esmaili_20071106.ppt
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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
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
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
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