Applied Science and Convergence Technology 2022; 31(6): 156-160
Published online November 30, 2022
Copyright © The Korean Vacuum Society.
Department of Energy Convergence Engineering, Cheongju University, Cheongju 28503, Republic of Korea
We investigated the optical and electrical characteristics of Sn-doped indium tin oxide (ITO) thin films. Bi-layer ITO/Sn film samples were prepared by RF sputtering followed by in-situ annealing. Three different conditions with respect to the Sn layer were prepared on a soda-lime substrate: thickness of 0.0 (only glass substrate), 1.0, and 2.0 nm, respectively. ITO films of 170 and 400 nm thickness were deposited on Sn/soda lime glass successively at a substrate temperature of 260 °C. ITO/Sn bi-layer films were in-situ annealed at both 260 and 400 °C for 30 min under 6 × 10−6 Torr. We studied the optical and electrical properties of the prepared Sn-doped ITO films under various growth conditions, ITO thickness and Sn thickness, and annealing temperature. The best results were obtained from the ITO (400 nm)/Sn (2.0 nm) bi-layer sample annealed at 400 °C, for which average visible transmittance of ~88 % in a wavelength range of 380-780 nm and a sheet resistance R□ of 5.88 Ω were obtained.
Keywords: Indium tin oxide/Sn bi-layer, Sn-doping, RF-sputtering, In-situ annealing, Sheet resistance
Transparent conducting oxide (TCO) is a material used as a transparent electrode in light-based devices such as flat-panel displays and solar cells that has a high optical transmittance of more than 80 % in visible light and a resistivity of less than 10−3 Ω·cm [1–5]. Among TCOs, indium tin oxide (ITO) is the most widely used material at present because it has high optical transmittance and excellent electrical conductivity and is also chemically stable [6–10]. However, as high-efficiency devices such as solar cells, photodiode, and displays have emerged, it has become more important for TCOs to have high transmittance and low resistance simultaneously. Achieving this with the existing pre-made ITO alone is difficult. In general, when the thickness of ITO increases, its optical transmittance decreases but the electrical conductivity increases. As such, there is a limit to simultaneously improving the light transmittance and electrical conductivity, which is crucial for practical applications [11,12].
ITO, which is created by putting Sn in the In position in In2O3, has more free electrons inside due to Sn having one more free electron than In, resulting in higher electrical performance. In other words, it is obtained by mixing indium oxide with tin oxide, and is optimized by doping Sn+4, which has one extra free electron than In+3, thereby increasing the number of free electrons to lower its resistivity. By doping Sn to In2O3, a material having excellent optical and electrical properties can be produced, but if Sn is inserted in amounts beyond a certain level, a stable oxide such as In4Sn3O12, Sn2Ox, or the like, is formed to suppress the movement of free charge and this deteriorates its electrical performance. The proper Sn content is known to be about 5-10 % [13–19]. When ITO is manufactured, a circular or rectangular target is employed, and the ITO target is manufactured by a sintering method in which raw material powder is mixed in a predetermined ratio, molded and sintered using a dry method or a wet method, and finally ground by a machine [20,21]. Therefore, once manufactured as a target, it is difficult to change the concentration of Sn, and it is also difficult to adjust the concentration of Sn to its limit.
In thisstudy, we propose a strategy for Sn-doping of a commercial ITO film at a composition of 10 wt.% in the form of a bi-layer thin film of ITO/Sn treated by in-situ annealing. To prepare samples to be evaluated we used RF sputtering equipment and then conducted in-situ annealing under a high vacuum. The Sn-doped ITO thin films showed a significant (roughly two-fold) improvement in sheet resistance without the lowering optical transmittance compared to undoped ITO. We present the details of the Sn-doped ITO fabrication and observation of optical and electrical properties according to the growth conditions of different material combinations.
Figure 1 shows a schematic of the sputter system for thin film production . For this study, we used a commercial ITO target prepared with a weight ratio of In2O3 and SnO2 of 90:10 [10 wt.%, ANP], and a target of Sn with a purity of 99.99 % (iTASCO). The base pressure of the sputter chamber was maintained below 6.0 × 10−6 Torr through overnight pumping. A plasma gas of argon (Ar, 99.999 %) was used at a flow rate of 30 sccm and the sputter process pressure was 10 mTorr. The sputter guns for ITO and Sn were operated by respective RF power supplies operated at 13.56 MHz. The RF power was 100 and 30 W and the target-to-substrate distance was 68 and 75 mm for the 10 wt.% ITO target and Sn target, respectively. In the RF sputtering system, a halogen lamp heater is located at the position of the ITO target. When necessary for annealing, the samples were set under the heater and exposed to the desired annealing temperatures,
Figure 2 schematically illustrates the process of Sn doping into the ITO film performed in this experiment. Prior to thin film deposition, a 20 mm × 20 mm × 0.7 mm soda lime glass (SLG) substrate was ultrasonically cleaned with acetone, ethanol, and de-ionized water for 10 min successively and then dried with N2 gas. First, a Sn film was deposited at ambient temperature on an SLG substrate, and then an ITO thin film was fabricated on it successively, and finally the bi-layer ITO/Sn film was heat-treated to finish the Sn-doping process through an in-situ annealing process.
For fine control of Sn deposition, we rotated the substrate through Sn plasma such that Sn deposition took place only when passing over the target at a constant speed and multiple passes resulted in a thick layer. When an SLG substrate was passed 100 times, we obtained a Sn film of 47.8 nm, for which the Sn growth rate per pass (or rotation) was estimated as 0.5 nm, as determined from the cross-sectional SEM result shown in Fig. 3(a). Figure 3(b) shows a surface morphology consisting of Sn grains densely and completely covering the surface of the glass substrate without pin holes. In this experiment, the amount of Sn sputtering in an ambient temperature was adjusted as follows:
In-situ annealing was performed for the diffusion of Sn into the sample, where a small amount of Sn was laid on the ITO film. The annealing temperature was selected over the melting point of Sn 232 °C, and the optical and electrical characteristics according to the annealing temperature were measured at
As for sample characterization, the film growth morphology was analyzed using a field emission scanning electron microscope (SEM) (Jeol, JSM-7610F). We also examined the crystallographic properties of the films by using high-angle X-ray diffraction (XRD, Rigaku diffractometer) with CuKα radiation (λ = 1.54 Å). The optical transmittance was investigated in a wavelength of 300 to 1400 nm using UV-visible spectroscopy (Hitachi, UH4150; Shimadzu). From the optical spectra, the average visible transmittance (AVT) was evaluated in a wavelength range of 380 to 780 nm. The electrical properties of the Sn-doped ITO thin film were measured using a 4-point probe (Ecopia, HMS-3000).
In order to determine the thickness effect for ITO crystal growth, films of ITO without adding extra Sn were investigated by using XRD. The XRD patterns of the ITO thin films with
When a certain amount or more of Sn is doped into a film prepared from an as-received 10 wt.% ITO target, the oxide would interrupt the flow of electrons, thereby lowering the conductivity. In consideration of this, optical and electrical characteristics in accordance with the thicknesses of the Sn thin film and the ITO thin film were also compared by doping Sn in ITO of both 170 and 400 nm, respectively.
A TCO film such as ITO should have high light transmittance. Although the sheet resistance would be lowered through Sn doping, it is necessary to investigate the effect of Sn doping on the transmittance of thin films. Figure 5 presents the optical transmittance spectra of ITO films depending on the annealing condition of layer thickness
From the optical transmittance spectra in Fig. 5, we evaluated the AVT in the range of 380 nm ≤ λ ≤ 780 nm. Figure 6 presents the result of AVT vs. Sn thickness,
To investigate the electrical properties of Sn-doped ITO, we employed Hall measurement using a 4-point probe (Ecopia, HMS-3000). Table II is a summary of all the properties according to the ITO thickness and annealing temperature. In Fig. 7, the electrical properties of (a) carrier concentration, n, (b) electron mobility, µ and (c) resistivity, ρ are plotted as a function of Sn thickness,
For the mobility, asshown in Fig. 7(b), the undoped ITO prepared from the as-received 10 wt.% ITO target showed different values of 22.66, 18.61, and 34.22 cm2/V·s for type A, type B, and type C, respectively. The highest value of mobility was observed in the type C sample prepared at (400 nm, 400 °C). As
The resistivity of undoped ITO (i.e.,
Figure 8 presents graphs of the sheet resistance R□ vs. the ITO film condition for (a) a fixed ITO thickness of
In addition, we studied the change of R□ at the same
In this doping experiment, we observed that high annealing temperature accelerated the diffusion of added Sn into the ITO matrix material. Figure 8(a) shows the lowered sheet resistance for the samples of
In this study, we fabricated bi-layer thin films of ITO/Sn using RF sputtering and performed an in-situ annealing process at high temperature to enhance the doping of Sn into ITO thin films. The optical and electrical characteristics of Sn-doped ITO thin films prepared with different thickness and annealing temperature were investigated and analyzed according to the change of the added Sn film thickness. The results of this experiment show that it is possible to adjust the Sn concentration inside ITO to the limit while growing the ITO thin film. In order to allow use as a transparent electrode for opto-electronic devices, the optical transmittance must be high and the sheet resistance must be low at the same time. For the sample of Sn-doping to 10 wt.% ITO with
This research was partially supported by Cheongju University Research Scholarship Grants in 2022. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-RS-2022-00143178). This work was partly supported by a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (No. 20224000000070, Human Resource Training For Smart Energy New Industry Cluster).
The authors declare no conflicts of interest.