A photoconductance method has been used to study directly the barrier asymmetry in TaOx magnetic tunnel junctions. Due to optical electron-hole pair generation in the barrier itself and subsequent transport in the electric field, the sign and magnitude of the barrier asymmetry can be determined quite accurately. The reliability of the technique is demonstrated by the independence on the direction of illumination. The oxidation time where the asymmetry becomes zero is found to coincide with a maximum in the magnetoresistance ratio. This is argued to be due to the complete oxidation of the barrier material, resulting in a symmetric tunnel barrier.

A systematic study has been conducted on the dependence of the magnetoresistance ͑MR͒ ratio on the barrier thickness in TaO x -based magnetic tunnel junctions. The relatively low MR ratio ͑Ͻ10% ͒ for the thinnest barriers studied decreases rapidly with increasing thickness. From Rutherford backscattering analysis and current-voltage measurements evidence for a Ta thickness dependent oxidation rate has been found. Photoconductance spectra measured on the same junctions indicate changes in the hot-electron transport into the barrier, independent of the barrier asymmetry. These changes further indicate a modification of the junction structure with increasing Ta thickness, which can have a strong adverse effect on the spin polarization of the tunnel current.

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A very important process step in the fabrication of magnetic tunnel junctions (MTJs) is the application of a modest anneal step in the presence of a high magnetic field. Roughly, a doubling of the magnetoresistance (MR) ratio is commonly observed. We show that both AlOx as well as TaOx MTJs with Co90Fe10 electrodes have similar oxidation time and anneal temperature dependencies of the MR ratios. In both cases, the maximum MR ratio shifts to higher oxidation times with annealing. TaOx MTJs are, in this sense, good model systems. From photoconductance experiments we find that for TaOx MTJs, this shift in maximum MR is accompanied by a similar shift of the zero crossing of the oxidation time dependent barrier asymmetry. This directly supports the point of view that for obtaining the highest MR ratio one should anneal MTJs that would be characterized as “slightly overoxidized” in the as-deposited state. We argue that this result can be understood by a homogenization of the oxygen distribution in the barrier, and∕or a change of the bottom barrier-electrode interface.