ZnO2 (Zinc oxide)

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ZnO (Zinc oxide)

• by
• Alexander Glavtchev

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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,
  LEDs, 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
gt100cm2/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

• Can produce voltage output from applied stress
  (strain), or produce stress when voltage is
  applied.
• Nanobelts and various nanoscale features of ZnO
  give higher piezoelectric constants (likely due
  to less dislocations and the impurity-free
  single-crystalline structures).
• Piezoelectrics 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 LEDs
  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 LEDs.
• 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.
• Its 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)
  R829R858
• Nitrogen doped zinc oxide thin film.
  http//repositories.cdlib.org/lbnl/LBNL-54116,
  2003
• www.wikipedia.com