In collaboration with the Laboratory of Molecular Beam Epitaxy (MBE) at the Faculty of Physics at the University of Warsaw, our research on one-dimensional nanostructures, dubbed nanowires, has led to the creation of a novel type of nanowires. They consist of a hexagonal gallium arsenide core (occurring only in nanowires, in contrast to the cubic structure of GaAs bulk crystals and epitaxial layers) and a topological crystalline insulator shell of lead tin telluride. In our paper published in Scientific Reports (2024), we demonstrated that combining these two specific compounds in nanoscale structures resolves the issue of crystal lattice mismatch between the materials.
Gallium arsenide is a crystal semiconductor used e.g. in the production of transistors for telecommunication and solar panels. In our nanowires it has a visible hexagonal wurtzite crystal structure in contrast to the regular structure found in bulk crystals and epitaxial layers. We have used a topological insulator for the shell due to their unique physical characteristics – while having insulating properties in the bulk, their outer layer behaves as an electrical conductor.
Nanowires are special due to their high length-to-diameter ratio. This extremely thin and elongated form helps to minimize the misfit stress between their core and shell materials, which can considerably reduce the occurrence of defects. Significantly, one of the unique physical characteristics of the core-shell interface is the possibility to modify the properties of one material by the other, which allows to choose physical parameters appropriately for next-generation electronic and optoelectronic devices, various kinds of detectors and sensors, or even – quantum logic systems.
In our collaboration, we have grown hybrid nanostructures with topological crystalline insulator – (Pb,Sn)Te solid solution deposited on the sidewalls of hexagonal wurtzite (wz) GaAs core. In the case of thin layers, GaAs crystallizes in the cubic, zinc-blende structure. Such nanowires grow along [0001] direction of the hexagonal crystalline lattice. They have six sidewalls formed by {11-20} or {1-100} planes, depending on the MBE growth conditions.
In our paper published in Scientific Reports vol. 14, article No. 589 (2024) we have shown that the combination of (Pb,Sn)Te and wz-GaAs in the nanoscale heterostructures reduces the problem of the substantial lattice mismatch of both materials. The arrangement of materials proposed by us is analogous to layered, planar zb-(Pb,Sn)Te/wz-GaAs structures, where the high lattice mismatch in the range 10-12%, (depending on the chemical composition of (Pb,Sn)Te) inhibits the formation of defect-free heterostructures. In our case the lattice mismatch along the nanowire axis is reduced to about 2-4%. In case of planar structures the differences between the lattice parameters in two different directions, which can have different values, have to be taken into account. Considering the lattice mismatch in the case of core-shell nanowires the sidewalls of the core play the role of the “substrate” for the shell material. Consequently, due to the high aspect ratio of the nanowires the “widths” of the sidewalls are much reduced in comparison to their “heights” (measured along the nanowire axis). Hence the problem of minimizing the misfit stress in the nanowires is much less essential than in heterolayers. We have also shown that depending on the orientation of the wz-GaAs nanowire sidewalls two distinct orientations of (Pb,Sn)Te shells can be obtained: {100} or {110}. However, theoretically predicted topological states on the latter surface have not been experimentally observed yet.
Our results demonstrate the potential of one-dimensional nanoscale heterostructures which cannot be grown as planar layers, open up new possibilities, in our case – the multiplication of surfaces of topological crystalline insulator with topologically protected surface states. Nanowires are also an excellent research material for investigations of topologically protected one-dimensional hinge states at the edges of their sidewalls.
