Transcript esmaili_20071106.ppt

ENEE-698E
2nd presentation by:
Saeed Esmaili Sardari
November 06, 2007
Oxygen sensing characteristics of
individual ZnO nanowire transistors
Q. H. Li, Y. X. Liang, Q. Wan, and T. H. Wanga
Institute of Physics, Chinese Academy of Sciences
Beijing, 100080, China
APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 26 27
DECEMBER 2004
Outline
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Recap of previous presentation
ZnO nanostructures
Fabrication of individual ZnO nanowire FET
Theoretical analysis
 Response to oxygen
 Response to illumination
 Experimental results
 Discussion
 Summary
PEFET and nanoforce sensor based
on a single ZnO nanowire
ZnO nanostructures
 ZnO has the richest family of
nanostructures among all materials
 Specific growth conditions lead to
different structures with different
properties
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Nanowires/Nanotubes
Nanobelts
Nanocombs
Nanosprings
Nanocages
ZnO nanostructures
 Three types of fast growth directions along with polar
surfaces due to atomic terminations give us tuning
parameters to get different structures
ZnO nanostructures
Fabrication of individual ZnO
nanowire FET
 ZnO nanowires fabricated with thermal
evaporation of ZnO powders by flowing a
carrier gas with an oxygen concentration of
20% at 1100 degrees Celsius
 Gold electrodes deposited by e-beam
deposition on a thermally oxidized highly ndoped silicon substrate
 Au electrodes are 50 nm thick
 Oxide layer is 500 nm thick
 Nanowires are dispersed with sonication in
ethanol
 The spacing between electrodes is about 1 micron
Fabrication of individual ZnO
nanowire FET
FET characteristics
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n- or p- type
Carrier mobility
Carrier density
Threshold voltage
Transconductance
Gate capacitance
FET characteristics
FET characteristics
 n- type FET
 VTH = -6.2 V
 Gate capacitance
 The gate capacitance was estimated to be
6x10−17 F from the formula
CG~2pεε0L/ln(2h/r)
 Transconductance from the slope of the
curve = 79 ns
 Mobility = 6.4 cm2/Vs
 From dI/dVG=μ(CG/L2)VSD
 Carrier concentration = 2300 μm−1
 from the formula en=CL|VTH|
Theoretical analysis
 Oxygen sensing
 UV illumunation
 Ambient oxygen get adsorbed on the surface of
the nanowire and forms an ionic specie ( O-, O2, O2- ) The electron is captured from the
nanowire; thus, reduces the carrier density, and
increases the resistance of the channel
 UV illumination generates electron/hole pairs
where holes make the adsorbed ionic species
desorb and increases the conductance of the
channel
Reported results
Reported results
Discussion
 Experimental results confirms the
hypothetical theory
 Chemistry of the adsorption and the very
fine details of the process are not as
clear as regular chemical reactions
 Different models, and different theories
are suggested for the
resistance/conductance change
Summary
 An individual ZnO nanowire FET is
presented
 Transistors characteristics are examined under
different oxygen pressures
 Change in drain-source current
 Change in threshold voltage
 Sensing property of the FET is attributed to
the capture and/or release of nanowire
carriers due to adsorption and/or
desorption of oxygen at the surface of the
sensing wire
Questions
 Thanks