12/03/20071. ZnO (Zinc oxide) by Alexander Glavtchev.

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ZnO (Zinc oxide)

byAlexander 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, 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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PropertiesMelting 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 PropertiesThe 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 PropertiesCan 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 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