Improvement in structural, electrical, and optical properties of Al-doped ZnO nanolayers by sodium carbonate prepared via solgel method

Abstract

The significant improvement in crystallinity, electrical, and optical properties of aluminum-doped zinc oxide (AZO) thin films was attained by Na doping. Both Al and Na contents were introduced into ZnO host via facile and cost-effective solgel method. Ultra-thin films of Na-doped AZO (NAZO), with thickness of about 40 nm, were deposited on glass substrates via spin-coating technique. The obtained results showed low sodium incorporation reduces the crystal strain which in turn leads to increase crystallite size from 31 nm for AZO to 49 nm for 2 at% Na-doped AZO layer. In addition, 2 at.% of Na doping, that was determined as an optimum level of doping concentration, decreased the macroscopic surface roughness (MRs) without notable changes in surface micro-cracks, leading to remarkable enhancement of the layer conductivity (about 54%). On the other hand, high concentration of sodium content (10 at%) increased the crystal strain and reduced the crystallinity order of AZO layer which resulted in high MRs and also high electrical sheet resistance. Optical dispersion curve and real and imaginary parts of dielectric constant were obtained using our precise method. More interestingly, 2 at% of Na doping led to an increase in transparency of AZO layer in the visible range, while its extinction coefficient (light power absorption) was enhanced which can be regarded as advantage for such thin films. Therefore, the optimum Na content can provide higher optical absorption even with higher transparency by reducing the light scattering and reflection. This finding provides a useful procedure to prepare transparent oxide semiconductor layers with noticeable light absorption, which is very interesting for various applications such as solar cells and anti-reflection coatings.