Comparison of HfOx-based resistive memory devices with crystalline and amorphous active layers

Previous investigators have demonstrated metal/HfOx/Cu devices with stable, long-term read/write endurance, low switching energy, and high on/off ratios[1,2]. To the best of the authors' knowledge there has been no published research that compares the resistive switching properties of crystalline and amorphous HfO
_x
. The HfO
_x
for this work was deposited on top of an electroplated copper thin film by physical vapor deposition using two different O
₂
partial pressures. Analyses by X-ray diffraction (XRD) show the two films differ structurally. Figure 1 (left) shows the XRD data for sample A. Peaks characteristic of crystalline HfO
_x
and the underlying Cu substrate are evident. Figure 1 (right) shows only peaks associated with the Cu substrate, indicating the HfO
_x
film in sample B is amorphous. X-ray photoelectron spectroscopy (XPS) depth profiles indicate varying stochimetry of HfO
_x
in sample A and a consistent sub-stochimetric oxide in sample B(not shown). Ni/HfO
_x
/Cu devices were fabricated from both 50 nm thick amorphous and crystalline HfO
_x
films. Devices from A and B exhibited consistent bipolar resistive switching. Both crystalline and amorphous devices exhibited average electroforming voltages of <;20 V and <;11 V, respectively. Both devices showed similar set and reset voltages, as seen in Figure 2. By varying the set current compliance from 10 μA to 10 mA and top electrode size, our data suggests there is a difference in the on-state conduction mechanism between the amorphous and crystalline HfO
_x
devices. Figure 3 shows that the on-state resistance decreases with the set current density for both 40 μm and 100 μm top electrode sizes for sample B. With higher set current densities, the on-state resistance is expected to decrease when a metallic filament is the physical on-state conduction mechanism[3]. By comparison, our preliminary data for sample A suggests the on-state resistance is independent of the current density (not shown). Peliminary endurance data (Figure 4) shows that the number of cycles before failure scales as a function of top electrode size, proprtionally for sample B and inversly for sample A. In summary, the structure of the HfO
_x
plays a role in determing the forming voltage, on-state resistance, and the device endurance.