2D small hole polarons in Ga₂O₃ nanolayers

It is well known that in wide-bandgap oxides, such as Ga₂O₃, holes localize and distort the lattice, forming small polarons. The presence of such polarons impacts several material properties (e.g., thermal conductivity, carrier transport, and optical emissions), so that measurement of these quantities can be used to show that polarons are present. However, these are all indirect measurements and to date, no direct observation of the lattice distortion associated with a polaron has been observed.

Using density functional theory at the hybrid functional level we show that even in nanolayers of Ga₂O₃ polarons can form. One of the hole polarons, localized on an O(III) atom, is of particular interest, as its associated lattice distortion is large: one Ga atom moves by 1.28 Å. It is metastable, but once formed, its lifetime should be sufficiently long to allow experimental observation. This polaron occurs both in the bulk and in nanolayers, even in the presence of large strains, indicating that it should be present in a wide range of experimental setups, such as ultrathin Ga₂O₃ layers supported by a substrate. Moreover, we show that it can occur near the surface, facilitating experimental observation.

In conclusion, our calculations provide a roadway for the first direct observation of polarons.