Transition Metal Dichalcogenide Based Memory Devices and Transistors
thesisposted on 16.08.2019, 17:19 by Feng Zhang
Silicon based semiconductor technology is facing more and more challenges to continue the Moore's law due to its fundamental scaling limitations. To continue the pace of progress of device performance for both logic and memory devices, researchers are exploring new low-dimensional materials, e.g. nanowire, nanotube, graphene and hexagonal boron nitride. Transition metal dichalcogenides (TMDs) are attracted considerable attention due their atomically thin nature and proper bandgap at the initial study. Recently, more and more interesting properties are found in these materials, which will bring out more potential usefulness for electronic applications. Competing with the silicon device performance is not the only goal in the potential path finding of beyond silicon. Low-dimensional materials may have other outstanding performances as an alternative materials in many application realms.
This thesis explores the potential of TMD based devices in memory and logic applications. For the memory application, TMD based vertical devices are fully studied. Two-terminal vertical transition metal dichalcogenide (TMD) based memory selectors were firstly built and characterized, exhibiting better overall performance compared with some traditional selectors. Polymorphism is one of unique properties in TMD materials. 2D phase engineering in TMDs attracted great attention. While electric switching between semiconductor phase to metallic phase is the most desirable. In this thesis, electric field induced structural transition in MoTe2 and Mo1-xWxTe2 is firstly presented. Reproducible bipolar resistive random access (RRAM) behavior is observed in MoTe2 and Mo1-xWxTe2 based vertical devices. Direct confirmation of a phase transition from a 2H semiconductor to a distorted 2Hd metallic phase was obtained after applying an electric field. Set voltage is changed with flake thickness, and switching speed is less than 5 ns. Different from conventional RRAM devices based on ionic migration, the MoTe2-based RRAMs offer intrinsically better reliability and control. In comparison to phase change memory (PCM)-based devices that operate based on a change between an amorphous and a crystalline structure, our MoTe2-based RRAM devices allow faster switching due to a transition between two crystalline states. Moreover, utilization of atomically thin 2D materials allows for aggressive scaling and high-performance flexible electronics applications. Both of the studies shine lights on the new application in the memory field with two-dimensional materials.
For the logic application, the ultra thin body nature of TMDs allows for more aggressive scaling compared with bulk material - silicon. Two aspects of scaling properties in TMD based devices are discussed, channel length scaling and channel width scaling. A tunability of short channel effects in MoS2 field effect transistor (FET) is reported. The electrical performance of MoS2 flakes is governed by an unexpected dependence on the effective body thickness of the device which in turn depends on the amount of intercalated water molecules that exist in the layered structure. In particular, we observe that the doping stage of a MoS2 FET strongly depends on the environment (air/vacuum). For the channel width scaling, the impact of edge states in three types of TMDs, metallic Td-phase WTe2 as well as semiconducting 2H-phase MoTe2 and MoS2 were explored, by patterning thin flakes into ribbons with varying channel widths. No obvious charge depletion at the edges is observed for any of these three materials, which is different from what has been observed in graphene nanoribbon devices.