Metal Oxide Nanowires: : Synthesis, Characterization and Device Applications

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1 Metal Oxide Nanowires: : Synthesis, Characterization and Device Applications Jia Grace Lu Dept. of Chemical Engineering and Materials Science & Dept. of Electrical Engineering and Computer Science University of California, Irvine

2 Challenges and Prospect 4Kb Minimum Dimension (nm) 64Kb 1Mb 16Mb 256Mb # of Transistors/Chip Year 1.E+12 1.E+1 1.E+8 1.E+6 1.E+4 (courtesy ZL Wang)

3 There There s s Plenty of Room at the Bottom - Richard Feynman, 1959 Vision: To synthesize nanoscale building blocks with precisely controlled size and composition, and assemble them into larger structures with unique properties and functions.

4 Science and Engineering in the nano-world Nano Materials (nanotube, nanowires, ultra thin films) & Device Fabrication Characterization (Electron Microscopy, SPM, Low-T, High-B) Source Ni/Au Nanowire Gate Dran Ni/Au SiO 2 Nano Device Application (FET, Sensor, SET )

5 Semiconductor Nanomaterials Quasi-one one-dimensional semiconductor nanostructures: Q1D physical properties High aspect ratio Conductivity control (p-type/( n-type) Bandgap engineering Electronic and photonic device applications Field effect transistors Schottky-barrier rectifiers LED & Lasers Sensors Single electron transistors Memory cells Logic gates

6 Nano Materials Ga 2 O 3 B Fe 2 O 3 15 o Ag-TCNQ GaN ZnO

7 Motivation ZnO Nanowires Device applications: Transparent electronics UV light emitter and detector Chemical sensors Electro-mechanical devices Lower power consumption Higher sensitivity Better gate modulation Faster response time Tunable band gap λ D conducting channel

8 Synthesis and Characterization of Nanowires CVD synthesis Electrical transport Scanning surface potential microscopy Chemical sensing property Optical property

9 Single Crystal Nanowires : Vapor- Liquid-Solid Mechanism Catalyst Zn O 2 O 2 Catalyst: Au Alloy Supersaturation Crystal growth

Single Crystal Nanowires 3 (1) (11) ZnO Nanostructures JCPDS 89-51 Intensity (a.

Nano- Crystals collecting substrate Quartz vial Zinc powder VLS growth mechanism

TEM shows ZnO nanowires are single crystal.

10 Single Crystal Nanowires 3 (1) (11) ZnO Nanostructures JCPDS Intensity (a.u.) 2 Ar+O 2 1 (2) (12) (11) (13) (112) (2) (21) (22) (23) Open end of vial Nano- Crystals collecting substrate Quartz vial Zinc powder VLS growth mechanism SEM image of nanowires, with diameter ranging from ~2 to 1 nm High resolution TEM shows ZnO nanowires are single crystal. The distance between neighboring lattice planes is.52 nm, and growth direction is along [1] -- c axis θ C axis

11 Control of Nanostructure Morphology b3 b4 b2 b5 b1 Out Furnace O2 Gas Flow C QuartzTube B A Zn Quartz Vial

12 Control of Position - Si wafer - Resist: PMMA - E-beam lithography - Catalyst deposition Si wafer

13 Control of Position - Si wafer - Resist: PMMA - Ebeam lithography - Catalyst deposition - Liftoff: remove PMMA - CVD growth Si wafer

14 Control of Alignment Vertical aligned ZnO nanowires array has promising potentials for nanoelectronics/optoelectronics. Anodic alumina membrane is an ideal template to integrate vertical aligned nanowires. AAO template Bottom electrode Adhesion layer Al 2 O 3 substrate Short Sn nanorod Al 2 O 3 substrate 4 nm ZnO nanowires Home-made anodic alumina membrane (AAM), 5 micron thick. Al 2 O 3 substrate Schematic of nanowire synthesis process

15 Control of Electric Properties Furnace O2 Gas Flow Source Vds NW Drain QuartzTube C B A Zn Quartz Vial Vg SiO 2 P ++ Si Back Gate I ds (µa) Chip B Chip C Higher Conductivity I ds (na) Chip B Chip C V ds (V) V g,t(b) Vg,t(C) V g (V)

Electrode Contact using Ebeam-lithography Define alignment marks using ebeam lithography and metal deposition Disperse NWs on substrate and

16 Electrode Contact using Ebeam-lithography Define alignment marks using ebeam lithography and metal deposition Disperse NWs on substrate and locate the coordinates 1 st resist layer: MMA/MAA 2 nd resist layer: PMMA e - e - Ebeam writing Developing Metal deposition Liftoff SiO 2 /p++ Si

17 Electrode Contact using Photolithography UV isperse NWs pin coat resist over the resist with a mask UV Exposure Developing Metal deposition Liftoff SiO 2 /p++ Si

18 Band Diagram of Electrode Contact Work function (Φ) of materials: Before contact E c Φ Ti = 4.3 ev Φ Ni = 5.2 ev E F - Ni E F-ZnO E v Φ ZnO = 4.3 ev E ZnO = 3.4 ev After contact φ B E F - Ni F Ni > F ZnO > F Ti Ni ZnO Ti ZnO Schottky junction Ohmic contact

Nanowire Field Effect Transistor Capacitance per unit length: Source Nanowire Drain C / L 2πεε / ln(2h / r) Carrier concentration per unit length: Gate SiO 2 p ++ Si V ds n = C L V

19 Nanowire Field Effect Transistor Capacitance per unit length: Source Nanowire Drain C / L 2πεε / ln(2h / r) Carrier concentration per unit length: Gate SiO 2 p ++ Si V ds n = C L V gt e V g Mobility: µ = ( di / dv )/(2πεεV / Lln(2h / r)) g ds r: NW radius; L: NW channel length; h: gate oxide thickness; V gt : gate threshold voltage; di/dv g : transconductance.

20 Electrical Transport Properties Vds=1mV Vds=75mV Vds=5mV Vds=25mV (b) I (na) Vg=-6 V Vg=-4 V Vg=-2 V Vg= V Vg=2 V Vg=4 V Vg=6 V V ds (V) I-V curves under different gate voltages I (na) 16 8 V gt =-8.37V V g (V) Transconductance at different sourcedrain biases Three-terminal electrical measurements show n-type behavior. Carrier concentration n e ~ 1 7 cm -1 ; and mobility µ e ~ 8 cm 2 /V s

21 High Performance ZnO NW FET Ids (na) 2 1 Vg=-2V Vg=-16V Vg=-12V Vg= -8V Vg= -4V Vg= V Ids (na) 3 2 2mV 4mV 6mV 8mV 1mV Vds (mv) Vg (V) ZnO nanowires show significant enhancement in mobility after surface passivation. Q1D concentration ~ 9 x 1 7 cm -1 ; 3D concentration ~1.16 x 1 18 cm 3 Q1D carrier mobility ~ 12 cm 2 /Vs

22 Thermionic Emission I (na) K 333 K 373 K 428 K 473 K Conductance (ns) /T Vds (V) (c) Electric current was observed to increase monotonically with temperature. An effective energy barrier height of.3 ev is estimated from the thermionic emission model: I ~ exp(-φ b /kt).

Electrical transport through individual nanowires is characterized with

23 Vertical Nanowire Array Chemical vapor deposition is used to grow ZnO nanowires array. Electrical transport through individual nanowires is characterized with conductive AFM probes. Top view of ZnO nanowire array embedded in AAM. I-V characteristic of an individual vertical ZnO nanowire in AAM.

24 Scanning Surface Potential Microscopy The uniformity of nanowire and the property of electrode contact was studied using scanning surface potential microscopy by probing local electric potential distribution. Conductive AFM tip is biased with a DC signal and an AC signal. AC signal is used to drive tip resonant vibration and DC signal can be controlled by feedback loop to trace the surface potential on sample. Vac Vtip Vds ~ V Source dc F = = SiO 2 Back gate V dc V dc V ac dz tip V Drain sample

5 4 Ids (na) 3 2 Cross section analysis of surface potential image

25 Surface Potential and Tip Gating Drain V ds Source Scanning direction V g Topography and surface potential image of a ZnO nanowire FET. 5 4 Ids (na) 3 2 Cross section analysis of surface potential image gives the potential drop on device: V AB =.26 V; V BC = Time (s)

26 Back gating vs tip gating 2 Vds Source Nanowire Drain I (na) Vbg=-2 V Vbg=-1 V Vbg= V Vbg=1 V Vbg=2 V Vg SiO 2 Back gate Vds (V) Vg Vds Source SiO 2 Back gate Drain I (na) Vtg=-2V Vtg=-1V Vtg=V Vtg=1V Vtg=2V Vds (V) Electrical transport results are obtained for both AFM tip gating and Si back gating. It is observed that negative tip gating has significant local depletion effect on FET and increases the height of effective barrier for electron conduction. However,

27 Back gating Back gating vs tip gating negative Vg Ec (d) Tip gating E F positive Vg Drain NW Source E F φ bd V on Drain NW φ bs Source E F V on Drain NW φ bt φ bs Source E F Gate potential affect conductance of NW by band bending. Si Back gate potential affects the entire FET, but AFM tip gate potential only affects the local region and creates an energy barrier at negative Vg.

28 Chemical Sensing Nanowire Electrons.6 O2 Current transport Adsorbed molecules (H 2,O 2,NO 2,NH 3,CO, C 2 H 5 OH, ) G/G.4.2 Source Drain Gate Nanowire channel Depletion region 4 2 ppm 1ppm 2ppm NO NW radius (nm) I (µa) -2-4 I (µa) ppm 1 ppm 2 ppm Vds= 5 V Vg (V) Vds (V) Sensitivity: S = DG/Go increases with reduced nanowire radius.

29 NO 2 Oxiding Sensing 1 Start of NO 2 4 Current (na) ppm 2 ppm 5 ppm 1 ppm 2 ppm Time (s) Response time (s) T=A N NO 2 concentration (ppm) Response time is defined as time when conductance drops 1/e from maximum. Response time decreases with increasing concentration.

30 Gate Tunable Sensitivity NO 2. Chemical Sensing roperties of nanowire FET argely depend on gate otential. Gate adjusts the position f the Fermi level of the hannel, affect the surface rocesses. Gate voltage tunes the etection range. I (µa) I (µa) Vg = 3 V ppm 1 ppm 2 ppm 3 ppm Vds (V) ppm.2 ppm.4 ppm.6 ppm Vg = -5 V Vds (V) I (µa) δ G/G Vds (V) a S V g + b ppm 1ppm 2 ppm 5 ppm 1 ppm 2 ppm Vg= V Sensitivity to 5 ppm NO Vg (V) Z. Fan and J. G. Lu, APL ( 25).

31 Electrically Refreshable Sensing At room temperature, chemisorbed molecules are quite difficult to be desorbed. Thermal desorption and ultra-violet illumination are conventional ways to refresh nanowire. For oxidizing gas, a strong negative gate field greatly facilitates desorption and is capable to quickly refresh nanowire sensor. I (na) I (na) 1 ppm NO2 Vg= V ppm NO 2 T = 453 K T = 3 K Time (s) Vg= -6 V T = 3 K

32 Distinguishability Temporal response of NO 2 and NH 3 to a negative gate voltage pulse demonstrates a potential distinguishable sensing. I (na) ppm NO 2,k=.12 5 ppm NO 2,k=.1 1 ppm NO 2,k=.16 I (na) ppm NO 2 recovery region Time (s).5% NH 3, k=.27-4 Vg =-2 V Vg = V Time (s) I (na) 4 2 1% NH 3, k=.25 2% NH 3,k= Time (s)

33 Gate Refresh Voltage S D g SiO 2 p ++ Si V ds Different activation energy for NO 2 and NH 3 adsorption. Distinguishable gate refresh voltage.

34 Reducing Sensing Properties Exposure nanowire to CO at 5K in O 2 ambient increase nanowire conductance by reaction: CO 2 + O CO + e Exposure nanowire to NH 3 at 5K in Ar ambient increase nanowire conductance also, but by electron direct transferring from NH 3 to nanowire. I (na) % O 2 in.5% CO off.5% CO on.5% CO on.5% CO off.5% CO on.5% CO on T=5K.5% NH 3 on Time (s) 1% NH 3 on.5% CO off T= 5K 1% NH 3 on I (na) 45 E c E f E c E f µ NH3 µ NH3 4.5% NH 3 off 1% NH 3 off 1% NH 3 off E v 3 K E v 5 K Time (s)

35 Optical Property of ZnO Current (na) Wavelength (nm) Vds=.2 V Input optical wavelength (nm) Photoluminescence and photoconductivity spectrum of single ZnO nanowire PL intensity (a. u.) Current (na) Current (na) Air Turn off laser Vacuum Turn on laser (min) Time (s) Due to the adsorption of oxygen: O 2 + e? O - 2 T d ~ 8 s in air T d > 1 hr in vacuum (T d is defined as the time for current drop from I max to I max /e)

36 Photoconductivity Current (na) Current (na) With 633 nm illumination Without 633 nm illumination Gate Voltage (V) With 633 nm illumination Without 633 nm illumination Current (na) (b) I=A P.43 V ds = 2 V Drain voltage (V) Laser power (mw) Light illumination results in an increase in nanowire carrier concentration and conductance, and a slight decrease in mobility. Power law dependence of photocurrent to the input optical power is due to the finite number of impurity states.

37 Photoluminescence Diameter dependence Temperature dependence (data taken at Göttingen University) nm Dia(1-2nm) Dia(2-1nm) Dia(6-2nm) 2 Intensity / a.u nm 392nm Intensity / a.u. 3 K 2 K 1 K 2 12 K Wavelength/ nm energy / ev Blue-shift as diameter decreases LT PL spectroscopy indicate the nanowires are of high crystal quality.

38 Polarization Dependent Photodetection E Light E? I? cos 2 q broadband light NW Nanowire diameter << l E? Maximum E -? Minimum, E i ~ 2ε2. E e /(ε+ε ) Current (na) V ds = 5 V E E 365 nm UV V ds = 2 V Polarization Angle (degree) Nanowire conductance is minimized when the incident field is polarized perpendicular to the long axis. Such anisotropy has potential application for polarization dependent photodetector and optical gated switch.

39 Summary on ZnO Nanowire Single crystal ZnO nanowires are synthesized and configured as field effect transistors. Electrical transport studies show ZnO nanowires are n-type semiconductor. Scanning surface potential microscopy demonstrates that electrical property of the nanowire is uniform. Chemical sensing results indicate the future application of ZnO FET as a self-contained, gate tuned, and gate refreshable chemical sensor. Optical studies show its potential applications in nanoscale optoelectronics.

40 Gallium Oxide Ceramic metal oxide n -type semiconducting at high temperature wide bandgap ~ 4.9eV Carrier concentration ~ C cm - 3 at 13 cm (M. Fleischer and H.Meixner, J. Appl. Phys., 74, 3, 1993) Base-centered monoclinic structure a =12.23 Å,, b =3.4 Å,, c =5.8 Å Potential Applications: High temperature gas sensor Optoelectronics device

41 Electron Microscopy A B 15 o Au Dimension: length~1-3 3µm, diameter~2-2nm 2nm VLS (Vapor-Liquid Liquid-Solid) catalytic growth mechanism Base-Centered Monoclinic crystalline structure Several growth direction can be observed: A. [111] spacing ~.25nm B. [1] spacing ~.55nm Others [2], [-13][

42 XRD & PL Characterizations Intensity (a.u.) 1 (2,-22) (111) (-41,4) (-111) (-311,-42,22) (6,-312,112) (51) Ga 2 O 3 nanowires JCPDS (43,512,-712) Intensity (a.u.) 6 3 1K 15K 2K 25K 3K 414nm q (degree) Wavelength (nm) CuKα (λ=1.54 Å) source As grown nanowire sample matches the β-ga 2 O 3 diffraction pattern Single phase The strongest peak is (111) Blue-Green broad emission is contributed by Gallium-oxygen vacancy pair (acceptor-donor) formed trapped excitons x x ( V, V ) + V ( V, V ) + V o Ga o o + hν (T. Harwing, et al., J. Solid State Chem., 24, 255, 1978) (Vasil tasiv et al., Ukr. Fiz. Zh., 33, 132, 1988) o Ga

43 Extrinsic Doping Intrinsic: It shows very low conductance (insulating, high resistance~1 16 Ω) ) at room temperature type doping: Sb 5+, W 6+, etc n-type doping: p-type doping: Zn atoms can be acceptors Ionic crystal radii size: Zn 2+ :.74nm, Ga 3+ :.62nm (R. D. Shannon, Acta Crystallographica,, A32, 751, (1976) Ga o 2ZnO = 2Zn ' + V + 2O o Setup (diffusion doping): Pure Zn powder in Ar inert ambient, 45? for 15 minutes

Transport Properties (I-V) Current (pa) 3 2 1 Vg= 3V Vg= V Vg=-3V Vds -1 SiO 2 Vg -2-1 1 2 Bias Voltage Vds (Volt) p ++ Si Improved transport properties - up to 3 order increase in magnitude p-type

44 Transport Properties (I-V) Current (pa) Vg= 3V Vg= V Vg=-3V Vds -1 SiO 2 Vg Bias Voltage Vds (Volt) p ++ Si Improved transport properties - up to 3 order increase in magnitude p-type semiconducting behavior - from gate voltage dependence Carrier concentration Carrier mobility p = µ Vg di dv h = (th) 2πεε e ln(2h / r) g L ln(2h / r) 2πεε V ds (R.Martel et al., Appl. Phys. Lett., 73, 2447, 1998)

45 Transport Properties (I-Vg) 8 Current (na) 6 4 di/dvg= -.988x1-9 A/V Source Vds NW Drain 2 Vg(th)= V Vg Back Gate Gate Voltage Vg (Volt) Transconductance di/dv g = A/V Threshold Voltage V g (th) = V ex. h = 2nm, r = 6nm, L = 2.12µm, ε = 3.9 (permittivity of SiO 2 ) p = cm -1 ( cm -3 ) µ h = cm 2 /Vs

46 Extrinsic Doping - Fe 2 O 3 Nanobelts 9 6 Vg = 1 V Vg = V Vg = -1 V I (pa) E F Vacuum I (pa) Vds φ Ni Ni Fe 2 O 3 φ Fe 2 O 3 Ec 2 1 (b) V ds = 2. V I (na) Vg=-1 V Vg=-5 V Vg= V Vg=1 V I (na) Vds (V) Vds=5V V g (V) Vg (V) Vacuum As-grown Fe 2 O 3 nanobelts are configured as field-effect transistors showing n-type behavior. For the p-type Zn doping, the nonlinear I-V arises from the Schottky contact formation due to nanobelt Fermi level shifts as a result of doping. E F φ Ni Ni Fe 2 O 3 φ Fe2 O 3 Ec E V p-type Zn doping

Small wonders, endless frontiers nanotechnology will be central to the next epoch of the information age, and will be as revolutionary as science and technology at

47 Small wonders, endless frontiers nanotechnology will be central to the next epoch of the information age, and will be as revolutionary as science and technology at the micron scale have been since the early 7s. John Armstrong, 1991 S D Vg SiO 2 p ++ Si V ds Vcc Top electrodes AAO template Bottom electrodes Vo Nanowires Vi1 Vi2 Vc

Department of Electrical Engineering IIT Madras

Department of Electrical Engineering IIT Madras Sample Questions on Semiconductor Devices EE3 applicants who are interested to pursue their research in microelectronics devices area (fabrication and/or