Ga₂O₃, with a wide band gap of 4.2–5.3 eV, exhibits large breakdown field, high dielectric constant and Baligaʼs figure of merit.^[1–4] Meanwhile, Ga₂O₃ can work steadily in extreme environment, such as high temperature, high electric field, and high energy radiation. Ga₂O₃ with such properties has a wide range of potential applications in high-power electronic devices, deep ultraviolet (DUV) solar-blind photodetectors, gas sensors, and transparent electrodes on optoelectronic devices.^[5–13] Ga₂O₃ Schottky barrier diodes have a high breakdown voltage.^[14] The β-Ga₂O₃ field-effect transistors (FETs) exhibits a large on/off current rations ( ).^[15] Rectifiers and enhancement-mode FETs based on Ga₂O₃ can be used under higher electric field than either SiC or GaN based devices.^[16] DUV solar blind photodetectors have important civil and military applications, such as missile tracking, short-range secure communication, ultraviolet astronomy, ozone holes monitoring, corona detection, and so on.^[17–20]

Bendable, lightweight, flexible, sensitive, and wearable, electronic devices are urgently needed in emerging technological applications.^[21–24] Flexible and transparent two-dimensional (2D) materials have been employed and developed in electronic technologies, including wearable energy-harvesting systems, folding electronic devices, curved screen electronic devices, soft portable devices, and rollup displays flexible Ga₂O₃ thin film photodetectors.^[17] The Ga₂O₃ thin film layer deposited directly on the flexible polymer materials was non-crystalline, and the amorphous films were unstable for long-term device applications.^[25–30] The Ga₂O₃ thin film layer can be fabricated by integrating exfoliated Ga₂O₃ micro-layers,^[31] but the thicknesses and dimensions of the Ga₂O₃ film layer were uncontrollable with the mechanical exfoliation method. In addition, the edge of the film is not intact.

In this paper, a millimeter sized (5 mm ×8 mm) free-standing Ga₂O₃ thin film was synthesized, and the flexible Ga₂O₃ solar-blind UV photodetector was fabricated. The Ga₂O₃ thin film layer transferred to the flexible substrates in this research was crystalline and its thickness was controllable. In addition, the edge of the film was intact. The experimental results demonstrated that the bending of the film had no obvious influence on its electrical performance. This provides an effective transfer method to fabricate crystalline Ga₂O₃ thin film onto flexible substrates for applications in flexible and wearable electronics.

The SAO buffer layer (BL) was deposited on silicon substrates by radio frequency (RF) magnetron sputtering, and then Ga₂O₃ thin film was grown on the SAO BL. The base pressure in chamber was 1×10⁻⁴ Pa, and the distance between SAO target (99.99% purity) and substrates was 6 cm. The substrates temperatures were 750 °C. The Sr₃Al₂O₆ (SAO)^[32,33] BL was deposited under a working pressure of 0.4 Pa and RF power of 120 W. The Ga₂O₃ thin film was deposited under a working pressure of 0.8 Pa and RF power of 70 W. Then the Ga₂O₃/SAO bilayer thin films on the Si substrates was immersed into deionized water. The crystal Ga₂O₃ thin film separated from substrates when SAO film was etched in water.^[31,33] Finally, the free-standing Ga₂O₃ thin film was transferred to the flexible substrates polyethylene terephthalate (PET). The crystal structure of the as-grown film was investigated by transmission electron microscopy (TEM). UV-visible (UV-vis) absorption spectrum was taken using Hitachi U-3900 UV-vis spectrophotometer. The current–voltage (I–V) of the Ga₂O₃ film-based detectors were measured by Keithely 2450 source meter. To test the electric properties of the flexible Ga₂O₃ film, interdigital Ti/Au electrodes were deposited on the film by radio frequency magnetron sputtering using shadow mask. The electrode fingers of shadow mask were wide and long with a spacing gap.

Figure 1(a) is the flow chart of the process to fabricate free-standing Ga₂O₃ films on PET. Figure 1(b) shows the photograph of free-standing Ga₂O₃ thin film floating on the surface of the deionized water. A millimeter sized (5 mm ×8 mm) free-standing Ga₂O₃ thin film was obtained. Figure 1(c) is the optical microscope image of free-standing Ga₂O₃ thin film in the water, and it appeared that the edge of film is intact. The free-standing Ga₂O₃ thin film can be transferred to any substrates. A photograph of Ga2O membrane transferred onto the flexible PET is shown in Fig. 1(d).

Fig. 1.

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	Fig. 1. (a) Schematics of the free-standing Ga2O thin film fabrication process. (b) Photograph of the free-standing thin film on the surface of the water. (c) An optical microscope image of the edge of free-standing Ga2O3 thin film. (d) An image of the device fabricated by transferring free-standing Ga2O3 film onto the PET substrates.

Figure 2(a) shows the structure and morphology of the Ga₂O₃ thin film characterized by transmission electron microscopy (TEM). The high-resolution TEM (HRTEM) image shows that the Ga₂O₃ thin film was polycrystalline. The polycrystalline structure was further confirmed by selected area electron diffraction (SAED) pattern in the illustration. Figure 2(b) shows the top SEM morphologies of the Ga₂O₃ thin film. It is obvious that the film was dense. The grain size of film was about dozens of nanometers. Figure 2(c) gives the ultraviolet–visible (UV) absorbance spectrum of the free-standing Ga₂O₃ thin film. The spectrum of the measured sample exhibits a sharp absorption edge at wavelength of ∼260 nm. The band gap of β-Ga₂O₃ thin film is about 4.75 eV obtained by fitting the extrapolating linear region. The plot (ahv)² versus hv is shown in the inset of Fig. 2(c), where h is Planckʼs constant, a is the absorption coefficient, and v is the frequency of the incident photon. Figure 2(d) shows the XPS spectrum with the binding energy in range from 1111 eV to 1155 eV. The energy peak of Ga 2p_1/2 and Ga 2p_3/2 are centered, which is attributed to the presence of gallium oxide and not gallium metal or others.

Fig. 2.

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	Fig. 2. (a) The HRTEM image of Ga2O3 thin film; inset: the corresponding SAED pattern. (b) The top surface view SEM image. (c) The UV absorbance spectrum of Ga2O3 thin film. (d) XPS spectra of Ga 2p core levels.

To investigate photoelectric properties of the film, flexible Ga₂O₃ based solar-blind UV photodetector was fabricated and a three-pair interdigital Ti/Au electrodes were deposited on top of the β-Ga₂O₃ thin film to construct a MSM structure device. The thickness of Ti layer and Au layer were 30 nm and 100 nm, respectively. The electrode fingers were wide, long, spacing gap, and 3 pair fingers. The 365 nm and 254 nm wavelength were used as light sources, respectively. The effective irradiated area was ∼0.03 cm². A lamp with a center wavelength of 254 nm and 365 nm was used as a UV illumination source. In Fig. 3(a), I–V curve measured under the 365 nm light (0.5 mW/cm² intensity level on the sample surface) does not show significant difference to the I–V curve measured in dark, which suggests that the Ga₂O₃ thin film was not sensitive to 365 nm light. In contrast, the current shown a sharp jump when the device was exposed to 254 nm light. Figure 3(b) displays the I–V curves of the Ga₂O₃ film under rigid, bending upward and bending downward states. The curved Ga₂O₃ film was considered to be on a circumference of radius r, whose value implies the degree of bending. The curvature degree of Ga₂O₃ film were , r = 5 mm, and r = −5 mm, respectively. Seen from Fig. 3(b), the Ga₂O₃ thin film exhibits almost the same electrical performance on the flexible PET, whether or not the film was bending. This implies that bending stress and bending direction had no influence on electrical performance of the films. The ignorable difference in these curves may be ascribed to the contact conditions between the Ga₂O₃ thin film and the electrodes. The insets shows the photographs of the flexible Ga₂O₃ thin film in the case of rigidity, bending upward and bending downward, respectively.

Fig. 3.

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Fig. 3. I–V characteristics curves: (a) at different light intensity; (b) under the states of the rigid, bend upward, and bend downward. The insets show the photographs of the flexible Ga2O3 thin film at the rigid, bend upward, and bend downward. Time-dependent photoresponse: (c) experimental curve and fitted curve of the current rise and decay process to 254 nm illuminations under the state of rigid. (d) The I–t curve under 254 nm light illumination.

Figure 3(c) shows the time-dependent photoresponse performance of flexible Ga₂O₃ thin film under the case of rigid. The Ga₂O₃ thin film was tested under 254 nm light by on/off switching and with an applied bias of 5 V. The intensity level of 254 nm illumination source on the sample surface was 1 mW/cm². The results displayed a good repeatability, lower dark current and high photo-conductivity of the free-standing Ga₂O₃ film based device. The flexible Ga₂O₃ film based device yields a high level of signal-to-noise ratio under DUV illumination. Moreover, the device shows a short rise time and fall time under 5 V bias voltage. The quantitative analysis of the current rise and decay process involves the fitting of the photo-response curve with a exponential relaxation equation:^[33]

In summary, a free-standing Ga₂O₃ thin film has been fabricated using Sr₃Al₂O₆ as a sacrificial layer. The free-standing Ga₂O₃ thin film was polycrystalline, thickness was controllable, and the film edge was intact. Additionally, the photoelectric performances testing of the flexible Ga₂O₃ photodetector showed that the free-standing Ga₂O₃ thin film had a similar electrical performance whether bending or not, meaning no degradation after bending. The experimental results demonstrated that it was a practical way to fabricate free-standing Ga₂O₃ thin films by using Sr₃Al₂O₆ as a sacrificial layer. The free-standing Ga₂O₃ film can be used to fabricate flexible and wearable electronic devices by transferring it to flexible substrates for application.