2, Northwestern University, Evanston, Illinois, United States
3, Northwestern University, Evanston, Illinois, United States
The famous phrase "the interface is the device"  is exemplified in two-dimensional (2D) materials, reminding us that the challenge of controlling the interface chemistry and structure is a differentiating opportunity for engineering 2D semiconductor devices. Nearby ions, molecules, or compounds have been demonstrated to dope 2D semiconductors without introducing chemical or structural defects, yet these surface charge transfer doping schemes employ molecular species that are overly sensitive to environmental changes such as temperature and humidity.  Here we report that controlling the stoichiometry of amorphous molybdenum oxide (MoOx) deposited by atomic layer deposition (ALD) on monolayer MoS2, a representative transition metal dichalcogenide (TMD), modulates its carrier density. Concurrently, MoOx with a dielectric constant exceeding 5 encapsulates the TMD and mitigates the influence of defects. We employ a low-oxidation-state molybdenum precursor (Mo(NMe2)4) enabling ALD at temperatures below 100 °C to provide a encapsulation scheme for TMDs that is gentle and compatible with nanolithographic patterning . X-ray photoelectron spectroscopy (XPS) reveals that the ALD oxidant (here H2O and ozone) and process conditions control the MoOx stoichiometry (film oxidation). Intrinsic properties including the optical absorption edge and resistivity vary monotonically with the oxygen content in MoOx. To evaluate doping, we probe the MoS2 carrier density with spectroscopic (Raman) and field-effect transistor (FET) measurements before and after ALD MoOx deposition on monolayer (and few layer) MoS2. Depending on the stoichiometry of the MoOx dielectric overlayer, the threshold voltage of MoS2 FETs shifts to the negative (Vth ≤ -75 V) or to the positive (Vth ≥ 80 V) with respect to the back gate voltage suggesting p- or n-type doping of the underlying TMD over a wide range (ΔND= 7×1012 cm-2). Interestingly, the effect of MoOx on the carrier density and mobility in MoS2 is reversible; selective etching of the MoOx layer in KOH reproduces the initial current-voltage characteristics, and Raman spectra suggest that this process maintains the van der Waals interface without significant chemical changes. Combining control of carrier concentration with selected area deposition via lithography provides an attractive route to the formation of p-n junctions for logic and other applications.
 Kroemer, H. Nobel Lecture 2000.
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