  The quantum mechanical tunnelling process conserves the quantum properties of
the particle considered. As applied to solid-state tunnelling (SST), this
physical law was verified, within the field of spintronics, regarding the
electron spin in early experiments across Ge tunnel barriers, and in the 90s
across Al2O3 barriers. The conservation of the quantum parameter of orbital
occupancy, as grouped into electronic symmetries, was observed in the '00s
across MgO barriers, followed by SrTiO3 (STO). Barrier defects, such as oxygen
vacancies, partly conserve this electronic symmetry. In the solid-state, an
additional subtlety is the sign of the charge carrier: are holes or electrons
involved in transport? We demonstrate that SST across MgO magnetic tunnel
junctions (MTJs) with a large resistance-area (RA) product involves holes by
examining how shifting the MTJ's Fermi level alters the ensuing barrier heights
defined by the barrier's oxygen vacancies. In the process, we consolidate the
description of tunnel barrier heights induced by specific oxygen-vacancy
induced localized states. Our work opens prospects to understand the concurrent
observation of high TMR and spin transfer torque across MgO-based nanopillars.
