Improved conductivity of aluminum-doped ZnO: The effect of hydrogen diffusion from a hydrogenated amorphous silicon capping layer

Plasma-deposited aluminum-doped ZnO (ZnO:Al) demonstrated a resistivity gradient as function of the film thickness, extending up to about 600 nm. This gradient decreased sharply when the ZnO:Al was capped by a hydrogenated amorphous silicon layer (a-Si:H) and subsequently treated according to the solid phase crystallization (SPC) procedure at 600 degrees C. The resistivity reduced from 1.2.10(-1) to 2.6.10(-3) Omega.cm for a film thickness of 130 nm, while for thicker films the decrease in resistivity was less pronounced, i.e., a factor of 2 for a film thickness of 810 nm. While the carrier concentration was not affected, the mobility significantly increased from 7 to 30 cm(2)/V.s for the thick ZnO:Al layers. This increase was ascribed to the passivation of grain boundary defects by hydrogen, which diffused from the a-Si:H toward the ZnO:Al during the SPC procedure. The passivation effect was more pronounced in thinner ZnO:Al layers, characterized by a smaller grain size, due to the presence of large grain boundaries. For thicker films with grain sizes up to 200-300 nm the mobility became progressively less affected by the presence of grain boundaries. Therefore, the hydrogen-induced improvement in conductivity was less significant for the thick ZnO:Al films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692439]