  Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous
CoFeB electrodes received much attention due to their high tunnel magneto
resistance ratio at room temperature. One important parameter for achieving
high tunnel magneto resistance ratios is to control the boron diffusion from
the electrodes especially during post growth annealing. By high resolution
transmission electron microscopy and electron energy loss spectroscopy
techniques we show that the cap layer material adjacent to the electrodes and
the MgO deposition method are crucial to control boron redistribution. It is
pointed out, that Ta cap layers acts as sinks for boron during annealing in
contrast to Ru layers. Furthermore, radio frequency sputtered MgO tunneling
barriers contain a rather high concentraion of boron in trigonal
[BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beam
evaporated MgO which is virtually free from any boron. Our data further
indicate that neither boron nor oxygen-vacancy-related gap states in the bulk
of MgO barriers affect spin polarized transport for tunnel magneto resistance
ratios at the level of 200%.
