Study On Electron Transport Mechanisms In MoS₂Field Effect Transistors

Two-dimensional Transition metal dichalcogenides (TMDCs) have attracted great interest among both scientists and engineers, due to their outstanding electrical, optical, mechanical, chemistry physical properties. Especially MoS2, as a2dimensional semiconductor with large bandgap (1.8eV for single layer MoS2), has potential applications in low-power, high-on/off ratio devices. Also, MoS2shows superior immunity to short channel effects down to100nm channel length and compatibility to traditional semiconductor process. It is worthy to note that the parameter-mobility-determines MoS2’s upward road to practical applications.Up to now, the mobility of most reported back-gated MoS2field effect transistors is between10-1～101cm2·V-1·s-1, which is far lower than upper limitation by photo phonon scattering (-410cm2·V-1·s-1for single-layer MoS2). Some transport mechanisms have been proposed, however, a systematically research on carrier transport in MoS2, as well as the clarification of possible scattering centers, is demanding. In view of above discussion, we have studied electrical transport mechanism in MoS2field effect transistors. The main contents are as followed:(1) We have demonstrated back-gated MoS2field effect transistors with high on/off ratio (～107) by photolithography, e-beam evaporation of metal electrode, lift-off. We show that chemisorption of oxygen and water from the ambient causes degradation of device conductance by up to～100times, but could be reversibly recovered by a simple VA process. The VA process can also significantly reduce the commonly observed variations in MoS2device performances.(2) We, for the first time, have directly observed intrinsic sulfur vacancies on single-layer MoS2surface by high-resolution transmission electron microscopy (HRTEM). The density of sulfur vacancies is estimated to～3.6x1013cm-2(the average distance between sulfur vacancies a～1.7nm), by averaging different places on several samples. Moreover, we have calculated the effects of sulfur vacancy on single-layer MoS2’s electronic structure by first-principle calculation:sulfur vacancy introduces defect states in band gap of MoS2and the electron density is limited to the scope around each defect state with the radiusξ～0.6nm. Furthermore, we have measured the transport properties of single-layer MoS2transistors from300K to7K and simulated the data (in low-density regime) in high-temperature and low-temperature regime by nearest-neighbor hopping model and2dimensional Mott’s variable range hopping model, respectively. With the increase of carrier density, electron transport has changed from localized state-limited hopping to extended state transport.Overall, the electrical transport in MoS2field effect transistors is limited by electron hopping over defect states introduced by sulfur vacancies on MoS2surface. This understanding of electrical transport in MoS2is conduced to purposely improving mobility of MoS2field effect transistors to a higher degree.