Study of boron diffusion in MgO in CoFeB∣MgO film stacks using parallel electron energy loss spectroscopy

Abstract

Boron diffusion in MgO has been investigated in annealed film stacks of sputtered CoFeB∣MgO using transmission electron microscopy and parallel electron energy loss spectroscopy. The analyses show significant B movement when the films are annealed, with the formation of BOx complexes. Characteristic diffusion lengths have been estimated in films annealed at the commonly employed temperature range of 300–400°C for the fabrication of magnetic tunnel junctions. An activation energy of 1.3eV (±0.4eV) has been extracted from these data that represent B diffusion in MgO through vacancies and defect states mediated by the formation of BOx complexes.

The role of Mg interface layer in MgO magnetic tunnel junctions with CoFe and CoFeB electrodes

“…[41][42][43] The boron diffusion into MgO in the other studies could be related to the CoFeB crystallization process during annealing, 44,45 but in our cases as-grown samples show highly textured crystallinity by reactive magnetron sputtering of a Mg target. 5,23 As a result of boron diffusion away from MgO in our samples, Co and Fe atoms become rich and oxidized at the interface, and TSP values decrease with thin Mg interface layer after annealing.…”

Boron diffusion in magnetic tunnel junctions with MgO (001) barriers and CoFeB electrodes

“…13 A ternary Mg-B-O phase could form in the presence of Mg + states in MgO but this would require a high density of oxygen vacancies. 15 Oxygen deficient MgO layers may absorb some boron, at the cost of lowering the barrier height. 8 An MgO barrier, which can absorb boron, could help to realize double MTJs with CoFeB middle layers.…”

Depth analysis of boron diffusion in MgO/CoFeB bilayer by x-ray photoelectron spectroscopy

“…11 However, to end up this discussion, further experiments are required. In any case, the large concentration of B species in MgO should introduce a high density of defect states in the MgO band gap 10 and influence the transport property of the CoFeB/MgO/CoFeB MTJs as recently evidenced in our work. 25 …”

“…[6][7][8][9][10][11][12][13][14][15][16] However, different conclusions came out from the experiments. Some groups reported that a considerable quantity of B diffuse into the MgO barrier after annealing, [6][7][8][9][10][11] while some others observed that boron tends to segregate at the MgO/CoFeB interface and form BO x after the crystallization of CoFeB layer. [12][13][14][15] Moreover, recently, the group of Miyajima et al 16 found that the crystallization of CoFeB layers was strongly dependent on the capping materials.…”