image: (a) Schematic illustration of the integration challenges in compute/memory/storage systems, highlighting the importance of BEOL-safe thermal budget (≤ 350°C) for embedded non-volatile memory. The proposed strategy employs precisely controlled in-situ La doping, achieving robust ferroelectricity, high remanent polarization (2Pr), and low operation field (Eop) under low-temperature annealing, as compared to conventional HZO. (b) Conceptual comparison between pure HZO and La-doped HZO devices.
Credit: Science China Press / Authors
Hafnium-based FE materials have attracted particular interest owing to their low power consumption, strong compatibility with complementary metal-oxide-semiconductor (CMOS) processes and proven scalability to below 3 nm. However, when the thermal budget is reduced, conventional HZO films exhibit a reduced FE‐phase fraction, leading to increased operating voltage (Vop), diminished remanent polarization (Pr), slower switching speed and suppressed reliability. These drawbacks limit their large-scale and high-density integration. Although several approaches have been reported to enhance ferroelectricity and reliability at reduced thermal budgets, challenges remain in simultaneously achieving low-voltage operation, strong ferroelectricity, and back-end-of-line (BEOL) compatibility.
In a new study, researchers report an in-situ lanthanum (La) doping strategy that introduces La during atomic layer deposition (ALD) with atomic-layer precision. By programming the ALD pulse sequence of La2O3, HfO2, and ZrO2, the team prepared lanthanum-doped Hf0.5Zr0.5O2 (La: HZO) films with controlled La concentrations while keeping the film thickness at 10 nm.
The approach enables ferroelectric switching under markedly reduced annealing temperatures. With 0.44% La doping, the La: HZO capacitors show pronounced ferroelectric hysteresis after annealing below 300°C, achieving a 2Pr of 27.8 μC/cm2. Under the annealing temperature of 350℃, the optimized capacitors further demonstrate 2.0 V operation, strong ferroelectricity with 2Pr of 37.5 μC/cm2, and fast switching down to 446 ns.
Reliability metrics also improve. The La: HZO capacitors exhibit a record-high breakdown voltage of 5.73 V, endurance exceeding 1011 cycles without hard breakdown, and substantial retention. These combined improvements address a key challenge for integrating HfO2-based ferroelectrics into advanced technology nodes, where low-voltage write and low thermal budgets must be satisfied simultaneously.
To understand why La helps at low thermal budgets, the team performed first-principles calculations and correlated them with structural and electrical characterization. The calculations indicate that La incorporation promotes oxygen-vacancy formation, which is associated with stabilizing the ferroelectric orthorhombic phase and enhancing ferroelectricity in La: HZO. Together, the experimental benchmarks and mechanistic insights provide guidance for designing low-power, high-speed embedded ferroelectric memories compatible with the BEOL process.