Electronic and optical properties of fluorine-doped tin oxide films

Thin films of fluorine-doped ${\mathrm{SnO}}_{\mathrm{2}}$ have been prepared by deposition on borosilicate glass using the spray-pyrolysis technique. The effect of doping on the concentration and mobility of the charge carriers (electrons) as well as the resistivity of the films has been studied. The undoped films had a resistivity of a few m Ω cm; this could be reduced by a factor of 10 by doping. The electron mobility in undoped films was about $\text{3\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/Vs}$ but could be improved by a factor of 5 to 6 by doping. The doping yield was about 2.3%. The high quality films which were deposited for photovoltaic applications had a sheet resistance of $R_{\square }\text{=2\hspace{0.17em}Ω/}\mathrm{sq}$ and an average transmittance, in the visible region, of $\text{T=85\%}$ for a thickness of 1.1 μm. Their figure of merit is one of the highest values reported: ${\mathrm{\phi =T}}^{10}{\mathrm{/R}}_{\square }\text{≈0.1\hspace{0.17em}}\mathrm{S}.$ The optical dispersion of our films can be explained perfectly by classical models. In the wavelength region of $\text{λ<0.580\hspace{0.17em}μ}\mathrm{m},$ the refractive index, $\mathrm{N,}$ for undoped and doped films can be given by ${\text{N=[1+λ}}^2{\text{/(0.370λ}}^2{\text{−0.0105)]}}^{\text{1/2}},$ where λ is in μm. From the study of dispersion and the plasma resonance frequency, the numerical values at optical frequencies of the dielectric constant, electron mobility, and electron effective mass were determined as 3.70, $\text{9.3–11.8\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/Vs},$ and ${\text{(0.26–0.45)m}}_0,$ respectively, where $m_0$ is the mass of free electrons. From the variation of direct and indirect optical transition energies with the carrier concentration, the density-of-states effective masses for electrons and holes were obtained as ${\text{0.85\hspace{0.17em}m}}_0$ and ${\text{0.78\hspace{0.17em}m}}_0,$ respectively. These studies revealed a direct energy bandgap of 4.11 eV for ${\mathrm{SnO}}_{\mathrm{2}}$ in addition to a defect band situated 0.45 eV above the valence band edge.