Polytypism in few-layer gallium selenide
Soo Yeon Lim, Jae-Ung Lee, Jung Hwa Kim, Liangbo Liang, Xiangru Kong, Thi Thanh Huong Nguyen, Zonghoon Lee, Sunglae Cho, Hyeonsik Cheong
Abstract
(X = S and Se). Four polytypes of bulk GaSe, designated as β-, ε-, γ-, and δ-GaSe, have been reported. Since different polytypes result in different optical and electrical properties even with the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We performed polarized Raman measurements on GaSe and found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even with the same thickness due to different stacking sequences of the polytypes. By comparing the ultra-low-frequency Raman spectra with the theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the stacking sequences of trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.