- Email: [email protected]
Preparation and optimization of SnOx thin film by solution method at low temperature
Superlattices and Microstructures 139 (2020) 106400
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Preparation and optimization of SnOx thin film by solution method at low temperature Honglong Ning a, Xu Zhang a, Shuang Wang a, Rihui Yao a, b, *, Xianzhe Liu a, Danqing Hou a, Qiannan Ye a, JinXiong Li a, Jiangxia Huang a, Xiuhua Cao c, **, Junbiao Peng a a
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China b Guangdong Province Key Lab of Display Material and Technology, Sun Yat-sen University, Guangzhou, 510275, China c State Key Laboratory of Advanced Materials and Electronic Components, Fenghua Electronic Industrial Park, No. 18 Fenghua Road, Zhaoqing, 526020, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnOx thin films Spin coating Plasma pretreatment Precursor concentration Rotating speed
SnOx films were prepared by sol-gel spin coating technology on glass substrates at low temper ature. Through the optimization of the solution process, the quality of the film was effectively improved. And the effects of rotating speed, substrate pretreatment and precursor concentration and other parameters on the films were emphatically discussed. It is found that the formation of the impurity particles and holes can be reduced and the film roughness can be decreased by treating the substrate with plasma. Adding a low-speed spin coating process before high-speed spin coating can significantly reduce the roughness of film. The roughness of film decreases, and the edge shrinkage phenomenon of film is improved with the increasing spin-coating speed. The thickness of the film increases linearly with the solution concentration, but the high concentration of precursor is easy to lead to the cracks of films. Based on the optimized solution process, SnOx films with flat, smooth surface (Rq ¼ 0.25 nm) and high transparency (visible light transmittance >90%) can be prepared at low temperature, which is expected to be used in devices based on transparent films.
1. Introduction The transparent oxide semiconductor materials are widely used in optoelectronic devices due to their good electrical conductivity and transparency, and its applications can be divided into transparent conductive oxides (TCO) and transparent semiconducting oxides (TSO). The first group are highly doped degenerate semiconductors owned very high conductivity and excellent transparency, which are usually used as transparent electrodes in many transparent electronic devices like smart windows [1], displays [2], solar cells [3] etc. The second group are doped in the semiconducting range only and are employed in electronic devices [4–6], such as thin film transistors [7], sensors [8,9]. Tin oxide (SnOx) materials have great potential for application in electrode materials [10–12] and * Corresponding author. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China. ** Corresponding author. E-mail addresses: [email protected] (R. Yao), [email protected] (X. Cao). https://doi.org/10.1016/j.spmi.2020.106400 Received 27 September 2019; Received in revised form 20 December 2019; Accepted 14 January 2020 Available online 18 January 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 1. (a): The XPS full spectrum of the SnOx films; (b): the Sn 4d core level spectrum (left side figure) and valence band spectrum (right side figure) of the SnOx films; (c) the Sn 3d5/2 spectrum of the SnOx films; (d): The XRD spectrum of the SnOx film.
electronic devices [13–15] because of their good electrical conductivity, high transparency, non-toxicity, and strong mechanical properties. SnOx is usually present in two oxidation states, namely tin oxide (SnO2) and stannous oxide (SnO) [16], which can be used as n-type and p-type semiconductor materials, respectively. Compared with other n-type transparent oxide semiconductor materials (such as In2O3, ZnO), SnO2 has a relatively wide direct band gap (3.6–4.0 eV), excellent optical transmittance (>85%), high chemical stability, which is considered to be a potential material that is expected to achieve fully transparent optoelectronic devices [17,18]. Stannous oxide (SnO) is an IV-VI oxide semiconductor material with large optical band gap, good optical transmittance, considerable hole mobility, non-toxicity, and content, etc [16,19] It has been widely used in the field of electronic and optoelectronic devices, and is considered to be one of the most promising p-type TSO materials [20–22]. In recent years, the preparation of SnOx films mainly rely on physical vapor deposition (PVD), including pulsed laser deposition [23], electron beam evaporation [24], magnetron sputtering [25] and so on. However, the above methods have problems such as expensive equipment, complicated operation, strict preparation conditions, and high cost. In contrast, the sol-gel spin coating technology is simple, safe, low cost, suitable for doping, easy to control processing parameters, no need for vacuum and convenient for preparation of large-area film [26,27], etc. At present, it has been widely used in laboratory research [28–30]. However, the quality of SnOx films prepared by spin coating technology still has a certain gap compared with the vacuum method, and problems such as impurities, holes, cracks, edge shrinkage, etc often occur in the process of preparing films by solution method. For thin-film devices, factors such as surface roughness of the film affect the performance of the device. Therefore, obtaining a high-quality film is the basis for the design and manufacture of the thin film device. However, in many current researches, annealing at high temperature is the most common way to eliminate these physical defects and improve the quality of the film, but high temperature is adverse for flexible devices. It is very meaningful to realize the preparation of high-quality SnOx film at low temperature. Because the quality of films often depends on the process parameters of spinning process [31]. In this paper, we are committed to optimizing the solution process to prepare high-quality SnOx films. It was found that reasonable regulation of precursor concentration, treatment of substrate with plasma, and adding a low-speed spin coating process and other means could facilitate the preparation of flat, smooth, highly transparent SnOx films at low temperature. 2. Experiment 2.1. The preparation of SnOx films The solution was prepared with tin chloride dihydrate (SnCl2�2H2O, Aladdin reagent, analysis pure 98%) as solute and ethylene glycol methyl ether (CH3OCH2CH2OH, analysis pure, Tianjin Damao chemical reagent factory) as solvent. The solution was stirred for 4 h at room temperature and allowed to age for 24 h. The glass substrate is 1 cm � 1 cm in size, and the back of the substrate is marked to distinguish the front and back of the film. The substrate was ultrasonically washed with tetrahydrofuran, deionized water, iso propanol, deionized water and isopropyl alcohol in that order, and then dried. The precursor solution was filtered by an organic phase 2
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 2. (a) Schematic diagram of spinning process; (b) the AFM image of SnOx film prepared by rotary coating at the speed of 500 rpm for 6 s and 6000 rpm for 20 s; (c) the AFM image of SnOx film prepared by rotary coating at the speed of 6000 rpm for 20 s.
needle filter with a diameter of 0.22 μm to clean larger particles, and then 50 μL precursor solution was taken to drop on a clean glass substrate, the sol-gel spread evenly over the glass substrate with a KW-4a spin coater, after that, the sol-gel was annealed at 100 � C for 5 min. SnOx films were prepared based on different spin coating speeds, substrate treatment schemes, and solution concentrations. 2.2. Characterization methods The chemical composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS) (model: Thermo ESCALAB 250Xi). The structure of the films was analyzed by X-ray diffraction (XRD) with Cu Kα (λ ¼ 0.154056 nm) (model: Empyrean). Polarizing microscope (POM) (model: Nikon Ds-Fi1), white light interferometer (WLI) (model: Vecco NT9300) and atomic force microscope (AFM) (model: BY3000) were used to observe the surface morphology of the films. And the thickness, density and roughness of the films were measured with X-ray reflection system (XRR) (model: Empyrean). And the ultraviolet–visible spectrophotometer (model: shimazu uv-2600) was used to characterize the transmission spectrum of thin films. This paper analyzes the chemical composition, structure, morphology and optical properties of the SnOx films. And the influence of various parameters of solution method on the properties of thin films was studied, we committed to the prepare high-quality SnOx films at low temperature by optimizing the so lution process. 3. Results 3.1. Chemical composition and structure In order to avoid the influence of pollutants on the surface of SnOx thin film, the surface was etched by Ar ion (Ep ¼ 3000 eV, t ¼ 30 s) before XPS characterization. Fig. 1 (a) shows the XPS full spectrum of the film. In addition to Sn and O, there are trace elements of C and a small amount of Cl in the film, indicating that after the annealing at 100 � C, the organic components in the film is significantly removed, but there are still some residual components of Cl. Fig. 1 (b) shows the Sn 4d core level spectrum and valence band spectrum of this film, the left side shows the Sn 4d core level spectrum and the right side shows the valence band spectrum. According to the report of Kachirayil J. Saji et al. [32], it is easier to distinguish the valence state of Sn by comparing the difference of binding energy between the peak of Sn 4d core level spectrum and the valence band, the binding energy difference of SnO2 is 21.1 ev or 21.5 ev, and that of SnO is 23.1 ev or 23.7 ev [33], different preparation conditions may lead to some differences in values, in Fig. 1 (b), the characteristic peaks of SnO2 and SnO appear simultaneously in the valence band, indicating that both SnO2 and SnO coexist in the film. Fig. 1 (c) shows the Sn 3d5/2 spectrum of this film, and the composition of the film is further analyzed by XPS-peak-differentation-imitating analysis, the combined energy of the peaks is 487.30 eV, 486.60 eV and 486.00 eV, respectively, the former two correspond to SnO2 [34,35], and the latter to SnO [36], the semi-quantitative analysis by Avantage software showed that 3
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 3. The POM and WLI images of SnOx films prepared at different rotating speeds: (a, d) 3000 r/min. (b, e) 5000 r/min. (c, f) 6000 r/min.
the atomic ratio of SnO and SnO2 in the film was 5.51% and 94.49% respectively. During the preparation of SnOx film, the following reactions occurred [16]: SnCl2 ⋅ 2H2 O þ 2ROH → SnðOHÞ2 þ 2RCl þ 2H2 O
(1)
SnðOHÞ2 → SnO þ H2 O
(2)
SnO is metastable under normal environmental conditions and is prone to oxidation in the presence of air [37]: (3)
SnOðSÞ þ 0:5O2ðgÞ ↔ SnO2ðSÞ
In order to better understand the crystallization of the SnOx films, XRD was used to characterize the SnOx films. The test results were shown in Fig. 1 (d), there were no diffraction peaks found, indicating that the SnOx film prepared at 100 � C did not crystallize. 3.2. Rotation speed In order to explore the influence of “low speed þ high speed spin” and “high speed spin” on the film formation, the SnOx films were prepared with above two spinning processes, which were named as sample 1 and sample 2 respectively. The preparation process is shown in Fig. 2 (a). In the low-speed rotary coating stage, the rotation speed was 500 r/min and lasted for 6 s. And for high-speed rotary coating stage, the rotation speed is 6000 r/min and lasts for 20 s. The surface morphology of the films was characterized by atomic force microscope (AFM), as shown in Fig. 2 (b) and Fig. 2 (c). And the scanning area of the images is 5 μm � 5 μm. The surface roughness of the SnOx film is analyzed by the analysis software attached to the AFM test system. And the root mean 4
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 4. The relationship between the thickness and density of the film and the rotating speed of the spin coating.
Fig. 5. The POM and AFM images of SnOx films prepared by different pretreatment methods: (a, c) Plasma treatment; (b, d) No plasma treatment.
square roughness (Sq) of sample 1 is only 0.26 nm, the Sq of the sample 2 is 3.75 nm. The thickness and density of the films were characterized by XRR. And the thickness of the sample 1 is 72.27 nm, the density was 3.69 g/cm3; the thickness of the sample 2 is 65.83 nm, and the density was 3.560 g/cm3. This phenomenon can be explained as: For sample 1, in the low-speed spin coating stage, the solution is slowly spread over the entire substrate plane due to inertia, forming a relatively complete wet coating. Under the in fluence of the rapid flow of air above the wet coating, the evaporation rate of the solvent is accelerated, causing the concentration of the wet coating rises rapidly, the viscosity increases, the film is initially formed, and the viscous resistance is not easily changed sharply. In the next high-speed spin coating stage, the initially formed film tends to be smooth and uniform. For sample 2, if high-speed spin coating is directly carried out, the wet coating is not formed initially and is subject to a huge inertia and the opposite effect of viscous resistance. In the initial stage, inertia is the dominant factor, and a large amount of solution is scooped out. Due to the accelerated evaporation rate of the solvent, the viscous resistance increases sharply, and the resultant force at any point above the coating also changes greatly, resulting in the uneven distribution of the wet coating; Besides, because the upper solvent of the wet coating volatilizes quickly and forms dry film easily, while the lower solution is still flowing, the fluid dynamic instability leads to uneven film [38], resulting in increased roughness. Therefore, setting a low-speed spin coating stage before high-speed spin coating can not only reduce the waste of the precursor solution, but also obtain a denser and smoother film. In order to investigate the effect of rotation speed on the film in the high-speed spin coating stage, the surface morphology of the films was characterized by POM and WLI. The overall morphology characterized by POM was shown in Fig. 3(a)-(c); the WLI images of the edge region of the films are shown in Fig. 3(d)–(f). It can be seen that the film is distributed with a certain inclination angle with the 5
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 6. The POM images of the SnOx films with different concentrations of precursor solutions.
Fig. 7. The relationship between the thickness of the film and the concentration of the precursor.
horizontal surface in the region X away from the boundary around the glass. The edge of the film shrinks from the outside to the inside, forming a “peak” with a height of Z, which is called the edge contraction of the film. It is found that with the increase of spinning speed, the length (X) and height (Z) of film edge shrinkage gradually become smaller, indicating that high spinning speed can greatly improve the phenomenon of edge shrinkage. The thickness and density of films prepared at different speeds was characterized with XRR, the test results are shown in Fig. 4. It can be seen that the thickness of films decreases with the increase of the rotating speed, but the decreasing trend gradually becomes slow. According to relevant reports [39], the thickness of the film obtained by spinning meets the following relationship: sffiffiffiffiffiffi 1 3η h¼ (4) ω 4ρt where ω is the spin speed, η is the solution viscosity, ρ is the solution density, and which is affected by solvent evaporation. Related studies [40] have also shown that for a given solution, the thickness of coating decreases with increasing spin coating speed, which is especially noticeable at slower speeds. And the density of the film increases as the rotational speed increases. Besides, according to the test report based on XRR, the root mean square roughness of the films prepared at different rotational speeds were 0.39 nm, 0.37 nm and 0.26 nm, corresponding to 3000 r/min, 5000 r/min and 6000 r/min, respectively. It can be seen that in the high-speed spin coating stage, as the rotational speed increases, the roughness of the film gradually decreases, this is related to the improvement of the edge shrinkage of the film. But if the spin-coating speed is too high, the loss of the precursor solution will be aggravated, and the roughness will be reduced to a certain extent, and the influence of the change on the performance of the thin film device is weakened [41]. 3.3. Plasma treatment In order to improve the wetting effect and adhesion of the solution on the substrate, the substrate was modified with plasma for 10 min with a power of 120 W, and then a layer of SnOx film was prepared on the substrate by spin coating. Fig. 5 shows POM and AFM images of SnOx films prepared by plasma and non-treatment. The films shown in Fig. 5(b) mainly have three types of defects, namely solid particle contamination (red area), hole (blue area), and pinhole (green area). Solid particle contamination is caused by solid impurities (>100 nm in diameter) attached to the surface of the substrate before spin coating, since the particle diameter is much larger than the film thickness, the film is penetrated; Because the solution may produce a small number of tiny bubbles during filtration, these bubbles break up and form pinholes during annealing. The large holes in the film are due to poor contact between the solution and the substrate surface. As shown in Fig. 5 (a), the SnOx film prepared on substrate treated with plasma has no large solid particle pollution and no holes, 6
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 8. The transmittance spectrum of the SnOx films with different concentrations of precursor solutions.
only a few pinholes, and relatively good uniformity in the edge area. It indicates that plasma treatment can effectively remove large particles on the substrate surface, reduce surface contact angle and enhance surface characteristics [42–44]. AFM was performed to characterize the morphology of the films, as shown in Fig. 5 (c) and Fig. 5 (d). The root mean square roughness were 0.25 nm and 0.31 nm, respectively, which confirmed that plasma pretreatment on the substrate was conducive to optimizing the surface morphology of the film. 3.4. Precursor solution concentration SnOx films were prepared with three precursor solutions of different concentrations, and the surface morphology of the films was characterized by POM, as shown in Fig. 6 (a) - (c). It was found that cracks appeared in some areas of the thin films prepared with high concentration solution. This can be explained as: with the increase of precursor concentration, the thickness of the film increases, and the internal stress in the film increases, resulting in the appearance of cracks in the film [45]. The thickness of SnOx films were characterized by XRR, as shown in Fig. 7. It can be found that the thickness (y) of the film increases linearly with the concentration (x) of the precursor solution, and the fitting result is y ¼ 111.18xþ3.5616 (R2 ¼ 0.9983), which supports the previous idea that the film cracked due to the increase of thickness. The phenomenon that the thickness of film increases linearly with the precursor concentration can be explained: in the process of annealing, the solvents and water volatilize, most of the ions other than tin ions and oxygen ions are removed, and the final thickness of the film is mainly determined by the concentration of tin ions and oxygen ions; on the other hand, the viscosity of the solution is positively correlated with the concentration [46], the high precursor solution has a large viscous drag and less loss during the spin coating process, so the deposited film is thicker. In order to further explore the influence of precursor concentration on the morphology, the morphology of SnOx films was char acterized by AFM, and the root mean square roughness of the films were 0.69 nm, 0.43 nm and 0.82 nm, respectively. It was found that the roughness first decreased with increasing concentration and then increased. When the concentration of the solution is low, the viscous resistance is not balanced with the centrifugal force during the short spin coating process, and the film cannot form a uniform and dense structure, this phenomenon will improve as the concentration of the precursor increases, but when the concentration of the precursor is too high, cracks will appear on the surface of the film, which will lead to the increase of the roughness of the film. The transparency of the SnOx films was measured by uv–vis spectrophotometer, and the transmission spectrum of the SnOx films was shown in Fig. 8. The transmittance of all the samples in the visible light band exceeded 89%, and the transmittance of the SnOx films prepared by the precursor solution with a concentration of 0.1 mol/L even exceeded 97%, this has application value in many transparent devices. 4. Conclusion In this paper, we optimized the process for preparing SnOx thin films by spin coating. Finally, a smooth, dense and transparent SnOx film was successfully prepared at low temperature. The following conclusions were obtained: ①Setting a low-speed spin coating stage before the high-speed spin coating can significantly reduce the roughness of films; High rotational speed can reduce edge shrinkage and roughness of the films. ②Plasma pretreatment on the substrate is advantageous for improving the contact characteristics and optimizing the quality of films. ③In a certain concentration range, the thickness of the film increases linearly with the concentration of the precursor, but cracks and other defects will appear at a high concentration. ④According to the following process: the substrate is first treated with plasma at a power of 120 W for 10 min, and the precursor concentration of 0.5 mol/L is spin-coated at 500 r/min for 6 s, and then spin-coated at 5000 r/min for 20 s, finally annealed at 100 � C. A flat, smooth (Rq ¼ 0.25 nm) and highly transparent (visible light transmittance >90%) amorphous SnOx film could be prepared. This study provides a reference for the preparation of high-quality SnOx films by spinning coating at low temperature. It is useful for 7
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
similar research and is expected to be used in multi-layer film transparent devices. Author statement I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere. All the authors listed have approved the manuscript that is enclosed. No conflict of interest statement Authors have no conflict of interest to declare. Acknowledgements This work was supported by Key-Area Research and Development Program of Guangdong Province (No. 2019B010934001), Na tional Natural Science Foundation of China (Grant No. 51771074 and 61574061), the Major Integrated Projects of National Natural Science Foundation of China (Grant No. U1601651), Science and Technology Project of Guangzhou (No. 201904010344), the Fundamental Research Funds for the Central Universities (No. 2019MS012), 2019 Guangdong University Student Science and Technology Innovation Special Fund (“Climbing Plan” Special Fund) (No. pdjh2019a0028 and pdjh2019b0041), National College Students’ Innovation and Entrepreneurship Training Program (No. 201910561005 and 201910561007), South China University of Technology 100 Step Ladder Climbing Plan Research Project (No. j2tw201902475 and j2tw201902203). References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials (1995) 433. [2] Y. Tak, K. Kim, H. Park, et al., Criteria for ito (indium–tin-oxide) thin film as the bottom electrode of an organic light emitting diode, Thin Solid Films 411 (2002) 12–16. [3] X. Liu, A. Ng, Y.H. Ng, et al., Effect of electrical properties, transmittance and morphology of ito electrode on polymer solar cells characteristics, Proceedings of SPIE - The International Society for Optical Engineering 8626 (2013). [4] V.M. Kalygina, T.Z. Lygdenova, Y.S. Petrova, et al., Influence of the substrate material on the properties of gallium-oxide films and gallium-oxide-based structures, Semiconductors 53 (2019) 452–457. [5] V.M. Kalygina, Y.S. Petrova, I.A. Prudaev, et al., Stability of electrical characteristics of mos structures based on gallium oxide, Russ. Phys. J. 59 (2016) 757–761. [6] A.Y. Polyakov, N.B. Smirnov, I.V. Shchemerov, D. Gogova, S.A. Tarelkin, S.J. Pearton, Compensation and persistent photocapacitance in homoepitaxial Sndoped b-Ga2O3, J. Appl. Phys. 123 (2018) 115702. [7] H. Mun, H. Yang, K. Char, Effect of a Ru doped SnO2-x buffer layer on thin-film transistors based on SnO2-x channel layer, in: APS March Meeting 2015, 2015. [8] N.K. Maksimova, A.V. Almaev, E.Y. Sevastyanov, Effect of additives Ag and rare-earth elements Y and Sc on the properties of hydrogen sensors based on thin SnO2 films during long-term testing, Coatings 9 (2019) 423. [9] A.V. Almaev, V.I. Gaman, Characteristics of hydrogen sensors based on thin tin dioxide films modified with gold, Russ. Phys. J. 60 (2017) 1081–1087. [10] A.A. Yadav, E.U. Masumdar, A.V. Moholkar, et al., Electrical, structural and optical properties of SnO2:F thin films: effect of the substrate temperature, J. Alloy. Comp. 488 (2009) 350–355. [11] I.Y.Y. Bu, Sol–gel deposition of fluorine-doped tin oxide glasses for dye sensitized solar cells, Ceram. Int. 40 (2014) 417–422. [12] B. Koo, H. Ahn, Structural, electrical, and optical properties of Sb-doped SnO2 transparent conductive oxides fabricated using an electrospray technique, Ceram. Int. 40 (2014) 4375–4381. [13] P. Hsu, W. Chen, Y. Tsai, et al., Sputtering deposition of p-type SnO films using robust Sn/SnO2 mixed target, Thin Solid Films 555 (2014) 57–61. [14] A. Chin, W.S. Cheng, C.F. Lu, et al., High mobility SnO2 TFT for display and future IC, in: Active-matrix Flatpanel Displays & Devices, 2016, https://doi.org/ 10.1109/AM-FPD.2016.7543695. [15] Y. Liu, C. Yin, Z. Zhang, et al., The investigation of hydrogen gas sensing properties of saw gas sensor based on palladium surface modified SnO2 thin film, Mater. Sci. Semicond. Process. 60 (2017) 16–28. [16] M. Marikkannan, V. Vishnukanthan, A. Vijayshankar, A novel synthesis of tin oxide thin films by the sol-gel process for optoelectronic applications, AIP Adv. 5 (2015), 027122. [17] S. Lin, Y. Tsai, K. Bai, Structural and physical properties of tin oxide thin films for optoelectronic applications, Appl. Surf. Sci. 380 (2016) 203–209. [18] L. He, Q. Cao, X. Feng, Structural, optical and electrical properties of epitaxial rutile sno2 films grown on mgf2(110) substrates by mocvd, vol. 44, 2018, pp. 869–873. [19] J. Du, C. Xia, Y. Liu, Electronic characteristics of p -type transparent SnO monolayer with high carrier mobility, Appl. Surf. Sci. 401 (2017) 114–119. [20] P.C. Hsu, W. Chen, Y. Tsai, et al., Fabrication of p-type SnO thin-film transistors by sputtering with practical metal electrodes, Jpn. J. Appl. Phys. 52 (2013), 05DC07. [21] J. Zhang, J. Yang, Y. Li, High performance complementary circuits based on p-SnO and n-IGZO thin-film transistors, Materials 10 (2017) 319. [22] Q. Lei, W. Liu, Y. Pei, et al., Trap states extraction of p-channel SnO thin-film transistors based on percolation and multiple trapping carrier conductions, Solid State Electron. 129 (2016). [23] H. Jadhav, S.R. Suryawanshi, M.A. More, et al., Pulsed laser deposition of tin oxide thin films for field emission studies, Appl. Surf. Sci. 419 (2017). [24] Y.A. El-gendy, Effects of film thickness on the linear and nonlinear refractive index of p-type SnO films deposited by e-beam evaporation process, Phys. B Condens. Matter 526 (2017) 59–63. [25] H. Yabuta, N. Kaji, R. Hayashi, et al., Sputtering formation of p-type SnO thin-film transistors on glass toward oxide complimentary circuits, Appl. Phys. Lett. 97 (2010), 072111 (3 pp.)–-072111 (3 pp.)072111 (3 pp.). [26] H. Guendouz, A. Bouaine, N. Brihi, Biphase Effect on Structural, Optical, and Electrical Properties of Al-Sn Codoped Zno Thin Films Deposited by Sol-Gel SpinCoating Technique, vol. 158, 2018. [27] K. Verma, B. Chaudhary, V. Kumar, et al., Influence of fe-doping on the structural, optical and luminescent behavior of zno thin films deposited by spin coating technique, Vacuum 146 (2017) 478–482, https://doi.org/10.1016/j.vacuum.2017.06.033. [28] F. Tang, C. Mei, P. Chuang, et al., Valence state and magnetism of Mn-doped PbPdO2 nanograin film synthesized by sol-gel spin-coating method, Thin Solid Films 623 (2017) 14–18. [29] Z.Y. Lee, S.S. Ng, F.K. Yam, Growth mechanism of indium nitride via sol–gel spin coating method and nitridation process, Surf. Coat. Technol. 310 (2017) 38–42.
8
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
[30] A.R. Nimbalkar, M.G. Patil, Synthesis of ZnO thin film by sol-gel spin coating technique for H2S gas sensing application, Phys. B Condens. Matter 527 (2017) 7–15. [31] T. Wang, X. Zhao, H. Yang, et al., On the roles of hec in pechini sol-gel method: enhancement of stability, wettability of the sol and surface roughness of Bi2212 film, Ceram. Int. 44 (2018) 12144–12148. [32] K.J. Saji, Y.P.V. Subbaiah, K. Tian, et al., P-type SnO thin films and SnO/ZnO heterostructures for all-oxide electronic and optoelectronic device applications, Thin Solid Films 605 (2015) 193–201. [33] J.M. Themlin, M. Chta B, L. Henrard, et al., Characterization of tin oxides by x-ray-photoemission spectroscopy, Phys. Rev. B Condens. Matter 46 (1992) 2460–2466. [34] T. Voscoboinikov Süzer, K.R. Hallam, et al., Electron spectroscopic investigation of sn coatings on glasses, vol. 355, 1996, pp. 654–656. [35] M.A. Stranick, A. Moskwa, SnO2 by xps, Surf. Sci. Spectra 2 (1993) 50–54. [36] R.O. Ansell, T. Dickinson, A.F. Povey, et al., Cheminform abstract: x-ray photoelectron spectroscopic studies of tin electrodes after polarization in sodium hydroxide solution, vol. 8, 1977, pp. 1360–1364. [37] C.M. Campo, J.E. Rodríguez, A.E. Ramírez, Thermal behaviour of romarchite phase SnO in different atmospheres: a hypothesis about the phase transformation, Heliyon 2 (2016) e00112. [38] P. Fowler, C. Ruscher, J. Forrest, Controlling marangoni-induced instabilities in spin-cast polymer films: how to prepare uniform films, Eur. Phys. J. E 39 (2016) 90. [39] X. Wang, F. Shi, X. Gao, et al., A sol–gel dip/spin coating method to prepare titanium oxide films, Thin Solid Films 548 (2013) 34–39. [40] R. Balzaroyti, C. Cristiani, L. F Francis, Spin coating deposition on complex geometry substrates:influence of operative parameters, Surf. Coat. Technol. 330 (2017). [41] C.P.T. Nguyen, J. Raja, S. Kim, et al., Enhanced electrical properties of oxide semiconductor thin-film transistors with high conductivity thin layer insertion for the channel region, Appl. Surf. Sci. 396 (2017) 1472–1477. [42] J. Rao, L. Bao, B. Wang, Plasma surface modification and bonding enhancement for bamboo composites, Compos. B Eng. 138 (2018) 157–167. [43] K.N. Pandiyarj, A. A Kumar, M. C Ramkumar, Effect of cold atmospheric pressure plasma gas composition on the surface and cyto-compatible properties of low density polyethylene (ldpe) films, vol. 16, 2016, pp. 784–792. [44] A. Eshaghi, M. Mesbahi, A.A. Aghaei, Influence of physical plasma etching treatment on optical and hydrophilic MgF2 thin film, Optik 161 (2018) 1–7. [45] M.H. Zhao, R. Fu, D. Lu, et al., Critical thickness for cracking of Pb(Zr0.53Ti0.47)O3 thin films deposited on Pt/Ti/Si(100) substrates, Acta Mater. 50 (2002) 4241–4254. � [46] B. Aguila, D, A. Vasco, P. Galvez, Effect of temperature and cuo-nanoparticle concentration on the thermal conductivity and viscosity of an organic phasechange material, Int. J. Heat Mass Transf. 120 (2018) 1009–1019.
9
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Preparation and optimization of SnOx thin film by solution method at low temperature Honglong Ning a, Xu Zhang a, Shuang Wang a, Rihui Yao a, b, *, Xianzhe Liu a, Danqing Hou a, Qiannan Ye a, JinXiong Li a, Jiangxia Huang a, Xiuhua Cao c, **, Junbiao Peng a a
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China b Guangdong Province Key Lab of Display Material and Technology, Sun Yat-sen University, Guangzhou, 510275, China c State Key Laboratory of Advanced Materials and Electronic Components, Fenghua Electronic Industrial Park, No. 18 Fenghua Road, Zhaoqing, 526020, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnOx thin films Spin coating Plasma pretreatment Precursor concentration Rotating speed
SnOx films were prepared by sol-gel spin coating technology on glass substrates at low temper ature. Through the optimization of the solution process, the quality of the film was effectively improved. And the effects of rotating speed, substrate pretreatment and precursor concentration and other parameters on the films were emphatically discussed. It is found that the formation of the impurity particles and holes can be reduced and the film roughness can be decreased by treating the substrate with plasma. Adding a low-speed spin coating process before high-speed spin coating can significantly reduce the roughness of film. The roughness of film decreases, and the edge shrinkage phenomenon of film is improved with the increasing spin-coating speed. The thickness of the film increases linearly with the solution concentration, but the high concentration of precursor is easy to lead to the cracks of films. Based on the optimized solution process, SnOx films with flat, smooth surface (Rq ¼ 0.25 nm) and high transparency (visible light transmittance >90%) can be prepared at low temperature, which is expected to be used in devices based on transparent films.
1. Introduction The transparent oxide semiconductor materials are widely used in optoelectronic devices due to their good electrical conductivity and transparency, and its applications can be divided into transparent conductive oxides (TCO) and transparent semiconducting oxides (TSO). The first group are highly doped degenerate semiconductors owned very high conductivity and excellent transparency, which are usually used as transparent electrodes in many transparent electronic devices like smart windows [1], displays [2], solar cells [3] etc. The second group are doped in the semiconducting range only and are employed in electronic devices [4–6], such as thin film transistors [7], sensors [8,9]. Tin oxide (SnOx) materials have great potential for application in electrode materials [10–12] and * Corresponding author. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China. ** Corresponding author. E-mail addresses: [email protected] (R. Yao), [email protected] (X. Cao). https://doi.org/10.1016/j.spmi.2020.106400 Received 27 September 2019; Received in revised form 20 December 2019; Accepted 14 January 2020 Available online 18 January 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 1. (a): The XPS full spectrum of the SnOx films; (b): the Sn 4d core level spectrum (left side figure) and valence band spectrum (right side figure) of the SnOx films; (c) the Sn 3d5/2 spectrum of the SnOx films; (d): The XRD spectrum of the SnOx film.
electronic devices [13–15] because of their good electrical conductivity, high transparency, non-toxicity, and strong mechanical properties. SnOx is usually present in two oxidation states, namely tin oxide (SnO2) and stannous oxide (SnO) [16], which can be used as n-type and p-type semiconductor materials, respectively. Compared with other n-type transparent oxide semiconductor materials (such as In2O3, ZnO), SnO2 has a relatively wide direct band gap (3.6–4.0 eV), excellent optical transmittance (>85%), high chemical stability, which is considered to be a potential material that is expected to achieve fully transparent optoelectronic devices [17,18]. Stannous oxide (SnO) is an IV-VI oxide semiconductor material with large optical band gap, good optical transmittance, considerable hole mobility, non-toxicity, and content, etc [16,19] It has been widely used in the field of electronic and optoelectronic devices, and is considered to be one of the most promising p-type TSO materials [20–22]. In recent years, the preparation of SnOx films mainly rely on physical vapor deposition (PVD), including pulsed laser deposition [23], electron beam evaporation [24], magnetron sputtering [25] and so on. However, the above methods have problems such as expensive equipment, complicated operation, strict preparation conditions, and high cost. In contrast, the sol-gel spin coating technology is simple, safe, low cost, suitable for doping, easy to control processing parameters, no need for vacuum and convenient for preparation of large-area film [26,27], etc. At present, it has been widely used in laboratory research [28–30]. However, the quality of SnOx films prepared by spin coating technology still has a certain gap compared with the vacuum method, and problems such as impurities, holes, cracks, edge shrinkage, etc often occur in the process of preparing films by solution method. For thin-film devices, factors such as surface roughness of the film affect the performance of the device. Therefore, obtaining a high-quality film is the basis for the design and manufacture of the thin film device. However, in many current researches, annealing at high temperature is the most common way to eliminate these physical defects and improve the quality of the film, but high temperature is adverse for flexible devices. It is very meaningful to realize the preparation of high-quality SnOx film at low temperature. Because the quality of films often depends on the process parameters of spinning process [31]. In this paper, we are committed to optimizing the solution process to prepare high-quality SnOx films. It was found that reasonable regulation of precursor concentration, treatment of substrate with plasma, and adding a low-speed spin coating process and other means could facilitate the preparation of flat, smooth, highly transparent SnOx films at low temperature. 2. Experiment 2.1. The preparation of SnOx films The solution was prepared with tin chloride dihydrate (SnCl2�2H2O, Aladdin reagent, analysis pure 98%) as solute and ethylene glycol methyl ether (CH3OCH2CH2OH, analysis pure, Tianjin Damao chemical reagent factory) as solvent. The solution was stirred for 4 h at room temperature and allowed to age for 24 h. The glass substrate is 1 cm � 1 cm in size, and the back of the substrate is marked to distinguish the front and back of the film. The substrate was ultrasonically washed with tetrahydrofuran, deionized water, iso propanol, deionized water and isopropyl alcohol in that order, and then dried. The precursor solution was filtered by an organic phase 2
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 2. (a) Schematic diagram of spinning process; (b) the AFM image of SnOx film prepared by rotary coating at the speed of 500 rpm for 6 s and 6000 rpm for 20 s; (c) the AFM image of SnOx film prepared by rotary coating at the speed of 6000 rpm for 20 s.
needle filter with a diameter of 0.22 μm to clean larger particles, and then 50 μL precursor solution was taken to drop on a clean glass substrate, the sol-gel spread evenly over the glass substrate with a KW-4a spin coater, after that, the sol-gel was annealed at 100 � C for 5 min. SnOx films were prepared based on different spin coating speeds, substrate treatment schemes, and solution concentrations. 2.2. Characterization methods The chemical composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS) (model: Thermo ESCALAB 250Xi). The structure of the films was analyzed by X-ray diffraction (XRD) with Cu Kα (λ ¼ 0.154056 nm) (model: Empyrean). Polarizing microscope (POM) (model: Nikon Ds-Fi1), white light interferometer (WLI) (model: Vecco NT9300) and atomic force microscope (AFM) (model: BY3000) were used to observe the surface morphology of the films. And the thickness, density and roughness of the films were measured with X-ray reflection system (XRR) (model: Empyrean). And the ultraviolet–visible spectrophotometer (model: shimazu uv-2600) was used to characterize the transmission spectrum of thin films. This paper analyzes the chemical composition, structure, morphology and optical properties of the SnOx films. And the influence of various parameters of solution method on the properties of thin films was studied, we committed to the prepare high-quality SnOx films at low temperature by optimizing the so lution process. 3. Results 3.1. Chemical composition and structure In order to avoid the influence of pollutants on the surface of SnOx thin film, the surface was etched by Ar ion (Ep ¼ 3000 eV, t ¼ 30 s) before XPS characterization. Fig. 1 (a) shows the XPS full spectrum of the film. In addition to Sn and O, there are trace elements of C and a small amount of Cl in the film, indicating that after the annealing at 100 � C, the organic components in the film is significantly removed, but there are still some residual components of Cl. Fig. 1 (b) shows the Sn 4d core level spectrum and valence band spectrum of this film, the left side shows the Sn 4d core level spectrum and the right side shows the valence band spectrum. According to the report of Kachirayil J. Saji et al. [32], it is easier to distinguish the valence state of Sn by comparing the difference of binding energy between the peak of Sn 4d core level spectrum and the valence band, the binding energy difference of SnO2 is 21.1 ev or 21.5 ev, and that of SnO is 23.1 ev or 23.7 ev [33], different preparation conditions may lead to some differences in values, in Fig. 1 (b), the characteristic peaks of SnO2 and SnO appear simultaneously in the valence band, indicating that both SnO2 and SnO coexist in the film. Fig. 1 (c) shows the Sn 3d5/2 spectrum of this film, and the composition of the film is further analyzed by XPS-peak-differentation-imitating analysis, the combined energy of the peaks is 487.30 eV, 486.60 eV and 486.00 eV, respectively, the former two correspond to SnO2 [34,35], and the latter to SnO [36], the semi-quantitative analysis by Avantage software showed that 3
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 3. The POM and WLI images of SnOx films prepared at different rotating speeds: (a, d) 3000 r/min. (b, e) 5000 r/min. (c, f) 6000 r/min.
the atomic ratio of SnO and SnO2 in the film was 5.51% and 94.49% respectively. During the preparation of SnOx film, the following reactions occurred [16]: SnCl2 ⋅ 2H2 O þ 2ROH → SnðOHÞ2 þ 2RCl þ 2H2 O
(1)
SnðOHÞ2 → SnO þ H2 O
(2)
SnO is metastable under normal environmental conditions and is prone to oxidation in the presence of air [37]: (3)
SnOðSÞ þ 0:5O2ðgÞ ↔ SnO2ðSÞ
In order to better understand the crystallization of the SnOx films, XRD was used to characterize the SnOx films. The test results were shown in Fig. 1 (d), there were no diffraction peaks found, indicating that the SnOx film prepared at 100 � C did not crystallize. 3.2. Rotation speed In order to explore the influence of “low speed þ high speed spin” and “high speed spin” on the film formation, the SnOx films were prepared with above two spinning processes, which were named as sample 1 and sample 2 respectively. The preparation process is shown in Fig. 2 (a). In the low-speed rotary coating stage, the rotation speed was 500 r/min and lasted for 6 s. And for high-speed rotary coating stage, the rotation speed is 6000 r/min and lasts for 20 s. The surface morphology of the films was characterized by atomic force microscope (AFM), as shown in Fig. 2 (b) and Fig. 2 (c). And the scanning area of the images is 5 μm � 5 μm. The surface roughness of the SnOx film is analyzed by the analysis software attached to the AFM test system. And the root mean 4
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 4. The relationship between the thickness and density of the film and the rotating speed of the spin coating.
Fig. 5. The POM and AFM images of SnOx films prepared by different pretreatment methods: (a, c) Plasma treatment; (b, d) No plasma treatment.
square roughness (Sq) of sample 1 is only 0.26 nm, the Sq of the sample 2 is 3.75 nm. The thickness and density of the films were characterized by XRR. And the thickness of the sample 1 is 72.27 nm, the density was 3.69 g/cm3; the thickness of the sample 2 is 65.83 nm, and the density was 3.560 g/cm3. This phenomenon can be explained as: For sample 1, in the low-speed spin coating stage, the solution is slowly spread over the entire substrate plane due to inertia, forming a relatively complete wet coating. Under the in fluence of the rapid flow of air above the wet coating, the evaporation rate of the solvent is accelerated, causing the concentration of the wet coating rises rapidly, the viscosity increases, the film is initially formed, and the viscous resistance is not easily changed sharply. In the next high-speed spin coating stage, the initially formed film tends to be smooth and uniform. For sample 2, if high-speed spin coating is directly carried out, the wet coating is not formed initially and is subject to a huge inertia and the opposite effect of viscous resistance. In the initial stage, inertia is the dominant factor, and a large amount of solution is scooped out. Due to the accelerated evaporation rate of the solvent, the viscous resistance increases sharply, and the resultant force at any point above the coating also changes greatly, resulting in the uneven distribution of the wet coating; Besides, because the upper solvent of the wet coating volatilizes quickly and forms dry film easily, while the lower solution is still flowing, the fluid dynamic instability leads to uneven film [38], resulting in increased roughness. Therefore, setting a low-speed spin coating stage before high-speed spin coating can not only reduce the waste of the precursor solution, but also obtain a denser and smoother film. In order to investigate the effect of rotation speed on the film in the high-speed spin coating stage, the surface morphology of the films was characterized by POM and WLI. The overall morphology characterized by POM was shown in Fig. 3(a)-(c); the WLI images of the edge region of the films are shown in Fig. 3(d)–(f). It can be seen that the film is distributed with a certain inclination angle with the 5
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 6. The POM images of the SnOx films with different concentrations of precursor solutions.
Fig. 7. The relationship between the thickness of the film and the concentration of the precursor.
horizontal surface in the region X away from the boundary around the glass. The edge of the film shrinks from the outside to the inside, forming a “peak” with a height of Z, which is called the edge contraction of the film. It is found that with the increase of spinning speed, the length (X) and height (Z) of film edge shrinkage gradually become smaller, indicating that high spinning speed can greatly improve the phenomenon of edge shrinkage. The thickness and density of films prepared at different speeds was characterized with XRR, the test results are shown in Fig. 4. It can be seen that the thickness of films decreases with the increase of the rotating speed, but the decreasing trend gradually becomes slow. According to relevant reports [39], the thickness of the film obtained by spinning meets the following relationship: sffiffiffiffiffiffi 1 3η h¼ (4) ω 4ρt where ω is the spin speed, η is the solution viscosity, ρ is the solution density, and which is affected by solvent evaporation. Related studies [40] have also shown that for a given solution, the thickness of coating decreases with increasing spin coating speed, which is especially noticeable at slower speeds. And the density of the film increases as the rotational speed increases. Besides, according to the test report based on XRR, the root mean square roughness of the films prepared at different rotational speeds were 0.39 nm, 0.37 nm and 0.26 nm, corresponding to 3000 r/min, 5000 r/min and 6000 r/min, respectively. It can be seen that in the high-speed spin coating stage, as the rotational speed increases, the roughness of the film gradually decreases, this is related to the improvement of the edge shrinkage of the film. But if the spin-coating speed is too high, the loss of the precursor solution will be aggravated, and the roughness will be reduced to a certain extent, and the influence of the change on the performance of the thin film device is weakened [41]. 3.3. Plasma treatment In order to improve the wetting effect and adhesion of the solution on the substrate, the substrate was modified with plasma for 10 min with a power of 120 W, and then a layer of SnOx film was prepared on the substrate by spin coating. Fig. 5 shows POM and AFM images of SnOx films prepared by plasma and non-treatment. The films shown in Fig. 5(b) mainly have three types of defects, namely solid particle contamination (red area), hole (blue area), and pinhole (green area). Solid particle contamination is caused by solid impurities (>100 nm in diameter) attached to the surface of the substrate before spin coating, since the particle diameter is much larger than the film thickness, the film is penetrated; Because the solution may produce a small number of tiny bubbles during filtration, these bubbles break up and form pinholes during annealing. The large holes in the film are due to poor contact between the solution and the substrate surface. As shown in Fig. 5 (a), the SnOx film prepared on substrate treated with plasma has no large solid particle pollution and no holes, 6
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
Fig. 8. The transmittance spectrum of the SnOx films with different concentrations of precursor solutions.
only a few pinholes, and relatively good uniformity in the edge area. It indicates that plasma treatment can effectively remove large particles on the substrate surface, reduce surface contact angle and enhance surface characteristics [42–44]. AFM was performed to characterize the morphology of the films, as shown in Fig. 5 (c) and Fig. 5 (d). The root mean square roughness were 0.25 nm and 0.31 nm, respectively, which confirmed that plasma pretreatment on the substrate was conducive to optimizing the surface morphology of the film. 3.4. Precursor solution concentration SnOx films were prepared with three precursor solutions of different concentrations, and the surface morphology of the films was characterized by POM, as shown in Fig. 6 (a) - (c). It was found that cracks appeared in some areas of the thin films prepared with high concentration solution. This can be explained as: with the increase of precursor concentration, the thickness of the film increases, and the internal stress in the film increases, resulting in the appearance of cracks in the film [45]. The thickness of SnOx films were characterized by XRR, as shown in Fig. 7. It can be found that the thickness (y) of the film increases linearly with the concentration (x) of the precursor solution, and the fitting result is y ¼ 111.18xþ3.5616 (R2 ¼ 0.9983), which supports the previous idea that the film cracked due to the increase of thickness. The phenomenon that the thickness of film increases linearly with the precursor concentration can be explained: in the process of annealing, the solvents and water volatilize, most of the ions other than tin ions and oxygen ions are removed, and the final thickness of the film is mainly determined by the concentration of tin ions and oxygen ions; on the other hand, the viscosity of the solution is positively correlated with the concentration [46], the high precursor solution has a large viscous drag and less loss during the spin coating process, so the deposited film is thicker. In order to further explore the influence of precursor concentration on the morphology, the morphology of SnOx films was char acterized by AFM, and the root mean square roughness of the films were 0.69 nm, 0.43 nm and 0.82 nm, respectively. It was found that the roughness first decreased with increasing concentration and then increased. When the concentration of the solution is low, the viscous resistance is not balanced with the centrifugal force during the short spin coating process, and the film cannot form a uniform and dense structure, this phenomenon will improve as the concentration of the precursor increases, but when the concentration of the precursor is too high, cracks will appear on the surface of the film, which will lead to the increase of the roughness of the film. The transparency of the SnOx films was measured by uv–vis spectrophotometer, and the transmission spectrum of the SnOx films was shown in Fig. 8. The transmittance of all the samples in the visible light band exceeded 89%, and the transmittance of the SnOx films prepared by the precursor solution with a concentration of 0.1 mol/L even exceeded 97%, this has application value in many transparent devices. 4. Conclusion In this paper, we optimized the process for preparing SnOx thin films by spin coating. Finally, a smooth, dense and transparent SnOx film was successfully prepared at low temperature. The following conclusions were obtained: ①Setting a low-speed spin coating stage before the high-speed spin coating can significantly reduce the roughness of films; High rotational speed can reduce edge shrinkage and roughness of the films. ②Plasma pretreatment on the substrate is advantageous for improving the contact characteristics and optimizing the quality of films. ③In a certain concentration range, the thickness of the film increases linearly with the concentration of the precursor, but cracks and other defects will appear at a high concentration. ④According to the following process: the substrate is first treated with plasma at a power of 120 W for 10 min, and the precursor concentration of 0.5 mol/L is spin-coated at 500 r/min for 6 s, and then spin-coated at 5000 r/min for 20 s, finally annealed at 100 � C. A flat, smooth (Rq ¼ 0.25 nm) and highly transparent (visible light transmittance >90%) amorphous SnOx film could be prepared. This study provides a reference for the preparation of high-quality SnOx films by spinning coating at low temperature. It is useful for 7
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
similar research and is expected to be used in multi-layer film transparent devices. Author statement I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere. All the authors listed have approved the manuscript that is enclosed. No conflict of interest statement Authors have no conflict of interest to declare. Acknowledgements This work was supported by Key-Area Research and Development Program of Guangdong Province (No. 2019B010934001), Na tional Natural Science Foundation of China (Grant No. 51771074 and 61574061), the Major Integrated Projects of National Natural Science Foundation of China (Grant No. U1601651), Science and Technology Project of Guangzhou (No. 201904010344), the Fundamental Research Funds for the Central Universities (No. 2019MS012), 2019 Guangdong University Student Science and Technology Innovation Special Fund (“Climbing Plan” Special Fund) (No. pdjh2019a0028 and pdjh2019b0041), National College Students’ Innovation and Entrepreneurship Training Program (No. 201910561005 and 201910561007), South China University of Technology 100 Step Ladder Climbing Plan Research Project (No. j2tw201902475 and j2tw201902203). References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials (1995) 433. [2] Y. Tak, K. Kim, H. Park, et al., Criteria for ito (indium–tin-oxide) thin film as the bottom electrode of an organic light emitting diode, Thin Solid Films 411 (2002) 12–16. [3] X. Liu, A. Ng, Y.H. Ng, et al., Effect of electrical properties, transmittance and morphology of ito electrode on polymer solar cells characteristics, Proceedings of SPIE - The International Society for Optical Engineering 8626 (2013). [4] V.M. Kalygina, T.Z. Lygdenova, Y.S. Petrova, et al., Influence of the substrate material on the properties of gallium-oxide films and gallium-oxide-based structures, Semiconductors 53 (2019) 452–457. [5] V.M. Kalygina, Y.S. Petrova, I.A. Prudaev, et al., Stability of electrical characteristics of mos structures based on gallium oxide, Russ. Phys. J. 59 (2016) 757–761. [6] A.Y. Polyakov, N.B. Smirnov, I.V. Shchemerov, D. Gogova, S.A. Tarelkin, S.J. Pearton, Compensation and persistent photocapacitance in homoepitaxial Sndoped b-Ga2O3, J. Appl. Phys. 123 (2018) 115702. [7] H. Mun, H. Yang, K. Char, Effect of a Ru doped SnO2-x buffer layer on thin-film transistors based on SnO2-x channel layer, in: APS March Meeting 2015, 2015. [8] N.K. Maksimova, A.V. Almaev, E.Y. Sevastyanov, Effect of additives Ag and rare-earth elements Y and Sc on the properties of hydrogen sensors based on thin SnO2 films during long-term testing, Coatings 9 (2019) 423. [9] A.V. Almaev, V.I. Gaman, Characteristics of hydrogen sensors based on thin tin dioxide films modified with gold, Russ. Phys. J. 60 (2017) 1081–1087. [10] A.A. Yadav, E.U. Masumdar, A.V. Moholkar, et al., Electrical, structural and optical properties of SnO2:F thin films: effect of the substrate temperature, J. Alloy. Comp. 488 (2009) 350–355. [11] I.Y.Y. Bu, Sol–gel deposition of fluorine-doped tin oxide glasses for dye sensitized solar cells, Ceram. Int. 40 (2014) 417–422. [12] B. Koo, H. Ahn, Structural, electrical, and optical properties of Sb-doped SnO2 transparent conductive oxides fabricated using an electrospray technique, Ceram. Int. 40 (2014) 4375–4381. [13] P. Hsu, W. Chen, Y. Tsai, et al., Sputtering deposition of p-type SnO films using robust Sn/SnO2 mixed target, Thin Solid Films 555 (2014) 57–61. [14] A. Chin, W.S. Cheng, C.F. Lu, et al., High mobility SnO2 TFT for display and future IC, in: Active-matrix Flatpanel Displays & Devices, 2016, https://doi.org/ 10.1109/AM-FPD.2016.7543695. [15] Y. Liu, C. Yin, Z. Zhang, et al., The investigation of hydrogen gas sensing properties of saw gas sensor based on palladium surface modified SnO2 thin film, Mater. Sci. Semicond. Process. 60 (2017) 16–28. [16] M. Marikkannan, V. Vishnukanthan, A. Vijayshankar, A novel synthesis of tin oxide thin films by the sol-gel process for optoelectronic applications, AIP Adv. 5 (2015), 027122. [17] S. Lin, Y. Tsai, K. Bai, Structural and physical properties of tin oxide thin films for optoelectronic applications, Appl. Surf. Sci. 380 (2016) 203–209. [18] L. He, Q. Cao, X. Feng, Structural, optical and electrical properties of epitaxial rutile sno2 films grown on mgf2(110) substrates by mocvd, vol. 44, 2018, pp. 869–873. [19] J. Du, C. Xia, Y. Liu, Electronic characteristics of p -type transparent SnO monolayer with high carrier mobility, Appl. Surf. Sci. 401 (2017) 114–119. [20] P.C. Hsu, W. Chen, Y. Tsai, et al., Fabrication of p-type SnO thin-film transistors by sputtering with practical metal electrodes, Jpn. J. Appl. Phys. 52 (2013), 05DC07. [21] J. Zhang, J. Yang, Y. Li, High performance complementary circuits based on p-SnO and n-IGZO thin-film transistors, Materials 10 (2017) 319. [22] Q. Lei, W. Liu, Y. Pei, et al., Trap states extraction of p-channel SnO thin-film transistors based on percolation and multiple trapping carrier conductions, Solid State Electron. 129 (2016). [23] H. Jadhav, S.R. Suryawanshi, M.A. More, et al., Pulsed laser deposition of tin oxide thin films for field emission studies, Appl. Surf. Sci. 419 (2017). [24] Y.A. El-gendy, Effects of film thickness on the linear and nonlinear refractive index of p-type SnO films deposited by e-beam evaporation process, Phys. B Condens. Matter 526 (2017) 59–63. [25] H. Yabuta, N. Kaji, R. Hayashi, et al., Sputtering formation of p-type SnO thin-film transistors on glass toward oxide complimentary circuits, Appl. Phys. Lett. 97 (2010), 072111 (3 pp.)–-072111 (3 pp.)072111 (3 pp.). [26] H. Guendouz, A. Bouaine, N. Brihi, Biphase Effect on Structural, Optical, and Electrical Properties of Al-Sn Codoped Zno Thin Films Deposited by Sol-Gel SpinCoating Technique, vol. 158, 2018. [27] K. Verma, B. Chaudhary, V. Kumar, et al., Influence of fe-doping on the structural, optical and luminescent behavior of zno thin films deposited by spin coating technique, Vacuum 146 (2017) 478–482, https://doi.org/10.1016/j.vacuum.2017.06.033. [28] F. Tang, C. Mei, P. Chuang, et al., Valence state and magnetism of Mn-doped PbPdO2 nanograin film synthesized by sol-gel spin-coating method, Thin Solid Films 623 (2017) 14–18. [29] Z.Y. Lee, S.S. Ng, F.K. Yam, Growth mechanism of indium nitride via sol–gel spin coating method and nitridation process, Surf. Coat. Technol. 310 (2017) 38–42.
8
Superlattices and Microstructures 139 (2020) 106400
H. Ning et al.
[30] A.R. Nimbalkar, M.G. Patil, Synthesis of ZnO thin film by sol-gel spin coating technique for H2S gas sensing application, Phys. B Condens. Matter 527 (2017) 7–15. [31] T. Wang, X. Zhao, H. Yang, et al., On the roles of hec in pechini sol-gel method: enhancement of stability, wettability of the sol and surface roughness of Bi2212 film, Ceram. Int. 44 (2018) 12144–12148. [32] K.J. Saji, Y.P.V. Subbaiah, K. Tian, et al., P-type SnO thin films and SnO/ZnO heterostructures for all-oxide electronic and optoelectronic device applications, Thin Solid Films 605 (2015) 193–201. [33] J.M. Themlin, M. Chta B, L. Henrard, et al., Characterization of tin oxides by x-ray-photoemission spectroscopy, Phys. Rev. B Condens. Matter 46 (1992) 2460–2466. [34] T. Voscoboinikov Süzer, K.R. Hallam, et al., Electron spectroscopic investigation of sn coatings on glasses, vol. 355, 1996, pp. 654–656. [35] M.A. Stranick, A. Moskwa, SnO2 by xps, Surf. Sci. Spectra 2 (1993) 50–54. [36] R.O. Ansell, T. Dickinson, A.F. Povey, et al., Cheminform abstract: x-ray photoelectron spectroscopic studies of tin electrodes after polarization in sodium hydroxide solution, vol. 8, 1977, pp. 1360–1364. [37] C.M. Campo, J.E. Rodríguez, A.E. Ramírez, Thermal behaviour of romarchite phase SnO in different atmospheres: a hypothesis about the phase transformation, Heliyon 2 (2016) e00112. [38] P. Fowler, C. Ruscher, J. Forrest, Controlling marangoni-induced instabilities in spin-cast polymer films: how to prepare uniform films, Eur. Phys. J. E 39 (2016) 90. [39] X. Wang, F. Shi, X. Gao, et al., A sol–gel dip/spin coating method to prepare titanium oxide films, Thin Solid Films 548 (2013) 34–39. [40] R. Balzaroyti, C. Cristiani, L. F Francis, Spin coating deposition on complex geometry substrates:influence of operative parameters, Surf. Coat. Technol. 330 (2017). [41] C.P.T. Nguyen, J. Raja, S. Kim, et al., Enhanced electrical properties of oxide semiconductor thin-film transistors with high conductivity thin layer insertion for the channel region, Appl. Surf. Sci. 396 (2017) 1472–1477. [42] J. Rao, L. Bao, B. Wang, Plasma surface modification and bonding enhancement for bamboo composites, Compos. B Eng. 138 (2018) 157–167. [43] K.N. Pandiyarj, A. A Kumar, M. C Ramkumar, Effect of cold atmospheric pressure plasma gas composition on the surface and cyto-compatible properties of low density polyethylene (ldpe) films, vol. 16, 2016, pp. 784–792. [44] A. Eshaghi, M. Mesbahi, A.A. Aghaei, Influence of physical plasma etching treatment on optical and hydrophilic MgF2 thin film, Optik 161 (2018) 1–7. [45] M.H. Zhao, R. Fu, D. Lu, et al., Critical thickness for cracking of Pb(Zr0.53Ti0.47)O3 thin films deposited on Pt/Ti/Si(100) substrates, Acta Mater. 50 (2002) 4241–4254. � [46] B. Aguila, D, A. Vasco, P. Galvez, Effect of temperature and cuo-nanoparticle concentration on the thermal conductivity and viscosity of an organic phasechange material, Int. J. Heat Mass Transf. 120 (2018) 1009–1019.
9