Morphotropic Phase Boundary of HfZr O Thin Films for Dynamic Random Access Memories.

The utilization of the morphotropic phase boundary (MPB) between the newly found ferroelectric orthorhombic phase and the tetragonal phase in an HfO-ZrO solid solution is suggested for a high-capacitance dielectric capacitor. Being different from other high- k dielectrics, where the k value decreases with decreasing film thickness, these films (Hf/Zr ratio = 6:4, 5:5, 3:7) showed increasing k values with decreasing film thicknesses in the ∼5-20 nm range. Among them, HfZrO and HfZrO films showed 47 and 43 peak k values at 6.5 and 9.2 nm thicknesses, respectively, suggesting the involvement of the MPB phenomenon. For the systematic understanding of this peculiar phenomenon, the phase evolution of the HfO-ZrO solid solution is presented based on experimental observations. The detailed electrical tests of the films with different compositions and thicknesses demonstrated that the characteristic feature of this material system is consistent with the involvement of the MPB depending on the composition and thickness. Through the optimization of the annealing process for crystallization, a 0.62 nm minimum equivalent oxide thickness was reported for the 6.5 nm thick HfZrO film, which is highly promising for the future dynamic random access memories. This work provided a breakthrough method for overcoming the fundamental limitation of a decreasing k value with a decreasing film thickness of other high- k dielectrics.