The Functional Microstructure of Al/AlOx/Al Tunnel Junctions

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

A Method for Producing Site-Specific TEM Specimens from Low Contrast Materials with Nanometer Precision

A method that enables high precision extraction of transmission electron microscope (TEM) specimens in low contrast materials has been developed. The main idea behind this work is to produce high contrast markers on both sides of and close to the area of interest. The markers are filled during the depositing of the protective layer. The marker material can be of either Pt or C depending on which one gives the highest contrast. It is thereby possible to distinguish the location of the area of interest during focused ion beam (FIB) milling and ensure that the TEM sample is extracted precisely at the desired position. This method is generally applicable and enables FIB/scanning electron microscope users to make high quality TEM specimens from small features and low contrast materials without a need for special holders. We explain the details of this method and illustrate its potential by examples from three different types of materials

Nanoscale interaction layer at the interface between Al films and SiO2 substrates of Al/AlOx/Al Josephson tunnel junctions

An interaction layer is found at the Al/SiO2 interface in Al/AlOx/Al tunnel junctions grown on SiO2 substrates. The amorphous intermixing layer has an average thickness of about 5 nm. We present the detailed structure of this interfacial layer as determined by transmission electron microscopy. The layer contains alumina with aluminum being octahedrally coordinated according to electron energy loss spectroscopy analysis rather than tetrahedrally coordinated, where the latter coordination is the most common type in amorphous alumina. Depth profiles of the Al-O and Si-O bonding characteristics were also investigated using energy loss near edge structure

Direct observation of the thickness distribution of ultra thin AlOx barriers in Al/AlOx/Al Josephson junctions

We have directly measured the thickness distribution of the tunnel barriers in state-of-the-art Al/AlOx/Al tunnel junctions. From the distribution we can conclude that less than 10% of the junction area dominates the electron tunnelling. The barriers have been studied by transmission electron microscopy, specifically using atomic resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging. The direct observation of the local barrier thickness shows a Gaussian distribution of the barrier thickness variation along the junction, from ~1 to ~2nm. We have investigated how the thickness distribution varies with oxygen pressure (Po) and oxidation time (to) and we find, in agreement with resistance measurements, that an increased to has a larger impact on barrier thickness and its uniformity compared to an increased Po

The Functional Microstructure of Al/AlOx/Al Tunnel Junctions

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

Monetite Superstructures Displaying Mineral Bridges and Chirality via Biomimetic Mineralization in Lyotropic Liquid Crystalline Phases

Monetite (CaHPO4) superstructures having structural intricacy and chirality were formed using liquid crystals (LCs). The growth of monetite in LCs was shown to be self-organized via reaction-diffusion and responsive to physicochemical cues, allowing us to sculpture the subunits and introduce an additional structural hierarchy by simply adjusting the reaction parameters. We identify two growth modes for monetite subunits, kinetic-limited growth of plank and diffusion-limited growth of filaments, and show that the modulation of supersaturation (by tuning initial ionic strength or pH) can be decisive for the crystal growth: following one mode or oscillating between the two. Particularly, the filamentous structure connecting aligned subunits presented in monetite grown from lamellar (L-alpha) LC possessed multiple mineral bridges rendering longitudinal alignment of subunit crystals and added a unique example of self-organized pattern formation in crystal superstructure

The atomic details of the interfacial interaction between the bottom electrode of Al/AlOx/Al Josephson junctions and HF-treated Si substrates

The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. Anamorphous layer with a thickness of ~5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. Theobservations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions

Correlation between Al grain size, grain boundary grooves and local variations in oxide barrier thickness of Al/AlOx/Al tunnel junctions by transmission electron microscopy

A thickness variation of only one \uc5ngstr\uf6m makes a significant difference in the current through a tunnel junction due to the exponential thickness dependence of the current. It is thus important to achieve a uniform thickness along the barrier to enhance, for example, the sensitivity and speed of single electron transistors based on the tunnel junctions. Here, we have observed that grooves at Al grain boundaries are associated with a local increase of tunnel barrier thickness. The uniformity of the barrier thickness along the tunnel junction thus increases with increasing Al grain size. We have studied the effect of oxidation time, partial oxygen pressure and also temperature during film growth on the grain size. The implications are that the uniformity improves with higher temperature during film growth