Wake-up, Retention and Fatigue in Wurtzite Ferroelectric Films

Ferroelectric wurtzite films typically show large polarizations of 70 – 140 μC/cm², coupled with coercive fields of 2 – 6 MV/cm. These films are appealing for non-volatile memory elements in ferroelectric random access memories, ferroelectric tunnel junctions, and ferroelectric diodes. It has been found that Al_0.93B_0.07N and Zn_1-xMg_xO thin films often undergo a wakeup process associated with development of mobile interfaces on bipolar cycling. The number of cycles required for wakeup is a strong function of the switching frequency and field, with more cyles required for wakeup as the excitation frequency increases. The activation energies for wake-up typically exceed those for the activation energy for the coercive field of a fully woken-up sample. Both types of films showed excellent retention of the stored polarization state. For example, in opposite state (OS) measurements even after 3.6×10⁶ sec (1000 hr.) at 200°C, the OS signal margin still exceeded 200 μC/cm²in Al_0.93B_0.07N. The predicted OS retention is 82% after 10 years baking at 200^oC. Zn_1-xMg_xO is even more robust in opposite state retention. A key challenge is to increase the cycle lifetime by prevention of premature dielectric breakdown events.