Characterization of aluminum oxide tunnel barriers by combining transport measurements and transmission electron microscopy imaging

Abstract

We present two approaches for studying the uniformity of a tunnel barrier. The first approach is based on measuring single-electron and two-electron tunneling in a hybrid single-electron transistor. Our measurements indicate that the effective area of a conduction channel is about one order of magnitude larger than predicted by theoretical calculations. With the second method, transmission electron microscopy, we demonstrate that variations in the barrier thickness are a plausible explanation for the larger effective area and an enhancement of higher order tunneling processes.

Dependence of the scatter of the electrical properties on local non-uniformities of the tunnel barrier in Nb/Al-AlOx/Nb junctions

“…This distribution (black bars in Fig. 6) does not follow a Gaussian distribution unlike the results of the two recent studies, 30,31 which performed cross-section HRTEM analysis of thermally grown AlO x tunnel barrier. We believe that the reason for this dissimilarity is that the mentioned studies 30,31 analyzed the thickness distribution of great number of points in their samples, and also, the type of junctions was different (Al/AlO x /Al 30 and Cu/AlO x /Al 31 ) from our junctions.…”

Stability, sub-gap current, 1/f-noise, and elemental depth profiling of annealed Al:Mn-AlOX-Al normal metal-insulator-superconducting tunnel junctions

“…7. We do not have direct evidence what causes this increase, but point out that transmission electron microscopy results on annealing of magnetic tunnel junctions with AlOx barriers 42 have shown homogenization of the tunneling barrier, which could explain the increase as the thinnest barrier regions dominating conductance 40,43 are removed. For a number of both Al-AlO x -Al and Al:Mn-AlO x -Al junctions, we also studied the stabilization properties of the junctions after annealing, for some junctions for over five years, with some representative data shown in Fig.…”

“…It was also shown that for typical non-annealed junctions, the barrier fluctuations can increase the Andreev current by a large factor ∼ 60, based on the barrier thickness variations observed with TEM imaging. 43 Thus, if the barrier becomes more uniform after annealing as has been seen in some cases, 42 strong improvements in the suppression of Andreev-current are expected. …”

“…39 It was already demonstrated in Ref. 43 that due to the 1/R 2 T dependence, Andreev sub-gap current is sensitive to the fluctuations of the tunnel barrier properties. It was also shown that for typical non-annealed junctions, the barrier fluctuations can increase the Andreev current by a large factor ∼ 60, based on the barrier thickness variations observed with TEM imaging.…”

An Algorithm on Direct Tunneling Current Model Based on DIBL Effect

“…I gso and I gdo are parasitic leakage currents through gate-to-S/D extension overlap region; and I gc is the gate-to-inverted channel tunneling current. Part of I gc is collected by the source (I gs ) while the rest goes to the drain (I gd ) [11,12]. Considering overall leakage currents, the relation between direct tunneling current and dielectric thickness is studied in detail.…”