Carriers Dynamics Of 2D TMDs

2D materials,especially semiconducting transition metal dichalcogenides(TMDs),are attracting more interest among both scientists and engineers,due to their outstanding electrical,optical,mechanical and chemical properties.And with their unique tunable bandgap dependent on the numbers of layers and thin structure,they are considered promising candidates for next generation electronic and optoelectronic applications.Therefore,understand deeply the carrier dynamics in material,which is a base of designing new 2D TMDs application and improving performance of devices.Among all the TMDs materials,MoS2 have attracted the most attention and research.The study about its electrical carriers transport based on MoS2 field effect transistor(FET)has been reported.However,one of the factors potentially limiting the use of MoS2 for low-power applications is its relatively low phonon-limited mobility?200-400 cm2 V-1 s-1 at room temperature.Compared with MoS2,WS2 has a similar structure with a lower effective mass and the predicted room-temperature phonon-limited electron mobility in monolayer WS2 is over 1000 cm2 V-1 s-1,which is the highest among semiconducting TMDs.However,so far,the reported mobility of single-layer WS2-based FET is still low.Therefore,we firstly foucs on the carriers transport in single-layer WS2-based FET.We have demonstrated backgated WS2 field effect transistors by using electron beam lithography(ebl),e-beam evaporation of metal electrode,lift-of.We firstly compare electrical performance of two devices with different dielectric constant substrates,SiO2,Al2O3,which are fabricated by using atom layer deposition(ALD).We show that high-k substrate can effectively improve the mobility and cause metal-insulator transition(MIT),And with MPS chemical treat,the mobility can reach up to 83 cm2 V-1 s-1(room temperature)and 337 cm2 V-1 s-1(low temperature),and its threshold voltage of MIT decrease to 75v.We also use a theoretical model to analyze the data,which is only include two factors:charge traps and charge impurities(CI).The fitting result shows that combination of a thin layer of Al2O3 and thiol chemical functionalization can significantly improve the mobility of WS2 FETs by reducing the density of charge traps and CI.Furthermore,we also study the effect of high-k substrate by using pump-probe ultrafast spectrum technology.The CVD MoSe2 are transferred to other substrates with different dielectric constant:SiO2(K =3.9)?Al2O3(K = 10)?HfO2(k = 16.5).The result shows that the delay time ?3 corresponding to interband recombination process decrease with increasing dielectric constant,from 171 ps to 71 ps to 23 ps.And the same rule is also observed in different pump light wavelength test result.Overall,the carriers transport process and photo-induced carrier dynamics are always affected by interface effect of substrates and traps.Understanding these processes,is a basic question of improving devices' performance and designing new photoelectric devices.