Chemical and structural analysis of low-temperature excimer-laser annealing in indium-tin oxide sol-gel films
Graphical abstract
Excimer laser annealing of indium-tin oxide sol-gel thin films. (a) Schematic diagram of ELA process in ITO thin film, (b) XRD patterns ranged from 50 to 240 mJ/cm2, and cross-sectional TEM images at (c) 100 mJ/cm2, (d) 150 mJ/cm2 and (e) 240 mJ/cm2. The inset shows low-magnification TEM images in the corresponding regions. The scale-bars correspond to 10 nm (50 nm for the insets).
Introduction
Transparent conducting oxide (TCO) thin films have been widely used in optoelectronic devices, such as solar cells and organic light-emitting diodes (OLEDs) [[1], [2], [3], [4], [5]]. Indium–tin oxide (ITO) is the most widely used TCO material, owing to its low resistivity (∼10−4 Ω cm), low extinction coefficient in the visible region (∼0.001), and wide optical band gap (3.5–4.3 eV) [[5], [6], [7]]. In order to meet the increasing demands for a larger device size and mitigate the impact of the increasing cost of indium, alternative TCO materials, such as doped zinc oxide [[8], [9], [10], [11]], graphene [[12], [13], [14], [15]], and BaSnO3 [16] are studied with a varying degree of success. Another requirement for the TCO films emerges from the widespread use of flexible devices with plastic substrates. Various fabrication methods, such as sol-gel, sputtering, and pulsed laser deposition have been investigated for this purpose [5,[17], [18], [19], [20], [21]]. It is worth noting that, for flexible substrates, the entire fabrication process has to be maintained at temperatures below 200 °C, as most flexible substrates cannot withstand higher temperatures [22].
One of the promising low-temperature processes is irradiation of solution-processed TCO films with ultraviolet (UV) light [17,23,24]. The absorption of continuous UV light or intense UV laser pulses breaks the molecular bonding and eliminates the organic species. Combined with the inevitable local heating at the film surface upon the UV irradiation, these processes may condense the solution-coated films and transform their physical properties. While continuous UV lamp irradiation requires a relatively large processing time [17,23], excimer-laser annealing (ELA) with intense UV laser pulses can significantly shorten the processing time by tuning the laser intensity and pulse repetition rate. Although the ELA process has been well established for poly-Si film fabrication and used extensively in the semiconductor industry [[25], [26], [27]], its effects on the chemical, structural, and electrical properties of TCO films have not been fully understood yet.
In this study, we analyzed the chemical and structural properties of sol-gel ITO thin films during ELA for better understanding of the electronic changes. We irradiated KrF excimer laser pulses on sol-gel-coated ITO films; the electrical properties of the films were measured as a function of the total deposited laser energy. After 240 mJ/cm2 of deposited ELA energy density, the ITO films exhibited good electrical properties as confirmed by Hall measurements: a resistivity of 5.75 mΩ cm, carrier density of 1.66 × 1020 cm−3, and mobility of 5.84 cm2/V. We elaborately analyzed the chemical compositions and oxidation states of the irradiated films using x-ray photoemission spectroscopy (XPS), which revealed not only the carbon reduction, but also the metal oxidations and their crystallization. Both macroscopic and microscopic structural evidences observed with x-ray diffraction (XRD) and transmission electron microscope (TEM) showed an enhanced crystallization with the increase of the laser energy density, consistent with the XPS results. We discuss the comprehensive correlations between the observed chemical and structural changes and the electrical properties in the sol-gel ITO films during the ELA.
Section snippets
Experiments
We prepared an indium precursor solution by dissolving In(NO3)3·xH2O into 2-methoxyethanol (2-ME) solvent at a concentration of 0.3 M and stirring at 75 °C for one day. For a tin precursor solution, SnCl2 was dissolved at the same condition; however, it was stirred at room temperature for 3 h [6,28]. They were mixed at a metal atomic ratio of 9:1 and further stirred at room temperature for 3 h. The resulting solution was spin-coated on SiO2/Si substrates at 3000 rpm for 30 s and subsequently
Electrical properties
Table 1 shows the measured electrical resistivities, carrier concentrations, and mobilities of the ELA-processed ITO thin films as a function of the deposited laser energy. As the laser energy density increases, the resistivity decreased gradually, reaching 5.75 mΩ cm at 240 mJ/cm2, which is comparable to the values of sol-gel ITO films fabricated by conventional sol-gel methods followed by high-temperature annealing (>400 °C). The decrease of the resistivity of the ELA-treated ITO thin films
Discussion
These experimental results allowed us to understand the laser-induced annealing process in the sol-gel ITO films and to construct more comprehensive picture of the ELA, as schematically shown in Fig. 4(a). In the initial ELA, small granular phases are crystallized as the precursors and organic solvents are evaporated. The KrF excimer laser pulses with photon energy of 5 eV and a duration of ∼20 ns have sufficient energies to break the chemical bonding of the organic species. At this initial
Conclusion
We expansively analyzed the chemical and structural properties for understanding the ELA process of the sol-gel-coated ITO films. After 240 mJ/cm2 of applied laser energy density, the electrical properties of ITO films become comparable to the heat-treated sol-gel films. The elaborate XPS analysis revealed that the ELA diminishes carbon contents in favor of both oxidation and crystallization. The XRD and TEM results commonly showed an enhanced crystallization with the increase of the laser
Acknowledgement
This work is supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT and Future Planning, Korea (NRF-2017R1C1B2004927 and NRF-2017R1A2B4009260). This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning, Korea (2009-0082580). This research was also supported by the Ministry of Trade, Industry and Energy (MOTE,
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