Effect of natural oxidation conditions on low resistance spin tunnel junctions

Spin dependent tunnel junctions with AlOx barrier were fabricated by in situ natural oxidation of a 7 Å (or 8 Å) thick Al film. Junctions were deposited on top of preannealed, ion-beam smoothed, 600 Å thick Al bottom electrodes. By changing the oxidation time from 10 min to 10 h with the oxygen pressure of 10 and 100 Torr, the top-pinned junctions have resistances varying from 31 to 406 Ω μm2 and the tunneling magnetoresistance (TMR) varies from 8.5% to 19.6% for the as deposited state. For the bottom-pinned junctions, the oxygen pressure was varied from 0.5 to 100 Torr, and oxidation time ranged from 5 min to 2 h. Bottom-pinned junction resistances as low as 10–12 Ω μm2 were obtained with TMR ranging from 14% to 17% for the junctions oxidized at the lower pressure (0.5 Torr). Rutherford backscattering analysis (RBS) indicates an O/Al ratio of 1.29±0.34 denoting incomplete oxidation of the Al for junctions oxidized at the lower pressures and short times. Junctions oxidized at higher pressures (⩾10 Torr) can reach 25%–30% TMR, with resistances ranging from 30 to 70 Ω μm2. RBS shows near stoichiometric Al2O3 oxide composition (O/Al=1.51±0.43) in these barriers.