1. Wide Bandgap Semiconductor Nanowires for Sensing S.J. Pearton 1, B.S. Kang 1, B.P.Gila 1, D.P. Norton 1, L.C.Tien 1, H.T.Wang 2, F. Ren 2, Chih- Yang Chang 3,G.C. Chi 3,Wei-Ming Wang 3 and Li- Chyong Chen 4 1 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, U.S.A 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, U.S.A. 3 Department of Physics, National Central University, Jhong-Li 320, Taiwan 4 Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

2. GaN Applications Blue/violet/white/UV LED Blue/green/UV lasers High power microwave transistors Robust sensors

3. GaN NWs grown by catalytic chemical vapor deposition

4. 500μm 5μm Ti/Au Pad SiN x /Si FESEM image & CL spectrum of a single GaN NW with two electrodes

5. Gate voltage-dependent I-V sd curves of a single GaN NW The carrier mobility is estimated at 30 cm 2 /V·s. The carrier concentration is estimated to be 2×10 17 cm -3

6. InN NWs grown by catalytic thermal-CVD HRTEM image XRD spectrum

7. Temperature-dependent I-V curve of a InN NW

8. f Appl. Phys. Lett. 64, p1508-1510 (1994) g Solid-state Electronics, 39, p1289-1294 (1996) h J. Vac. Sci. Technol. B, 14, p3520-3522 (1996) i This work Resistivity comparison between thin film and nanowire (n-type GaN and InN) thin filmnanowire resistivity (Ω cm) contact resistivity (Ω cm 2 ) resistivity (Ω cm) contact resistance (Ω) n-GaN4.4×10 -2 a 3~7×10 -6 a,b 56 ~ 1.24×10 -4 c,d,e X InN 2.1~ 3.1×10 -3 f,g,h 1.8×10 -7 f 4×10 -4 i 2i2i a Solid State Electron 41, p165-168 (1997) b Appl. Phys. Lett. 70, p57-59 (1997) c Appl. Phys. Lett. 85, p1636-1638 (2004) d Nano Lett. 2, p101-104 (2002) e Nano Lett. 3, p1063-1066 (2003)

9. 1010 Single Crystal Nanowire TEM image of an individual ZnO Nanowire. An estimated diameter of the wire is 20 nm. A small particle embedded at the tip of the wire is Ag or Ag-Zn alloy. HR-TEM image and selected area diffraction (SAD) of the nanowire indicates that it is a single crystal ZnO. 0002

10. Heterostructured nanowires Type I Type II Core (Zn,Mg)O (Hexa.) Sheath (Zn,Mg)O (Hexa.) Zn 1-x Mg x O (x <0.02) (Hexa.) (Mg,Zn)O (cubic) Radial heterostructureAxial heterostructure ZnO (Zn 1-X Mg X )O ZnO (Zn 1-X Mg X )O Growth condition -. Zn : 3 × 10 -6 mbar -. Mg : 4 × 10 -7 mbar -. O 3 /O 2 : 5 × 10 -4 mbar, -. Tg= 400  C Growth condition -. Zn : 3 × 10 -6 mbar -. Mg : 2 × 10 -7 mbar -. O 3 /O 2 : 5 × 10 -4 mbar, -. Tg= 400  C

11. Type I - Radial heterostructured nanowire Core (Zn,Mg)O (Hexa.) Sheath (Zn,Mg)O (Hexa.) -. Nanowire is crystalline with the wurtzite crystal structure maintained throughout the cross- section. -. The higher contrast for the center core region clearly indicates a higher cation atomic mass. -. Core : zinc-rich Zn 1-x Mg x O -. Sheath : Mg-rich Zn 1-y Mg y O

12. 10 nm ab 0002 1120 b Type II - Radial heterostructured (Zn,Mg)O/(Mg,Zn)O nanowire Zn 1-x Mg x O (x <0.02) (Hexa.) (Mg,Zn)O (cubic) [0001] (1ī1) (200) (11ī) Compositional line scan across the nanowire (STEM) -. Core : Zn 1-x Mg x O Hexagonal Wurtzite structure -. Sheath (Shell): Mg 1-x Zn x O Cubic Rock salt structure Zn Mg

13. (Mg,Zn)O nanowire (cubic rock salt structure) 200 020 B=[001] 2.04 Å Position across nanowire(nm) Intensity(arb.) Growth condition -. Zn : 3 × 10 -6 mbar -. O 3 /O 2 : 5 × 10 -4 mbar, -. Mg : 8 × 10 -7 mbar -. T g = 400  C

14. ZnO hexagonal wurtzite st. (Mg,Zn)O cubic rock salt st. (Zn 1-x Mg x )O/(Zn 1-x Mg x )O hexa. / hexa. wurtzite / wurtzite Radial heterostructured (Zn,Mg)O  Zn = 3 × 10 -6  O 3 /O 2 = 5 × 10 -4  Mg = none  Zn = 3 × 10 -6  O 3 /O 2 = 5 × 10 -4  Mg = 8 × 10 -7  Zn = 3 × 10 -6  O 3 /O 2 = 5 × 10 -4  Mg = 2 × 10 -7  Zn = 3 × 10 -6  O 3 /O 2 = 5 × 10 -4  Mg = 4 × 10 -7 [unit: mbar] Tg= 400  C core /sheath (Zn 1-x Mg x )O / (Mg,Zn)O hexa. / cubic wurtzite / rock salt st. core /sheath Nanowires vs Zn, Mg pressures III

15. Fabrication of ZnO nanowire device Insulator Electrode (Al/Pt/Au) Al/Pt/Au ZnO Nanowire -. Fundamental understanding of transport -. Nanoelectronics -. Nano sensors (UV, chemical, bio.)  Motivation -. Electrode : Al/Pt/Au by sputtering -. Diameter of ZnO nanowire : 130 nm -. Channel Length : 3.7  m  Structure of Nanodevice

16. Prototype device fabrication sequence Design and Deposit Alignment Marks Deposit SiO 2 Evaporation & Nanowires Deposition Find Nanowires Relative To Alignment Marks Spin PMMA Resist E-beam Write Aligned Pattern And Develop Deposit Metal And Lift Off Ethanol and Nanowire Suspension

17. UV Response of single ZnO nanowire Dark UV 366nm on off UV 366nm at V D 0.25V

18. I=I o (e qV/nkT -1) Ideality factor = 1.1 Forward Bias Al/Pt/Au Pt/Au (schottky contact) Reverse Bias Pt/ZnO nanowire Schottky Diode

19. Depletion-mode ZnO nanowire field-effect transistor Source Drain Gate Oxide Nanowire Si Insulator (SiO 2 ) Source (Al/Pt/Au) Drain (Al/Pt/Au) Gate(Al/Pt/Au) Nanowire Gate oxide ((Ce,Tb)MgAl 11 O 19 )

20. electrode (Al/Pt/Au) Nanowire Si Insulator (SiO 2 ) Microchannel pH Sensing with Single ZnO Nanowire

21. Hydrogen Detection Hydrogen has been used as fuels in many NASA’s space exploration missions. President Bush’s Hydrogen Fuel Initiative in 2003. Why hydrogen sensing? –Safety! –Production, Storage, Transport Hydrogen concentration in air reaches a dangerous level at 4%. ppm-level detection is needed.

22. Simple Fabrication Process Direct deposition of metal contacts on the silicon substrate with nanorods. No need to go through sonication and E-beam lithography to fabricate the sensors. The sensor has better sensitivity (more nanorods combined). Al/Pt/Au

23. Hydrogen-Selective Sensing at Room Temperature with ZnO Nanorods

24. Hydrogen-selective gas sensing at 25C with Pd/ZnO nanorods

25. Wireless Hydrogen Sensor System Prototype – powered by battery 916 MHz TX RX Micro- controller Low-noise Op Amp Micro- controller 16x1 LCD Remote SensorCentral Station

26. Self-Powered Wireless Sensor Use energy from ambient –Solar, vibration, ambient RF radiation Use energy supplied locally –Hydrogen flow, micro fuel cell, acoustic, thermal gradient Use energy supplied remotely –Wireless power supply (wireless power transmission)

27.  High quality,single-crystal growth of wide bandgap semiconductor nanowires  Bimodal growth of cored ZnO/(Zn,Mg)O heterostructured nanowires.  Type I -. Core : Zn 1-x Mg x O (x < 0.02), Hexagonal wurtzite structure -. Sheath : Zn 1-x Mg x O (x >> 0.02), Hexagonal wurtzite structure  Type II -. Core : Zn1-xMgxO (x < 0.02), Hexagonal wurtzite structure -. Sheath : (Mg,Zn)O, Cubic rock salt structure  (Mg,Zn)O nanowires having cubic rock salt structure Conclusions  Functional Nano-devices  Pt/ZnO nanowire Schottky Diode  Depletion-mode GaN and ZnO nanowire field-effect transistor  UV, pH, & gas sensors from GaN,InN and ZnO nanowires