Abstract
Time-dependent dielectric breakdown (TDDB) lifetime of ultrathin (1 nm) MgO in spin-transfer torque magnetoresistive random access memory (STT-MRAM) devices has recently been shown to be driven by factors other than voltage alone. This study focuses on the specific role of asymmetry in the current flow for different polarity pulsing modes of voltage stress on the TDDB lifetime of 1-nm MgO. Numerical analysis, based on a 3-D heat-diffusion equation and spintronic simulations, has been performed to characterize the temperature rise in the devices for precise correction of self-heating to obtain a correct interpretation of MgO TDDB. It is shown that the different lifetimes for the positive and negative modes can be attributed to different temperature increases arising from self-heating. While the positive and negative modes displayed a non-Arrhenius behavior, the bipolar mode showed an Arrhenius trend in which we observed a unique bimodal behavior of TDDB activation energy ( E-a) as a function of stress voltage in the ultrathin MgO stack. We discuss the role of additional driving forces, such as current, self-heating, charge trapping, and interface strain governing the breakdown mechanism along with the voltage effect.