Two-dimensional Electronic Transport induced by Surface Electron Accumulation in MoS2 Layered Crystals and Nanostructures

M. D. Siao 蕭名登¹, W. C. Shen 沈韋竹², Ruei-San Chen^1*, Z. W. Chang 張子韋³, M. C. Shih 施敏全⁴, Y. P. Chiu 邱雅萍^3,4, C. M. Cheng 鄭澄懋^5,6

¹Graduate Institute of Applied Science and Technology 應用科技研究所, National Taiwan University of Science and Technology 國立臺灣科技大學, Taipei, Taiwan
²Department of Electronic Engineering 電子工程系, National Taiwan University of Science and Technology 國立臺灣科技大學, Taipei, Taiwan
³Department of Physics 物理學系, National Taiwan Normal University 國立臺灣師範大學, Taipei, Taiwan
⁴Department of Physics 物理學系, National Taiwan University 國立臺灣大學, Taipei, Taiwan
⁵National Synchrotron Radiation Research Center 國家同步輻射研究中心, Hsinchu, Taiwan
⁶Department of Physics 物理學系, National Sun Yat-Sen University 國立中山大學, Kaohsiung, Taiwan

* Presenter:Ruei-San Chen, email:rsc@mail.ntust.edu.tw

Because the surface-to-volume ratio of quasi two-dimensional (2D) materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layer semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, the results of this study revealed that the surface of high-quality synthesized molybdenum disulfide (MoS2) is a major n-doping source. The surface electron concentration of MoS2 is nearly four orders of magnitude higher than that of its inner bulk. A substantial thickness-dependent electronic transport in MoS2 nanoflakes beyond that of the quantum confinement scale was observed. The transfer length method was used to determine the current transport in MoS2 following a 2D behavior rather than the conventional 3D mode. Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) measurements confirmed the presence of surface electron accumulation (SEA) in this layer material. Notably, the pronounced n-doping characteristic was not observed in the cleaved fresh surface of MoS2. The in-situ ARPES measurement indicates that the cleaved MoS2 fresh surface exhibits a near intrinsic state without SEA. The SEA is formed gradually in the MoS2 surface due to desulphurization at room temperature and even at a very low temperature of 85 K. The surface defects created by sulfur vacancies have been confirmed to be the major physical origin of the SEA phenomenon. In addition, field-effect transistor measurement points that the MoS2 nanoflakes with fresh surface exhibit much higher carrier mobility and lower concentration compared to those with pristine surface. Ohmic contact fabrication of the MoS2 nanostructure devices with very low contact resistance using the focused-ion beam technique was also demonstrated. The study provides an unprecedented insight into transition metal dichalcogenide (TMD) layer materials, which have important implications in material processing, device design, and fundamental research. The presence of SEA could also explain previously reported anomalies in MoS2 devices, such as high residual electron concentration and low Schottky barrier height.

Keywords: molybdenum disulphide, two-dimensional transport, surface electron accumulation, scanning tunneling microscopy, angle-resolved photoemission spectroscopy