Raman Studies on Polytypism in Layered Gallium Selenide

Gallium selenide (GaSe) is one of layered group-III metal monochalcogenides, which has an indirect bandgap of 3.0 eV in monolayer and a direct bandgap of 2.0 eV in bulk phase unlike other conventional transition metal dichalcogenides (TMDs) such as MoX₂ and WX₂ (X=S and Se). Since GaSe has high photo-responsivity and external quantum efficiency (EQE) in the UV-range, it can be used as a photodevice such as a photodetector [1]. In bulk phase, four polytypes designated as β-, ε-, γ-, and δ-GaSe have been reported. Since different polytypes result in different optical and electrical properties even for the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even for the same thickness due to GaSe polytypism. By comparing the ultra-low-frequency Raman spectra with theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the polytypes for trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.

[1] PingAn Hu et al., ACS nano 6 5988 (2012).