## 3 Results
The Ga\({}_{2}\)O\({}_{3}\) PD reached 2.01 \(\times\) 10\({}^{-6}\) A under the same measurement conditions, which demonstrates an increase of 2 orders of magnitude. In Figure 2c,d, the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD exhibits a photo response to 254 nm, 365 nm, and  
405 nm with a light density of 500 \(\upmu\)W/cm\({}^{2}\), 800 \(\upmu\)W/cm\({}^{2}\), and 800 \(\upmu\)W/cm\({}^{2}\), respectively. When the light was illuminated from the CsPbBr\({}_{3}\) side, as in Figure 2c, the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD showed currents of 2.39 \(\times\) 10\({}^{-7}\) A, 7.45 \(\times\) 10\({}^{-8}\) A, and 9.66 \(\times\) 10\({}^{-8}\) A under a bias voltage of 5 V for the light wavelengths of 254 nm, 365 nm, and 405 nm, respectively. When the light was illuminated from the sapphire side, as in Figure 2d, the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD showed a current of 2.03 \(\times\) 10\({}^{-6}\) A, 7.11 \(\times\) 10\({}^{-8}\) A, and 6.18 \(\times\) 10\({}^{-8}\) A under a bias voltage of 5 V for the light wavelengths of 254 nm, 365 nm, and 405 nm, respectively. These data have been collated in Table 1. For the sake of description, light illuminated from the sapphire side and from the CsPbBr\({}_{3}\) side are abbreviated as \"from the sapphire side\" and \"from the CsPbBr\({}_{3}\) side\". Figure 1: (**a**) The preparation process of the PDs. (**b**) SEM image of the CsPbBr\({}_{3}\)/Ga\({}_{2}\)O\({}_{3}\)/Sapphire film. The inset figures are the EDX (Energy-Dispersive X-ray Spectroscopy) of the cross-section and the SEM of the surface, respectively. (**c,d**) XRD spectra of the CsPbBr\({}_{3}\)/Ga\({}_{2}\)O\({}_{3}\)/Sapphire film. (**e**) UV-vis transmittance spectra of the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film and the Ga\({}_{2}\)O\({}_{3}\)/IZO/CsPbBr\({}_{3}\) on sapphire; the inset plot is the (\(\alpha\)hv)\({}^{2}\) as a function of photon energy. As shown in Table 1, the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD demonstrated similar response properties under 365 nm and 405 nm lights from both the CsPbBr\({}_{3}\) side and the sapphire side. This is because Ga\({}_{2}\)O\({}_{3}\) cannot absorb long-wavelength lights, which can only be absorbed by CsPbBr\({}_{3}\) regardless of whether they come from the CsPbBr\({}_{3}\) side or the sapphire side. Under the 254 nm light's illumination, the current's response exhibited an order-of-magnitude change. That is to say, the light current from the sapphire side increased tenfold compared to the CsPbBr\({}_{3}\) side. This phenomenon was maintained after multiple measurements. This is because the 254 nm light can be absorbed by both the Ga\({}_{2}\)O\({}_{3}\) and CsPbBr\({}_{3}\) films. When the 254 nm light was illuminated from the CsPbBr\({}_{3}\) side, it was absorbed by the CsPbBr\({}_{3}\) film. When the 254 nm light is was illuminated from the sapphire side, it was absorbed mainly by the Ga\({}_{2}\)O\({}_{3}\) film. This indicates that the responsivity in the deep-ultraviolet (UV) wavelength region was mainly determined by the absorption of Ga\({}_{2}\)O\({}_{3}\) in the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD. With the introduction of the Ga\({}_{2}\)O\({}_{3}\) layer, the photocurrent increased from 10\({}^{-8}\) to 10\({}^{-7}\) A at a bias voltage of 5 V when compared with the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD and the CsPbBr\({}_{3}\)-only device. This obviously indicates that the ability of the CsPbBr\({}_{3}\) film to absorb UV light increased with the help of \(\beta\)-Ga\({}_{2}\)O\({}_{3}\).