Controlling boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing by variation of cap layer materials and MgO deposition methods

摘要

Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphousCoFeB electrodes received much attention due to their high tunnel magnetoresistance ratio at room temperature. One important parameter for achievinghigh tunnel magneto resistance ratios is to control the boron diffusion fromthe electrodes especially during post growth annealing. By high resolutiontransmission electron microscopy and electron energy loss spectroscopytechniques we show that the cap layer material adjacent to the electrodes andthe MgO deposition method are crucial to control boron redistribution. It ispointed out, that Ta cap layers acts as sinks for boron during annealing incontrast to Ru layers. Furthermore, radio frequency sputtered MgO tunnelingbarriers contain a rather high concentraion of boron in trigonal[BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beamevaporated MgO which is virtually free from any boron. Our data furtherindicate that neither boron nor oxygen-vacancy-related gap states in the bulkof MgO barriers affect spin polarized transport for tunnel magneto resistanceratios at the level of 200%.