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nanostructure on polymeric template
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ScienceDirect Materials Today: Proceedings 18 (2019) 1517–1523
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ICN3I-2017
Structure and morphology of atomic layer deposition grown ZnO thin film / nanostructure on polymeric template Ajaib Singha, Aakash Mathura, Dipayan Pala, Amartya Senguptab, Rinki Singhc and Sudeshna Chattopadhyaya,c,d,* a
Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Indore 453552, India b Department of Physics, Indian Institute of Technology Delhi, Delhi 110016, India c Centre for Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Indore 453552, India d Discipline of Physics, Indian Institute of Technology Indore, Indore 453552, India
Abstract Zinc oxide (ZnO) was grown on polymer template, namely Poly (methyl methacrylate) (PMMA), by atomic layer deposition (ALD) technique. The detailed structural characterizations of the composite ZnO/PMMA film, through X-ray scattering techniques, indicate that on the PMMA template crystalline ZnO thin film was formed with its c-axis oriented along the surface normal. Combination of X-ray reflectivity (XRR) and atomic force microscopy (AFM) studies demonstrate the formation of high quality ZnO thin film (about 25 nm thick) on PMMA template, with more than 92% coverage and low surface/interface roughness. AFM results clearly show that the ZnO film consists of almost uniform nanoparticles with ~24-30 nm in-plane size distribution, which is very much consistent with the observed size of the ZnO crystallites obtained from X-ray diffraction (XRD) study. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: ZnO thin film; ZnO-polymer nanocomposites; X-ray reflectivity (XRR); Structure-morphology
* Corresponding author. Tel.: +91-732-4306-544 E-mail address: [email protected], [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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1. Introduction Metal oxides such as ZnO, ITO, CuO, TiO2, SnO2 ,Fe2O3 [1] represent an assorted and appealing class of materials which have been a studied intensively in past years, because of their potential application in photonic crystals [2], dye sensitized solar cells [3], sensors [4] etc. In particular ZnO is a multifunctional n-type semiconductor having wide band gap (3.4 eV), high exciton binding energy (60 meV), effective ultraviolet absorbance and good chemical stability [5]. By virtue of these excellent optical properties, ZnO is known as vital material for light emitting devices (LEDs) [6], semiconductor laser [7] and solar cells [8]. For numerous applications such as smart card, electronic maps, flat panel displays and other large area applications flexible and light weight devices are needed. Recently, flexible polymer substrates have received great attention due to their specific properties such as low weight, small volume, easy packaging and assorted design possibilities. Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon based electronics cannot provide a solution. Among available polymers, Poly (methyl methacrylate) PMMA is abundantly used in many applications, due to its transparency to visible light, high chemical stability and low cost, and can be used as polymer to form composite with ZnO for its application in near infrared reflectance, UV-shielding and photocatalytic activity [9, 10]. Apart from these application PMMA has applications in the field of biomedical, sensing technology, solar energy and as polymer electrolyte in battery [9]. Moreover, PMMA has been already evaluated to be a potential candidate as the organic solar cells application and organic field effect transistors (FETs) devices due to its high resistivity, suitable dielectric constant, chemical inertness, thermal stability and high mechanical flexibility [11, 8]. PMMA as the prototype, has already shown its ability to work as a suitable template for growth of metal nanoparticles [12]. Number of deposition techniques have been explored to grow ZnO thin films, for example, atomic layer deposition (ALD) [13, 14], molecular beam epitaxy (MBE) [15], pulsed laser deposition (PLD) [16], magnetron sputtering [17] and dual ion beam sputtering (DIBSD) [18] . Among all these techniques, atomic layer deposition (ALD) has been considered as a promising method to deposit ZnO thin film. Thin film deposited through ALD shows excellent film deposition conformality due to the surface-saturated and self-limiting reaction mechanism [13, 14]. These advantages make ALD a novel and robust method to deposit ZnO films into extremely high-aspect-ratio structures at different temperature and variety of substrates [19]. Atomic layer deposition is one of the robust techniques, which has the potential to deposit the materials at very low energy without perturbing the polymeric template. The reproducible economical fabrication of metal oxide-polymer nanocomposites thin film with tuned optical properties for advanced technological applications remains a major challenge. Film thickness of the thin films has great impact on the optical and morphological properties of the material, which in turn makes them very attractive for various technological applications [20, 21]. There have been several reports concerning the details of the structural and optical properties of ALD grown ZnO thin films [22-24]. However, there are rare reports for ALD grown ZnO thin film on polymer template. In this article, we investigated the structure, morphology and film quality of ALD grown ZnO thin film on PMMA template (ZnO/PMMA), where PMMA template was grown on Si substrate. Thickness, electron density profile, surface/interface roughness of the ZnO/PMMA films were explored using XRR technique. Structure and morphology of ZnO/PMMA system has been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM), also compared with the results for the simultaneously grown ZnO film, directly on Si substrate. 2. Material and methods Poly (methyl methacrylate) (PMMA) of molecular weight 350,000 was spin-coated on polished silicon substrates (Si (100)) from toluene solution using a spin coater, Apex Instruments Co. Pvt. Ltd., model spinNXG-P2H to form PMMA templates (~70 nm thick PMMA films). Prior to the PMMA deposition, the silicon wafers were cleaned in a piranha solution (3:1 mixture of sulfuric acid and 30% hydrogen peroxide) to remove any organic contamination, followed by ultra-sonication in acetone and ethanol and then keeping them in de-ionized (DI) water, and finally drying up in nitrogen (N2, 99.999% purity). ZnO thin films (of thickness ~25 nm) were grown on PMMA templates (PMMA on Si substrate) at room temperature by using BENEQ TFS-200 ALD reactor, and for comparison purpose
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ZnO was also grown simultaneously on Si substrate. Diethyl zinc (DEZn, Zn(C2H5)2, Sigma-Aldrich) and DI water were used respectively as precursors for zinc and oxygen. In initial stage the DEZ precursor get kinetically trapped on the PMMA surface. The trapped DEZ precursor can subsequently further react with water and start the further subsurface reactions [25]. The overall mechanism in shown in schematic 1, while reaction occurring during ZnO formation is as follows [19, 26]. ZnO 2 1 And the two ALD half reactions involved are [27] Zn
Zn Zn
O
Zn
2 3
Nitrogen (N2, 99.999% purity) was used both as a carrier and purging gas. Hereafter PMMA/Si and ZnO/PMMA/Si samples will be referred as PMMA and ZnO/PMMA films respectively.
Schematic 1: Schematic of ALD Grown ZnO: (a) DEZ precursor pulse; (b) excess precursor and reaction by-products are purged with inert carrier gas N2; (c) H2O precursor pulsed and reaction with surface; (d) excess precursor and reaction by-products are purged with N2.
For investigating film thickness X-ray reflectivity (XRR) has been employed. X-ray reflectivity data for PMMA/Si and ZnO/PMMA/Si samples were collected using a Rigaku SmartLab automated multipurpose X-ray diffractometer with CuK radiation ( = 1.54 Å) operating at 30 kV and 40 mA. Under the specular condition with incident angle i equal to the scattering angle f, and i varying from 0° to 4°, X-ray reflectivity study has been done. Using this geometrical requirement, the momentum transfer vector satisfy equation, q = kf – ki with kf(i) being the scattered (incident) X-ray wave vector. The only non-zero component along the normal to the sample surface, qz (= (4π/λ) sin i), which was varied from 0.028 to 0.4 Å-1. At critical angle c, all X-rays propagate along sample surface, such that i = c and qc = (4π/λ) sin c . Further critical momentum vector qc is related to electron density of material by 16 / , where re = 2.818 × 10-15 m is the Thomson’s classical radius and is the electron density of the material [28]. Reflectivity data for PMMA and ZnO/PMMA films were analyzed by using Parratt formalism, where each film is divided into layers of fixed thickness (d), average electron density () and interfacial roughness (), and used as fit parameters [13, 29]. This method recursively solves Fresnel equations at each interface to result in varying electron density of each layer. The crystalline structures of the as deposited ZnO thin films were characterized by X-ray diffraction (XRD) with CuK radiation (=1.54 Å) using Rigaku SmartLab automated multipurpose X-ray diffractometer. The surface morphology of the films was investigated by atomic force microscope (AFM), operating in the tapping mode with Si cantilevers of about 10 nm tip radius, using a Nanoscope IV multimode SPM.
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3. Results and discussion 3.1. Thickness, electron density profile, and surface–interface roughness of PMMA and ZnO/PMMA X-ray reflectivity (XRR) provides detailed information about film thickness, density profiles normal to the surface, surface and interface roughness of the multilayer systems. Fig. 1(a) shows the X-ray reflectivity (data in open circle) of PMMA and ZnO/PMMA films along with corresponding best fits using Parratt formalism (in line), while extracted EDP across film depth from best fit, is shown in Fig. 1(b). Inset of Fig. 1(a) shows the zoomed portion of reflectivity data at low qz, indicating the clear enhancement of qc value for ZnO/PMMA film (qcB ~ 0.043 Å-1) with respect to the PMMA film (qcA ~ 0.032 Å-1). Reflectivity profiles have been normalized by Fresnel reflectivity, RF (where RF qz-4). The comparison of extracted electron density profiles for ZnO/PMMA and pristine PMMA films, shown in Fig. 1(b), indicate that room temperature ALD grown (1000 cycles) ZnO forms a thin layer of high quality ZnO film of 26 nm thickness, on 65 nm thick PMMA template, with more than ~92% coverage and reasonably low ZnO/PMMA interface roughness, ~ 6 Å. Also, the results exhibit that the electron density of the PMMA film, which is ~ 0.32 eA-3, remains unchanged after ZnO film deposition. The sharp interface at ZnO/PMMA and the unaffected electron density of PMMA clearly indicate that there is no significant perturbation in underneath PMMA template due to the ZnO film deposition through ALD technique.
Fig. 1.(a) X-ray reflectivity results for PMMA and ZnO/PMMA films: Fresnel normalized X-ray reflectivity (R/RF) data (open circles) and best fits (lines through data) for PMMA and ZnO/PMMA; inset shows the zoomed portion of XRR data, showing the change in qc values; (b) Extracted electron density profiles (EDPs) across film depth, from XRR data fits.
3.2. Structure and morphology : XRD and AFM Fig. 2(a) shows XRD pattern of ZnO/PMMA system at room temperature. Three peaks appear at 2 = 31.7°, 34.3°, and 36.2° for ZnO/PMMA which correspond to (100), (002) and (101) directions of hexagonal wurtzite ZnO crystal structure (JCPDS PDF Card No. 01-079-2205). ZnO (002) peak shows stronger intensity compared to ZnO (101), whereas the standard powder XRD pattern of bulk ZnO (hexagonal wurtzite bulk ZnO) shows the maximum intensity for the ZnO (101) peak (JCPDS PDF Card No. 01-079-2205), as shown in Figure 2(a), (b) and (c). Figure 2 (a) and (b) indicate that the ALD grown ZnO on PMMA and Si show the preferred orientation along <0002> direction, where the ZnO on PMMA template is less textured in comparison to Si template. The average crystallite size for ZnO thin film is calculated from Debye Scherer equation given by [30]
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(4)
Where ‘’ is the X-ray wavelength, ‘’ is the Bragg angle, and ‘’ is the FWHM of the (002) peak (measured in radians). The estimated average ZnO crystallite size was 24 nm. The results indicate that the ALD grown ZnO thin films contain crystallites of ~24 nm diameter, having preferential orientation with the c-axis perpendicular to the surface of the system.
Fig. 2. X-ray diffraction pattern of (a) ZnO thin film grown on PMMA template (ZnO/PMMA); (b) ZnO/Si; (c) Reference data: Standard diffraction pattern of hexagonal wurtzite ZnO (JCPDS PDF Card No. 01-079-2205).
The surface morphology of the ZnO film has been studied by AFM. Fig. 3(a) shows AFM micrograph of the ZnO/PMMA system. The AFM image indicates the formation of ZnO nanoparticles of spherical shape, with average particle size (in-plane) about 30 nm with a surface roughness of about 4 nm. Whereas ZnO on Si template shows lower surface roughness, of about 2 nm, and smaller average particle size (in-plane), ~24 nm, as shown in Figure 3(b). The AFM results are very much consistent with the observations from XRR and XRD for ZnO/PMMA sample.
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Fig. 3. AFM images: (a) ZnO thin film grown on PMMA template (ZnO/PMMA) and (b) ZnO/Si. (Inset shows the zoomed portion of the image).
4. Conclusion ALD grown ZnO thin film on PMMA template shows high film quality, in terms of high coverage, conformality, surface and interface roughness. The detailed characterization of structure and morphology of the ZnO/PMMA system were analyzed by XRR, XRD and AFM techniques. The ZnO film is crystalline in nature with preferred orientation, having c-axis perpendicular to the PMMA surface. X-ray reflectivity results confirm the growth of ZnO thin film on PMMA template, without any significant perturbation in the underneath polymer template, either in thickness or electron-density, indicating the importance of such systems in different application like flexible electronics, curved organic light emitting diode (OLED). Acknowledgements We would like to acknowledge IIT Indore for all kinds of support to this work. Authors would like to thank the Nanoscale Research Facility (NRF) at IIT Delhi for AFM measurements. We acknowledge support by Council of Scientific and Industrial Research (CSIR), India, Project No. 03 (1310)/14/EMR-II and Department of Science and Technology (DST), India, Project No. SB/S2/CMP-077/2013. References [1] E. Comini, Anal. Chim. Acta 568 (2006) 28-40. [2] D.P. Puzzo, M.G. Helander, P.G. O’Brien, Z. Wang, N. Soheilnia, N. Kherani, Z. Lu, G.A. Ozin, Nano Lett. 11 (2011) 1457-1462. [3] U. Mehmood, S.-u. Rahman, K. Harrabi, I.A. Hussein, B.V.S. Reddy, Adv. Mater. Sci. Eng. 2014 (2014) 12. [4] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1869-1871. [5] W. Zhong Lin, J. Phys. Condens. Matter 16 (2004) R829. [6] D.C. Kim, W.S. Han, B.H. Kong, H.K. Cho, C.H. Hong, Physica B 401 (2007) 386-390. [7] U. Ozgur, D. Hofstetter, H. Morkoc, Proc. IEEE 98 (2010) 1255-1268. [8] Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, Adv. Mater. 21 (2009) 4087-4108. [9] U. Ali, K.J.B.A. Karim, N.A. Buang, Polym. Rev. 55 (2015) 678-705. [10] H. Wang, P. Xu, S. Meng, W. Zhong, W. Du, Q. Du, Polym. Degrad. Stab. 91 (2006) 1455-1461. [11] W.H. Lee, J.A. Lim, D. Kwak, J.H. Cho, H.S. Lee, H.H. Choi, K. Cho, Adv. Mater. 21 (2009) 4243-4248. [12] L.-B. Zhong, J. Yin, Y.-M. Zheng, Q. Liu, X.-X. Cheng, F.-H. Luo, Anal. Chem. 86 (2014) 6262-6267. [13] D. Pal, A. Mathur, A. Singh, J. Singhal, A. Sengupta, S. Dutta, S. Zollner, S. Chattopadhyay, J. Vac. Sci. Technol., A 35 (2017) 01B108. [14] D. Pal, J. Singhal, A. Mathur, A. Singh, S. Dutta, S. Zollner, S. Chattopadhyay, Appl. Surf. Sci. 421 (2017) 341-348. [15] Y. Chen, D. Bagnall, H.-j. Koh, K.-t. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912-3918. [16] X. Sun, H.S. Kwok, J. Appl. Phys. 86 (1999) 408-411. [17] A. Singh, S. Schipmann, A. Mathur, D. Pal, A. Sengupta, U. Klemradt, S. Chattopadhyay, Appl. Surf. Sci. 414 (2017) 114-123. [18] F. Quaranta, A. Valentini, F.R. Rizzi, G. Casamassima, J. Appl. Phys. 74 (1993) 244-248. [19] S.M. George, Chem. Rev. 110 (2009) 111-131. [20] X. Hao, J. Ma, D. Zhang, T. Yang, H. Ma, Y. Yang, C. Cheng, J. Huang, Appl. Surf. Sci. 183 (2001) 137-142. [21] L. Miao, S. Tanemura, M. Tanemura, S.P. Lau, B.K. Tay, J. Mater. Sci. Mater. Electron. 18 (2007) 343-346. [22] P.-Y. Lin, J.-R. Gong, P.-C. Li, T.-Y. Lin, D.-Y. Lyu, D.-Y. Lin, H.-J. Lin, T.-C. Li, K.-J. Chang, W.-J. Lin, J. Cryst. Growth 310 (2008) 3024-3028. [23] E. Przeździecka, W. Paszkowicz, E. Łusakowska, T. Krajewski, G. Łuka, E. Guziewicz, M. Godlewski, Semicond. Sci. Technol. 24 (2009) 105014.
A. Singh et al. / Materials Today: Proceedings 18 (2019) 1517–1523 [24] T. Singh, T. Lehnen, T. Leuning, D. Sahu, S. Mathur, Appl. Surf. Sci. 289 (2014) 27-32. [25] M. Napari, J. Malm, R. Lehto, J. Julin, K. Arstila, T. Sajavaara, M. Lahtinen, J. Vac. Sci. Technol., A 33 (2015) 01A128. [26] R.W. Johnson, A. Hultqvist, S.F. Bent, Materials today 17 (2014) 236-246. [27] A. Afshar, K.C. Cadien, Appl. Phys. Lett. 103 (2013) 251906. [28] S. Chandran, N. Begam, J. Basu, J. Appl. Phys. 116 (2014) 222203. [29] L.G. Parratt, Phys. Rev. 95 (1954) 359. [30] S.S. Kumar, P. Venkateswarlu, V.R. Rao, G.N. Rao, Int. Nano Lett. 3 (2013) 30.
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Structure and morphology of atomic layer deposition grown ZnO thin film / nanostructure on polymeric template Ajaib Singha, Aakash Mathura, Dipayan Pala, Amartya Senguptab, Rinki Singhc and Sudeshna Chattopadhyaya,c,d,* a
Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Indore 453552, India b Department of Physics, Indian Institute of Technology Delhi, Delhi 110016, India c Centre for Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Indore 453552, India d Discipline of Physics, Indian Institute of Technology Indore, Indore 453552, India
Abstract Zinc oxide (ZnO) was grown on polymer template, namely Poly (methyl methacrylate) (PMMA), by atomic layer deposition (ALD) technique. The detailed structural characterizations of the composite ZnO/PMMA film, through X-ray scattering techniques, indicate that on the PMMA template crystalline ZnO thin film was formed with its c-axis oriented along the surface normal. Combination of X-ray reflectivity (XRR) and atomic force microscopy (AFM) studies demonstrate the formation of high quality ZnO thin film (about 25 nm thick) on PMMA template, with more than 92% coverage and low surface/interface roughness. AFM results clearly show that the ZnO film consists of almost uniform nanoparticles with ~24-30 nm in-plane size distribution, which is very much consistent with the observed size of the ZnO crystallites obtained from X-ray diffraction (XRD) study. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: ZnO thin film; ZnO-polymer nanocomposites; X-ray reflectivity (XRR); Structure-morphology
* Corresponding author. Tel.: +91-732-4306-544 E-mail address: [email protected], [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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1. Introduction Metal oxides such as ZnO, ITO, CuO, TiO2, SnO2 ,Fe2O3 [1] represent an assorted and appealing class of materials which have been a studied intensively in past years, because of their potential application in photonic crystals [2], dye sensitized solar cells [3], sensors [4] etc. In particular ZnO is a multifunctional n-type semiconductor having wide band gap (3.4 eV), high exciton binding energy (60 meV), effective ultraviolet absorbance and good chemical stability [5]. By virtue of these excellent optical properties, ZnO is known as vital material for light emitting devices (LEDs) [6], semiconductor laser [7] and solar cells [8]. For numerous applications such as smart card, electronic maps, flat panel displays and other large area applications flexible and light weight devices are needed. Recently, flexible polymer substrates have received great attention due to their specific properties such as low weight, small volume, easy packaging and assorted design possibilities. Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon based electronics cannot provide a solution. Among available polymers, Poly (methyl methacrylate) PMMA is abundantly used in many applications, due to its transparency to visible light, high chemical stability and low cost, and can be used as polymer to form composite with ZnO for its application in near infrared reflectance, UV-shielding and photocatalytic activity [9, 10]. Apart from these application PMMA has applications in the field of biomedical, sensing technology, solar energy and as polymer electrolyte in battery [9]. Moreover, PMMA has been already evaluated to be a potential candidate as the organic solar cells application and organic field effect transistors (FETs) devices due to its high resistivity, suitable dielectric constant, chemical inertness, thermal stability and high mechanical flexibility [11, 8]. PMMA as the prototype, has already shown its ability to work as a suitable template for growth of metal nanoparticles [12]. Number of deposition techniques have been explored to grow ZnO thin films, for example, atomic layer deposition (ALD) [13, 14], molecular beam epitaxy (MBE) [15], pulsed laser deposition (PLD) [16], magnetron sputtering [17] and dual ion beam sputtering (DIBSD) [18] . Among all these techniques, atomic layer deposition (ALD) has been considered as a promising method to deposit ZnO thin film. Thin film deposited through ALD shows excellent film deposition conformality due to the surface-saturated and self-limiting reaction mechanism [13, 14]. These advantages make ALD a novel and robust method to deposit ZnO films into extremely high-aspect-ratio structures at different temperature and variety of substrates [19]. Atomic layer deposition is one of the robust techniques, which has the potential to deposit the materials at very low energy without perturbing the polymeric template. The reproducible economical fabrication of metal oxide-polymer nanocomposites thin film with tuned optical properties for advanced technological applications remains a major challenge. Film thickness of the thin films has great impact on the optical and morphological properties of the material, which in turn makes them very attractive for various technological applications [20, 21]. There have been several reports concerning the details of the structural and optical properties of ALD grown ZnO thin films [22-24]. However, there are rare reports for ALD grown ZnO thin film on polymer template. In this article, we investigated the structure, morphology and film quality of ALD grown ZnO thin film on PMMA template (ZnO/PMMA), where PMMA template was grown on Si substrate. Thickness, electron density profile, surface/interface roughness of the ZnO/PMMA films were explored using XRR technique. Structure and morphology of ZnO/PMMA system has been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM), also compared with the results for the simultaneously grown ZnO film, directly on Si substrate. 2. Material and methods Poly (methyl methacrylate) (PMMA) of molecular weight 350,000 was spin-coated on polished silicon substrates (Si (100)) from toluene solution using a spin coater, Apex Instruments Co. Pvt. Ltd., model spinNXG-P2H to form PMMA templates (~70 nm thick PMMA films). Prior to the PMMA deposition, the silicon wafers were cleaned in a piranha solution (3:1 mixture of sulfuric acid and 30% hydrogen peroxide) to remove any organic contamination, followed by ultra-sonication in acetone and ethanol and then keeping them in de-ionized (DI) water, and finally drying up in nitrogen (N2, 99.999% purity). ZnO thin films (of thickness ~25 nm) were grown on PMMA templates (PMMA on Si substrate) at room temperature by using BENEQ TFS-200 ALD reactor, and for comparison purpose
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ZnO was also grown simultaneously on Si substrate. Diethyl zinc (DEZn, Zn(C2H5)2, Sigma-Aldrich) and DI water were used respectively as precursors for zinc and oxygen. In initial stage the DEZ precursor get kinetically trapped on the PMMA surface. The trapped DEZ precursor can subsequently further react with water and start the further subsurface reactions [25]. The overall mechanism in shown in schematic 1, while reaction occurring during ZnO formation is as follows [19, 26]. ZnO 2 1 And the two ALD half reactions involved are [27] Zn
Zn Zn
O
Zn
2 3
Nitrogen (N2, 99.999% purity) was used both as a carrier and purging gas. Hereafter PMMA/Si and ZnO/PMMA/Si samples will be referred as PMMA and ZnO/PMMA films respectively.
Schematic 1: Schematic of ALD Grown ZnO: (a) DEZ precursor pulse; (b) excess precursor and reaction by-products are purged with inert carrier gas N2; (c) H2O precursor pulsed and reaction with surface; (d) excess precursor and reaction by-products are purged with N2.
For investigating film thickness X-ray reflectivity (XRR) has been employed. X-ray reflectivity data for PMMA/Si and ZnO/PMMA/Si samples were collected using a Rigaku SmartLab automated multipurpose X-ray diffractometer with CuK radiation ( = 1.54 Å) operating at 30 kV and 40 mA. Under the specular condition with incident angle i equal to the scattering angle f, and i varying from 0° to 4°, X-ray reflectivity study has been done. Using this geometrical requirement, the momentum transfer vector satisfy equation, q = kf – ki with kf(i) being the scattered (incident) X-ray wave vector. The only non-zero component along the normal to the sample surface, qz (= (4π/λ) sin i), which was varied from 0.028 to 0.4 Å-1. At critical angle c, all X-rays propagate along sample surface, such that i = c and qc = (4π/λ) sin c . Further critical momentum vector qc is related to electron density of material by 16 / , where re = 2.818 × 10-15 m is the Thomson’s classical radius and is the electron density of the material [28]. Reflectivity data for PMMA and ZnO/PMMA films were analyzed by using Parratt formalism, where each film is divided into layers of fixed thickness (d), average electron density () and interfacial roughness (), and used as fit parameters [13, 29]. This method recursively solves Fresnel equations at each interface to result in varying electron density of each layer. The crystalline structures of the as deposited ZnO thin films were characterized by X-ray diffraction (XRD) with CuK radiation (=1.54 Å) using Rigaku SmartLab automated multipurpose X-ray diffractometer. The surface morphology of the films was investigated by atomic force microscope (AFM), operating in the tapping mode with Si cantilevers of about 10 nm tip radius, using a Nanoscope IV multimode SPM.
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3. Results and discussion 3.1. Thickness, electron density profile, and surface–interface roughness of PMMA and ZnO/PMMA X-ray reflectivity (XRR) provides detailed information about film thickness, density profiles normal to the surface, surface and interface roughness of the multilayer systems. Fig. 1(a) shows the X-ray reflectivity (data in open circle) of PMMA and ZnO/PMMA films along with corresponding best fits using Parratt formalism (in line), while extracted EDP across film depth from best fit, is shown in Fig. 1(b). Inset of Fig. 1(a) shows the zoomed portion of reflectivity data at low qz, indicating the clear enhancement of qc value for ZnO/PMMA film (qcB ~ 0.043 Å-1) with respect to the PMMA film (qcA ~ 0.032 Å-1). Reflectivity profiles have been normalized by Fresnel reflectivity, RF (where RF qz-4). The comparison of extracted electron density profiles for ZnO/PMMA and pristine PMMA films, shown in Fig. 1(b), indicate that room temperature ALD grown (1000 cycles) ZnO forms a thin layer of high quality ZnO film of 26 nm thickness, on 65 nm thick PMMA template, with more than ~92% coverage and reasonably low ZnO/PMMA interface roughness, ~ 6 Å. Also, the results exhibit that the electron density of the PMMA film, which is ~ 0.32 eA-3, remains unchanged after ZnO film deposition. The sharp interface at ZnO/PMMA and the unaffected electron density of PMMA clearly indicate that there is no significant perturbation in underneath PMMA template due to the ZnO film deposition through ALD technique.
Fig. 1.(a) X-ray reflectivity results for PMMA and ZnO/PMMA films: Fresnel normalized X-ray reflectivity (R/RF) data (open circles) and best fits (lines through data) for PMMA and ZnO/PMMA; inset shows the zoomed portion of XRR data, showing the change in qc values; (b) Extracted electron density profiles (EDPs) across film depth, from XRR data fits.
3.2. Structure and morphology : XRD and AFM Fig. 2(a) shows XRD pattern of ZnO/PMMA system at room temperature. Three peaks appear at 2 = 31.7°, 34.3°, and 36.2° for ZnO/PMMA which correspond to (100), (002) and (101) directions of hexagonal wurtzite ZnO crystal structure (JCPDS PDF Card No. 01-079-2205). ZnO (002) peak shows stronger intensity compared to ZnO (101), whereas the standard powder XRD pattern of bulk ZnO (hexagonal wurtzite bulk ZnO) shows the maximum intensity for the ZnO (101) peak (JCPDS PDF Card No. 01-079-2205), as shown in Figure 2(a), (b) and (c). Figure 2 (a) and (b) indicate that the ALD grown ZnO on PMMA and Si show the preferred orientation along <0002> direction, where the ZnO on PMMA template is less textured in comparison to Si template. The average crystallite size for ZnO thin film is calculated from Debye Scherer equation given by [30]
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0.9 / cos
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(4)
Where ‘’ is the X-ray wavelength, ‘’ is the Bragg angle, and ‘’ is the FWHM of the (002) peak (measured in radians). The estimated average ZnO crystallite size was 24 nm. The results indicate that the ALD grown ZnO thin films contain crystallites of ~24 nm diameter, having preferential orientation with the c-axis perpendicular to the surface of the system.
Fig. 2. X-ray diffraction pattern of (a) ZnO thin film grown on PMMA template (ZnO/PMMA); (b) ZnO/Si; (c) Reference data: Standard diffraction pattern of hexagonal wurtzite ZnO (JCPDS PDF Card No. 01-079-2205).
The surface morphology of the ZnO film has been studied by AFM. Fig. 3(a) shows AFM micrograph of the ZnO/PMMA system. The AFM image indicates the formation of ZnO nanoparticles of spherical shape, with average particle size (in-plane) about 30 nm with a surface roughness of about 4 nm. Whereas ZnO on Si template shows lower surface roughness, of about 2 nm, and smaller average particle size (in-plane), ~24 nm, as shown in Figure 3(b). The AFM results are very much consistent with the observations from XRR and XRD for ZnO/PMMA sample.
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Fig. 3. AFM images: (a) ZnO thin film grown on PMMA template (ZnO/PMMA) and (b) ZnO/Si. (Inset shows the zoomed portion of the image).
4. Conclusion ALD grown ZnO thin film on PMMA template shows high film quality, in terms of high coverage, conformality, surface and interface roughness. The detailed characterization of structure and morphology of the ZnO/PMMA system were analyzed by XRR, XRD and AFM techniques. The ZnO film is crystalline in nature with preferred orientation, having c-axis perpendicular to the PMMA surface. X-ray reflectivity results confirm the growth of ZnO thin film on PMMA template, without any significant perturbation in the underneath polymer template, either in thickness or electron-density, indicating the importance of such systems in different application like flexible electronics, curved organic light emitting diode (OLED). Acknowledgements We would like to acknowledge IIT Indore for all kinds of support to this work. Authors would like to thank the Nanoscale Research Facility (NRF) at IIT Delhi for AFM measurements. We acknowledge support by Council of Scientific and Industrial Research (CSIR), India, Project No. 03 (1310)/14/EMR-II and Department of Science and Technology (DST), India, Project No. SB/S2/CMP-077/2013. References [1] E. Comini, Anal. Chim. Acta 568 (2006) 28-40. [2] D.P. Puzzo, M.G. Helander, P.G. O’Brien, Z. Wang, N. Soheilnia, N. Kherani, Z. Lu, G.A. Ozin, Nano Lett. 11 (2011) 1457-1462. [3] U. Mehmood, S.-u. Rahman, K. Harrabi, I.A. Hussein, B.V.S. Reddy, Adv. Mater. Sci. Eng. 2014 (2014) 12. [4] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1869-1871. [5] W. Zhong Lin, J. Phys. Condens. Matter 16 (2004) R829. [6] D.C. Kim, W.S. Han, B.H. Kong, H.K. Cho, C.H. Hong, Physica B 401 (2007) 386-390. [7] U. Ozgur, D. Hofstetter, H. Morkoc, Proc. IEEE 98 (2010) 1255-1268. [8] Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, Adv. Mater. 21 (2009) 4087-4108. [9] U. Ali, K.J.B.A. Karim, N.A. Buang, Polym. Rev. 55 (2015) 678-705. [10] H. Wang, P. Xu, S. Meng, W. Zhong, W. Du, Q. Du, Polym. Degrad. Stab. 91 (2006) 1455-1461. [11] W.H. Lee, J.A. Lim, D. Kwak, J.H. Cho, H.S. Lee, H.H. Choi, K. Cho, Adv. Mater. 21 (2009) 4243-4248. [12] L.-B. Zhong, J. Yin, Y.-M. Zheng, Q. Liu, X.-X. Cheng, F.-H. Luo, Anal. Chem. 86 (2014) 6262-6267. [13] D. Pal, A. Mathur, A. Singh, J. Singhal, A. Sengupta, S. Dutta, S. Zollner, S. Chattopadhyay, J. Vac. Sci. Technol., A 35 (2017) 01B108. [14] D. Pal, J. Singhal, A. Mathur, A. Singh, S. Dutta, S. Zollner, S. Chattopadhyay, Appl. Surf. Sci. 421 (2017) 341-348. [15] Y. Chen, D. Bagnall, H.-j. Koh, K.-t. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912-3918. [16] X. Sun, H.S. Kwok, J. Appl. Phys. 86 (1999) 408-411. [17] A. Singh, S. Schipmann, A. Mathur, D. Pal, A. Sengupta, U. Klemradt, S. Chattopadhyay, Appl. Surf. Sci. 414 (2017) 114-123. [18] F. Quaranta, A. Valentini, F.R. Rizzi, G. Casamassima, J. Appl. Phys. 74 (1993) 244-248. [19] S.M. George, Chem. Rev. 110 (2009) 111-131. [20] X. Hao, J. Ma, D. Zhang, T. Yang, H. Ma, Y. Yang, C. Cheng, J. Huang, Appl. Surf. Sci. 183 (2001) 137-142. [21] L. Miao, S. Tanemura, M. Tanemura, S.P. Lau, B.K. Tay, J. Mater. Sci. Mater. Electron. 18 (2007) 343-346. [22] P.-Y. Lin, J.-R. Gong, P.-C. Li, T.-Y. Lin, D.-Y. Lyu, D.-Y. Lin, H.-J. Lin, T.-C. Li, K.-J. Chang, W.-J. Lin, J. Cryst. Growth 310 (2008) 3024-3028. [23] E. Przeździecka, W. Paszkowicz, E. Łusakowska, T. Krajewski, G. Łuka, E. Guziewicz, M. Godlewski, Semicond. Sci. Technol. 24 (2009) 105014.
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