## 3 Results  
The preparation procedures and the structure configurations of Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) PDs are shown in Figure 1a. At high temperatures, small droplets of the gallium-acetylacetonate aqueous solution produced a chemical reaction on the surface of the sapphire that caused the Ga\({}_{2}\)O\({}_{3}\) film to grow. Then, the IZO layer was used as an electrode between the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) and the CsPbBr\({}_{3}\) films. After spin coating and annealing, the CsPbBr\({}_{3}\) perovskite was deposited on the IZO/\(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film. Figure 1b shows the cross-section of the CsPbBr\({}_{3}\)/Ga\({}_{2}\)O\({}_{3}\)/Sapphire film. The thickness of the Ga\({}_{2}\)O\({}_{3}\) film was about 420 nm and that of the CsPbBr\({}_{3}\) was about 410 nm. The results of an EDX (Energy-Dispersive X-ray Spectroscopy) also showed a clear demarcation. Although the CsPbBr\({}_{3}\) was grown on the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\), the CsPbBr\({}_{3}\) film was composed of the grain's boundaries, as shown in the inset image. This indicates that every grain shows good contact with the others. As shown in Figure 1c, the XRD spectra exhibited a single \(\beta\)-crystal phase for the grown Ga\({}_{2}\)O\({}_{3}\). The peaks of 18.8\({}^{\circ}\), 38.5\({}^{\circ}\), and 59.6\({}^{\circ}\) could be assigned to the \((-201)\), \((-402)\), and \((-603)\) planes of the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\), respectively. This is consistent with our previous report that Mist-CVD is capable of growing single \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) crystals [26; 27]. Additionally, a part of the data from the XRD have been enlarged in Figure 1d. There are some peaks (around 15.6\({}^{\circ}\), 22.1\({}^{\circ}\), and 31.1\({}^{\circ}\)) which prove that the CsPbBr\({}_{3}\) film was successfully deposited on the surface. Moreover, the UV-vis spectra of the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film and the CsPbBr\({}_{3}\)/IZO/\(\beta\)-Ga\({}_{2}\)O\({}_{3}\) sample are demonstrated in Figure 1e. The light could transmit through the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film after about 250 nm. After 300 nm, the light transmission of the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film was around 80%. The bandgap of \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) can be calculated using the following formula [28; 29]:  
\[\mathrm{E_{g}}\approx 1240/\lambda\]  
where \(\lambda\) is the absorption cutoff edge of the light, and \(\mathrm{E_{g}}\) is the bandgap of the semiconductor material. The \(\mathrm{E_{g}}\) of the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) film is about 4.95 eV. The inset plot also exhibits this \(\mathrm{E_{g}}\). Because the bandgap of CsPbBr\({}_{3}\) is the smallest bandgap in structure of the CsPbBr\({}_{3}\)/IZO/\(\beta\)-Ga\({}_{2}\)O\({}_{3}\), the the CsPbBr\({}_{3}\)/IZO/\(\beta\)-Ga\({}_{2}\)O\({}_{3}\) sample shows transmittance after 500 nm. This is in line with the bandgap of CsPbBr\({}_{3}\), which has a value of about 2.40 eV. In these results, the CsPbBr\({}_{3}\) shows light absorption at the UV wavelength, but the \(\beta\)-Ga\({}_{2}\)O\({}_{3}\) cannot absorb light after 258 nm. Figure 2a demonstrates the dark currents of the Ga\({}_{2}\)O\({}_{3}\) PD and Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD. The Ga\({}_{2}\)O\({}_{3}\) PD reached a dark current of 7.00 \(\times\) 10\({}^{-11}\) A at a \(-\)5 V bias. However, after the successful deposition of the CsPbBr\({}_{3}\), the dark current of the Ga\({}_{2}\)O\({}_{3}\)/CsPbBr\({}_{3}\) heterojunction PD exhibited a small increase and achieved 2.56 \(\times\) 10\({}^{-10}\) A at a \(-\)5 V bias. This phenomenon is attributable to the introduction of defects with a polycrystalline-phase CsPbBr\({}_{3}\). Moreover, the CsPbBr\({}_{3}\) device was also prepared and achieved 2.09 \(\times\) 10\({}^{-8}\) A at a 5 V bias under 254 nm of illumination with a light intensity of 500 \(\upmu\)W/cm\({}^{2}\).