2. ENEE-698E 2 nd presentation by: Saeed Esmaili Sardari November 06, 2007

3. 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

4. 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

5. PEFET and nanoforce sensor based on a single ZnO nanowire

6. 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

7. ZnO nanostructures  Three types of fast growth directions along with polar surfaces due to atomic terminations give us tuning parameters to get different structures

8. ZnO nanostructures

9. 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

10. Fabrication of individual ZnO nanowire FET

11. FET characteristics  n- or p- type  Carrier mobility  Carrier density  Threshold voltage  Transconductance  Gate capacitance

12. FET characteristics

13.  n- type FET  V TH = -6.2 V  Gate capacitance The gate capacitance was estimated to be 6x10 −17 F from the formula C G ~2pεε 0 L/ln(2h/r)  Transconductance from the slope of the curve = 79 ns  Mobility = 6.4 cm 2 /Vs From dI/dVG=μ(CG/L 2 )V SD  Carrier concentration = 2300 μm −1 from the formula en=C L |V TH |

14. Theoretical analysis  Oxygen sensing  UV illumunation Ambient oxygen get adsorbed on the surface of the nanowire and forms an ionic specie ( O -, O 2-, O 2 - ) 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

15. Reported results

17. 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

18. 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

19. Questions  Thanks