Carrier Control And Device Applications Of Two Dimensional Transition Metal Dichalcogenides

Since the emergence of transition metal dichalcogenides?TMDs?in recent years,tremendous efforts have been involved to explore the interesting physical phenomena and potential electronic applications.Carrier control in semiconductors plays a crutial role for its foundamental research and device applications.Investigating the carrier control in TMDs materials will greatly benifit our understanding on two-dimensional materials.Here,we investigated the the carrier conrol in MoS₂,WSe₂ and ReS₂ with different techniques,and further explored their physical properties and device applications.At the beginning,we exfoliated mono-and bilayer MoS₂ onto degenerate doped Si wafers covered with 300 nm SiO2,and characterized the thickness as well as surface quality by Optical microscope,Raman spectra and atomic force microscope?AFM?.We performed the field effect measurements of both mono-and bilayer MoS₂ by utilizing the backgate.The current on/off ratio exceed 105,and the room temperature mobility is?20cm2 V-1s-1 for both mono-and bilayer MoS₂.We explored the transport properties of MoS₂?both mono-and bilayer?by using the powerful ionic liquid gating technique.With the high-density carriers induced into the channel by ionic liquid gating,an insulator-metal transition has been observed in monolayer MoS₂.While for bilayer MoS₂ under high carrier density,not only the metal-insulator transition,but also a superconducting transition emerged.In chapter four,we further investigated the superconductivity induced by ionic liquid gating in mono-and bilayer MoS₂.For bilayer MoS₂,the Berezinski-Kosterlitz-Thouless?BKT?transiton mode was used to study the superconducting behavior,which revealed the 2D nature of MoS₂ superconductivity.At low temperature,we found the vortices move through the sample by two modes,thermally activated flux flow?TAFF?and temperature-independent quantum tunneling?quantum creep?.Additionally,the critical temperature increased with increasing the ionic liquid gate bias?VLG? 10 V?.Superconductivity in monolayer MoS₂ was also achieved by accumulating carriers with ionic liquid gating.The critical carrier density for monolayer superconductiviting is?0.55 × 1014 cm-2,which is much smaller than bilayer case?1.23 x 1014 cm-2?.The ab initio calculations indicate that the gate-induced superconductivity for the doped monolayer MoS₂ originates from the phonon softening of low-frequency longitudinal and transverse acoustic phonon modes?LA and TA modes?at the K and M points.Our Raman spectra also suggests the existence of phonon softening when accumulating carriers into monolayer MoS₂.In chapter five,we investigated the Ta doped WSe₂ and the anisotropic field effect transistors based onReS₂.We achieved the Ta doped WSe₂ by chemical vapor transport?CVT?method,which is widely used in crystal growth.The few layer WSe₂ FETs show outstanding p-type behavior.The vertical stacked MoS₂/W0.99Ta0.01Se₂ heterojunction with an idieality factor of?1 and a rectification ratio of 105 was further demonstrated.The excellent performance in photodetection further illustrated the high quality of our p-n heterojunction.Next,we investigated the in-plane anisotropic properties of mono-and few-layer ReS₂.The mobilities variable at different crystal orientations result in different conduct behavior.Finally,we successfully demonstrated integrated digital inverters with good performance?the highest gain is?4.4?by utilizing two ReS₂ anisotropic field effect transistors.