Frost & Sullivan, 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 thermal stability.
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," explained Fortunato.
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 knowledge.'"
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: emf@fct.unl.pt