Microstructural Analysis and Optical Linearity and Nonlinearity of Nanostructured Al─Doped ZnO/SnO2 Thin Films

Abstract

Al─doped ZnO/SnO₂ (Al─ZnO/SnO₂) thin films are prepared using spray pyrolysis followed by an investigation of their microstructural and optical properties. Unlike ZnO, Al─doped ZnO (Al─ZnO), SnO₂ and Al─doped SnO₂ (Al─SnO₂) films, which exhibited polycrystalline structures with distinct peaks, Al─ZnO/SnO₂ films displayed a single sharp peak, indicating strong preferential orientation along the ZnO (100) plane. Scanning electron microscopy revealed spherical aggregates of random polycrystals in ZnO and SnO₂ samples, while Al─ZnO/SnO₂ films have more pores/voids and various nanostructures, including nanorods growing parallel to the substrate. These nanorods provided 1D conductive pathways that closed the open-circuits created by the pores/voids, thereby improving electron transport. The refractive index (n) and extinction coefficient (k) are evaluated using the Cauchy normal dispersion model, and the obtained values are used to determine other linear and nonlinear optical parameters. Al─ZnO/SnO₂ films exhibited low n (≈1.45) and k (≈0) in the visible region, an enhanced band gap (≈3.8 eV), and low Urbach energy (≈84 meV), which minimized light scattering losses, resulting in high visible region transmittance (≈90%). The synergy between high transparency and improved electrical conductivity inferred from the enhanced microstructural and optoelectronic properties makes these films promising candidates for use as transparent conducting electrodes in optoelectronic devices.