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¿°C. The resistivity reduced from 1.2¿·¿10-1 to 2.6¿·¿10-3 O¿·¿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 cm2/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