Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

Abstract: Strong electron-phonon interaction which limits electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in-situ Raman scattering from single layer MoS$2$ electrochemically top-gated field effect transistor (FET), we show softening and broadening of A${1g}$ phonon with electron doping whereas the other Raman active E${2g}^{1}$ mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why A${1g}$ mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single layer MoS$_2$-based FETs, which have a high on-off ratio and are of enormous technological significance.

• The paper demonstrates that the A1g phonon mode softens by ~4 cm⁻¹ and broadens by ~6 cm⁻¹ under electron doping, evidencing strong symmetry-dependent electron-phonon coupling.
• It employs in-situ Raman spectroscopy and DFT calculations to elucidate how system symmetry governs the differential responses of phonon modes in MoS2 transistors.
• Integrated experimental and theoretical insights establish Raman spectroscopy as a precise tool for probing carrier concentrations in advanced 2D electronic devices.

Symmetry-dependent Phonon Renormalization in Monolayer MoS$_2$ Transistors

This paper provides a detailed examination of the phonon renormalization in monolayer molybdenum disulfide (MoS$_2$) transistors, with a primary focus on the symmetry-dependent electron-phonon interactions. The research utilizes in-situ Raman spectroscopy to observe changes in the A$_{1g}$ and E$_{2g}^{1}$ phonon modes as a function of electron doping in a MoS$_2$ field-effect transistor (FET). Through both experimental measurements and first-principles density functional theory (DFT) calculations, the study explores how symmetry considerations influence the sensitivity of phonon modes to electron doping and highlights the implications for Raman-based characterization techniques in electronic devices.

Key Findings:

• Phonon Mode Sensitivity: The A$_{1g}$ phonon mode exhibits significant softening and broadening with electron doping, whereas the E$_{2g}^{1}$ mode remains largely unaffected. Experimentally, the phonon frequency of A$_{1g}$ decreases by approximately 4 cm$^{-1}$, and its linewidth broadens by 6 cm$^{-1}$ under electron doping levels around 1.8 $\times$ 10$^{13}$/cm$^2$. This contrast in phonon behavior is rationalized through symmetry analysis and corroborated by DFT calculations.
• Electron-Phonon Coupling (EPC): The A$_{1g}$ mode shows stronger coupling to electronic states than the E$_{2g}^{1}$ mode. This outcome is explained by the symmetry of the electronic states that are involved and the unique representation of the A$_{1g}$ mode, which aligns with the identity representation of the system's symmetry group. This symbiotic relationship allows for enhanced EPC in the A$_{1g}$ mode when electron doping increases.
• Theoretical and Experimental Concordance: Theoretical DFT calculations predict a softening of the A$_{1g}$ phonon by about 7 cm$^{-1}$, which aligns well with experimental observations. This agreement strengthens the argument about the role of symmetry and electron-phonon interactions in monolayer MoS$_2$.
• Raman Spectroscopy as a Characterization Tool: The sensitivity of the A$_{1g}$ phonon mode to electron doping underscores the efficiency of Raman spectroscopy in non-destructively probing carrier concentration in MoS$_2$ FETs. The results point toward Raman spectroscopy's potential utility in the analysis of device performance parameters such as mobility and the on-off ratio.

Implications and Future Directions:

The insights into phonon dynamics and doping effects in monolayer MoS$_2$ expand the understanding of electron-phonon interactions in two-dimensional materials, which are pivotal for the advancement of nano-electronic devices. The study could lead to optimized methods for characterizing and tuning device performance in MoS$_2$ and similar 2D materials. As the demand for high-mobility, low-power electronics increases, understanding and leveraging the interactions exhibited in systems like MoS$_2$ could pave the way for improved design and functionality in FETs and other semiconductor applications.

Future research could explore other 2D materials such as WS$_2$ and BN to determine whether these symmetry-dependent effects are generalizable across the class of transition metal dichalcogenides and related compounds. Further, expanding the scope of study to include temperature-dependent behavior and interactions with additional degrees of freedom in the lattice structure might provide deeper insights into controlling and exploiting electron-phonon interactions for technological applications.