Structure And Transport Properties Of Two Dimensional GaGeTe Material

Two-dimensional（2D）material has become a research hotspot in recent years due to its atomically flat surface,rich material and physical properties,such as high mobility,high mechanical strength and good flexibility.GaGeTe 2D material has attracted much attention because of its p-type carrier conducting property,high carrier concentration and high mobility,where the carrier concentration is high to 10¹⁹cm^-3 and mobility is7.83×10³ cm² v^-1 s^-1.Theoretical calculations exhibit that when the thickness of GaGeTe decreases from bulk to few layers（FL）,the bandgap would widen and there undergoes a transition from semimetal to the semiconductor.However,experimental studies have shown that bulk GaGeTe is an indirect semiconductor with a bandgap of1.12 e V.There is an absence of systematic studies on the electronic band structure and thickness dependence of the structure and transport properties on 2D GaGeTe.Based on that,in this paper,GaGeTe flakes with different thicknesses were prepared.The structure and transport properties of GaGeTe material with different thicknesses are systematically studied.The main contents and results of this paper are as follows:1.Two-dimensional GaGeTe flakes with different thicknesses from 2.2 nm（bilayer）to 80 nm were exfoliated and transferred to Si O₂/Si substrates,which have a clean and flat surface.Phonon modes strongly dependent on the layer thickness,the evolution of the structure properties of GaGeTe with thickness can be divided into three sections,i.e.,the thickness is above 15.1 nm,between 15.1 and 10 nm,and below 10nm.GaGeTe with thickness above 15.1 nm behaves bulk properties.Characteristic Raman-active modes located at 98.4,132.7 and 147.5 cm^-1 for few-layer（when the thickness is below 10 nm）GaGeTe flakes are demonstrated,indicating unique 2D structure properties.While the GaGeTe flakes with thickness from 15.1 to 10 nm exhibit both bulk structure properties and 2D ones.The variable temperature Raman spectra shows that for GaGeTe of different thickness sections,the dependence of phonon frequencies on temperature are appropriate to different models,which further confirms the evolution behavior of the above structure properties with thickness.2.Study on transport properties of two-dimensional GaGeTe materials with different thickness.It is found that Ti and Cr couldn’t form ohmic contact with GaGeTe,while Pd could form good ohmic contact with GaGeTe material of different thicknesses.The conductivity of GaGeTe material is strongly dependent on the thickness.When the thickness is above 15.1 nm,the conductivity is independent on the thickness.However,when the thickness decreases from 15.1 nm to 10 nm,the electrical conductivity is31.26 S/cm for the 15.1 nm thick flakes,and then exponentially drops to 4.35×10^-7S/cm for the 10 nm thick ones.The exponentially decreasing conductivity with thickness maybe related to the widening of band gap when thickness decreases.This result was confirmed by Kelvin Probe Force microscopy（KPFM）testing the relationship between work function and layer thickness.In addition,the transport results also indicate that there occurs a transition from semimetal to semiconductor in GaGeTe when the thickness decreases.The GaGeTe homojunctions were constructed by GaGeTe 2D materials of different thickness.The obvious rectification characteristics further confirm the relationship between band gap and thickness,and also show that GaGeTe materials of different thickness show different transport behavior.3.Preparation,electrical properties and photoelectric properties of GaGeTe field effect transistor（FETs）.FETs with different thickness of GaGeTe were fabricated by micromachining technology.The results show that few-layer GaGeTe exhibits p-type carrier conducting properties,and its mobility and carrier concentration are dependent on thickness.The mobility and carrier concentration of GaGeTe semiconductor decrease with the decrease of layer number.When the thickness decreases from 14.5nm to 10 nm,the mobility of GaGeTe semiconductor decreases from 0.15 cm² v^-1 s^-1to4×10^-3 cm² v^-1 s^-1,and carrier concentration decreases from 10¹⁸ cm^-3 to 10¹⁵ cm^-3.The photoelectric measurements show that the 8.3 nm thick GaGeTe transistor has obvious response to light.When the optical wavelength changes from 520 nm to 940nm,the responsivity and detectivity decrease exponentially.Under the illumination of light with a wavelength of 520 nm,power intensity of 434 m W cm^-2,the R and D*values of the device are 15 m A W^-1 and 2.8×10¹⁰ Jones,respectively.The response timesτ_R andτ_F are 6.34 ms and 6.8 ms,respectively.