Dian Song

Proper oxidation for spin-dependent tunnel junctions

From wafers where the AlOx barrier varies from junction to junction, we determine the proper oxidation conditions for spin-dependent tunnel junctions. We obtain large variation in the resistance×area product (R×A) of the junctions within a wafer by either using nonuniform plasma oxidation on uniform Al films, or using uniform oxidation on Al films with a wedge thickness profile. When plotted against R×A, the tunneling magneto-resistance (TMR) for all junctions on the wafer falls on one curve that exhibits a broad maximum in the TMR. We propose that this maximum is where most metal Al has been oxidized while the oxidation of the bottom electrode is minimal. With annealing, we achieved our highest TMR, 38%, in highly resistive Co–AlOx–Co junctions. The most conductive junctions we made have about 18% TMR and R×A of 140 Ω μm2. They are made by natural oxidation on about 5 A of Al. For barriers thinner than 13 A Al, we start to lose TMR for junctions larger than 2 μm2. This is possibly caused by pinholes in t...