Structural and electrical properties of atomic layer deposited Al-doped ZnO films

Structural and electrical properties of Al-doped ZnO (AZO) films deposited by atomic layer deposition (ALD) are investigated to study the extrinsic doping mechanism of a transparent conducting oxide. ALD-AZO films exhibit a unique layer-by-layer structure consisting of a ZnO matrix and Al₂O ₃ dopant layers, as determined by transmission electron microscopy analysis. In these layered AZO films, a single Al₂O₃ dopant layer deposited during one ALD cycle could provide ≈4.5 × 10¹³ cm^-2 free electrons to the ZnO. The effective field model for doping is suggested to explain the decrease in the carrier concentration of ALD-AZO films when the interval between the Al ₂O₃ layers is reduced to less than ≈2.6 nm (>3.4 at% Al). By correlating the electrical and structural properties, an extrinsic doping mechanism of ALD-AZO films is proposed in which the incorporated Al atoms take oxygen from the ZnO matrix and form doubly charged donors, such as oxygen vacancies or zinc interstitials. Layer-by-layer Al-doped ZnO films are deposited by atomic layer deposition to study the extrinsic doping mechanism of a transparent conducting oxide. A combined understanding of structural and electrical properties of these films based on an effective field model suggests that doubly charged donors, such as oxygen vacancies and zinc interstitials, would be formed by extrinsic doping of Al atoms.