1 Effects of CVD Growth P arameters on Global and Local Optical Properties of MoS 2 Monolayers

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Effects of CVD Growth Parameters on Global and Local Optical Properties of MoS2

Monolayers

Ana Senkić1,2⬧, Josip Bajo3⬧, Antonio Supina1,2, Borna Radatović1, and Nataša Vujičić1

1 Institute of Physics, Center for Advanced Laser Techniques and Center of Excellence for

Advanced Materials and Sensing Devices, Zagreb 10 000, Croatia

2 Faculty of Physics, University of Rijeka, Rijeka 51 000, Croatia

3 University of Vienna, Faculty of Physics, Vienna Center for Quantum Science and Technology

(VCQ), 1090 Vienna, Austria

Address correspondence to Ana Senkić, asenkic@ifs.hr; Nataša Vujičić, natasav@ifs.hr

⬧ These authors contributed equally to this work.

ABSTRACT

Semiconducting transition metal dichalcogenides (TMDs) combine strong light-matter interaction

with good chemical stability and scalable fabrication techniques, and are thus excellent prospects

for optoelectronic, photonic and light-harvesting applications. Controllable fabrication of high-

quality TMD monolayers with low defect content is still challenging and hinders their adoption

for technological application. The optical properties of chemical vapor deposition (CVD) grown

monolayer MoS2 are largely influenced by the stoichiometry during CVD by controlled

sulfurization of molybdenum (Mo) precursors. Here, we investigate how the sulfur concentration

influences the sample morphology and, both globally and locally, their optical response. We

confirm that samples grown under a Mo:S > 1:2 stoichiometric ratio have regular morphology

facilitated by a moderate coverage of triangular monocrystals with excellent optical response. Our

data-driven approach correlates growth conditions with crystal morphology and its optical

response, providing a practical and necessary pathway to address the challenges towards the

controlled synthesis of 2D TMDs and their alloys with desired optical and electronic properties.

KEYWORDS Molybdenum disulfide, Chemical Vapor Deposition (CVD), Growth Parameters,

Optical Properties of CVD MoS2

Introduction

The diversity of 2D material systems has been increasing substantially over the last two decades,

and each new system offers new and exciting research opportunities. Transition metal

dichalcogenides (TMDs) form a compelling class of 2D materials of significant interest due to

their thickness-dependent electrical and optical properties with potential applications in

optoelectronics, flexible electronics, chemical sensing and quantum technologies [1]–[9].

At the monolayer limit, the semiconducting TMDs exhibit direct band gaps within the visible range

and large exciton binding energies [10], [11]. The lack of inversion symmetry in the monolayer

results in unique optical selection rules, enabling circularly polarized light to selectively populate

degenerate valleys in the Brillouin zone, providing an additional degree-of-freedom for the

realization of valleytronic devices [3], [12]–[14]. Despite the vast amount of research conducted

on TMDs, mostly on MoS2, reproducible fabrication of high-quality TMD monolayers is still

challenging. Although the top-down method, such as exfoliation from bulk crystals [15], [16]

offers pristine sample quality and proves a satisfactory platform for one-off studies of properties,

it is neither scalable nor practical for TMD crystal incorporation into devices. To this end,

considerable effort has been taken towards controllable synthesis of monolayer materials using a

variety of techniques such as liquid exfoliation method [17]–[19], physical vapor deposition (PVD)

method, [20] chemical vapor deposition (CVD) method [21]–[25] and ultra-high vacuum (UHV)

molecular beam epitaxy (MBE) [26]–[29] method. As one of the bottom-up techniques, MBE

allows the epitaxial growth of 2D van der Waals materials and offers unparalleled control of

sample cleanliness and growth parameters due to the UHV environment. However, need for UHV

conditions makes it technically more demanding and time consuming. Another limitation of MBE

techniques is the need for the growth substrates that facilitates the MBE growth, which can be

incompatible with the desired application in optoelectronic devices [30]. The atmospheric pressure

CVD is the most widely used technique for the synthesis of TMDs layers because of its simplicity,

low costs, low precursor expenditure, fast growth rate and large domain sites making it compatible

with industry standards [31]. The CVD allows us an arbitrary choice of a substrate, as well as to

control the number of layers and domain size. However, sample uniformity and reproducibility

issues have been reported [32], with significant variations not only from the sample to the sample

but also across the single grain, posing a significant barrier for the implementation of CVD grown

TMDs in technological applications. Variations in optical response and charge transport

mechanisms in the CVD grown samples have been attributed to different origins, such as trapped

charges at sulfur vacancies [33], grain boundaries [34], trapped charges at the interface of MoS2

and oxide dielectrics [35], extrinsic disorder from adsorbates [36], and other defects within the

films. This calls for mensurable and controllable fabrication of TMD monolayers with high-quality

optical response and low defect density.

The scope of this work was to investigate how the sulfur concentration in correlation with Mo-

precursor concentration influences the morphology and optical response of MoS2 monolayers. The

CVD grown samples from liquid-based molybdenum (Mo) precursors were synthesized under

different growth conditions and their overall optical properties were investigated immediately

upon synthesis by measuring photoluminescence (PL) and Raman signal on different flakes across

the sample and from different sample zones. Described workflow includes sample synthesis under

certain growth conditions followed by optical spectra acquisition and subsequent data analysis

providing a crucial link to determine the next optimal CVD growth set of parameters to produce a

high-quality sample with desired morphology [37]. Such an approach, referred as “active

learning”, allows us to improve the knowledge about the future growth parameters space, its

resulting morphology accompanied by sample properties with as few experimental runs as possible

[38].

Here, the growth conditions are determinate by three parameters: growth temperature (TG), sulfur

temperature (TS) and inert gas flow (ζ). We noted how the optical properties of CVD grown

monolayer MoS2 are largely influenced by stoichiometry via controlled sulfurization of Mo-

precursors and we correlated the stoichiometry with the optical properties of CVD synthesized

monolayer MoS2 flakes prepared under varying degrees of MoO3 sulfurization. When the Mo:S

ratio changes and the amount of molybdenum increases, triangular flakes with Mo-zz termination

edge form. On the contrary, when the sulfur ratio increases, triangular flakes with only S-zz

termination edge form [39]. Therefore, controlled sulfurization of the Mo- precursors leads toward

high-quality single crystalline growth with no evidence of grain boundaries or extrinsic disorder

from the adsorbates, which have been confirmed with AFM and SEM measurements.

1. Materials and methods

1.1 MoS2 Synthesis

Figure 1 shows an illustration of the chemical vapor deposition (CVD) setup for MoS2 growth

along with a photograph of the sample depicting the droplet deposition contour in the wafer center

and the main steps of the synthesis at different stages. The wafer of 10 mm × 10 mm in size is

tentatively divided into 4 sample zones marked by dashed lines, with the first zone being upstream,

i.e., in the direction of the inert gas flow, as indicated with the arrow, see Fig. 1 b). The first zone

includes the region from the nearest wafer edge (with the respect to sulfur boat), over the droplet

edge and its depletion zone toward the end of the first quarter of the sample. The second zone

continues from the end of the zone 1 toward the droplet center; the third zone continues from the

droplet center up to the third quarter of the substrate and the fourth zone continues from the end

of the third zone over the outermost droplet edge, its depletion zone until the end the wafer,

therefore last quarter of the substrate. The four-zone approach enables us to examine how the sulfur

inflow influences crystal growth on different parts of the wafer, having in mind that the sulfur

precursor concentration is regulated by wafer distance from the sulfur boat (i.e. from the source of

the sulfur), its evaporation temperature and inert gas flow, under the assumption that Mo- precursor

and growth promoter are evenly deposited on the substrate over the all four zones. Since

evaporation temperature and inert gas flow can be precisely regulated, in order to ensure a

systematic growth method with high reproducibility over the whole wafer, with desired crystal

morphology and optical properties, the quality of the crystal was checked on a randomly chosen

flake from each of the four zones.

Prior to the synthesis, the Si/SiO2 substrate was a blow cleaned against dust particles using argon

gas. The silicon part of the substrate was heated with a propane flame in order to minimize any

organic impurities. As a Mo- precursor, we used a mixture of two deionized (DI) water-based

solutions in equal parts: ammonium heptamolybdate ((NH4)6Mo7O24) (AHM) solution (Kemika),

15.4 parts per million (ppm) and sodium-molybdate (Na2MoO4) solution (Merk’s Reagenzien),

15.4 ppm concentration. Our preliminary work showed that a mixture of these two solutions in the

same volume parts gives the best crystal morphology and good optical response [40]. When using

only AHM solution as the Mo- precursor, grown MoS2 flakes were relatively small and irregular

but having fair PL emission spectra. The density of crystal growth along the four substrate zones

was moderate and uniform. In the case when only sodium-molybdate solution was used, grown

MoS2 islands formed polycrystalline clusters and emission PL spectra were poor. The crystal

growth along the substrate zones was excessively dense, creating the inevitable bulky film

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1EffectsofCVDGrowthParametersonGlobalandLocalOpticalPropertiesofMoS2MonolayersAnaSenkić1,2⬧,JosipBajo3⬧,AntonioSupina1,2,BornaRadatović1,andNatašaVujičić11InstituteofPhysics,CenterforAdvancedLaserTechniquesandCenterofExcellenceforAdvancedMaterialsandSensingDevices,Zagreb10000,Croatia2FacultyofPhys...

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