Interaction of an Ultrathin Zinc Surface Passivation Layer with a Room Temperature-Deposited Al-Doped ZnO Film Leading to Highly Improved Electrical Transport Properties

In this study, we have demonstrated the interaction of an ultrathin Zn surface passivation layer with a room temperature (RT)-deposited Al-doped ZnO (AZO) film, leading to highly improved electrical transport properties. The resistivity of the AZO film increases monotonically with time in ambient conditions (from 0.02 to 0.33 Ω·cm), while overlaying a 4 nm Zn layer on AZO film stabilizes the resistivity to a value of 4.56 × 10^–3 Ω·cm, which decreases to a value of 2.40 × 10^–3 Ω·cm for a 5.3 nm Zn overlayer. The remarkable enhancement in the electrical stability and the conductivity value is attributed to a probable diffusion of Zn species into the AZO films, passivating Zn vacancy (V_Zn) and forming Zn interstitial (Zn_i), Zn_i-V_O donor complexes supported by RT Raman and X-ray photoelectron spectroscopy studies. Temperature-dependent resistivity measurement reveals the semiconducting behavior of the AZO films with 4 and 6 nm Zn overlayer, where the transport process is governed by thermally activated band conduction at and below RT (up to ∼247 K), followed by nearest-neighbor hopping and Mott variable range hopping mechanisms as the temperature goes down. The 5.3 nm Zn-coated AZO film shows metallic behavior at RT and a metal to semiconductor transition ∼220 K deviating from the Boltzmann conduction process due to electron–electron interaction phenomena.