Causes of ferroelectricity in HfO2-based thin films: An ab initio perspective
We present a comprehensive first principles study of doped hafnia in order to understand the for- mation of the ferroelectric orthorhombic grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal grains can transform into orthorhombic 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 → orthorhombic transformation. This also holds for Al dop- ing. 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  → monoclinic  transformation, out-of-plane 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 expla- nation 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.