Single-crystal, 4-inch and ultrathin gallium oxide for sundial-inspired high-dimensional solar-blind photodetection metasystem

Abstract

Ultra-wide bandgap gallium oxides offer tremendous possibilities to develop short-wave optoelectronic devices. However, it is formidably challenging to produce single-crystal gallium oxide wafer and develop high-performance high-dimensional optoelectronics. Here we show a liquid-metal-assisted strategy to directly synthesize and transfer single-crystal, large-area and ultrathin β-Ga₂O₃. Benefiting from the UV-exposure oxidation of liquid gallium and strong interaction with gallium, our β-Ga₂O₃ film shows a 4-inch wafer-scale size, a 7.5-nm thickness and a flexible transfer operation. The solar-blind β-Ga₂O₃ detector achieves high responsivity (16.3 A W⁻¹), fast response (<150 μs) and wide linear dynamic range (120 dB). By employing metasurface design, the anisotropy ratio reaches a record high value of 28.8 for Ga₂O₃-based detectors. Moreover, we develop a sundial-inspired metasystem to simultaneously detect the incident direction, polarization, and intensity of solar-blind irradiation. These findings illustrate the potential of high-quality Ga₂O₃ wafer for high-dimensional photodetection, paving the way for next-generation solar-blind communications.

Researchers show a liquid-metal-assisted strategy to synthesize and transfer 4-inch ultrathin β-Ga₂O₃ single crystal. The sundial-inspired metasystem can simultaneously detect the direction, polarization and intensity of solar-blind irradiation.

Introduction

Ultra-wide bandgap oxides are essential for next-generation integrated optoelectronic devices due to their high breakdown field, high mobility and outstanding stability^1–4. Gallium oxide (Ga₂O₃) is one of the most representative candidates, and its conventional preparation methods include edge-defined film-fed growth⁵, Czochralski process⁶, magnetron sputtering⁷, metal-organic chemical vapor deposition (MOCVD)⁸, molecular beam epitaxy (MBE)⁹ and even mechanical exfoliation (ME)¹⁰. Unfortunately, these strategies are natively difficult to achieve large-scale production, substrate independence, defect suppression, ultimate purity, and high crystallinity¹¹. For instance, the oxygen vacancies that are difficult to eliminate in MOCVD, the strong dependence of sapphire substrates in MBE, and the uncontrollable uneven thickness of samples in ME are all obstacles to the Ga₂O₃ preparation process to be overcome^12–14. Recently, room-temperature liquid metal offers an opportunity for the synthesis of gallium oxide with nano-scale thickness^15–17. However, the liquid-metal-printing strategy is currently encountering issues related to size, edges and stress^16,17. A high-quality, low-cost and lightweight production scheme is still absent, hence it is yet to fulfill the prerequisites of wafer-scale ultrathin Ga₂O₃ and fabrication platform compatible with industry-standard pilot lines.

Another major challenge of ultra-wide bandgap Ga₂O₃ is function consolidation within information and communication technology devices or systems, in which Ga₂O₃ primarily serves as a short-wave photosensitive material^18,19. Advanced independent functions based on these optoelectronic devices have been vigorously developed^20,21. The design of low-dimensional photodetector arrays further provides capabilities for imaging or optical path analysis, thereby benefiting the trajectory recognition and contour analysis of solar-blind light sources²². More fundamentally, asymmetric crystal structure and metasurface design may enable high-performance photodetectors with high-dimensional perceiving functions, including not only multispectral analysis and polarization photodetection but also the ability to capture abundant details of irradiation^23,24. Consequently, it is imperative and timely to integrate the above practical functions into one unique system, rather than relying on a complex modular assembly of individual functions.

Here, we report an explicit synthesizing-transferring technique of single-crystal, 4-inch and ultrathin (S4U) β-Ga₂O₃ with the assistance of liquid metal gallium to address numerous issues in the above-mentioned material preparation and functional design. The prepared single-crystal β-Ga₂O₃ film exhibits a wafer-scale size of 4-inch, a thickness of 7.5 nm and obvious intrinsic anisotropy. The UV-exposure oxidation strategy and covalent bonding interface between the metal and oxide jointly ensure the high crystallinity of S4U β-Ga₂O₃ during the subsequent transferring and patterning processes, providing a feasible solution for the design of high-power integrated circuits. The solar-blind photodetector based on S4U β-Ga₂O₃ exhibits a high responsivity (16.3 A W⁻¹) and fast response speed (<150 μs) within a wide linear dynamic range (120 dB), outperforming the reported Ga₂O₃ devices based on conventional process^7,9,19. Combining intrinsic anisotropy and metasurface design, the anisotropy ratio is specifically optimized to a record high value of 28.8 among gallium oxide films. Inspired by the sundial design, a high-dimensional photodetection metasystem has been innovatively constructed, displaying the function to simultaneously detect the incident direction, polarization, and intensity of unknown solar-blind irradiation.

Results

Liquid-metal-assisted synthesis and transfer of β-Ga₂O₃

Most ultra-wide bandgap semiconductors exist in the form of oxides, which have nonlayered crystal structures, featuring strong covalent or ionic bonds, etc²⁵. Therefore, classic exfoliation methods, such as mechanical and liquid-phase exfoliation, are difficult to prepare large-area, uniform, clean and ultrathin oxide films²⁶. Fortunately, natural oxide layers tend to cover the surface of metals against the atmospheric environment, which serves as a possible route for separating the native oxide from a metal to create 2D oxides. Research based on liquid metal Ga has reported that it can be used to print 2D gallium oxide films by using custom-made equipment^14,16,17. Yet, the incompletely oxidized 2D products (GaO_x) have anomalously high electrical conductivity, often requiring post-treatment to maintain stability^14,15.

As an alternative, we propose an oxidizing liquid metal UV-exposure scheme to produce single-crystal ultrathin β-Ga₂O₃ with a large area. As shown in Fig. 1a, the preparation process begins with the melting of metallic Ga under a stable substrate heating temperature (30 °C). Self-limiting nanoscale gallium oxide films can be formed within minutes under high-power UV irradiation (100 W). The external centrifugal force applied by a spin coater accelerates the separation of the surface ultrathin oxide film from liquid Ga (Fig. 1b)¹⁷. The central metal Ga of the original compound is effectively pushed to the edges, leaving the original top and bottom layer of 2D oxides in the center of the 6-inch Si/SiO₂ (280 nm) substrate. The oxide films adhere strongly to the substrate by the van der Waals force. To wash up the unoxidized liquid gallium, hot ethanol turbulence (60 °C) purifies the surface of the film, leaving the cooled 2D gallium oxide films. Last, the 800 °C post-annealing treatment is introduced to improve the crystallinity of the gallium oxides (Supplementary Fig. 1). Furthermore, the specific details of the experiment are explored based on the yield rate of the 2D film products. As shown in Fig. 1c, d, the mass (m) of the liquid metal Ga and the rotational speed (ω) of the spin-coating operation jointly determine the size and quality of films. Inappropriate quality and rotational speed can lead to collapse boundary and trapped residues. The most ideal conditions (ω = 200 r min⁻¹, m = 0.993 g) can lead to the successful fabrication of a uniform 4-inch 2D gallium oxide film with clear boundaries in the inset scanning electron microscope (SEM) of Fig. 1c. Figure 1e shows the in situ energy dispersive x-ray spectroscopy mapping of gallium and oxygen elements in 2D gallium oxide. The net atomic ratio of Ga to O is 40.55:59.45, which is close to the ideal stoichiometric ratio of 2:3. The in situ atomic force microscopy image in Fig. 1f and uniformity of thickness characterization in Supplementary Fig. 2 presents the firm and flat surface of the 2D Ga₂O₃ with 7.5-nm thickness and 0.12-nm roughness, which is of significant improvement compared to before annealing (Supplementary Fig. 3). High resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SAED) characterizations in Fig. 1g further confirm that the prepared ultrathin film is β-Ga₂O₃, which is recognized as the most stable phase for gallium oxides. In order to verify the crystalline uniformity of the β-Ga₂O₃ film, the strongest mode A⁽³⁾_g among all Raman vibration modes is selected as the polarization indicator^27,28. The angular-resolved polarized Raman spectroscopy (ARPRS) in Fig. 1h shows a highly consistent orientation of four-fold symmetry at different sampling points 1–13 in Fig. 1c under the parallel polarization detection architecture, confirming the regularity of the lattice arrangement in 2D β-Ga₂O₃²⁹. The inserted formula in Fig. 1h represents the relationship among the intensity of Raman vibration (I_//), the polarization angle (θ), and the fitting coefficient (b and c). Additional treatment proposed here can effectively improve crystallinity and reduce the density of oxygen vacancies (Supplementary Fig. 4). More detailed material characterizations are provided in Supplementary Fig. 5.

Fig. 1. The preparation and property analysis of single-crystal, 4-inch and ultrathin β-Ga₂O₃ film.

a The preparation of single-crystal, 4-inch and ultrathin β-Ga₂O₃ with spin-coating, oxidation, wash-up and post-annealing process. b The spin-coating mechanism of 2D oxide film. c The 4-inch uniform film and the exploratorily prepared samples with a collapse boundary, trapped residues. The inset is the local magnified scanning electron microscope image (scale bar: 1 inch). d The size and quality of the 2D film are influenced by the metal mass and rotational speed. e The in situ energy dispersive X-ray spectroscopy mapping of 2D oxide. f The β-Ga₂O₃ film with 7.5-nm thickness analyzed by in situ atomic force microscopy. The surface roughness is 0.12 nm. g The high-resolution transmission electron microscope and selected area electron diffraction characterizations of 2D single-crystal β-Ga₂O₃ film. h Raman spectrum of 2D β-Ga₂O₃ and the angular-resolved polarized Raman spectroscopy (ARPRS) analyze of A⁽³⁾_g vibration mode.

Nevertheless, the above platform for the synthesis of large-scale 2D β-Ga₂O₃ has not yet fully met the demands for customization and flexibility. Our research moves forward to the liquid metal-assisted transfer technology. It is reported that Ga₂O₃ bulk material is of an appropriate surface energy (0.34 J m⁻²), which is comparable to classic graphite and molybdenum sulfide bulk³⁰. In Fig. 2a, the differential charge density distribution shows the clear covalent characteristic of the interface between Ga and (100) β-Ga₂O₃. In this study, we adopt the improved mechanical exfoliation process, using the solidified liquid metal as the adhesive layer and directly tore off the synthesized S4U β-Ga₂O₃. Through this method, we not only exfoliate an intact rectangular β-Ga₂O₃ film onto a 4-inch Si/SiO₂ substrate (Fig. 2b), but also successfully construct a crossed ultrathin β-Ga₂O₃ homojunction (Fig. 2c). The more detailed experimental procedures are described in the “Methods” and Supplementary Fig. 6. The samples are in good accordance with the pre-prescribed edge by laser cutting. As the interaction between two-dimensional gallium oxide and metals can be disrupted by surface treatment or the introduction of impurities, the plasma treatment can effectively modify the exfoliated 2D β-Ga₂O₃ film, as the “NENU” pattern in Fig. 2d. In order to further verify the practicality of such transfer scheme for complex patterns, we also present the transfer results of fingerprint patterns using gallium oxide in Supplementary Fig. 7. Moreover, we successfully implemented the integrated circuit design based on S4U β-Ga₂O₃ by combining the lithography and plasma treatment schemes (Fig. 2e). As shown in the UV fluorescence microscope image, the clear boundaries provide excellent prerequisites for future electrode introduction and improved electrical contact^31,32. This integrated circuit design demonstrates excellent uniformity and stability (repeatability) of photoresponse (Supplementary Fig. 8), and also avoids any adverse reaction between gallium and other metal electrodes³³.

Fig. 2. The transfer strategy of single-crystal, 4-inch and ultrathin β-Ga₂O₃ using liquid metal.

a The differential charge density distribution of the interface between Ga and (100) β-Ga₂O₃. b Rectangular 2D β-Ga₂O₃ film transferred onto a 4-inch Si/SiO₂ substrate. The arrows along the edges indicate the direction of laser cutting. c The cross-stacked 2D β-Ga₂O₃ homojunction transferred and constructed by cooled liquid Ga. d β-Ga₂O₃ film with a “NENU” pattern after plasma treatment and transfer operation (scale bar: 10 mm). Patterned 2D β-Ga₂O₃ films can also be completely transferred onto flexible polydimethylsiloxane (PDMS) substrates. e The integrated circuit design of the β-Ga₂O₃ film (scale bar: 20 mm). The magnified fluorescence micrograph image shows clear boundaries between the β-Ga₂O₃ and substrate (scale bar: 500 μm).

Anisotropic solar-blind response optimized by metasurface

For practical applications, nature provides convenience. The ozone layer of the Earth almost completely absorbs the wavelength component of 200–280 nm in solar irradiation. In the natural environment of the earth’s surface, the background radiation of sunlight in this solar-blind region is extremely weak, and the solar-blind photodetection will not be influenced by the sunlight or other ambient light. Ultrawide bandgap (>4.5 eV) of Ga₂O₃ provides an ideal foundation for designing UV-sensitive devices. We first construct the prototype metal-semiconductor-metal horizontal detectors based on S4U β-Ga₂O₃ films. The response region (225–315 nm) covers most of the solar-blind wavelength band (200–280 nm) in Fig. 3a. The responsivity, defined as the ratio of net photocurrent to incident power in the active devices, reaches a peak of 16.3 A W⁻¹ around 265 nm wavelength. Hence, 265 nm is selected as the irradiation wavelength for subsequent opto-electric characterizations. Figure 3b presents the photocurrent density and responsivity as a function of incident power intensity under 265 nm irradiation and 2 V bias. Device shows a linear response and a wide linear dynamic range (LDR = 20 log (P_max/P_min) = 120 dB), in which P_max and P_min are the upper and lower limits of incident power intensity of the linear region^34,35. We also evaluate the response time (136.1/88.4 μs for rise/decay time, Fig. 3c), which is comparable to the current state-of-the-art for the photodetectors based on 2D-layered-semiconductors^35,36. Such excellent performance stems from the inherent photoconductive property of the UV-sensitive material. The photo-generated carrier transport process is not restricted by the interface scattering effects or the capture and release by the defect state of oxygen vacancy. In addition, by eliminating the post-annealing operation in the preparation process, we can also fabricate 2D amorphous Ga₂O₃ detectors. The performance of 2D amorphous Ga₂O₃ has been characterized and compared in Supplementary Fig. 9.

Fig. 3. Anisotropic solar-blind response of single-crystal, 4-inch and ultrathin β-Ga₂O₃.

a The responsivity and response region of the β-Ga₂O₃ detector. Single crystal β-Ga₂O₃ device exhibits intense solar-blind responsivity of 16.3 A W⁻¹ under 2 V bias. b The photocurrent density and responsivity as a function of incident power intensity in a 2D single-crystal β-Ga₂O₃ detector under 265 nm laser irradiation and 2 V bias. c The rise time (136.1 μs) and the decay time (88.4 μs) of the 2D β-Ga₂O₃ detector. d The polarization-dependent output curves under 265 nm linearly polarized laser irradiation with the power density of 100 mW cm⁻². The polarization angles of each curve differ by 15°. The inset energy band diagram illustrates the polarization-dependent optical transition selection rule in wave vector (k) space. e The scanning electron microscope image of metasurfaces. The electrodes in those metasurfaces are of different widths and periodicities. f The optimum anisotropy ratio of 28.8 with 265 nm electrode width and 0.5 duty ratio. The photocurrent in the polar coordinate pattern shows obvious polarization-dependent characteristics. g The anisotropy ratio of polarized photocurrent density was optimized to 28.8 by the metasurface design. Current result possesses the most comprehensive performance of devices based on Ga₂O₃, and it is closest to the desired corner, simultaneously achieving high responsivity, fast response speed and strong anisotropy^44–57.

The intrinsic crystal structure of β-Ga₂O₃ makes it possible for high-dimensional photodetection. Monoclinic β-Ga₂O₃ belongs to the C2/m space group in a low-symmetry structure, defining its anisotropic nature³⁷. The inset of Fig. 3d displays the energy band diagram of the 2D β-Ga₂O₃ along the (100) direction, which illustrates the polarization-dependent optical transition selection rule^38,39. Specifically, the direct transition only occurs within the wider bandgap (~4.7 eV) when the electric vector (E) of incident light is parallel to the b-axis (E//b-axis). In the opposite case of E//c-axis, the direct transition only occurs within the narrower bandgap (~4.5 eV). Such polarization-dependent selective absorption of β-Ga₂O₃ determines the subsequent polarization-sensitive photo-response with obvious anisotropy (see Supplementary Fig. 10 for the polarized absorption spectra). The polarization-dependent output curves can be well modulated by rotating the half-wave (λ/2) plate under 265 nm linearly polarized laser irradiation with a power density of 100 mW cm⁻². As shown in Fig. 3d, the maximum and minimum photocurrent density value occur at 90° (E//c-axis) and 0° (E//b-axis), respectively, as a result of the anisotropic absorptivity of β-Ga₂O₃ along different crystal axes mentioned above.

Metasurfaces, planar arrays of subwavelength nanostructures, have revolutionized light control and provided a promising solution to address the challenges of both performance enhancement and function integration⁴⁰. Patterning metallic, semiconducting, or dielectric layers of optoelectronic devices into nano/microstructures enables researchers to harness optical resonances (OR) for performance optimization^41,42. The rectangular or interdigitated electrode is an optimal photo-trapping structure here to efficiently conduct the photogenerated carriers, while its size and periodicity are also crucial factors (Fig. 3e)⁴³. As is well known, electromagnetic waves could form standing waves in a resonant cavity with dimensions matched with the incident wavelength, thereby localizing and enhancing the electromagnetic field. Herein, we have carried out electron beam lithography (EBL) to fabricate a batch of electrodes on the β-Ga₂O₃ films, whose horizontal width (250 nm, 255 nm, …, 275 nm, 280 nm) is close to the selected incident wavelength of 265 nm in terms of size. Considering the shielding effect of electrode size on the irradiation, the duty ratio (DR = w/p) also needs to be taken as a variable (varying from 0.2–0.8), which is defined as the ratio of electrode width (w) to the period (p). Figure 3e partially displays the SEM image of electrode arrays with different size and DR. It is found that under the size-condition of 265 nm in electrode width (equal to the wavelength of incident light) and a DR of 0.5, the anisotropy ratio (AR, defined as the ratio of the maximum to minimum polarization photocurrent) of this polarization detection reaches the optimum value of 28.8 by OR (Fig. 3f). Such optimization of anisotropy ratio is in good agreement with the simulation results in Supplementary Fig. 11. Although it is difficult to conduct optoelectronic characterization in the practice operation, we still managed to design a unit structure for detecting circular polarized light in solar-blind region (Supplementary Fig. 12). In Fig. 3g, the AR of polarized photocurrent density is significantly optimized to a record high value of 28.8 by this elaborate metasurface design. In order to evaluate the overall performance of the solar-blind photodetector based on gallium oxide, we have summarized the three key performance parameters (responsivity, response time and anisotropy ratio) of these devices^44–57. This work possesses the most comprehensive performance that is closest to the desired corner, simultaneously achieving high responsivity, fast response speed and strong anisotropy. Even though other individual micro-nano structures possess extremely high anisotropy, due to the size-effect of absorption cross-section limiting the optical focusing operation in the invisible region, they are not operation-friendly anyhow⁵⁸. Indeed, our results of polarization photodetection have already surpassed those of the mainstream low-dimensional materials (Supplementary Fig. 13).

Sundial-inspired photodetection metasystem

Inspired by the sundial designed to roughly determine the time by the relative position of the sun in the sky (Fig. 4a), this work moves forward to utilize spatial relationships to analyze the information of solar-blind irradiation through using projections within the 3D space. Therefore, we have constructed a Ga₂O₃-based versatile metasystem decoding high-dimensional solar-blind optical information. As shown in Fig. 4b, the metasystem is constructed by two parts. On the one hand, amorphous Ga₂O₃ film covers and encapsulates a micron-sized silica cylinder in the vertical direction, while a pair of Au electrodes is set at both ends, forming an amorphous Ga₂O₃ detector with a wide LDR. Complete preparation and performance are elaborated in detail in the Supplementary Figs. 14, 15. On the other hand, an S4U β-Ga₂O₃ film is prepared and transferred onto the horizontal plane via the liquid metal oxidation and post-annealing strategy mentioned above. 150 pairs of Au electrodes with a width of 265 nm and DR of 0.5 are processed into striped patterns by EBL on top of the horizontal β-Ga₂O₃ film, forming the metasurface with a series of classic metal-semiconductor-metal photodetectors in concentric-sector shape (see “Methods” section for more detailed experimental procedures).

Fig. 4. The function demonstration of the sundial-inspired high-dimensional solar-blind photodetection metasystem.

a Sundial designed to roughly determine the time by determining the relative position of the sun. b The deconstructed schematic diagram of solar-blind Ga₂O₃ detection metasystem. c The scanning electron microscope image of the meta-unit and metasystem. The meta-unit is composed of 150 pairs of Au electrodes with a width of 265 nm and a duty ratio of 0.5. The detection metasystem consists of 36 meta-units. d Technical route of high-dimensional solar-blind photodetection. e Irradiation intensity is calculated by the linear relationship with the photocurrent. The photocurrent density of the Ga₂O₃ cylinder detector is independent of polarization direction. f The judgment of polarization direction by evaluating the current density value in different meta-units. The asymmetry factor among the meta-units reaches 17. g The incident direction is identified by the metasystem. The spatial angle α and polar angle β of solar-blind irradiation are identified by analyzing the distribution of current density and the tangent geometric relationship between shadow length and device height.

To describe the detection metasystem more clearly, the 3D polar coordinate system (r, α, β) has been introduced in Fig. 4b. The origin is defined at the center of the bottom of the amorphous columnar detector, and three variables r, α and β represent polar radius, the spatial angle between the wave vector and the polar coordinate plane, and the polar angle, respectively. The direction of the c-axis of β-Ga₂O₃ in this section is defined as the 0° polar angle.

Figure 4c shows the SEM image of this solar-blind detection meta-unit and the whole metasystem. Herein, a meta-unit (one separate sector area) consists of 150 detectors with clear and distinct boundaries (meta-unit in Fig. 4b, c). The whole solar-blind detection metasystem is composed of 36 meta-units, forming a complete circular pattern (metasystem in Fig. 4b, c). Thus, the corresponding polar angle β is a series of discrete values (±180°, −170°, …, −10°, 0°, 10°, …, 170°). Through this design, high-dimensional optical information of unknown linearly polarized solar-blind irradiation, such as incident power intensity, polarization orientation, and the direction of wave vector (namely, the direction of light propagation), could be simultaneously acquired by this sundial-inspired metasystem, as the technical route shown in Fig. 4d. The detailed detection mechanisms are explained as follows.

First, the amorphous Ga₂O₃ cylinder detector is employed to identify the power density of incident polarized light because of its wide LDR (110 dB, Supplementary Fig. 15). As shown in the illustration in Fig. 4e (top right), the incident power of solar-blind irradiation could be easily deduced by the linear relationship (I = AP^α, α ~ 1) between power (P) and current (I). The data in Fig. 4e clearly demonstrates the photocurrent density results independent of the polarization angle θ, which is defined as the angle between the electric field E and the vertical projection of the silica cylinder in the wavefront plane. As shown, the native amorphous feature makes the Ga₂O₃ cylinder detector insensitive to the light polarization, which is beneficial to exactly determining the incident light power^59–61.

Next, the polar angle β-dependent photoresponse is measured and shown in Fig. 4f. In this case, the polarization angle θ of solar-blind irradiation is set as 0°, and the polar radius r is fixed as 12.12 μm (Supplementary Fig. 16). Photocurrent density of different meta-units shows notable differences due to the variations in their polar angles. The maximum photocurrent density is recorded at the direction of β = ±180°. This is because optical resonance between neighboring Au electrodes enhances the photoresponse of meta-unit detector, similar to the situation described in Fig. 3e. The asymmetry factor of photoresponse, defined as the ratio of maximum photocurrent to the minimum one collected from all devices with different polar angles, reaches as high as ~17, indicating the excellent capability of this metasystem in identifying linear polarization state of incident solar-blind irradiation. In this situation, the linear polarization angle θ equals to the polar angle β_max corresponding to the meta-unit with the maximum current value (θ = β_max). It is noted that the photocurrent density at β = 0° is seriously decreased due to the shadow effect of the standing amorphous Ga₂O₃ cylinder, which in fact could help us analyze the propagation direction of incident light (see Supplementary Fig. 17 for detailed discussion for polarization photodetection).

To further determine the incident direction of solar-blind light, the current densities of each device within each meta-unit are collected in Supplementary Fig. 18. Particularly, the uniformly irradiated area exhibits a highly consistent spatial photocurrent density distribution. On the contrary, the current of some units exhibits abnormal distribution due to the shadows of the vertical amorphous Ga₂O₃ detector, as shown in the specific area of Fig. 4g. In these specific areas, the current value suddenly increases along the radial direction, and we designate the polar angle β of meta-unit in which the most obvious current change occurs as the azimuth angle of incident light in 2D plane. Subsequently, this study focuses on the meta-unit shaded and covered by the shadow of the amorphous Ga₂O₃ cylinder (top inset in Fig. 4g). Under the circumstance of grazing incidence, the shadow length of the cylindrical projection could be figured out by the distribution distance of dark current density along the polar radius. The spatial angle α could be conveniently calculated through the tangent geometric relationship (tanα = H/L) between the height (H) of the cylinder detector and shadow length (L), as shown in the lower inset of Fig. 4g, and the minimum resolution can be as low as 0.06°. As a proof of concept, an incident light with a spatial angle of 7° is introduced as an example. It could be clearly observed that the current density shows a sudden increase as the polar radius extends owing to the switch from dark current to the photocurrent (the black curve in Fig. 4g). Therefore, we can easily obtain the shadow length of amorphous Ga₂O₃ cylinder and calculate the spatial angle α in vertical plane (~6.8°, the bottom curve in Fig. 4g). In general, both spatial angle α and polar angle β (azimuth angle in 2D plane) can collaboratively help us approximately determine the propagation direction of incident light.

Based on the above presentation, the high-dimensional optical information (e.g., power density, polarization and incident direction) of unknown linearly polarized solar-blind irradiation can be well decoded with the current Ga₂O₃-based photodetection metasystem. As an application-oriented for actual communication purposes, we also propose a potential solar-blind polarization communication strategy with better security and anti-interference capability in a more concise manner (Supplementary Figs. 19–21)⁶².

Discussion

In summary, we have reported a holistic framework to synthesize single-crystal, 4-inch and ultrathin β-Ga₂O₃ wafer via ultraviolet exposure of liquid metal gallium and post-annealing process. The prepared β-Ga₂O₃ wafer can also be flexibly transferred in a specific pattern through covalent bonding with the solidified metallic Ga, providing an effective manner to construct front-end device architectures for integration. The photodetector based on S4U β-Ga₂O₃ achieves an ultimate comprehensive performance (16.3 A W⁻¹ of high responsivity, response time less than 150 μs and 120 dB of wide linear dynamic range in the solar-blind region). The anisotropy ratio has been optimized to a record high value of 28.8 due to the intrinsic anisotropy of β-Ga₂O₃ and metasurface structure. Such a technical approach not only alleviates the trade-off problem between fast response and high responsivity but also expands the polarization dimension in the Ga₂O₃-based photodetection system. Inspired by sundial design, we have constructed a high-dimensional solar-blind photodetection metasystem, displaying the practical function of simultaneously detecting the incident direction, polarization, and intensity of solar-blind irradiation. Although further investigations are still needed, our preparation technology of S4U β-Ga₂O₃ and metasystem design both explore and advance a promising production route towards next-generation solar-blind communication architecture.

Methods

Materials preparation and characterization

The S4U Ga₂O₃ films were fabricated by UV exposing liquid gallium. 50 g Ga was positioned in a glass container and heated to 30 °C. Owing to the low phase transformation point (29.76 °C for the melting point), bulky gallium melted into a liquid state. Gallium droplets were applied onto a 6-inch Si/SiO₂ (280 nm) substrate. According to Cabrera-Mott growth kinetic model, self-limiting nanoscale gallium oxide films can be formed in a few minutes. Moreover, high-power UV irradiation (>10⁷ mW cm⁻², 265 nm LED) provided additional reactive oxygen, which was beneficial for reducing the density of oxygen vacancies in gallium oxide. The subsequent spin-coating operation and wash-up operation with 60 °C ethanol both separated the surface ultrathin oxide film from Ga droplets, leaving 2D amorphous Ga₂O₃ attached to the substrate by van der Waals forces. Finally, post-annealing treatments at 800 °C in the air significantly improve the crystallinity, transforming the amorphous films into a 2D β-Ga₂O₃ film.

The prepared S4U Ga₂O₃ films were transferred onto various substrates by using a laser-marking machine (New Industries Optoelectronics Technology, UV-50) and a dry-transfer stage (Metatest, E1-G) for subsequent material characterization. Plasma treatment (Saiaote Technology, SAT-5D) is adopted for patterned processing with a customized mask (“NENU”). Furthermore, for intricate circuit designs, the photoresist serves as a sacrificial layer, isolating gallium oxide and the liquid metal Ga. Liquid metal smeared on the surface of S4U Ga₂O₃ is instantly cooled down to 5 °C and then solidified in a few minutes. Using polydimethylsiloxane (PDMS) and other adhesive materials (Scotch tape), the patterned materials can be efficiently released together with the solidified gallium. Wash-up operation with hot ethanol is also applicable to remove surface gallium.

The morphology, binding energy and thickness characterizations of the Ga₂O₃ films were respectively characterized using transmission electron microscopy (TEM, JEOL JEM-2100), X-ray photoelectron spectroscopy (XPS, ESCALAB 250) and atomic force microscopy (AFM, Bruker, Dimension Icon). The Raman signals were collected by a Raman spectroscopy (Horiba, Jobin Yvon) and polarization module (Thorlabs), excited by laser with 532 nm wavelength. The crystal structures of β-Ga₂O₃ were determined by a D/max 2500 X-ray diffraction spectrometer (XRD, Rigaku) with Cu Kα radiation (λ = 1.5418 Å). The atomic ratio of β-Ga₂O₃ was analyzed by an energy dispersive spectrometer (EDS) with a scanning electron microscope (SEM, FEI, Quanta 250 FEG). In addition, the absorption spectrum was collected by a spectrophotometer (Andor Technology Kymera193i) under the illumination of an Xe lamp, which could be combined with a polarizer to indicate the direction of the axis of the β-Ga₂O₃.

Photodetector fabrication and testing

As the initial attempt, we chose Au electrodes (thickness of 50 nm) and adjusted their spatial position on S4U β-Ga₂O₃. It was found that the carriers have a higher mobility along the b-axis, which is consistent with our previous work. Subsequent experiments also followed this direction to fabricate the devices. Furthermore, we have designed a photodetection meta-unit with special subwavelength-structure electrodes, approaching the selected excitation wavelength (265 nm). Devices are fabricated by EBL. Specifically, PMMA E-Beam Resists (950 K) were spin-coated using a spin coater (pre-rotation at 600 rpm for 6 s and main rotation at 6000 rpm for 60 s, PMMA’s thickness is about 200 nm). Then, the soft bake procedure was performed to bake the device on a hot plate at a temperature of 150 °C for 180 s. Next, Au electrodes were patterned using the electron beam lithography system (RAITH eLINE Plus, acceleration voltage is 10 kV, beam current is 0.2 nA), and methyl isobutyl ketone (MIBK) as the developer. At last, the Au electrodes (50 nm) were deposited by an electron beam evaporator. So far, a series of electrodes with different specifications has been fabricated, forming a metasurface with photo-trapping structures.

The optoelectronic characterization of the 2D Ga₂O₃ polarization photodetectors were carried out by an integrated optoelectronic testing system (Metatest Scanpro 100), which were equipped with a series of source-meters (Keithley 2600 series), a Xenon lamp as light source (wavelength region: 200–1100 nm) and a spectrophotometer (Andor Technology Kymera193i). Moreover, several polarized laser units with different wavelengths (213 nm, 265 nm, 320 nm, 405 nm, and 532 nm) were introduced and coupled in the integrated measurement platform for device testing. The response time was measured by a preamplifier (Stanford Research Systems SR570) and a high-resolution oscilloscope (Picoscope 4224), guaranteeing the precision of response time (less than 1 μs). The polarization direction of the incident light was modulated by rotating a UV half-wave plate (Thorlabs).

The simulation of optical resonance

To simulate the optical response of electrode arrays on Ga₂O₃ layers with polarization incident light, we constructed a two-dimensional model using the software COMSOL Multiphysics and the finite element method. The electric field distribution across the device cross-section was further investigated under 265 nm polarized light, with polarization directions perpendicular to and parallel to the antenna orientation, respectively. Periodic boundary conditions were set in the x-direction, representing a periodical structure, while a perfectly matched layer condition was implemented in the y-direction. The mesh size of the model was generated by using the self-adaptation method to ensure that the mesh size is small enough for an accurate simulation result. The refractive index of the substrate SiO₂ is taken as 1.48. The thickness of the Ga₂O₃ layer is set as 7.5 nm. The thickness of the Au layer is set as 50 nm, and the width of the Au nanoantenna is set as 265 nm at a duty ratio of 0.5. The Ga₂O₃ layer is regarded as an anisotropic material exhibiting specific differences in in-plane conductivity (σ) between the σ_|| and σ_⊥ planes⁶³.

Metasystem development and demonstration

Based on the detection meta-unit with a high anisotropy ratio, the metasystem combines the advantages of amorphous Ga₂O₃ and single-crystal β-Ga₂O₃. An amorphous Ga₂O₃ detector was constructed based on a vertical custom-made SiO₂ cylinder (diameter of 5 μm and height of 12 μm), and required no annealing operation. In the horizontal direction, 2D β-Ga₂O₃ films were fabricated by UV exposing liquid gallium. A circle pattern in a 2D β-Ga₂O₃ film was created by a high-energy laser marking. The outer side 2D β-Ga₂O₃ film was transferred on to the substrate passing through an amorphous Ga₂O₃ sample with liquid metal assistance (Fig. 4b). The 2D β-Ga₂O₃ film was then evenly divided into 36 sectors, and section occupied 10° in the polar coordinate system. Moreover, electrodes were fabricated and arranged radially in sequence by electron-beam lithography. In order to maintain the structure while ensuring the irradiation area, we used commercial ultra-fine (75 nm) probes to accurately measure the current density of each detector. Probes could move in pairs on the integrated optoelectronic test platform. During the experiment, the solar-blind polarized light source was fixed on the mechanical arm, and the direction angle was measured using a protractor (KDS, DAS-F51) with a precision of 0.1°. The incident light power is measured by a commercial power meter (Thorlabs, C-Series). A half-wave (λ/2) plate was used to adjust the direction of the electric vector (polarization direction) of the incident light, ensuring the spot position and power remain steady.

Author contributions

W.Z. and J.S. contributed equally to this work. W.Z., J.S., W.L., H.X., Y.L. and C.-W.Q. conceived the project. W.Z. J.S., W.L., and X.L. fabricated the metasystem. W.Z., J.S., J.J. and J.F. measured the optical and optoelectrical performance. W.L., H.X., Y.L. and C.-W.Q. supervised the project and revised the manuscript. W.Z., J.J. and X.Z. achieved the COMSOL simulation and analysis. W.Z., J.J. and W.L. carried out the material analysis. W.Z., J.S., W.L., H.X. and C.-W.Q. co-wrote the paper, with inputs from all co-authors. All authors discussed the results and commented on the paper.

Peer review

Peer review information

Nature Communications thanks Zeng Liu and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

This work was supported by the National Key R&D Program of China (No. 2023YFB3610200), the National Natural Science Fund for Distinguished Young Scholars (No. 52025022), the Program of National Natural Science Foundation of China (Nos. 62574038 and 62275045), the 111 Center (B25030), and the Fund from Jilin Province (Grant No. SKL202602014JC). C.-W.Q. acknowledges the support of NSTIC Grant from A*STAR, Singapore (WBS: A-8003984-00-00).

Data availability

The source data generated in this study are provided in the Supplementary Information/Source Data file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-75058-w.