Physical properties and heterojunction device demonstration of aluminum-doped ZnO thin films synthesized at room ambient via sol-gel method

ZnO and some of its ternary wide-bandgap alloys offer interesting opportunities for designing materials with tunable band gaps, strong piezoresistivity and controlled electrical conductance with high optical transparency. Synthesizing these materials on arbitrary substrates using low-cost and unconventional techniques can help in integrating semiconductors with different physical, electrical, and optical characteristics on a single substrate for heterogeneous integration of multifunctional devices. Here we report the successful synthesis of aluminum (Al) doped ZnO (AZO) thin films on soda-lime glass, silicon and fluorine doped tin oxide (FTO) pre-coated glass substrates by using sol-gel deposition method at ambient condition. X-ray diffraction (XRD) analysis revealed that varying degree of Al doping significantly impacts the crystal orientation, semiconductor bandgap and optical transparency of the film. Crystal structure of the film is also found to be strongly correlated to the characteristics of the substrate material. The impact of heating rate during post annealing process is studied and optimized in order to improve the surface morphology of the deposited films. Optical characterizations have revealed that bandgap energy of AZO films can be tuned between 3.30 eV and 4.1 eV as the Al concentration is varied from 1% to 20%. Similarly, electrical characteristics of these films indicate that 1% AZO film has the lowest resistivity of 2.2 × 10 ^-2 Ω cm. Finally, 1% Al doped AZO thin film was used in fabricating a p-Si/n-AZO heterojunction, which exhibited good diode characteristics with more than five orders of magnitude rectification ratio and an ideality factor of 3.28.