  Gallium selenide (GaSe) is a novel two-dimensional material, which belongs to
the layered III-VIA semiconductors family and attracted interest recently as it
displays single-photon emitters at room temperature and strong optical
non-linearity. Nonetheless, few-layer GaSe is not stable under ambient
conditions and it tends to degrade over time. Here we combine atomic force
microscopy, Raman spectroscopy and optoelectronic measurements in
photodetectors based on thin GaSe to study its long-term stability. We found
that the GaSe flakes exposed to air tend to decompose forming firstly amorphous
selenium and Ga2Se3 and subsequently Ga2O3. While the first stage is
accompanied by an increase in photocurrent, in the second stage we observe a
decrease in photocurrent which leads to the final failure of GaSe
photodetectors. Additionally, we found that the encapsulation of the GaSe
photodetectors with hexagonal boron nitride (h-BN) can protect the GaSe from
degradation and can help to achieve long-term stability of the devices.
