Technology and TEM characterization of Al doped ZnO nanomaterials

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Technology and TEM

characterization of Al

doped ZnO nanomaterials

國立成功大學 (NCKU)

材料科學及工程系 (MSE)

劉全璞 (Chuan-Pu Liu)

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Outline

• Introduction of ZnO

• Doping ZnO nanomaterials in CVD

• Al doped ZnO Nanowires

• Al doped ZnO Nanotube/Nanowire

junctions

• Al doped ZnO rectangular nanorods

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• Introduction of ZnO

• Wurtzite structure

• Wide and direct band-gap (3.3ev) semiconductor

• Polar surface (0001)/(0001) – Novel nanostructures

• High exciton binding energy (60mev)

• Candidate for optoelectronic devices and many more

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• Piezoelectric material

(surface acoustic wave devices )

• Varistor (bulk or thin film)

• Gas sensor (H₂ , CO and H₂O)

• DMS (diluted magnetic semiconductor)

• TCO (transparent conductive oxide) • Light-emitting devices

( nano-structure and thin film)

• Applications of ZnO

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Method: thermal evaporation

O₂gas

Ar gas

Pumping

x

Source: Zn or Zn+Al mixed powder

Substrate: Si (100) Alumina boat Quartz tube Pressure meter Adaptor x mm Heater

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D(T) = D₀ exp(-ΔH /RT)

V_tot =V_Zn + ∑ V_Zn-Al _{V: vapor pressure}

Simple and new concept –

Alloying-evaporation deposition (AED)

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TEM characterization of Al:ZnO nanowires alloying treatment temperature of 420o_C

First to demonstrate ZnO nanostructures doped with Al from TEM Al mapping

Al / (Al+Zn) : 2.5 at.%

determined by EELS

Applied Physics Letters, 88, 023111 (2006)

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Core

-

loss analysis for Zn and Al by EELS

Beam energy (keV): 200 Convergence angle (mrad):1 Collection angle (mrad):10 Cross model: hydrogenic

Background fit model: power law Signal width: 200eV

Elem. Areal density (atoms/nm**2) Zn 1.42e+015 ± 1.4e+014 / sum(spec) Al 2.27e+014 ± 2.3e+013 / sum(spec) Relative quantification:

Elem. Atomic ratio (/Zn) Percent content Zn 1.00 ± 0.000 86.25 ± 11.5 Al 0.16 ± 0.023 13.75 ± 1.8

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TEM of Al:ZnO nanowire/nanotube junction structures alloying treatment temperature of 500oC

ÖAl concentration in the ZnO nanostructures could be adjusted by varying the temperature of alloying treatment

Al / (Al +Zn) : 12 at.%

determined by EELS

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Possible reasons for the formation of

nanowire/nanotubes junction structures

Al:ZnO nanotube Al:ZnO necking region

Al:ZnO nanowire d ₍₀₀₀₁₎ = 5.16 Å

d ₍₀₁₁₀₎ = 2.82 Å

Al concentration increase

Lattice mismatch Nanotube formed to release the strain

Lattice constant decreases 2% from

Lattice constant decreases 2% from nanowire nanowire to to nanotubenanotube

d ₍₀₀₀₁₎ = 5.11 Å d ₍₀₁₁₀₎ = 2.81 Å

d ₍₀₀₀₁₎ = 5.05 Å d ₍₀₁₁₀₎ = 2.81 Å

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CL of ZnO and Al:ZnO 1D nanostructures

Blue-shift of UV peak due to Al incorporation

Green/UV decrease due to Al incorporation

A decrease in oxygen vacancy ion due to Al incorporation concentration

V··_{o +Al}

2O3 Æ 2Al·Zn +3Oo

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gas flow Source

A

B

C

D

1.5 cm 1.5 cm Time Temperature O₂ injection 650 °C, 3 torr, 1 hour Ar injection Substrate :Si (100) Source : Al+Zn

Horizontal tube furnace system

500 °C, 30 min

Doping-induced rectangular nanorods

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SEM results

Lengths of 5–10 microns, widths of 20–200 nm

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TEM results

¾the atomic ratio of Al to (Al+Zn) in the ZnO

rectangular nanorods and nanosheets from TEM EELS results is around

6.13 and 4.07 at.%, respectively.

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Growth mechanism

[01-10]

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More doping-induced nanostructures

100 nm c-axis (c) Root: Zn:O ≈ 52.4 : 47.6 Tip:Zn:O ≈ 48.7 : 51.3 Two-luminescenced Nanorods

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Low EELS of (ZnAl)O nanobelts by STEM SI C D C D off Edge center O-2p Zn-3d BPR O-2s ? Surface plasma -related? Al related?

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roi@8 a b Eg threshold O-2p _Zn-3d BPR Al related? O-2s ? Surface plasma -related? SPR a b off Edge center Low EELS of (ZnAl)O nanowire by STEM SI

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Mapping for the unknown peak energy at 4-5 eV

Only occurring from the edge

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Local [Al] variation

Lateral Axial

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100nm

(a)

100nm

(b)

~64～

Selective Growth of ZnO nanorods on ZnO

dots

(1) Growth of ZnO pyramids by RF sputtering on Si(111)

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(a)

(a) (b)

(2) Growth of ZnO nanowires only on pre-synthesized ZnO dots

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TEM characterisation of ZnO nanowires and the interface

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Conclusions

• Al doping technology proposed for growing ZnO nanomaterials by CVD

• Successful Al doping into ZnO nanowires

• Doping induced Nanotube/nanowir, rectangular nanorods, and two-luminescenced nanorods

• TEM characterization of microstructure and electronic structures of Al doped ZnO nanowires • Selective growth of Al-doped ZnO nanowires on ZnO pyramids

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Acknowledgement

Students:

Students: Wang Wang ReiRei--ChiChi; ; JiannJiann--Han Han Huang (Huang (黃黃俊俊翰翰 ), Yu

), Yu--Han Han Liao Liao ((廖廖譽譽翰翰) and ) and JiannJiann--Lin Kuo Lin Kuo ((郭郭建建麟

麟)₎

The work was supported by NSC, Taiwan and

The work was supported by NSC, Taiwan and Air Air Force, USA

Force, USA..

The authors thank the

The authors thank the center for _{center for}micro/nano _micro/nanoscience and _{science and}

technology

technology, National Cheng Kung University, Taiwan for , National Cheng Kung University, Taiwan for the provision of the HRTEM.