Transition Metal Doping of MoS₂: A Correlated Experimental and Theoretical Study

Abstract

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) layered materials covalently bonded within the layers, but with only weak van-der-Waals (vdW) interactions between individual monolayers [1]. Substantial progress has been made in better understanding of these materials, from the nature of their defects [2] to the achievement of large area epitaxially grown films [3-5]. These materials have attracted great attention for applications such as next-generation electronics - including sub-60 mV sub-threshold slope transistors, flexible electronics, and optoelectronics, novel applications in spintronic devices [6] and the use of heterostructures of TMDs for tunnel field effect transistors (TFETs) [7]. However, issues including control of channel and source/drain doping have impeded their implementation into device. Similar to three-dimensional semiconductors, doping of the TMDs is required to modulate carrier concentration, to achieve Ohmic contacts, and to generate n-type and p-type materials which are required for complementary metal-oxide-semiconductor (CMOS) technology and TFET applications. One of the most studied TMDs is MoS2. The transition metals Nb and Re are two candidate dopants for MoS2 with theoretical results showing their suitability as p- and n-type dopants, respectively [8]. Experimental results have confirmed that Nb substitutes at the Mo-site and acts as a p-type dopant in MoS2 [9-11]. While Re has been confirmed as an n-type dopant, with the additional benefit of reducing sulfur vacancies and defect-related gap states [12-13]. This study reports on the band structure and electrical characteristics of doped and unintentionally doped chemical vapor transport (CVT) grown MoS2 bulk crystals. We present a direct determination of the valence band structure of the MoS2 and the impact of transition metal doping (Nb and Re) using high-resolution angle-resolved photoemission spectroscopy (ARPES). Structural defects in the form of vacancies are widely known to strongly alter the MoS2 electronic structure. Therefore, we have performed highly-efficient density functional theory (DFT) based simulations to provide insight into the impact of vacancies in addition to the incorporation of transition metal dopants on the MoS2 band structure. Unfolded band structures obtained through our simulations [14], in comparison with the experimentally obtained occupied band structure of doped and un-doped MoS2 have shown excellent agreement and revealed that there has been significant distortion to the band structure due to the presence of vacancies, as well as the introduction of degenerate Nb-doping. Scanning tunneling microscopy (STM) studies revealed high quality crystals with point defects established by our first-principle calculations to be mainly Mo vacancies. We also report our Hall effect analysis to obtain the electrical metrics for the crystals. Secondary ion mass spectrometry (SIMS) shows the impurities present in the crystals, which is then used to explain the difference in transport behaviour and dopant types between similar crystals from different material sources. Figure 1. (a) Unfolded band structure of un-doped MoS2 with Mo vacancy obtained by DFT calculations shown using contour plot of total weight intensity. Black curve: primitive-cell of pristine MoS2 band structure. Mo vacancy induced localized states are located close to the valence band edge - shown by white rectangles. (b) ARPES spectra for a non-intentionally-doped MoS2 crystal acquired along the high symmetry Γ-K direction overlapped with DFT results showing excellent agreement considering the effect of a Mo vacancy. (c) DFT-obtained STM images of a pristine MoS2 (left), and Mo vacancy in MoS2 (right). (d) Charge density difference between pristine MoS2 and MoS2 with Mo vacancy. Red and blue indicate charge accumulation and depletion, respectively, at Mo vacancy sites. Figure 2. In situ STM, XPS and LEED measurements of MoS2. (a) Large-scale STM image with bright and dark defects (300×300 nm). (b) STM image (100×100 nm) with line profiles over the defects 1, 2, and 3. (c) STM image illustrates atomic resolution with interatomic distance 0.32 nm. (d) & (e) corresponds to the binding energies of Mo 3d and S 2s, and S 2p core levels, respectively. (f) LEED showing highly ordered structure. References: [1] Applied Materials Today, 9, 504, 2017. [2] ACS Nano, 8, 2880, 2014. [3] 2D Materials, 4, 045019, 2017. [4] 2D Materials, 4, 025044, 2017. [5] ACS Nano, 9, 474-80, 2015. [6] Nature Nanotechnology, 7, 699-712, 2012. [7] Applied Physics Letters, 103, 053513, 2013. [8] Physical Review B, 88, 075420, 2013. [9] AIP Advances, 6, 025323, 2016. [10] Nano Lett, 14, 6976-6982, 2014. [11] Applied Physics Letters, 104, 092104, 2014. [12] Applied Physics Letters, 111, 203101, 2017. [13] Advanced Functional Materials, 28, 1706950, 2018. [14] npj 2D Materials and Applications, 3, 33, 2019. Figure 1