Characterization of hafnium oxide resistive memory layers deposited on copper by atomic layer deposition

Hafnium oxide-based resistive memory devices have been fabricated on copper bottom electrodes. The HfO_x active layers in these devices were deposited by atomic layer deposition (ALD) at 250 °C with tetrakis(dimethylamido)hafnium(IV) as the metal precursor and an O₂ plasma as the reactant. Depth profiles of the HfO_x by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a copper concentration on the order of five atomic percent throughout the HfO _x film. In addition to the Cu doped HfO_x, a thin layer (20 nm) of Cu_xO is present at the surface. This surface layer is believed to have formed during the ALD process, and greatly complicates the analysis of the switching mechanism. The resistive memory structures fabricated from the ALD HfO_x exhibited non-polar resistive switching, independent of the top metal electrode (Ni, Pt, Al, Au). Resistive switching current voltage (I-V) curves were analyzed using Schottky emission and ionic hopping models to gain insight into the physical mechanisms underpinning the device behavior. During the forming process it was determined that, at voltages in excess of 2.5 V, an ionic hopping model is in good agreement with the I-V data. The extracted ion hopping distance ∼ 4 Å was within the range of interatomic spacing of HfO₂ during the forming process consistent with ionic motion of Cu^{2 +} ions. Lastly the on state I-V data was dominated at larger voltages by Schottky emission with an estimated barrier height of ∼ 0.5 eV and a refractive index of 2.59. The consequence of the Schottky emission analysis indicates the on state resistance to be a product of a Pt/Cu₂O/Cu filament(s)/Cu₂O/Cu structure.