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Interfacial Oxidation Enables Charge-Transfer Contacts and Degenerate n-Doping in Monolayer MoS$_2$

Marco Bianchi, Daniel Lizzit, Alberto Turoldo, Ezequiel Tosi, Paolo Lacovig, Monika Schied, Davide Curcio, Charlotte E. Sanders, Silvano Lizzit, Philip Hofmann

Abstract

High contact resistance remains a central obstacle to the integration of two-dimensional (2D) semiconductors in electronic devices. Recent advances have demonstrated that contact performance can be dramatically improved through interface engineering, including the use of group-V semimetals and charge-transfer contacts based on strong interfacial doping. Here, we show that controlled interfacial oxidation provides an effective route to convert a semimetal contact into a charge-transfer contact that degenerately $n$-dopes single layer MoS$_2$. Using a combination of angle-resolved photoemission spectroscopy, X-ray photoelectron diffraction, low-energy electron diffraction and scanning tunnelling spectroscopy, we demonstrate that putting single layer MoS$_2$ in contact with a pristine Bi layer merely results in weak doping, whereas oxidation of the Bi layer leads to a pronounced occupation of the MoS$_2$ conduction band with an electron density on the order of $10^{13}$~cm$^{-2}$. The cause of this strong electron doping is the fact that an ultrathin $β$-Bi$_2$O$_3$ layer forms below the MoS$_2$ and that this has a particularly low work function, thereby acting as an efficient electron donor to MoS$_2$. Interfacial oxidation thus emerges as a powerful design knob for engineering charge-transfer contacts to 2D semiconductors.

Interfacial Oxidation Enables Charge-Transfer Contacts and Degenerate n-Doping in Monolayer MoS$_2$

Abstract

High contact resistance remains a central obstacle to the integration of two-dimensional (2D) semiconductors in electronic devices. Recent advances have demonstrated that contact performance can be dramatically improved through interface engineering, including the use of group-V semimetals and charge-transfer contacts based on strong interfacial doping. Here, we show that controlled interfacial oxidation provides an effective route to convert a semimetal contact into a charge-transfer contact that degenerately -dopes single layer MoS. Using a combination of angle-resolved photoemission spectroscopy, X-ray photoelectron diffraction, low-energy electron diffraction and scanning tunnelling spectroscopy, we demonstrate that putting single layer MoS in contact with a pristine Bi layer merely results in weak doping, whereas oxidation of the Bi layer leads to a pronounced occupation of the MoS conduction band with an electron density on the order of ~cm. The cause of this strong electron doping is the fact that an ultrathin -BiO layer forms below the MoS and that this has a particularly low work function, thereby acting as an efficient electron donor to MoS. Interfacial oxidation thus emerges as a powerful design knob for engineering charge-transfer contacts to 2D semiconductors.
Paper Structure (9 sections, 9 figures)

This paper contains 9 sections, 9 figures.

Table of Contents

  1. Introduction
  2. Results
  3. Conclusion
  4. Methods
  5. Bi intercalation in ARPES
  6. Structural determination using scanning tunnelling microscopy and low energy electron diffraction
  7. XPD simulations of alternative structures
  8. Work function measurements
  9. First-Principles Calculations of the Work Function

Figures (9)

  • Figure 1: (Color online) Photoemission intensity along the $\mathrm{K}-{\Gamma}-\mathrm{K}^{\prime}$ direction (upper panels) and momentum-dependent photoemission intensity at the Fermi level and 200 meV below the SL MoS$_2$ valence band maximum, as marked by the dashed red lines in the upper panels (lower panels). (a) As-grown epitaxial single layer MoS$_2$ on Au(111). (b) Bi intercalated SL MoS$_2$. (c) SL MoS$_2$ on bismuth oxide on Au(111). Black, yellow and blue curved lines indicate the dispersion near the SL MoS$_2$ valence band maxima for the clean, Bi-intercalated and Bi oxide substrate, respectively. The red line in panel (c), upper row, is an energy distribution curve taken at the $\mathrm{K'}$ point, along the vertical black line in order to estimate the position of the valence band maximum for the Bi oxide case. (d) Table with the measured work functions $\Phi$ for the indicated systems and the calculated work function for Bi-terminated $\beta$-Bi$_2$O$_3$. The value range for SL MoS$_2$ is from Refs. Robinson:2015aaChoi:2014aa.
  • Figure 2: Bi intercalation and oxidation tracked by XPS on the (a) S $2p$ and Bi $4f$ core levels at h$\nu=260$ eV and (b) Mo $3d$ and S $2s$ core levels at h$\nu=360$ eV: Pristine SL MoS$_2$ on Au (black line); Bi intercalated (yellow line); oxidized system for different oxidation steps: in vacuum and in air (light and dark blue respectively).
  • Figure 3: XPD characterization of the interface. (a) Structural model for the $\beta$-Bi$_2$O$_3$(201) surface assumed to be placed under SL MoS$_2$ in the Bi 4f$_{7/2}$ multiple scattering simulations. Stereographic projections of the modulation function for (b) Mo $3d_{5/2}$ on $\beta$-Bi$_2$O$_3$ along with a simulation for free-standing SL MoS$_2$, (c) Bi $4f_{7/2}$ for $\beta$-Bi$_2$O$_3$ below SL MoS$_2$ with a simulation for $\beta$-Bi$_2$O$_3$ without the SL MoS$_2$ and (d) Bi $4f_{7/2}$ for $\beta$-Bi$_2$O$_3$ on Au(111) without SL MoS$_2$, along with a corresponding simulation. Note that the simulations in panels (c) and (d) differ by the number or rotational domains (3 vs. 6).
  • Figure 4: Bi intercalation at an intermediate state observed by ARPES. The panels correspond to those in Fig. 1 of the main paper but for partially intercalated single layer MoS$_2$. The original bands and shifted bands are marked by black and yellow curved lines, respectively.
  • Figure 5: Bi intercalation and oxidation tracked by LEED and STM, following the same steps as shown in Fig. 1 of the main paper. (a), (b) Results for pristine SL MoS$_2$ on Au(111). (c), (d) Partially and (e), (f) fully Bi-intercalated situation. (g), (h) Corresponding data for SL MoS$_2$ on bismuth oxide. Scanning parameters: (b): $I_t=350$ pA, $V_t=111$ mV, (d) $I_t=260$ pA, $V_t=507$ mV, (f) $I_t=280$ pA, $V_t=730$ mV, (h): $I_t=360$ pA, $V_t=1044$ mV, (i): $I_t=730$ pA, $V_t=1022$ mV.
  • ...and 4 more figures