Dopant induced band gap broadening and conductivity enhancement of Al doped Zn_0.85Ca_0.15O thin films

Influence of Al doping on the properties of band gap engineered Ca²⁺ substituted ZnO thin films were investigated. Al doped Zn_0.85Ca_0.15O films were prepared on glass substrates by spray pyrolysis technique and obtained lower resistivity (~ 10⁻³ Ωcm) and widened band gap. Films were polycrystalline and showed hexagonal wurtzite structure irrespective of doping. Due to the textured growth, crystallinity first improved and then decreased. Elemental analysis confirmed the incorporation of Ca and Al in ZnO lattice. Optical transmission in the visible and near infrared region varied in accordance with crystallinity and degradation at higher doping level is related to the scattering of photons by various disorders and to free carrier absorption. Optical energy gap increased with doping concentration due to Burstein-Moss effect as evident from Hall measurements. At higher doping level, carrier concentration and mobility decreased due to the limit of solid solubility and ionized impurity scattering respectively, but energy gap increased due to high polycrstallinity.