For the miniaturization of dynamic random-access memory (DRAM), it is necessary to obtain low equivalent oxide thickness (EOT) and low leakage capacitor materials with complementary metal-oxide semiconductor (CMOS) process compatibility. Herein, we explore the influence of the dose of H2O reactants on the phase structure and electrical characteristics of Hf0.3Zr0.7O2 (HZO) thin films during the atomic layer deposition (ALD). By controlling the H2O dose as well as other conditions including inserting alumina, adjusting annealing temperature and electric field cycling, a high dielectric constant of ∼45 (EOT ∼ 0.54 nm) and low leakage current density (<7.7 × 10−8 A/cm2 at ±0.5 V) are achieved in the TiN/Al2O3 (∼0.3 nm)/Hf0.3Zr0.7O2 (∼6 nm)/TiN capacitor near the morphotropic phase boundary (MPB). Furthermore, the dielectric constant remains above 40 with endurance >1012 cycles (even extrapolated to 1015 cycles under voltage pulse of 0.5 V @10 MHz) at both room temperature and 85 °C. These results are helpful for achieving a high dielectric constant and low leakage for future generation DRAM capacitor materials.