WIRELESS INFORMATION NETWORK LABORATORY ......WIRELESS INFORMATION NETWORK LABORATORY Introduction: ZnO Materials • II-VI compound semiconductor: – Direct bandgap, with E g @3.32

WIRELESS INFORMATION NETWORK LABORATORY

Device Research for the MUSE Initiative

Dr. Yicheng Lu, Dr. Nuri W. EmanetogluMs. Jian Zhong, Ms. Ying Chen

WINLAB, Electrical and Computer Engineering Dept.Rutgers University

May 13, 2004

This work has been supported by the NJCST Excellence Center for Research (MUSE) grant, NJ Nanotechnology Consortium, US AFOSR, US Army CECOM, and NSF.


NJCST R&D Excellence Center

MULTI-MODAL WIRELESS INTEGRATED SENSOR-ON-SILICON (MUSE) TECHNOLOGY


Motivation: Why ZnO?• Crystal structure: new material for the “blue revolution”,

buffer layer for GaN• Piezoelectricity: large k2, for SAW, BAW and MEMS• Optical properties: large and direct bandgap, large

exciton bonding energy, for UV lasers, sensors, modulators.

• Chemical and biochemical: catalysis, for sensors and fuel cell.

• Transparent and conductive oxide.• Multifunctional: MITSAW chip, tunable, integratable.


Introduction: ZnO Materials• II-VI compound semiconductor:

– Direct bandgap, with Eg ≅3.32 eV.– Bandgap engineering: alloy with Cd or Mg

to tailor bandgap from 2.8eV to 4.0eV.• Multi-functional:

– Hexagonal wurtzite class crystal => piezoelectricty with large coupling coefficient.

– Large and fast photoconductivity => optical sensing.

– Al or Ga doping => transparent conductive oxide.

– Li & Mg doping => ferroelectric.– Alloyed with Mn => magnetic oxide

semiconductor.• Integrate electrical, optical and

piezoelectrical properties => MITSAW chip technology

Zinc

Oxygen

[0 0 0 1]

[2 -1 -1 0]

[1 1 -2 0]

[-1 2 -1 0]


Fundamental Properties of MgxZn1-xO• ZnO can be alloyed with MgO and CdO

to produce the ternary compounds MgxZn1-xO and CdxZn1-xO.

• MgxZn1-xO/ZnO heterostructures have many device applications.

• The bandgap Eg : 2.8 eV to 4.0 eV.• MgxZn1-xO can to made as a

multifunctional material:- piezoelectric.- ferroelectric ((M)xZn1-xO, with M = Li,

Mg). - diluted magnetic semiconductor.

• Physical properties can be tailored by controlling Mg composition.

∆Ec=0.9∆Eg

∆Ev=0.1∆Eg

Mg0.33Zn0.67O,Eg = 4.0eV

ZnO,Eg = 3.3eV


Structure of ZnO films on r-sapphire

• The as-grown film on r-sapphire is dense and very smooth• The c-axis of ZnO is in the plane of the film• Interface is sharp and semi-coherent• The total misfit accommodated by strained regions

2.81 nm

ZnO

Sapphire

( )1011( )0112

(1100)

(1 1 2 0 )


MgxZn1-xO Film Grown on r-Al2O3

Mg0.25Zn0.75O on r-Al2O3 Mg0.25Zn0.75O on r-Al2O3

� Film is smooth, dense and in epitaxial quality as shown in cross-sectional FE-SEM image.

� The film composition is determined by RBS.


Photoresponse of ZnO• Polycrystalline ZnO as a photoconductor1,2 :

– Fast process: Solid State process, [hν → h+ + e-]– Slow process:

• Oxygen adsorption, O2(g) + e- → O2-(ad)

• Photodesorption, h+ + O2-(ad) → O2(g)

• High quality epitaxial ZnO produces large and fast photoresponse:

– Reduce grain boundaries– Reduce defects induced recombination– Reduce electron concentration by N-doping

1. Y. Takahashi, M. Kanamori, A. Kondoh, H. Minoura and Y. Ohya, Jpn. J. Appl. Phys. 33, 6611 (1994).

2. D. H. Zhang, J. Phys. D Appl. Phys. 28, 1273 (1995).


Spectral Response and Speed of a Photoconductive UV Detector

A. Spectral ResponseCutoff Wavelength: 370nmVisible rejection ratio > two orders of

magnitude

B. Photocurrent vs. Response Time Rise Time: 1µµµµsFall Time: 1.5µµµµs

0 1 2 3 4 5

0

1

2

3

Pho

tocu

rren

t (nA

)Time (µµµµs)

Bias: 5VOptical Pulse:• <100fs• ~5.6fJ

250 300 350 400 450 500 550

1

10

100

Bias: 5VPhotoresponsivity:

~ 400A/W

Pho

tore

spon

sivi

ty (A

/W)

Wavelength (nm)


Spectral Response and Speed of a Schottky UV Detector

A. Spectral Response Cutoff Wavelength: 370 nmVisible rejection ratio > three orders of magnitude

Phot

ocur

rent

(mA

)

Time (nSec)

TIME ( µµµµSec )PH

OT

OC

UR

RE

NT

(m

A )

Ag-ZnO-Ag Schottky Photodetector

B. Photocurrent vs. Response Time Rise Time: 12222nsFall Time: 50ns

300 320 340 360 380

10-3

10-2

10-1

100

Ag-ZnO-Ag Schottky Photodetector

RE

SP

ON

SIV

ITY

( a.

u. )

WAVELENGTH ( nm )


MgxZn1-xO-based UV Sensor

Advantages:• Wide and direct band gap (3.3eV)• Eg tunable from 3.3 to 5.8 eV by alloying ZnO with MgO to form

MgxZn1-xO.• Large photoresponse• High photoconductivity


Novel UV-SAW Photodetector• UV photons generate e-h pairs in mesa structure, increasing

conductance.• Electrical field accompanying SAW interacts with carriers,

slowing acoustic wave.• Results in a phase shift/time delay across device.• Output in frequency domain. Can be read out wirelessly.


Review of UV SAW Photodetectors• GaN UV SAW detectors:

– GaN/c-Al2O3 SAW device as photodetector, used as phase shifting element in oscillator circuit (365 nm, 60 kHz @ 221.3 MHz). D. Ciplys, R. Rimeika, M.S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J.

Yang, and M.A Khan, Appl. Phys. Lett., 80, 2020 (2002).

– GaN/c-Al2O3 device where SAW is used to transport e-h pairs generated in sensing area to MSM detector.T. Palacios, F. Calle, J. Grajal, Appl. Phys. Lett., 84, 3166 (2004).

• ZnO/LiNbO3 UV SAW detector– ZnO thin film deposited on LiNbO3 SAW device. used as

phase shifting element in oscillator circuit (365 nm, 170 kHz @ 37 MHz, 40 mW/cm2). Sharma and K. Sreenivas, Appl. Phys. Lett., 83, 3617 (2003)


ZnO/r-Al2O3 SAW UV Detector Advantages

• The Sezawa wave mode in the ZnO/r-Al2O3 system has higher acoustic velocity and effective coupling compared to GaN/c-Al2O3 and ZnO/LiNbO3.

• This leads to larger tunability, hence higher sensitivity to UV light.

• The higher coupling also lends itself to a lower loss, more efficient zero-power remote wireless sensors.


SAW Interaction with thin conductive film• Current induced only in thin mesa layer, which has

conductance σd

• Conductance at maximum loss: σm = vocεo(ε1+ε2)• Frequency independent.• Velocity change:

• Attenuation:

voc = free surface SAW velocity λ = SAW wavelengthvsc = electrically shorted surface SAW velocityK2

eff = effective electro-mechanical coupling

2md

2eff

ococ

sc�

)�/(�11

Kv�v

vvv

+==−

2)�/(�1

�/�

�

�K�

md

md2eff

+=


Sensor Output: Time Delay and Phase Shift

• Time delay:

• Phase shift:

Vg = group velocity Vp = phase velocity

��

�

�

��

�

�−=−=

ocg,scg,mesaocscmax v

1v

1Ltt�t

2eff

p

mesa2effmesa

maxp

pmesamax K

vLf

�2

K�

L2�

v

�v

�

L2�� ==

��

�

�

��

�

�=ϕ

��

�

�

��

�

�=

p

pmesa

v

�v

�

L2��ϕ


UV SAW Photodetector Device Response• Devices with different acoustic wavelength (λ = 8,12,16 µm)

and delay length (L = 1.2, 1.7 mm)• (a): λ = 12 µm; L = 1.2 mm; dark vs. microscope lamp• (b): λ = 8 µm; L = 1.7 mm; dark vs. 0.81 mW/cm2 and 2.32

mW/cm2 UV (λ = 365 nm) illumination

(a) (b)


UV SAW ∆φ and ∆IL Response• Phase shift, and insertion loss change of λ = 8 µm, L = 1.7

mm device.• No significant change above 400 nm.• Device response increases significantly at band edge (λlight =

372 nm)• Equivalent to ZnO PC & PV photodetectors

(a) (b)


UV SAW Photodetector Applications • Zero-power wireless remote sensors for:

– Early missile threat warning– Chemical and biological agent detection– Engine/flame detection – Furnace monitoring– UV dosimetry– Ozone/pollution monitoring– Indoor plant growth

Interrogation pulse

Sensor response

Antenna

SAW IDT

RF stage

DSP unit

Control unit(e.g. PC)

Interrogation unit Wireless UV-SAW Sensor

Semiconducting ZnO mesa

Reflector array


ZnO Monolithically Integrated Tunable SAW Sensor

• Features and Advantages:

• (1) Tunability:– High K2 and high vSAW, leading to large tuning ranges and high

operating frequencies.

• (2) Multifunctionality and multimode operation:– Novel acoustic, electronic, and optical devices and applications.

• gas sensors, liquid environment sensors, optical operation in UV.

• (3) Integration of XYZ-on-a-chip: monolithically integrated in one material system.

MgxZn1-xO

R-Plane Sapphire

IDT (Al)

MgxZn1-xO/ZnO QWGate electrode

IDT (Al)

Piezoelectric ZnO layer


ZnO MITSAW Applications• Applications & opportunities:

– Voltage controlled oscillators.– Adaptive and tunable filters.– Zero-power remote wireless sensors.– Fixed and tunable optical delay lines.– Tunable, multi-mode chemical/

biochemical sensors.(sensor expected to grow to ~$3-5 B in 2005, and to ~$10 B in 2010 by the most conservative estimates)

• Example: Novel MITSAW Biosensor:– Resettable and tunable, therefore increasing the sensor’s lifetime– Dual SAW modes operation (gas-phase and liquid-phase sensing);– Operating in UV and acoustic mode, increasing accuracy.– Can be integrated with Si IC: sensor-on-chip; lab-on-chip.

Gate voltageinput

REF.

2DEGmesa

SAWIDT

2DEGGround

Sensing device with chemicallyselective receptor coating

Sensoroutput

Mixer

2DEGmesa


Other Wireless SAW Sensor Applications

• Chemical/ biochemical sensors for homeland defense, military, environmental protection

• Particle monitor• Temperature and pressure sensor• Viscosity sensor (pipelines, etc.) • Non-destructive evaluation (cracks, fissures,

etc. in pipelines, walls, etc.)


Conclusions

1. Materials: Tailored and Multi-Functional • MOCVD growth of high quality epitaxial ZnO film on r-Al2O3 and

MgxZn1-xO film on r-Al2O3;• Achieved multifunctional ZnO and MgxZn1-xO: semiconducting,

transparent-and -conductive, piezoelectric,ferroelectric;• Material properties can be tailored by varying Mg composition in

MgxZn1-xO, as well as using multi-layer structures.

2. Sensor Devices: Tunable and Multi-Mode• ZnO/r-Al2O3 and MgxZn1-xO/r-Al2O3 SAW Devices;• ZnO- and MgxZn1-xO-based UV Sensors (photoconductive and

schottky);• Prototype zero-power, wireless UV SAW photodetector;• Prototype MITSAW device for sensing and circuit applications.

WIRELESS INFORMATION NETWORK LABORATORY Patents• “High Contrast, Ultrafast Optically Addressed Ultraviolet Light Modulator Based Upon Optical Anisotropy in ZnO Films Grown on R-plane Sapphire” (with M. Wraback, H. Shen, S. Liang* and C.R. Gorla*), US Patent No.6,366,389 (April 2, 2002)

• “Monolithically Integrated Tunable Surface Acoustic Wave Technology and Electrical Systems Provided Thereby” (with N.W. Emanetoglu), US Patent No. 6,559,736 B1 (May 6, 2003)

• “Surface Acoustic Wave Technology and Sensors Provided Thereby”, (with N.W. Emanetoglu), US Patent # 6,621,192 B2, Sept. 16, 2003.

• “Tailoring Piezoelectric Properties Using MgxZn1-xO and MgxZn1-xO/ZnO Structures”, (with N.W. Emanetoglu), US Patent # 6,716,479 , April 6, 2004.

• "Room-temperature ZnO Spintronics" (with Pan Wu) , filed in April, 2002.• “Fabrication of Ag Schottky Diodes on MgxZn1-xO”, (with H. Sheng, S. Muthukumar&, N.W. Emanetoglu, J. Zhong), filed in May, 2002

• "Biosensors Using ZnO-based Nanostrutures" (with Z.Zhang, H.Shang, N.W. Emanetoglu, M. Inouye and O. Mironitchenko), field in May, 2002

• “Selective Growth and Fabrication of ZnO Single Nanotip and Nanotip Arrays”, (with S. Muthukumar, N.W. Emanetoglu.), filed in Sept., 2002

• “Tailoring Piezoelectric Properties Using ZnO and AlxGa1-xN (0 x 1) Multilayer Structures”, (with Ying Chen and N.W. Emanetoglu), Invention Disclosure, filed June 2003.


2003-2004 Publication List• “ZnO Nanotips Grown on Si Substrates by Metalorganic Chemical Vapor Deposition”, accepted

to appear in the Journal of Electronic Materials, 2004 (J. Zhong, S. Muthukumar, G. Saraf, H. Chen, Y. Chen, Y. Lu)

• “Li Diffusion In Epitaxial ZnO Thin Films”, accepted to appear in the JEM, 2004 (P. Wu, J. Zhong, N.W. Emanetoglu Y. Chen, S. Muthukumar, Y. Lu)

• “Wet Chemical Etching of (11-20) ZnO Films”, accepted to appear in the JEM, 2004, (J. Zhu, N.W. Emanetoglu, Y.Chen, B. V. Yakshinskiy, Y.Lu)

• “Metalorganic Chemical Vapor Deposition and Characterization of Epitaxial MgxZn1-xO (0<x<0.33) Films on r-Sapphire Substrates”, J. Crys. Growth, vol. 261,no.2-3, pp. 316-23 (S. Muthukumar, Y. Chen, J. Zhong, F. Cosandey, Y. Lu, T. Siegrist)

• “Characterization of MgxZn1-xO Bulk Acoustic Wave Devices”, IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, vol. 50, no. 10, pp. 1272-8, Oct. 2003 (R.H. Wittstruck, X. Tong, N. W. Emanetoglu, P. Wu, J. Zhu, Y. Lu, A. Ballato)

• “Ga-doped ZnO single-crystal nanotips grown on fused silica by metalorganic chemical vapor deposition”, Appl. Phys. Lett., vol. 83, n. 16, pp.3401-3, Oct. 2003 (J. Zhong, S. Muthukumar, Y. Chen, Y. Lu, H. M. Ng, W. Jiang, E. L. Garfunkel)

• “Surface Acoustic Waves in ZnO/AlxGa1-xN/c-Sapphire Structures”, Y. Chen, N.W. Emanetoglu, Y. Chen, G. Saraf, Y. Lu, A. Parekh, V. Merai, M. Prophristic, D. Lu, D.S. Lee, E. A. Armour, Proc. of 2003 IEEE International Ultrasonics Symposium, pp.2130-2133, 2003

• “SAW Analysis of the MgxZn1-xO/SiO2/Si System”, H. Wu, N.W. Emanetoglu, G.Saraf, J. Zhu, P. Wu, Y. Chen,Y. Lu, Proc. of 2003 IEEE International Ultrasonics Symposium, pp. 897-900,2003


Acknowledgements• Postdoctoral/Research Associates

– Dr. S. Liang - Dr. C.R. Gorla– Dr. X. Tong - Dr. S. Feng– Dr. Y. Chen - Dr. N.W. Emanetoglu

• Graduate Students– S. Muthukumar– H. Sheng - P. Wu - G. Saraf– J. Zhong - J. Zhu - Y. Chen– Z. Zhang - R. Wittstruck - K. Wu– H.Chen

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WIRELESS INFORMATION NETWORK LABORATORY ......WIRELESS INFORMATION NETWORK LABORATORY Introduction: ZnO Materials • II-VI compound semiconductor: – Direct bandgap, with E g @3.32