The Functional Microstructure of Al/AlOx/Al Tunnel Junctions

Doctoral thesis, 2013

Josephson junction as a novel superconducting electronic component plays an important role in most small-scale superconducting systems. Particularly the Al/AlOx/Al tunnel junction, a high-performance junction made with a well-established microfabrication technology, is widely used as a building block of many quantum circuits. However, the performance of these devices is limited by unwanted coupling between the device and environment, decoherence and different types of noise. The origin of noise and decoherence in superconducting devices containing tunnel junctions has attracted a lot of attention but there is no concurrency on the subject. Many investigations have been carried out using electrical measurements and computer simulations. However, very little information is available about the detailed microstructure of the tunnel junctions and therefore the physical origin of noise is still unknown. Consequently, it is important to understand the microscopic nature of the tunnel junctions and correlate the structural information to electrically measured characteristics of the devices such as noise, RC (resistance capacity) products, sub gap current and decoherence times. The present work concerns the functional nanostructure of Al/AlOx/Al tunnel junctions and how it develops during the fabrication of the tunnel junctions. The unique aspect of this work is the ability to directly correlate the local structure to properties on the nanoscale and the close collaboration and interplay between the research groups fabricating the junctions, characterising the properties of the junctions and performing high resolution imaging and spectroscopy of the same individual junctions. The functional microstructure, i.e. the microstructural constituents that determine the properties, has been identified. Information is provided about the fine scale microstructure of the individual patterned and electrically characterised nanodevices. An important aspect is the control of the evolution of the microstructure in order to control the properties on a local scale and not only the average structure. Information about how to obtain a more uniform morphology and internal structure of the tunnel barriers is deduced from the studies. The high spatial resolution microstructural investigations have been performed using imaging and spectroscopy by scanning electron and transmission electron microscopy.