Compliance‑Free ZrO₂/ZrO₂₋ₓ/ZrO₂ Tri‑Layer Resistive Memory Enables Controllable Interfacial Multistate Switching

Background

Advances in non‑volatile memory demand higher density, speed, and lower power. Resistive random‑access memory (RRAM) based on metal oxides stands out for its scalability, CMOS compatibility, and low energy operation [3]. The dominant switching mechanism involves migration of oxygen vacancies under an electric field, leading to either filamentary or interfacial (homogeneous) conduction [4‑6]. Filamentary switching relies on conductive filaments that bridge electrodes, whereas interfacial switching is governed by the distribution of vacancies at an interface, producing a uniform current spread [5‑6]. The mode observed depends on the device architecture; an oxygen‑gradient profile along the vertical axis often favors interfacial behavior [7‑8], while recent studies demonstrate coexistence of both modes within a single material [9‑11].

Multistate storage—using intermediate‑resistance states (IRS) between the high‑resistance state (HRS) and low‑resistance state (LRS)—offers a promising route to increase storage density and enable neuromorphic synapses [19‑23]. Conventional multistate RRAM requires current compliance during the SET process to prevent device damage, complicating circuit design and raising fabrication costs [29‑30]. A compliance‑free architecture that still delivers multistate operation would thus represent a significant advance.

ZrO₂ is an attractive switching layer due to its thermodynamic stability, simple stoichiometry, and CMOS compatibility [31‑33]. Introducing an interfacial Zr layer to create a ZrO₂/ZrO₂₋ₓ/ZrO₂ heterostructure can enhance switching characteristics [34‑35]. Here we present such a tri‑layer device that exhibits both interfacial and filamentary switching, with a controllable transition between them, and demonstrates compliance‑free, multistate behavior.

Methods

Tri‑layer films (20 nm/5 nm/20 nm) were deposited by medium‑frequency plasma‑assisted magnetron sputtering (Leybold Optics HELIOS Pro XL) at room temperature. A rotating substrate (180 rpm) ensured uniformity. The Zr layer was first sputtered (2000 W, Ar) and subsequently oxidized in an O₂ plasma to form the stoichiometric ZrO₂ layers. By temporarily turning off the O₂ flow for 20 s, an oxygen‑deficient ZrO₂₋ₓ layer was created in the middle. Structural analysis used XRD (grazing incidence, θ = 1°), XPS (Al–Kα), and TEM (JEOL 2100/ARM200F) with ADF imaging and EDX mapping.

Device fabrication involved a TiN bottom electrode (200 nm, reactive sputtered in N₂), a patterned SiO₂ isolation layer, the tri‑layer switching stack, and a TiN top electrode (200 nm). Electrical testing employed a Keithley 4200 system; voltage sweeps were performed at 0.5 V s⁻¹ with the top electrode biased.

Results and Discussion

XRD of the as‑deposited tri‑layer shows peaks at 28.2° and 29.8°, corresponding to monoclinic and tetragonal ZrO₂ phases, confirming the coexistence of stoichiometric and oxygen‑deficient layers [34]. XPS depth profiling reveals a ZrO₂₋ₓ layer in the center, with TiOₓNᵧ interdiffusion at the TiN/ZrO₂ interface, which acts as an oxygen reservoir [38‑39]. TEM/ADF imaging confirms the layered structure and the presence of the interfacial TiOₓNᵧ layer.

Electrical measurements (Figure 4) show a two‑step forming process under a negative bias (I_CC = 1 mA), indicating sequential filament formation in the two outer ZrO₂ layers. The device then operates in the interfacial mode: the RESET current decreases gradually, and both HRS and LRS currents scale with electrode area, confirming non‑filamentary conduction [12]. The SET process exhibits self‑compliance, eliminating the need for external current limiting [29‑30].

Applying a more negative bias with 20 mA compliance triggers a second forming step (Figure 5a), creating a continuous filament across the tri‑layer. The device then switches in a filamentary mode, evidenced by sharp SET and RESET transitions and distinct resistance distributions (Figure 5b). Both modes follow the space‑charge‑limited current (SCLC) model with traps, but the filamentary LRS shows an additional Child’s law region absent in the interfacial mode, indicating trap‑mediated conduction [40‑43].

Comparative studies with a single‑layer 40 nm ZrO₂ device (Figure 6) confirm that the interfacial switching requires the embedded ZrO₂₋ₓ layer; the single‑layer device exhibits only filamentary behavior. The tri‑layer structure therefore enables the dual‑mode operation observed.

The switching mechanism is illustrated in Figure 7: during forming, oxygen ions move toward the bottom electrode, generating two weak filaments and a low‑resistance state. Positive bias pulls ions back, oxidizing the ZrO₂₋ₓ layer and raising resistance. The second forming step creates a strong filament that bypasses the ZrO₂₋ₓ layer, switching the device into filamentary mode.

In the interfacial regime, the device can store multiple resistance levels by varying the RESET voltage (Figure 8a). Cycling tests over 100 cycles (Figure 8b) show reproducible switching, underscoring the reliability of this multistate memory approach.

Conclusions

We demonstrate a ZrO₂/ZrO₂₋ₓ/ZrO₂ tri‑layer RRAM that transitions controllably between interfacial and filamentary modes. The embedded ZrO₂₋ₓ layer is essential for interfacial switching, while intrinsic series resistors from two weak filaments enable compliance‑free operation. By tuning the RESET voltage, the device achieves stable, multistate resistance, positioning it as a promising candidate for next‑generation high‑performance non‑volatile memory.

Abbreviations

ADF‑STEM:

Annular dark‑field scanning transmission electron microscopy

EDX:

Energy‑dispersive X‑ray spectroscopy

HRS:

High‑resistance state

IRS:

Intermediate‑resistance state

LRS:

Low‑resistance state

RRAM:

Resistive random‑access memory

SCLC:

Space charge limited current

XPS:

X‑ray photoelectron spectroscopy

XRD:

X‑ray diffraction