During the last two years the main focus of development of ferroelectric HfO2 based materials is the detailed understanding of the ferroelectric properties in thin doped HfO2 layers. A variety of dopant materials (Si, Al, Ge, Y, Gd, La and Sr) were studied in addition to a mixed Hf1-xZrxO2. A different ionic radius, smaller or larger than Hf and different valency can impact the phase formation (Figure 1). 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 or oxygen concentration at the phase boundary between the monoclinic and the tetragonal/cubic phase and is enhanced through mechanical confinement. Scanning transmission electron microscopy and electron diffraction methods confirmed the structure. Continuous research is ongoing aimed to understand the root cause of this previously unknown phase. In particular, the effect of interplay between the influence of different dopants and the amount of oxygen vacancies or interstitials present in the layer on phase formation is under investigation. A detailed analysis of Lanthanum doping confirmed the interaction between both. In fact, it was shown that two three-valent Lanthanum dopant atoms within a HfO2 based layer introduced one oxygen vacancy. Surface and interface energy of these grains together with residual stress generated during growth and crystallization anneal can be additional critical parameters. Ab initio simulations by partners at the Munich UAS and University of Connecticut confirmed the influence of the above mentioned factors 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.
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 Lanthanum doped HfO2 with TiN electrodes. Piezo-response force microscopy (Oak Ridge Nat. Laboratory/Univ. Nebraska) in conjunction with transmission electron microscopy (North Carolina State University) measurements revealed domains within single grains with a diameter of ~20-30 nm for 10 nm thick films. The polycrystalline structure of the 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 and Curie temperature.
Future studies will focus on the structural basis of the ferroelectric properties and their impact on the ferroelectric switching behavior and how device cycling performance can be improved.
Dr. Uwe Schroeder
Fraunhofer IPMS-CNT, Dresden (Germany), RWTH Aachen (Germany), UAS Munich (Germany), GLOBALFOUNDRIES Dresden (Germany), IMEC (Belgium), Oak Ridge National Labs (USA), Dalin University (China), North Carolina State University Raleigh (USA), Tokyo Institute of Technology (Japan)