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 Al2O3 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 approximate to 4.5 x 10(13) 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 Al2O3 layers is reduced to less than approximate to 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