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