talks with ScienceWatch.com and answers a few
questions about this month's Emerging Research Front Paper
in the field of Materials Science.
Article: A comprehensive review of ZnO materials
U;Alivov, YI;Liu, C;Teke, A;Reshchikov, MA;Dogan,
S;Avrutin, V;Cho, SJ;Morkoc, H
Journal: J APPL PHYS, 98 (4): art. no.-041301 AUG 15
Addresses: Virginia Commonwealth Univ, Dept Elect Engn, Med
Coll Virginia Campus, Richmond, VA 23284 USA.
Virginia Commonwealth Univ, Dept Elect Engn, Richmond, VA
Virginia Commonwealth Univ, Dept Phys, Richmond, VA 23284
Why do you think your paper is highly
This review article represents a cohesive treatment of all aspects of zinc
oxide (ZnO), a semiconductor material which is widely investigated for
applications in photonics, transparent electronics, acoustics, and sensing.
Even though first investigations of this material date back to the 1930s,
availability of high-quality substrates in the last couple of decades and
reports of p-type conduction and ferromagnetic behavior, both of which,
however, still remain controversial, have fuelled the renewed interest in
Especially if reliable and reproducible p-type conductivity in ZnO is
achieved, this material system can provide optical emitters superior to
today's devices. Our paper received such a high level of citation due to
the fact that it is a well-organized and comprehensive review of this
highly attractive field.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
"Successful commercialization of
ZnO-based devices will have a direct impact
on our daily lives."
Our paper provides a complete synthesis of all the recent progress made by
many of the leading experts in the field and a critical review of the
published results. It is gratifying to state that we owe so much to our
coworkers and colleagues who contributed to the field of ZnO and in
particular in our efforts to bring this paper to the service of all
researchers and students.
Would you summarize the significance of your paper in
My colleagues and I reviewed all physical properties and the applications
of the ZnO semiconductor material system, which is attractive for a variety
of applications in optoelectronics and electronics. Despite the progress
made in the research phase, there are still a number of important issues
that need to be investigated further before this material can be
transitioned to commercial use.
However, there are some niche applications of ZnO, which are not addressed
by other semiconductor systems of today. These include transparent
thin-film transistors used in liquid crystal displays, transparent contacts
applicable to all types of optical devices, including solar cells, gas, and
bio-sensors based on nanostructures, and, ideally, thresholdless laser
structures exploiting the large exciton-binding energy of ZnO, which makes
it the best photon emitter among all semiconductors. This review paper
discusses all these prospects and addresses present problems.
How did you become involved in this research and were
any particular problems encountered along the way?
I first became involved with the ZnO material during my graduate studies.
In 1999, we had measured absorption coefficients and refractive indices,
which are essential for designing optical devices, especially lasers, of
ZnO and MgZnO thin films.
For over 10 years, I have been doing research on optoelectronic devices
based on Gallium nitride (GaN), another semiconductor whose applications
overlap considerably with ZnO. However, GaN device technology is much more
mature as GaN-based very high performance electronic and optical devices,
including power field effect devices, light-emitting diodes, and blue laser
diodes (those in game consoles and blu-ray players), have already been
The highly ionic nature of ZnO with large electron–phonon coupling
and low thermal conductivity does not bode well for electronic devices.
Additionally, and more importantly, a lack of credible p-type doping
hampers the thought of widespread optical emitters based on ZnO, especially
when there is such a stiff competition from GaN.
Until p-type conductivity is realized for ZnO, applications of this
interesting material system in devices will be limited. I should also
mention that, even though nanostructures seem a little easier to produce
with ZnO, it remains to be seen whether nanostructures, in general, would
really make inroads in the area of devices.
Where do you see your research leading in the
Our current efforts are focused on understanding fundamental obstacles to
obtaining reliable p-type conductivity in ZnO. Future work related to this
material system also involves microcavities, which are quite promising for
very low-threshold lasers operating in the UV and blue regions of the
optical spectrum. If reliable p-type ZnO is obtained, we may see ZnO-based
optical emitters revolutionize the optoelectronics industry.
Do you foresee any social or political implications for
Successful commercialization of ZnO-based devices will have a direct impact
on our daily lives. As an example, if the p-type conductivity problem is
resolved, ZnO's relatively low production cost and superior optical
properties, when compared to other competing semiconductors, mainly GaN,
may lead to a more efficient solid-state lighting technology based on ZnO.
The expected economic and social impacts are tremendous, since such a
development would reduce energy needs, and therefore, environmental
pollution significantly, while improving the quality of life.
Ümit Özgür, Ph.D.
Department of Electrical and Computer Engineering
Virginia Commonwealth University Web