Large Area Single Crystal (0001) Oriented MoS2 Thin Films (1302.3177v3)

Abstract: Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, and potential to open up new areas of physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area electronic grade single crystal Molybdenum Disulfide (MoS2) thin films with the highest mobility reported in CVD grown films so far. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) orientated sapphire yielded highly oriented single crystal MoS2 films for the first time. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.

• The paper demonstrates that optimized CVD processes using elevated temperatures and (0001) oriented sapphire substrates yield MoS2 films with crystallinity comparable to bulk samples.
• High-resolution XRD and Raman spectroscopy confirmed that films grown at 900°C and 1100°C achieve structural quality on par with natural MoS2.
• Enhanced electron transport properties, including high field-effect mobility and space charge limited conduction, suggest significant potential for advanced electronic applications.

An Analysis of Large Area Single Crystal (0001) Oriented MoS Thin Films

The research presented in the paper addresses a significant challenge in the synthesis of two-dimensional (2D) materials: the attainment of large area, high-crystal-quality Molybdenum Disulfide (MoS$_2$) thin films. This paper focuses on the use of chemical vapor deposition (CVD) techniques to grow single-crystal MoS$_2$ films, oriented along the (0001) direction, achieving structural qualities comparable to those of reference geological samples. By presenting these findings, the authors contribute to the field of layered metal dichalcogenide materials with potential implications for electronic and optoelectronic devices.

A pivotal aspect of the research is the role of growth temperature and substrate choice on the quality of the MoS$_2$. The experiments demonstrate that higher temperatures and epitaxially suitable substrates, such as (0001) oriented sapphire, are critical for achieving optimal film quality. Samples grown at temperatures ranging from 500°C to 1100°C revealed a notable enhancement in crystallinity and film orientation with increasing temperature. Specifically, the samples grown at 900°C and 1100°C matched the structural integrity of bulk MoS$_2$, as confirmed through techniques such as high-resolution X-ray diffraction, Raman spectroscopy, and transmission electron microscopy.

Raman spectroscopy provided insight into the vibrational modes of the MoS$_2$ films, with the E$_{2g}^1$/A$_{1g}$ intensity ratio serving as an indicator of film quality. For the highest quality samples (C and D), this ratio aligns closely with that of natural bulk MoS$_2$, emphasizing the high structural quality achieved. Furthermore, X-ray diffraction analysis reinforced these findings, where higher temperature samples exhibited clear diffraction peaks indicating superior crystalline order.

Electron transport properties were also evaluated, highlighting significant findings regarding the films' electrical behavior. The I-V characteristics displayed space charge limited transport, indicative of low background carrier density—a desirable trait for field-effect transistors. The high field-effect mobility observed is a notable achievement relative to other CVD-grown films, suggesting that such films could bridge the gap between experimental results and practical applications in electronics.

The implications of this research extend into the theoretical and practical realms, suggesting pathways for synthesizing other 2D chalcogenide materials. By leveraging epitaxial growth techniques on hexagonal substrates, the paper signals potential advancements in heterostructure fabrication, thereby expanding the functionality of these 2D materials.

In conclusion, the paper presents a comprehensive exploration of scalable methods for the creation of high-quality MoS$_2$ films. These findings are pivotal for advancing the integration of 2D materials into electronic systems, potentially offering new avenues for research into their properties and applications. Future work might delve into engineering heterostructures or alloys with mixed chalcogen compositions, paving the way for innovation in semiconductor technologies.