Oxidation process of AlOx-based magnetic tunnel junctions studied by photoconductance

Abstract

The oxidation process of Co/AlOx/Co magnetic tunnel junctions has been investigated by photoconductance, in addition to traditional transport measurements. The shape of the photoconductance curves is explained within the framework of a simple qualitative model, assuming an oxidation time dependent imbalance of the incident forward and reverse hot electron fluxes, as well as inelastic scattering processes in the oxide. Due to the large sensitivity of the technique, the presence of unoxidized Al beneath the barrier layer can be monitored very accurately. The disappearance of a negative contribution to the photocurrent indicates the complete oxidation of the barrier layer, which coincides with the maximum magnetoresistance. From a Fowler analysis, the barrier height is determined as a function of oxidation time. The observed disagreement of the effective barrier heights determined by this technique and those found by Simmons fits demonstrates the added value of photoconductance studies.

Barrier thickness dependence of the magnetoresistance in TaOx magnetic tunnel junctions

“…The latter process can possibly be affected by an asymmetric barrier profile, as previously shown for underoxidized AlO x MTJs. 5 For high ͑h Ͼ 4.2 eV͒ photon energies the sign of the additional photocurrent is completely determined by the barrier asymmetry, 22 which is given in Fig. 1͑c͒.…”

“…For each Ta thickness, a monotonic increase with oxidation time is observed, similar to that commonly observed for AlO x . 5,19 This is related to the fact that for longer oxidation times, more and more oxygen is incorporated in the barrier, improving the insulating properties of the junction. However, for the same oxidation time the R ϫ A product decreases with increasing Ta thickness.…”

“…5 By using a barrier material with a much lower bandgap, a reduced barrier height might be expected. 11 Alternatively it might be of extrinsic origin, related to a lower structural quality of the tunnel barrier, e.g., caused by defects or impurities.…”

Change of the barrier potential shape in magnetic tunnel junctions due to an anneal treatment

“…8,9 From these earlier studies, photoconductance seems therefore a suitable technique to directly study the influence of an anneal treatment in more detail.…”

“…Further details of the used measurement schemes can be found elsewhere. 8,9 Subsequently, the sample is subjected to a next anneal step, at a temperature that is 25°C higher than during the previous anneal. To investigate the role of the oxygen distribution during annealing in more detail, the junctions are studied both as a function of anneal temperature as well as a function of the oxidation time.…”