Zinc-Oxide Films Are Transparent And Conductive
an international auditing and consulting company
from England, has highlighted in their magazine,
for the second time, the excellence of the
research done by investigators at CEMOP and
those belonging to the Department of Science
and Materials' Engineering.
Because of numerous applications
in displays, solar cells, electrochromic devices,
antistatic windows, heatable glasses, and so on,
transparent conducting oxides (TCOs) are regarded
as promising compounds. While previous research
on TCOs has delved into compounds such as indium
tin oxide (ITO) and fluorine tin oxide (FTO),
zinc oxide thin films have not been thoroughly
researched. Meanwhile, zinc oxide thin films have
attracted increasing interest because of advantages
such as low cost, resource availability (for example,
zinc is far more readily available than indium),
nontoxicity, and relatively high chemical and
Despite the attractiveness,
a number of issues hinder the practical applicability
of this compound. One of these is that non-doped
zinc presents a high resistivity due to low carrier
concentration. Aluminium (Al), Indium (In), and
Gallium (Ga) have been used as dopants in zinc
oxide-based films, but Al has been most widely
researched. However, Al presents a very high reactivity
causing oxidation during film growth--this may
become a serious problem later on. Therefore researchers
are exploring the use of new dopants.
Researchers at the New University
of Lisbon have been successful in substituting
Al with Ga for more than a year, and have reported
encouraging results. Professor Elvira Fortunato--the
lead researcher--says that "gallium was chosen
because defects are fewer. Moreover, there was
the problem of alumina as a result of oxidation
of aluminium oxide in the aluminium-doped films."
Ga is less reactive and therefore more resistant
to oxidation than Al. Moreover, the slightly shorter
bond length of Ga-O compared with Zn-O (1.92 versus
1.97 angstrom) is advantageous as it allows only
minimum deformation of the ZnO lattice even when
there is a high Ga concentration.
Fortunato's team deposited
gallium-doped zinc oxide (GZO) films onto soda
lime substrates through radio frequency (RF) magnetron
sputtering. The process was carried out at room
temperature under argon (Ar) pressure between
0.15 to 1.2 Pa and RF power of 175 W. Being able
to deposit highly conductive and transparent thin
films on plastic substrates at room temperature
was the greatest single challenge in this research.
"We adapted the RF sputtering
system, which we made ourselves, to deposit gallium
rather than aluminum by changing the target material.
Instead of using ZnO:A1203, I now use Zno:Ga203,"
Film thickness was measured
using a surface profilometer while surface morphologies
were studied using a field effect scanning electron
microscope (SEM). Electrical parameters such as
electrical resistivity, free carrier concentration,
and Hall mobility were inferred using the four
point probe method while optical transmittance
was measured using a double beam spectrophotometer.
Using high growth rates (above
280 angstrom per minute), researchers were able
to obtain high conducting and transparent GZO
films. The films presented an overall transmittance
in the visible spectrum of about 90%. Fortunato
avers that her "group has achieved a world
record with gallium doped ZnO films." The
researchers have determined the resistivity of
such films to be in the range of 10-4 to 2´10-4
ohm-cm; the lowest reported for films deposited
at room temperature.
For higher sputtering pressures,
an increasing resistivity was observed both due
to a decrease in the mobility and carrier concentration,
associated to a change in the surface morphology.
These results indicate the attractiveness of using
such inexpensive films for the production of transparent
electrodes on flexible optoelectronic devices.
The next stage in the gallium-doped
zinc oxide research being conducted at the New
University of Lisbon is finding ways to perform
p-type oxide semiconductors, according to Fortunato.
"LEDs, photodetectors, solar cells, and lasers
are among the devices that would benefit from
the films being developed by the Portuguese research
team, whose work is being supported by a variety
of national and trans European agencies. Fortunato
is optimistic about the potential of the work,
stating, "there are no impossibles. To quote
Einstein, `Imagination is more important than
Details: Elvira Fortunato, Associate Professor,
Department of Science of Materials, Faculty of
Science and Technology, New University of Lisbon,
2829-516 Caparica, Portugal. Phone: 21-294-85-62.
Fax: 21-294-85-58. E-mail: email@example.com