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Topics Covered
Background
Zinc Oxide (ZnO)
Applications of Zinc Oxide
Current Challenge with
Zinc Oxide
Benefits of Zinc Oxide
Opportunities of Zinc Oxide as Both Semiconductor and
Conductor
Properties of Zinc Oxide
Physical and Optical Properties of Zinc Oxide
Trends in Zinc Oxide Manufacturing Process
Applications of ZnO as a Conductor
Applications
of ZnO as a Semiconductor and Other Applications
Technical
Issues with ZnO
Summary
Background
NanoMarkets is a leading provider of market and technology
research and industry analysis services for the thin film, organic and printable
electronics businesses (which we refer to as TOP Electronics.) Since the firm’s
founding, NanoMarkets has published over two
dozen comprehensive research reports on emerging technology markets. Topics
covered have included sensors, displays, OLEDs, HB-LEDs, e-paper, RFID,
photovoltaics, smart packaging, novel battery technologies, printed
electronics, organic electronics, emerging memory and storage technologies
and other promising technologies. Our client roster is a who’s who of companies
in specialty chemicals, materials, electronics applications and manufacturing.
NanoMarkets also hosts a blog at www.nanotopblog.com where we discuss technology trends, company announcements and
the industry’s on-going progress.
Zinc Oxide (ZnO)
Zinc
Oxide (ZnO) is emerging as a material of interest for a variety of
electronic applications. It can be used in a large number of areas, and unlike
many of the materials with which it competes, ZnO is
inexpensive, relatively abundant, chemically stable, easy to prepare and
non-toxic. Most of the doping materials that are used with ZnO are also
readily available.
Applications of Zinc Oxide
At present, the most widely publicized application for ZnO is an ITO
replacement for displays and photovoltaic panels, where ZnO could
lower costs of transparent conductors. But new applications for ZnO are much
broader than that. In addition to its conductive nature, ZnO also can
be used as a semiconductor for making inexpensive transistors for disposable
electronics or even low-cost LEDs. ZnO is also finding applications in thin-film batteries, and
ZnO's
ability to be engineered into interesting nanostructures hints at new
applications down the road. ZnO already is being tapped in spintronics.
Current Challenge with Zinc Oxide
However, technical difficulties must be addressed before ZnO is able to
reach its full potential. One important challenge is that there is as yet no
stable p-type ZnO semiconductor. Technical hurdles aside, the number of
patent filings of ZnO uses in electronics continues to grow and NanoMarkets
believes that ZnO will represent a substantial market as an electronic
material over the next eight years.
Benefits of Zinc Oxide
As mentioned above, ZnO has several advantages over its competitors; it is
inexpensive, relatively abundant, chemically stable, easy to prepare and
non-toxic. One of the strongest market opportunities for ZnO is a
cost-effective replacement for ITO, which costs (99.99 percent purity or higher)
over $700/kg. The cost of indium metal, which as of this writing is over
$1,000/kg, accounts for a large share of the production costs. This compares to
zinc, which is traded at less than $1 on the London Metal Exchange, and although
there is a cost associated with processing zinc metal into high purity zinc
oxide powder, the overall cost is a fraction of the current cost of ITO.
Additionally, its abundance and chemical stability has made ZnO a material
of interest as a replacement for toxic, expensive GaAn transistors in the LED
space.
Another benefit of ZnO is that it can be processed using various manufacturing
process. This compares to ITO, which is typically sputtered—a costly, wasteful
process that causes interfacial damage. The sputtered layer is also adversely
affected with each annealing, etching and drying stage causing brittleness and
adhesion issues, all of which negatively impact the performance of the films.
The ability to use chemical vapor deposition (CVD) or metal organic chemical
vapor deposition (MOCVD) techniques is particularly attractive, resulting in
better step coverage, higher deposition rates, improved composition grading and
the elimination of interfacial damage. Less costly process methods add to the
attractiveness of using ZnO for a variety of applications.
Opportunities of Zinc Oxide as Both Semiconductor and Conductor
One of the benefits of ZnO is that it can be used as a conductor and a semiconductor.
While NanoMarkets expects close to 70 percent of the applications in this report
will favor ZnO as a conductor, the potential of ZnO as a
semiconductor certainly is noteworthy. ZnO is a good
conductor because of its environmental stability, low resistivity and high
transparency, not to mention its low cost and abundance. The low cost naturally
makes ZnO
attractive as a semiconductor. As a semiconductor however, there are still
technical issues in the ability to achieve repeatable, stable p-type film.
Properties of Zinc Oxide
In addition to cost savings, ZnO offers the
following properties.
- High carrier mobility
- Transparency
- Wide band gap
- Low temperature process
Physical and Optical Properties of Zinc Oxide
The high carrier mobility is directly linked to transparency, which makes it
fully possible for ZnO to compete with existing silicon materials. The wide band
gap is important because it opens the possibility of creating Ultra Violet (UV)
LEDs and white LEDs with superior color purity. Low temperature processing is
preferred in some applications such as OLEDs. ZnO has a
direct band gap energy of 3.37 eV at room temperature, and exciton and biexciton
energies of 60 meV and 15 meV, respectively. Epitaxy will likely further improve
ZnO's
exciton properties, which directly relates to the optical properties in
photovoltaics and displays.
Zinc
oxides as transparent semiconductors are attracting interest mainly because
there has been a sharp jump in the need for higher carrier mobility of
transparent semiconductors. The carrier mobility determines transparent TFT
characteristics. This is now exceeding the carrier mobility of materials such as
low-temperature poly-Si (LTPS) and amorphous Si used in LCD panels.
Trends in Zinc Oxide Manufacturing Process
Transparent semiconductors such ITO as GaN and diamond are already well known
but they come at high material costs, and the manufacturing processes used to
make these semiconductors pose significant problems for their use in transparent
electronic devices, which demand relatively large screens such as displays.
Currently, the materials are far from ideal for some of the fastest growing
applications sectors in which transparent conductors are used. Some crack when
used in the current generation of touch screen displays and it is likely to do
so in next-generation rollable displays. Next-generation screens include large
flat panel displays whose geometry is not limited to flat surfaces, but may take
on curved or cylindrical configurations. The active element flexible film design
also can be used for electroluminescent tape, signage, for flexible glue-on
displays and for video displays such as workstations, HDTV, theater screens and
billboards.
Applications of ZnO as a Conductor
Most of the present applications for ZnO are as a
conductive film. As research continues to refine the processes for manufacturing
ZnO as a
thin film it is becoming clear that this inexpensive abundant material may be
suited for a number of applications. Without doubt, displays are the leading
application where ZnO is being used as a replacement conductive material.
Indium tin oxide has been the transparent conductor of choice for many
display applications due to its combination of environmental stability,
relatively low electrical resistivity and high transparency. However, ITO is far
from the perfect solution to many transparent conductor needs driving the need
for ITO substitutes. Due to the high cost of indium and ITO's reliance on
sputtering, ZnO becomes an attractive replacement. Most proposals for
metal oxide TCOs dispense with the indium altogether. Materials that have been
considered for TCOs include variations on tin oxide or ZnO,
especially the latter. ZnO-based materials that have been considered or used for TCOs
include zinc oxide itself, Mg-doped zinc oxide (MZO), Al-doped ZnO (AZO),
Ga-doped ZnO (GZO), Al-doped MZO (AMZO), gallium-doped ZnO (AGO) and
gallium-doped MZO (MMZO). Indium doped ZnO is also used, although this, of
course, brings with it the high cost of indium. Again, it does not take much to
understand why ZnO is attractive. ZnO is
inexpensive, relatively abundant, easy to prepare, and non-toxic. The use of ZnO
and derivatives for TCOs does not impact the price of zinc in the way that the
use of ITO impacts the price of indium because the use of ZnO as a TCO
is an unimportant application from the perspective of the ZnO business as a
whole. In addition, with the possible exception of Ga, most of the doping
materials used with ZnO are also readily available. Finally, all the materials
based on ZnO are thermally and chemically stable.
Photovoltaics and LEDs are both on high growth market trajectories and are
becoming favorable applications for ZnO as a
conductive coating. Several solar companies are basing their PV technology on
copper indium gallium deselenide (CIGS). Characteristic to the CIGS process, all
companies use molybdenum (Mo) as the back contact deposited by sputtering, and
the majority use ZnO as the front contact deposited either by sputtering or
CVD.
Another potential market for ZnO as a
conductor is the thin-film battery market, which is growing steadily. Thin-film
batteries are best suited where small power sources are needed and need to be
manufactured in a variety of shapes and sizes to fit in obscure wasted space
locations. ZnO is currently being successfully used as a printed
conductive coating for thin film batteries. Other present day successful uses
for ZnO as a conductive coating include EMI and RFI coatings and shielding.
Applications of ZnO as a Semiconductor and Other Applications
Some of the potential applications for ZnO as a
conductor also lend themselves to ZnO as a
semiconductor. These include photovoltaics and LEDs, which
could become favorable applications for ZnO as a
semiconductor. However, there are technical difficulties still being worked out
for ZnO.
With regard to photovoltaics, the band gap leaves little of the solar spectrum
to be absorbed. Since the semiconductors are transparent to light with energy
less than the band gap, they only absorb photons with energy greater than the
band gap. ZnO has a bandgap of 3.37 eV leaving very little of the solar spectrum
able to be absorbed.
ZnO
presently struggles to fulfill the needs of the LED industry
as an actual light emitter because of the need for stable repeatable p-type ZnO, but a
number of institutions appear to be close to solving this issue. This is a case
of a clear market in need of a technological breakthrough. ZnO offers
phosphor-free spectral coverage coupled with quantum efficiency approaching near
90 percent, making it an attractive replacement for traditional GaN LEDs.
Other areas of interest that are still being perfected include using ZnO as low
cost TFTs for display backplanes. ZnO-TFTs will
pave the way for large-scale macro-electronics such as electronic paper,
flexible/wearable electronics and conformable 3D imaging. Flexible transistors
will be used primarily in active-matrix backplane electronics of displays in
terms of low voltage driving ability.
Gas monitoring devices are in demand for a rapidly growing range of
applications. Metal oxide based chemical sensors have been used extensively for
the detection of toxic pollutant gases, combustible gases and organic vapors.
The main advantages of chemical sensors are their low price, small size, high
sensitivity and low power consumption. Semiconducting metal oxides SnO2 and ZnO have been
explored as gas sensing detectors. ZnO has
demonstrated properties of unique nanostructures such as nanocombs, nanorings,
nanohelixes/nanosprings, nanobelts, nanowires and nanocages and properties for
novel applications as sensors and biomedical transducers.
There are other markets attracting the attention of ZnO, including
spintronics and smart textiles. Spintronics is a nascent field exploiting the
spin of electrons rather than their charge. Investigations of cobalt-doped zinc
oxide have shown promise in providing a diluted magnetic semiconductor. The
smart textiles industry, also a nascent market, is experimenting with ZnO grown
microarrays as part of flexible polyester filaments.
To summarize, ZnO is emerging as a promising material in a variety of
electronics applications. The number of patent filings by academia and industry
continues to grow, and it is hopeful that some of the technical challenges will
be solved allowing ZnO to become ubiquitous in next-generation displays, solar
panels and lighting.
Technical Issues with ZnO
The most serious obstacle to date, which limits ZnO's
potential for certain applications, is the ability to achieve a stable
commercially viable process for p-type ZnO. Despite
several years of research, the cause of these problems is still the subject of
controversy. Low impurity solubility, excessive acceptor ionization energy and
possible compensating mechanisms are three main factors making p-type doping of
ZnO difficult.
Therefore it should come as no surprise that there is a lot of research
activity by industry, university and government in this area. Several methods
incorporating buffer layers, dopants and growth techniques are being
investigated.
Fundamentally the research is based on the fact that LEDs require
both positively and negatively charged semiconducting materials. In an LED, when
an electron meets a hole, it falls into a lower energy level and releases energy
in the form of a photon of light.
The University of California in San Diego (UCSD) and Sanyo Electric Co. are
two companies with patents in this area of p-type ZnO. UCSD has
created p-type nanowires from doped ZnO crystals
with phosphorus using CVD. The addition of phosphorus atoms to the ZnO crystal
structure has lead to p-type material through the formation of a defect complex
that increases the number of holes relative to the number of free electrons.
Sanyo Electric has been awarded a patent for the fabrication of p-type ZnO by doping
ZnO with an
alkali metal and hydrogen. Other methods explored include growing ZnO films by
MOCVD on GaN wafers.
Summary
NanoMarkets opinion is that the technology is just a couple of
years away from being refined, which will open up new market opportunities for
ZnO,
particularly in the LED space.
Source: "Markets for Zinc Oxide in Electronics", Market Report by
Nanomarkets
For more information on this source please visit NanoMarkets