Electrical, structural, and optical properties of ITO thin films prepared at room temperature by pulsed laser deposition
Introduction
Indium tin oxide (ITO) is widely used as a transparent conducting electrode in optoelectronic devices, because the films have high transmittance in the visible region and low resistivity. Generally, it is well known that the low resistivity value of ITO films is due to a high carrier concentration caused by both oxygen vacancies and substitutional tin dopants. Also, the high transparency in the visible and near-IR region is caused by wide band gap. There are several deposition techniques used to grow ITO thin films including evaporation [1], [2], sputtering [3], [4], chemical vapor deposition (CVD) [5], and pulsed laser deposition (PLD) [6], [7], [8]. In comparison with other techniques, PLD has several advantages. The composition of the films is quite close to that of the target, and films crystallize at low substrate temperature due to the high kinetic energies of the atoms and ionized species in the laser-ablated plasma [7], [8]. Recently, the development of a deposition process at low temperature is required for application to film growth on a heat-sensitive substrate [9].
In this paper, we report on the investigation of the electrical, structural, and optical properties of ITO thin films prepared by PLD at room temperature, as a function of oxygen pressure and rapid thermal annealing (RTA) temperature.
Section snippets
Experimental
The ITO thin films were deposited at room temperature by using PLD with a Nd:YAG laser (355 nm, 5 Hz, and FWHM of 6 nm). The target used in this study was sintered ITO pellet containing 5 wt.% SnO2. The ablated material was deposited on 1.25 cm × 1.25 cm glass substrates (microscope cover glass, Marienfeld). The substrates were ultrasonically cleaned in acetone and methanol, rinsed in deionized water, and subsequently dried in a flowing nitrogen gas before deposition. The basal vacuum in the chamber
Result and discussion
Fig. 1 illustrates the variation of resistivity, carrier density, and Hall mobility as a function of oxygen pressure for 400 nm-thick ITO films grown at room temperature. It can be seen that resistivity is strongly influenced by the oxygen pressure during the film growth. High resistivity films were obtained at low oxygen pressure and it rapidly decreased to a lowest value of 2.4 × 10−3 Ω cm at an oxygen pressure of 2.0 Pa. This decrease in resistivity may be due to the reduction of severe oxygen
Conclusion
Indium tin oxide thin films have been deposited by PLD on glass substrates. The electrical, structural, and optical properties of these films were investigated as a function of the oxygen pressure and RTA temperature. All of the ITO films grown by PLD were found to be n-type semiconductors. An increase in oxygen pressure from 1.3 × 10−4 to 6.7 Pa reduced lattice structural disorders and hence increased conductivity. However, a further increase in oxygen pressure resulted in less oxygen vacancies
Acknowledgement
This work was supported by KOSEF through National Core Research Center for Nanomedical Technology (R15-2004-024-0000-0).
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