Deep electron traps in HfO2-based ferroelectrics: (Al/Si-doped) HfO2 versus HfZrO4

•Concentration of deep electron traps is in the range of 1019 cm−3 posing challenges for electrostatic control of HfO2- and HZO-based ferroelectric devices.•Trap energy distribution universally exhibit three characteristic optical energy levels: around 2 eV, 3 eV, and 4 eV below the oxide CB.•The observed traps are likely intrinsic to HfO2 lattice rather than related to dopants or ZrO2 sub-network.