2D Chalcogenide Epitaxy by Gas Source Chemical Vapor Deposition

Tanushree H. Choudhury

Research Associate, Thin Films MOCVD2D Crystal Consortium – Materials Innovation Platform

2018 2DCC Webinar

April 24, 2018

2DCC Framework2DCC Framework…… where we fit in!

Gas Source Chalcogenide

CVD

EllipsometryRaman

SpectroscopyPhotoluminescence

mip.psu.edu

Thin Film - MOCVD PersonnelFaculty

Joan Redwing Joshua Robinson

Postdoctoral Scholar

Mikhail Chubarov

Graduate Students

Natalie BriggsGaSe

Azimkhan KozhakhmetovNbS2, Epi-graphene

Xiaotian ZhangWSe2

Jeffrey KronzCVD Graphene

Kehao ZhangPowder Vaporization MoS2

Research Associate

Tanushree Choudhury

F. Xia, et al. Nature Photonics 27 (2014) 899

Layered materials….beyond graphene

Covalent bonds within layer

Layers held together by van der Waals forces

Can form stable monolayer and few layer films

Bulk CrystalsFilms

Wide range of electronic properties

Transition metal dichalcogenides

(TMDs)MoS2, WSe2, WS2, MoSe2, WTe2, etc.

Layer number-dependent properties

A.K. Geim and I.V. GrigorievaNature 499 (2013) 419

Heterostructures without lattice matching

Interlayer exciton coupling

P. Rivera et al.Nature Comm. 6 (2015) 6242

2D TMDs – Intriguing Properties & Physics

Valley polarization

Linyou CaoMRS Bulletin 40 (2015) p. 592.

Xiao, et al. Phys. Rev. Lett. 108 (2012) 196802

Coulomb engineering

A. Raja, et al.Nature Comm. 6 (2017) 15251

A. Splendiani, et al.Nano Lett 10 (2010) 1271

2D tunnel diodes and transistors

T. Roy, et al. ACS Nano 9 (2015) 2071.

Flexible electronicsTom Jackson (PSU)

2D TMDs – Promising Applications

Exciton-based photonics

Linyou CaoMRS Bulletin 40 (2015) p. 592. D. Jariwala, et al.

ACS Photonics 4 (2017) 2962.

Thin film photovoltaics

Excitonic transistorsSaptarshi Das (PSU)

Chem/bio sensors

F. Perkins, et al.Nano Lett. 13 (2013) 668.

2D TMDs – How to get monolayers?

J. Wang et al. in “2D Materials – Synthesis, Characterization and Potential Application,” (2016)

15 x 22 mm MoS2 ($300) – 2dsemiconductors.com

+ =

Exfoliation of bulk crystals

Powder vapor transport

F. Zhang, et al. 2D Materials 4 (2017) 025029

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 detectors2016 2DCC Webinar: MOCVD Growth of 2D Chalcogenides: Challenges and Opportunities

Substrate

H2 Carrier Gas

W, Mo, S, Se Precursors

Gas Source CVD/Metalorganic CVD

• Cold wall vertical and horizontal reactor designs

• CFD simulations used to model fluid, heat and mass transport

Tungsten W(CO)6, WCl6Molybdenum Mo(CO)6Niobium NbCl5Selenium (CH3)2Se, H2SeSulfur (C2H5)2S, H2S

• Focused on understanding impact of precursor chemistry on growth and properties of TMD films

Substrates for TMD “Epitaxy”

D. Dumcenco, et al. ACS Nano (2015)

MoS2 on (0001) sapphireSubstrate/Film a (Å)

Sapphire 0.476SiC 0.307GaN 0.319hBN 0.250MoS2 0.316WS2 0.316WSe2 0.332

Mismatch~-0.4%(3x3) MoS2/(2x2)Al2O3

Mismatch~13%

D. Ruzmetov, et al. ACS Nano (2015)MoS2 on (0001) GaN

Epitaxial growth of large area TMD monolayers

• Growth of large area, single crystal TMD monolayers and heterostructures needed to advance device technologies

• Transition metals and chalcogens have very different vapor pressures

• Suitable substrate needed to provide template for “epitaxy”

• Growth conditions must promote lateral growth of domain edges with minimal bilayer nucleation

Challenges/requirements:

epitaxial orientation lateral domain growth coalescence

Three Step Process for WSe2 Monolayers

Nucleation Stage Ripening Stage Lateral Growth Stage30 s 5 min 45 min

Substrate temperature: 800˚C, H2Se flow rate 7 sccm (Se/W~26,000), Reactor pressure: 700 Torr

Nucleation Stage: • High initial partial pressure of W(CO)6 to

drive nucleation.

Ripening Stage:• Annealing in H2Se to promote surface

diffusion and uniform WSe2 domain size.

Lateral Growth Stage:• Lower partial pressure of W(CO)6 to enable

lateral domain growth without additional nucleation.

X. Zhang, et al. Nano Lett. 18 (2018) 1049

1 μm

Ripening of WSe2 islandsNucleation stage: 30 sec at W(CO)6=1.2x10-3 sccm, H2Se=7 sccm, 800oC, 700 TorrRipening stage: 0 to 45 mins at H2Se=7 sccm only

Ripening and Surface Diffusion

𝑟𝑟 𝑡𝑡 = 𝑟𝑟 0 1 +𝑡𝑡𝜏𝜏

1/3𝜌𝜌 𝑟𝑟 = 0, 𝑡𝑡 = 𝜌𝜌 𝑟𝑟 = 0,0

1

1 + 𝑡𝑡𝜏𝜏

2D Ripening Model:

Domain size (r): Cluster density (ρ):

Estimate of surface diffusion coefficient:

• Diffusing species may be adatoms or small clusters

LD ≈ ⁄1 2 Lav where Lav ≈ ⁄1 𝜌𝜌LD = 2 𝐷𝐷𝑡𝑡

𝜌𝜌 ≈1

16𝐷𝐷𝑡𝑡 𝐷𝐷 ≈ 1.2x10−14 ⁄𝑐𝑐𝑐𝑐2 𝑠𝑠𝑠𝑠𝑐𝑐

*similar to Pt cluster diffusivity on sapphire at 600oC (P. Wynblatt, et al. Prog. Solid State Chem 9, 21 (1975)).

Lateral Growth of WSe2 Islands

Coalescence of monolayer in < 1 hour Bilayer growth rate increases after

~50% monolayer coverage

Preferred direction Commensurability

Nucleation stage: 30 sec at W(CO)6=1.2x10-3 sccm, H2Se=7 sccm, 800oC, 700 TorrRipening stage: 10 mins at H2Se=7 sccm onlyLateral growth stage: 5-45 mins at W(CO)6=2.7x10-4 sccm, H2Se=7 sccm, 800oC, 700 Torr

Coalesced film

1L

2L

Lateral Growth – Effect of Substrate Temperature

Lateral growth rate independent of temperature from ~700-900oC

Suggests growth limited by mass transport of W-precursor to surface

Shape evolution related to Se:W ratio

Increase H2Se to 10 sccm

S. Wang, Chem. Mater. 2014, 26 (22), 6371–6379

Nucleation stage: 30 sec at W(CO)6=1.2x10-3 sccm, H2Se=7 sccm, 800oC, 700 TorrRipening stage: 10 mins at H2Se=7 sccm only, 800oC, 700 TorrLateral growth stage: 10 mins at W(CO)6=2.7x10-4 sccm, H2Se=7 sccm, 600-900oC, 700 Torr

Epitaxial Orientation – In-Plane XRD• Conventional Bragg-Brentano XRD challenging for 2D monolayers

Epitaxial Relationship: (11�20)WSe2 // (11�20)sapphire

ω-2θBragg-Brentano

In-Plane XRDφ-scan FWHM=0.28o

M. Chubarov, et al. Nanotech. 29 (2018) 055706.

TEM Characterization of WSe2

• Water-based transfer process used to remove WSe2 from sapphire

(developed by Fu Zhang and Nasim Alem at Penn State)

anti-phase boundary

• Anti-phase boundaries in WSe2 due to coalescence of 0o and 60o domains

F. Zhang, et al. Nanotech. 29 (2017) 025602

Grain boundaries in TMD monolayers

H. Yu et al. Small 2017, 13, 1603005

0° + 60° → anti-phase grain boundary (metallic)0° + 0° → perfect crystal

0° + 0°

0° + 60°

Epitaxy of WSe2 on hBN flakes

0 50 100 150 200 250

Cou

nts

Angle (degree)

Three-step growth: 60 sec. nucleation, 10 mins ripening, 45 mins lateral growth at 800oC, 7 sccm H2Se

fully coalesced region

2 µm

Add TEM from Fu

~80% preferred orientation

2 nm2 nmReduced number of anti-phasegrain boundaries for WSe2 on hBN

Fu Zhang and Nasim Alem

Anti-phase boundary No grain boundary

0o + 60o 0o + 0o

1.45 1.50 1.55 1.60 1.65 1.70 1.75

Inte

nsity

(a.u

.)

Energy (eV)

1.65

1.45 1.50 1.55 1.60 1.65 1.70 1.75

Energy (eV)

Nor

mal

ized

Inte

nsity

(a.u

.)

1.64

1.62

1.60

0 1 2 3 40

1

2

3

4

5

x-axis (µm)

y-ax

is (µ

m)

1.58

1.60

1.62

1.64

1.66

0 1 2 3 4 50

1

2

3

4

5

x-axis (µm)y-

axis

( µm

)

1.58

1.60

1.62

1.64

1.66FWHM ≈ 55 meV

PL peak = 1.606 ± 0.017 eV

FWHM ≈ 90-130 meV

Photoluminescence

PL peak = 1.656 ± 0.002 eV

WSe2 on sapphire

1L

2L

1L2L

WSe2 on hBN

Promising approach but limited by lack of large area hBN substrates or templates

Process scale-upUpgraded III-V MOCVD system - Converted to chalcogenides

Glove box for sample loading/unloading

Integrated toxic gas detection system

Gas panel with vent/run manifolds, 6 bubblers for liquid/solid sources, 4 gas source lines (H2Se, H2S)

Cold wall reactor for 2” diameter substrates with rotation

WS2 on 2” sapphire

Material capabilities: WS2, MoS2, NbS2

WSe2, MoSe2, NbSe2

WTe2, MoTe2

Doping, alloys, heterostructures

1L

2L

Room temperature PL

WS2 epitaxy on sapphire

750°C

800°C

850°C

Pressure: 50 Torr, Duration: 60 min, W(CO)6 = 5.7x 10-4 sccm, H2S = 160 sccm, S:W = 270000

M. Chubarov, et al. Nanotech. 29 (2018) 055706.

900°C

950°C

1000

°C

Pressure: 50 Torr, Duration: 60 min, W(CO)6 = 5.7x 10-4 sccm, H2S = 160-320 sccm

1L

2L

The chalcogen concentrations plays an important role in obtaining epitaxy.

(3030) α – Al2O3

(1010) WS2

(1010) WS2

H2S= 320 sccm

H2S= 160 sccm

Edge

Center

Edge

Center

Edge

1L

2L

(Mo,W)S2 monolayers, alloys and heterostructures

Photoluminescence: (Mo,W)S2

850oC

In-Plane XRD: WS2 on sapphire

1L

2L

sapp

hire

λ = 532 nm

WS2

MoS2

(Mo,W)S2 alloysAFM: WS2 on sapphire

MoS2/WS2 heterostructures

Growth temperature: 850-1000°C, Reactor Pressure: 50 Torr,W(CO)6 = 10-4-10-5 sccm, Mo(CO)6 = 10-3-10-2 sccm H2S = 400 sccm, H2 carrier gas

ADF-STEM image (S. Bachu, D. Hickey and N. Alem)

Mo (dark)

WS2

Defect mediated growth

Growth of TMDPlasma treatment to

induce defects

Pristine Plasma treatedPressure: 50 Torr, Duration: 20 min, W(CO)6 = 5.7x 10-4 sccm, H2S = 400 sccm

Epitaxial graphene

In collaboration with Prof. Joshua Robinson, Prof. Adri van Duin and Prof. Eric Hudson (PSU)

Study interaction of defects with precursors to control nucleation sites.

Reactor modelling and simulations

ReaxFF calculations of gas phase kinetics – Prof. Adri van DuinCFD simulations Prof. Yuan Xuan (PSU)

Modelling of gas phase chemistry and flow dynamics to develop predictive capacity

In-situ Characterization

Features

• Cold wall stainless steel chamber• 2 inch wafer growth with mechanical wafer rotation• Transfer chamber with robot arm to transfer sanple between chambers• Ellipsometer for in-situ growth monitoring• Additional analysis chamber for Raman/PL without ambient exposure

Tungsten W(CO)6, WCl6Molybdenum Mo(CO)6Niobium NbCl5Selenium (CH3)2Se, H2SeSulfur (C2H5)2S, H2S

Installation Mid 2018

A national user facility focused on advancing the synthesis of 2D chalcogenides2D chalcogenide monolayers, surfaces and interfaces are emerging as a compelling class of systems with transformative new science that can be harnessed for novel device technologies in next-generation electronics.

An NSF user facility with broad access:

• Open calls for user proposals,• No user fees for U.S. academic or government use• Access to a team of local experts • Webinars, workshops, website resources• Partnership opportunities with PUI, MSI

For more information: mip.psu.edu

State-of-the-art synthesis and in-situ characterization equipment

External user requests: Mono/Bilayer WS2, MoS2, WSe2.

Research proposals: Te based TMDs will be explored.