Carrier generation in highly oriented WO3 films by proton or helium implantation

Implantation of ${\mathrm{H}}^{+}$ or ${\mathrm{He}}^{+}$ ions into highly oriented ${\mathrm{WO}}_3$ films, which were deposited on sapphire R-plane substrates by the pulsed laser deposition method, was carried out to generate carrier electrons. X-ray diffraction measurements revealed that the films consisted of only the two types of grains that were epitaxially grown with their c axes perpendicular to the surface of the substrate. After implantation to a dose of ${\text{1×10}}^{17}\hspace{0.17em}{\mathrm{cm}}^{\text{−2}},$ a remarkable increase in conductivity at 300 K was observed to 200 Scm⁻¹ for ${\mathrm{H}}^{+}$ implantation or to 3 Scm⁻¹ for ${\mathrm{He}}^{+}$ implantation. The efficiency of carrier generation was ∼60% for ${\mathrm{H}}^{+}$ implantation or ${\text{∼10}}^{\text{−3}}\%$ for ${\mathrm{He}}^{+}$ implantation. This striking difference demonstrates that the chemical effect of bronze formation is much more effective for carrier generation in ${\mathrm{WO}}_3$ than is the physical effect of oxygen vacancy formation by nuclear collision processes. The resulting conductivity of the oriented films was higher by 1–3 orders of magnitude than that of the corresponding polycrystalline film prepared by the rf sputtering method. Hall mobility in the oriented films (∼1 cm² V⁻¹ s⁻¹) was as high as that (1–2 cm² V⁻¹ s⁻¹) in single-crystal ${\mathrm{Na}}_{\text{0.40}}{\mathrm{WO}}_3$ or ${\mathrm{WO}}_{\text{2.99}}$ and was higher by several orders of magnitude than that in polycrystalline films. The results of Hall voltage and x-ray diffraction measurements led to the conclusion that the higher conductivity in the highly oriented films mainly originates from an increase in carrier mobility, which results from decreases in grain boundary effects as scattering centers.