Optimized HfO2/TiO2/HfO2 Trilayer RRAM: Electrode-Dependent Bipolar Switching and Enhanced Uniformity via Atomic Layer Deposition

Abstract

We fabricated HfO₂/TiO₂/HfO₂ trilayer resistive RAM (RRAM) on Pt‑ and TiN‑coated Si substrates with Pt top electrodes using atomic layer deposition (ALD). Both symmetric Pt/Pt and asymmetric Pt/TiN devices exhibit classic bipolar resistive switching. In the low‑resistance state (LRS) the conduction is Ohmic, while in the high‑resistance state (HRS) space‑charge‑limited current (SCLC) dominates. The choice of bottom electrode strongly influences the electroforming polarity, the ON/OFF resistance ratio, and the spread of set/reset voltages. Pt/TiN devices achieve a reduced negative forming voltage (−3.7 V), a narrower voltage distribution, and a modest ON/OFF ratio of 10². This behavior is attributed to the oxygen‑reactive TiN acting as an oxygen reservoir, which facilitates lower forming voltages and more uniform switching parameters.

Background

Resistive random‑access memory (RRAM) promises high density, low power, and non‑volatility, positioning it as a candidate to replace flash memory. Transition‑metal‑oxide‑based RRAMs are especially attractive because they are compatible with CMOS processing and can be deposited with atomic precision via ALD. The resistive switching (RS) in these devices is governed by oxygen‑vacancy filaments; however, uniformity of RS parameters (ON/OFF ratio, set/reset voltages, endurance) remains a challenge. Trilayer oxide stacks (e.g., Al₂O₃/HfO₂/Al₂O₃) have demonstrated improved uniformity due to interfacial engineering. Yet, the role of electrode material, especially the bottom electrode, on RS characteristics is not fully understood. This study explores how Pt versus TiN bottom electrodes affect the RS behavior of HfO₂/TiO₂/HfO₂ trilayers grown by ALD.

Methods

Two substrates were used: commercial Si/SiO₂/Ti/Pt and laboratory‑grown Si/SiO₂/TiN. TiN (30 nm) was deposited by plasma‑enhanced ALD at 400 °C using TiCl₄ and NH₃. On both bottom electrodes, 5 nm HfO₂/10 nm TiO₂/5 nm HfO₂ trilayers were grown at 250 °C by thermal ALD with TEMAH, TiCl₄, and H₂O precursors. A 100 nm Pt top electrode (150 µm diameter) was sputtered through a shadow mask. Characterization included AFM for surface roughness, XPS for chemical states and depth profiling, and electrical measurements with a Keithley 4200 under a 10 mA compliance. The top electrode was biased while the bottom electrode was grounded.

Results and Discussion

Device Structure and Morphology
AFM revealed RMS roughness of 0.39 nm for Pt and 0.87 nm for TiN. Correspondingly, the HfO₂/TiO₂/HfO₂ stack on Pt showed smoother topography (0.68 nm) versus 1.3 nm on TiN.

Electrical Switching
The symmetric Pt/Pt device required a +7 V forming step and exhibited a reset at −0.8 V and set at +2.0 V, yielding an ON/OFF ratio of ~10⁵. In contrast, the Pt/TiN device formed at −3.7 V, set at −1.5 V, and reset at +1.5 V, with a ratio of ~10². The lower forming voltage and tighter voltage spread in Pt/TiN arise from TiN’s higher oxygen affinity, which modulates vacancy distribution and acts as an oxygen reservoir.

Uniformity
Statistical analysis over 200 cycles showed that Pt/TiN devices had set voltages clustered between −0.8 and −1.8 V and reset voltages between +1.3 and +1.8 V, whereas Pt/Pt devices displayed a broader spread. Endurance tests revealed that Pt/Pt maintained >10⁵ OFF/ON ratio after 200 cycles, whereas Pt/TiN showed a reduced ratio (~10²) and greater HRS variability. Retention measurements up to 10⁴ s confirmed stable ON/OFF states for both stacks, projecting >10‑year retention.

Conduction Mechanism
Double‑log I–V plots confirm Ohmic behavior (slope ~1) in LRS and SCLC (slope ~2) in HRS up to a critical voltage. The transition to a steep current rise at ~6.8 V (Pt/Pt) or ~1.85 V (Pt/TiN) marks the filament formation threshold.

Oxygen Vacancy Distribution
XPS depth profiling indicated that the TiO₂ interlayer hosts the highest vacancy concentration (~34 %). The underlying HfO₂ near Pt had ~6 % vacancies, while that near TiN had ~9 %. The top HfO₂ exhibited only ~2 % vacancies. TiN’s chemical activity promotes a higher vacancy density in adjacent layers, which explains the lower forming voltage and enhanced uniformity in Pt/TiN devices.

Electrode‑Dependent Electroforming
In Pt/Pt stacks, positive forming yields a modest voltage (+7 V) due to easier oxygen release at the upper HfO₂. Negative forming requires a higher voltage (−11 V) because oxygen must migrate through the lower HfO₂, leading to cell degradation. In Pt/TiN stacks, negative forming at −3.7 V is feasible because TiN absorbs O²⁻ ions, mitigating O₂ release and acting as an oxygen reservoir. Positive forming at +14 V causes irreversible breakdown, as the accumulated filament cannot be ruptured.

Conclusions

HfO₂/TiO₂/HfO₂ trilayer RRAM fabricated by ALD demonstrates reliable bipolar switching. The bottom electrode critically governs electroforming polarity, ON/OFF ratio, and voltage dispersion. Pt/TiN devices benefit from a reduced forming voltage (−3.7 V) and tighter set/reset distribution due to TiN’s oxygen‑reservoir effect and higher vacancy density. These findings highlight the importance of electrode selection and trilayer engineering in designing next‑generation, uniform, and scalable RRAM devices.