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Metalorganic Chemical Vapor Deposition (MOCVD) Growth of 2D Chalcogenides:
Challenges and Opportunities
Joan M. RedwingSept. 27, 2016
Outline
Overview of 2DCC goals & facilities Chalcogenides - What are the materials of interest? Synthesis methods for transition metal dichalcogenides (TMDs) MOCVD equipment and process challenges CVD/MOCVD of TMDs – status of the field The 2DCC platform: MOCVD capabilities, current & future
2D monolayers, surfaces and interfaces are emerging as a compelling class of systems with transformative new sciencethat can be harnessed for novel device technologies.
2D Materials for Next Generation Electronics
TIs & TMDs: steep slope transistors
(< 60 mV/decade) Topological spintronics: efficient spin transfer torque for
low power MRAM (fJ/switch)
Nature (2014)(Ralph & Samarth)
Nature Comm. (2015)(Robinson)
Hybrid 2D devices
cf. Fiori et al., Nature Nano. (2014)
2D Crystal Consortium (2DCC) Platform
Develop custom deposition tools with in situ and real time characterization of monolayer and few layer films.
Unique capabilities in simulation of reaction kinetics through first principles + reactive potential approach
The Layered Chalcogenide Families
2D chalcogenides offer a wide range of electronic and optical properties for devices.
TMD Synthesis “Atlas”
S. Das, J.A. Robinson, M. Terrones, et al.Annual Review of Materials Research, 45 , 1-27 (2015)
Powder Vaporization vs. CVD What’s the difference?
Chem. Mater., 2014, 26 (22), pp 6371–6379
Powder Vaporization vs. CVD: What’s the difference?
Nature Materials, doi:10.1038/nmat4064 (2014)
Nature Materials 13, 1135–1142 (2014)
WS2-WSe2 lateral heterostructures
MoS2-MoSe2 lateral heterostructures
Nature Nanotechnology 9, 1024–1030 (2014)
CVD/MOCVD Equipment Basics
Sources (liquid and solid) are outside chamber in temperature and pressure-controlled “bubblers” to precisely control source vapor pressure.
Hydride gases (SiH4, NH3, AsH3, H2Se, H2S, etc.) used to minimize carbon incorporation.
Vent/run assembly –used to establish steady state gas flows and switching.
Reaction chamber –typically cold wall to prevent source pre-reaction.
Pressure control of chamber –typically from milliTorr to atmospheric pressure.Image: Wikipedia
Highly toxic!
Pyrophoric!
Ventilated enclosure with toxic gas & flame detectors
State-of-the-art MOCVD
Computational fluid dynamics simulation of momentum, heat and mass transport
In-situ monitoring –optical reflectance, ellipsometry & curvature
Computer control and tracking of all gas flows, temperatures, pressures, etc.
agnitron.comitn.liu.se.edu
Sandia.gov coewww.rutgers.edu
MOCVD/CVD Process Challenges - TMDs
Element Melting Temp (oC)
Vapor Press (Torr)
(@500oC)W 3422 1.78x10-47
Mo 2617 1.97x10-27
Nb 2468 2.97x10-38
S 115 >750
Se 221 49
Te 450 0.91
Limited surface mobility
High surface desorption rate
W, MoS, Se
A route to single crystal monolayer films
What is needed?• Substrate to serve as template for “epitaxy”• Oriented nuclei sufficient thermal energy for rotation• Low nucleation density low gas phase supersaturation• Lateral growth of nuclei long surface diffusion lengths• Coalescence of domains with few low angle grain boundaries• No additional nucleation of domains
MOCVD of WSe2 – Effect of Substrate Temperature
400 oC 600 oC 800 oC
200 nm 200 nm200 nm
PW(CO)6 = 3x10-4 TorrSe:W ratio = ~6500
Higher substrate temp higher surface diffusion length larger domains
Substrate
H2, W(CO)6, DMSe Sources: • W(CO)6 (25oC, 740 Torr)• (CH3)2Se (DMSe, 25oC, 740 Torr)• UHP H2 carrier gasReactor pressure=700 TorrTotal gas flow rate=450 sccmDeposition time=30 minSapphire substrate
MOCVD of WSe2 – Effect of Se/W Ratio
3332
200 nm
1666
200 nm
833
200 nm
PW(CO)6 = 3x10-4 Torr, Substrate temp=800oC
Higher Se/W ratio Larger domains/lower nucleation density
4999
200 nm
6665
200 nm
8331
200 nm
Increase Se/W further domains become defective
MOCVD of WSe2 with DMSe ((CH3)2Se)800°C, 700 Torr, PW(CO)6 = 6x10-4 Torr, Se:W=3332
High concentration of CH3*
Deposits defective graphene on sapphire
Defectivegraphene?
Auger spectra:
CVD of WSe2 with H2Se
No carbon!
Toxicity of H2Se! – 50 ppb (TWA), 0.3 ppm (LC50)
800oC, 700 TorrPW(CO)6 = 6x10-4 Torr, Se/W~20,000
• Can use much higher Se/W ratios• Higher nucleation density• Coalesced films
MOCVD of WS2: DES ((C2H5)2S) and H2S
H2S
• Domains are larger with H2S than with DES.• Carbon present in films grown by DES reduces
the lateral growth.
DES = 1.1 sccmS:W = ~6500
H2S = 10 sccmS:W = ~60000
DESA1gE1
2g
Carbon
800°C, 50 Torr, PW(CO)6 = 4.25x10-5 Torr
CVD of MoS2 – Controlling NucleationV.K. Kumar, S. Dhar, T.H. Choudhury, S.A. Shivashankar and S. RaghavanNanoscale 7, 7802 (2015)
S. Dhar, et al. Phys. Chem. Chem. Phys. 18, 14918 (2016)
Hot Wall CVD Reactor
Gas phase supersaturation:
MOCVD of MoS2 – Controlling NucleationK. Kang, et al. Nature 520, 7549, 656 (2015).
• Low gas phase supersaturation• Long growth time for monolayer
(~ 26 hours)
Hot Wall CVD ReactorMoS2 and WS2
CVD/MOCVD of WSe2 – Nucleation Step
• Reduce growth temperature initially increase nucleation density
• Increase W(CO)6 initially increased nucleation density
S.M. Eichfeld, et al. 2D Materials 3, 025015 (2016)
0 sec 10 sec 60 sec
4 µm 4 µm4 µm
Substrates for TMD Epitaxy
D. Dumcenco, et al. ACS Nano (2015)
MoS2 on (0001) sapphire
Substrate/Film a (Å)(0001)Sapphire 0.476
(0001)SiC 0.307(0001)GaN 0.319
MoS2 0.316WS2 0.316
WSe2 0.332
Mismatch~-0.4%(3x3) MoS2/(2x2)Al2O3
Mismatch~13%
D. Ruzmetov, et al. ACS Nano (2015)MoS2 on (0001) GaN
Epitaxy of WSe2 on Sapphire
WSe2 on annealed sapphireSources: W(CO)6, H2Se, H2 800°C, 700 TorrPW(CO)6 = 2.7x10-4 Torr, Se/W~26,000
Sapphire annealed at 1150°C in air for 8 hr
Epitaxy of TMDs - 2D Heterostructures
• Still challenging to grow monolayer heterostructures over larger areas• Advantage of CVD – in-situ heterostructure growth (without air exposure)
Y.C. Lin, et al. Nature Comm. 6, 7311 (2015)
H.M. Hill, et al. Nano Lett. 16, 4831 (2016)
• Electronic structure of 2D heterostructuresdependent on layer orientation
MoS2/WSe2 and WSe2/MoSe2 heterostructures
CVD/MOCVD – Other Layered Chalcogenides
Bi2Se3 Bi2Te3 NbS2
H.W. Jin, et al. Nanoscale 7, 17359 (2015)
S. Zhao, et al. 2D Materials 3, 025027 (2016)
J.E. Brom, et al. Appl. Phys. Lett. 100, 162110 (2012)
• Develop in-situ optical characterization to monitor film growth at monolayer level• Understand the role of the substrate in nucleation and epitaxy • Explore precursors and processes to achieve self-limiting layer-by-layer growth• Investigate growth of “new” 2D films and heterostructures• Develop reproducible processes for uniform, wafer-scale (2” diameter) 2D films• Design user-friendly equipment to promote accessibility to external users
2DCC Goals - MOCVD
MOCVD #1
Horizontal, cold wall reactor capable of growth on substrates up to 1x1 cm. Gas panel with six bubbler manifolds and constant temp baths for liquid/solid sources Gas cabinets, toxic gas detectors and effluent scrubber for H2S and H2Se gas sources LabView control system for fully automated operation with recipe mode. Current precursors: W(CO)6, Mo(CO)6, NbCl5, (C2H5)2S, (CH3)3In, (CH3)3Sb Current processes developed for WS2 and MoS2
Increase quartz tube to accommodate 2” diameter substrates w/rotation. Add glove box to minimize air exposure of samples and reactor chamber. Incorporate mass spectrometer on reactor for real time gas analysis.
Used Emcore III-V MOCVD system Converted to chalcogenides
Current Capabilities
Scheduled Upgrades
MOCVD #2
Raman
PL
Load lock and sample transfer stage
In situ spectroscopic ellipsometry for growth rate monitoring
8 bubbler stations and 4 gas sources including H2Se and H2S
Wide range of growth temperatures (200oC-1000oC) and reactor pressures (50 Torr to 1400 Torr)
In situ Raman and photoluminescence (PL) for monolayer film analysis
Mass spectrometer for analysis of gas phase chemistry
Custom designed chalcogenide MOCVD system Focus will be on incorporation of in-situ optical characterization
CVD/MOCVD Personnel
Prof. Joan RedwingMatSE/EE
Prof. Joshua RobinsonMatSE
Dr. Tanushree ChoudhuryResearch Associate
Natalie BriggsGraduate Student
Prof. Mauricio TerronesPhysics/MatSE/Chem
Dr. Sarah EichfeldResearch Associate
Xiaotian ZhangGraduate Student
Dr. Bhakti JariwalaPostdoctoral Scholar
How do I become a 2DCC user?Proposals are accepted through an online proposal submission portal Get started today at the Become a User tab on the 2DCC website.
The 2DCC accepts two types of proposals which align with the 2DCC scope and capabilities (which evolve over time):Research Projects Proposals can describe synthetic, characterization and/or theory efforts
that are performed by 2DCC staff and/or users who come to the facility. Typical project duration is 1 to 2 years. Proposals consist of 3 page (max) project description plus NSF-style bio. Proposals are reviewed by external experts.Request for Standard Samples Request for samples that are routinely fabricated by the 2DCC. Current
List provided on the 2DCC website. Proposals consist of 1 page (max) description plus NSF-style bio. Requests are reviewed internally.
