Tunnel barrier's property in magnetic tunnel junctions probed by Raman spectroscopy

By monitoring the intensity of the weak Raman peak from the Si substrate, the transmittance of the multi-layer structure could be deduced. Since the samples are identical, except for the thickness of the top aluminum layer, any difference in the intensity of the Si Raman peak is due to the difference in the transmittance of the top aluminum (oxide) layer. It was found that the intensity decreases monotonically with the aluminum layer thickness, which means that there is a substantial underoxidation. Furthermore, the intensity for the 1.6 nm sample is measurably lower than that for the 0.8 nm sample, which indicates that even the "optimized" 1.6 nm sample may have some degree of under-oxidation. This work demonstrates that Raman spectroscopy could be a useful tool in probing the oxidation state of the aluminum oxide tunnel barriers.