[Paper Review] Raman and Photoluminescence Study of Dielectric and Thermal Effects on Atomically Thin MoS2

Rusen Yan, Simone Bertolazzi+8 more|arXiv (Cornell University)|Nov 17, 2012

2D Materials and Applications16 citations

TL;DR

This study investigates dielectric and thermal effects on monolayer MoS2 using Raman and photoluminescence (PL) spectroscopy, comparing samples with and without HfO2 capping via atomic layer deposition. It finds that Raman red shifts arise primarily from modulation doping by HfO2, while PL red shifts are mainly due to strain induced by the dielectric layer, highlighting the need for caution in interpreting spectroscopic data due to strain-dependent band structure changes.

ABSTRACT

Atomically thin two-dimensional molybdenum disulfide (MoS2) sheets have attracted much attention due to their potential for future electronic applications. They not only present the best planar electrostatic control in a device, but also lend themselves readily for dielectric engineering. In this work, we experimentally investigated the dielectric effect on the Raman and photoluminescence (PL) spectra of monolayer MoS2 by comparing samples with and without HfO2 on top by atomic layer deposition (ALD). Based on considerations of the thermal, doping, strain and dielectric screening influences, it is found that the red shift in the Raman spectrum largely stems from modulation doping of MoS2 by the ALD HfO2, and the red shift in the PL spectrum is most likely due to strain imparted on MoS2 by HfO2. Our work also suggests that due to the intricate dependence of band structure of monolayer MoS2 on strain, one must be cautious to interpret its Raman and PL spectroscopy.

Motivation & Objective

• To understand the influence of dielectric capping (HfO2) on the electronic and vibrational properties of monolayer MoS2.
• To disentangle the contributions of strain, doping, thermal effects, and dielectric screening on Raman and PL spectra.
• To provide a reliable interpretation framework for spectroscopic measurements in atomically thin MoS2, given its strong strain dependence.
• To guide dielectric engineering in 2D semiconductor devices by identifying dominant physical mechanisms in HfO2-capped MoS2.

Proposed method

• Atomic layer deposition (ALD) was used to grow HfO2 on monolayer MoS2 to apply controlled dielectric capping.
• Raman spectroscopy was performed to analyze phonon modes and detect strain, doping, and dielectric screening effects.
• Photoluminescence (PL) spectroscopy was used to probe the bandgap and electronic transitions in MoS2.
• Spectroscopic data from HfO2-capped and uncapped MoS2 samples were compared to isolate the effects of dielectric capping.
• Theoretical considerations of strain, doping, and dielectric screening were used to interpret experimental shifts in Raman and PL peaks.

Experimental results

Research questions

• RQ1 What causes the observed red shift in the Raman spectrum of MoS2 after HfO2 capping?
• RQ2 What is the dominant origin of the PL peak red shift in HfO2-capped monolayer MoS2—doping, strain, or dielectric screening?
• RQ3 How do thermal effects influence the Raman and PL spectra of atomically thin MoS2?
• RQ4 To what extent does dielectric screening from HfO2 alter the electronic structure of monolayer MoS2?
• RQ5 How does strain in MoS2 affect its band structure and spectroscopic response?

Key findings

• The Raman G-mode peak in MoS2 exhibits a red shift upon HfO2 capping, primarily attributed to modulation doping induced by the dielectric layer.
• The photoluminescence (PL) peak red shift is most likely caused by mechanical strain imparted on the MoS2 layer by the HfO2 capping layer.
• Dielectric screening from HfO2 has a minor contribution to the observed spectral shifts compared to doping and strain effects.
• The study demonstrates that strain significantly alters the band structure of monolayer MoS2, complicating the interpretation of spectroscopic data.
• The results emphasize the importance of accounting for strain and doping when analyzing Raman and PL spectra in 2D transition metal dichalcogenides.