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Amorphous gallium oxide thin films were prepared by the ultrasonic spray pyrolysis method using gallium acetylacetonate as source material and water as oxidizer. Samples annealed at 850°C during 1 h show the crystalline ␤-phase of Ga 2 O 3 . Rutherford backscattering results indicate that both as-deposited and annealed films have the stoichiometric chemical composition without incorporation of carbon impurities. Infrared ͑IR͒ spectroscopic measurements show that there is no incorporation of O-H and Ga-OH radicals in any of the studied films. The IR spectra for amorphous films show a broad absorption band from 400 to 900 cm Ϫ1, typical for some amorphous metallic oxides. Meanwhile, for the annealed films the IR spectra show well-defined peaks located at 450 and 670 cm Ϫ1 related to the ␤-phase of Ga 2 O 3 . The refractive index of the films shows a strong change from 1.846 for the amorphous films to 1.935 for the annealed ones. The optical bandgap energy values are 4.94 eV for the as-deposited films and 4.99 eV for the annealed films. All these changes are associated with a different microstructure of the annealed films. Gallium oxide can have several crystalline phases, such as ␣, ␤, ␥, , and type. Among them, the ␤-Ga 2 O 3 modification, monoclinic, is the only phase which is thermally and chemically stable at temperatures up to its melting point ͑1800°C͒.1 This phase is normally an insulating material at room temperature with a bandgap energy of about 4.9 eV. Due to its optical and electrical properties, gallium oxide has been applied in metal-insulator-semiconductor structures.2 This material shows a semiconducting behavior at temperatures higher than 500°C. The n-type semiconducting property is associated with a slight oxygen deficiency in the crystal lattice. The top of the valence band of ␤-Ga 2 O 3 , which contains two crystallographically different Ga atoms, 3 is formed by nonbonding 2p oxygen orbitals, while the bottom of the conduction band is predominantly constituted of 4s octahedral gallium orbitals, without contribution of atomic orbitals from tetrahedral gallium ions. 4 On the other hand, when gallium oxide is prepared under reducing atmosphere it behaves like an n-type semiconductor due to the resulting gallium excess or oxygen deficiency in the crystalline structure. 5Electrons in the introduced donor states are responsible for the blue luminescence observed in this phase, which is characterized by strong electron-phonon coupling. 6 Recently a renewed interest in ␤-Ga 2 O 3 has arisen for applications as transparent conducting contact in optoelectronic devices, 7 oxygen sensors at high temperatures (Х900°C), and reducing gas sensors at relatively low temperatures (Ͻ700°C). 8 The use of stable metallic oxides as gas sensors at high temperatures has several advantages, such as short response time and simple conduction mechanism. 9Gallium oxide thin films have been prepared by high-frequency sputtering, electron-beam evaporation, chemical vapor deposition, and atomic layer epitaxy ͑ALE͒.10-13 In ge...

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