Technology and TEM characterization of Al doped ZnO nanomaterials

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)

Outline Introduction of ZnO Doping ZnO nanomaterials in CVD Al doped ZnO Nanowires Al doped ZnO Nanotube/Nanowire junctions Al doped ZnO rectangular nanorods Conclusions

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

Applications of ZnO Piezoelectric material Varistor (surface acoustic wave devices ) (bulk or thin film) Gas sensor (H 2, CO and H 2 O) DMS (diluted magnetic semiconductor) TCO (transparent conductive oxide) Light-emitting devices ( nano-structure and thin film) Doping is required for functional devices

Method: thermal evaporation O 2 gas Quartz tube Heater x x mm Pressure meter Pumping Ar gas Substrate: Si (100) Source: Zn or Zn+Al mixed powder Adaptor Alumina boat

Simple and new concept Alloying-evaporation deposition (AED) D(T) = D 0 exp(-δh /RT) V tot =V Zn + V Zn-Al Zn: Al= 93: 7 wt. % V: vapor pressure

TEM characterization of Al:ZnO nanowires alloying treatment temperature of 420oC Al / (Al+Zn) : 2.5 at.% determined by EELS First to demonstrate ZnO nanostructures doped with Al from TEM Al mapping Applied Physics Letters, 88, 023111 (2006)

Core-loss analysis for Zn and Al by EELS 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 Beam energy (kev): 200 Convergence angle (mrad):1 Collection angle (mrad):10 Cross model: hydrogenic Background fit model: power law

TEM of Al:ZnO nanowire/nanotube junction structures alloying treatment temperature of 500 o C Al / (Al +Zn) : 12 at.% determined by EELS Al concentration in the ZnO nanostructures could be adjusted by varying the temperature of alloying treatment

Possible reasons for the formation of nanowire/nanotubes junction structures d (0001) = 5.16 Å d (0110) = 2.82 Å d (0001) = 5.11 Å d (0110) = 2.81 Å d (0001) = 5.05 Å d (0110) = 2.81 Å Al:ZnO nanowire Al:ZnO necking region Al:ZnO nanotube Lattice constant decreases 2% from nanowire to nanotube Al concentration increase Lattice mismatch Nanotube formed to release the strain

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 2 O 3 2Al Zn +3O o (Ryoken et al, J. Mater. Res., 20, 10, 2005)

Doping-induced rectangular nanorods Temperature O 2 injection 650 C, 3 torr, 1 hour 500 C, 30 min Ar injection Substrate :Si (100) Source : Al+Zn Time Horizontal tube furnace system 1.5 cm D C Source B gas flow A 1.5 cm Nanotechnology, in press, 2008 Jan

SEM results Lengths of 5 10 microns, widths of 20 200 nm thicknesses of 30 150 nm

TEM results the 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.

Growth mechanism [01-10]

More doping-induced nanostructures (c) c-ax is Root: Zn:O 52.4 : 47.6 Tip:Zn:O 48.7 : 51.3 100 nm Two-luminescenced Nanorods

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

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

Mapping for the unknown peak energy at 4-5 ev Only occurring from the edge

Local [Al] variation Axial Lateral

Selective Growth of ZnO nanorods on ZnO dots (1) Growth of ZnO pyramids by RF sputtering on Si(111) (a) (b) ~64~ 100nm 100nm

TEM characterisation of ZnO pyramids

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

TEM characterisation of ZnO nanowires and the interface

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

Acknowledgement Students: Wang Rei-Chi Chi; Jiann-Han Huang ( 黃俊翰 ), Yu-Han Liao ( 廖譽翰 ) and Jiann-Lin Kuo ( 郭建麟 ) The work was supported by NSC, Taiwan and Air Force, USA. The authors thank the center for micro/nano science and The authors thank the technology,, National Cheng Kung University, Taiwan for the provision of the HRTEM.