Thickness-dependent semimetal-to-semiconductor transition in two-dimensional GaGeTe

Two-dimensional GaGeTe flakes with different thicknesses from 80 to 2.2 nm (bilayer) were exfoliated and transferred to a SiO2/Si substrate. A series of samples with different thicknesses were prepared and identified by optical microscopy, atomic force microscopy, and Raman spectrum. Raman modes strongly dependent on the layer thickness and characteristic Raman-active modes for few-layer (FL) GaGeTe flakes are demonstrated. These vibration modes of FL GaGeTe show a linear red-shift phenomenon with increasing temperature and their full width at half maximum of the Raman mode exhibits a weak temperature dependence below 200 K, and then, a linear increase with temperature. The electrical conductivity is 96.48 S/cm for 74 nm flakes and drops exponentially to 2.27 × 10-7 S/cm for 7 nm ones because of the bandgap widening with the decrease of layer thickness, which is evidenced by the work function increase from 4.4 to 4.96 eV, when the thickness decreases from 80 to 2.2 nm. Moreover, the electrical conductivity performs two different temperature dependence behaviors on the thickness, indicating a transition from semimetal for bulk to semiconductor for FL GaGeTe, which agrees well with that of the theoretical calculation.