Ferroelectricity in Atomic Layer Deposited Hf_(1-x)Zr_xO_2 Nanoscale Films: Characterization by Synchrotron Grazing Incidence X-ray Diffraction and Polarization Measurements

摘要

Bulk hafnium oxide (HfO_2) is known to form a stable monoclinic phase at room temperature and pressure, tetragonal and cubic phases at elevated temperatures, and orthorhombic phase at higher pressures. Nano-crystalline HfO_2, due to surface energy effects, is also known to form a stable tetragonal phase at lower temperatures. The presence of a capping layer on nanoscale films could lead to a suppression of the monoclinic phase along with a significant increase in the dielectric constant. Doping the HfO_2 with other materials, such as Si, Zr, Al, La, Gd, Sr, and Y, has been shown to induce a ferroelectric orthorhombic phase in HfO_2. Theoretical studies by Materlik et al., predicts undoped HfO_2 to be ferroelectric when the grain sizes are less than 4 nm. In case of Hf_(1-x)Zr_xO_2 with x = 0.5, a slightly higher upper limit of the grain size (～ 5nm) is predicted as a requirement to show ferroelectric properties. In addition, the transition to a ferroelectric phase is expected to happen under asymmetric stress conditions in both HfO_2 and ZrO_2 nanoscale films. The well-developed process and integration schemes for HfO_2 as a gate dielectric material in semiconductor devices makes it an attractive candidate to study the formation and origin of the ferroelectric phase in detail. Ferroelectric HfO_2-based films have technological applications including ferroelectric field-effect-transistor (FeFET) memory and low power consumption FETs based on the use of ferroelectric negative differential capacitance.