Influence of structural changes on electrical properties of Al:ZnO films

Influence of structural changes on electrical properties of Al:ZnO films

Materials Letters 258 (2020) 126641 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue In...

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Materials Letters 258 (2020) 126641

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Influence of structural changes on electrical properties of Al:ZnO films Osman Urper, Nilgun Baydogan ⇑ Istanbul Technical University, Energy Institute, Maslak, Istanbul 34469, Turkey

a r t i c l e

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Article history: Received 27 July 2019 Received in revised form 5 September 2019 Accepted 7 September 2019 Available online 7 September 2019 Keywords: Sol-gel Optoelectronic Thin films Structural characterization

a b s t r a c t This paper presents a study of Al-doped ZnO films deposited at room temperature by a cost-effective solgel dip-coating technique. The influence of Al concentration on crystallinity, electrical and optical properties of films annealed in nitrogen and oxygen atmosphere has been studied. The resistivity has been decreased from 1.1–1.6  104 X.cm to 0.73–0.93  104 X.cm with the increase of Al concentration from 0,8 to 1,2 at. %, respectively. Transmission spectrum displays that optical transmittance has been displayed between 85% and 90% by increasing Al concentration. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Zinc Oxide (ZnO), as one of promising semiconductive metal oxide, has obtained considerable attention because of its optoelectronic properties. ZnO possesses a wide direct band gap of 3.37 eV and large exciton binding energy 60 MeV which means that excitons can exist in a material at room temperature [1]. ZnO has great potential by doping for optoelectronic applications. Depend on dopants (such as Al), the films demonstrate high conductivity, good optical transmittance and enhanced band gap [2]. Several techniques have been used for fabrications of Al doped ZnO thin films such as spray pyrolysis, magnetron sputtering, electrochemical deposition and sol-gel processing [3]. Sol-gel technique has gained more interest, due to its low cost, simplicity and easy for doping incorporation. The primary objective of the study is the forefront of applied research in the field of optoelectronic applications on Al:ZnO thin film. The structural changes with optoelectronic properties are indicated novel conclusive information to solve resistivity problems depending on comparative analysis of the structural changes of Al:ZnO derived under different atmospheric conditions [4].

(0.5 M) was dissolved in ethanol and DEA (Diethanolamine) was added as a stabilizer which exhibited catalyst activity during the dissolving process. The mixture was stirred at 60 °C on a stirrer until obtained the transparent solution. Four different Al concentration (0, 8–1, 0–1, 2–1, 6 at. %) were carried out. The solution was aged for 24 h at room temperature. For preparing thin films, the cleaned substrates were immersed in a solution seven times to obtain thin film layers in oxygen at room temperature. The preheating process was applied in a furnace for 10 min at 300 °C in oxygen to evaporate the solvent and chemical inorganics. The process for each layer was repeated seven times and annealed for 60 min at 550, 600, 700, 800 and 900 °C in oxygen and nitrogen atmospheric conditions. The XRD patterns were recorded by Xray Diffractometer (XRD) via Cu Ka radiation (k = 1.5418 A). The surface morphology and particle distribution were characterized by JEOL 6335F SEM. The thickness measurement was performed by a surface profilometer (Veeco Dektak-6 M). The electrical resistivity was determined by a four-point resistivity probe Keithley 4200.

3. Results and discussions 2. Experimental Al:ZnO films were fabricated by the sol-gel process using zinc acetate dehydrate [Zn(CH3COO)2.2H2O] as Zn precursor, ethanol as a solvent, Aluminum nitrate [Al(NO3)3.9H2O] as Al source. Zn ⇑ Corresponding author. E-mail address: [email protected] (N. Baydogan). https://doi.org/10.1016/j.matlet.2019.126641 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

Fig. 1a shows XRD patterns of zinc oxide thin films, including various Al doping (0.8–1.0–1.2–1.6 at. %), deposited on the glass substrate at 700 °C. The diffraction peaks at (1 0 0), (0 0 2), and (1 0 1) planes are matched via wurtzite ZnO which close to standard diffraction pattern (JCPDS Card No. 80-00075). The intensity of diffraction peaks at (0 0 1) and (0 0 2) exhibit c-axis preferred orientation. Although, peak intensity decreased with increasing of Al dopant concentration which was depended on difference

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Fig. 1. X-Ray diffraction patterns of ZnO:Al thin films doped with (a) four different Al amount, (b) annealed in oxygen at five different temperature values, (c) annealed in nitrogen and oxygen, d) Average Crystallite Size 1.2 at. % Al doped ZnO:Al thin films annealing with variety temperatures, e) The change of ZnO crystallite size with doping content.

between ionic radiuses of Zn (0,074 nm) and Al (0,054 nm) ions (Fig. 1e), hexagonal wurtzite structure of doped ZnO films gradually changed when Al3+ ions substituting Zn2+ ions. The crystallite size derived from XRD analysis results was determined according to Debye-Scherrer method [5]. A relationship is available between crystal size and X-ray diffraction patterns intensity and the layer number and diffraction intensity of (0 0 2) diffraction plane increased with the thickness. (0 0 2) diffraction peak features was analyzed using relative intensity i(0 0 2) = I(0 0 2)/[I(1 0 0) + I (0 0 2) + I(1 0 1)] [6]. The sol-gel process presented excellent control of the stoichiometry of precursor solutions, ease of compositional modification, as the particle size of nanoparticles synthesized by sol-gel process was controlled by concentration of reactants (precursor and solvent measured amounts sensitively). The reactants strongly affected the crystallographic orientation and morphology of the resultant ZnO films. The concentration of

catalyst (aluminum (Al)-doped ZnO) for moles and atomic grams were measured in the solution at sol-gel immersion technique. Hence changes in average crystallite size evaluated practically depending on relative intensity in Table 1. Table 1a has explained the changes of particle size with dopant concentration. Grain boundary decreased with increasing particle sizes by higher annealing temperature in Table 1b. The increasing Al dopant, which has smaller radius than the host atoms, decreased the film crystalline and average particle sizes. Therefore, crystallite size reduced because of the lattice distortion caused by radius difference between the Al dopant and the replaced atom in ZnO. Because of the crystalline average size should be feasible rate, the available results achieved via 1,2 at. % Al concentration. Increasing Al concentration (over 1,2%) caused crystalline distortion due to the difference between Al, Zn ionic radius. On the other hand, peak intensity increased and film peak position has reached a suitable

Table 1 Relative intensities changes in the peak diffractions and average crystallite sizes (a) depending on Al concentration of the Al:ZnO thin film, and (b) depending on annealing temperature at 1.2 at. % Al concentration. Dopant Concentration Al at. % (a) 0.8 1.0 1.2 1.6 Annealing Temp. (°C) (b) 550 600 700 800 900

Relative Intensity (1 0 0)

Crystallite Size (nm) (1 0 0)

Relative Intensity (0 0 2)

Crystallite Size (nm) (0 0 2)

Relative Intensity (1 0 1)

Crystallite Size (nm) (1 0 1)

74 73 95 67

41.12 30.22 28.45 19.32

98 84 86 79

42.21 30.54 27.52 20.15

100 100 100 100

43.10 31.45 29.04 21.07

Relative Intensity (1 0 0)

Crystallite Size (nm) (1 0 0)

Relative Intensity (0 0 2)

Crystallite Size (nm) (0 0 2)

Relative Intensity (1 0 1)

Crystallite Size (nm) (1 0 1)

55 70 61 72 57

17.90 22.14 29.80 33.22 36.01

100 94 97 96 84

22.80 25.58 28.36 39.47 43.42

71 100 100 100 100

21.84 29.00 43.00 58.00 66.00

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Fig. 2. SEM images ZnO:Al thin films annealed at various temperatures in different ambient a) 700 °C in nitrogen, b) 700 °C in oxygen, c) 800 °C in oxygen, d) 900 °C in oxygen.

Fig. 3. a) The optical transmittance spectrum of Al doped ZnO film annealed at 700 °C in oxygen, b) The optical bandgap plot for Al doped ZnO concentration at 700 °C in oxygen, c) The changes of the electrical resistivity of the Al doped ZnO thin films annealed at 700 °C in Nitrogen and Oxygen ambient.

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agreement with film crystallinity at between 550 and 900 °C in oxygen by increasing the annealing temperature (Fig. 1b). The film crystallinity enhanced Al (1.2 at. %) doped film samples (in Fig. 1c) presented a comparison of film samples for the annealed in nitrogen and oxygen at 700 °C. Because changes of crystalline size were between 20 and 70 nm and rise in film crystalline size tent to increase with the rise of annealing (Fig. 1d). There were no significant differences between oxygen and nitrogen atmosphere through their resistivity and optical transmittance [7]. Fig. 2 exhibits surface morphology from SEM images at 1.2 at. % Al amount of film annealed under different atmosphere conditions at 700 °C. Nanocluster spherical Al:ZnO particles were derived after the annealing treatment. The average size of particles was 41 nm (in nitrogen) and 28 nm (in oxygen) (Fig. 2a, b). Microstructure density increased with high dopant content. Due to the increasing dopant concentration, the grain size was decreased and it was obtained small grain which declined pore numbers. Many pores present considerable important for transition or blocking for electron-hole between electron-hole sources and substrate [8]. Even the film exhibit well-regulated grain boundary at 700 °C, however; the film has behaved as a trapezium crystal at 800 °C– 900 °C (Fig. 2c, d). High transparency for TCOs s is one of the most important factor, the optical transmittance determined via a spectrometer. According to the Al contents, Al: ZnO films exhibited high transmittance above 80% in the range of visible spectrum above 650 °C (Fig. 3a). Bandgap energies calculated from the absorption edges of 700 °C (Fig. 3b). The absorption edge for direct interband is determined in Eq. (1) [9].

ahv ¼ C hv  Eg

1=2

;

ð1Þ

where C is a constant for a direct transition, and ɑ is the optical absorption coefficient. The optical energy gap (Eg) can be calculated via (ɑhv)2 vs. hv for direct transitions (Fig. 3b), and the energy gap was determined via linear absorption edge part of the curve using Eq. (1) [10]. The optical band gap energy increases from 3.29 to 3.38 via increasing Al concentration (in Fig. 3). The sheet resistance of Al-doped ZnO decreased resistivity with increasing Al concentration from 0.8 to 1.2 at. %. According to increasing Al concentration from 1.2 to 1.6 at. %, the resistivity gradually decreased because of the gap differences between atomic radius which caused segregation at the grain boundary and decreased the carrier mobility (Fig. 3c). As a result, the lowest resistivity was obtained for 1.2 at. % concentration at 0.73 and 0.93  104 (X.cm) for films annealed in oxygen and nitrogen. 4. Conclusion Al:ZnO thin film was fabricated using sol-gel dip-coating method for optoelectronic applications. The film samples had a hexagonal wurtzite structure. The transmittance was higher than

85% in the visible region and the band gap energy increases from 3.29 to 3.38 eV by increasing Al concentration. The lowest resistivity was obtained between 0.73 and 0.93  104 X.cm. Thin-film with low resistivity (0.73  104 X.cm) was promisingly applied for optoelectronic devices derived by a basic and inexpensive method. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Authors thank to Prof. Dr. Huseyin CIMENOGLU for his support about fabrication process at laboratory in Materials Engineering, ITU, Turkey. This work was financially supported by ITU Scientific Research Projects Foundation, BAP, Turkey, Project No: 41978 for Ph.D. Thesis. References [1] O. Martínez, J.L. Plaza, J. Mass, B. Capote, E. Diéguez, J. Jiménez, Luminescence of pure and doped ZnO films synthesized by thermal annealing on GaSb single crystals, Superlattices Microstruct. 42 (2007) 145–151, https://doi.org/ 10.1016/j.spmi.2007.04.024. [2] M. Wang, K.E. Lee, S.H. Hahn, E.J. Kim, S. Kim, J.S. Chung, E.W. Shin, C. Park, Optical and photoluminescent properties of sol-gel Al-doped ZnO thin films, Mater. Lett. 61 (2007) 1118–1121, https://doi.org/10.1016/ j.matlet.2006.06.065. [3] O. Urper, O. Karacasu, H. Cimenoglu, N. Baydogan, Annealing ambient effect on electrical properties of ZnO:Al/p-Si heterojunctions, Superlattices Microstruct. 125 (2019) 81–87, https://doi.org/10.1016/j.spmi.2018.10.027. [4] M.G. Nair, M. Nirmala, K. Rekha, A. Anukaliani, Structural, optical, photo catalytic and antibacterial activity of ZnO and Co doped ZnO nanoparticles, Mater. Lett. 65 (2011) 1797–1800, https://doi.org/10.1016/ j.matlet.2011.03.079. [5] P. Swapna, S. Venkatramana Reddy, Synthesis and Characterization of Al Doped And (Co, Al) codoped ZnO Nanoparticles via Chemical co-precipitation Method, Asian J. of Nanosci. Mater. 2 (2018) 111–119, https://doi.org/10.26655/ AJNANOMAT.2019.1.8. [6] Sumetha Suwanboon, The properties of nanostructured ZnO thin film via SolGel coating, Naresuan Univ. J. 16 (2) (2008) 173180. [7] M. Murugesan, D. Arjunraj, J. Mayandi, V. Venkatachalapathy, J.M. Pearce, Properties of Al-doped zinc oxide and In-doped zinc oxide bilayer transparent conducting oxides for solar cell applications, Mater. Lett. 222 (2018) 50–53, https://doi.org/10.1016/j.matlet.2018.03.097. [8] M.C. Jun, S.U. Park, J.H. Koh, Comparative studies of Al-doped ZnO and Gadoped ZnO transparent conducting oxide thin films, Nanoscale Res. Lett. 7 (2012) 639, https://doi.org/10.1186/1556-276X-7-639. [9] O. Karacasu, Nanocrystalline ZnO: Al Thin Films Prepared By Sol-Gel Dip Coating Technique and ZnO:Al/P-Si Heterojunctions, MSc. Thesis, Thesis No. 310405, Istanbul Technical, University, Institute of Science and Technology, 2010, https://tez.yok.gov.tr/UlusalTezMerkezi/tezSorguSonucYeni.jsp. [10] E. Ziegler, A. Heinrich, H. Oppermann, G. Stover, Electrical properties and nonstoichiometry in ZnO single crystals, Phys. Stat. Sol. A 66 (1981) 63, https://doi. org/10.1002/pssa.2210660228.