Structural and Optical Characteristics of Gallium Oxide Thin Films Deposited by Ultrasonic Spray Pyrolysis

Abstract

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...

Light-emitting Ga-oxide nanocrystals in glass: a new paradigm for low-cost and robust UV-to-visible solar-blind converters and UV emitters

“…The reported E G values in Ga 2 O 3 phases, mainly from measurements of the optical absorption edge of thin lms, lie in the range from 4.2 to 5.1 eV. [43][44][45][46][47][48][49][50][51] Such a spread of values is related to the variability of oxygen stoichiometry in Ga-oxides, the larger the oxygen de-ciency, the smaller the effective bandgap. 32 Additional spread of E G values can be expected in NCs, in principle, from quantum connement (QC).…”

FPLAPW study of the structural, electronic, and optical properties of Ga2O3: Monoclinic and hexagonal phases

“…We would like to note that the experimental values of the band gap of Ga 2 O 3 lie in the range from 4.23 to 4.99 eV for thin films [6,11,[27][28][29]33,39], and from 4.6 to 5.1 eV for single crystals [4,7]. In Figs It has been pointed out before that some covalent bonding character between Ga and O may exist [24].…”

“…On the theoretical side, there are relatively fewer theoretical studies [21][22][23][24][25][26] on Ga 2 O 3 specially in its ␣ phase, in spite of the availability of numerous experimental studies [6,7,11,[27][28][29][30][31][32][33][34][35][36][37][38][39]. The electronic structure and optical properties of ␤-Ga 2 O 3 has been calculated by Kenji Yamaguchi using the full-potential linearized augmented plane wave (FPLAPW) [23].…”

The role of the oxide shell in the chemical functionalization of plasmonic gallium nanoparticles

“…The formation of the Ga 2 O 3 shell is therefore essential for the actual plasmonic response and stability of Ga NPs, but little attention has been paid to it so far. Ga 2 O 3 is a semiconductor whose wide band gap enables its use as transparent semiconducting oxide because of its low absorbance in a wide spectral range from the middle UV to the middle IR 10 . Its most stable crystalline phase is monoclinic β-Ga 2 O 3 , which has a low enthalpy of formation compared to other metal oxides (-1089.1 kJ/mol) 11 .…”