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

TL;DR

This study investigates how different cap layer materials and MgO deposition techniques influence boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing, impacting their tunnel magneto resistance performance.

Contribution

It reveals that cap layer choice and MgO deposition method significantly control boron diffusion, which is crucial for optimizing tunnel magneto resistance in magnetic tunnel junctions.

Findings

Ta cap layers act as boron sinks during annealing.

Electron beam evaporated MgO contains less boron than RF sputtered MgO.

Boron and oxygen vacancies do not significantly affect TMR ratios at 200%."

Abstract

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%.