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

Abstract

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)2S (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/H2 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 (≳600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 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.

Gas‐Phase Alkali Metal‐Assisted MOCVD Growth of 2D Transition Metal Dichalcogenides for Large‐Scale Precise Nucleation Control

“…We deduce that the intermediate reaction processes are as follows: (i) Mo(CO) 6 and SP were decomposed into Mo and Na 2 CO 3 , respectively, in gas‐phase due to the low thermal decomposition temperature of Mo(CO) 6 and SP. [ 27,36 ] (ii) Mo and Na 2 CO 3 were adsorbed and then diffused on the substrate, and then Na 2 MoO 4 nuclei were formed through heterogeneous nucleation, which is caused by the chemical reaction between Na 2 CO 3 admolecules and Mo adatoms on the substrate. After the nucleation process, the solid Na 2 MoO 4 nuclei melted to form a liquid droplet.…”

“…(iii) Simultaneously, due to the high decomposition temperature of (C 2 H 5 ) 2 S, (C 2 H 5 ) 2 S diffused into the substrate and then was decomposed into S‐containing molecules such as hydrogen sulfide or ethyl mercaptan by substrate‐mediated thermal decomposition at the growth temperature of 600 °C. [ 36 ] The S‐containing molecules were adsorbed and then diffused on the substrate. (iv) MoS 2 was grown as a result of surface diffusion and surface reaction of molten Na 2 MoO 4 liquid and S‐containing admolecules on the substrate.…”

Characterization of MOCVD-Prepared CIS Solar Cells

“…In Figure 2b, for the sample prepared via three-stage MOCVD (black), the peak at approximately 43.2 • is ascribed to MOCVD-processed Mo. The use of Mo(CO) 6 (a carbonyl group precursor) appears to result in carbon contamination and, thereby, a different crystal structure [22][23][24]. However, it is difficult to distinguish between the αand β-CIS phases based only on XRD characterization, because these phases are crystallography coherent.…”

Detailed study on MOCVD of wafer-scale MoS2 monolayers: From nucleation to coalescence

“…Despite some valuable theoretical modelling and calculation results [4][5][6][7], it is still challenging to identify the impact of multiple factors and conditions during development and optimization of growth processes. Up to now, a large number of process details and growth parameters have been reported to influence the final output of MOCVD processes, including substrate pre-treatment [8][9][10][11], growth temperature [12][13][14], selection and mass flow rate of precursors [13,[15][16][17][18][19][20]. To complicate things further, strongly different reactor geometries from horizontal [14] and vertical [17] flow, multi-wafer planetary [8,12,15] to showerhead [16,19,20] obviously lead to in part fundamentally contradictory findings, e.g.…”

“…Up to now, a large number of process details and growth parameters have been reported to influence the final output of MOCVD processes, including substrate pre-treatment [8][9][10][11], growth temperature [12][13][14], selection and mass flow rate of precursors [13,[15][16][17][18][19][20]. To complicate things further, strongly different reactor geometries from horizontal [14] and vertical [17] flow, multi-wafer planetary [8,12,15] to showerhead [16,19,20] obviously lead to in part fundamentally contradictory findings, e.g. concerning the selection of carrier gas of either N 2 or H 2 .…”