Impedance spectroscopic analysis on effects of partial oxidation of TiN bottom electrode and microstructure of amorphous and crystalline HfO₂ thin films on their bipolar resistive switching

The electrical resistance switching (RS) properties of amorphous HfO ₂ (a-HfO₂) and crystalline HfO₂ (c-HfO ₂) thin films grown on a TiN substrate via atomic layer deposition were examined using DC current-voltage (I-V) sweep and AC impedance spectroscopic (IS) analyses. The rapid thermal annealing of the a-HfO ₂ film at 500 °C under a N₂ atmosphere for 5 min crystallized the HfO₂ film and induced an interfacial TiON barrier layer. The a-HfO₂ sample showed fluent bipolar RS performance with a high on/off ratio (∼500), whereas the c-HfO₂ sample showed a much lower on/off ratio (<∼10), but its switching uniformity was better than that of a-HfO₂. Such critical differences can be mainly attributed to the absence and presence of the TiON barrier layer in the a-HfO₂ and c-HfO₂ samples, respectively. The AC IS especially enabled the resistance states of the HfO₂/Pt interface and the HfO ₂/TiN interface to be separately examined during one complete switching cycle of each sample. Although the Pt/c-HfO₂ interface has a Schottky barrier in the pristine state, it disappeared once the c-HfO ₂ was electroformed and was not recovered even after the reset step. In contrast, the Pt/a-HfO₂ interface partly recovered the Schottky barrier after the reset.