Characterization of aluminum oxide tunnel barriers by combining   transport measurements and transmission electron microscope imaging
  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. 
T. Aref, A. Averin, S. van Dijken, A. Ferring, M. Koberidze, V. F.   Maisi, H. Nguyen, R. M. Nieminen, J. P. Pekola, L. D. Yao
cond-mat.mes-hall