Microscopic mechanism of imprint in hafnium oxide-based ferroelectrics

Hafnia-based ferroelectrics have greatly revived the field of ferroelectric memory (FeRAM), but certain reliability issues must be satisfactorily resolved before they can be widely applied in commercial memories. In particular, the imprint phenomenon severely jeopardizes the read-out reliability in hafnia-based ferroelectric capacitors, but its origin remains unclear, which hinders the development of its recovery schemes. In this work, we have systematically investigated the imprint mechanism in TiN/Hf_0.5Zr_0.5O₂ (HZO)/TiN ferroelectric capacitors using experiments and first-principles calculations. It is shown that carrier injection-induced charged oxygen vacancies are at the heart of imprint in HZO, where other mechanisms such as domain pinning and dead layer are less important. An imprint model based on electron de-trapping from oxygen vacancy sites has been proposed that can satisfactorily explain several experimental facts such as the strong asymmetric imprint, leakage current variation, and so forth. Based on this model, an effective imprint recovery method has been proposed, which utilizes unipolar rather than bipolar voltage inputs. The remarkable recovery performances demonstrate the prospect of improved device reliability in hafnia-based FeRAM devices.