Intentional carbon doping reveals CH as an abundant charged impurity in nominally undoped synthetic WS2 and WSe2

K A Cochrane,F Zhang,J B Neaton,A Kozhakhmetov,J A Robinson,J-H Lee,C Dong,M Terrones,B Schuler,T Zhang,A Weber Bargioni

doi:10.1088/2053-1583/ab8543

Abstract

Understanding the physical properties and controlling the generation of intrinsic and extrinsic defects is central to the technological adoption of 2D materials in devices. Here we identify a charged carbon-hydrogen complex at a chalcogen site (CHX) as a common, charged impurity in synthetically grown transition metal dichalcogenides (TMDs). This conclusion is drawn by comparing high resolution scanning probe microscopy measurements of nominally undoped and intentionally carbon doped TMD samples. While CH impurity densities in undoped CVD-grown WS2 and MOCVD-grown WSe2 can range anywhere from parts per million to parts per thousand, CH densities in the percentage levels were selectively generated by a post-synthetic methane plasma treatment. Our study indicates that methane plasma treatment is a selective and clean method for the controlled introduction of a charged carbon-hydrogen complex at a surface chalcogen site, a defect that is commonly present in synthetic TMDs.

Highlights

Chemical doping is a vital technology to tune the transport and optoelectronic properties of semiconductors through band-gap engineering
Charged defects can be detrimental to intrinsic 2D material functionality, for instance by acting as strong scattering centers limiting charge carrier mobility [6, 10, 22], or modifying luminescence by enhancing charge recombination [23] and shortening exciton lifetimes. [24, 25] We recently found that the negative charge trapped at the defect site gives rise to a series of hydrogenic bound and resonant states originating from hole localization in different valleys
Monolayer islands of WS2 and WSe2 on graphene on silicon carbide (SiC) substrates were prepared by chemical vapor deposition (CVD) [26,27] and metalorganic chemical vapor deposition (MOCVD) [21,28] respectively

Summary

Introduction

Chemical doping is a vital technology to tune the transport and optoelectronic properties of semiconductors through band-gap engineering. Dopants in two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) exhibit significantly more localized wavefunctions and higher binding energies deeper in the band gap [1–3]. This poses severe limitations to transferring the concept of chemical doping to low-dimensional material systems, but offers exciting possibilities to create surfacebound atomic quantum systems by chemical design rules [4, 5]. For both classical optoelectronic and quantum photonic applications, it is desirable to suppress the intrinsic disorder in 2D semiconductors to an absolute minimum. Avoiding or passivating any deleterious defects that act as charge traps [6, 7], add to charge scattering [8] and recombination [9, 10], and cause decoherence effects by fluctuating magnetic fields [11] is an essential step for their technological adoption. [12–14] a detailed understanding of types and properties of impurities present in synthetically grown 2D materials is essential

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Results

Conclusion