Imaging oxygen vacancy dynamics in nanoscale ReRAM devices*

Non-volatile resistive memory (ReRAM) is considered a potential successor to flash memory, but important facts about the microscopic physics underlying this technology remain controversial. For instance, in valence change ReRAM (VCM), which is thought to switch through the motion of oxygen vacancies, it is not clear how the conducting path forms. We microfabricated clean, functional, transmission electron microscopy (TEM)-compatible Ti/HfO₂/Pt VCM devices and cycled them numerous times in situ. Imaging these devices with standard scanning TEM (STEM), we found that the current pathways are effectively invisible. However, employing STEM electron beam induced current (EBIC) imaging made the switching processes obvious. Our STEM EBIC system can be configured to achieve sensitivity to EBIC signals arising from traditional sources such as electric fields, as well from novel sources related to temperature, electrical potential, and secondary electron emission. Cycling the device between the ON and OFF states, we mapped the conducting path morphology at various stages with STEM EBIC.