Exploring the direction-dependency of conductive filament formation and oxygen vacancy migration behaviors in HfO2-based RRAM

Oxygen vacancy (VO) defects play an essential role in governing the conductivity of semiconductor materials. The direction-dependency of oxygen vacancy conductive filament (CF) formation and VO migration behaviors in HfO2-based resistive random access memory (RRAM) were systematically investigated through first-principles calculations. The energetic and electronic structural analyses indicate that the continuous distribution of 3-fold oxygen vacancy (VO3) or 4-fold oxygen vacancy (VO4) is more favorable for the CF formation along [010] and [001] directions, and a continuous distribution between VO3 and VO4 in the m-HfO2 system can also combine to promote the formation of CFs along a particular direction. Furthermore, the high annealing temperature and low oxygen partial pressure (PO2) could effectively reduce the VO formation energy and promote the formation of CFs, resulting in a lower applied voltage of the devices. Our results indicate that q = 0 and q = +2 are the most probable charge states for VO3 and VO4 in m-HfO2. Subsequently, it is found that the low activation energy of VO originates from the +2q charged VO3 or VO4 migrating in the CFs along a particular crystallographic [001] direction. The diffusion coefficient (D) of the oxygen atom along the [001] direction is much higher than that of all the other possible pathways considered, due to the lower energy barrier. This demonstrates that the growth of CFs is potentially direction-dependent, and that a lower forming voltage and lower SET voltage are required when the CFs are grown along a particular direction in RRAM devices. The present work would help to provide a fundamental guide and new understanding for the development and application of HfO2-based RRAM.