Transcript Metal-free-catalyst for the growth of Single Walled Carbon

Metal-free-catalyst for the growth
of Single Walled Carbon
Nanotubes
P. Ashburn, T. Uchino, C.H. de Groot
School of Electronics and Computer Science
D.C. Smith, G. N Ayre, K.N. Bourdakos
School of Physics and Astronomy
A.L. Hector, B. Mazumder
School of Chemistry
University of Southampton
Contents
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Aims
New CNT growth process using Ge
Initial results
Possible mechanisms
Refinement of CNT process
Recent results
Conclusions
Aims
Standard Growth Methods
• Require
– Metal catalyst nanoparticles
• Metals include Fe, Ni, Co and others
• Particles need to be a few nanometers in diameter
– Carbon containing gas
• SWNT favoured by smaller non-conjugated molecules
– Energy to decompose the feedstock
• Thermal energy – CVD
• RF – Plasma enhanced CVD
• Often include
– Hydrogen –
• encourages SWNT growth
– Oxidising agent –
• appears to regenerate catalyst
Non-metallic routes to SWNT
• But standard standard growth methods have
disadvantage of using metal catalysts
• Metals are lifetime killers in silicon & also
degrade yield
• Non-metallic catalyst is desirable for silicon
compatibility
• Aim of this research is to search for non-metallic
routes to SWNT growth
A New CNT Growth Method
using Germanium
New Germanium based route to
SWNT
Start with either
SiGe (30% Ge) layer grown on silicon
Or
Stranski-Krastanow Ge (d = 20-250nm) dots on silicon
Pre-Treat Substrates
• Carbon ion implant – 31016 cm-2 30keV
• Strip native oxide with HF vapour
• Chemically oxidise with H202
Gas inlet
(CH4, Ar, H2)
Quartz tube
Gas
outlet
Substrate
Ceramic
boat
Oven
Temperature (C)
Chemical Vapour Deposition
1000
850
RT
30
90
60
Time (min)
Two Step Process
Anneal – 1000oC, H2 300 sccm, Ar 1000 sccm, 10min
Growth – 850oC, CH4 1000 sccm, Ar 300 sccm, 10 min
Initial results
SEM Post Growth: SiGe Samples
• Two types of fibre observed after growth
(a) As-grown
• “Fat” Curly Fibres
• Removed by HF vapour etch
• Oxide nanofibers
• “Thin” straighter fibres
• Remain after HF vapour etch
500 nm
(b) After HF Treatment
•Carbon nanotubes
500 nm
Raman Spectra
(a)
G band
lexcitation = 633nm
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Clear G-band signal
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No D- band observed
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Radial
breathing
modes
indicate SWNT with diameters
in range 1.2-1.6nm
CNTs
Intensity (arb. units)
•
1200
1400
RBM
1600
Si
1800
(b)
CNTs
-100
-200
-300
-400
(c)
• Thick fibres give broad peak at
1400 cm-1 similar to ones
reported for amorphous carbon.
Thick fibers
1200
1400
1600
1800
Raman shift (cm-1)
Intensity (arb. units)
Ge dot samples
-250
1200
-200
-150
-100
Raman shift (cm-1)
1400
1600
Raman shift
CNTs on Ge dots
1800
(cm-1)
Raman spectra of Ge dot sample
TEM Images
(b)
(a)
10 nm
A bundle of SWNTs
50 nm
Oxide nanofibers
Possible Mechanisms
Analysis of Experimental Data
Sample
Implant
Pre-treatment
Growth Gas
CNT growth
1
C ion
H2O2
CH4, H2
CNTs
2
C ion
-
CH4, H2
Lower CNT
density
3
C ion
H2O2
Ar, H2
No CNTs, oxide
fibres
4
No
H2O2
CH4, H2
No CNTs
5
No
-
CH4, H2
No CNTs
Further Analysis
• Evidence of C diffusion to surface
• C expected to aid nanotube growth
1023
102
Ge
1022
(a)
(b)
Ge
C
O
101
C
1021
100
O
1020
10-1
1019
10-2
1018
10-3
1017
10-4
0
40
80
0
40
SEM
80
Depth (nm)
After implant
Ge content (%)
C, O Concentration (cm -3)
•Ge nanoparticles formed during pre-anneal
1 µm
After pre-heating
AFM
Possible Mechanisms
• Vapour-liquid-solid growth one possibility:
Ge nanoparticle would be seen at tip of
nanotube
• Nanotube growth from root another
possibility
• TEM shows no evidence on particle at tip
Refinement of CNT Process
Issues
•Ge nanoparticles responsible for growth
•But need to control nanoparticle size
•Ge implantation widely used to create Ge
nanoparticles in oxide
•Has advantage that nanoparticle size
controlled by implant dose and anneal
conditions
Ge Nanoparticle Fabrication
Ge implant
SiO2
Si
Anneal
SiO2
Si
Oxide etch
Ge nanoparticles
Si
Recent Results
Ge Nanoparticles
3E16cm-2 Ge implant
No C implant
600C anneal
3E16cm-2 Ge implant
No C implant
1000C anneal
Ge Nanoparticle Sizes
3E16cm-2 Ge implant
No C implant
600C anneal
3E16cm-2 Ge implant
C implant
600C anneal
•600C anneal gives ~2nm Ge nanoparticles
•C implant gives smaller Ge nanoparticles
After Nanotube Growth
3E16cm-2 Ge implant
No C implant
600C anneal
Carbon nanotubes formed
3E16cm-2 Ge implant
No C implant
1000C anneal
No carbon nanotubes formed
•New process allows nanotube growth
without C implant
Analysis of Experimental Data
Pre-anneal
CNT growth
CNT area density (μm in length / μm2)
Temperature
Time
Temperature
Time
without C+
C+
1100 ℃
5 min
850 ℃
20 min
No CNTs
No CNTs
1050 ℃
5 min
850 ℃
20 min
No CNTs
0.6 ± 0.1
1000 ℃
5 min
850 ℃
20 min
No CNTs
2.7 ± 0.8
950 ℃
10 min
850 ℃
20 min
0.6 ± 0.2
2.0 ± 0.6
900 ℃
10 min
850 ℃
20 min
3.5 ± 1.0
4.1 ± 1.2
850 ℃
10 min
850 ℃
20 min
1.3 ± 0.1
1.2 ± 0.3
N/A
N/A
1000 ℃
20 min
No CNTs
No CNTs
N/A
N/A
950 ℃
20 min
1.6 ± 0.5
3.2 ± 2.6
N/A
N/A
900 ℃
20 min
No CNTs
0.4 ± 0.1
N/A
N/A
850 ℃
20 min
No CNTs
No CNTs
•Carbon implant widens process window for
nanotube growth
Effect of Carbon Implant
#2 no C+ implant
#2 C+ implant
RBM
G
Si
G
RBM
Si
100 200 300 400 500
Intensity (arb. units)
Intensity (arb. units)
Si
Si
RBM
100 200 300 400 500
D
M
1200
1400
1600
Wave number
1800
(cm-1)
No C implant
2000
1200
1400
1600
1800
2000
Wave number (cm-1)
C implant
•No D band for C implanted samples
•Carbon implant improves nanotube “quality”
Conclusions
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Developed a new route to SWNT growth
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Evidence shows Ge nanoparticles key to growth
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SWNTs produced have diameter range 1.2 -1.6nm
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SWNTs are “highly quality” as measured by Raman
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Implanted Ge nanoparticles give more reproducible
SWNT growth.
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C implant widens process window for SWNT growth &
improves nanotube “quality”