Hafnium Oxide Based Ferroelectric Materials

During the last two years the main focus in the project was on a detailed understanding of the ferroelectric properties in thin doped HfO₂ layers. A variety of dopant materials (Si, Al, Ge, Y, Gd, La and Sr, see Fig. 1) with a crystal radius ranging from 50 to 130 pm was studied in addition to a mixed Hf_1-xZr_xO₂. Deposition techniques included atomic layer deposition and physical vapor deposition.
The ferroelectric orthorhombic Pca2₁ phase of HfO₂ is formed when the material is crystallized with a certain dopant composition at the phase boundary between the monoclinic and the tetragonal/cubic phase and is enhanced when a mechanical confinement is used. Scanning transmission electron microscopy (STEM) and electron diffraction methods confirmed the structure. The polarization hysteresis for all dopants showed a maximum remanent polarization value between 15-40 μC/cm², depending on the dopant material. The highest values were obtained for Gadolinium doped HfO₂ with TaN and Lanthanum doped HfO₂ with TiN electrodes. Continuous research is ongoing to understand the root cause of this previously unknown phase.

Piezo-response force microscopy in conjunction with transmission electron microscopy (TEM: IFW Dresden and North Carolina State University) measurements revealed domains within single grains with a grain diameter of ~20-30 nm for 10 nm thick films (Fig. 2). Polycrystalline films caused a varying polarization orientation within the layer. The size distribution of the grains follows a Poisson distribution resulting in a grain size dependent coercive field und Curie temperature. Ab initio simulations by partners at the Munich UAS and University of Connecticut confirmed the influence of dopant concentration, film stress, surface energy of the grains, and oxygen vacancies on the phase stability of ferroelectric HfO₂. A qualitative model describing the influence of these basic parameters on the crystal structure of HfO₂ was proposed.

In addition, the influence of these parameters on the field cycling behavior was examined. This revealed the wake-up, internal bias field (Fig. 3), fatigue and retention effects in doped HfO2 are dominated by charge trapping effects at defects mainly located in the HfO₂/electrode interface, in addition to field induced phase transition. With the understanding of the basic parameters to form the ferroelectric phase of HfO₂ the typical switching characteristics of ferroelectric HfO₂ based non-volatile memory devices can be explained.
Future studies will focus on the structural basis of the ferroelectric properties and their impact on the ferroelectric switching behavior and how this cycling performance can be improved.

Contact: Dr. Uwe Schroeder