  The ability to emit narrow exciton lines, preferably with a clearly defined
polarization, is one of the key conditions for the use of nanostructures based
on III-VI monochalcogenides and other layered crystals in quantum technology to
create non-classical light. Currently, the main method of their formation is
exfoliation followed by strain and defect engineering. A factor limiting the
use of epitaxy is the presence of different phases in the grown films. In this
work, we show that control over their formation makes it possible to create
structures with the desired properties. We propose Ga$_2$Se$_3$/GaSe
nanostructures by van der Waals epitaxy with a high VI/III flux ratio as a
source of narrow exciton lines. Actually, these nanostructures are a
combination of allotropes: GaSe and Ga$_2$Se$_3$, consisting of the same atoms
in different arrangements. The energy position of the narrow lines is
determined by the quantum confinement in Ga$_2$Se$_3$ inclusions of different
sizes in the GaSe matrix, similar to quantum dots, and their linear
polarization is due to the ordering of Ga vacancies in a certain crystalline
direction in Ga$_2$Se$_3$. Such nanostructures exhibit single-photon emission
with second-order correlation function $g^{(2)}(0)\sim$0.1 at 10 K that makes
them promising for quantum technologies.
