ZnO2 _Zinc oxide_

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					12/03/2007   1
ZnO (Zinc oxide)




          by
  Alexander Glavtchev
             Why is ZnO interesting?
Widely used:
• medicinal purposes (colds, rashes, antiseptics, sunscreen lotions)
• used in manufacturing of rubber as rubber cure (or as filler)
• pigment for paints and coatings
• in electronics, used mainly in laser diodes, LED’s, transparent thin
  film coatings, and various piezoelectrics

Bright future:
• shows promising signs in the field of nanotechnology, UV detecors,
   nanoscale detectors and actuators
• direct bandgap semiconductor that could replace silicon as the main
   substrate in chip manufacturing (if it can be easily/cheaply p-doped)
• dual semiconductor and piezoelectric properties!!

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                          Properties
Melting Point: 1975 °C
High electron mobility: >100cm2/Vs
High exciton binding energy: ~60meV (electron-hole binding energy)
Direct bandgap: 3.3eV




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             Piezoelectric Properties
 The output amplitude is related to
 the input signal by:




 Lithium niobate (LiNbO3) and
 Lithium tantalate (LiTaO3) are
 currently two very-widely used
 piezoelectric crystals due to their
 high piezoelectric strain coefficients.




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             Piezoelectric Properties
ØCan produce voltage output from
applied stress (strain), or produce
stress when voltage is applied.
ØNanobelts and various nanoscale
features of ZnO give higher
piezoelectric constants (likely due to
less dislocations and the impurity-free
single-crystalline structures).
ØPiezoelectrics used in: sensors
(acoustic and electronic, as pickups in
electric guitars, detection/generation
of sonar waves, etc.); actuators (high-
precision motors, loudspeakers,
atomic force microscope probe
control); possible future use in
vibration and noise reduction
(housing, automobiles).
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         Semiconductor Properties
• Zinc oxide is a direct wide-bandgap semiconductor (~3.3-3.4eV).
• Allows for efficient photon emission, as in LED’s or laser diodes
  (rather than phonon emission with energy loss and heat generation).
• Can easily be n-doped with aluminum, indium, or extra zinc.
• Possesses high electron mobility and photoconductivity – can help
  speed up currents in semiconductor devices.

Drawbacks:
• p-doping is currently very difficult and inefficient and has prevented
  mass manufacturing of ZnO-based wafers.
• High-purity ZnO grown on substrates other than sapphire has been
  challenging.


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                 Nano-Structures
• Zinc oxide shows great potential for nanoscale electro-mechanical
  fabrication.
• Highly-symmetric, singly-crystalline nanoneedles, nanowires,
  nanobelts, nanorings, nanohelixes, nanocombs, etc.
• Hexagonal (wurtzite) structure helps lattice-matching and
  controlled growth.
• Positive Zn surfaces and negative O surfaces create electric dipoles
  that facilitate polarization growth along certain directions and
  planes under applied voltage and temperature.




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             Nano-Structures




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                          Summary
•   ZnO displays dual semiconductor and piezoelectric properties.
•   Used in laser diodes and LED’s.
•   Potential to be used as a wide-bandgap semiconductor.
•   Widely used in many other fields (medicine, farming, pigments).
•   Zinc oxide nanostructure growth is heavily researched presently.
•   The substance likely has the largest variety of nanostructures (and
    their associated properties) among all known materials.
•   It’s hexagonal lattice can easily match catalysts’ lattice structure
    and facilitate controlled growth patterns.
•   Structures like nanowires, nanobelts and nanorings are of great
    interest in photonics research, optoelectronics, nanotechnology, and
    biomedicine.


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                                    Sources
•   Nanoarchitectures of semiconducting and piezoelectric zinc oxide. JOURNAL OF APPLIED
    PHYSICS 97, 044304 s2005d
•   Piezoelectric Characterization of Individual Zinc Oxide Nanobelt Probed by Piezoresponse
    Force Microscope. Nano Lett., Vol. 4, No. 4, 2004
•   Nanostructures of zinc oxide. Zhong Lin Wang. Materials Today, June 2004.
•   Photonic band structure of ZnO photonic crystal slab laser. JOURNAL OF APPLIED
    PHYSICS 98, 103102 2005
•   Deformation-Free Single-Crystal Nanohelixes of Polar Nanowires. Nano Lett., Vol. 4, No. 7,
    2004
•   Zinc oxide hexagram whiskers. APPLIED PHYSICS LETTERS 88, 093101 2006
•   Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter
    16 (2004) R829–R858
•   Nitrogen doped zinc oxide thin film. http://repositories.cdlib.org/lbnl/LBNL-54116, 2003
•   www.wikipedia.com




12/03/2007                                                                                  14

				
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