterostructure photodetector with zero-dimensional (0D) platinum quantum dots deposited on the Ga₂O₃ channel. This structure enhanced DUV light absorption and the injection of photogenerated carriers, resulting in a responsivity of 4.49 A/W and a response time of 0.35 s. Oh et al. [13] further reported a Ga₂O₃ solar-blind DUV photodetector with transparent and conductive graphene electrodes, where the modulation of the contact barriers at the graphene/Ga₂O₃ interface allowed the device to reach a high responsivity of 29.8 A/W. However, studies indicate that while high responsivity in these devices is often attributed to surface states and defects, these same characteristics can increase recombination rates of photogenerated carriers, ultimately leading to longer response times [14]. Despite progress in this area, optimizing both responsivity and response time simultaneously remains a major challenge for current Ga₂O₃ photodetectors. Therefore, the development of innovative Ga₂O₃ photodetector structures is urgently needed.

In this work, Ga₂O₃ solar-blind DUV photodetectors with a suspended structure were proposed and constructed. The photodetector exhibits a high R of 1.51 × 10¹⁰ A/W, a sensitive detectivity (D^{\ast }) of 6.01 × 10¹⁷ Jones, a large external quantum efficiency (EQE) of 7.53 × 10¹² %, and a fast rise time (τ_r) of 180 ms under 250-nm illumination. Notably, the Ga₂O₃ photodetector demonstrates both a high R of 1.51 × 10¹⁰ A/W and a rapid response time of 180 ms under ultra-weak power intensity (P_in) excitation of 0.01 μW/cm². This improvement is attributed to the combined effects of reduced interface defects, enhanced carrier transport, efficient carrier separation, and increased light absorption enabled by the suspended structure. These findings offer a promising approach for achieving high responsivity and high-speed performance in Ga₂O₃-based photodetectors, advancing their potential for practical applications.

Figure 1A shows a schematic diagram of the Ga₂O₃ photodetector with a suspended structure. The corresponding scanning electron microscope (SEM) image of the fabricated Ga₂O₃ photodetector was shown in Fig. 1D, clearly showing the suspended structure and the overall morphology of the device. The Ga₂O₃ channel dimensions were precisely measured, with a length of 10.2 μm and a width of 2.6 μm, ensuring the appropriate aspect ratio for optimal device performance. To create the suspended structure, reactive ion etching (RIE) equipment was employed, utilizing CF₄ and Ar gases as etching agents. This process enabled the precise definition of the trench beneath the Ga₂O₃ channel. Following the etching process, the trench depth was measured using an atomic force microscope (AFM), as shown in Fig. 1B. The AFM results revealed a well-defined trench with a depth of 28 nm and smooth, straight sidewalls, which are crucial for maintaining mechanical stability and enhancing the optoelectrical properties of the suspended structure. In addition to trench characterization, the thickness of the Ga₂O₃ channel was measured using AFM, as depicted in Fig. 1C, where the thickness of Ga₂O₃ film was found to be 340 nm. Besides, the optical microscopy image of the fabricated Ga₂O₃ photodetector with a suspended structure was displayed in the inset of Fig. 1C. It can be found that the Ga₂O₃ channel was perfectly transferred to the top of the suspended structure without introducing notable mechanical deformation or damage. Furthermore, the surface morphology of the Ga₂O₃ channel was characterized using AFM, with the results shown in Fig. S1. The surface was found to be highly uniform, with a root mean square roughness of 2.04 nm. Such a low roughness indicates minimal surface defects, which is essential for reducing carrier recombination and improving the overall responsivity and speed of the photodetector.

Next, comprehensive physical characterizations of the Ga₂O₃ channel were performed to assess its structural and optical properties. Synchrotron-based grazing incidence x-ray diffraction (GIXRD) measurements were initially conducted to evaluate the crystalline structure of the Ga₂O₃ channel. Figure 2A shows the 2D-GIXRD pattern, where the presence of sharp and well-defined diffraction spots indicates the high crystalline quality of the Ga₂O₃ channel, signifying its superior structural quality [15]. Following this, the azimuthally integrated 1D-GIXRD spectrum was extracted from Fig. 2A, as shown in Fig. 2B. This spectrum reveals diffraction peaks corresponding to the monoclinic β-phase of Ga₂O₃, which are in agreement with the standard PDF cards (No. 41-1103) [16]. The peaks observed at 30.1°, 30.5°, 31.5°, and 37.4° were indexed to the (400), (−401), (−202), and (401) crystal planes, respectively, confirming the material's monoclinic structure. Notably, the absence of any impurity-related peaks further demonstrates the high phase purity and crystalline quality of the Ga₂O₃ channel. To further explore the optical properties, Raman spectroscopy was performed with a 532-nm laser excitation. The resulting Raman spectrum, shown in Fig. 2C, exhibits distinct peaks in the range of 100 to 850 cm⁻¹. Eleven characteristic Raman modes were identified at 114, 145, 169, 200, 320, 346, 416, 477, 631, 658, and 767 cm⁻¹, corresponding to B_g^1, B_g^2, A_g^2, A_g^3, A_g^4, A_g^5, A_g^6, B_g^4, B_g^5, A_g^9, and A_g^{10} vibration modes, respectively [17]. These results are consistent with previously reported data for Ga₂O₃, reinforcing the excellent single-crystalline quality of the channel. In addition, photoluminescence (PL) spectroscopy with a 325-nm laser excitation was performed, as shown in Fig. 2D. A prominent PL emission peak was observed at around 420 nm, corresponding to UV-blue emission. The strong intensity of this peak is indicative of high crystallinity and a low concentration of oxygen vacancies and defects within the Ga₂O₃ channel [18], further supporting the high-quality fabrication of the material. High-resolution transmission electron microscopy (HR-TEM) was also employed to explore the morphology and crystal structure in more detail. Figure 2E shows a cross-sectional HR-TEM image of the Ga₂O₃ channel, where the observed lattice fringe spacings of ~0.58 nm and ~0.61 nm correspond to the (001) and (200) atomic planes, respectively [19]. This confirms that the Ga₂O₃ channel adopts a monoclinic crystal structure, imaged along the [010] zone axis. Moreover, the selected-area electron diffraction (SAED) pattern, inset in Fig. 2E, further corroborates the high symmetry and excellent single-crystal quality of the Ga₂O₃ channel [20]. Finally, the optical transmission properties of the Ga₂O₃ channel were examined. Figure 2F presents optical transmission spectrum measured over the range of 200 to 650 nm. The Ga₂O₃ channel demonstrates high transmittance in the visible light range and a sharp absorption edges in the DUV region, indicating good optical quality and a low density of deep-level defects. Additionally, E_g can be obtained by Tauc plot formula: (αhv)² = C(hv − E_g), where α is the absorbance coefficient, h is the Planck's constant, and v is the frequency of the excitation light. The calculated bandgap was 4.67 eV, which aligns closely with previously reported values for Ga₂O₃ [21]. These findings collectively highlight the excellent structural and physical properties of the Ga₂O₃ channel, underscoring its potential for high-performance applications in DUV photodetectors and other optoelectronic devices.

Then, the electrical and photoelectrical properties of the fabricated Ga₂O₃ photodetector with a suspended structure were systematically measured. Figure 3A illustrates the experimental setup for photoelectrical testing, where DUV light is perpendicularly directed onto the surface of the device, covering the entire Ga₂O₃ channel. Figure 3B displays the current–voltage (I–V) characteristics of the Ga₂O₃ photodetector under different P_in conditions, ranging from 0 to 5 μW/cm² at a wavelength of 250 nm. Under dark conditions, the I–V curve displays linearity and symmetry, indicating the formation of good ohmic contacts between the electrodes and the Ga₂O₃ channel. This was achieved by using a Ti/Al/Ni/Au electrode stack, followed by rapid thermal annealing at 470 °C for 60 s. During the annealing process, Al atoms diffuse rapidly into the Ti layer, forming a Ti–Al intermetallic phase with a low work function, which creates chemically stable contacts and induces oxygen vacancies at the Ga₂O₃/electrode interface [22]. These oxygen vacancies promote ohmic contact formation and greatly reduce the contact resistance, causing the device to exhibit a high dark current of 0.52 mA at V = 0.1 V. In terms of photoelectrical performance, the current increases with higher P_in under 250-nm illumination, as evident in Fig. 3B, demonstrating the high DUV sensitivity of the Ga₂O₃ device. Notably, the device shows substantial photocurrent (I_ph), where I_ph represents the photocurrent generated under each illumination intensity even under very weak illumination conditions, such as P_in = 0.01 mW/cm². The extracted values I_ph for various P_in levels are plotted in Fig. 3C, where the Ga₂O₃ photodetector generates I_ph values of 40, 89, 107, 156, 188, and 224 μA at V = 0.1 V for P_in values of 0.01, 0.1, 0.3, 1, 2, and 5 μW/cm², respectively. The increase in I_ph with increasing P_in is attributed to the excitation of more photogenerated carriers, which enhances photoconductivity [23]. To investigate the relationship between I_ph and P_in, Fig. 3D presents a plot of I_ph versus P_in at V = 0.1 V, fitted using the power law formula of I_ph ∝ {\mathit{AP}}_{{\mathrm{in}}^{\alpha }}, where A is a constant and \alpha is the index of power law. Ideally, \alpha should be equal to 1, which indicates a linear response. However, in this work, the obtained \alpha value is 0.29, which is considerably less than 1, confirming the presence of electron-hole recombination centers caused by defects in the Ga₂O₃ channel l, as previously confirmed by PL measurements [24]. Although this sublinear response indicates some recombination losses, it offers a notable advantage in terms of higher sensitivity at low light intensities, where even small increases in incident power result in relatively larger changes in photocurrent. This characteristic makes the Ga₂O₃ photodetector particularly suitable for detecting weak ultraviolet signals, demonstrating high responsivity under ultra-low illumination conditions, which is critical for applications requiring enhanced sensitivity in low-light environments.

To further quantify the optoelectronic performance, key metrics such as R, D^{\ast }, and EQE are calculated. The responsivity R, which represents the photocurrent generated per unit incident light power on the effective area, was determined using the following equation [25]:where I_ph is the photogenerated current, P_in is the light power density, and S is the effective area of the Ga₂O₃ photodetector. Figure 3E plots the maximum responsivity (R_max) for different P_in conditions, showing a clear trend where R_max decreases with increasing P_in. This decline is likely due to the saturation of photogenerated carriers at higher light intensities, reflecting the complex interplay of generation, separation, and transport processes of photocarriers [26]. The Ga₂O₃ photodetector achieves R values of 1.51 × 10¹⁰, 3.36 × 10⁹, 1.35 × 10⁹, 5.89 × 10⁸, 3.54 × 10⁸, and 1.69 × 10⁸ A/W for P_in values of 0.01, 0.1, 0.3, 1, 2, and 5 μW/cm², respectively. Remarkably, the device achieves an exceptionally high R of 1.51 × 10¹⁰ A/W under very low illumination (P_in of 0.01 μW/cm²), which is attributed to the suspended structure. This design minimizes direct contact between the Ga₂O₃ channel and the substrate, thereby reducing carrier trapping by interface defects and improving carrier collection efficiency [27].

Another parameter detectivity D^{\ast }, another key parameter, is used to describe the sensitivity of the device relative to the noise level, assuming that shot noise dominates. D^{\ast } is calculated using the following equation [25]where e is the carrier charge (1.6 × 10⁻¹⁹ C) and I_dark is the dark current. As shown in Fig. 3E, D^{\ast } reaches 6.01 × 10¹⁷ Jones under the lowest P_in condition of 0.01 μW/cm². Even under higher P_in, the device maintains D^{\ast } values in the order of 10¹⁶ Jones, indicating excellent signal detection capabilities with minimal noise interference. Finally, the external quantum efficiency EQE of the Ga₂O₃ photodetector was calculated, which represents the ratio of the number of converting absorbed photons to electrons and the total number of excitation photons. The EQE can be extracted from the equation [25]:where h is the Planck's constant (6.626 × 10⁻³⁴ Js), c is the speed of light (3.0 × 10⁸ m/s), e is the elementary charge, and λ is the illumination wavelength (250 nm). As shown in Fig. 3F, the maximum EQE is as high as 7.53 × 10¹² % under P_in = 0.01 μW/cm² at V = 0.1 V. However, as P_in increases, the EQE gradually decreases, likely due to the increased recombination rate of photogenerated carriers at higher light intensities. In summary, the Ga₂O₃ photodetector with a suspended structure exhibits excellent photoelectrical performance, particularly in terms of high DUV sensitivity and the ability to detect weak signals. These characteristics, combined with its superior structural properties, make the device highly promising for DUV photodetection applications.

Following the analysis of the device's overall photoelectrical performance, the transient response behavior was investigated to further understand the dynamic characteristics of the Ga₂O₃ photodetector. Figure 4A shows the transient photoresponse properties of the Ga₂O₃ photodetector with a suspended structure under 250-nm illumination, with an incident P_in of 0.01 μW/cm² at V = 0.1 V. The illumination was cycled on and off every 20 s, with a total cycle period of 40 s. The device exhibited stable and repeatable ON-OFF switching behavior, indicating a strong and consistent photoresponse. To evaluate the τ_r and the decay time (τ_d) of the Ga₂O₃ photodetector with a suspended structure, Fig. 4B provides a detailed view of the rise and decay edges of the photoresponse curve from Fig. 4A. The rise time τ_r, defined as the time required for the photocurrent to increase from 10% to 90% of its maximum value after the light is turned on, was measured at 0.18 s. Similarly, the decay time τ_d, defined as the time it takes for the photocurrent to decrease from 90% to 10% of its maximum value after the light is turned off, was measured at 2.3 s. These values indicate a fast response to illumination changes, making the device highly suitable for real-time DUV detection applications. To gain deeper insight into the enhanced photoelectrical properties attributed to the suspended structure, Fig. 4C presents the energy band diagram of the Ga₂O₃ device under 250-nm illumination. In the suspended structure of the Ga₂O₃ photodetector, the design minimizes direct contact between the Ga₂O₃ channel and the substrate. This architectural approach plays a crucial role in enhancing the device's performance by reducing interface defects and suppressing carrier recombination that typically occurs at the interface in conventional devices. Interface defects commonly introduce deep-level traps that act as recombination centers for photogenerated carriers, increasing dark current and degrading detectivity [28]. In contrast, the suspended Ga₂O₃ structure effectively lowers the density of these defects, thereby reducing the dark current and enhancing the signal-to-noise ratio, which is critical for high-performance photodetectors [27]. Moreover, the suspended structure greatly improves carrier transport and collection efficiency. In conventional Ga₂O₃ devices, band bending near the interface can hinder carrier mobility by creating potential barriers that trap carriers, leading to recombination. In the suspended structure, as illustrated in the energy band diagram in Fig. 4C, this band bending is largely avoided, leading to a relatively flat band alignment [29]. The flat band configuration allows photogenerated carriers to experience minimal energy loss during transport while also reducing the bandgap distortion caused by substrate-induced thermal noise or interface scattering. As a result, the photogenerated electrons and holes migrate more efficiently toward their respective electrodes, thereby enhancing the overall carrier mobility and improving the device's responsivity. When exposed to 250-nm DUV illumination, high-energy photons are absorbed by the Ga₂O₃ material, generating electron-hole pairs. In the suspended Ga₂O₃ device, the absence of interface traps drastically reduces recombination losses, allowing a larger fraction of the photogenerated carriers to contribute to the photocurrent. The suspended structure creates a strong electric field between the electrodes, efficiently separating the photogenerated electrons and holes and driving them toward the anode and cathode, respectively. This reduction in recombination, coupled with enhanced carrier transport, directly leads to the fast rise time τ_r of 0.18 s observed in Fig. 4B [30]. Furthermore, the suspended structure enhances light absorption efficiency by preventing direct interaction between the Ga₂O₃ channel and the substrate. This design minimizes parasitic absorption losses, which are common in substrate-supported devices, thereby ensuring that more incident photons are available to generate electron-hole pairs. The increased absorption of photons further boosts the photocurrent and enhances the device's responsivity [31]. The combined effects of efficient carrier separation, reduced recombination, and enhanced light absorption result in remarkable improvements in both responsivity and response time, thereby greatly enhancing the overall photoelectric performance of the device. Figure 4D provides a benchmark comparison of the responsivity and response time of Ga₂O₃ photodetectors reported in the literature [6,32–52]. Typically, there is a trade-off between responsivity and response time in photodetector design, where improvements in one parameter often come at the cost of the other. However, the suspended Ga₂O₃ photodetector presented in this work achieves both ultra-high responsivity and rapid response times, surpassing many of the previously reported devices. This highlights the potential of substrate-free or suspended device structures in achieving high-performance Ga₂O₃ photodetectors for next-generation DUV sensing applications.

In summary, high-performance Ga₂O₃ photodetectors with a suspended structure were successfully demonstrated for the first time, showcasing exceptional sensitivity to ultra-weak DUV light, with the ability to detect illumination levels as low as 0.01 μW/cm². Notably, the photodetector achieves a high responsivity of 1.51 × 10¹⁰ A/W, a sensitive detectivity of 6.01 × 10¹⁷ Jones, a large external quantum efficiency of 7.53 × 10¹² %, and a fast rise time of 180 ms under 0.01 μW/cm² illumination at V = 0.1 V. These remarkable improvements are primarily attributed to the reduction of interface defects, improved carrier transport, efficient carrier separation, and enhanced light absorption enabled by the suspended structure. This research offers a viable alternative for designing and optimizing high-performance Ga₂O₃ solar-blind photodetectors, advancing the field of optoelectronics.