Tungsten Incorporation into Gallium Oxide: Crystal Structure, Surface and Interface Chemistry, Thermal Stability, and Interdiffusion

Tungsten (W) incorporated gallium oxide (Ga₂O₃) (GWO) thin films were deposited by radio-frequency magnetron cosputtering of W-metal and Ga₂O₃-ceramic targets. Films were produced by varying sputtering power applied to the W-target in order to achieve variable W-content (0–12 at. %) into Ga₂O₃ while substrate temperature was kept constant at 500 °C. Chemical composition, chemical valence states, microstructure, and crystal structure of as-deposited and annealed GWO films were evaluated as a function of W-content. The structural and chemical analyses indicate that the samples deposited without any W-incorporation are stoichiometric, nanocrystalline Ga₂O₃ films, which crystallize in β-phase monoclinic structure. While GWO films also crystallize in monoclinic β-Ga₂O₃ phase, W-incorporation induces surface amorphization as revealed by structural studies. The chemical valence state of Ga ions probed by X-ray photoelectron spectroscopic (XPS) analyses is characterized by the highest oxidation state, i.e., Ga³⁺. No changes in Ga chemical state are noted for variable W-incorporation in the range of 0–12 at. %. Rutherford backscattering spectrometry (RBS) analyses indicate the uniform distribution of W-content in the GWO films. However, XPS analyses indicate the formation of mixed valence states for W ions, which may be responsible for surface amorphization in GWO films. GWO films were stable up to 900 °C, at which point thermally induced secondary phase (W-oxide) formation was observed. A transition to mesoporous structure coupled with W interdiffusion occurs due to thermal annealing as derived from the chemical analyses at the GWO films’ surface as well as depth profiling toward the GWO–Si interface. Surface imaging analyses indicate thermally induced morphological changes are dependent on W-concentration in the GWO films. Thermally induced diffusion of W in the film is responsible for the observed formation of pores of variable size; the maximum pore radius noted was ∼27 nm for GWO films with highest W-content. The electronic charge redistribution appears to be dominated by the hydroxyl groups and W-chemistry as evident in XPS analyses. RBS data indicate that the extent of diffusion and intermixing layer depth are dependent on W-content in the GWO films. Thermally induced W-diffusion and depth penetration into the Si substrate with Si–W–Ga₂O₃ intermixing at the interface is evident only in GWO samples with highest (12 at. %) W-incorporation. A model has been formulated to account for the mechanism of W-incorporation, thermal stability, and interdiffusion via pore formation in GWO films.