  Reversible resistive switching between high-resistance and low-resistance
states in metal-oxide-metal heterostructures makes them very interesting for
applications in random access memories. While recent experimental work has
shown that inserting a metallic "oxygen scavenger layer" between the positive
electrode and oxide improves device performance, the fundamental understanding
of how the scavenger layer modifies heterostructure properties is lacking. We
use density functional theory to calculate thermodynamic properties and
conductance of TiN/HfO$_2$/TiN heterostructures with and without Ta scavenger
layer. First, we show that Ta insertion lowers the formation energy of
low-resistance states. Second, while the Ta scavenger layer reduces the
Schottky barrier height in the high-resistance state by modifying the interface
charge at the oxide-electrode interface, the heterostructure maintains a high
resistance ratio between high- and low-resistance states. Finally, we show that
the low-bias conductance of device on-states becomes much less sensitive to the
spatial distribution of oxygen removed from the HfO$_2$ in the presence of the
Ta layer. By providing fundamental understanding of the observed improvements
with scavenger layers, we open a path to engineer interfaces with oxygen
scavenger layers to control and enhance device performance. In turn, this may
enable the realization of a non-volatile low-power memory technology with
concomitant reduction in energy consumption by consumer electronics and
significant benefits to society.
