Spray Angle Dependence for the Growth of Terrace-truncated Nanocone Structure of Gallium-doped Zinc Oxide by Advanced Spray Pyrolysis Deposition Technique

Rough-surfaced nanocone structures are preferred for use as transparent conductive oxides due to their high optical transparency and electrical conductivity. Structural properties of Ga-doped ZnO terrace-truncated nanocones, which were grown by advanced spray pyrolysis deposition technique, vastly changes with the spraying angle. In the present study, the effect of the spray angle on terrace-truncated nanocone structure formation was investigated. Spray pyrolysis deposition technique was used to grow the nanostructure as the growth rate can be controlled easily. The prepared samples were characterized using X-ray diffraction spectroscopy (XRD) and scanning electron microscopy (SEM) techniques. Optical and electrical properties were investigated by the UV-visible spectrum and four-probe method. The lowest spray angle of 15º showed homogeneous and hexagonal shaped nanocone structure with an average top diameter of 22.8 nm and an average height of 240 nm. An excellent transparent conductive oxide behavior was obtained from the sample synthesized at the lowest spray angle of 15^o with high conductivity of 2.5×10³ ?^-1 .cm^-1 and high transparency of 82% in the visible range.

Ga-doped ZnO; Spray angle; Spray pyrolysis; Terrace-truncated nanocone structure; Transparent conductive oxide material

Semiconductor oxides, which have high electrical conductivity as well as high transparency in the visible range, are considered transparent conductive oxide (TCO) materials. They have a wide range of commercial applications such as smart devices, liquid crystal displays (LCDs), light emitting diodes (LEDs), touch panels, etc. (Wu et al., 2008; Liu et al., 2010; Yan et al., 2015). In general, ITO, SnO₂, Ga₂O₃, In₂O₃, and CdO are extensively used as TCO materials. Among them, ITO is the most well-established TCO material as it has excellent transparent conducting performances.

However, there is a high demand for new TCO materials due to the scarcity and lower stability of indium in hydrogen plasma. Impurity-doped ZnO is commonly used as an optional TCO material, as an alternative to ITO (Look, 2001; Rao and Kumar, 2010; Bedia et al., 2014; Bramantyo et al., 2019). ZnO is an n-type II-VI semiconductor with unique physical and chemical properties such as direct wide band gap (3.37 eV), large exciting binding energy at room temperature (~60 meV), high thermal stability, and nontoxicity (Yim et al.,2007; Fernández and Gandía, 2012; Moditswe et al., 2016). The conductivity of ZnO is caused by ionization of zinc interstitials and oxygen vacancies. The carrier formation by ionization of Zn interstitial is the preponderant mechanism for intrinsic ZnO (Yim et al., 2007). To enhance the electrical conductivity and optical transparency, ZnO is doped with impurities such as B, Al, Ga, Sn, Y, Sc, Ti, or Zr (Look, 2001; Yim et al., 2007; Rao and Kumar, 2010; Moditswe et al., 2016). Even though Al and Ga attained the dominant attention as dopants for ZnO, Ga is considered the preferable dopant because of its similarity in both covalent and ionic radii (0.62 Å and 1.26 Å) with that of Zn (0.74 Å and 1.31 Å) (Bedia et al., 2014). Moreover, covalent bond length of Ga–O (1.92 Å) is comparable with the covalent bond length of Zn–O (1.97 Å) with respect to Al–O (2.7 Å) and In-O (2.1 Å) (Le et al., 2010). Because of this, lattice distortion is possessed at a minimal value even ZnO is highly doped with Ga (Look, 2001). Moreover, when comparing with Al, Ga has high electronegativity, high stability to moisture, and lower reactivity and diffusivity (Look, 2001; Fernández and Gandía, 2012). It is considered that doped Ga atoms replace Zn host atoms and expand free electron density, which increases the electrical conductivity. There are many reports on the formation of various kinds of impurity-doped ZnO structures, such as nanorods, nanoflakes, nanobelts, nanoparticles, nanocones, etc. Among them, it is widely accepted that the 1-D nanorod structure is the most suitable layout for dye sensitized solar cell applications because of its high surface-to-volume ratio.

However, some researchers have reported about the importance of ZnO nanocone structure, which could increase the light transparency with respect to nanorods, by reducing the scattering (Lao et al., 2003; Gao et al., 2006; Yin et al., 2012; Li et al., 2015; Han et al., 2018). Various methods have been developed to produce ZnO thin films, such as pulsed laser deposition, metal-organic chemical vapor deposition (MOCVD), spray pyrolysis, sputtering, sol-gel, and chemical bath deposition techniques (Hu and Gordon, 1992; Hirata et al., 1996; Chen et al., 1998; Sholehah and Yuwono, 2015). Spray pyrolysis deposition (SPD) technique has several advantages over other methods, such as simplicity, low cost, ability for large area deposition, and high homogeneity (Yadav et al., 2010). One of the most important advantages of SPD technique is its ability to change the growth rate easily.

In this study, we have used the rotational, pulsed, and atomized spray pyrolysis deposition technique (RPASP). This method has numerous advantages over normal spray pyrolysis deposition techniques as we can optimize the device according to the requirements by changing the parameters. This novel device is capable of individually controlling spray time, time interval during each spray, rotation speed, number of rotations, distance between the nozzle tip and glass substrate, and spray angle. In general, the spray angle is considered as a critical factor in spray pyrolysis deposition technique. Bandara et al. (2016) also reported about the importance of spray angle for the growth of fluorine-doped zinc oxide 1-D nanostructures. In this study, we have investigated the effect of spray angle for the growth and properties of terrace-truncated nanocone structure of Ga-doped ZnO by RPASP deposition technique, as no proper study has been reported to the best of our knowledge.

In this study, we investigated the spray angle dependency for the growth of Ga-doped ZnO nanostructure by advanced spray pyrolysis deposition technique. The average top diameter and the nanostructure density of Ga-doped ZnO were 22.8 nm and 195 per µm², 36.9 nm and 230 per µm², 38.3 nm and 138 per µm² at spraying angles of 15^o, 30^o, and 45^o, respectively. The physical properties of nanostructures were vastly changed with the spraying angle, as the horizontal and the vertical components of the velocity of vaporized particles were changed. Terrace-truncated nanocone structures were observed by FE-SEM images and the uniform distribution of Ga in ZnO crystal structures were confirmed by EDX mapping. According to the XRD spectra, the growth of nanostructure was favored along the c-axis, which is perpendicular to the FTO glass substrate. The highest optical transmittance of 82% in the visible range was attained by the sample prepared at the lowest spraying angle. The terrace-truncated nanocone structures support to increase the optical transmittance by reducing the light scattering, as suggested. The optical transmittance was decreased by increasing the spraying angle, due to the formation of structural defects such as nanoplates. The foremost electrical conductivity of 2.5×10³ ?^-1 cm^-1 was observed on the Ga-doped ZnO sample that was synthesized at the lowest spraying angle. The optimum transparent conductive oxide properties of high optical transmittance at the visible range as well as high electrical conductivity were attained by the Ga-doped ZnO nanostructure grown at the lowest spraying angle of 15^o.

    We gratefully acknowledge Prof. Masaru Shimomura, for his great support throughout the research. We also like to show our gratitude to Dr. Hirulak Siriwardena for his support on this research paper.

Attanayake, S.L.B., Murakami, K., Okuya, M., 2019. Synthesis and Characterization of Al Doped ZnO Terrace-truncated Nanocone Structure by Advanced Spray Pyrolysis Deposition Technique. Japanese Journal of Applied Physics, Volume 58(8), pp. 080904-1–080904-3

Bandara, A., Okuya, M., Shimomura, M., Murakami, K., Rajapakse R.M.G., 2016. Effect of Spray Directions on the Crystal Growth of Fluorine-doped Tin Oxide One-dimensional Nanostructured Thin Films. Journal of Advances in Physics, Volume 12(1), pp. 2347–3487

Bedia, F.Z., Bedia, A., Aillerie, M., Maloufi, N., Genty, F., Benyoucef, B., 2014. Influence of Al-doped ZnO Transparent Contacts Deposited by a Spray Pyrolysis Technique on Performance of HIT Solar Cells. Energy Procedia, Volume 50, pp. 853–861

Bramantyo, A., Murakami, K., Okuya, M., Udhiarto, A., Poespawati, N.R., 2019. Morphological and Structural Study of Vertically Aligned Zinc Oxide Nanorods Grown on Spin Coated Seed Layers. International Journal of Technology, Volume 10(1), pp. 611–622

Chen, Y., Bagnall, D. M., Koh, H., Park, K., Hiraga, K., Zhu, Z., Yao, T., 1998. Plasma Assisted Molecular Beam Epitaxy of ZnO on c-plane Sapphire: Growth and Characterization. Journal of Applied Physics, Volume 84(7), pp. 3912–3918

Fernández, S., Gandía, J.J., 2012. Texture Optimization Process of ZnO: Al Thin Films using NH₄Cl Aqueous Solution for Applications as Antireflective Coating in Thin Film Solar Cells. Thin Solid Films, Volume 520(14), pp. 4698–4702

Gao, P.X., Mai, W., Wang, Z.L., 2006. Superelasticity and Nanofracture Mechanics of ZnO Nanohelices. Nano Letters, Volume 6(11), 2536–2543

Han, S., Akhtar, M.S., Jung, I., Yang, O., 2018. ZnO Nanoflakes Nanomaterials via Hydrothermal Process for Dye Sensitized Solar Cells. Materials Letters, Volume 230, pp. 92–95

Hirata, G.A., McKittrick, J., Cheeks, T., Siqueiros, J.M., Diaz, J.A., Contreras, O., 1996. Synthesis and Optelectronic Characterization of Gallium Doped Zinc Oxide Transparent Electrodes. Thin Solid Films, Volume 288(1–2), pp. 29–31

Hsiao, C.H., Huang, C.S., Young, S.J., Guo, J.J., Liu, C.W., Chang, S.J., 2013. Optical and Structural Properties of Ga-doped ZnO Nanorods. Journal of Nanoscience and Nanotechnology, Volume 13(12), pp. 8320–8324

Hu, J., Gordon, R.G., 1992. Textured Aluminum-doped Zinc Oxide Thin Films from Atmospheric Pressure Chemical-vapor Deposition. Journal of Applied Physics, Volume 71(2), pp. 880–890

Lao, J.Y., Huang, J.Y., Wang, D.Z., Ren, Z.F., 2003. ZnO Nanobridges and Nanonails. Nano Letters, Volume 3(2), pp. 235–238

Le, H.Q., Lim, S.K., Goh, G.K.L., Chua, S.J., Ong, J., 2010. Optical and Electrical Properties of Ga-Doped ZnO Single Crystalline Films Grown on MgAl₂O₄(111) by Low Temperature Hydrothermal Synthesis. Journal of the Electrochemical Society, Volume 157(8), pp. H796–H800

Li, C., Lin, Y., Li, F., Zhu, L., Sun, D., Shen, L., Chen, Y., Ruan, S., 2015. Hexagonal ZnO Nanorings: Synthesis, Formation Mechanism and Trimethylamine Sensing Properties. RSC Advances, Volume 5(98), pp. 80561–80567

Liu, H., Avrutin, V., Izyumskaya, N., Özgür, Ü., Morkoç, H., 2010. Transparent Conducting Oxides for Electrode Applications in Light Emitting and Absorbing Devices. Superlattices and Microstructures, Volume 48(5), pp. 458–484

Look, D.C., 2001. Recent Advances in ZnO Materials and Devices. Materials Science and Engineering: B, Volume 80(1–3), pp. 383–387

Moditswe, C., Muiva, C.M., Juma, A., 2016. Highly Conductive and Transparent Ga-doped ZnO Thin Films Deposited by Chemical Spray Pyrolysis. Optik, Volume 127(20), pp. 8317–8325

Rao, T.P., Kumar, M.C.S., 2010. Physical Properties of Ga-doped ZnO Thin Films by Spray Pyrolysis. Journal of Alloys and Compounds, Volume 506(2), pp. 788–793

Rouhi, J., Mamat, M.H., Raymond Ooi, C.H., Mahmud, S., Mahmood, M.R., 2015. High Performance Dye-sensitized Solar Cells based on Morphology-controllable Synthesis of ZnO–ZnS Heterostructure Nanocone Photoanodes. PLoS One, Volume 10(4), pp. 1–14

Sholehah, A., Yuwono, A.H., 2015. The Effects of Annealing Temperature and Seed Layer on the Growth of ZnO Nanorods in a Chemical Bath Deposition Process. International Journal of Technology, Volume 6(4), pp. 565–572

Visser, D., Ye, Z., Prajapati, C.S., Bhat, N., Anand, S., 2017. Investigations of Sol-Gel ZnO Films Nanostructured by Reactive Ion Beam Etching for Broadband Anti-reflection. ECS Journal of Solid State Science and Technology, Volume 6(9), pp. P653–P659

Wu, G.M., Lin, H.H., Lu, H.C., 2008. Work Function and Valence Band Structure of Tin-doped Indium Oxide Thin Films for OLEDs. Vacuum, Volume 82(12), pp. 1371–1374

Yadav, A.A., Barote, M.A., Masumdar, E.U., 2010. Studies on Nanocrystalline Cadmium Sulphide (CdS) Thin Films Deposited by Spray Pyrolysis. Solid State Sciences, Volume 12(5), pp. 1173–1177

Yan, M., Zhang, Q., Zhao, Y., Yang, J., Yang, T., Zhang, J., Li, X., 2015. Applications of Transparent Conducting Oxides in Organic Light Emitting Devices. Journal of Nanoscience and Nanotechnology, Volume 15(9), pp. 6279–6294

Yim, K., Kim, H.W., Lee, C., 2007. Effects of Annealing on Structure, Resistivity and Transmittance of Ga-doped ZnO Films. Materials Science and Technology, Volume 23(1), pp. 108–112

Yin, Z., Liu, X., Wu, Y., Hao, X., Xu, X., 2012. Enhancement of Light Extraction in GaN-based Light-emitting Diodes using Rough Beveled ZnO Nanocone Arrays. Optics Express, Volume 20(2), pp. 1013–1021