Ultrathin Al₂O₃ Layers Enhance Mg‑Doped LiNbO₃ Film Reliability and Fatigue Endurance

Abstract

We fabricated bilayer capacitors comprising 5 % Mg‑doped LiNbO₃ single‑crystal films (200 nm) and ultrathin Al₂O₃ tunnel switch layers (2–6 nm) using ion slicing and atomic‑layer deposition. Transient domain‑switching current measurements show that the P‑V hysteresis loops become symmetric under a single‑pulse (type II) regime, thanks to a built‑in electric field produced by asymmetric Au/Pt electrodes and the compensating internal imprint field. The Al₂O₃ layer behaves as an ideal tunnel switch: it turns on during ferroelectric switching, providing series resistance that suppresses by‑electrode charge injection, and turns off afterward, acting as a high‑resistive dielectric. Consequently, the fatigue endurance of the Mg‑doped LiNbO₃ capacitors improves progressively with increasing Al₂O₃ thickness, reaching over 10⁴ cycles. These results offer a viable strategy to enhance the reliability of ferroelectric devices for non‑volatile memory applications.

Background

Lithium niobate (LiNbO₃) single‑crystal films are prized for their outstanding electro‑optic, acousto‑optic, and domain‑switching properties, enabling high‑frequency oscillators, modulators, and data‑storage devices. Wafer‑scale LiNbO₃‑on‑insulator (LNOI) has emerged as a platform for integrated photonics and memory, fabricated via ion implantation and wafer bonding. However, LiNbO₃ suffers from imprint hysteresis and limited fatigue endurance caused by interfacial passive layers and by‑electrode charge injection, which compromise polarization retention in non‑volatile memory applications.

Previous work has shown that inserting an ultrathin Al₂O₃ dielectric between the ferroelectric layer and the electrode can act as a tunnel switch: it conducts during polarization reversal and becomes insulating afterward, thereby blocking unwanted charge injection and improving device reliability. This study extends that concept to Mg‑doped LiNbO₃ films, which are known to exhibit reduced imprint due to Li‑site vacancies and oxygen‑related defects that trap space charge.

Methods

The 5 % Mg‑doped LiNbO₃ films were produced by ion slicing of bulk Z‑cut crystals followed by bonding to a SiO₂‑buffered substrate. A 5 nm Cr adhesion layer and 100 nm Pt bottom electrode were sputtered before bonding. After polishing to 200 nm, ultrathin Al₂O₃ layers (2–6 nm) were deposited by ALD (TFS‑200) using diethyl zinc and de‑ionized water at 200 °C. Au square top electrodes (1 × 10⁻⁴ cm²) were patterned by lift‑off. Film thicknesses were verified by spectroscopic ellipsometry; crystallinity and interface quality were assessed by X‑ray diffraction (Cu Kα) and cross‑sectional SEM.

Domain‑switching currents were recorded using an Agilent 8114A pulse generator (10 ns rise time) and a LeCroy 6054 oscilloscope, with the circuit resistance (50 Ω) and pulse width (1 µs) kept constant. Two pulse sequences were employed: (i) double pulses of opposite polarity with a 5 s interval (type I), and (ii) a single pulse superimposed on a negative DC bias (type II). The latter captures the full switching transient within a single pulse.

Results and Discussion

X‑ray diffraction confirms that the LiNbO₃ films retain a single‑phase rhombohedral structure, with no secondary phases. SEM cross‑sections reveal sharp interfaces between LN, Pt, Cr, and SiO₂.

Domain‑switching current transients exhibit a plateau whose height scales linearly with applied voltage, indicating a well‑defined coercive voltage (≈ 25 V) that is independent of the maximum applied voltage in Mg‑doped films. Unlike undoped LiNbO₃, the Mg‑doped samples show symmetric P‑V loops under type II pulses, attributed to the built‑in field from the Au/Pt electrode asymmetry that partially screens the depolarization field.

Introducing Al₂O₃ layers modifies the switching dynamics: the switching current plateau remains linear in voltage, but the initial resistance increases with Al₂O₃ thickness, reflecting the series resistance introduced by the tunnel switch. Interfacial capacitance remains nearly constant (~1.4 nF), confirming that Al₂O₃ behaves as a resistor during switching. Under non‑switching conditions, impedance measurements reveal a linear 1/Cᵗᵒᵗ versus Al₂O₃ thickness, confirming the dielectric nature of the Al₂O₃ layer (ε≈7.9) and a ferroelectric capacitance of ~14 pF.

Fatigue testing under 1 µs pulses with 0.5 s period shows that devices with 6 nm Al₂O₃ endure >10⁴ cycles with minimal loss of remnant polarization, whereas devices without Al₂O₃ fail earlier due to charge injection. The coercive voltage increases modestly with Al₂O₃ thickness (≈ 25 V to 34 V), a trade‑off that can be mitigated by optimizing ALD conditions to reduce defects.

Conclusions

We have demonstrated that ultrathin Al₂O₃ tunnel switch layers inserted between Mg‑doped LiNbO₃ films and Au electrodes yield symmetric P‑V hysteresis loops and significantly enhance fatigue endurance. The Al₂O₃ layer functions as a dynamic resistor during switching and as an insulating dielectric afterward, effectively suppressing by‑electrode charge injection. This bilayer architecture provides a practical route to robust ferroelectric memory devices with improved reliability.