Defect-Engineered TiO₂₋ₓ Channels for Vertically Stacked ECRAM Synaptic Arrays via In-Situ Atomic Layer Reduction

Jaehyeong Lee^a, Sangwon Lee^b, Hyeongjin Moon^a, Geongu Han^c, Jiwoo Eun^a, Hongju Kim^a, Jongchan Ryu^a, Jihwan An^b^,^d, Yun Seog Lee^a

^aDepartment of Mechanical Engineering, Seoul National University, Seoul, 08826 Republic of Korea

^bGraduate School of Semiconductor Technology, Pohang University of Science and Technology, Pohang, 37673, Republic of Korea

^cDepartment of Manufacturing Systems and Design Engineering, Seoul National University of Science and Technology, Seoul, 01811 Republic of Korea

^dDepartment of Mechanical Engineering, Pohang University of Science and Technology, Pohang, 37673 Republic of Korea

Proceedings of Neuronics Conference 2026 (Neuronics26)

Seoul, Korea, Republic of, 2026 September 8th - 10th

Organizers: Valeria Bragaglia and Seyoung Kim

Poster, Jaehyeong Lee, 030
Publication date: 24th July 2026

Electrochemical random-access memory (ECRAM) offers linear, symmetric, low-energy conductance modulation for analog in-memory computing, yet planar ECRAM density (~10 F²) and channel-length-limited switching remain barriers to scalable neuromorphic hardware. Vertically stacked ECRAM (VECRAM) addresses both: verticalization reduces the unit-device footprint to 4 F², and stacking N levels on a shared gate pillar further lowers it to 4 F²/N, while the short vertical channel accelerates ionic switching. Realizing this requires a channel process that is simultaneously conformal on high-aspect-ratio sidewalls and tunable in oxygen-vacancy (V₀) content — mandating an all-atomic-layer-deposition (ALD) active stack. However, ALD growth is intrinsically near-stoichiometric, which suppresses the analog programmability that stacking is meant to exploit.

We resolve this with an in-situ atomic layer reduction (ALR) process for PEALD TiO₂₋ₓ channels: a brief Ar-plasma reduction step (0–45 s) inserted after each oxidation half-cycle within an (ABC)ₘ supercycle, decoupling oxidation and reduction while preserving self-limiting growth. XPS shows the Ti³⁺/V₀ fraction rising monotonically with plasma exposure, and GIXRD/HR-TEM reveal a concurrent amorphous-to-nanocrystalline transition with local anatase A(101) order. Electrochemical impedance spectroscopy (EIS) confirms that the oxygen-ion migration barrier is minimized at Ar plasma 30 s exposure and re-increases with further Ar plasma exposure time.

This V₀ tuning directly controls synaptic behavior: first-pulse conductance overshoot and update nonlinearity (NL) are both minimized at the optimal ALR condition (NL reduced from ~0.45 to ~0.13), tracking the channel activation energy. In device-in-the-loop MNIST simulation, the measured pulse response improves the error rate from 37.6 % (near-stoichiometric) to 10.5 %, approaching the 3.75 % floating-point limit. Building on this channel, we demonstrate an all-ALD N = 2 stacked VECRAM primitive with conformal sidewall coverage, establishing in-situ oxygen-vacancy engineering as a route to combine analog linearity with the density of vertical stacking.

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