[Paper Review] Spatially Resolved Photo-Excited Charge Carrier Dynamics in Phase-Engineered Monolayer MoS2

Hisato Yamaguchi, Jean‐Christophe Blancon+10 more|arXiv (Cornell University)|Dec 17, 2014

2D Materials and Applications2 references4 citations

TL;DR

This study investigates photo-excited charge carrier dynamics in phase-engineered monolayer MoS2 using spatially resolved scanning photocurrent microscopy and photoluminescence imaging. It demonstrates that transforming the 2H-phase (semimetallic) to 1T-phase (metallic) reduces Schottky barrier heights from ~200 meV to a few meV, enhancing photoresponsivity by over an order of magnitude, which enables efficient, low-power optoelectronic devices with Ohmic-like contacts.

ABSTRACT

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS2) under photo-excitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is observed towards the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are few meV for 1T- and ~200 meV for 2H-contacted devices. We also demonstrate that a reduction of native Schottky barriers in a 1T device enhances the photo responsivity by more than one order of magnitude, a crucial parameter in achieving high performance optoelectronic devices. The obtained results pave a pathway for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where Ohmic contacts are necessary for achieving high efficiency devices with low power consumption.

Motivation & Objective

• To understand the intrinsic optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs), particularly monolayer MoS2.
• To investigate how phase engineering—specifically the transformation from 2H-phase to 1T-phase—affects charge carrier dynamics under photo-excitation.
• To determine the spatial distribution and magnitude of Schottky barrier heights at metal contacts in MoS2 devices.
• To evaluate the impact of reduced Schottky barriers on photoresponsivity and device performance.
• To establish a pathway toward achieving Ohmic contacts in 2D TMD-based optoelectronic devices for low-power, high-efficiency applications.

Proposed method

• Scanning photocurrent microscopy with diffraction-limited spatial resolution was used to map photocurrent generation along the channel of CVD-grown monolayer MoS2 devices.
• Photoluminescence imaging was employed to correlate optical emission with local electronic structure and phase distribution.
• Phase-engineering techniques were applied to locally transform 2H-phase MoS2 into 1T-phase, creating distinct regions with different electronic properties.
• The position- and bias-dependent photocurrent measurements were analyzed to extract Schottky barrier heights at the metal contacts.
• A comparative analysis was performed between 2H-phase (semiconducting) and 1T-phase (metallic) MoS2 devices to assess differences in charge transport and photoresponse.

Experimental results

Research questions

• RQ1 How does the local phase of monolayer MoS2 (2H vs. 1T) influence the spatial distribution of photo-generated charge carriers?
• RQ2 What is the magnitude of Schottky barrier heights at metal contacts in 2H-phase and 1T-phase MoS2 devices?
• RQ3 How does phase engineering affect the photocurrent generation profile along the device channel?
• RQ4 To what extent does reducing Schottky barriers enhance photoresponsivity in 2D TMD-based devices?
• RQ5 Can phase-engineered 1T-MoS2 achieve near-Ohmic contact behavior for improved optoelectronic performance?

Key findings

• The Schottky barrier height at the contact interface is reduced to a few meV in 1T-phase MoS2, compared to ~200 meV in 2H-phase MoS2, indicating a significant reduction in injection barriers.
• Photocurrent generation shifts from the Schottky contact regions in 2H-phase MoS2 to the center of the channel in 1T-phase MoS2, confirming the suppression of Schottky barriers.
• The photoresponsivity of 1T-phase MoS2 devices is enhanced by more than one order of magnitude compared to 2H-phase devices.
• Spatially resolved measurements confirm that the 1T-phase transformation locally suppresses Schottky barriers, enabling more efficient charge extraction.
• The results demonstrate that phase-engineered 1T-MoS2 can serve as a viable platform for achieving low-resistance, Ohmic-like contacts in 2D semiconductor devices.