Carbon Incorporation in MOCVD of MoS2 Thin Films Grown from an Organosulfide Precursor

With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)(6) and diethyl sulfide (CH3CH2)(2)S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)(6)/DES/H-2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)(6) precursor ratios, we observed that temperatures above DES pyrolysis onset (greater than or similar to 600 degrees C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H-2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-to-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices.

Automated summary

The role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin film growth was investigated, revealing the formation of nanographitic carbon under high temperature and excessive precursor ratios competing with MoS2 growth. Introducing H2 gas significantly hindered DES pyrolysis, reducing carbon incorporation in MoS2 films. The C content in MoS2 films affects photoluminescence and trion-to-exciton ratio, providing insights into process-induced C impurity doping in MOCVD-grown 2D semiconductors and its impact on device functionality.