Consumer electronics such as TV

1

DEPARTMENT OF
PHYSICS
Improving the Electrical Properties of Zinc Oxide
Leonel Hernandez Advisor Dr. Gonzalez
Department of Physics
Abstract
Results
Materials and Methods
After having annealed the different samples and
testing them for conductivity, a few were chosen
to be reduced. The reduced samples were retested
for conductivity. Below is a graph showing the
increase in conductivity, after annealing.
Consumer electronics such as TVs and personal
computers with flat-panel displays are part of a
multi billion-dollar industry which is still
growing. These electronics use transparent
semiconductors to display images.  Zinc oxide
(ZnO) is an attractive transparent conductor that
can potentially substitute the commercially used
materials at a reduced cost.  A good transparent
conducting oxide is characterized by
high-electrical conductivity and optical
transparency.  Since ZnO in its pure form is an
insulator our aim was to improve its electrical
properties and to understand the mechanism
responsible for such a desirable effect.  ZnO
powders with various impurity levels and
different grain sizes were prepared and exposed
to reducing environments to improve their
electrical properties. The electrical
conductivity and Seebeck coefficient of the
samples were measured before and after
reduction.  For some samples, the conductivity
increased up to 1400 .  The atomic and
micro-structural properties of these powders were
determined by x-ray fluorescence, x-ray
diffraction and scanning electron microscopy and
correlated to the electrical properties.  The
impurities and grain size distribution in these
zinc oxide materials had an effect on their
electrical behavior.  Studying the direct
correlation between the atomic-scale structure
and the desirable macroscopic electrical
properties of zinc oxide can lead to a better
understanding and a more effiient optimization of
this material for commercial devices.
ZnO oxide powder from different manufacturers
was used each with different purity levels and
grain size. Two, labeled N1 and N2, have grain
size on the scale of nanometers, while the
others, B1, B2, and B3, have larger grain size.
The manufacturers provided a purity level of
above 99. Each material was labeled based on
purity level, with 1 being the most pure. Purity
was experimentally measured and B3 and N1 had
many impurities.
Synthesis
Since we sought to find the conductivity and
thermopower of the materials, the powder had to
be formed into a solid object. It was pressed
into small pellets, of 6 mm diameter and about
0.16 grams.
The powders had to be macerated with acetone and
then dried prior to pressing for homogenization
purposes. The same procedure was used to press
all the pellets. A stainless steel die with an
inner chamber of radius 3 mm was used to hold the
powder.
From the table above we can see that there is a
definite increase in conductivity after
reduction. Samples from the B1 batch showed an
increase of up to more than 1400.
The graph below shows the data for the
thermopower before and after reduction. For the
most part the samples had a very small change in
their thermopower. The negative values indicate
that the samples are n-type. The smaller
magnitudes correspond to a higher electron
concentration.
The press used at just over 1000 lbs
Annealing
Once the pellets were pressed, in order to
increase the particle-particle contact, they were
placed in a high temperature furnace. This
process, called annealing, increases their
durability, allowing us to test their properties
without damaging the pellets.
Introduction
The pellets were baked in the furnace for a
couple of days at 1200 degrees Celsius. The
maximum temperature achievable by the furnace is
1500 degrees.
Transparent conducting oxides (TCOs) are used in
many industrial applications. They are used
because of their conductivity and transparency in
the visible region. Zinc oxide (Zn0) has the
potential to become a very good TCO, with further
research. It is relatively cheaper than other
TCOs and is non toxic.
Forming Gas Reduction
Once annealed, about half of the pellets were
chosen to be reduced in a forming gas chamber.
Forming gas is a mixture of 4 hydrogen in
nitrogen, and is commonly used to dissipate
moisture and oxygen. We reduced these pellets in
order to determine the effect that forming gas
reduction has on thermopower and conductivity. In
theory, gas reduction will remove some oxygen
molecules from ZnO, increasing the carrier
concentration of electrons and thus conductivity.
Applications of TCOs include LCDs and solar
cells.
SEM pictures of B2 samples. The picture above is
before reduction at 20K x zoom and the picture to
the right is after reduction at only 5K x zoom.
The reducing chamber in which our samples were
placed. They were held at about 500 degrees
Celcius under forming gas.
Testing the Electrical Properties
After the pellets were annealed they were tested
for their conductivity using a four-point probe
and for thermopower using the soldeing iron.
Conclusions The
results from the tests conducted indicate that
forming gas reduction improved the conductivity
in some cases up to more than 1400.The purest
samples had better electrical properties. More
experiments are being carried out to reproduce
these measurements, and similar results have been
obtained. Recent efforts are focusing on
optimizing the synthesis procedure and reduction
conditions to further improve the electrical
performance of zinc oxide
Objective
Zinc oxides atomic structure, as a crystal, is
regular and repeating. This rigid pattern binds
all of the materials electrons to the atoms.
Atomic defects or irregularities in this atomic
structure may increase the electron
concentration, and hence the material's ability
to conduct. The objective is to explore the
defect structure of ZnO, analyzing influential
factors and synthesis procedures.
Pellets were placed between the soldering iron
and metal block with two thermocouple
attachments. To test the thermopower we placed
the pellet between two thermocouples. A
temperature difference was created by placing the
pellet on top of a metal block initially at room
temperature and connecting a modified soldering
iron to the top. This allows us to measure the
carrier concentration of each pellet, which
helped us determine the effect of the gas
reduction process.
The four-point probe setup with the probe on the
far left and the current source and voltmeter in
the center and right, respectively. By applying
different currents to the two outer probes and
using a voltmeter to measure the voltage across
the two inner probes we were able to determine
the pellets conductivity.
Acknowledgements
I would like to thank Dr. Gabriela
Gonzalez-Aviles for giving me the opportunity to
work on this research project from which I have
gained a great amount of knowledge. I would also
like to thank the following peers who also worked
on this research project Alexander Slawik, Tom
McManus, and Jared Hennen.
represent O2- represent Zn2
Side view