High-performance dynamic random access memory capacitor with an equivalent oxide thickness of 0.31 nm <i>via</i> stepwise cycling in Y-doped Hf _0.5 Zr _0.5 O ₂ thin films

This study investigates the field-induced ferroelectric (FFE) characteristics and dynamic random access memory (DRAM) performance of Y-doped Hf_0.5Zr_0.5O₂ (Y:Hf_0.5Zr_0.5O₂) thin films grown by atomic layer deposition (ALD). Compared with undoped Hf_0.5Zr_0.5O₂, the Y:Hf_0.5Zr_0.5O₂ film exhibited suppressed ferroelectric orthorhombic phase and stabilized FFE tetragonal phase, resulting in double hysteresis loop characteristics in the polarization-electric field curves. These changes were attributed to substitutional diffusion of Y ions introduced by a single ALD cycle of Y₂O₃ inserted in the middle of the film. However, the onset field of the FFE effect from the pristine film was too high for DRAM application. To address this issue, a stepwise cycling method was proposed, consisting of an initial short high-field cycling step (6 MV cm^-1, 10⁵ cycles, 1 second) followed by subsequent cycles at gradually decreased field amplitudes (5 MV cm^-1, 10⁵ cycles, 1 second → 4 MV cm^-1, 10⁷ cycles, 100 seconds). This approach effectively shifted the FFE switching peaks toward lower fields, enabling charge boosting at low voltage (±0.8 V) while minimizing increases in remanent polarization and leakage current density (<i>J</i>). Consequently, the stepwise cycled 5.5-nm-thick Y:Hf_0.5Zr_0.5O₂ film achieved a high dielectric permittivity (<i>k</i>) of ∼68 and a record-low equivalent oxide thickness (EOT) of ∼0.31 nm among dielectric thin films satisfying the DRAM <i>J</i> criterion (<i>J</i> < 10^-7 A cm^-2 at 0.8 V). The substantial EOT reduction with stepwise cycling was enabled by the low-voltage charge-boosting effect, which enhanced the field-induced polarization response. These improvements were attributed to dopant-induced local structural inhomogeneity and effective redistribution of double positively charged oxygen vacancies. The EOT values were sustained with only slight degradation over 10⁹ cycles of 0.8 V operation and fully recovered after short high-field cycling.