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Aluminium oxide (AlO$_\mathrm{x}$) tunnel junctions are important componentsin a range of nanoelectric devices including superconducting qubits where theycan be used as Josephson junctions. While many improvements in thereproducibility and reliability of qubits have been made possible through newcircuit designs, there are still knowledge gaps in the relevant materialsscience. A better understanding of how fabrication conditions affect thedensity, uniformity, and elemental composition of the oxide barrier may lead tothe development of lower noise and more reliable nanoelectronics and quantumcomputers. In this paper we use molecular dynamics to develop models ofAl-AlO$_\mathrm{x}$-Al junctions by iteratively growing the structures withsequential calculations. With this approach we can see how the surface oxidegrows and changes during the oxidation simulation. Dynamic processes such asthe evolution of a charge gradient across the oxide, the formation of holes inthe oxide layer, and changes between amorphous and semi-crystalline phases areobserved. Our results are widely in agreement with previous work includingreported oxide densities, self-limiting of the oxidation, and increasedcrystallinity as the simulation temperature is raised. The encapsulation of theoxide with metal evaporation is also studied atom by atom. Low density regionsat the metal-oxide interfaces are a common feature in the final junctionstructures which persists for different oxidation parameters, empiricalpotentials, and crystal orientations of the aluminium substrate.

Simulating the fabrication of aluminium oxide tunnel junctions