Strongly polarized narrow exciton lines from Ga$_2$Se$_3$/GaSe allotropic nanostructures

The ability to emit narrow exciton lines, preferably with a well-defined polarization, is one of the key conditions for the use of nanostructures based on III-VI monochalcogenides in nanophotonics and quantum technologies (for single photon emitters). At present, the main method of their formation is the exfoliation of layered crystals, supplemented by strain and defect engineering. The factor limiting the use of epitaxy is the presence of different polytypes and structural phases in the grown films. However, control over their formation can make it possible to create structures with desired properties. In this work, we propose a Ga$_2$Se$_3$/GaSe nanostructure fabricated by van der Waals epitaxy with a high VI/III flux ratio as a source of strongly polarized narrow lines of exciton emission. In fact, this nanostructure is formed from allotropes: GaSe and Ga$_2$Se$_3$, which consist of the same atoms in different arrangements. The energy position of the narrow lines is determined by the quantum confinement in ultrathin Ga$_2$Se$_3$ inclusions of different sizes in the GaSe matrix (barrier), and their linear polarization is due to the ordering of Ga vacancies in a certain crystal direction in Ga$_2$Se$_3$. Using transmission electron microscopy, Raman spectroscopy, and micro-photoluminescence, the morphology of Ga$_2$Se$_3$/GaSe nanostructures, the temperature and temporal behavior of the narrow lines have been established. Our results open the way to the creation of allotropic nanostructures capable, in particular, of providing narrow-line emission.