We have investigated high-quality MgO tunnel junctions with a range of barrier thickness in order to identify the underlying physical mechanism responsible for dielectric breakdown. Two types of dielectric breakdown (“soft” and “hard”) were observed. Soft breakdown was observed in a few percent of the devices. This breakdown mode is not intrinsic and is attributed to tunnel junction imperfections. The hard breakdown occurs because a critical electric field is reached across the tunnel barrier. Other possible breakdown mechanisms, such as thermally driven mass diffusion or charge trapping, were not consistent with the hard dielectric breakdown data and were ruled out.

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