Abstract
This study examined the optical and electrical properties of indium tin oxide (ITO) films treated with a hydrogen plasma and a post-HCl solution. ITO films, 200 nm in thickness, were deposited on polyethylene terephthalate (PET) substrates by the radio frequency magnetron sputtering method. The samples were then exposed to an inductively coupled hydrogen plasma and this was followed by HCl : DI solution treatment. The work function of the ITO surface was decreased from 4.94 to 4.61 eV without thermal annealing. The sheet resistance was decreased due to the formation of a high density of O–H bonds and oxygen vacancies. Although the transmittance of the hydrogen-plasma-treated sample was decreased drastically by the formation of In/Sn clusters, enhanced transmittance and increased optical band gap without a deterioration of the electrical properties could be achieved effectively by removing the In/Sn clusters using a HCl solution.
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1. Introduction
Various types of transparent conducting oxide (TCO) thin films, such as impurity-doped indium oxide, tin oxides and zinc oxides, have been used widely as transparent electrodes for many optoelectronic devices [1]. In particular, indium tin oxide (ITO) is used widely as an electrode material on account of its good electrical conductivity and high transparency in the visible region [2–5]. Recently, inverted organic-polymer photovoltaics (OPV) with metal-modified ITO as the transparent cathode with a low work function have attracted increasing attention due to reduced charge recombination, resulting in an increase in power conversion efficiency [6–11]. On the other hand, it has been difficult to achieve improved optical and electrical properties of ITO deposited on flexible substrates, such as polyethylene terephthalate (PET) and polyether sulfone (PES), due to the poor thermal endurance of organic substrates. An Ar/H2 plasma treatment and post-thermal annealing are performed to improve the carrier concentration of ITO films, which leads to a rather low contact resistance [12–14].
On the other hand, hydrogen can serve as a donor in ZnO [15], leading to an enhanced carrier concentration of ZnO, which was confirmed experimentally by Lee et al using a hydrogen plasma treatment [16].
This paper reports, the effects of H2 plasma and subsequent HCl treatment on the optical and electrical properties of ITO grown on a PET substrate. In particular, the changes in work function, as well as the surface morphology, chemical composition of the surface, and mechanism of low sheet resistance of ITO treated with an inductively coupled hydrogen plasma and a HCl solution are described.
2. Experimental
A 200 nm thick ITO film was deposited directly on a PET substrate using a 2 inch diameter ITO target (10 wt% SnO2 + 90 wt% In2O3) in a home-made radio frequency (RF, ν = 13.56 MHz) magnetron sputtering system at room temperature, a power density of 19.7 W cm−2, working pressure of 2 mTorr and Ar gas flow of 25 standard cubic centimetres per min (sccm) for 36 min. After ITO sputtering, the ITO film grown on PET was treated with hydrogen in a home-made inductively coupled plasma (ICP) system, equipped with a 1 kW ICP power supply at a frequency of 13.56 MHz. The hydrogen plasma treatment was performed at a hydrogen flow rate of 85 sccm, working pressure of 5.7 × 10−3 Torr and ICP source power of 200 W for 2 min at room temperature. Bias voltage was not applied to reduce the damage by ion bombardment. To remove the metallic clusters formed on the ITO surface after the plasma treatment, a wet treatment using a HCl solution (36.5% of HCl : DI 1 : 10) was carried out for 30 s. X-ray diffraction (XRD) confirmed that all the films were amorphous even after the hydrogen plasma and HCl treatment. The sheet resistance of the treated samples was measured using a four-point probe (FPP-RS8 (1 G) 1 mΩ/sq made by Dasol ENG). The resistivity and sheet carrier concentration were measured using a Hall measurement system in the van der Pauw configuration. X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS, respectively) were used to examine the surface composition and change in work function of the ITO films. Field-emission scanning electron microscopy (FE-SEM) and ultraviolet–visible (UV–VIS) spectroscopy were used to analyse the surface morphology and transmittance of the samples, respectively.
3. Results and discussion
As shown in figure 1, the resistivity (Rs) of H2-treated and H2/HCl-treated ITO was decreased significantly from 1.3 × 10−2 Ω cm of the reference sample to 2.7 × 10−3 Ω cm and 4.05 × 10−3 Ω cm, respectively. The sheet carrier concentration (Ns) of the films treated with H2 and H2/HCl increased significantly, in accordance with the results of sheet resistance, suggesting that the improved electrical properties originated from a reaction between the hydrogen plasma and the ITO surface. Hydrogen itself can serve as a shallow donor [15] and was confirmed experimentally to be responsible for the enhanced n-type conductivity of undoped ZnO [16]. This suggests that the improvement in Rs was the result of an increased sheet carrier concentration by the hydrogen plasma treatment. On the other hand, Rs of HCl-treated ITO was not improved but rather increased to 2.3 × 10−2 Ω cm. This suggests that the increase in Rs in the HCl-treated ITO film was presumably due to the formation of a more oxidized surface by HCl [17, 18] or remaining byproducts such as InClx [18]. On the other hand, the Hall mobility of H2- and H2/HCl-treated ITO decreased, which is possibly due to the increased carrier concentration on the surface of the amorphous ITO film, which is similar to the result of crystalline ITO [19].
Figure 1. Sheet carrier concentration, specific resistivity and Hall mobility of the surface-treated ITO as a function of the treatment method.
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Standard imageFigure 2 shows the FE-SEM images of the as-deposited, H2-, H2/HCl- and HCl-treated ITO. The hydrogen plasma changed the ITO surface to a rough surface covered with a high density of In/Sn clusters due to oxygen removal. The density of the In/Sn clusters was reduced considerably by the HCl treatment. Compared with the as-deposited ITO films, the surfaces treated with HCl were relatively unchanged.
Figure 2. FE-SEM images of (a) as-deposited, (b) H2-treated (c) H2/HCl-treated and (d) HCl-treated ITO samples.
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Standard imageThe transmittance of the TCO is an important factor for display and solar cell applications. Figure 3 shows the transmittance of each sample. The transmittance of the H2-plasma-treated sample was quenched rapidly to about 25% of the as-deposited sample due to the scattering of incident light by In/Sn clusters on the surface. After the H2/HCl treatment, the transmittance of the sample increased drastically to the value of the as-deposited sample but was rather low compared with the as-deposited sample with a blue shift, presumably due to the remaining metallic clusters. On the other hand, the transmittance of the wet-treated sample without a prior plasma treatment was relatively unchanged. These results, along with the result of figure 1, strongly suggest that the elimination of In/Sn clusters by a HCl treatment is an effective method after hydrogen plasma treatment for the purpose of achieving high transmittance and that In/Sn metal clusters do not contribute to the enhanced electrical properties of hydrogen-plasma-treated ITO.
Figure 3. Transmittance of each sample after different treatment methods.
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Standard imageFigure 4 presents the secondary electron cutoff data obtained by UPS to measure the work function (Φ) of the ITO surfaces. The work function was calculated using the formula, Φ = hυ − |EF − Ecutoff|, where hυ is the photon energy (He I, 21.21 eV), EF is the Fermi energy and Ecutoff is the binding energy cutoff. Table 1 lists the work function variation of the ITO surface after each treatment. The maximum shift in the secondary electron cutoff was observed in the H2/HCl-treated sample, and the total shift towards a higher binding energy was 0.33 eV. On the other hand, the ITO surface treated with the HCl solution showed a work function of 5.01 eV. It is noteworthy that the effective work function of ITO on PET substrates could be changed from 4.94 to 4.61 eV by the H2 plasma and a post-wet treatment (H2/HCl-treated sample) without the need for a buffer layer, such as TiO2, ZnO and Cs2CO3, particularly for the fabrication of inverted OPV [7–11].
Figure 4. Secondary electron cutoff data of ITO on a PET substrate after different treatment methods, observed by UPS.
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Standard imageTable 1. Work function of the as-deposited, H2-, H2/HCl- and HCl-treated ITO surfaces.
| Treatment | As-deposited | H2 | H2/HCl | HCl |
|---|---|---|---|---|
| Work function (eV) | 4.94 | 4.7 | 4.61 | 5.01 |
The electrical characteristics of ITO are strongly affected by oxygen depending on the number of oxygen vacancies and O–H bonds [20]. XPS was used to examine the O-related bonding configuration of the treated ITO film surfaces. Figure 5 shows the O1s peak of each sample. The O peak of the ITO films can be deconvoluted into three components. The dashed lines centred at 529.68 and 531.02 eV were assigned to oxygen in oxide lattices without and with oxygen vacancies, respectively [21–23]. An additional peak located at 531.92 eV was assigned to O–H bonding [21]. As shown in figure 5, the intensity of the O peak at 531.0 eV, which was assigned to the peak from the oxygen vacancy region, increased after the H2 plasma treatment (figures 5(a) and (c)). This suggests that the concentration of oxygen vacancies was increased, resulting in the formation of In/Sn clusters on the ITO surface. Moreover, the intensity of the O–H peak at 531.9 eV increased more significantly than that of the oxygen vacancies, indicating that O–H bond formation is more dominant than the formation of oxygen vacancies by the hydrogen plasma treatment of ITO. A similar result was also reported for IGZO films in that the intensity of the oxygen vacancy peak decreased, whereas the intensity of the O–H peak increased at high plasma power [24]. In addition, the intensity of the peak at 529.6 eV increased slightly after the HCl treatment due to severe oxidation of the surface. Compared with the O1s peak shape from the H2-treated sample, post-HCl treatment in the H2/HCl sample does not affect the peak intensity of O–H bonds, indicating that the concentration of O–H bonds could be preserved. It is known that the H+ ion binds to an oxygen vacancy in the film or forms an O–H bond due to the stabilization of hydrogen [25]. In addition, the intensity variation of O–H bonds correlated with the change in sheet carrier concentration in figure 1, suggesting that the increased concentration of O–H bonds is a key factor in improving the electrical properties of ITO films.
Figure 5. XPS O1s peaks with (a) H2/HCl-treated, (b) HCl-treated, (c) H2-treated and (d) as-deposited ITO, respectively.
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Standard image4. Conclusion
The resistivity of ITO grown on PET was decreased significantly from 1.3 × 10−2 to 4.05 × 10−3 Ω cm by an inductively coupled H2 plasma and post-HCl treatment without the need for a metal layer or thermal annealing. XPS showed that oxygen vacancies and O–H bonds were formed by a reaction with hydrogen in the plasma. The O–H bonds can lead to an increase in carrier concentration on the ITO surface, thereby enhancing the electrical properties of ITO. Although the formation of In/Sn clusters by the hydrogen plasma caused the transmittance of the sample to deteriorate, these metallic clusters could be removed effectively by a HCl treatment without deterioration in the electrical and optical properties. The H2/HCl treatment decreased the work function of the surface of ITO by about 0.33 eV, compared with that of the as-deposited sample.
Acknowledgments
This study was supported by the Ministry of Knowledge Economy (MKE) through the project of GTFAM (B0009040) Regional Innovation Center (RIC) and Korea Institute for Advancement of Technology (KIAT) through the Inter-ER Cooperation Projects.