The ferroelectric behavior of capacitors based on hafnium oxide dielectrics will be reported. Thin films of 7-30 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remanent polarization up to 25 μC/cm² and a high coercive field of about 1 MV/cm was observed. Doping of HfO₂ by different dopants with an atomic radius ranging from 110 pm (Si) to 188 pm (Gd) was evaluated and in all cases ferroelectric behavior was verified by polarization-voltage hysteresis, small signal capacitance-voltage characteristics, and for Si as a dopant also by piezoelectric measurements. Polarization retention and endurance was characterized on capacitor and transistor structures.