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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 : [email protected]
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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
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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
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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
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Introduction
Low temperature growth of GaSe
High temperature growth of GaSe
Two-step growth of GaSe
Summary
24
Outline
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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