Effect of annealing and applied bias on barrier shape in CoFe/MgO/CoFe tunnel junctions

Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions (MTJs) as a function of annealing and in situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen, and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the as-grown sample. In situ off-axis electron holography was employed to image the barrier potential profile of a MTJ directly, with the specimen under electrical bias. Varying the bias voltage from −1.5 to $+$1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface.

Figure 1

(a) Low-magnification TEM image of the MgO-based MTJ device multilayer structure. HREM images close to the barrier area of (b) as-grown and (c) annealed samples. The FTs of MgO layer are shown as insets in (b) and (c).

Figure 2

(a) TEM image of an in situ sample with five separate pillars. (b) TEM image showing the morphology of one pillar in contact with the Au probe during $I$-$V$ measurement.

Figure 3

Plots of current density versus voltage for the as-grown and annealed samples.

Figure 4

(a) Phase-shift image of the as-grown MTJ reconstructed from the hologram. (b) Line scan of the phase-shift profile averaged over 200 pixels parallel to the barrier layer. Profile corresponds to the boxed region in (a).

Figure 5

(a) Composite elemental of O (blue), Fe (green), and Co (red) obtained from EFTEM data; normalized intensity profiles for (b) Co, (c) Fe, and (d) O.

Figure 6

(a) Low-magnification TEM image of the areas used for in situ biased electron holography experiments. (b) Line scan of the phase-shift profile averaged over 200 pixels parallel to the barrier layer for different applied bias voltages of −1.5, 0, and $+$1.5 V taken from region A. The lower panel shows a schematic illustrating the barrier potential shape under (c) negative bias, (d) zero bias, and (e) positive bias.

Figure 7

The averaged phase-shift profile under different biased voltages of −1.5, 0, and $+$1.5 V taken from region B of Fig. 6a.