In this work, a novel fabrication technique is presented, which allows to construct topological insulator - superconductor (TI-SC) hybrid devices of high quality under UHV conditions. A stencil mask is applied to the substrate before growth of (Bi,Sb)2Te3 TI thin films by means of molecular beam epitaxy. The shadow mask is used for stencil lithography of superconductive electrodes on top of the topological thin films. Finally, the ready devices are covered with a capping layer to protect the surfaces of TI and S from oxidation at ambient conditions.
Besides the full device fabrication under UHV conditions, it is important to choose the right material combination. While superconductive aluminum (Al) electrodes on top of (Bi,Sb)2Te3 thin films yielded a low interface quality, measurements on (Bi,Sb)2Te3 films with niobium (Nb) electrodes, however, provided a high interface transparency. In radio frequency experiments a missing first Shapiro step was detected in our Nb - (Bi,Sb)2Te3 - Nb junctions, indicating signatures of Majorana bound states.
To exploit our stencil technique to more complex device layouts for proposed topological quantum computational applications, the mask was attached to pre-patterned Si substrates. This allowed us to combine selective area growth of topological nanostructures and stencil lithography of superconductive electrodes with nm precision, paving the way for fabrication of elaborated Majorana devices.