Molecular Beam Epitaxy of GaSe using valved Se

cracking source

Choong Hee Lee1, Sriram Krishnamoorthy1, Dante J. O'Hara2, Roland K. Kawakami2,3, Siddharth Rajan1

06/22/2016 1 Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA 2 Program of Materials Science and Engineering, University of California, Riverside, CA 92521 3 Department of Physics, The Ohio State University, Columbus, OH 43210

Contact : lee.5107@osu.edu

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Introduction

Low temperature growth of GaSe

High temperature growth of GaSe

Two-step growth of GaSe

Summary

2

Outline

/ 25 3

2D material family

A large family of 2D materials covers the band gaps from zero to UV region

2D materials can be metallic, dielectric, semiconducting

Combining different 2D materials can create heterostructures or superlattices

TMD (transitionmetal dichalcogenides)

IIIA chalcogenides IVA chalcogenides

h-BN

X-ene b-P

Xie et al., Nanoscale 7.44 18392-18401 (2015).

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Integration of different band gap materials 3D wide band gap : high breakdown fields 2D narrow band gap : Atomically thin with low Rsheet

MoS2/GaN Tunnel junction → Wed. 1:30 pm, Clayton Hall, Room 128

MoS2/GaN based HBT → Wed. 1:50pm, Clayton Hall, Room 128 4

Motivation : 2D/3D heterostructure

Lee II, Edwin W., et al. Applied Physics Letters 107.10 (2015): 103505.

Krishnamoorthy, Sriram, et al. "High Current Density 2D/3D Esaki Tunnel Diodes." arXiv preprint arXiv:1606.00509 (2016).

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What do we need for devices? → Large area high quality film

Molecular Beam Epitaxy (MBE) enables high purity low background materials

Sharp heterojunctions

In situ monitoring RHEED Raman spectroscopy

5

2D material growth using MBE

Westwood, W. D. Microelectronic Materials and Processes. Springer Netherlands, 133-201 (1989).

2D MBE system in OSU

/ 25 6

Van der Waals epitaxy using MBE

van der Waals epitaxy (vdWE) introduced by Atsushi Koma

Lattice mismatch can be removed if two materials do not have chemical bonding.

2D/3D heterostructures are also possible

1) Koma, Atsushi, Kazumasa Sunouchi, and Takao Miyajima. Microelectronic Engineering 2.1 (1984): 129-136. 2) Koma, Atsushi. Thin Solid Films 216.1 (1992): 72-76.

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Why GaSe ? → Understand the growth of vdWE

Layered 2D hexagonal binary chalcogenide semiconductor

Repeating unit cell of Se-Ga-Ga-Se (1ML)

Direct band gap of ~1eV (Bulk) / Indirect bandgap of ~2 eV (1ML)

Not stable in ambient conditions 7

Introduction to GaSe Not stable in air

Beechem et al. Applied Physics Letters 107.17 173103 (2015).

https://www.webelements.com

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Able to achieve single crystal GaSe on passivated-GaAs1 and unpassivated-Si2 substrate.

Growth diagram for GaSe on Si was proposed.2

In-plane disordered GaSe on sapphire(0001) has been grown.3,4

Today’s talk : Single crystal GaSe growth on GaN substrate

8

Previous MBE growth of GaSe

3Wu et al. physica status solidi (a) 212.10 2201-2204 (2015).

2Eddrief, M., et al Journal of crystal growth 135.1 1-10 (1994).

1Ueno, Keiji, et al. Japanese journal of applied physics 30.8A L1352 (1991).

4Chegwidden, Scott, et al. Journal of Vacuum Science and Technology-Section A-Vacuum Surfaces and Films 16.4 2376-2380 (1998).

GaAs (111)B [10-1] GaAs (111)B [11-2]

GaSe [10-1] GaSe [11-2]

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Introduction

Low temperature growth of GaSe

High temperature growth of GaSe

Two-step growth of GaSe

Summary

9

Outline

/ 25

Source material Ga : standard Knudsen thermal effusion cell Se : valved cracker source

Sample preparation Solvent cleaning Baked at 400 oC & 850 oC

Growth condition Se=1E-6 Torr Temperate : 350 oC - 500 oC Ga:Se = 1:10 – 1: 200

Substrate c-plane Sapphire c-plane GaN

Goal : large area sc-GaSe growth

10

GaSe growth condition using MBE Se valved cracker source

Effusion cell for Ga

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Substrate : c-plane Sapphire Growth study for GaSe

The Ga2Se3 phase is detected at very low Ga flux (1:200 flux ratio) due to Ga deficiency

As Ga flux increases, (002) diffraction peak for GaSe was observed.

11

XRD results for GaSe/Sapphire

Ga:Se BEP ratio 350 oC 400 oC 450 oC 500 oC

1:10 1:50

1:100

1:200

10 15 20 25 30 35 40 45

2theta (degree)

Ga:Se=1:200

Ga:Se=1:100

Coun

ts (a

.u.)

Ga:Se=1:50

GaSe (006)

Ga:Se=1:10

GaSe (002) GaSe (004)

GaSe (002)GaSe (004)

Ga2Se3 (111)

Sapphire (006)GaSe (004)GaSe (002)

Tsub=400 oC

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Clear streaky RHEED patterns were observed at certain Ga:Se flux ratios for a given growth temperature (Boxed with red lines).

Excess Se or Ga was found to make the surface rougher.

With very high Ga flux, dimming of the RHEED pattern was observed.

12

RHEED images for GaSe Ga:Se

BEP ratio Tsub=350 oC Tsub=400 oC Tsub=450 oC Tsub=500 oC

1:50

1:100

1:200

No RHEED Spotty Dimmed Streaky

Spotty Spotty

Spotty Spotty Streaky Dimmed

Spotty

Sapphire [10-10]

Gase [11-20]+[10-10]

Ga2Se3

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[10-10] streak spacing is larger by a factor of 3 than [11-20] pattern.

The presence of GaSe streaky RHEED at all azimuthal rotations.

The GaSe film shows in-plane structural disorder.

13

GaSe RHEED image

[11-20]

[10-10]

[10-10]

[11-20]

Wu et al. physica status solidi (a) 212.10 2201-2204 (2015).

𝑚𝑚

𝑎𝑎 = 3𝑚𝑚

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Required Ga flux for smooth surface reduces as temperature increases

Excess Ga results in rougher surfaces.

Smooth atomic steps with the step height measured to be ~0.8 nm

Large area coverage 14

AFM images for GaSe

Ga:Se BEP ratio Tsub=350 oC Tsub=400 oC Tsub=450 oC Tsub=500 oC

1:50

1:100

1:200 RMS=9nm RMS=22 nm

RMS=203nm RMS=25nm

RMS=2nm

RMS=28nm

RMS=8 nm

RMS=2nm

2nm/min

1.3nm/min

1nm/min

RMS=45nm RMS=160nm

RMS=1nm Ga2Se3

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At the line, Streak RHEED patterns and smooth surface

Above the line, Se sufficient → Ga flux limits the growth rate Spotty RHEED, 3D islands formed

Below the line, Ga sufficient → Se flux limits the growth rate Dimmed RHEED and rough surface

Narrow growth window

15

Schematic of growth diagram

350 400 450 500

BEP

ratio

(Se/

Ga)

Growth temperature (oC)

3D growth

Rough surface

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Growth condition 400 oC, Ga:Se=1:100

Smooth terrace morphology

(0002) oriented GaSe and in-plane disordered crystal structure

Low temperature growth → complete surface coverage

Need high temperature growth. 16

GaSe growth on GaN substrate with low Se flux

5 10 15 20 25 30 35 40 45 50

GaSe (008)

Sapphire (006)

GaN (002)

GaSe (004)

Coun

ts (a

.u.)

2theta (degree)

160226 GaSe on GaN

GaSe (002)

Gase [11-20]+[10-10]

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Introduction

Low temperature growth of GaSe

High temperature growth of GaSe

Two-step growth of GaSe

Summary

17

Outline

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Phase pure GaSe film was obtained at high temperature growth

18

High temperature GaSe growth on GaN

10 20 30 40 50

2θ (degree)

Ga:Se=1:400, 500 oC∗

∗∗

∗

∗∗

∗

∗

∗∗

∗∗

∗∗

∗

∗∗

∗

∗

∗∗

∗

∗

∗∗

Inte

nsity

(a.u

.)

1:200, 500 oC

1:100, 500 oC

1:400, 550 oC

GaSe (004)

1:200, 550 oC

Ga2Se3 (111)

GaSe (002)

∗∗

∗

∗

1:100, 575 oC

Intermediate growth temperature induced mixed phase of GaSe and Ga2Se3

Pure Ga2Se3 phase at high Se rich growth condition

Tsub : 400 oC → 500-575 oC Se flux : 1E-6Torr → 1E-5 Torr

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Six-fold symmetry hexagonal structure

GaSe and GaN hexagonal unit cells are aligned [11–20]GaSe//[11–20]GaN [10–10]GaSe//[10–10]GaN

Aligned triangular islands formed

High temperature growth → single crystal

Incomplete surface coverage → Two-step growth (High temp + Low temp) 19

Single crystal GaSe growth Tsub=575 oC Ga:Se=: 1:100 (Se=1e-5 Torr) Growth time : 2hrs

GaSe [11-20] GaSe [10-10]

GaN [11-20] GaN [10-10]

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Introduction

Low temperature growth of GaSe

High temperature growth of GaSe

Two-step growth of GaSe

Summary

20

Outline

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Engineering the growth

Additional GaSe layer was grown at low temperature for improved surface coverage

Rough surface morphology Spotty + streaky mixed patterns

Six-fold symmetry still sustained

21

Two step growth of sc-GaSe

Gase [11-20] 1st Gase [10-10] 1st

GaN [11-20] GaN [10-10]

Gase [11-20] 2nd Gase [10-10] 2nd

GaN

1st step GaSe (575 oC, 1E-5 Se flux)

2nd step GaSe (400 oC, 1E-6 Se flux)

Single crystal

Film coverage

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Higher order peak of (008) for GaSe after two-step growth

Six-fold symmetry of hexagonal structure of sc-GaSe

Unit cell of GaSe aligned to that of GaN substrate

22

Crystal structure of two-step growth of GaSe

10 20 30 40 50

GaS

e (0

08)

Sapp

hire

(006

)

GaN

(002

)

GaS

e (0

04)

Inte

nsity

(a.u

.)

2θ (degree)

GaSe on nucleation layer Nucleation layer

GaS

e (0

02)

Sapp

hire

(006

)

GaN

(002

)

GaS

e (0

04)

GaS

e (0

02)

-180 -120 -60 0 60 120 180

Inte

nsity

(a.u

.)Phi (degree)

GaN (102) GaSe (103)

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Faint terrace morphology with small islands RMS surface roughness : 1.35 nm

Strong Raman signals from GaSe after two-step growth

Improved the surface coverage of GaSe over GaN substrate

23

AFM and Raman data of GaSe

100 150 200 250 300 350

A21g

E22g

E22g

A11g

A21g

E21g

A11g

2nd step GaSe

1st step GaSe

Inte

nsity

(a.u

.)

Raman shift (cm-1)

GaN/Sapphire substrateE2

L (GaN)

0 2 4 6 8 10 12 14 16 18 2002468

101214161820

µm

µm

350.0

550.0

750.0

950.0

1150

13501500

A11g

/ 25

Introduction

Low temperature growth of GaSe

High temperature growth of GaSe

Two-step growth of GaSe

Summary

24

Outline

/ 25

Low temperature growth Smooth stpe flow morphology In-plane disordered

High temperature growth Single crystal GaSe Incomplete coverage

Two-step growth Single crystal GaSe Smooth surface Complete coverage

25

Conclusion

GaSe [11-20] GaSe [10-10]

-180 -120 -60 0 60 120 180

Inte

nsity

(a.u

.)

Phi (degree)

GaN (102) GaSe (103)

Gase [11-20]+[10-10]

0 2 4 6 8 10 12 14 16 18 2002468

101214161820

µm

µ m350.0

550.0

750.0

950.0

1150

13501500

A11g