Raman scattering studies on very thin layers of gallium sulfide (GaS) as a function of sample thickness and temperature

Gallium sulfide is a semiconducting material with a layered structure and a characteristic low interlayer interaction. Because of weak van der Waals forces, GaS crystals are relatively easy to exfoliate to very thin layers. In this work nanometric-GaS layers were obtained by a micro-mechanical exfoliation process and were transferred to Si/SiO2 substrate. The thickness of these layers was estimated from AFM measurements. Raman spectra were collected for different layer thicknesses ranging from one layer to bulk crystal. An analytical function fitted to experimental data is proposed to determine layer thickness from Raman measurements. For the first time, the Raman position and the FWHM of the main Raman peaks were measured on very thin GaS layers as a function of temperature in the range from 80 to 470 K. The first order temperature coefficients of the A 1g Raman peaks were determined. Phonon decay due to anharmonic processes at temperatures above 300 K in layers of thickness below 4 nm was observed. Contribution of optical phonon scattering processes to thermal properties of very thin GaS layers is discussed.