Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique

Al doped SnO2 thin films have been synthesized by a sol-gel dip coating technique with different percentages of Al on glass and silicon substrates. X-ray diffraction studies confirmed the proper phase formation in the films and atomic percentage of aluminium doping in the films was obtained by energy dispersive X-ray studies. SEM studies showed the particle sizes lying in the range 100-150 nm for the undoped films and it decreased with increase of Al doping. Optical transmittance spectra of the films showed high transparency (similar to 80%) in the visible region and the transparency increases with the increase of Al doping in the films. The direct allowed bandgap of the films have been measured for different Al concentration and they lie within the range of 3.87-4.21 eV. FTIR studies depicted the presence of Sn-O, Al-O, bonding within the films. The room temperature electrical conductivities of the films are obtained in the range of 0.21 S cm(-1) to 1.36 S cm(-1) for variation of Al doping in the films 2.31-18.56%. Room temperature Seebeck coefficients, S-RT of the films were found in the range +56.0 mu VK-1 to -23.3 mu VK-1 for variation of Al doping in the films 18.56-8.16%. It is observed that the Seebeck coefficient changes its sign at 12.05% of Al in the films indicating that below 12.05% of Al doping, SnO2:Al behaves as an n-type material and above this percentage it is a p-type material.