Gallium oxide (Ga₂O₃) is a wide bandgap (4.8 eV) semiconductor oxide with the advantages of high transparency and excellent chemical and thermal stability. Therefore, Ga₂O₃ thin film has a wide range of applications in metal oxide field effect transistors, photodetectors and so on. However, the large bandgap of Ga₂O₃ is unfavorable to the conductivity, which limits the application of Ga₂O₃ film in optoelectronic devices. The optical and electrical properties of Ga₂O₃ can be significantly improved by elemental doping, thereby enhancing device performance. The lattice deformation of Ti-doped Ga₂O₃ (TGO) is small due to the close matching of the Shannon ion radii (0.060 5 nm, 0.042 nm) of Ti⁴⁺ in octahedral and tetrahedral coordination with Ga³⁺ (0.062 nm, 0.047 nm). The reported plasma-enhanced atomic layer deposition at 120 ℃ for the preparation of TGO films requires four precursors: triethyl gallium, oxygen plasma, titanium tetraisopropoxide (TTIP) and H₂O. Using H2O as an oxidizer requires long purging times after water vapor exposure and brings in hydroxyl (-OH) impurities at deposition temperatures below 150 ℃. Compared with H₂O, O₃ has stronger oxidability and higher volatility and does not introduce impurities. In order to avoid the problems caused by using H₂O as precursor. TiO₂, Ga₂O₃ and TGO films are prepared by thermal atomic layer deposition using Trimethylgallium (TMG) and Tetrakis-dimethyl-amido Titanium (TDMAT) as precursor sources and O₃ as reaction gas at 250 ℃. The Ti-doped Ga₂O₃ concentration is adjusted by designing the Ga₂O₃/TiO₂ cycle ratio. TGO thin films form sandwich structure through different cycles (9, 6 and 3) of Ga₂O₃ and 1 cycle of TiO₂. The growth rates of Ga₂O₃ and TiO₂ measured by spectroscopic ellipsometry are 0.037 nm/cycle and 0.08 nm/cycle, respectively. The growth rate of TGO film is lower than the theoretical calculated value due to the delayed growth of Ga₂O₃ nucleation caused by the decrease of surface reactive site density after TiO₂ growth. The results of X-ray photoelectron spectroscopy show that the concentration of Ti in the film increases with the decrease of Ga₂O₃/TiO₂ cycle ratio, the binding energy of O 1s, Ga 2p and Ti 2p shifts to the lower, which is attributed to the replacement of some sites of Ga by Ti atoms, indicating that Ti elements are successfully doped into Ga₂O₃ films. The core level spectra of TiO₂ and Ga₂O₃ show the presence of Ti⁴⁺ and Ga³⁺ ions in the films. In the O 1s core level spectra of TGO films, Ga-O bonding decreases with increasing Ti-O bonding content, indicating the formation of Ga₂O₃-TiO₂ composites in the TGO films. The absence of diffraction peaks in the grazing incidence X-ray diffraction spectra indicates that the deposited Ga₂O₃ and TGO films are amorphous. The surface root mean square roughness of 0.377 nm is observed by atomic force microscopy, indicating that the surface of the film is flat and smooth. This is attributed to the layer-by-layer growth of atomic layer deposition with the advantage of atomic-level thickness control. The TGO films exhibit high transparency in the visible region and strongly absorb ultraviolet light. With the increase of Ti doping concentration, the refractive index of TGO films increases from 1.75 to 1.99 due to chemical changes, the transmittance decreases due to the increase of extinction coefficient in the ultraviolet region, the absorption edge appears red-shifted and the optical bandgap decreases from 4.9 eV to 4.3 eV. The reduced band gap of TGO films can extend the sensitive region of optoelectronic devices to longer wavelengths. The optical band gap of thin films measured by spectrophotometric and X-ray photoelectron spectroscopy shows consistent results. The comprehensive analysis shows that Ti doping has a significant impact on the optical properties of Ga₂O₃.