Defects and doping in transition-metal dichalcogenide thin films grown by molecular-beam epitaxy

Abstract

Ultrathin layers of transition-metal dichalcogenides (TMDs) are an emerging class of two-dimensional (2D) materials with novel electronic, optoelectronic, spin- and valley-electronic properties. The past years have seen intensive research interests in the fundamental physics and promising applications in 2D TMDs. By scanning tunneling microscopy and spectroscopy (STM/S), recent studies have revealed both intrinsic point and line defects in as-grown TMD layers. These defects give rise to unique physical phenomena in low dimensions and dramatic changes on TMDs in terms of electronic and magnetic properties. Besides the control of intrinsic defects, intentional doping plays a critical role in tuning the 2D semiconductors’ conductivity, for purpose of ultrathin functional device applications. Motivated by these visions, this thesis will focus on the intrinsic defects and substitutional doping in TMDs. To be specific, the fundamental physical natural and formation mechanism of the defects will be firstly investigated by a combinational STM/S, scanning transmission electron microscopy (STEM) and density functional theory (DFT). The latter part of this thesis will concentrate on intentional doping effects on TMDs by the incorporation of group-V elements. The tuning, control and doping efficiency of the materials electronic properties have been explored using various characterization techniques. Domain boundaries (DBs) are ubiquitous one-dimensional (1D) defects in epitaxial TMDs. First principle calculations have shown the metallic nature of DBs and their atomic structures have been revealed by STEM. In MBE-grown MoSe2, DBs of the 4|4P structure are commonly observed. They are well-isolated 1D metals sitting on the van der Waals (vdW) substrates like graphene and surrounded by semiconducting MoSe2 domains with wide bandgap. Through a manual control on the defect density and DBs’ length, quantum well states are observed by STM/S on the DBs. Further studies on the relation between Coulomb blockade induced bandgap size with the defect length indicate the Tomonaga-Luttinger liquid (TLL) state confined along the wire. TLL is also confirmed by the power-law suppression of density-of-state (DOS) near the Fermi level and by the special DOS behavior near DB ends. The derived Luttinger parameter is around 0.28, indicating a strong electron-electron interaction within the 1D system. Angle-resolved photoemission spectroscopy (ARPES) spectra hint the energy band structures of DBs, consistent with spectra obtained by STS. Hole doping is achieved in MoSe2 monolayer by the substitutions of selenium atoms with nitrogen atoms. The doping process follows a post-growth nitrogen plasma treatment and the doping level are shown to be controllable by the adjustment of plasma treatment time. A catalytic formation mechanism of dual-defects is revealed through statistical analysis on the defect densities. By spectroscopic measurements, the impurity bands are observed near the valence band edge and such defect states are confirmed by DFT calculations. A gradually formed compressive strain is noted during the nitrogen plasma treatments. Doping of films during growth is also achieved by incorporation of phosphorous atoms in MoSe2 monolayer during MBE. The doping concentrations can be tuned by changing the P/Se flux ratio. The substitutions are verified by the spectroscopic detections of P atoms and by spectroscopic signature of Mo-P bonds. Compared with N doping, P behaves more like a shallow-level acceptor though both N and P are group-V elements. Inspired by the findings, a systematic analysis on the doping efficiency in 2D TMDs is discussed and several suggestions are proposed. Tensile strain is detected on the P-doped MoSe2 sample and has an impact on the electronic behavior of the system. Other types of unique defects in doped MoSe2 are also revealed, including Mo-adatoms and MoSe2 nanowires.