Accessing optoelectronic properties in atomic layered semiconducting materials

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Image 1: Spatially resolved intrinsic photocurrent response from MoS2. (Left sketch): The measurements were performed using a scanning photocurrent microscopy technique where a focused laser beam rasters on the sample surface and electrical response is measured. While maximum photocurrent generation occurs at the MoS2/metal contacts for a conventional device (bottom middle map), after the metallic phase transformation of regions underneath the metal electrodes, the photocurrent peak is observed towards the center of the device channel (top middle map), suggesting a strong reduction of native Schottky barriers. Circular insets illustrate Schottky barrier height derived from the optoelectronic measurements. The photo-­responsivity of MoS2 devices was enhanced by at least one order of magnitude after phase transformation (red) as compared with their conventional counterparts (black) (right graph).

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A Los Alamos-led collaboration investigated an emerging class of two-dimensional nanomaterials beyond graphene, called transition-metal dichalcogenides. These materials have the formula ME2, where M is a transition metal and E is sulfur, selenium, or tellurium. The researchers discovered how to access the intrinsic optoelectronic properties of transition-metal dichalcogenides. The materials could enhance high-performance semiconductor devices including those for energy harvesting applications. The journal ACS Nano published their findings.

Transition-metal dichalcogenides materials were developed to overcome the semi-metallic nature of graphene (an atomic layer-thick material made from carbon) that limits its application in optoelectronics. The transition-metal dichalcogenides exhibit relatively high mobility of carriers and enhanced light-matter interactions when reduced to a monolayer (0.6 nm). These properties are attractive for optoelectronic applications. However, the device performances have been hindered by the native energy barriers at the interfaces between transition-metal dichalcogenides and metal electrodes, which limit the extraction of charge carriers.

One of the chief challenges of transition-metal dichalcogenide research is to achieve devices with ohmic contacts, thereby gaining access to optoelectronic properties that cause devices to function without the extrinsic effects that could limit their performances. A research team from Los Alamos, Rutgers University, Rice University, and Pacific Light Technologies has measured the optoelectronic properties of transition-metal dichalcogenides. After a semiconducting to metallic phase transformation occurred in regions underneath the metal electrodes, the scientists could access the otherwise masked, intrinsic optoelectronic response of transition-metal dichalcogenides.

This research builds on previous Los Alamos-directed work on phase transformation to achieve low resistance contacts to atomic layered transition-metal dichalcogenides. The results, achieved at the Center for Integrated Nanotechnologies (CINT) at Los Alamos, are important for the fundamental understanding of intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides where ohmic contacts are necessary to achieve high-efficiency devices with low power consumption or minimum energy loss.

In the new research, the team used photoexcited molybdenum disulfide (MoS2). They characterized the material via correlated scanning photocurrent microscopy and photoluminescence imaging. The results demonstrated that a reduction of native Schottky barriers by phase transformation (from 200 meV down to few meV) enhances the photoresponsivity of a device by more than one order of magnitude. This is a crucial parameter in achieving high-performance optoelectronic devices. The study provides insights into the efficient and optimum design of high performance transition-metal dichalcogenide-based devices via simple and reliable procedures for forming ohmic-like contacts to access intrinsic optoelectronic properties.

Reference: “Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS2,” ACS Nano 9, 840 (2015): doi:10.1021/nn506469v. Authors include: Hisato Yamaguchi, Gautam Gupta, and Aditya Mohite (Materials Synthesis and Integrated Devices, MPA-11); Jean-Christophe Blancon and Jared Crochet (Physical Chemistry and Applied Spectroscopy, C-PCS); Rajesh Kappera and Manish Chhowalla (Rutgers University); Pulickel Ajayan and Jun Lou (Rice University); and Benjamin D. Mangum (Pacific Light Technologies). Jon Baldwin (MPA-CINT) performed metal electrode depositions, Chris Sheehan (MPA-CINT) conducted electron microscopy, and Darrick William (MPA-CINT) maintained the SEM system for e-beam lithography.

The Laboratory Directed Research and Development (LDRD) program funded the work. The research was performed in part at the Center for Integrated Nanotechnologies (CINT), a DOE Office of Science User Facility that Los Alamos National Laboratory and Sandia National Laboratoriesoperate for the DOE. Yamaguchi received a LANL Director’s Postdoctoral Fellowship. The effort supports the Lab’s Energy Security mission area and the Materials for the Future science pillar through research to make more energy efficient semiconductor devices, including those for energy harvesting applications. Technical contact: Aditya Mohite