Fabrication And Electrical Characteristics Ultra-short Channel MoS₂ Field Effect Transistors

Device scaling-down has long been the development tendency of semiconductor chip industry.However,conventional silicon-based transistors are approaching their physical limits after being scaled down for a long time.In this region,devices suffer from short channel effects that severely suppress devices performances.Consequently,looking for alternative materials has become a route to continue the device-scaling-down tendency.A potential solution is using recently discovered two-dimensional?2D?materials such as monolayer molybdenum disulfide?MoS₂?which is a one-atom-thick semiconductor with a direct band gap of 2.2 e V.With good electronic and optoelectronic properties and superior immunity to short channel effects,monolayer MoS₂ has become a promising material for future ultra-scaled electronics.In this thesis,we explored graphene contacted ultra-short channel monolayer field effect transistors?FETs?including the following two parts:1.Graphene/MoS₂ heterostructure fabrication.We have developed a simple and effective technique for 2D crystals transfer and fabricate graphene/MoS₂ heterostructure 2D heterostructures with clean interface.We pre-drop a right amount of Propylene Carbonate?PC?solution onto the substrate supporting a 2D materials?such as MoS₂?that is about to be transferred later.Heated to 80 ?C,the PC solution is solidified into a PC film which will act as carrier in the following transfer process.The 2D materials can be picked up from the substrates by peeling off the PC film.Then a 2D heterostructures can be formed by placing the PC carried 2D materials onto other 2D flakes?such as graphene?through our home-made transfer system with the alignment guided by optical microscope.2.Ultra-short channel MoS₂ FETs fabrication and electrical measurements.Graphene grain boundaries?GBs?can be accurately widened into few-nanometers-wide nanogaps using our previous developed technique of H2-plasma anisotropic etching on graphene.With nanoogapped graphene as contact electrodesand monolayer MoS₂ as channel materials,the channel lengths of the ultra-short channel devices can be defined as the widths of few-nanometers-wide graphene nanogaps.That is,the channel lengths of the graphene contacted ultra-short channel monolayer MoS₂ FETs can be aggressively scaled down to few nanometers.We fabricated graphene contacted monolayer MoS₂ FETs with channel lengths of 8 nm and 3.8 nm and performed the electrical measurements.The perfect linearity of their out-put curves indicates Ohmic contacts between graphene MoS₂ and revealing the superiority of graphene applied as contact electrodes in ultra-short channel MoS₂ FETs.For 8 nm device,the switching properties are quite well with an ON/OFF ratio of ¹06 and field effect mobility of 28cm2V-1s-1 which are comparable to those of long channel MoS₂ FETs,with no short channel effects exhibited.Although slight short channel effects tend to emerge with channel length shrunk down to 3.8nm,which is evidenced by the decreased ON/OFF of ¹05,the electrical properties are still acceptable for high performance transistors.In other words,MoS₂ FETs could possess respectable electrical properties while surpassing the 5-nm limit.These results indicate the great promise of MoS₂ in the future ultra-scaled electronics.