Polytypism in Few-Layer Gallium Selenide

TL;DR

This study uses polarized Raman spectroscopy, theoretical calculations, and electron microscopy to identify and differentiate polytypes of few-layer gallium selenide, revealing stacking sequence effects on vibrational modes and stability.

Contribution

It introduces a method combining Raman spectroscopy and microscopy to determine polytype stacking sequences in GaSe, advancing understanding of their optical and electrical properties.

Findings

Different ultra-low-frequency Raman spectra correspond to distinct stacking sequences.

AB-type stacking is more stable than AA'-type stacking in GaSe.

Correlation established between Raman spectra and stacking sequences.

Abstract

Gallium selenide (GaSe) is one of layered group-III metal monochalcogenides, which has an indirect bandgap in monolayer and direct bandgap in bulk unlike other conventional transition metal dichalcogenides (TMDs) such as MoX2 and WX2 (X=S and Se). Four polytypes of bulk GaSe, designated as beta-, epsilon-, gamma-, and delta-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 performed polarized Raman measurement on GaSe and found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even for the same thickness due to different stacking sequences of the polytypes. 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 stacking sequences for trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.