A Study On The Fabrication Of Monolayer MoS_ 2 FET And Its Stability

Since the discovery of graphene,the study of two-dimensional materials has attracted the attention of many researches,in which the MoS₂-based micro-field effect transistors and optoelectronic devices have potential applications because of the unique band structure and optical properties of the MoS₂ material.Compared to graphene,MoS₂ is a natural semiconductor.The MoS₂ bulk has an indirect band gap of 1.29 eV,while the monolayer MoS₂ has a direct band gap of 1.8 eV.However,the carrier mobility in monolayer MoS₂ is much lower than that in graphene.So far a lot of attempts have been made to improve the performance of the MoS₂-based FET devices,including decreasing electric contact resistance with different metal electrodes,screening the carrier scattering from the impurity charges to increase the mobility with high-k cap-layer,increasing the carrier concentration with ion liquid electrolyte as gate dielectrics.However,the studies on the stability of the MoS₂ FETs and the effects of the surface adsorption in ambient atmosphere are rare.Due to the large surface volume ration of the MoS₂ monolayer,the surface adsorption easily occurs in ambient.In this thesis,we attempted to clarify the reasons for the deterioration of MoS₂-based FET in air by investigating the variation of source-drain current with time in different atmosphere.The main contents of the thesis are as follows.First,we optimized the fabrication process of the monolayer MoS2-based FET.We analyzed and solved some problems encountered in the fabrication process of the monolayer MoS2-based FET,such as detachment of MoS2 monolayer from the underlying substrate,current leakage of SiO2 insulating layer,and residue of the photoresist on the monolayer MoS2 surface.We succeeded in fabricating the nonolayer MoS2 FET with good performance.Our as-made FET is sensitive to the illumination.The carrier mobility calculated from the transfer curve is in accord with the literature.Next,we investigated the time evolution of the source-drain current of the monolayer MoS2 FET in vacuum,N2,O2 and air,respectively.It was found that surface adsorption of oxygen and water vapor greatly decreases the carrier mobility,which is dominantly responsible for the observed decrease in the source-drain current.In contrast,the carrier density is only slightly affected.That is,the adsorbed oxygen and water act as effective scattering centers,not effective carrier donors.At last,we studied the response of MoS2 FET to light illumination.We found that light illumination could promote the surface desorption of oxygen and water.