Causes of ferroelectricity in HfO₂-based thin films: an ab initio perspective

Abstract

We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal [100] → orthorhombic [001] transformation. This also holds for Al doping. We also show that for HfxZr1−xO2, where we have studied x = 1.00, 0.75, 0.50, 0.25, 0.00, the value x = 0.50 provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] → monoclinic [100] transformation, out-ofplane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an explanation for common experimental observations for the optimal ranges of doping in Si:HfO2, Al:HfO2 and HfxZr1−xO2. We also present model thin film heterostructure computations of Ir/HfO2/Ir stacks in order to isolate the interface effects, which we show to be significant. ferroelectricity in HfO 2 thin films"

Effect of Dopant Ordering on the Stability of Ferroelectric Hafnia

“…Let us emphasize our results for the 6.25% concentration, as they capture very well the main message of this work. In this case, the open circles in Figure a pertain to isolated‐impurity calculations as those typically reported in previous DFT studies . The corresponding formation energies largely reflect the energy differences between the pure polymorphs, marked by the colored arrows at in the figure; hence, the PE‐m state prevails.…”

Ferroelectric Properties and Polarization Fatigue of La:HfO2 Thin‐Film Capacitors

“…[ 4,5 ] In general, HfO 2 has three crystalline phases, that is, the room‐temperature monoclinic phase (m‐phase, space group: P 2 1 / c ) and the high‐temperature tetragonal (t‐phase, space group: P 4 2 / nmc ) and cubic (c‐phase, space group: Fm m ) phases. [ 6 ] But these phases are all centrosymmetric, forbidding ferroelectricity. The noncentrosymmetric ferroelectric phase is orthorhombic (o‐phase) with a space group of Pca 2 1 , in which the oxygen atoms shift along the c ‐axis, giving rise to spontaneous polarization.…”

Defect Engineering to Achieve Wake‐up Free HfO2‐Based Ferroelectrics

“…[ 2 ] These transition temperatures can be substantially altered by doping, mechanical stress, or surface manipulation and the high‐temperature phases are practically achieved in HfO 2 thin films at room temperature (RT). [ 3–14 ] A non‐centrosymmetric polar orthorhombic phase (Pca2 1 , o‐phase) is believed to be the structural origin of ferroelectricity in HfO 2 ‐based thin films. [ 2,15 ] Pca2 1 is extremely close in free energy (< k B T /5, where k B is the Boltzmann constant) to the equilibrium nonpolar phases.…”

“…The t‐ to o phase transition causes ≈1% decrease in volume. [ 8 ] Batra et al [ 5 ] predicted a transition from the nonpolar to polar Pca2 1 phase by applying a strong electric field combined with the application of an appropriate strain. Therefore, the electric field can provide an additional driving force to induce ferroelectricity in HfO 2 ‐based thin films.…”