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.
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