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

Abstract

Tungsten (W) incorporated gallium oxide (Ga 2 O 3 ) (GWO) thin films were deposited by radio-frequency magnetron cosputtering of W-metal and Ga 2 O 3 -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 Ga 2 O 3 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 Ga 2 O 3 films, which crystallize in β-phase monoclinic structure. While GWO films also crystallize in monoclinic β-Ga 2 O 3 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., Ga 3+ . 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−Ga 2 O 3 intermixing at the interface is evident only in GWO samples with highest (12 at. %) Wincorporation. A model has been formulated to account for the mechanism of W-incorporation, thermal stability, and interdiffusion via pore formation in GWO films.

Oxygen Sensors Based on Thin Films of Gallium Oxide Modified with Silicon

“…The roughness of such films was 0.0783 microns. It should be noted that an increase in grain size was observed for thin films of Ga2O3 films obtained by RF magnetron sputtering and upon doping with Nb and W [8,9]. From our results, it follows that modifying of films with silicon promoted the formation of larger grains of Ga2O3.…”

Molybdenum Incorporation Induced Enhancement in the Mechanical Properties of Gallium Oxide Films

“…Efforts in this study are particularly directed to determine the Mo concentration in the GMO films. The Ga, Mo, and O core levels were used to follow the standard analytical procedures for the purpose. The chemical composition data GMO films are shown in Figure .…”

“…Furthermore, it has been reported and demonstrated in the literature that Ga 2 O 3 presents interesting physicochemical properties and device applications, which can be tuned by the presence of dopants. Improved catalytic activity for hydrogen production, reduction of carbon dioxide, and decomposition of organic compounds are some of the most important ones to mention . However, in all these applications, fundamental understanding of the physics and chemistry of Ga‐oxide‐based nanomaterials so as to provide a better control on the interplay between surface/interface structure, thermodynamic conditions, chemical processes, and kinetics is the key to achieving enhanced efficiency as well as control over the environmental pollutants.…”

Chemical composition tuning induced variable and enhanced dielectric properties of polycrystalline Ga2‐2xWxO3 ceramics

“…We paid utmost attention to the tailoring the optical band gap by customizing the deposition temperature and/or fabrication method for different cations in Ga 2 O 3 . Sustained efforts have proved a red shift in optical bandgap as reported for W‐incorporated β ‐Ga 2 O 3 39 and Ti‐incorporated β ‐Ga 2 O 3 40 polycrystalline thin films. Detailed structural analyses of Fe‐alloyed Ga 2 O 3 reveal the effect of Fe 3+ substitution in Ga 3+ in octahedral and tetrahedral positions owing to its closeness in Shannon ionic radii 43 .…”