Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure (1607.08092v1)

Abstract: Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homo-junctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in-situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.

• The paper demonstrates that remote N₂ plasma exposure successfully induces covalent nitrogen doping in MoS₂, enabling controlled p-type behavior.
• Remote plasma exposure induces measurable compressive strain in the MoS₂ lattice, as confirmed by Raman spectroscopy and first-principles calculations.
• Electrical characterizations reveal that doped MoS₂ maintains stable FET threshold voltages, indicating robust device performance under strain.

Covalent Nitrogen Doping and Compressive Strain in MoS₂ by Remote N₂ Plasma Exposure

The paper discusses a novel method for covalent nitrogen doping in MoS₂ using remote nitrogen plasma exposure and the resultant compressive strain. Transition metal dichalcogenides (TMDs), like MoS₂, are increasingly important for scaling semiconductor channels in field-effect transistors (FETs), but effective doping methods are critical to improving their utility in electronic devices. Covalent doping stands out due to its potential for stable, controllable atomic integration, previously complicated by non-covalent bonding volatility and extrinsic factors affecting doping levels.

Remote nitrogen plasma exposure is explored as a viable method for introducing nitrogen into MoS₂ as a chalcogen substituent and potential p-type dopant. Through in-situ X-ray photoelectron spectroscopy (XPS), the presence of covalently bonded nitrogen was verified, showing sulfur substitution by nitrogen. This technique offers preeminence due to its controlled doping concentration, enhanced by varying plasma exposure time—a benefit over other dopants, such as Nb, used for Mo substitution.

Electrical characterization reveals p-type doping effects aligning with theoretical predictions and experimental observations. Remote nitrogen plasma doping manages to sustain MoS₂'s electronic performance, with field-effect transistors (FETs) displaying consistent threshold voltages without increase in scattering. This approach achieves good thermal stability of the Mo-N bond and provides a morphologically stable MoS₂ surface under observed conditions.

Importantly, the paper elucidates how nitrogen doping through this technique induces compressive strain on MoS₂, providing a first of its kind report of strain induced by single-atom doping in TMD materials. The paper uses Raman spectroscopy to characterize shifts associated with strain due to Mo-N bonds, which results in contraction across the MoS₂ lattice. Such insights position nitrogen-doping as a method capable of strain modulation, potentially affording optical band gap tuning. The presence of cracks observed at higher nitrogen concentrations could suggest a mix of strain and other mechanical or chemical factors possibly enhancing crack propagation.

First-principles calculations support experimental data, establishing a clear correlation between nitrogen concentration and strain magnitude. The estimations indicate a logarithmic relationship between quantitative nitrogen exposure and resultant compressive effects. These findings propose an ability to predictably manipulate the strain-optical properties of MoS₂.

In closing, this research highlights a strategic methodology for introducing p-type dopants in TMDs, sustaining functional stability and device performance while exploring new physical phenomena, like strain induction. This research could significantly impact the future landscape of nanoscaled electronics, with potential applications including more efficient optoelectronic devices, sensors, and beyond. However, further research is required to fully explore nitrogen doping's bandgap effects and material properties modulation in two-dimensional systems.