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. WangaInstitute of Physics, Chinese Academy of SciencesBeijing, 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 n-doped 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
Top Related